Spring06

Yield Management Vo l u m e 8 I s s u e

1 S p r i n g 2 0 0 6 $ 5 . 0 0 US

S O L UYield Acceleration T Strategies I O N S for the Semiconductor Industry

SPECIAL FOCUS: Data to Decisions: Moving up the Knowledge Hierarchy for Enhanced Metrology Decision-making 15

6

COVER STORY — From rom Data ata to to Decisions ecisions Optimizing ptimizing Fin inFET Structures tructures with with Design esign-based based Metrology etrology

20 25

When hen to to Raise aise the the Red ed Flag lag 警告を出すタイミングを見極める

49

Automating utomating Investigation nvestigation Line ine Width idth Roughness oughness

of of

C O N T E N T S 6 Optimizing FinFET Structures with Design-based Metrology

60 In-chip Overlay Metrology in 90-nm Production

20 When to Raise the Red Flag

64 Reliable, Repeatable Wafer and Tool Dispositioning in 300 mm Fabs

For model-based biasing of a FinFET structure, design-based metrology (DBM) can be your methodology.

Disposition progressive mask defects before they impact the process window.

25 警告を出すタイミングを見極める

プロセスウィンドウに影響が出る前に進行性マス

In-die target insertion is an insurance policy against process issues, enabling in-die troubleshooting and potentially improving lot dispositioning.

Scrap or rework? An automated wafer and tool dispositioning system can deliver the fastest, most accurate go/no-go decisions

ク欠陥を取り除いてください。

33 Bridging the Gap Between Design and Mask

Reducing design respins through full-chip process window verification.

42 Opening the Window to Higher Parametric Yield at 32 nm

Strategies using design for manufacturability (DFM) and multi-variate advanced process control (APC) can be a powerful opponent against process window limitations.

49 Automating Investigation of Line Width Roughness

Full spectral analysis trumps the standard deviation in measuring and monitoring line width roughness.

Cover image by Carlos Hueso and Inga Talmantiene, KLA-Tencor and www.picturequest.com

56 Focusing on the Drifts

2

Cover

Compared with top-down CD SEM tools, spectroscopic ellipsometry provides a more effective way to generate insightful data for litho and etch process control.

Spring 2006

Yield Management Solutions

Stor y

15 From Data to Decisions

It’s all about the data. The right data leads you to decisions that drive process improvement in your fab.

S p r i n g

23 !LL CRITICAL DEFECTS ARE HIGHLIGHTED IN RED

2 0 0 6

44

50 B

A

Product 76 SpectraCD-XT Cost-effective Optical CD Metrology

76 TeraScan.ON CRITICAL DEFECTS STARlight-2 ARE HIGHLIGHTED IN

Cost-effective Reticle Contamination Inspection YELLOW OR GREEN

76 Viper 2435 Automated Wafer and Tool Dispositioning System

WEB Exclusives Predicting Line Edge Roughness through a Mechanistic Model Because the molecular nature of photoresist

materials can give rise to yield-limiting issues, considering this nature can help in the prediction of certain excursions, such as line edge roughness.

News 77 2367 UV Line Monitor for Rapid, Low Cost of Ownership Yield Ramp

77 eS32 C e-Beam Inspection for Faster Decision-making

77 DesignScan Lithography-aware Design Inspection

78 Candela CS20 High Brightness LED Production Monitor

78 P-16 and P-16OF Contact Stylus Profilers

79 MRW3 Quasi-static Tester Measurement System for MRAM and HDD Industries

79 KT Analyzer Parametric Analysis Solution

www.kla-tencor.com/magazine Leveraging Scatterometry to Enhance STI Etch Process Development Detecting across-chip variations is crucial

Yield Management Solutions is published by KLA-Tencor Corporation.

To receive Yield Management Solutions, subscribe online at: www.kla-tencor.com/magazine

to rapid STI etch process development. Scatterometry enables a robust, uniform, high-yielding STI process through multiple profile parameter measurements.

For literature requests, visit: www.kla-tencor.com/company/inquiry MG -YMSSPR- 02/06

Sections 4

For information, visit: www.kla-tencor.com

Editorial: And the Show Goes On

30 KLA-Tencor Rings the NASDAQ Closing Bell

Š2006 KLA-Tencor Corporation.

All rights reserved. Material may not be reproduced without permission from KLA-Tencor Corporation.

Products in this document are identified by trademarks of their respective companies or organizations.

46 Award: HongSun So, Samsung Best Engineer Honoree

Spring 2006

www.kla-tencor.com/magazine

3

Editorial S

e

c

t

i

o

n

s

And the Show Goes On Well, the show was an astounding success. The curtains rose on time, the players were brilliant, and each scene had us holding our breath. No, of course I’m not referring to the latest Broadway sellout, but to the recent Consumer Electronics Show. Dual-core processors are here to stay— turning us all into road warriors with on-the-go access to entertainment. It seems that every gizmo and gadget is all about power, performance, and price. Creative Zen Vision: M, touted as having the goods to give the iPod a run for its money, walked away with CNET’s Best in Show award.

This 30GB, 2.5-inch screen media powerhouse— which also supports album art, simultaneous photo viewing, and music playback, with at least four hours of battery life—costs a paltry $330. Power, performance, price. All in one little package.

And so my thoughts turn to SPIE Microlithography 2006. If CES is pure theatre, the SPIE community sits in the director’s chair. Showcasing both innovation and strategy, SPIE drives important decisions for our industry that are ultimately behind many of the products showcased at CES. Decisions that address questions like, which lithography process should I plan for on my roadmap: 193-nm immersion, EUV, e-beam, optical maskless, imprint or something entirely different? What XRET is best for a particular design? Is the design manufacturable? How forgiving is the process window?

4

Spring 2006

Yield Management Solutions

What’s the best strategy to control line edge roughness? What data is required to ascertain if the device will achieve performance specs?

Making knowledgeable decisions squarely rests on the quality of data and information one has. With this in mind, the focus of this issue is on enabling the most effective decision-making for patterning process control. “From Data to Decisions,” our cover story by litho guru Chris Mack, describes a conceptual framework for using metrology to systematically improve the progression from data to decision.

Progressive mask defects are an industry-wide reliability problem, particularly when the defects approach the critical state where the mask needs to be pulled out of production and sent for cleaning or repair. Promos Technologies’ “When to Raise the Red Flag” puts forth a new methodology for effective dispositioning of defective masks.

The concerns of leakage current at 65-nm is driving the adoption of FinFET structures. The decreasing size of these structures makes it particularly important to obtain good 2D and 3D pattern fidelity in lithography and etching. In “Optimizing FinFET Structures with Design-based Metrology,” IMEC examines the characterization of a detailed 2D layout and creation of a complete model of the lithographic process using design-based metrology.

Yield Management

solutions E d i t o r - i n -C h i e f Uma Subramaniam

Managing Editor Christine Young

Contributing Editors David Moreno, Lisa Garcia

Advanced fabs today require accurate and rapid disposition decision-making during manufacturing, as well as a quick assessment of tool and process module output. KLA-Tencor’s “Reliable, Repeatable Wafer and Tool Dispositioning in 300 mm Fabs,” argues the benefits of automated disposition versus manual disposition as a way to accelerate yield learning and improve fab productivity.

The extreme reticle enhancement technologies used at 65-nm and beyond increase the risk of design-related reticle defects. OPC verification before the mask-making step can help ensure design-intended device integrity. UMC’s “Bridging the Gap between Design and Mask” proposes a new methodology for full-chip process window monitoring.

I encourage you to carefully examine each article in this issue of Yield Management Solutions. Collectively, they address issues that are of grave concern to the lithography community.

Production Editors

Siiri Hage, Vidya Kumaravel

A r t D i r ec t o r a n d Production Manager

Harry Wichmann, Inga Talmantiene Design Consultants Carlos Hueso, Jovita Rinkunaite, Inga Talmantiene

Circulation Editor Nancy Williams

KLA-Tencor Worldwide Corporate Headquarters KLA-Tencor Corporation 160 Rio Robles San Jose, California 95134 408.875.3000

I n t e r n a t i o n a l O f f i ce s KLA-Tencor France SARL Evry Cedex, France 33 16 936 6969

See you at SPIE Microlithography 2006.

KLA-Tencor GmbH Munich, Germany 49 89 8902 170 KLA-Tencor (Israel) Corporation Migdal Ha’Emek, Israel 972 6 6449449 KLA-Tencor Japan Ltd. Yokohama, Japan 81 45 335 8200 KLA-Tencor Korea Inc. Seoul, Korea 822 41 50552

Uma Subramaniam Editor-in-Chief

KLA-Tencor (Malaysia) Sdn. Bhd. Johor Bahru, Malaysia 607 557 1946 KLA-Tencor (Singapore) Pte. Ltd. Singapore 65 6367 6788 KLA-Tencor Taiwan Branch Hsinchu, Taiwan 886 3 552 6128 KLA-Tencor Limited Wokingham, United Kingdom 44 118 936 5700

Spring 2006

www.kla-tencor.com/magazine

5

Lithography M

e

t

r

o

l

o

g

y

Optimizing FinFET Structures with Design-based Metrology Tom Vandeweyer, Christie Delvaux, Johan De Backer, and Monique Ercken, IMEC Gian Lorusso, Radhika Jandhyala, Amir Azordegan, Gordon Abbott, and Zeev Kaliblotzky, KLA-Tencor Corporation

Considering the engineering challenges in developing a reliable high-k gate stack that limits leakage current for planar transistors, fin field effect transistor (FinFET) structures may actually be needed at the 65-nm node. The decreasing sizes of FinFETs make it particularly important to obtain good 2D and 3D pattern fidelity in lithography and etching. This article examines characterization of a detailed 2D layout and creation of a complete model of the lithographic process using design-based metrology (DBM). This model can be used for model-based biasing of the FinFET structure.

Introduction

The characterization of fin field effect transistor (FinFET) structures, or other two-dimensional (2D) designs, becomes important with the decreasing sizes in future technologies. Robust measuring methods are therefore needed to characterize the changing shape of the structure during the different process steps. A good metrology approach is also important for the creation of robust simulation models. These models predict how a design will be patterned in resist.

Some typical concerns for FinFETs that need characterization are: the rounded corner (top-down view); fin width variation through pitch and as function of fin length; line edge roughness (LER); and sidewall roughness.

They all have an impact on the performance of the FinFET device. The magnitude of the rounded corner decreases the final length (or, source-drain distance) of the FinFET (Figure 1A). One of the problems stemming from this rounding phenomenon is the significant increase in fin width W when the fin length L is decreasing (Figure 1B). Variation in fin width, due to the rounding of the fin opening, will impact short channel effects. This effect increases for shorter fin lengths.1 As for most structures, critical dimension (CD) variations through pitch (Figure 1C) 6

Spring 2006

Yield Management Solutions

and as a function of fin length are undesirable, because they render the devices non-reliable.1 Since LER and sidewall roughness have an influence on electrical behavior, it’s also important to control them and keep them as low as possible.2,3 (LER and sidewall roughness will not be addressed in this paper).

Different methods can be used to reduce some of these effects.4 For example, adding serifs in combination with a conventional illumination, or applying strong off-axis illumination settings, like annular, will reduce the rounding of the corners. But, what will happen with the proximity behavior? The annular exposure setting will deteriorate the fin width variation through pitch, while the effect of using conventional illumination on the through pitch behavior will be smaller. Many variables play a role in the optimization of a 2D pattern, some with a larger effect than others. Two simple exposure settings will be tested in this first case: a conventional one and an annular one (the latter in combination with some basic serif introduction).

Since a full characterization of the 2D structure is wanted, design-based metrology (DBM) is introduced to decrease the effort that the creation of the measurement job takes.

DBM creates an automatic CD scanning electron microscope (SEM) job with hundreds of sites, starting from the design in GDS format. This development takes approximately one hour, whereas an engineer will spend several hours behind the SEM to create the job manually. DBM is used here in combination with an an off-line measure-

M

NM

B

i Ãi

t

r

o

l

o

g

y

For the baseline technology integration work (front-end of line (FEOL)), a 193 nm resist from JSR, AR237J at 230 nm film thickness (FT), is used on Brewer Science ARC29a organic bottom anti-reflective coating (BARC), FT = 77 nm. The stack for FinFET patterning (or, the active layer) is 65 nm silicon on 150 nm buried oxide (or, silicon-on-insulator (SOI) stack). A 60 nm TEOS oxide hard-mask (HM) is used during the patterning process for two reasons: to provide etch resistance for the silicon etching and to enable CD (HM) trimming.

A binary mask (BIM) is used to print an active pitch of 350 nm; the CD at mask level is 120 nm. The litho target is set at 100 nm. This target is chosen to have acceptable process latitudes (CD control) in litho.

A

NM

e

à >Ìi`

C Figure 1: The effect of the rounded corner: a) on the length of the FinFET. Illustrated with a 0.63NA, conventional 0.89s exposure (upper-left), and a 0.75NA, annular 0.89 outer-s and 0.65 inner-s (upper-right); b) as a function of the decreasing length of the FinFET; c) due to proximity effects, isolated features are printed smaller than the dense. In the FinFET structure, this proximity effect is seen between the inner (dense) and outer (semi-isolated) fins.

ment tool, enabling image-based measurements on SEM images, to further simplify the process. For two exposure settings, the 2D behavior of the FinFETs is studied intensively to build a resist model that will be used to optimize future reticle designs at IMEC. Experimental setup

All exposures are performed on an ASML PAS5500/1100 step-and-scan system, interfaced with a TEL Clean Track Act8. Maximum numerical aperture (NA) is 0.75. The total system is charcoal-filtered to prevent airborne base contamination. Top-down CD SEM metrology is done using a KLA-Tencor eCD-2 CD SEM tool.

Two different modules are used in the experiments.

On the one hand, different actual FinFET devices are explored for a full characterization. On the other hand, regular Mentor Graphics test pattern structures are used to build the resist model. Two parameters in the FinFET device are fixed: the width of the fins and the pitch, 120 nm and 350 nm, respectively (both 1X on reticle).

Three other parameters in the FinFET device are varied throughout the experiment. The first one is the length of the fin: the shortest is 180 nm and the longest is 1,45 µm (see Figure 6). The second variable is the biasing of the width of the outer fin in a multiple fin structure (see Figure 7). The outer fin width is varied from 90 nm to 150 nm in steps of 10 nm. The last variable is the placement of the serifs (See Table 1 and Figure 8).

The size of the serif is in all cases 90 nm by 90 nm. Two kinds of placements are present. In the first, the placement consists of the serif symmetric with respect to the corner (called OPC2), i.e. overlap in x- and y-direction is the same. The overlap is 75 nm, meaning that 15 nm of the original design is removed in both directions.

In the non-symmetric case (called OPC1), the overlap in y-direction is decreased; 40 nm is removed from the original design.

OPC0

No serifs

OPC1

OPC2

90 nm X 90 nm Non-symmetric (with respect to the corner)

90 nm X 90 nm Symmetric (with respect to the corner)

Table 1: Sizes and placement of serifs on the FinFET design.

Two exposure conditions are studied in more detail: a 0.63NA conventional 0.89s and a 0.75NA annular 0.89 outer s and 0.65 inner s.

Spring 2006

www.kla-tencor.com/magazine

7

M

e

t

r

o

l

o

g

y

Methodology

DBM methodology

The DBM tool used here was developed by KLA-Tencor.5 As input, it requires the GDS of the design and the coordinates of the measurement sites. With the coordinates of the sites of interest in the design, the DBM tool creates the pattern recognition templates for each site (Figure 2). The 2D pattern on the clip is compared with the 2D pattern on the wafer until an overlap between the two is found (the pattern recognition).

A

B

C

Figure 3: a) minimum gap; b) maximum gap; c) rounded corner.

At this point, the DBM tool has all it needs to create the automatic CD SEM measurement: coordinates of the positions and pattern recognition templates. If a measurement is needed, the tool defines by itself which measurment algorithm is used. There is also the possibility to acquire SEM images at each site for further analysis, as an off-line measurement on SEM images is available.

results to build a first resist model to optimize future FinFET designs.

A useful feature in this off-line measurement tool is the ability to measure several similar sites in batch mode.

Creation of the resist model

The software package used for the model building in this paper is Calibre WorkBENCH from Mentor Graphics.

The 216 sites in the Mentor Graphics line test module are automatically measured by combining the two tools described previously. For every change in exposure setting, resist and/or substrate stack, new measurements are needed for the calibration of the model.

Figure 2: Diagram of DBM tool creating a CD SEM job. As input, only a design in GDS format and coordinates of the measurement sites are needed.

The off-line measurement tool

KLA-Tencor has developed an off-line measurement tool that enables indirect measurements based on the saved SEM image. Possible measurement algorithms are: minimum or maximum gap width, the rounded corner algorithm, a contact-hole algorithm, and a line-width algorithm. All algorithms can measure multiple structures on an image, as shown in Figure 3. The minimum gap width algorithm is used later in the work to determine the smallest fin width. This approach is preferred because the standard line-width algorithm gives an average value of the line-width in the chosen measurement box. This means that any rounding along the length of the fin is not taken into account. The maximum gap width algorithm is applied to measure the fin length.

The rounded corner algorithm determines the magnitude of the rounding (top-down) of the corners in the FinFET device. Moreover, the goal is to use all these 8

Spring 2006

Yield Management Solutions

First a setup file is created, including the information of exposure setting and substrate. Then a default resist model is used to simulate the 216 sites of interest to compare them with the measured data. Immediately, the Model Flow Tool of Calibre creates a new resist model. A few iterations are needed to define the best agreement. Between two iterations, it is useful to check which sites have a good or bad correlation by using the Model Center of Calibre. The bad ones can be removed if there is doubt on the measured value. Finally, a bestfit resist model is defined.

Results and discussion

In total, 364 sites of interest are defined in the chip design to characterize the 2D behavior of the FinFET and to build the resist model. The CD SEM measurement was made by using the DBM tool since, as indicated before, it takes a lot of time to create the CD SEM measurement parameters manually.

Characterization of the FinFET structure

In this section, the FinFET will be described through discussion on: the rounded corner (top-down view), the fin width versus length, and fin width versus pitch.

M

One of the main concerns with decreasing the size of the fins is the magnitude of the rounded corner, since it has an impact on the effective length and width of the shorter fins. The rounding of the fin is characterized by the difference in area between the edge of the FinFET and the surrounding rectangle. The result for the four corners is added up. This can be done automatically by using the rounded corner algorithm on the off-line measurement tool.

By moving towards off-axis illumination settings such as annular or quasar, the corners become more squared (Figure 1A).4 As noted previously, two exposure conditions are used: a 0.63NA, conventional 0.89s and a 0.75NA, annular 0.89/0.65 so/sI. The magnitude of the rounded corners is smaller for the fins shorter than 750 nm when an annular exposure setting is used. This is not the case for the longer fins; both illumination settings give a similar result. The magnitude of the rounding increases with increasing fin length, independent of the exposure setting (Figure 4). The reason why a larger rounding is observed for the longer fin is that these corners are smoother. They have a longer tail compared to the shorter fins (Figure 5).

r

l

o

g

y

7AFER

/0#

/0#

/0#

Figure 6: The FinFETs with and without serifs on design, on reticle and printed in resist with a 0.75NA, annular 0.89/0.65 so/sI illumination setting.

/0# !NNULAR

/0# !NNULAR

/0# !NNULAR

i }Ì Ê­ ®

Figure 7: The magnitude of the rounded corner versus the fin length L for the different OPC versions (a 0.75NA, annular 0.89/0.65 so/sI).

/0# !NNULAR

o

2ETICLE

$ESIGN

/0# #ONV

i }Ì Ê­ ®

Figure 4: The magnitude of the rounding versus the fin length L for OPC0 (no serifs present). Two exposure settings are compared: a 0.63NA, conventional 0.89s and a 0.75NA, annular 0.89/0.65 so/sI.

>} ÌÕ`iÊ vÊ, Õ `i`Ê À iÀÊ­>Õ®

>} ÌÕ`iÊ vÊ, Õ `i`Ê À iÀ

t

Introduction of serifs decreases the magnitude of rounded corner. This can be seen immediately and visually in Top Down SEM (TD-SEM) (Figure 6). Analysis with the rounded corner algorithm results in the same conclusion (Figure 7). As before, the magnitude of the rounded

>} ÌÕ`iÊ vÊ, Õ `i`Ê À iÀÊÊ­>Õ®

Rounded Corner

e

/0# !NNULAR

/0# #ONV

i }Ì ­ ®

Figure 8: The magnitude of the rounded corner versus the fin length L for OPC1.

Figure 5: Rounding is more pronounced for the longer fins due to a longer tail

Two exposure settings are compared (0.63NA, conventional 0.89s and 0.75NA,

from one edge center to the other center.

annular 0.89/0.65 so/sI ). The same effect is observed for OPC2.

Spring 2006

www.kla-tencor.com/magazine

9

M

e

t

r

o

l

o

g

y

corners is smaller for the fins shorter than 750 nm when an annular exposure setting is used (Figure 8). But for the longer fins, both exposure settings give the same result when serifs are used, as has been seen when there are no serifs used. The use of off-axis illumination settings, like annular, helps in filtering out the part of the light falling in the NA pupil that is not relevant to the imaging of the densest structures. This improves the contrast, and thus the imaging, of smaller pattern details, like corners.

Width versus length of the fin

Another concern is the width variation as a function of varying fin length induced by optical proximity effects.

The length and width of the fin can be measured with a standard line/space width CD measurement algorithm or with the newly developed minimum/ maximum gap width algorithm.

Fins with a length below 530 nm are chosen to be checked for fin width variation caused by varying length. The same region will be used to characterize the width variation caused by the difference in pitch, seen in FinFET devices as the width variation between inner (dense) and outer (semi-isolated) fin.

As previously shown, introduction of serifs has a positive effect on the rounded corners. But how will it influence the width variation induced by the length of the fin? The length of the fin increases when serifs are used in the design. The increase is most pronounced for OPC1 (or, the non-symmetrically placed serifs).

The influence of the serifs on the width variation induced by the length of the fin is large. For example, together with conventional exposure, the width variation caused by the length has completely disappeared in the region of interest (Figure 10). So, this confirms what was stated before, “A part of this width difference is due to the impact of the rounded corner on the width for shorter fins�. In the case of OPC2 (or, symmetrical serifs) in combination with conventional exposure, the shorter fins become even smaller than the longer ones.

Also, when annular exposure is used, a large improvement is seen (Figure 10), but a width variation of 20 nm is still observed.

7IDTH NM

First, the influence of varying length on the width of the fins is checked. The range in width variation in the region of interest (180-530 nm length) is 60 nm for the conventional exposure and 71 nm for the annular exposure (Figure 9). Part of this difference is due to the impact of the rounded corner on the width for shorter fins. The larger range of the annular exposure setting (10 nm) is probably due to the larger proximity effect when an off-axis exposure setting is used.

,ENGTH NM /0# #ONV

Comparing the lengths of the fins for the two different exposures shows that for the annular setting the difference with the designed length is smaller.

/0# #ONV

/0# #ONV

A

/0# #ONVENTIONAL

7IDTH NM

/0# !NNULAR

7IDTH NM

,ENGTH NM

/0# !NNULAR

/0# !NNULAR

/0# !NNULAR

B

$ESIGNED ,ENGTH NM

Figure 9: Fin width as function of the designed length (OPC0). The change is larger for 0.75NA, annular 0.89/0.65 so/sI than for 0.63NA, conventional 0.89s.

10

Spring 2006

Yield Management Solutions

Figure 10: Smaller proximity effect for both OPC1 and OPC2, with: a) 0.63NA, conventional 0.89s; b) 0.75NA, annular 0.89 0.89/0.65 so/sI.

M

)NNER WIDTH /UTER WIDTH

t

r

o

l

o

g

y

For both exposure settings, the necessary bias is decreased when serifs are placed. For the conventional exposure, no bias is needed. Also, for the annular setting, the bias is decreased to 20 nm bias (an outer fin of 140 nm). The effect for the non-symmetric and symmetric serifs is approximately the same.

Building a resist model

/UTER WIDTH DESIGN NM /0# #ONV

/0# #ONV

/0# #ONV

A

)NNER WIDTH /UTER WIDTH

e

The combination of DBM with the off-line measurement tool is powerful for characterizing 2D patterns. It is also useful in retrieving the needed measurements for the creation of a resist model. On the used chip design to pattern the FinFET devices, there is also a Mentor Graphics line test pattern available. This pattern consists of 216 different sites: isolated lines, dense line patterns, and end-of-line structures (Figure 12). These structures have also been used to build a resist model.

/UTER WIDTH DESIGN NM /0# !NNULAR

/0# !NNULAR

/0# !NNULAR

B

A

B

C

D

Figure 11: Ratio of inner and outer fin width versus the designed width of the outer fin: a) 0.63NA, conventional 0.89s; b) 0.75NA, annular 0.89 0.89/0.65 so/sI.

Width versus pitch

Known proximity effects for standard line and space patterns will also play a role here and will become larger when off-axis illumination settings are used. In this paper, the pitch of the FinFET device is fixed at 350 nm, but the proximity effect is seen as a difference between inner and outer fin (Figure 1C). This result can be presented in two ways. The first approach is to plot the fin widths versus length. If the same width on design is used for inner and outer fin, the inner fin is printed larger than the outer fin. This is more pronounced for the shorter fins. A second representation is to plot the ratio between inner and outer fin width (Figure 11). In this way, it’s possible to define the best bias needed for the outer fin to print all fins on target (ratio = 1). When using conventional exposure, a bias of 10 nm on the outer fin (an outer fin of 130 nm) is enough to compensate for the optical proximity. When the annular setting is used, a bias of 30 nm is needed to compensate for this effect.

Figure 12: Example of the different patterns in the Mentor Graphics line test module: a) dense lines; b) dense line ends; c) an isolated line end; and d) an inverse line end.

As mentioned before, it is not really user-friendly to define the CD SEM measurement needed for this type of work manually. Thus, the DBM tool is used to automate it. The sites are grouped per similar design, because for most similar sites it is possible to do the analysis with the off-line measurement tool in batch mode. The CD data together with the coordinates and specifications of the sites are put in a data file format, readable as a sample file within Calibre.

Spring 2006

www.kla-tencor.com/magazine

11

M

e

t

r

o

l

o

g

y

Unfortunately, for every change in exposure setting, resist, and substrate stack, new measurements are needed for the recalibration of the model. The fitting is done using the Model Flow Tool and the Model Center Tool of Calibre WorkBENCH, as explained previously. Once a final model is defined, it is verified with the experimental data via two methods: top-down comparison of the model with the SEM image (a visual comparison) and a comparison of the actual measurements on wafer with measurement results retrieved from the simulated clips.

As an example, the results for 0.63NA, conventional 0.89s are shown (Figure 13). The correlation between the measured data and the model is 0.976.

A conventional illumination combined with serifs seems to be the best choice for the chosen design (120/350 nm - width/pitch). This exposure condition is beneficial for the decrease in width variation and to provide a less rounded corner. When smaller FinFETs are needed in the future (not only in width, but also in density), an annular setting will give better resolution and a bias will be needed to tackle the proximity effect.

The combination of DBM and off-line image-based measurements is also useful to retrieve the input needed for the creation of a resist model. It is shown that a resist model indeed can predict the patterning in resist (etch is not included in this paper).

The simulated 2D profiles overlap very well with the SEM images. A slight necking is observed on the simulated edge contour plots for the longer fins, but it is not present on the SEM images. This overlap illustrates the usefulness of this approach to check the accuracy of a resist model.

Acknowledgments

As a second check, actual top-down views on wafer are compared with simulated clips. The magnitude of the rounded corner gives similar results for both cases. For the longer fin lengths alone, the magnitude of the rounded corner is slightly smaller on the simulated clips than on the actual CD SEM images. This occurs since the tail (seen before on longer fins, Figure 5) on the simulated clips is not as long as on the real pattern in resist.

References

The model correlates very well with the actual data and will now be used to optimize future FinFET designs to decrease optical proximity effects.

Conclusions

The combination of design-based metrology with off-line image-based measurements is a useful tool to describe different parameters of a 2D pattern. The capability for 2D characterization is shown using a FinFET pattern. Rounded corner, width variation, introduction of serifs, biasing, and different exposure settings are the parameters that have been varied to gain an understanding of the patterning behavior of the 2D structure.

12

Spring 2006

Yield Management Solutions

The authors would like to thank Nadine Collaert (IMEC integration) and Ivan Pollentier for their useful discussions, as well as the algorithm group at KLA-Tencor for their support.

1. S. Xiong and J. Bokor, Sensitivity of double-gate and FinFET devices to process variations, IEEE Trans. Electron Dev. 50 (11), p2255-2261, 2003.

2. J. Croon et al., Line Edge Roughness: Characterization, Modeling and Impact on Device Behavior, IEDM, Electron Devices Meeting, p307-310, 2002 3. L.H.A. Leunissen et al., Full Spectral Analysis of Line Width Roughness, SPIE, Vol5752, p499-509, 2005 4. M. Ercken et al., Challenges in Patterning 45nm Node Multiple-Gate Devices and SRAM cells, Interface 2004 5. C Bevis “Design driven inspection or measurement for semiconductor using recipe� US 6,886,153 (2005)

This paper was originally published at INTERFACE 2005, the FUJI FILM Electronic Materials Microlithography Symposium.

May 22–24, 2006 Sheraton Hotel Boston, Massachusetts, USA www.semi.org/asmc

The 17th Annual IEEE/SEMI

ÂŽ

ASMC 2006

Advanced Semiconductor Manufacturing Conference

People Processes Controls ASMC 2006 continues the rich and established tradition of this premier conference dedicated to unveiling breakthroughs in semiconductor manufacturing from wafer fab productivity and profitability to advanced process controls and device yield.

With more than 90 peer-reviewed technical papers, and poster sessions, ASMC 2006 attracts engineers and managers from around the world.To register visit us at www.semi.org/asmc.

Media Sponsors:

Sponsored by:

Cover

Story

From Data to Decisions Chris A. Mack, Lithography Consultant John Robinson, KLA-Tencor Corporation

The value of metrology data is explored conceptually by describing the systematic progression from the data to a decision made in the fab. The Knowledge Hierarchy, a conceptual framework for understanding the increasing value of data as it becomes information, then knowledge, then a decision, is introduced. Carefully spelling out every step in the decision making process allows for an understanding of where the weak links in the chain are located, and which improvements will have the greatest impact on overall decision quality. This framework then allows one to properly assess the relationship between data quality and decision quality, and work towards systematically improving decision quality.

Introduction

How does one quantify the value of a metrology tool? Obviously this is a commonly asked question for both producers and purchasers of metrology equipment. While there are many answers and approaches used for specific applications of metrology data, there are some common themes that apply to all metrology value statements. Thinking about these commonalities over the last several years, we have developed a framework for understanding the value of metrology that we call “from data to decisions.”

A carpenter friend of ours is fond of saying, “Nobody wants a 1/4" drill. They want a 1/4" hole.” The drill is the most effective tool for getting the hole they really want, but the value comes from the hole. Likewise, nobody wants a metrology tool. What they really want is: • A process that’s in control and is predictable • Lower rework rates • Better bin sort (device performance) • Faster ramp to high yield • Sustained higher yields • Quick detection and elimination of yield crashes or potential yield crashes Metrology tools are just an effective means to achieve these primary goals of profitable semiconductor manufacturing. Obviously, if one is to express, and hopefully quantify, the value that a metrology tool adds to a fab, then one must clearly link the immediate use of the tool (collecting data of some sort) to the final goals of that use (improved fab profitability).

Spring 2006

www.kla-tencor.com/magazine

15

C

o

v

e

r

S

t

The Knowledge Hierarchy

Before tackling the problem of how to understand the value of metrology in a wafer fab, a few preliminary concepts and terms should be clarified.

What is the difference between data and information? Between information and knowledge? How are these concepts related? Data, information, knowledge, and finally acting on that knowledge to make a decision, form a chain of increasing value we call the Knowledge Hierarchy.

$!4!

).&/2-!4)/.

+./7,%$'%

To understand this hierarchy let’s define and give examples for each step. Data are, of course, the raw numbers or images provided by the measurement tool (from the Latin, it is “the thing given”). It is a collection of numbers with units and with known uncertainty (that is, known precision and accuracy). Information, on the other hand, is data in context. Information includes sufficient details of what was measured where and when so that it can be easily discerned from other similar collections of data. It is organized and accessible. Information may also include the filtering out of extraneous bits of the data or distilling the numbers down as much as possible (reporting a mean and standard deviation, for example). Knowledge is an interpretation of the information based on an understanding (that is, a model) of cause and effect. Whereas information answers the question of “what”, knowledge answers the question “why”. Finally, decision means acting on the knowledge obtained. In our fab context, this means acting with the intention of improving the fab’s bottom line (improving yield, bin sort, etc.).

16

Spring 2006

o

r

y

The table below provides a CD metrology example that clarifies the distinctions between data, information, knowledge, and decisions. Suppose a product lot uses a standard sampling plan where some number of CD measurements are made. The resulting collection of numbers, with their associated uncertainties (either measured or assumed), is the data.

As you might imagine, a collection of numbers out of context is next to useless. Thus, when the context is added (the targets were nominally 90 nm isolated lines in resist ar$%#)3)/. ranged along the scanner slit) and statistically interpreted (determining that the variation was statistically different from most other lots), the data becomes information. Already, the transformation of the data to information can be immensely useful.

But information alone is not enough.

What is causing the systematic ).&/2-!4)/. variation $!4! in CDs across the slit? In Table 1, the targets have been specifically designed for sensitivity to focus

errors. Adding other information (a separate measurement to monitor dose errors) and a previously calibrated model of how these targets vary with focus and exposure enables us to assign a cause to the variation seen in the data: there was a -80 nm focus tilt across the slit. Further, the model also allows us to estimate how much a process change might reduce the CD variation along the slit. This is knowledge. And it is powerful knowledge, because it allows us to understand what actions will cause what benefits. It allows us to make a decision.

From this example we can see that data adds value to the fab only when it moves up the Knowledge Hierarchy and enables a decision to be made.

The value to the fab is in the decision – or, more properly, in making the correct decision. Thus, the best way to judge the value of the data is to judge the value of the decision that the data enables. But we’re getting ahead of ourselves. First we must +./7,%$'% $%#)3)/. understand how to systematically move up the Knowledge Hierarchy from data to decisions.

Concept

Definition

Example (from CD metrology)

DATA

A collection of numbers, calibrated and with known repeatability

The measured CDs are 96.2 ± 0.9 nm, 94.4 ± 0.8 nm, etc.

INFORMATION

The right data, at the right time, in the right context, organized for access

Isolated 90 nm lines vary systematically by 6.5 nm across the slit for this lot

KNOWLEDGE

Interpretation of information to assign cause and effect

A -80 nm focal tilt adjustment would reduce the systematic CD variation across the slit from 6.5 nm to 2.1 nm

DECISION

Acting on the knowledge with Let’s make the focus tilt adjustment an expectation for benefit before the next lot is run because we believe it will have a positive, noticeable impact on yield

Table 1: Knowing how to reduce CD variation.

Yield Management Solutions

C

Moving up the Knowledge Hierarchy

The details of the Knowledge Hierarchy are very use-case dependent, with the most generic elements near the bottom and the most usecase specific elements at the decision stage. Data is characterized by accuracy and precision specifications that, in many cases, apply to a wide range of applications. Once the context is added to make information, however, we already have one or more decisions in mind. Targets to be measured are optimized for maximum sensitivity to some variables and minimum sensitivity to others, in order to be most useful in moving up the Knowledge Hierarchy.

Knowledge is extremely use-case dependent, working towards a specific decision. Thus, it seems that a systematic approach for defining a Knowledge Hierarchy must be top down: start with the question that you want to answer, the decision that you want to make.

Let’s consider an example. One of the most common and important use cases for parametric metrology data is lot dispositioning at photolithography: measure representative wafers from a lot after lithography but before etch to see if the lot should be reworked. Thus, the driving question (decision) is, “Should this lot be reworked?” Important and related questions are, “If this lot is reworked, what should be done differently?” and “Can we feed any information forward to the etch step that will improve the final results?” Let’s pick the first question, and see what is involved in making the decision. To do this, we must methodically list every step in the sequence of steps that go into that decision. On the right is an attempt at a fairly exhaustive listing of activities that go on, from start to finish, when making the lot go/no-go decision for the case of overlay.

o

v

e

r

S

t

o

r

y

I. Preparation A. Define the metrology tool to be used

• Required precision, accuracy and throughput

B. Design the measurement target C. Define the within-field sampling plan D. Put measurement targets in the chip design • Scribe kerf, interdie streets, within die

E. Define the full sampling plan

• Fields per wafer, wafers per lot, lot frequency

F. Create measurement recipe G. Create analysis recipe

• May include reticle data, lens distortion map

H. Create overlay spec for lot pass/fail

• Spec is intended to reflect device yield/performance • Spec may depend/influence sample plan, tool specs, analysis approach

I. Define the process (action plan) that applies the spec in production

II. Measurement A. Print wafers B. Transport wafers to overlay tool and load C. Select wafers to measure • May be manual or from host or recipe

D. Make measurements E. Perform analysis (usually automatically) F. Upload measurement and analysis results to host or 3rd party system

III. Analysis Method A. Method 1: compare raw data statistics to spec B. Method 2: compare modeled results (model coefficients, modeled max error, overlay limited yield) to specs C. Method 3: SPC-like analysis (check for out-of-control condition) D. Apply some combination of above methods E. Assess the quality of the data

IV. Decision Regimes

A. Obvious pass – send the lot on B. Obvious fail – rework the lot C. Gray Area Options

• Consider gray area as failure – rework the lot • Shrink the gray area – Make more measurements (repeat on same points, increase sample), possibly on a different tool – Change measurement algorithm for greater precision • Apply human judgement (last resort)

V. Decision Post-Mortem A. For reworked lots, how have things improved?

• Measure reworked lots • Compare new measurements to old • Did corrections work as expected?

B. For reworked lots, what is the root cause of the problem?

• What process changes would reduce rework rate?

• Are the processes and tools in control?

C. For passed lots, are things OK downstream?

• Correlation of overlay results to yield • Is the expected failure rate obtained?

D. Can the overall dispositioning process be improved?

• Relate results to fab metrics (yield, cycle time, throughput, CoO) • Time to results • Measurement costs • Cost of a bad decision

Spring 2006

www.kla-tencor.com/magazine

17

C

o

v

e

r

S

t

o

r

y

$!4!

Generally, there are five basic steps in the making of a standard production decision: preparation, measurement, analysis, decision, and post-mortem.

Preparation (or planning) is often underestimated in terms of its importance to the overall quality of the decision to be made. In particular, the sampling plan and the design of the measurement target will have a profound impact on the quality of the decision, often far in excess of the measurement uncertainty itself.

Proper preparation allows data to be seamlessly turned into information.

Of course, the measurement itself is important – no amount of planning or analysis can make up for uncertainty or inaccuracy in the raw data.

But a focus on measurement tool specifications out of context of the decision to be made has little value.

Analysis of the data (turning information into knowledge) assigns a probable cause to what is happening on the wafer. In overlay, it assigns correctables: if the lot had been run with these different settings, this is how much better things would have turned out. The decision is made by relating the knowledge of what could be done better with what is effectively a cost analysis: is it worth it to make a process change (e.g., to rework the lot)? The final step, after the decision has been made and implemented, is the post-mortem.

Have we learned anything from this experience that we can use to do things better next time?

As the example in the next section illustrates, there is a direct correlation between the basic way in which fab decisions are made and the process of moving up the Knowledge Hierarchy.

18

Spring 2006

).&/2-!4)/.

+./7,%$'%

$%#)3)/.

PREPARATION

System to turn data into information

MEASUREMENT

Create data

ANALYSIS

Turn information into knowledge

DECISION

Turn knowledge into action

POST-MORTEM

Improve the data to decision process

Assessing the value of data

Given a thorough understanding of how data moves up the Knowledge Hierarchy to become a decision, we now have a framework for how to assess the value of metrology. Data is valuable only in so much as it affects the quality of a decision. (Note that deciding to make no changes to the process is still an important decision to make, one that should be made actively, not passively.) If the process of making a decision is systematized (as is often the case in our increasingly automated fabs), it will be possible to make a quantitative correlation between data quality and decision quality.

For example, for the case of a go/ no-go lot dispositioning decision we break decision errors into alpha and beta errors. An alpha error (also called a type I error) is when we decide to rework a good lot. A beta (or type II) error is when a bad lot is not reworked. Each error has its own unique costs in the fab. The cost of making a bad decision is the cost of an alpha error multiplied by the probability that an alpha error will occur, plus the cost of a beta error times its probability of occurrence. Lowest overall cost occurs when the ratio of the probabilities of alpha errors to beta errors is equal to the ratio of the costs of beta errors to alpha errors. It is always easy to decrease one type of error at the expense

Yield Management Solutions

of the other, so good metrology and analysis will strive to both lower and balance the two types of errors. Given a systematic analysis and decision making method, one can relate the quality of the data and the actual problem occurrence probability to the probability of making alpha and beta errors. In that way, real dollar values can be a signed to measurements and to measurement specifications.

When determining correctables, data uncertainty can be related directly to uncertainty in the correctables (which then can be related to the value of the reworked lots, as well as the changing problem occurrence probability). Again, it is possible to use the systematic decision making process to quantify the impact of different measurement specifications, target quality, sampling plans, and data analysis models on the quality of the correctable decisions being made.

Quantifying the relationship between data quality and decision quality provides a tool for assessing the decision process as well. Given a certain data quality, what sampling, target design, and analysis method provides the needed decision quality?

Often metrologists focus on improving measurement precision when decision quality can more easily be improved with better preparation or analysis.

C

Conclusions

Metrology is valuable, which is why we make measurements. The value, however, can only be quantified by first describing the systematic progression from the data to a decision, then assessing the relationship between data quality and decision quality. The Knowledge Hierarchy is a conceptual framework for understanding the increasing value of data as it becomes information, then knowledge, then a decision. It maps directly to the actual steps used in a fab when metrology data drives decisions. Carefully spelling out every step in the decision making process allows for an understanding of where the weak links in the chain are located, and which improvements will have the greatest impact on overall decision quality. Metrology data only “adds value” to the wafers when it moves up the hierarchy to become a valuable decision.

o

v

e

r

S

t

o

r

y

Author’s note

Anybody who works for a metrology company has no doubt heard this refrain from a customer at some point or another: unlike process tools, metrology tools do not “add value” to the wafer. I’ve never quite understood what this comment means. Certainly it can’t mean that metrology tools aren’t valuable; otherwise, why would people buy them?

I think it means that the way in which metrology “adds value” to the wafer is more indirect, and thus harder to express in simple, short sentences. During my tenure at KLA-Tencor, I thought about this problem, and with my colleague John Robinson, developed a framework for understanding the value of metrology that we call “from data to decisions.”

- Chris Mack

Biographies Chris Mack was Vice President of Lithography Technology for KLA-Tencor from 2000 – 2005. He currently writes and consults in Austin, Texas.

John Robinson, Director of Marketing and Applications Development at KLA-Tencor, received his Ph.D. in physics at the University of Texas at Austin in 1995. Based in Austin, Texas, Robinson joined KLA-Tencor in 1997, and has been responsible for overlay, optical CD, and CD-SEM analysis products.

Spring 2006

www.kla-tencor.com/magazine

19

Lithography R

e

t

i

c

l

e

I

n

s

p

e

c

t

i

o

n

When to Raise the Red Flag Effective Dispositioning of Defective Masks Jerry Huang, Lan-Hsin Peng, and Chih-Wei Chu, ProMOS Technologies Kaustuve Bhattacharyya, Ben Eynon, Farzin Mirzaagha, Tony Dibiase, Kong Son, Jackie Cheng, Ellison Chen, and Den Wang, KLA-Tencor Corporation

Progressive mask defects are an industry-wide mask reliability problem, particularly when the defects approach the critical state where the mask either needs to be pulled out of production or sent for cleaning (or repair). This problem is especially troublesome with expensive high-end masks running deep ultraviolet (DUV) lithography. In these cases, the fab will want to sustain the problematic masks in production as long as possible, until just before the masks begin impacting the process window. This study found that while a small, growing defect may not print at the best focus exposure condition, it can still influence the process window, shrinking it significantly. Direct, high resolution reticle inspection enables early detection of these defects; however, fabs still need an effective means to disposition defective masks. A lithographic detector has been evaluated to see if it can predict the criticality of such progressive mask defects.

Examining the nature of mask defect growth

In a typical fab, many masks remain problem-free (clean) even after a large number of exposures. On average, about 1% of binary masks (at 365 nm lithography) and 6% to 15% of embedded phase shift masks (EPSMs) (using DUV lithography) show a defect growth problem through the duration of their usage in the fabs1,2. A direct, high resolution mask inspection can detect these defective masks effectively. But the nature of this defect growth can be severe on some masks, which means thousands of real crystal growth-type defects on the pattern side of masks. As one can imagine, this can

Figure 1. Progressive mask defects growth in fab.

20

Spring 2006

Yield Management Solutions

render the defect review session of these problematic masks to be quite complicated. However, a mask inspection tool such as KLA-Tencor’s TeraScan STARlight has the capability of binning the defects by size as well as by type (such as “on-chrome”, “on-clear”, “on-half-tone”, etc.). This helps to disposition masks that have a reasonable number of defects. When the total defect count grows on certain masks, however, traditional review techniques may result in a very lengthy review session.

Hence, it was decided that the new TeraScan STARlight-2 (SL2) should be used to evaluate a run-time mask error enhancement factor (MEEF) based detector that may isolate the defects of interest from those thousands of total defects caught by SL2.

In Figure 1 below, a fab’s incoming inspection indicates that a mask arrived clean, but after only 20 days of

R

e

t

i

c

production usage, the mask showed catastrophic defect growth. While defects on the mask image to the left can be easily reviewed individually (manually), the total count of the defects on the mask image to the right is so large that a manual review is difficult. A more automated way to review defects is needed.

Impact on the process window

Understanding the impact of small progressive defects on the process window is absolutely critical. Some critical mask defects will print on the wafer (collapse process window), and some will have no impact at all. But there will be many defects on these highly defective masks that will fall in between these parameters. They may not completely collapse the process window, but may reduce it. Criticality of mask defects has to be examined from this point-of-view, as any reduction in process window can potentially cause problems.

l

n

s

p

e

c

t

i

o

n

a big role. If the defect location and size are kept the same and the half pitch of the mask is reduced, the probability of the defect to cause bridging on the wafers will increase. This means a mask defect (in Figure 2, consider the 120 nm defect) that caused no problem in the past on older design nodes (like at the 135 halfpitch process) may now impact the process window on a new design node (90 nm half-pitch) due to denser geometry and higher MEEF. When the defect size becomes 200 nm, it completely collapses the process window.

Simulation Results at 193 nm Exposure and 0.75 NA

Figures 3 and 4 show the process window effect of a 120 nm defect on mask. With the tighter design rules, the volume of defects of concern continues to increase.

Focus

Exposure

It is also important to not only consider a defect from pure size, as contamination defects are not completely opaque and can be considered phase defects. A combination of defect size, transmission loss, and location should provide a good indication of the defect’s criticality. This makes MEEF-based binning necessary.

It can be seen from the simulation images in Figure 2 that the half pitch of the mask and defect location plays

I

e

Figure 3. A 120 nm mask defect did not impact the Focus process window for 135 nm half-pitch process.

Focus

Defect size (on mask)

K1 factor

135nm

120nm

0.52

135nm

200nm

0.52

135nm

320nm

0.52

90nm

120nm

0.35

Wafer image

Exposure

Mask defect

Exposure

Half pitch

Figure 4. A 120 nm mask defect reduced the process window significantly at 90 nm half-pitch process.

90nm

200nm

0.35

It can be seen that if the mask defect increases in size to 200 nm, there will be no process window left in the figure below for this 90 nm half-pitch process.

Figure 2. Mask defects being simulated on wafer — MEEF and defect size impact.

Spring 2006

www.kla-tencor.com/magazine

21

R

e

t

i

c

l

e

I

n

s

p

e

c

t

i

o

NM NODE %03- IS PRINTED USING A DOSE MATRIX

Focus

n

-ASK $EFECT $EFECT 0RINTED ON 7AFER

Exposure

"EST %XPOSURE #ONDITION .OMINAL DOSE

Figure 7. Mask defect impacting the process window — wafer images from CD Figure 5. A 200 nm mask defect collapses the process window at 90 nm

Developing the detector

half-pitch process.

Printed mask defect and the process window

A 110-nm node 193 EPSM was inspected and then exposed using a dose matrix. Dose was varied +/- 20%.

The results are as follows: -ASK

SEM tool.

-ASK $EFECT

$EFECT 0RINTED ON 7AFER

Litho3 and ReviewSmart on KLA-Tencor’s TeraScan mask inspection tool are the two detectors being characterized. This paper provides preliminary results from a few inspections performed with the new TeraScan STARlight-2 (SL2). A complete characterization was beyond the scope of this work. Further characterizations will be required in the near future to understand the implication of these detectors on various mask layers and nodes.

Litho3

A combination of defect size, transmission loss, and location should give a good indication of the defect’s criticality. So a MEEF-driven litho type detector was developed to use at run-time. With the proper setup, this detector is capable of binning critical defects under a single bin. This detector uses the following concepts: a. It contains a group of specialized detectors that operate run-time based on geometry and lithographic context

Figure 6. Mask defect printed on wafer.

A dose matrix exposure of this mask showed that even a small defect can impact the process window. The defect was visible at slight under-dose conditions. Even -5% dose variation from the best exposure condition enhanced the printability of the defect.

Substantial ongoing investigation is required to understand the printability of contamination defects. However, in this current work, the focus was kept on the development of an effective disposition method for mask defects (go / no-go criteria). For simplicity, all of the work here is based on an optical image from the mask (via the TeraScan inspection system), with the goal being to develop a run-time detector to provide speed.

22

Spring 2006

Yield Management Solutions

b. Defects of the same size and intensity will be binned differently based on the defect location MEEF; i.e., a certain defect located in a high MEEF area will go in a particular litho bin while a similar defect (in size and intensity) that is in a low MEEF area will remain excluded from this special bin.

If the litho detector is set up correctly, it will bin all the critical defects of interest in the special bin. The thresholds that control this bin are user definable from the setup. An initial evaluation showed encouraging results.

The mask below had roughly 700 defects. When the litho detector was used, some of these defects were binned in a special bin (run-time). All of the defects in

R

e

t

i

c

!LL CRITICAL DEFECTS ARE HIGHLIGHTED IN RED

.ON CRITICAL DEFECTS ARE HIGHLIGHTED IN YELLOW OR GREEN Figure 8. Litho3 detector binning the critical defects (in red) on a TeraScan SL2 system.

this special bin were critical in nature and can be seen below. These are all highlighted in red for visibility.

Non-critical defects are shaded in yellow and green. The user can tighten these special bin criteria so that fewer defects are shaded in red. But care must be taken to set these criteria correctly.

ReviewSmart

l

e

I

n

s

p

e

c

t

i

o

n

The ReviewSmart detector is developed based on the goal of more effective binning of similar defects3.

ReviewSmart identifies defects which are lithographically similar using a set of operator-specified thresholds.

These similar defects are then binned into a group. A large number of defects can be binned into only a few groups and within each group, defects are also ranked by severity. All of these happen at run-time from the main GUI (no extra simulations to run). The operator then just needs to classify one defect from each of these groups, rather than reviewing all defects and having the rest of the defects auto-classified. The following is an example from a highly defective mask where ReviewSmart was able to bin small crystal growth-type defects on isolated (non-critical) areas effectively, placing this type of defects in a single bin while keeping defects on dense geometry in a separate bin.

Conclusions

Progressive mask defects such as crystal growth and haze continue to threaten the industry. Resolution requirements have driven the IC industry to implement very low k1 lithography processes, which elevate the impact of mask errors as a result. Simulation data and then a real print-study on a 193 EPSM showed that some of the critical mask defects will print on wafer (collapsing process window) and some will have no impact at all. But there will be many defects on these highly defective masks that will fall in between these, impacting the process window if they print at slightly under-dosed conditions. Such defects may not completely collapse the process window, but they will reduce it. Criticality of mask defects has to be examined from this point-of-view, as any process window reduction can potentially cause a problem.

A certain percentage of masks (1% to 15%) show this progressive defect problem. High-resolution mask inspection will detect this defect problem. But initially, many of these defects are just forming and not so opaque in nature. During defect review on a mask inspection tool, sorting out critical defects from this ocean of nascent defects is no simple task. Disposition of such mask defects becomes a lengthy task when a large number of defects (mainly progressive

Figure 9. ReviewSmart binning of defects on TeraScan systems.

Spring 2006

www.kla-tencor.com/magazine

23

R

e

t

i

c

l

e

I

n

s

p

defects) are present on the mask. Effective disposition of a highly defective mask needs carefully set, productionworthy go / no-go criteria.

The new lithographic run-time detectors developed and tested during this work targeted only contamination defects. Some initial characterization will be required to fine-tune this detector in the fab. This study showed promise towards creating a helpful tool for mask disposition in the fabs. Efforts will continue over the coming months to perfect these detectors and their usage.

Acknowledgements

The authors would like to thank the following individuals for their contribution: William Volk, Qiang Li, Steven Labovitz, Ching Yun Hsiang, Paul Yu, Amir Azordegan, and Zhian Guo of KLA-Tencor Corp.

e

c

t

i

o

n

and Den Wang, Mask Defect Dispositioning, in 25th Annual BACUS Symposium on Photomask Technology, edited by J. Tracy Weed, Patrick M. Martin, Proc. of SPIE Vol. 5992, 59921X, (2005) CID# 599206 References 1. K. Bhattacharyya, M. Eickhoff, Mark Ma, Sylvia Pas, A Reticle Quality Management Strategy in Wafer Fabs Addressing Progressive Mask Defect Growth Problem at low k1 Lithography, Photomask Japan, 2005 2. K. Bhattacharyya, K. Son, B. Eynon, D. Gudmundsson, C. Jaehnert, D. Uhlig, A Reticle Quality Management Strategy in Wafer Fabs Addressing Progressive Mask Defect Growth Problem at low k1 Lithography, BACUS Symposium on Photomask Technology, 2004 3. P. Yu, V. Hsu, E. Chen, R. Lai, K. Son, W. Ma, P. Chang, J. Chen, Implementation of an Efficient Defect Classification Methodology for Advanced Reticle Inspection, Photomask Japan, 2005

Jerry Huang, Lan-Hsin Peng, and Chih-Wei Chu, Kaustuve Bhattacharyya, Ben Eynon, Farzin Mirzaagha, Tony Dibiase, Kong Son, Jackie Cheng, Ellison Chen,

2006

KLA-Tencor Trade Show Calendar February 21-22

SPIE Microlithography, San Jose, California

March 6-7

IC China, Shanghai, China

March 21-23

SEMICON China, Shanghai, China

June 6

IITC Hospitality Reception, Burlingame, California

July 12-14

SEMICON West, San Francisco, California

July 12

YMS West, San Francisco, California

24

Spring 2006

Yield Management Solutions Spring 2006

www.kla-tencor.com/magazine

43 33

R

e

Lithography

t

i

c

l

e

I

n

s

p

e

c

t

i

o

n

警告を出すタイミングを見極める 危険マスクを適切に判別するには Jerry Huang, Lan-Hsin Peng, and Chih-Wei Chu, ProMOS Technologies Kaustuve Bhattacharyya, Ben Eynon, Farzin Mirzaagha, Tony Dibiase, Kong Son, Jackie Cheng, Ellison Chen, Den Wang, KLA-Tencor 社

マスクの進行性欠陥は、マスクの信頼性に関わる業界全体の深刻な問題です。このような問題は、遠紫外線 (DUV) リソグラフィを実施している高コストのハイエンドマスクでは特に深刻です。そのような場合でも、工場 では、問題のマスクがプロセスウィンドウに影響を及ぼし始める直前まで使い続けることを望んでいます。この 研究によって、微小な進行性欠陥はフォーカス/露光条件が適切であればウェーハに転写されることはないが、 それでもプロセスウィンドウに影響を与え、プロセスウィンドウを大幅に狭めるということが明らかになりまし た。高解像度でのレチクルの直接検査により、これらの欠陥を早期に発見することはできますが、いまだに欠陥 マスクの効果的な判定方法を模索している工場が多いことも事実です。本文では、あるリソグラフィ欠陥検出ツ ールを評価して、このような進行性のマスク欠陥の致命度の予測可能性を考察した結果を報告します。 マスク欠陥の増大要因を調査する

一般的なウエハファブでは、多くのマス クは継続した使用後も異物問題と無縁で す。つまり、クリーンな状態を保ってい ます。平均では、波長365 nmリソグラフ ィによるバイナリマスクの約1%、また DUVリソグラフィによるハーフトーン位 相シフトマスク (EPSM) の約6~15%で、 製造工程でマスクを使用している間に欠 陥が増大する問題が発生しています1,2。 マスクを高解像度で直接検査すれば、こ のような欠陥マスクを適切に検出できま す。しかし、この欠陥増大の度合いは、 マスクによっては深刻なものになり、マ スクのパターン面に数千もの結晶成長型 の実欠陥が発生します。その結果、この

ような問題マスクの欠陥レビューセッションは非常 に難しくなります。KLA-TencorのSTARlight マスク 検査ツールは、欠陥のサイズやマスクのタイプ (遮 光部、透過部、ハーフトーンなど) 別に欠陥をビニ ングする機能があります。欠陥数がある程度であれ ば、この機能を使って効果的にマスクの良否判定を 行うことができます。しかし、総欠陥数が多い場 合、従来のレビュー技術によるレビューセッション は非常に長い時間を要します。今回、新登場のTeraScan STARlight (以下、“SL2”) のマスク誤差増大要 因 (MEEF) をベースとした検出ツールを評価して、 SL2で捕捉された数千単位の総欠陥数から重要な対 象欠陥のみを抽出できるかどうかを検証することに しました。 次の図1は、入荷時の検査ではクリーンな状態のマスク が、量産環境でマスクを20日間使用した後で、深刻な欠

Spring 2006

www.kla-tencor.com/magazine

25

R

e

t

i

c

l

e

I

n

s

p

e

陥増大が発生したことを示しています。左側にあるマ スク画像の欠陥は容易にひとつひとつレビューできます が、右側のマスク画像の総欠陥数は膨大で、人による個 々のレビューはほぼ不可能です。したがって、より自動 的に欠陥をレビューする方法が必要です。 プロセスウィンドウへの影響

プロセスウィンドウ内の微小な進行性欠陥の影響を 把握することは特別重要です。ウェーハに転写される (プロセスウィンドウを消滅させる) 重大なマスク欠陥 もあれば、まったく影響を与えない欠陥もあります。 しかし、このように欠陥の多いマスクでは、その多く が中間に属しています。このようなどちらともつかな い欠陥はプロセスウィンドウを完全に消滅させないま でも、狭める可能性があります。少しのプロセスウィ ンドウの縮小も潜在的な問題を抱えるため、この視点 からマスク欠陥の致命度を検証する必要があります。

c

i

o

n

ことがわかります。欠陥の発生場所とサイズが同じで もマスクのハーフピッチが小さいと、欠陥がウェーハ 上にブリッジを引き起こす確率は高くなります。これ は、旧デザインノード (135 nmハーフピッチプロセスな ど) では問題を発生しなかったマスク欠陥 (図2の欠陥 サイズ120 nmに注目) が、プロセスパターンの微細化と MEEF値の増加によって、先端デザインノード (90 nmハ ーフピッチ) のプロセスウィンドウに影響を与えるかも しれないことを意味します。欠陥サイズが200 nmになる と、プロセスウィンドウは完全に消滅してしまいます。

波長193nm露光とNA0.75によるシミュレーション結果

図3および4は、マスク上の120 nm欠陥がプロセスウィン ドウに与える影響を示しています。デザインルールの高 集積化によって、欠陥の致命度は増大します。

焦点

露光

また、異物欠陥の中には透明なものがあり、この場合は 位相欠陥と解釈されるため、純粋にサイズの点から欠 陥を検証するだけでは不十分です。欠陥サイズ、透過 損失、および欠陥の発生場所を考慮して初めて欠陥の致 命度を正確に把握できることを念頭に置く必要がありま す。それが、MEEF測定ベースのビニング機能が必須で ある理由です。 図2のシミュレーション画像を見ると、マスクおよび欠 陥の発生場所のハーフピッチが大きな影響を与えている

t

図3:ハーフピッチ135 nmプロセスでは、120 nmマスク欠陥はプロセスウィンド Focus ウに影響を与えない

Mask defect

Defect size (on mask)

K1 factor

135nm

120nm

0.52

135nm

200nm

0.52

135nm

320nm

0.52

90nm

120nm

0.35

Wafer image

Exposure

Half pitch

図4:ハーフピッチ90 nmのプロセスでは、120 nmマスク欠陥がプロセスウ ィンドウを大幅に縮小

90nm

200nm

0.35

図2:ウェーハ上でシミュレーションしたマスク欠陥 – MEEF値と欠陥サイズの影響

26

Spring 2006

Yield Management Solutions

次の図では、マスク欠陥のサイズが200 nmまで大 きくなると、ハーフピッチ90 nmのプロセスではプ ロセスウィンドウが完全に消滅することがわかり ます。

R

e

t

i

c

l

e

I

n

s

p

e

c

t

i

o

n

図7:プロセスウィンドウに影響を与えるマスク欠陥 – CD SEMツールによる ウェーハ画像 図5:90 nmハーフピッチプロセスでは、200 nmマスク欠陥がプロセスウィ ンドウが完全に消滅させる

マスク欠陥の転写とプロセスウィンドウ 110 nmノードによる波長193 nmのEPSMを検査し、露光 量マトリックスに従ってマスクを露光しました。露光 量は±20%変化させました。結果は次のとおりです。

欠陥検出ディテクターを開発する

今回の研究で評価対象としたのはKLA-TencorのTeraScanマスク検査ツールの、Litho3およびReviewSmartとい う機能です。本文では、新しいTeraScan STARlight (SL2) を 使用した暫定的な結果を報告します。本格的な評価は 本研究の範囲外であり、多様なマスクレイヤおよびノ ードに対する同検出ツールの効果を把握するには、今 後も広範囲な評価が必要になると思われます。

Litho3

欠陥サイズ、透過損失、および欠陥の発生場所とい う情報が揃わないと、欠陥の致命度を正確に予測す ることはできません。そのため、検査時に自動的に MEEF値に連動して実行するリソグラフィタイプ検 出ディテクター(Litho3) を開発しました。Litho3は、 適切にセットアップすることによって、重大欠陥を 1つのビンに集約することができます。Litho3は、次 のコンセプトに基づいて開発されています。

図6:ウェーハにマスク欠陥が転写

露光量マトリックスに従ってこのマスクを露光し た結果、微細な欠陥でもプロセスウィンドウに影 響を及ぼすことが判明しました。露光量が少し でも不足すると、マスク欠陥の転写が認められま す。つまり、最適な露光条件の露光量からわずか 5%減少しても、欠陥の転写性が増加しました。 異物欠陥の転写性を予測するには、今後も継続的な 調査が必要です。本研究において筆者らが目的とし ているのは、マスク欠陥の適切な判断方法を確立す る、つまり、マスクが量産に適しているかどうかの 判断の目安となる基準を設定することでした。本研 究では、TeraScan検査装置から得られたマスクの光 学画像を使用し、処理能力の高い欠陥検出およびレ ビュー機能を開発することを目的としました。

a. マスクパターンおよびリソグラフィのコンテキ ストに連動して検査実行時に動作する専用ディ テクターを有すること。 b. 同じサイズおよび強度の欠陥でも、欠陥の発生 場所のMEEF値に基づいてそれぞれ異なるビンに 分類すること。つまり、MEEF値が高い領域にあ る欠陥を専用リソグラフィビンに分類し、サイ ズと信号強度は同じでもMEEF値が低い領域にあ る欠陥はこの専用ビンには分類しない。 Litho3を正しくセットアップすれば、重要な欠陥は この専用ビンに分類されます。このビンを制御する しきい値はセットアップ時にユーザが定義できま す。初期評価では期待の持てる結果が出ました。 次の図に示すマスクには約700の欠陥があります。 Litho3を実行することにより、これらの欠陥の一部を専 Spring 2006

www.kla-tencor.com/magazine

27

R

e

t

i

c

l

e

I

n

s

p

e

c

t

i

o

n

ReviewSmart

ReviewSmart検出ツールは、類似する欠陥をさらに 効果的にビニングする目的で開発されました3。ReviewSmartは、オペレータが指定したしきい値を使用す ることによって、リソグラフィの観点からどの欠陥 が類似しているかを特定します。さらに、これらの 類似欠陥は同グループにビニングされます。多数の 欠陥でもわずか数個のグループにビニングされ、各 グループに分類された欠陥は重大度別にランク付け されます。これらはすべて、検査実行時並行して実 行され、しかも検査時間にほとんど影響がありませ ん。検査終了後、オペレータはグループ分けされた 欠陥をグループごとに分類していきます。以下は欠 陥の多いマスクのサンプルです。このサンプルでReviewSmartは、重要度の低いオープンエリア上の微小な 結晶成長型欠陥と、高集積度パターン上の欠陥を効 果的にビニングし、それぞれの欠陥を別個のグルー プに分類しています。 結論 図 8:TeraScan SL2システムのLitho3ディテクターによる重大欠陥のビニン グ (赤色)

用ビンに分類することができました。この専用ビンに 分類された欠陥は現実に重大であり、次の図のように 表出しました。重大欠陥はわかりやすいように赤で示 してあります。重大欠陥以外の欠陥は黄色と緑色で示 しています。専用ビン基準を厳しく設定して赤いビン に分類する欠陥を少なくすることもできますが、その 際は設定を慎重に行う必要があります。

結晶成長やヘイズなどの進行性のマスク欠陥は引き 続き業界にとって脅威となっています。解像度要求 が厳しくなり、IC業界は非常に低いK1係数を持つリ ソグラフィプロセスを導入しようとしています。そ の結果、マスク上の誤差がウエハ上のパターンに与 える影響が大きくなっています(MEEF値の増大)。波 長193 nmのEPSMに関するシミュレーションデータや 実際の転写テストによって、マスク上の欠陥によっ てはウェーハに転写されてプロセスウィンドウを消 滅させるものと、まったく影響を及ぼさないものが あることが判明しました。しかし、 これらの中間に属する欠陥でも、露 光量が少しでも不足するとプロセス ウィンドウへの影響があることも明 らかになっています。このような欠 陥はプロセスウィンドウを完全に消 滅させないまでも縮小させます。プ ロセスウィンドウの縮小が問題を引 き起こすという可能性を考慮し、こ の観点からマスク欠陥の致命度を検 証する必要があります。 進行性欠陥の問題は、マスクの特 定の割合 (1~15%) で認められます。 高解像度によるマスク検査を実施 すれば、このような欠陥問題を検 出することは可能ですが、初期段 階では欠陥が形成途上にあり、光 を通してしまいます。マスク検査 ツールによる欠陥レビューにおい て、このような初期段階にある大

28

Spring 2006

Yield Management Solutions

R

e

t

i

c

量の欠陥から重大欠陥を識別することは容易ではあ りません。マスク上に多数の欠陥(基本的に進行性欠 陥)が発生している場合、マスク欠陥の分類作業は非 常に長い時間を要します。欠陥の多いマスクを適切 に判断するには、マスクが量産に適しているかどう かの判断の目安となる基準を慎重に設定する必要が あります。 本研究で開発して試験を実施した新しい“Litho3”リソ グラフィ欠陥検出ディテクターは、異物欠陥のみを 検査対象としています。量産環境にこのツールを導 入するには、同ツールの微調整のために初期評価が 必要となるでしょう。本研究により、製造工場での マスクの良否判断に役立つツールの開発に明るい前 途を見出すことができました。ただし、理想的なデ ィテクターセッティングを開発して用途を体系化す るために、今後数ヶ月で取り組む予定です。 謝辞

本研究に取り組むにあたり、次の方々から支援、協力を いただきました。この場を借りて感謝の意を表します。 KLA-TencorのWilliam Volk氏、Qiang Li氏、Steven Labovitz氏、Ching Yun Hsiang氏、Paul Yu氏、Amir Azordegan氏、Zhian Guo氏。

l

e

I

n

s

p

e

c

t

i

o

n

Farzin Mirzaagha氏、Tony Dibiase氏、 Kong Son氏、 Jackie Cheng氏、Ellison Chen氏 およびDen Wang氏、 第25回フォトマスク技術に関する年次BACUSシ ンポジウムにおける、写真・光化学計測技術者 協会事務弁護士Patrick M. Martin氏、J. Tracy Weed氏 編纂危険マスクの判別 Vol. 5992, 59921X, (2005) CID# 599206 参考文献 1. K. Bhattacharyya, M. Eickhoff, Mark Ma, Sylvia Pas, A Reticle Quality Management Strategy in Wafer Fabs Addressing Progressive Mask Defect Growth Problem at low k1 Lithography, Photomask Japan, 2005 2. K. Bhattacharyya, K. Son, B. Eynon, D. Gudmundsson, C. Jaehnert, D. Uhlig, A Reticle Quality Management Strategy in Wafer Fabs Addressing Progressive Mask Defect Growth Problem at low k1 Lithography, BACUS Symposium on Photomask Technology, 2004 3. P. Yu, V. Hsu, E. Chen, R. Lai, K. Son, W. Ma, P. Chang, J. Chen, Implementation of an Efficient Defect Classification Methodology for Advanced Reticle Inspection, Photomask Japan, 2005

Jerry Huang氏、Lan-Hsin Peng氏 および Chih-Wei Chu氏、Kaustuve Bhattacharyya氏、 Ben Eynon氏、

Spring 2006

www.kla-tencor.com/magazine

29

KLA-Tencor Rings the On Nov. 18, 2005, KLA-Tencor executives gathered at the NASDAQ offices in Times Square to ring the bell that closed the day’s trading activity. The New York City event marked a passing of the torch from retiring CEO Ken Schroeder to newly appointed CEO Rick Wallace.

Ken Schroeder bids NASDAQ a fond farewell.

(L-R) John Kispert, Ken Levy, Rick Wallace, and Ken Schroeder team up to ring in a new era for KLA- Tencor.

30

Spring 2006

Yield Management Solutions

NASDAQ Closing Bell

In NASDAQ tradition, signatures are gathered for display on the Market Site Tower — the epicenter of financial news and events.

This modern day icon is a seven-story high-tech electronic display that illuminates Times Square 24 hours a day.

Spring 2006

www.kla-tencor.com/magazine

31

w w w . k l a - t e n c o r. c o m / l i t h o

]SYV TEXXIVRMRK TVSGIWW GSRXVSP VIWSYVGI

0)%62 136) 0EXIWX WXVEXIKMIW JSV 6IXMGPI 5YEPMX] %WWYVERGI 0MRI[MHXL ERH 3ZIVPE] 'SRXVSP 73098-32 8330/-87 )EW] EGGIWW XS SYV TSVXJSPMS SJ TEXXIVRMRK TVSGIWW GSRXVSP XSSPW

( %8%

- 2 * 3 6 1 %8 - 3 2

/ 2 3; 0 ) ( + )

()'-7-327

6)+-78)6 *SV YTGSQMRK ;IFMREVW ERH )ZIRXW %7/ 8,) )<4)68 .YWX GPMGO XS VIUYIWX ]MIPH GSRWYPXMRK SV ETTPMGEXMSRW &/1W 79&7'6-&) *SV IEVP] RSXM´GEXMSR SJ YTHEXIW

/0% 8IRGSV 'SVTSVEXMSR

Lithography D F M

Bridging the Gap Between Design and Mask Physics-based Model for OPC Verification Yung Feng Cheng, Yueh lin Chou, Chuen Huei Yang and CL Lin, UMC Corporation Bo Su, Gaurav Verma, William Volk, Mohsen Ahmadian, Hong Du, Abhishek Vikram, Scott Andrews, KLA-Tencor Corporation

Disparate inspection strategies have given way to various approaches for verifying post-OPC designs for manufacturing.

However, a new paradigm has emerged in design verification that moves OPC verification from the design plane to the wafer plane, where it really matters. KLA-Tencorâ&#x20AC;&#x2122;s DesignScan system inspects the OPC decorated design by simulating how the design will be transferred to the reticle layer and how that reticle will be imaged into resist across the full focus-exposure calibration window. Building on this paradigm is a new methodology on process window monitoring for OPC databases using DesignScan and report results for a chip. This methodology will be explored in this article, along with new applications in areas such as reticle target CD specification.

Introduction

As technology progresses towards 65 nm and beyond, optical lithography faces increased difficulties, and reticle enhancement techniques (RET) become imperative for multiple process layers. With RET implementation, especially the optical proximity correction (OPC) technique, the mask layout deviates further from the design-intended layout. Thus, the need for linking design space with process space to ensure designintended device integrity is even greater.

For the OPC decorated design database, it is important to verify the OPC before the mask-making step. This verification step can help ensure that there are no design-related defects, and it can provide a reasonable process window for a given process. Design rule or optical rule checkers have typically been used successfully to find design-related defects at best focus and exposure conditions. However, for full chip process window verification, there was no such system available until now. The main requirements for process window monitoring are good resist modeling and inspection speed.

We previously reported an integrated approach using DesignScan. In the article, we detail DesignScanâ&#x20AC;&#x2122;s defect detection capabil-

ity, model accuracy by comparison to wafer scanning electron microscopy (SEM) images, and simulation speed1. We also propose new use cases for DesignScan for design-based process monitoring and control, as well as process window impact-based mask specifications.

Uncovering pattern-dependent systematic defects

DesignScan 290 is a new inspection system from KLATencor that detects pattern-dependent (feature-based) systematic defects in the lithography process window of the post-RET design. The inspection is accomplished by simulating the transfer of the design to the reticle plane and subsequently projecting the reticle image onto the photoresist. The simulations are conducted at nominal condition and at user-defined off focus-exposure conditions through the process window and beyond. The normal DesignScan inspection is a two-step process. First comes the best focus and exposure (F0E0) inspection (the simulated image to database inspection), in which the simulated resist images at the best focus and exposure condition are compared to the pre-OPC design database (design intent) to detect possible defects due to OPC decoration. Second is the process window inspection (the simulated image to simulated image inspection), which uses the best focus and exposure condition resist images as the reference. In this phase, each simulation within the process window is compared to the one at nominal conditions to detect any unacceptable variation in pattern fidelity.

Spring 2006

www.kla-tencor.com/magazine

33

D F M

The DesignScan system consists of hardware and software components, many of which are based on the KLA-Tencor TeraScan die-to-database reticle inspection system and the KLA-Tencor PROLITH simulation software. The system is configurable for a selected throughput target.

Prior to the DesignScan inspection, the post-RET/OPC designs are converted to a proprietary KLA-Tencor format optimized for inspection throughput. During inspection, the converted design is segmented into patches and fed to the proprietary image computer, which is used for both the simulation of the design patches and the detection of defects. The image computer is a multiple processorbased supercomputer capable of simulating multiple jobs simultaneously. Design patches that fail within the process window are noted as defective not only within the process window, but anywhere in the FE space which is simulated. Defect reports are sent to the operator consol, which is also used for inspection setup and review.

The simulation models three distinct steps in the lithography process: the transfer of the pattern to the reticle, the formation of the aerial image, and the formation of the resist image. The models for the reticle manufacturing process and aerial image formation, which uses vector imaging, are based on mature TeraScan technology used for die-to-database reticle inspection. The resist image formation and development are based on a proprietary fast resist model and PROLITH technology, also well developed and tested.

Each of the three models is calibrated or specified separately and could be decoupled, due to the physics-based nature of those models. Physics-based models are used to simulate reticle images from the design. The aerial image model is specified by providing the following basic illumination parameters: illumination wavelength, parametric or measured source shape, and objective lens NA. The resist model is calibrated by matching physical parameters to tuning data. The tuning data consists of focus-exposure matrix (FEM) critical dimension (CD) data from one-dimensional lines and spaces at various pitches and sizes. Due to physics-based resist modeling, any FE condition can be selected for simulation within the calibration space (tested up to 2x process window).

By contrast, empirical models have to be calibrated at each FE condition at which the simulation has to be done. Also, physics-based models require recalibration only if the physics of the process is changed, the polygons can be changed arbitrarily, and no recalibration is required. For empirical models used by other OPC verification tools, a recalibration is required if the physics or the polygons are changed. This leads to a significantly 34

Spring 2006

Yield Management Solutions

reduced calibration burden for physics-based models as compared to the burden required for maintaining empirical models.

Creating the right recipe

At runtime, the user creates an inspection recipe through a menu-driven graphical user interface (GUI) shown in Figure 1. It is at this point that the design database to be inspected and the previously calibrated or specified models are recalled. The user also selects the range of focus and exposure conditions to be used in the inspection during this step. Each focus and exposure condition to be simulated can be defined individually and need not be a regular array of points. Any segment of the design that has been converted to the DesignScan format can be inspected. The system supports the definition of multiple inspection areas, each of which can have independent defect sensitivity settings. The sensitivity setting can be changed for each detector and each area independently. The runtime setup process typically takes less than 10 minutes.

Once the recipe is defined, the inspection process begins and each patch of the design is processed through the simulation and defect detection models. It is at this stage that the various simulations for each design patch are compared to the simulation at nominal conditions.

The defect detection algorithm checks for the following types of defects: bridges, breaks, extra and missing printed features, minimum resist width and minimum space width (CD variation defects), line-end shortening

Figure 1: The DesignScan setup GUI. The recipe is created in a series of steps that begin by selecting the design to be inspected.

D F M

for prioritizing the structures for corrections. It also enables correction of those defects which have the biggest process window impact first, since they are the process window limiters. Without such a guide, one may correct those defects which are not the process window limiters; as a result, no process window improvements will be seen.

Experiments: SRAM test database and Metal 1 layer

Figure 2: The DesignScan review GUI. This interface is configurable and multiple configurations can be saved as templates.

(LES), and interlayer overlap (ILO) defects. The defect detection is based on any topographical change between a test resist profile and the reference resist profile A proprietary defect binning model is used to group all lithographically identical defects. This greatly improves review efficiency. During inspection, DesignScan applies lithography hierarchy to perform binning, i.e., patterns are compared within the lithographically significant distance for matching and binning. The efficiencies gained by the defect binning system will be dependent on how the pattern repeats in the design, among other factors.

The binning is particularly useful when certain defective pattern is repeating, like in memory devices. In the example shown below, using the SRAM design database, a reduction in the number of unique defects by a factor of one and half order of magnitude has been observed.

An SRAM test database, called SMP, was chosen for the test. This SMP database is designed using the 65 nm design rule, with a minimum half pitch of 90 nm. The database is decorated using model-based OPC, and the Metal 1 layer serves as the test layer. Two FEM wafers were printed using UMCâ&#x20AC;&#x2122;s Metal 1 process with resist calibration patterns and the SMP test database patterns.

The focus and exposure range extends beyond the process window. Four line/space patterns are used for model calibration. While the model accuracy was 4.1 nm when this work was originally completed, we have made significant improvements on model accuracy and recently demonstrated 2.5 nm for a poly-silicon layer. We expect to achieve better than 2 nm model accuracy. A Bossung plot of the measured CDs from the FEM wafers and the DesignScan model-predicted CDs for a dense line is shown in Figure 3.

The SMP database is 3.9 x 8.6 mm2 in size on a wafer scale. A typical DesignScan inspection with 9 FE conditions takes about 62 minutes for this database. The FEM data: space 100p 190.data observation model prediction

140

120

cd (nm)

Review can occur after or concurrent with the inspection. The defect review application shown in Figure 2 can be used to display a wide range of defect-related information, such as database images, mask images, aerial images, and resist images. The display is userconfigurable and a configuration can be saved as a template and recalled later. The defects can be sorted by process window impact, as well as defect type or defect ID. It is very useful for process window monitoring to sort defects based on the process window impact. Such capability is only possible by DesignScan because one can select any FE grid for simulation within the calibration window. By knowing where a defect appears in FE space and its relative location to the nominal condition, DesignScan can easily obtain the process window impact separation and sort defects based on the process window impact. Such sorting provides a powerful guide

160

100

80

60

40

-200

-150

-100

-50

0

50

100

150

200

250

defocus

Figure 3: The Bossung plot of the measured CD data and DesignScan simulated CD data from the model calibration for a dense line.

Spring 2006

www.kla-tencor.com/magazine

35

D F M

system specifies that, for an 8 mm x 8 mm database, simulation with 9 FE conditions finishes within 2.5 hours. Nine FE conditions are a minimum requirement to adequately monitor the designâ&#x20AC;&#x2122;s performance across the process window by varying focus, dose, and both (diagonally) around the nominal condition.

Design RET application DesignScan 9 FE inline inspection.

Defect found?

Fix Yes

In OPC database process window inspection, the DesignScan PW inspection. following process flow is proposed (see Figure 4): Hot spot found. after OPC decoration, perform a DesignScan inline inspection at 9 FE conditions (3x3). If there Mask tape out Pass on hot spot Local fix few hot are defects found within the process window, information to wafer spots to extend then either the RET has to be modified to fix process monitoring process window the defects or the process has to be re-centered for the design, until no defects are present in the Figure 4: The process flow chart in design process window monitoring for DesignScan. process window. Subsequent to the design being clean in the process window, DesignScan process proportional to the number of defects found. Extending monitoring inspectionâ&#x20AC;&#x201D;pushing FE conditions defocus 100 nm and dose 2 mJ/cm2 further out uncovfor simulation beyond the edge of the process window ered no defects [see Figure 5 (b)]. and to find hot spots (weak spots) in the chipâ&#x20AC;&#x201D;should be run. Depending on the FE conditions (distance from In order to find weak spots, the FE conditions are the process window) and the number and nature of hot extended even further outside the process window. Gap spots found, one of the following actions can be selected: defects start to appear on the high dose side, and bridge send to mask tape out since it passed inline inspection defects appear on the low dose side [see Figure 5 (c)]. or perform local fixes to a few hot spots to extend the 1332 thin line and gap defects were found at a dose process window further (optimization); at the end, pass 14.3% higher than the nominal dose and at 180 nm dethe hot spot information downstream to monitor those focus. The proprietary advanced defect binning (ADB) hot spots in mask making and wafer processing (helping capability reduced the total defects to 21 unique groups, wafer process control), since these patterns are the most due to the repeating pattern nature of the memory sensitive and first to fail in the design due to process device. Similarly, 2374 thin trench and bridge defects window drift or change. were found at a dose 21.4% lower than the nominal dose and at 180 nm defocus. DesignScan ADB reduces Results and analysis the total defects to 27 unique groups. Since gap defects appear closer (14.3% vs. 21.4% relative to the nominal 1. Process window monitoring The SMP test database was already well debugged using wafers before it was inspected on DesignScan. However, the process could be improved and completed faster with a simulation-based system like DesignScan.

DesignScan inline inspection found no defects within the process window, as shown in Figure 5 (a). The FE conditions to be simulated are selected at the edge of the process window and to match the wafer FE conditions (for verification purpose only). DesignScan Figure 5: The standard DesignScan inline inspection on SMP. No defects were found (represented by green dots) color codes the defect map based on within process window (a). Pushing FE points out further in focus and dose still uncovered no defects (b). In order defect types (green means no defects to find weak spots, the FE points need to be extended by nearly 2x the process window (c). found), and the size of the circle is 36

Spring 2006

Yield Management Solutions

D F M

respectively. By pushing the dose away from the nominal dose a little further, the thin line defects become gaps as shown in Figure 6 (e). The gap defects appear at the exact locations and FE conditions as predicted by DesignScan. The SEM results confirm the weakness in those defective locations and the simulated resist images match wafer SEM images as well. DesignScan predicts the gap defect onset at 180 nm defocus, while the SEM image shows that the gap defects occur between 180 nm to 230 nm defocus (granularity limited due to the focus stepping distance for the FEM wafers).

Figure 6: DesignScan simulated resist images at nominal condition (a) and at defective condition (b). Corresponding SEM images from a FEM wafer at nominal condition (c) and at defective condition. Increase the dose further (e), and the predicted thin line defects become gaps, confirming the weakness of those locations.

Figure 7 shows another example of thin line/gap defect, indicated by a red dot in the DesignScan simulated resist image. The corresponding wafer SEM image confirms the defect. DesignScan predicts the gap defect onset at 180 nm defocus, while the SEM image shows the gap defects occuring between 180 nm to 230 nm defocus.

Those thin line defects become gap defects when the FE point is pushed further away from the nominal focus and exposure (F0E0). Given the fact that CD curves in a Bossung plot for a given feature do not cross, the weakest features will have the largest CD changes and become breaks/bridges first when moving FE away from F0E0. The onset of breaks/bridges is relatively easy to determine from SEM images. Thus, the predicated FE condition in which a break/bridge defect occurs can be directly compared to the FE condition where such a defect occurs on a FEM Figure 7: Another example of thin line/gap defect (left) and its corresponding wafer image (right). wafer through SEM review (subject to the increThe thin line breaks at the exact place that DesignScan predicted would be a weak point. mental changes of focus and dose of FEM). Due to systematic and random errors in mask making, dose) to the optimal condition, they are the weakest in optical imaging systems, and in resist image structures and also the process window limiter. Figure formation, there is FE offset between the simulated 6 shows an example comparing a group of thin line resist images from design database to the actual resist defects predicted by DesignScan and the corresponding images at defective locations. We have established the wafer SEM images. In Figure 6 (a) and (b), the resist DesignScan prediction offset specification on FE condiimages from DesignScan at nominal condition (a) and tions where a defect will occur based on internal tests: defective condition (b) are shown, where the white areas within the process window, the dose offset should be indicate areas without photoresist (spaces) and the dark within 2.5% and the focus offset should be within areas represent the areas covered with photoresist (lines).

30 nm; outside the process window, the dose offset At high dose defective condition, DesignScan found should be within 5% and the focus offset should be multiple defects (indicated by red dots) due to the thinwithin 50 nm due to increased line/space changes as a ning of certain resist lines as shown in Figure 6 (b). To function of dose and/or focus change. The confirmed compare DesignScan simulation results, the corresponddefects found by DesignScan on the SMP database are ing wafer SEM images are shown in Figure 6 (c) and (d) all within the offset specifications. for the nominal condition and the defective condition, Spring 2006

www.kla-tencor.com/magazine

37

D F M

tive weakness of several defects as a function of focus and dose changes is indicated. There are four defects within the field of view (FOV), labeled from the top to the bottom as d1 to d4 (see the lower left corner resist image). The upper right corner image is the closest to the nominal conditions, and the lower left image is the farthest from the nominal conditions. It is clear from Figure 8 (a) that d2 is the weakest and d3 is the strongest among the four. By knowing the relative weakness of those defects, a designer can then prioritize the structures for corrections. Fixing the weakest structures will help extend the process window until the next weakest becomes the weakest. Fixing the second weakest structures first will not extend the process window, since the process window is dictated by the weakest structures.

B

C

D

Figure 8: In order to better match the simulation result and the wafer SEM image, DesignScan simulated near the onset of a group of defects in finer 3x4 FE grid: every 0.5 mJ/cm2 in dose in vertical direction and every 10 nm in defocus in horizontal direction as shown in (a). A corresponding SEM image matches the bottom simulated image in 180 nm defocus column, which is enlarged in (c).

B

Since DesignScan can simulate whole or part of the SMP database anywhere within the resist model calibration conditions (FE range), it is quite useful in certain cases to separate defects by their weakness (where in FE space a defect occurs relative to the nominal condition). Figure 8 shows an example of a group of bridge defects near the onset at much finer FE grid. Again, the fast simulation speed allows us to do such a fine FE grid (18 FE points) in 67 minutes for the SMP test database.

The fine grid FE simulation pinpoints where a defect starts to appear; thus, in the case of Figure 8, the rela38

Spring 2006

Yield Management Solutions

C

D

Figure 9: Another bridge defect example. DesignScan correctly predicted the weak points. Fine FE grid simulation tells relative weakness of two defective locations.

D F M

The fine FE grid inspection can also help system developers to gauge the accuracy of the model prediction.

By matching the catastrophic defective condition of predicted defect onset to that of a FEM wafer, the defect prediction offset can be established. For this particular group of defects, the dose offset is 3.6% of nominal dose and the focus offset is 10 nm [see Figure 8 (b) and (c)].

Figure 9 shows another example for bridge defects. The fine FE grid simulation clearly distinguishes the relative weakness of two defects within the FOV, and helps to match the simulated resist image to the wafer SEM image [see Figure 9 (b) and (c)].

Table 1 summarizes the confirmation results for all gap defects and bridge defects. 20 out of 21 groups of gap defects and 27 out of 27 groups of bridge defects are confirmed by the wafer SEM images. &% RANGE $EFECT TYPE !$" #ONFIRMED /FFSET NM

.!

NM GAPS

NM

BRIDGES

NM

Table 1: The defect confirmation summary.

All of the defect information can be used in a subsequent mask-making process (pattern-dependent CD control) and in wafer processing monitoring (patterndependent yield loss). Since we know that the gap defects are the weakest and are the process window limiters, it is critical to achieve tight CD control in a mask-making process in the regions that contain those gap defects. The CD control in other regions is less critical; thus, the CD control specifications can be loosened. In wafer processing (lithography and etch), these gap defectivity locations (structures) will be the first to fail if the process condition deviates away from the nominal condition, which can cause yield loss. Such information will greatly increase the efficiency of wafer processing monitoring.

2. New DesignScan detectors-LES and ILO

We tested the newly developed line-end shortening (LES) detector on the SMP database. The detector can be enabled and disabled in recipe setup. DesignScan found no LES defects within the process window for SMP. 490 LES defects were detected outside the process window at 21.4% lower than nominal dose and 180 nm defocus using a detection threshold of 60 nmâ&#x20AC;&#x201D;an adjustable parameter. When a line-end shortening exceeds 60 nm relative to the reference line end, the simulated resist image at nominal condition, or for the nominal condi-

One group of gap defects (31 total instances) predicted

by DesignScan cannot be conclusively confirmed by SEM images as shown in Figure 10. Thus, 3675 out of 3706 (99.16%) defects have been confirmed by wafer SEM images. In the simulation model calibration process, we intentionally tune the data so that we over-predict defects. By doing so, we increase the alpha risk (false alarm, detecting more nuisance defects), but minimize the beta risk (failure to detect, not miss a real defect), since missing a real defect is more costly than a false alarm.

Figure 11: An example of an LES defect detected by DesignScan. The DesignFigure 10: DesignScan predicted a defect (left) and its corresponding SEM image (right). The SEM image cannot conclusively confirm the defect in this case.

Scan simulated resist images at defective (left) and reference (right) conditions are shown on top. The green lines represent the design feature edges and the purple lines indicate resist edges. The corresponding wafer SEM images are shown below at the defective (left) and the reference (right) conditions.

Spring 2006

www.kla-tencor.com/magazine

39

D F M

tion compared to the drawn/decorated database, the line end will be flagged as defective. More LES defects can be detected by lowering the LES detection threshold.

ADB reduces the defects into 21 groups. SEM review images have confirmed all 21 group LES defects, and Figure 11 shows an example of an LES defect in a zoomin view. DesignScan predicated images match the wafer SEM images well. In all detected LES defect cases, no or inadequate OPC for the line ends are identified. Based on the DesignScan LES findings, the OPC at those line ends can be significantly improved to avoid severe LES. Another newly developed 2D DesignScan defect detection capability is interlayer overlap (ILO) detector. The interlayer overlap detector can be activated and the detection parameter adjusted independently for the layer below and the layer above.

For the SMP database, we will focus on contact to Metal 1 layer overlap testing. Metal 1 is the layer of interest and the overlap error between the contact and Metal 1 is caused by line-end shortening of metal lines. DesignScan found 60 ILO defects with the default parameter setting at 17.8% lower than nominal dose and 180 nm defocus, and 343 ILO defects at 21.4% lower than nominal dose and 180 nm defocus, along with other defects. Figure 12 shows an example of an ILO defect (red circle at right) detected by DesignScan with SMP. The simulated resist image at nominal condition is at left and the resist image at defective condition is at right.

The green lines represent the metal database and the blue squares are contacts.

be implemented is determination of the reticle target CD specification through process window simulation.

This can be achieved by simply biasing the polygons of the post-OPC database in both directions (+/-) during the data conversion phase and re-inspecting the database. By evaluating the process window impact of the biased database, the CD tolerance specifications for different regions of the database can be determined.

The SMP test database in our study was biased+/-5 nm, +/- 10 nm, and +/- 15nm. Inline 9 FE condition DesignScan inspection still yields zero catastrophic defects. However, the gap defects (Figure 6 shows some of the examples) appear early on the high dose side for positive biases (white space increases) as shown in Figure 13. No bridge defects appear in the FE range inspected on the low dose side for negative biases (white space decreases) up to 15 nm. For concept demonstration purposes, we stopped at 15 nm bias, since higher bias may require the re-adjustment of the nominal condition and resist model recalibration.

Figure 13: Defect map of SMP with 5 nm, 10 nm, and 15 nm biases (increase white area). Thin line (blue) and gap (orange) defects appear closer to the nominal condition as the bias increases, which means the process window gets smaller.

Figure 12: An example of an interlayer overlap defect (indicated by the red circle) of SMP between the contact layer (blue squares) and the Metal1 layer (green lines). The resist edges are enhanced with purple lines for the nominal condition (left) and defective condition (right).

3. Mask CD target specification

One of the new applications for which DesignScan can 40

Spring 2006

Yield Management Solutions

Not only are those gap defects the weakest in the SMP database and the process window limiters, they also dictate the reticle CD specifications. Maintaining good CD control in the gap defective locations is much more critical than in other regions, the foundation for pattern-dependent reticle CD specifications. We can also conclude that for the SMP database, the reticle CD control is asymmetric: for the tested metal layer case, maintaining tighter CD control is required on the positive bias side rather than on the negative bias side.

To avoid such CD control asymmetry and also extend the process window, re-centering the process is recommended. For the SMP test database, lowering the nominal dose from 28 mJ/cm2 to 27 mJ/cm2 will help to reduce defect-related yield loss, assuming no other

D F M

negative effect to the device functionality. Typically, the process window center, which is the center of the process window used for all designs, is determined using test structures. This is inadequate because the test structures do not always adequately represent the full database and also because the behavior of all designs is not identical through a process window. Using DesignScan, the process center can be determined and optimized for each individual design, providing the largest possible process window and, hence, yield.

In this illustration we have discussed the mask CD specification use case through open and bridge defects. Similar analysis can be performed for CD variations through the use of DesignScan CD simulation capability.

Summary

We have demonstrated a methodology for monitoring the performance of a post-OPC design database across and beyond the process window using DesignScan. The ability of simulating resist images at any arbitrary FE location (within model calibration range) using one calibrated model and fast inspection speed allows DesignScan to perform full process window monitoring and optimization. The methodology in this paper has been demonstrated using a 90 nm half pitch SRAM Metal 1 database. In this database, no defects were found within the process window. Gap defects were found and confirmed on the wafer 14.3% away from the nominal dose and at 180 nm defocus. Bridge defects were confirmed on the wafer 21.4% away from the nominal dose and at 180 nm defocus. These patterns are the weakest across the FE conditions. If there is a change in the process, these locations will fail first; hence, these locations should be used for inline monitoring of the lithography process for systematic changes. This database has a fairly robust process window; however, the process window can also be further improved by fixing these potentially yield-limiting locations that have been identified by DesignScan.

In this article, we have demonstrated additional use cases for DesignScan. The solution can be used for design-based mask CD specification determination and design-based process centering. Users can fit the process to the design or vice versa. We have also demonstrated the methodology for using DesignScan for design-based process control and monitoring.

Acknowledgements

The authors would like to thank the UMC lithography and OPC team and KLA-Tencor DesignScan engineering team for their support.

Yung Feng Cheng, Yueh lin Chou, Chuen Huei Yang and CL Lin, Bo Su, Gaurav Verma, William Volk, Mohsen Ahmadian, Hong Du, Abhishek Vikram, Scott Andrews,Reducing Design Respins, in 25th Annual BACUS Symposium on Photomask Technology, edited

by J. Tracy Weed, Patrick M. Martin, Proc. of SPIE Vol. 5992, 59921X, (2005) CID# 599244 Reference 1. W. Howard, et al, â&#x20AC;&#x153;Inspection of integrated circuit databases through reticle and wafer simulation: An integrated approach to Design for Manufacturing (DFM)â&#x20AC;?, Proc. SPIE vol. 5756, pp 61-72 (2005).

Spring 2006

www.kla-tencor.com/magazine

41

Lithography M

e

t

r

o

l

o

g

y

Opening the Window to Higher Parametric Yield at 32 nm DFM and APC Strategies Tackle Process Window Limitations Kevin M. Monahan, KLA-Tencor Corporation © 2005 IEEE. Kevin M. Monahan, Enabling DFM and APC Strategies at the 32nm Technology Node. Reprinted, with permission, from International Symposium on Semiconductor Manufacturing (ISSM) 2005 Conference.

Most semiconductor manufacturers expect 193-nm immersion lithography to remain the dominant patterning technology through the 32-nm technology node. Even now, the interaction of more complex designs with shrinking process windows is severely limiting parametric yield. The industry is responding with strategies based upon design for manufacturability (DFM) and multi-variate advanced process control (APC). The primary goal of DFM is to enlarge the process yield window, while the primary goal of APC is to keep the manufacturing process in that yield window. This article discusses new and innovative process metrics, including virtual metrology, that will be needed for yield at the 32-nm technology node.

Introduction

Enabling DFM and APC strategies with metrology depends on innovation1. As a minimum, DFM will require feeding forward design intent, simulator output, layout clips, and design rule-check (DRC) hot spots to expedite setup of measurement tools. Current DRC and aerial image modeling at best focus and exposure conditions is increasingly unreliable. In the future, process windowaware approaches will require powerful full-chip simulators that can accurately predict and measure developed patterns in resist, along with accurate measurement feedback to calibrate the printability simulator.

To control development costs, the conversion of data to information, knowledge, and decisions must be taken as far upstream as possible.

On the process control side, implementing an APC strategy requires feeding forward both process context and measurement data.

In the future, we know that process context and measurement data must increase dramatically to support multi-variate control at the lot, wafer, field, die, and intra-die levels.

In addition, yield and performance losses 42

Spring 2006

Yield Management Solutions

are often caused by process integration issues or combinations of profile, shape, roughness2, thickness, and pattern placement errors. For these applications, new measurement types are required, creating a concomitant need to decrease the cost and increase the yieldrelevance of each measurement. In addition, combined dispositioning and parametric yield analysis will require data from multiple metrology tools.

New metrology requirements

The case for linking design, layout, mask, and wafer processes with metrology is compelling. Greater complexity is offset by the advantage of greater access to adjustment; an array of strategies generates higher return than just one. The increasing metrology needs of DFM and APC can be met by innovations in the measurement of pattern shape, profile, overlay, thickness, composition, and electrical properties. As an example, some of the key dependencies for device performance are given by the transistor drive current equation below:

( )

Id=1 W • ( e • µ) • ( V- V ) 2 t 2 L •T

Drive current at saturation depends on physical dimensions such as gate width W, gate length L, and gate oxide thickness T. It can limit the speed and, therefore,

M

the average selling price of a device. Drive current also varies with electrical properties such as channel electron mobility µ, gate oxide dielectric constant x, and threshold voltage Vt. These, in turn, are affected by such factors as strain, composition, and transistor architecture. The following sections will explore innovations in physical dimension and pattern placement metrology.

B

C

D

Figure 1: (a) Design-intent layout prior to simulation. (b) Noise-free SEM image simulation. (c) SEM image for comparison with layout.

From CD to Shape Metrology

e

t

r

o

l

o

g

y

From CD to Profile Metrology

Scatterometry-based CD (SCD) metrology will evolve into more yield-relevant “profile metrology” and may become the reference tool for calibrating the CD SEM. SCD is able to accurately reproduce cross-section profiles imaged in a transmission electron microscope (TEM) (Figure 2a). The ability of SCD based on spectroscopic ellipsometry (SE) to accurately measure footing and notching at the base of gate structures has led to twofold improvements in correlation to electrical L-poly and drive current (Figure 2b). For this reason, SCD tools are currently displacing other metrology tools in feedforward APC applications from lithography to etch. In control applications for shallow-trench isolation (STI), significant cost savings have been realized by metrology convergence. SCD tools are currently displacing CD SEM, AFM profile, and SE film thickness tools for the control and monitoring of isolation (Figure 2c). The benefits are lower cost, shorter cycle time, and greatly reduced temporal, spatial, and technology de-correlation for the more performance-relevant compound measurements such as aspect ratio.

Scanning electron microscopy (SEM)-based CD metrology will evolve into more yield-relevant “shape metrology”. Measurements such as line-end shortening, miniFrom Scribe Line to In-chip mum gap, line roughness, and feature rounding will Traditional box-in-box (BiB) overlay metrology will be performed as commonly as standard CDs are today. evolve into more yield-relevant, grating-based overlay To decrease cost, multiple measurements will be made metrology (AIM). This will take measurement of pattern within the same image field to assess local pattern variation and provide robust averages for APC. OPC printability verification will be an increasingly critical DFM application for the SEM. Design-based metrology (DBM) will enable both APC and DFM strategies. Briefly, DBM is the use of design-intent and hot-spot information to define perfor B A mance-relevant measurement locations within a semiconductor device. A particularly powerful implementation3 is shown in Figures 1a-1c. It solves one of the most vexing DBM problems by simulating a noise-free SEM image from design input. The simulated image is then used for robust C pattern recognition and precise location of measurement Figure 2: (a) SCD profiles on TEM cross-sections. (b) SCD and SEM correlation to L-poly. (c) Simultaneous SCD sites. measurements on STI.

Spring 2006

www.kla-tencor.com/magazine

43

M

e

t

r

o

B

l

o

C

g

y

D

Figure 3: (a) Traditional box overlay target. (b) Robust grating overlay target. (c) Small in-chip overlay target.

placement error to new levels of accuracy and enable combined CD and overlay dispositioning. At the 32-nm node, BiB overlay metrology (Figure 3a) will suffer from extreme process sensitivity, particularly with respect to reticle fabrication error, asymmetric deposition and etching, and chemical mechanical planarization (CMP).

Grating-based overlay technology (Figure 3b) can decrease process-induced measurement error by a factor of two. Remaining pattern placement error, including unmodeled intra-chip error, will be addressed with tiny in-chip grating targets4 (Figure 3b). These enable significant reduction of model residuals, the largest remaining source of overlay measurement error. In some cases, such small overlay targets may be combined with lineend-shortening (LES) targets that are used to monitor focus and exposure excursions in lithography cells. The benefits are lower cost per yield-relevant measurement and higher temporal, spatial, and technology correlation for rootcause analysis.

ment targets is critical; therefore, calibrated lithography models, such as PROLITH, will be employed to assist in the initial target optimization. These models must use realistic mask data and comprehend the most aggressive resolution enhancement technologies (Figure 4a).

Second, they must provide accurate results for immersion lithography at 193 nm (Figure 4b). Third, they must enable rigorous virtual metrology to supplement actual physical measurement (Figure 4c). The benefits are lower cost per measurement, inline validation of physical metrology, and upstream pattern analysis to reduce design, mask, and wafer-level yield loss.

Conclusions

This article identified four trends that address the metrology needs of semiconductor manufacturing at the 32-nm technology node. In particular, the focus has been on the physical parametric measurements that will enable future APC and DFM strategies. The key trends are: • The transition of the SEM from CD metrology to a more yield-relevant and cost-effective shape metrology utilizing critical design data.

• The transition of SCD from CD metrology to more yield-relevant profile metrology for gates, STI, sidewall spacers, and contact holes.

From Actual to Virtual Metrology

Traditional process modeling and simulation will evolve into yieldpredictive “virtual metrology”.

Even now, the measurement technologies discussed (SEM, SCD, and AIM) rely to some extent on simulation. Simulated SEM images assist with design-based pattern shape metrology. Rigorous coupled wave (RCW) algorithms generate libraries of ellipsometric spectra for comparison with actual SCD data. Overlay simulators5 predict the optical signatures of innovative overlay targets in order to maximize sensitivity and minimize response to process noise. Finally, robust printability of SEM, SCD, and AIM measure44

Spring 2006

B

A

C Figure 4: (a) Chrome-less phase mask structure. (b) Polarization simulation for I-193nm. (c) Calibrated virtual metrology simulation.

Yield Management Solutions

M

• The transition of overlay metrology from box-in-box targets in the scribe line to more yield-relevant AIM grating targets inside the chip.

e

t

r

o

l

o

g

y

References 1. K. Monahan and B. Trafas, SPIE Vol. 5756, 2005.

2. P. Leunissen, et al., SPIE Vol. 5752, 2005.

• The transition from actual to virtual metrology using calibrated process simulators, such as PROLITH and its derivatives.

3. L. Capodieci, et al., KLA-Tencor YMS West 2005.

Acknowledgements

5. L. Seligson, et al., SPIE Vol. 5752, 2005.

4. P. Leray, et al., SPIE Vol. 5752, 2005.

The author thanks Brian Trafas, Murali Narasimhan, Umar Whitney, John Robinson, Amir Azordegan, Gian Lorusso, Matt Hankinson, Ted Dziura, David Tien, Mike Adel, Chris Sallee, and Dan Wack for valuable inputs. © 2005 IEEE. Kevin M. Monahan, Enabling DFM and APC Strategies at the 32nm Technology Node.

Reprinted, with permission, from International Symposium on Semiconductor Manufacturing (ISSM) 2005 Conference.

Reticle Yield Management Seminar A valuable venue for innovative ideas

KLA-Tencor’s Yield Management Seminars (YMS) focus on the latest solutions and strategies for accelerating yield through critical technology transitions. Participants have the unique opportunity to learn and gather information from several leading experts in the field.

Date: Time: Location:

Monday, September 18, 2006 12:30-5:30 Monterey, California

Call for future papers

Papers should focus on using KLA-Tencor tools and solutions to enhance yield through increased productivity and performance. If you are interested in presenting a paper at one of our upcoming Yield Management Seminars, please submit a one-page abstract to: Nancy Williams by fax at (408) 875-4144 or email at nancy.williams@kla-tencor.com.

YMS at a Glance Date

Location

July 12

San Francisco, California

August

Singapore; Taiwan

Awards S

e

c

t

i

o

n

s

CUSTOMER COMMITMENT COMMENDED

WITH SAMSUNG BEST ENGINEER HONOR

Applications Engineer Helps Optimize Puma 9000 Performance at Memory Production Line For HongSun So, being a successful senior product applications engineer at KLA-Tencor Korea involves more than technical expertise. Success is also driven by a deep understanding of the customer’s real problems. Assigned to work with the Samsung Electronics Semiconductor Memory Division on its Puma 9000 implementation, So has clearly made an impression. For his dedication, the Samsung MI group has honored So with its Samsung Best Engineer award.

“Receiving Samsung’s award is an honor to the role of the applications engineer,” said So, a KLA-Tencor employee for the past five years. “And it means that I need to continue to work hard and do good work to keep this honor.”

So considers as a key responsibility his efforts in helping Samsung speed their production ramps. When a tool encounters an issue, he’ll typically stand by long after a service engineer has resolved the problem, just to be sure performance is maintained.

He’ll perform applications tests promptly, and he’s committed to writing the optimal recipes to help the customer monitor unique killer defects in production. For the most part, he considers himself to be a voice of the customer, gathering and delivering customer feedback to KLA-Tencor engineering and marketing teams to help enhance the tools.

46

Spring 2006

Yield Management Solutions

“Every day, I meet or call Samsung to maintain our good communication flow as well as our relationship,” explained So. “When the customer successfully applies some of the Puma performancerelated tips I provide, they express their gratitude.”

In the fourth quarter of Fiscal Year 2005, So received a KLA-Tencor General Manager Award for his “unequivocal devotion to work.” Exhibiting strong leadership within the local applications group, So has mentored and trained local applications engineers as well as customers, helping them make optimal use of their Puma 9000 tools.

Prior to being assigned to Samsung, So was the owner of successful Puma evaluation projects for R&D at Hynix. There, he completed 10 layer evaluations within a three-week period—which was considered by the customer to be unprecedented.

For the Samsung honor, So was chosen from a pool of 370 applications and customer service engineers from more than 20 vendors. Notes So, “Having ownership of one’s job and product is very important in the eyes of a customer. I always try to understand the customer’s situation and requests, and work hard alongside the customer to resolve any issues or develop ways to more effectively find unique defects.”

Accelerating Yield

K-T Certified

TM

Refurbished price. Winning performance.

The one-stop, worry-free source for fully refurbished KLA-Tencor tools ➤ Guaranteed to meet or exceed original product performance specifications ➤ Short lead times and KLA-Tencor expertise meets your manufacturing requirements with no surprises ➤ Includes performance and reliability-enhancing software and hardware upgrades ➤ Largest supply of KLA-Tencor refurbished tools ➤ Service and applications packages designed to fit your needs ➤ Full asset management program for when you need help managing your surplus assets ➤ For more information, visit:

w w w. k l a - t e n c o r. c o m / c e r t i f i e d ©2004 KLA-Tencor Corporation.

®

Accelerating Yield®

Particle on-epi: � bright scatter � dark reflected � same optical size

Particle in-epi: ©2005 KLA-Tencor Corporation.

� bright scatter � dark reflected � smaller scatter signature (film thicker over particle)

Do you know the three W’s of epi-layer inspection? Only Candela finds where it is, what it is, and when it occurred. ™

Differentiating between subtle optical characteristics can provide critical information on defects. A particle under the epi layer is a very different problem than a particle on the surface. Our Optical Surface Analyzers (OSA) are unique surface inspection systems that employ a combination of measurement technologies to automatically detect and classify a variety of defects. Defects are binned by size into user-defined categories, and displayed on a defect map. The OSA images remain linked to the report, for quick and effective review. � Automatically classifies particles and scratches as “on” versus “in or under” the epi layer � User-defined defect classifications allow automated detection and reporting of unusual defect types � Crystal defects such as dislocations and polytype changes are automatically detected and counted � Manual or automated cassette-to-cassette operation � Accommodates wafer sizes from 50 to 300 mm � For more product information, go to:

www.kla-tencor.com/candela 48

Spring 2006

Yield Management Solutions

Lithography M

e

t

r

o

l

o

g

y

Automating Investigation of Line Width Roughness Full Spectral Analysis Enables Benchmarking of New Resists L.H.A. Leunissen, M. Ercken and J. A. Croon, IMEC

G.F. Lorusso, H. Yang, A. Azordegan and T. DiBiase, KLA-Tencor Corporation

One of the most commonly used estimators of line width roughness (LWR) is the standard deviation. However, this approach is incomplete and ignores a substantial amount of information. As an alternative, full spectral analysis can be used to investigate and monitor LWR. A variety of estimatorsâ&#x20AC;&#x201D;including standard deviation, peak-to-valley, average, correlation length, and Fourier analysisâ&#x20AC;&#x201D;have been implemented online on CD SEM. The algorithms were successfully tested against e-beam written LWR patterns, both deterministic and random. This methodology, which allows a fully automated investigation of LWR, was used to monitor LWR over a long period of time, benchmark new resists, and to investigate the effect of LWR on device performance and yield.

Introduction The importance of LWR for future technology nodes has been demonstrated in various experimental and theoretical investigations.

However, although it is clear that LWR will influence device performances and production yield, not to mention metrology strategies, the quantitative details are still

controversial. This information is crucial in order to define specific guidelines and monitoring criteria. Furthermore, it has been recently pointed out that a measurement of LWR alone does not guarantee the full characterization of the physical phenomenon under investigation. A full spectral analysis is needed. In the case of self-affine edges, for example, it has been demonstrated that a set of three parameters is required to define the physical system: the LWR standard deviation s, the correlation length x, containing spectral information on the edges, and the fractal exponent a, related to the kind of diffusion process involved in the physical creation of the edges.

In this study, we use full spectral analysis of LWR to investigate the effect of LWR on yield, precision, and resist characterization. All of the algorithms used in this investigation are now implemented online on the eCD series of KLA-Tencor CD SEMs. The availability of these algorithms on the monitoring tool is essential in order to allow LWR characterization in a production environment. A purposely designed LWR standard, created by means of direct e-beam writing, has tested the performance of the measurement algorithms in terms of accuracy and precision.

In order to experimentally evaluate the effect of LWR on yield, direct e-beam writing was used to create devices with varying degrees of LWR. This allowed us to estimate the experimental parameters in the proposed LWR model, so we could specify the requirements needed to set up a monitoring procedure. This article reports LWR monitoring data over a period of about four months.

The experimental results of the monitoring confirmed various assumptions of the model, such as the normal distribution of the LWR population and the validity of the self-affine hypothesis. The procedure reported here is intended to describe the various steps needed to design LWR monitoring in a production environment.

Spring 2006

www.kla-tencor.com/magazine

49

M

e

t

r

o

l

o

g

y

Although the influence of LWR on yield attracts much attention, the relationship between line edge roughness (LER) and precision is critical and often underestimated.

This investigation compares the functional dependence of critical dimension (CD) precision on LWR, correlation length and gate placement error, and compares the theoretical predictions with experimental measurements.

Finally, an online LWR algorithm is used to quantify the difference in performances of resists and topcoats for immersion lithography. These results indicate that LWR measurement algorithms are a critical metrology tool in a variety of applications not strictly related to process monitoring.

Experimental conditions

The measurement algorithms used to characterize LWR in this work are now fully available on the KLA-Tencor CD SEMs in the eCD family. They are capable of measuring various LWR estimators, such as sigma, peakto-valley, and average delta, as well as correlation length and power spectrum. They allow exporting of all of the edge information for further analysis. The accuracy and reliability of the algorithms were tested by means of e-beam written LER standards. Figure 1 shows an image of a deterministic LER standard. The use of these standards allowed fine-tuning and characterization of the performances of the algorithms. The LWR precision for two consecutive measurements in the same site was estimated to be 0.2 nm.

Figure 1: SEM image of a LWR standard written by e-beam lithography (Leica VB6-HR). These standards were used to tune and characterize the online algo-

The resist line patterns were generated with additional programmed LER using e-beam lithography. On both sides of the original line, small rectangles were taken, of which the dose was altered. The dose was randomly varied between all of the small rectangles, thus resulting in a varying linewidth. The nominal linewidth was close to 62 nm. The rectangles presented here have a length of 60 or 120 nm, resulting in a range of correlation lengths. The dose range was chosen to have various LWR. After the e-beam lithography step, the processing was continued with etching of the poly-Si, resist strip, and SiON removal.

In order to determine the effect of gate placement error on precision for a sample with known roughness characteristics, we simulated the effect of gate misplacement on a set of experimental results on a sample with well characterized LWR and correlation. The experimental results were then compared with the analytical prediction.

For the experimental LWR monitoring, IMEC’s current standard C013 gate process monitor (130 nm node design rules) was chosen. Litho target CD was 110 nm.

Targeted gate etch CD was 70 nm. To achieve this 40 nm CD loss during etch, resist and hardmask trim was applied during the etch process. This monitor was run bi-weekly. In each monitor batch, two wafers were selected for measuring the LWR (only after etch). Per wafer, five dies were measured (center, north, east, south, and west) and for each die, the LWR measurement was averaged over 20 sites along a long line. This procedure was applied seven times.

Currently, IMEC is screening several dedicated ‘wet’ resists and topcoats for the start-up of the immersion program on their ASML XT:1250Di. Results shown in this article were still performed on the ASML PAS5500/1100 ‘dry’ system. All resists were exposed on ARC29A (Brewer/Nissan Chemical) as organic bottom anti-reflective coating. Resist thickness applied was 150 nm. Vendor-recommended process conditions were applied. All resist coating was done manually (pipets). Target pitch resolution (150 nm) was achieved using dipole illumination, 0.89/0.65s @ 0.75 NA. The topcoats investigated also needed to be applied manually. An optimal thickness was chosen for reflection control.

rithms, as well as to create devices with various degrees of LWR.

LWR description

In order to test the yield model, and estimate its parameters, transistors with additional LWR were created.

50

Spring 2006

Yield Management Solutions

Many factors contribute to LWR, including resist composition1, aerial image contrast1,2,3,4 development1,4 and

M

process conditions3, 4, 5. The LWR is usually characterized by the 3s value, where the standard deviation (s) is defined as:

(1)

where dW(zi) is the deviation from the average linewidth dW and N is the number of measurement points.

It is not sufficient to measure only the 3s variation5, 6, 7.

A more complete description of the LWR can be obtained by measuring the full spatial frequency dependence of the roughness. All this makes the comparison and quantification of line edge roughness more difficult.

Besides s, the spatial frequency components can be resolved using a power spectral density (PSD) function, height-height correlation function, or by assuming a first-order autoregressive process. The spatial frequency dependence can be described by two additional parameters that quantify the spatial aspects of LWR: the roughness exponent (a) and the correlation length (x).

The roughness exponent is associated with the fractal dimension D (a=2-D)8 and its physical meaning is that it gives the relative contribution of high frequency fluctuations to LWR. Large values of a signify less high frequency fluctuations. On the other hand, the correlation length denotes the distance after which the edge points can be considered uncorrelated.

Impact of LWR on yield

Poly gates of devices exhibit a certain linewidth roughness. Generally, parameters that describe the operation of the MOS device are a function of its gate length (L) (Note that the gate length of a MOSFET is equal to the width of the poly line). Roughness causes this length to locally vary over the width (W) of the device. This roughness causes an increase in parameter fluctuations and off-state current. It can also give rise to yield loss.

In this section we will calculate the impact of LWR on yield for devices. The CD distribution as found experimentally in Figure 4 and Figure 8 shows that the population is normally distributed, which results in the following probability density function

(2)

with <L> the average linewidth and sLWR the local CD variation. We take the following variable DL=L-<L>/sLWR

e

t

r

o

l

o

g

y

having dDL=dL/sLWR. We assume that a segment causes the whole device to fail if its length is smaller than a certain critical length (Lcrit). Therefore, we need the cumulative distribution function, which gives the probability that L<Lcrit. The chance that this happens for one segment (psegt) is:

(3)

The device contains W/x segments depending on the width. When W<x, the chance that one device is failed is equal to pseg (min{1,W/x]=1). To calculate the chance that a device is not working (pdev), the device is divided into segments with width x. Each segment is considered to have a constant gate length that varies with sLWR. This yields:

(4)

Finally, the yield of a circuit with Ndev identical devices can be calculated to be:

(5)

It is assumed that the circuit only functions when all of its devices are working. Transistors with different line edge roughness were used to test this yield model. A rather arbitrary failure criterion was chosen, namely the appearance of punch-through, which will demonstrate the mode nicely. Punch-through reduces the control of the gate on the off-state current, which causes it to bend upwards. Mathematically, this translates into: the device fails when d2lnID/dVGS2 > 0 at VGS=0 V10. Figure 2 shows the fraction of working transistors as a function of the gate width for smooth transistors and the two cases where extra roughness was introduced. The correlation length x and LWR s are indicated in the plot, as determined by SEM pictures. The induced correlation length is increased with respect to the initial values because they are convoluted with the intrinsic correlation length of the resist (about 40 nm). It is seen that Equation (5) gives a reasonably good fit to the experimental data with Lcrit@50 nm (the accuracy is somewhat limited due to a sample size of only ~60 devices per point). It follows that Lcrit= Lnominal - 16.5 nm.

Spring 2006

www.kla-tencor.com/magazine

51

M

e

t

r

o

l

o

g

y

LWR monitoring

,72 NM

X NM

9IELD ; =

3 S ,72 NM X NM

3 S,72 NM X NM

.DEV ; = Figure 2: Relative amount of 62-nm gate length transistors that do not suffer from

In Figure 4, we show the LWR distribution obtained during our monitoring for about 600 measurements.

The population is clearly normal, as evidenced by the Gaussian fit. We estimated an average LWR of 2.3 nm, with a standard deviation for the individual measurement of 0.34 nm. As a consequence, the precision of the estimated LWR is 0.04 nm. These results confirm the assumption on normality of the LWR stochastic process, used earlier to calculate the effect on the yield. Beside LWR, we also monitored correlation length and fractal exponent. The correlation length resulted in 34.5 nm, with a precision of 1.2 nm on this estimate, while the fractal a was 0.55, with a precision of 0.009.

punch-through, as a function of the number of devices for three gate patterning

processes with intrinsicly different LER characteristics.

Technology Transistor 3sLWR (<L> - Lcrit)/3 Gate Length Node per Chip (ITRS) [nm] (calc.) [nm] [nm]

22 nm 35 nm 45 nm 65 nm 90 nm 130 nm

8.85E+09 4.42E+09 2.21E+09 1.11E+09 5.53E+08 2.76E+08

0.70 1.00 1.40 2.00 3.00 4.60

0.76 1.10 1.52 2.12 3.13 5.50

9 13 18 25 37 65

EBUB mU

$PVOUT

Using the simple model, we want to determine the impact on a large-scale design. Therefore, we consider a digital circuit with a large number of devices and assume W=x =L using the numbers as given in Table 1.

-83 ON

Figure 4: Experiments found distribution of the LWR (solid line) and a fitted Gaussian (dashed line).

Inserting the numbers in Equations (3)-(5), we calculate the yield for a given sLWR. Figure 3 displays the yield for the applied parameters for the 130 nm node down to the 22 nm node, respectively. We consider here a device failed when more than 0.1% of its components do not satisfy the requirement for critical length.

Figure 5 shows the combined results for the first four different monitoring days, 100 measurements each.

The information on the LWR can be extracted by using three different characterization functions: (a) structure function, (b) power spectrum, and (c) dependence of the &

&

ON ON ON

&

&

S ON

X ON

X ON

&

S

&

X

&

ON

S

Table 1: Predicted paremeters for future technology nodes.

:JFME

X ON

( S B V

ON ON

&

S & 1 L B V

T - ON

&

& B

&

S-83 ON

Figure 3: Yield as a function of different technology nodes as calculated using Eq.(5). The parameters as defined in Table 1 are used.

52

Spring 2006

Yield Management Solutions

S ON

C

&

D

&

L ON

- ON

Figure 5: Typical behavior of the three different characterization functions: (a) structure function, (b) power spectrum, and (c) dependence of the LWR on gate length.

M

LWR on gate length. The plot shows the average functions measured in each monitoring. The repeatability of the four estimators is evident: the different monitoring yields essentially the same curve, since the stochastic process is essentially the same. There are, however, differences in between the three approaches. The structure function is definitively a better estimator for the correlation length and the fractal dimension as compared to the power spectrum or the s(L), and it contains information on each of the three parameters.

The results in Figure 5 demonstrate experimentally that the LWR is indeed a self-affine process, as previously assumed. The three curves are essentially identical to the simulations by Constantoudis et al. in13, thus confirming that the full description is achieved with a three-parameter model, where the descriptors are s, a, and x. The results also validate the hypothesis that the knee in the power spectrum is indeed related to the correlation length.

e

t

r

o

l

o

By using our estimate of s and x, we define the yield criteria from an earlier section of this article for a CD=75.8 nm (Figure 6).

S-83 ON

y

for distances smaller than the correlation length (x) there exists a dependency of the data. We assume a first-order autoregressive process R(x) as defined by

(6)

with sinf representing the sigma at the low frequency limit (long box length). Then, the following two-parameter equation can be derived that fits the sigma variation with increasing box length L12, 14:

(7) Therefore, if the smeas(L) measured for a given length is smaller than sinf, the portion of the LWR that is not measured is the CD variation (sCDU) contribution of the resist that can be calculated using

We notice also that the estimate of the sigma from the edges appears to be slightly smaller as compared to the one from the three estimator functions (2.3 nm versus 2.5 nm). This is indeed accounted by the autoregressive model of s(L). The monitored sinf=2.5 nm after correcting for the limited field of view.

g

(8)

In the next section of this article, we discuss the effect of LWR, correlation length, box length, and positioning error on CD precision. Suppose that the box length of the measured line is large such that s(L)~sinf. The consequence is that repetitive measurements yield the same results. Repeating measurements for shorter lengths result in a spread of the determined smeas.

For example, we re-measure the CD of a line, but the line is shifted with respect to the original measurement position. The error with respect to the original measurement position has the same form as Equation (7), but L is replaced by the positioning error (Dxpos) and sinf is replaced by sCDU. The correlation length is related to the resist and is unaltered.

(9)

.POJUPSJOH $ZDMF

Figure 6: The dots indicate the monitoring data with error bars. The different dashed lines represent the yield criteria for a 50%, 95%, and 99%, respectively.

LWR and precision

It is demonstrated that the measured LWR (smeas) changes when the box length is increased. Investigation of points along the edge shows that for larger distance (x) between two measurements, the data is independent. However,

If Dxpos=0, additional CD measurement yields identical results. For Dxpos>>x the measurements are independent and the spread is sCDU. We take a measurement box of L=200 nm and measure the LWR to verify the model experimentally. Figure 7 shows the experimental findings and the theoretical results from Equation (9). The parameters in Equation (9) are x =44.3 nm and sinf=5.5 nm as found from earlier experiments using this resist.

Spring 2006

www.kla-tencor.com/magazine

53

M

e

t

r

o

l

o

g

y

for immersion lithography on the ASML XT 1250i at IMEC (exposures are performed on the ASML /1100 â&#x20AC;&#x2DC;dryâ&#x20AC;&#x2122;). The Gaussian behavior is again evident. Each measurement is performed over 340 sites, and the precision of the estimated average is less than 0.1 nm, thus allowing clear, rigorous discrimination between resist type. The measurements were fully automated, and they took less than one hour per resist. The mean LWR for the five resists is given in Table 2:

.PEFM &YQFSJNFOU

$% 1SFDJTJPO ON

$ Y ON

Figure 7: CD repeatability as a function of the positioning error along the edge

Resist:

sLWR [nm]

LS2

LS3

LS6

LS7

LS8

4.68

3.30

3.61

3.90

2.90

Table 2: Mean s for five resists as evaluated on the ASML /1100 â&#x20AC;&#x2DC;dryâ&#x20AC;&#x2122;.

of the line.

The theoretical results match the experimental data.

These findings show that for a given box length the precision of a CD measurement is limited due to the positioning accuracy of the CD-SEM. If better LWR reproducibility is required, then the box length should be increased or the repositioning at the exact same position needs to be improved.

Besides average information on the roughness across the wafer, it is possible to obtain wafer mapping of the LWR. Figure 9 radial LWR maps for different resists are given as an example. The data clearly indicate a spike in LWR of the resist L4 close to the center of the wafer, most probably related to problems in the coating procedure.

-4 -4 -4

Resist characterization

1

,3

,3

,3

COUNTS A U

-4

,3 ,3

Figure 9: Radial 3 sLWR maps for different resists.

Conclusions

S

NM ,72

Figure 8: Measured distribution of the sLWR for 5 different resists.

Spring 2006

%JF

54

-4

S-83 ON

The algorithms used to characterize roughness are being applied to a variety of use cases, in addition to process control. In particular, we reported recently9,15 on LWR measurement characterizing LER transfer to etch and roughness variation caused by 193 nm shrinkage. We discuss here the use of the LER algorithm applied to benchmarking of resist and topcoats. The results in Figure 8 show the LWR measured on five different resists, being screened to define the candidate of choice

Yield Management Solutions

The effects of line edge roughness on the intrinsic MOS transistor performance and yield have been investigated in a 130 nm CMOS technology with gate lengths ranging down to 50 nm. From the experiment, it follows that Lcrit ~ Lnominal - 16.5 nm. This margin can be increased by: reducing _LWR or increasing _. However, the latter â&#x20AC;&#x2DC;solutionâ&#x20AC;&#x2122; will also give rise to increased parameter fluctuations. A better approach might be to

M

actually try to decrease _ so far that further processing more efficiently smooths out the roughness, which, in its turn, leads to a lower value of _LWR. The experiment for a large-scale digital design shows that the devices have to be able to cope with acceptable device deviations for a given _LWR. It also has been demonstrated that the online metrology can be applied with high accuracy for the screening of new resist materials.

Acknowledgements

The authors would like to thank Christie Delvaux, Nadia Vandenbroeck, and Frieda Van Roey for all the practical work and assistance in the many measurements. The authors are indebted to the European Commission and the Medea+ organization, for the funding of the European project IST-1-507754-IP (More Moore). The authors would also like to thank the IMEC lithography department, including the industrial affiliates based at IMEC and the IIAP partners of the 193-immersion program.

L.H.A. Leunissen, G.F. Lorusso, M. Ercken, J. A. Croon, H. Yang, A. Azordegan, T. DiBiase, Full Spectral Analysis of Line Width Roughness in SPIE 2005 Metrology, Inspection, and Process Control for Microlithography XIX, Proceedings of SPIE Vol. 5752, pgs. 578-590 (2005) References 1. W.G. Lawrence, “Spatial Frequency Analysis of Line Edge Roughness in Nine Chemically Related Photoresists”, Proc.

SPIE 5039, 713 (2003) 2. J. Shin, G. Han, Y. Ma, K. Moloni and F. Cerrina, “Resist Line Edge Roughness and Aerial Image Contrast”, J. Vac. Sci. Technol. B19, 2890 (2001) 3. H.P. Koh, Q.Y. Lin, X. Hu and L. Chan, “Effect of Process Parameters on Edge Roughness of Chemically Amplified Resists”, Proc. SPIE 3999, 240 (2000) 4. S. Masuda, X. Ma, G. Noya and G. Pawlowski, “Lithography and Line Edge Roughness of High Activation Energy Resists”, Proc. SPIE 3999, 252 (2000) 5. M. Ercken, G. Storms, C. Delvaux, N. Vandenbroeck, P. Leunissen and I. Pollentier, “Line Edge Roughness and its Increasing Importance”, Proceedings of Interface (2002)

e

t

r

o

l

o

g

y

6. G.P. Patsis, V. Constantoudis, A. Tserepi and E. Gogolides, “Quantification of Line Edge Roughness of Photoresists. Part I: A Comparison Between Off-line and On-line Analysis of Top-down SEM Images”, J. Vac. Sci. Technol. B21, 1008 (2003) 7. G. P. Patsis, V. Constantoudis, A. Tserepi, and E. Gogolides, Grozdan Grozev, and T. Hoffmann, “Roughness Analysis of Lithographically Produced Nanostructures: Off-line Measurement and Scaling Analysis”, Microelectronic Engineering 678: 319-25 (2003) 8. A.-L. Barabasi and H.E. Stanley, “Fractal Concepts in Surface Growth”, Cambridge University Press, Cambridge, England, 1995 9. L.H.A. Leunissen, G. F. Lorusso, T. DiBiase, H. Yang, A. Azordegan, “On-line Spectral Analysis of Line Edge Roughness: Algorithms Qualification and Transfer to Etch”, Semiconductor Fabtech (to be published) 10. J.A. Croon, L.H.A. Leunissen, M. Jurczak, M. Benndorf, R. Rooyackers, K. Ronse, S. Decoutere, W. Sansen and H.E. Maes, “Experimental Investigation of the Impact of Line-edge Roughness on MOSFET Performance and Yield,” Proc. ESSDERC, 227 (2003) 11. V. Constantoudis, G. P. Patsis, L. H. A. Leunissen, and E. Gogolides, “Line Edge Roughness and Critical Dimension Variation: Fractal Characterization and Comparison using Model Functions”, J. Vac. Sci. Technol. B 22, 1974 (2004) 12. L.H.A. Leunissen, W.G. Lawrence and M. Ercken, “Line edge roughness: Experimental Results Related to a Two-parameter Model” Microelectron. Eng. 73-74, 265 (2004).

13. V. Constantoudis, G. P. Patsis, L. H. A. Leunissen, and E. Gogolides, “Toward a Complete Description of Line Width Roughness: a Comparison of Different Methods for Vertical and Spatial LER and LWR Analysis and CD Variation”, Proc.

SPIE 5375, 967 (2004) 14. J.A. Croon, W. Sansen, and H.E. Maes, “Matching Properties of Deep Sub-micron MOS Transistor” Springer (to be published) 15. L.H.A. Leunissen, M. Ercken, M. Goethals, S. Locorotondo, K. Ronse, G.B. Derksen, D. Nijkerk, G.F. Lorusso, “Transfer of Line Edge Roughness During Gate Patterning Process” Proceedings of International Symposium on Dry Process (ISBN 4-9900915-7-4), 1 (2004)

Spring 2006

www.kla-tencor.com/magazine

55

Critical Dimension M

e

t

r

o

l

o

g

y

Focusing on the Drifts Spectroscopic Ellipsometry-based APC for Consistent Device Performance W. Lin, S. Liao, R. Tsai, M. Yeh, C. Hsieh, Y. Yu, and B.S. Lin, United Microelectronics Corporation S. Fu and T.G. Dziura, KLA-Tencor Corporation

Lot-to-lot after-develop inspection (ADI) critical dimension (CD) data are generally used to tighten the variation of exposure energy of an exposure tool through an automated process control (APC) feedback system. With decreasing device size, the process window of an exposure tool becomes smaller. Therefore, whether the ADI CD can actually reveal the real behavior of a scanner becomes a more critical question, especially for the polysilicon gate layer. CD SEM has generally been chosen as the metrology tool for this purpose, but top-down CD SEMs do have their limitations. Spectroscopic ellipsometry-based scatterometry technology, commonly referred to as SpectraCD, provides an alternative. In this study, SpectraCD, in contrast with CD SEM, improved the linearity of the correlation between ADI after-etch inspection (AEI) CDs from 0.4 to 0.8. The resulting data provided sufficient motivation to switch the APC feedback system from CD SEM to SpectraCD.

Introduction

APC has been in place for some time in semiconductor fabs and has proven to be crucial for achieving the required manufacturing tolerances for advanced technology nodes. The technique has been applied in CD control using CD SEM data as input, which the APC system then uses to adjust the scanner dose for subsequent lots. This algorithm works well provided that the focus-exposure window is periodically monitored1. As design rules continue to shrink, the demands on metrology performance are increasing, motivating process engineers to evaluate alternative CD measurement technologies such as SpectraCD (SCD), which is a model-based scatterometry tool utilizing spectroscopic ellipsometry. SCD has demonstrated good precision and throughput performance, and is being evaluated for inline process control2. The tool reports multiple profile features (CD, height, sidewall angle 56

Spring 2006

Yield Management Solutions

(SWA)) which can in principle be used to control multiple aspects of the process. This article describes the results of employing SCD in a 130-nm APC system.

CD measurement with SpectraCD

SpectraCD is a model-based metrology tool for measuring CD and profile of structures. It uses a broadband light source to collect spectroscopic ellipsometry data, reporting this as spectral variation in ‘alpha’ and ‘beta’ (analogous to the ellipsometric quantities tan Y and cos D). Data is collected from grating targets that contain the device structure of interest. The gratings may be one-dimensional (line) or two-dimensional (contact arrays). Measuring the device profile consists of computing a diffraction spectrum and fitting the resulting alpha-beta spectrum to the data collected, then reporting the profile parameters that give the best fit to the data. The model is computed either offline (library mode) or in real-time regression mode (CDExpress).

The tool is also a complete films characterization and measurement platform.

M

e

t

r

o

l

o

g

y

Monitoring ADI and ACI processes

4$% .$%

Z Y

3

&YQPTVSF FOFSHZ

Figure 3. Correlation between SCD MCD and exposure energy, measured over the time period shown in Figs. 1 and 2.

&UDIFS "

.$%

Data were collected from a 130-nm polysilicon process from 211 product lots; two scanners, six etch chambers, three CD SEMs, and one SCD tool were involved in this study. In the initial APC algorithm, CD SEM CD data was fed back to the scanner to control exposure; no feedback was employed to control scanner focus. SCD measurements were made in parallel with the data collected by the APC system. It was observed that the ADI MCD (CD at 50 percent of height) measured by SCD exhibited a systematic trend over a certain group of lots (Fig. 1), implying that one or more of the scanner parameters was drifting (while APC was enabled). Another possibility was that SCD was measuring the lots incorrectly (for whatever reason). Several tests were then performed to isolate the root cause. Figure 2 shows the trend in scanner exposure energy during the same time period.

The change in MCD is clearly anti-correlated with the change in scanner dose, indicating that the SpectraCD had correctly flagged a dose drift. In fact, if the change in MCD is normalized to the change in dose (Figure 3),

&UDIFS #

&UDIFS "

"$* "%*

4$% .$%

-PU

Figure 4. Lot-to-lot trends in photo and etch MCD compared over the same time period, as measured by SCD. The results indicate that the changes flagged by SCD in photo were replicated in etch.

.$% QU ."

-PU

the result (8.6 nm/mJ) agrees quite well with the calibration obtained independently with a single matrix wafer (7.1 nm/mJ).

Figure 1. ADI MCD trend over 135 lots (data and 5-lot moving average) as

%PTF

4$% .$%

measured by SCD, while under CD SEM-based APC control.

.$% %PTF

-PU

Figure 2. Trend of scanner dose and ADI MCD from SCD measured over 135

A comparison of the lot-to-lot trends at ADI and ACI gave additional confidence that SCD measured the lots correctly. Figure 4 shows the trend in ADI MCD and ACI MCD from the same lots, processed by one scanner and three different etch chambers. The changes in CD track very well between photo and etch; R2 correlation values ranged from 0.8 to 0.87. It is expected that any CD changes occurring in photo would be transferred to etch in the absence of any feed forward correction. Finally the SCD WA trend (Fig. 5) indicated good stability of the scanner focus, eliminating that as a possible cause of the MCD drift. The measured SWA 3s ~ 0.7 deg; combining this with a known 0.8 deg SWA drift per 0.1 Âľm of focus drift gives a focus stability < 0.1 Âľm.

lots. There is a clear correlation between the MCD and dose over time.

Spring 2006

www.kla-tencor.com/magazine

57

M

e

t

r

o

l

o

g

y

CFTU GPDVT Â&#x2DC;N

48" EFH

CFTU GPDVT Â&#x2DC;N

48" QU ."

-PU

Figure 5. Lot trend of the ADI SWA measured by SCD, indicating good stability of scanner focus.

"%* "$* FUDI CJBT

can be drawn: (1) the CD SEM is capturing real photo and etch CD variation, and the SCD measurement is less sensitive to these changes, or (2) SCD is measuring the lots correctly. We rejected conclusion (1) because it is unlikely that the SCD measurement is insensitive for both ADI and ACI in a way that gives good correlation between photo and etch through the entire range of CD variation. Also, the CD SEM CD measurement is known to be affected by any SWA variation; for poly lines ~ 2 nm/deg of CD shift has been measured3, and this can increase the measured lot-to-lot variation. The possibility that there was an issue with one particular CD SEM was considered; therefore, a performance comparison was conducted with that CD SEM on several other tools. The correlation between etch and photo was consistently better on the SCD tool; R2 varied from 0.1-0.38 for the CD SEMs and 0.8-0.87 for SCD was already mentioned. Testing was conducted using the SCD tool with the APC system.

$%

APC control with SCD

-PU

Figure 6. ADI and ACI lot trends measured by CD SEM. The ACI CD value has been shifted by the mean etch bias.

The greater measured correlation coefficient between dose and MCD (0.785) compared to SWA and MCD (0.177) lends more weight to the conclusion that real changes in dose drove drift in MCD, which were flagged by the SCD measurement.

The performance of the APC system with SCD data input was evaluated from ADI control to Lcap.

Figure 7 shows the ADI MCD lot trend before and after switching to SCD APC. A dramatic improvement in the lot-to-lot 3s of 53 percent is observed. This was achieved with no change in etch bias and an improvement in etch bias variation as well. Figure 8 shows the etch bias with and without SCD APC. The etch bias is essentially unchanged, and the etch bias 3s is reduced by almost 24 percent. With these improvements in CD control, one would expect that significant improvements at electrical test would be realized as well. This was confirmed by Lcap data taken under the

$%4&. CBTFE "1$

4$% CBTFE "1$

A close examination of the CD SEM data from the same lots proves instructive. Figure 6 shows ADI and ACI lot trends, with the mean value of the ACI CD shifted by the mean etch bias in order to overlay the two curves for comparison. For the most part, the data indicates that the CD at etch follows that at photo, but there are some clear excursions (highlighted in the figure), and the post-etch CD variation is larger. In fact, the etch bias lot-to-lot 3s is more than 70 percent greater on the CD SEM. There are several possible conclusions that 58

Spring 2006

Yield Management Solutions

.$%

Comparing SCD and CD SEM performance

.$% QU ."

-PU

Figure 7. Comparison of APC system performance when the metrology input was changed to SCD.

M

e

t

r

o

l

o

g

y

$%4&. CBTFE "1$

4$% CBTFE "1$

-DBQ

&UDI CJBT ON

Z Y 3

"%* .$%

-PU

Figure 8. Etch bias for the case of CD SEM-based APC and SCD-based APC.

Figure 10. Correlation over several lots of Lcap with ADI MCD measured by SCD.

Acknowledgment 4$% CBTFE "1$

-DBQ

$%4&. CBTFE "1$

-DBQ QU ."

W. Lin, S. Liao, R. Tsai, M. Yeh, C. Hsieh, Y. Yu, B.S. Lin, S. Fu, T.G. Dziura, Feasibility of Improving CD SEM-based APC System for Exposure Tool by Spectroscopic Ellipsometry-based APC System in SPIE 2005 Data Analysis and Modeling for Process Control II, Proceedings of SPIE Vol. 5755, pgs. 138-144 (2005) References 1. K. Monahan, â&#x20AC;&#x153;Microeconomics of Accelerated Shrinks in Demand-Limited Markets,â&#x20AC;? ISSM 2001.

-PU

Figure 9. Lcap lot trend under CD SEM-based APC and SCD-based APC.

two APC systems. Figure 9 shows that a 45 percent reduction in Lcap variation was achieved by converting to a SCD-based APC system. The correlation between ADI MCD, measured on a grating target, and Lcap, measured in a test key, is good, confirming that good front-end control using SCD results in consistent device performance.

Conclusions

The data presented in this study demonstrates the applicability of SpectraCD for litho and etch process control, especially when used as the data source for the fab APC system. SCD is capable of flagging process drift with good precision. Dose calibrations agree with established metrology, making transfer of process control to SCD seamless. The high measurement stability at both photo and etch result in stable front-end process control and good final electrical device performance.

2. R. M. Peters, R. H. Chiao, T. Eckert, R. Labra, D. Nappa, S. Tang, and J. Washington, â&#x20AC;&#x153;Production Control of Shallow Trench Isolation (STI) at the 130nm Node Using Spectroscopic Ellipsometry Based Profile Metrologyâ&#x20AC;?, Proc. SPIE, vol. 5375, pp. 798-806 (2004).

3. V. Ukraintsev, â&#x20AC;&#x153;Effect of bias variation on total uncertainty of CD measurementsâ&#x20AC;?, Proc. SPIE, vol. 5038, pp. 644-650 (2003).

4. W. Lin, S. Liao, R. Tsai, M. Yeh, C. Hsieh, Y. Yu, B.S. Lin, S. Fu, T.G. Dziura, Feasibility of Improving CD SEM-based APC System for Exposure Tool by Spectroscopic Ellipsometrybased APC System in SPIE 2005 Data Analysis and Modeling for Process Control II, Proceedings of SPIE Vol. 5755, pgs. 138-144 (2005)

Spring 2006

www.kla-tencor.com/magazine

59

Overlay M

e

t

r

o

l

o

g

y

In-chip Overlay Metrology in 90-nm Production Bernd Schulz, Rolf Seltmann, and Joerg Paufler, AMD Philippe Leray, IMEC Aviv Frommer, Pavel Izikson, Elyakim Kassel, and Mike Adel, KLA-Tencor Corporation © 2005 IEEE. Bernd Schulz, Rolf Seltmann , Joerg Paufler, Philippe Leray, Aviv Frommer, Pavel Izikson, Elyakim Kassel & Mike Adel, In-chip overlay metrology in 90nm production. Reprinted, with permission, from International Symposium on Semiconductor Manufacturing (ISSM) 2005 Conference.

While scanner aberration-induced pattern placement errors (PPE) can be measured, simulated, and validated by scanning electron microscope (SEM), the magnitude of the effect on late-generation scanners is small — of the order of ~1.5 nm in one line peak to peak across the slit. In-die overlay data contains additional sources of variation beyond PPE, and the results have been verified by SEM. Current practices based on linear models do not capture in-die variations as large as 5 nm, which significantly impacts model residuals. Currently, in-die target insertion is an insurance policy which enables in-die troubleshooting when process issues are suspected, and will potentially improve lot dispositioning in the future.

Introduction

Overlay is traditionally measured with metrology structures located in the four corners of a reticle field, consistent with the assumption of an overlay model with linear field dependence. This assumption dictates that the four corner measurements represent the extremes of overlay of all structures inside the reticle field. A number of recent studies have indicated that the discrepancies from field linearity may no longer be negligible when considering overlay control requirements at 65 nm. In order to be able to characterize and monitor overlay at multiple field locations, it has become necessary to insert metrology structures into locations other than the field edge scribe lines.

Standard size overlay structures are difficult to insert in-chip and represent a barrier to verification of the above assumption. This study reports on results of in-chip overlay metrology on product wafers using microAIM targets of 13x13 µm2.

on real product reticles. The size of the mark under test is about 13x13 µm2, compared with the standard size of 27x27 µm2. The design of the reference layer has a NS (non-segmented) outer and segmented inner. As will be described later, this mark can be used as a PPE monitor prior to the subsequent processing and lithographic steps. The current layer design consists of a NS inner, so the complete mark behaves as a NS AIM overlay mark. This is shown in the SEM images in Figures 3 and 4. Insertion locations were selected in areas of the device without electrical functionality.

These areas are typically tiled with dummy areas. Some of the dummy tiles are removed to place the overlay

'JOHFS UBSHFUT JO EJF 1SPEVDU

'JOHFS UBSHFUT JO EJF TFBM 1SPEVDU

Metrology target insertion into product dies

This section describes the methods used for designing and laying out micro-AIM marks 60

Spring 2006

Yield Management Solutions

Figure 1: Alternative micro-AIM layout strategies: in-die method on the left and internal scribe method on the right.

M

e

t

r

o

l

o

g

y

1SFDJTJPO

"*. TDSJCF MJOF N"*. TDSJCF MJOF N"*. JO EJF #J# TDSJCF MJOF

ON

9

Figure 2: Optical image of micro-AIM target inserted in-die as shown on the left in Figure 1.

:

Figure 5: 3sigma precision performance of standard AIM, scribe line micro-AIM, and in-die micro-AIM targets compared with scribe line box in box.

9 :

$% 4&. 0WFSMBZ

Figure 3: SEM images of micro-AIM

Figure 4: SEM image of micro-AIM

target in photoresist.

target after second lithography and

Metrology performance on in-die targets

In order to enable metrology target insertion in multiple field locations, target size needs to be substantially reduced. In this study the actual target area was reduced by a factor of four compared with the standard scribe line targets. One of the potential risks associated with this drastic reduction is the possible degradation in overlay mark fidelity (OMF) and precision due to reduced spatial and temporal information content.

The data in Figure 5 indicates that although some degradation is observed, the performance is still deep within the specifications of the Archer AIM tool. The in-die optical metrology results were also validated by a correlation study with CD SEM-based overlay results after etch, as shown in Figure 6. A Mandel regression

subsequent etch.

marks (See optical image in Figure 2). An alternative insertion strategy also tested was based on internal scribe lines within a six-die field. These two insertion strategies are shown diagrammatically in Figure 1.

Figure 6: CD SEM overlay versus imaging overlay results from in-die targets.

was performed on the data sets to evaluate linearity and biases, and the results in Table 1 indicate a good linearity with a small but significant bias in the x direction.

This is potentially attributed to CD SEM bias as no tool induced shift (TIS) correction is applied, as opposed to the case in optical metrology where TIS is routinely measured and corrected.

X overlay

Slope

Offset [nm]

R2

0.99 Âą 0.07

-1.4

0.88

Y overlay

0.94 Âą 0.11

0.2

0.77

Table 1: Orthogonal regression parameters for correlation data in Figure 6.

Spring 2006

www.kla-tencor.com/magazine

61

M

e

t

r

o

l

o

g

y

Aberration-induced pattern placement errors across the field

Exposure tool optical aberrations have been demonstrated in the past to be a key contributor to in-die PPE. It is also known that the magnitude of PPE is dependent on pitch and feature size1,2. Furthermore, Kye et. al have shown that such errors are also dependent on illumination conditions, such as coherency and illumination shape. Ueno et. al have published a methodology for characterizing with high precision the field dependence of PPE using grating-based simultaneous overlay marks. In the current work, such simultaneous marks have been reduced in size to 13x13 microns in order to fit into the available space at in-die locations while maintaining withinspecification metrology performance. We have also performed full electromagnetic simulations utilizing aberration measurements (taking into account Zernikes Z5 to Z15). The PPE sensitivity of the segmented and non-segmented parts of the grating to individual aberrations was calculated. As expected a near linear response to Zernikes was found with negligible crosstalk between individual aberrations. Figure 7 and Figure 8 display the results of these simulations compared with on-product measurements from wafers from two different generation exposure tools (PAS/1100 & XT1250).

11& ON

(Y direction) across the slit for two subsequent generations of scanners.

11& ON

911& ON

4MJU QPTJUJPO <NN>

Figure 9: CD SEM validation data for PPE in the x direction, measured on the same lot as the data in Figure 6, after etch.

11& 9 (FO "

4JNVMBUJPO 11& 9 (FO #

4JNVMBUJPO

Figure 7: Simulated and measured pattern placement error dependence (X direction) across the slit for two subsequent generations of scanners.

Spring 2006

$% 4&. (FO # BU '*

4MJU QPTJUJPO NN

62

11& 9 (FO # BU '*

11& 9 (FO "

4JNVMBUJPO 11& 9 (FO #

4JNVMBUJPO

Figure 8: Simulated and measured pattern placement error dependence

4MJU QPTJUJPO NN

These results demonstrate that this effect can be measured and correlated well to simulations. In three out of four cases, the mean discrepancy was less than 0.5 nm and below 1 nm in the fourth case. More significantly, it is also observed that X-PPE peak-to-peak variation across the slit was reduced from 4 nm to 1.5 nm from

Yield Management Solutions

one exposure tool generation to the next. Further validation of the PPE data was also achieved by performing CD SEM measurements. In this case, optical and CD SEM metrology was performed after etch, since photoresist coverage prohibits SEM-based overlay metrology immediately subsequent to lithography. The results are shown in Figure 8 where a mean offset of ~ 0.5 nm is observed between the SEM-based and optical metrology. A bias is also observed between the optically detected PPE before and after etch of about a nanometer. Such etch-induced PPE have been observed in our previous work and will be the subject of further endeavor. As we shall see in the next section, despite the impressive improvement in aberration-induced placement errors, significant high order structure is still observed in the overlay behavior across the scanner field, indicating that we need to continue the search for the dominant in-field contributors.

M

e

t

r

o

l

o

g

y

Full-field modeling

In this phase, the in-die overlay measurements were compared with the predicted overlay based on a model generated using a standard four-corner sampling plan as follows. In a first step, the in-die high density sampled data set was used to generate an interfield (wafer-level) model. This model was removed from the raw data and the remaining intra-field data was averaged over all fields to generate an “in-die map”. This map was then compared with (i.e. subtracted from) the linear intrafield model generated using a standard four-corner sampling plan.

This methodology was used on several data sets from production lots on different products and in each case, a high order fingerprint was observed for the in-die map.

In some cases, this map displayed systematic deviations from the linear model of up to 5 nm. It was also observed that this signature varied in the scan direction, further supporting the conjecture that aberration-induced PPE is not the primary cause of high order intra-field overlay.

In particular, a correlation was observed between the in-die map and the position within the individual chip.

ON

Figure 10: Systematic in-die residuals over scanner field on product, remaining after removal of standard four-corner linear model.

Acknowledgements Conclusions

Aberration-induced pattern placement errors have been characterized on product wafers for two different scanner generations. We observe their magnitude to be reduced to the extent that the systematic residuals of in-die overlay measurements cannot be explained by aberration-induced PPE alone. Other error sources are still a significant contribution to the total residual error and need to be further characterized.

In-die micro overlay targets can be used to refine the current overlay models. Sampling plans for overlay measurements with a combination of conventional and in-die overlay structures will become an important compromise between throughput in production and improved feedback and lot dispositioning.

© 2005 IEEE. Bernd Schulz, Rolf Seltmann , Joerg Paufler, Philippe Leray, Aviv Frommer, Pavel Izikson, Elyakim Kassel & Mike Adel, In-chip overlay metrology in 90nm production. Reprinted, with permission, from International Symposium on Semiconductor Manufacturing (ISSM) 2005 Conference.

This work was performed with the help of an R&D grant from the European Community under contract # IST-001854 Project OCSLI. References 1. C. Progler, S. Bukofsky, and D. Wheeler, “Method to budget and optimize total device overlay,” in Optical Microlithography XII, Luc Van den hove, Editor, Proceedings of SPIE Vol. 3679, 193-207 (1999).

2. Atsushi Ueno, Kouichirou Tsujita, Hiroyuki Kurita, Yasuhisa Iwata, Mark Ghinovker, Jorge M. Poplawski, Elyakim Kassel, and Mike E. Adel, “Improved overlay metrology device correlation on 90-nm logic processes”, Proc. SPIE vol. 5375, 222 (2004) 3. Jongwook Kye, Mircea Dusa, Harry J. Levinson “Linewidth asymmetry study to predict aberration in lithographic lenses”, Proceedings of, SPIE vol. 4346-134 (2001).

Spring 2006

www.kla-tencor.com/magazine

63

Process I

n

s

p

e

c

t

i

o

n

Reliable, Repeatable Wafer and Tool Dispositioning in 300 mm Fabs Bruce Johnson, Rebecca Pinto, Ph.D, and Stephen Hiebert , KLA-Tencor Corporation

Advances in wafer fabrication along with rising economic pressures on chipmakers have created greater challenges in the dispositioning of wafers and process tools. Such a climate has rendered it nearly impossible for manual disposition inspection to deliver even adequate results in a manufacturing environment or for process tool requalification. Manual inspectors often miss gross process problems, passing wafers downstream, where they will later be scrapped or create yield loss.Automated disposition, on the other hand, can integrate into a fab’s defect analysis infrastructure to enable better yield learning. These advantages, plus an automated system’s capability for high sampling, make it suitable for a low cost of ownership inspection strategy.

Introduction

Advanced fabs require accurate and rapid disposition decision-making during manufacturing, as well as a quick assessment of tool and process module output. Operators at manual or semi-automated inspection stations have historically done much of this, but these methods have been ineffective for quite some time. Manual inspections are expensive, and the results are well known to be unreliable. This is especially true for advanced 300 mm manufacturing, where vanishingly small device features, factory automation, and large wafer surfaces challenge the ability of the operator to assess the wafer and lot; these conditions place large quantities of valuable wafers at risk.

There are cases in most fabs where an operator has missed process or tool errors which have resulted in litho hot spots, CMP underpolish, scratches, underetch, splashback, coating failures, and many other types of gross errors. These can happen randomly on one wafer in a lot, on some pattern of wafers within a lot (such as every other wafer due to process tool chamber/stage configuration), or on a whole production lot. Most fabs have had significant yield hits from lots 64

Spring 2006

Yield Management Solutions

which were, for example, not coated with resist, but which were not recognized or sampled by the inspector and ultimately had to be scrapped. The cost of a small inspection error – missing a significant, but challenging-to-detect process error – can be very high.

In fabs with large product mixes, such as a foundry or a development line, each lot may represent all of the material for a specific customer. The loss of that lot, especially if it happens late in the device manufacturing process, can be devastating to both the fab and its relationship with the fab’s customer. For some smaller fabless semiconductor customers, it can almost be fatal because of the cycle time hit on a key part. Yet, these failures do happen with surprising regularity when fabs are not able to sample at the level and sensitivity required to capture critical excursions consistently and early on. Manual inspection has resulted in many cases of missed problems and resultant loss because of its inability to reliably find important defects. Automated inspection, on the other hand, is well suited to performing the disposition job. Its major strengths are: • Good sensitivity to detect defects of all types • Consistent results from tool to tool, day to day, and fab to fab • High throughput to adequately sample every lot

I

n

s

p

e

c

3FQFBU FYQPTVSF

t

i

o

n

4QMBTI CBDL

4DSBUDI %FGPDVT 'JMN 13 QSPCMFN TIPU EFGPDVT %FWFMPQ /( 6O o FYQPTVSF )PU TQPU 1PPS DPBUJOH

/PUDI EJTDPMPS

1BSUJDMF

Figure 1. Examples of product wafer frontside litho errors targeted at disposition inspection. (Problems routinely missed by inspection operators are highlighted in red.)

• Low cost of operation • Factory automation to work within a fully automated fab • Detailed, documented results compatible with fab defect analysis systems This paper examines these aspects of disposition inspection for both manual and automated approaches.

Low-overhead automatic recipe creation

Recipe creation with an automated wafer and tool disposition system can offer many economic advantages for a fab: • Simple, reality-based recipe design and user interface • Derivative recipes (layout, layer,

The job of disposition

The vast majority of production lots are good, and should be passed on to the next processing step. However, every once in a while, there may be a processing problem which impacts any of:

disposition rules) • Low skill level required, especially for derivate recipes • Automated recipe completion at first run

• A large portion of a wafer • One or several wafers within a lot

• Consistent, fast results between recipes

• An entire lot • Multiple lots These problems may come from the challenging process windows of today’s advanced technology, material problems, process recipe errors, process drift, operator error, or production tool errors.

Dispositioning vs. defect line monitoring

The disposition inspection’s goal is to quickly find these larger problems and to identify if there is an overall problem with the process which requires attention.

It is not the goal of the inspection to find smaller subtle Spring 2006

www.kla-tencor.com/magazine

65

I

n

s

p

e

c

t

i

o

n

Integrating with the automated factory

Even the best manual inspector does not run

â&#x20AC;˘ Engineering disposition â&#x20AC;&#x201C; for issues which the operator might not be positioned to handle

on factory automation, and s/he needs to be

In addition, the disposition result may flag the need to stop the process or production tool, either by a qualified operator or the responsible engineer.

in proximity to the wafers, which violates the goals of a highly automated fab. Automated inspection, on the other hand, is fully compatible: â&#x20AC;˘ Automatic material handling â&#x20AC;˘ Remote review â&#x20AC;˘ Status reporting â&#x20AC;˘ Remote host control â&#x20AC;˘ Alarms for wafer and lot results â&#x20AC;˘ Alarms for tool status

problems which are ongoing yield detractors, such as is done with defect line monitor, or with film thickness, overlay error, and CD measurement. These inspections and measurements are typically done on a small statistical sampleâ&#x20AC;&#x201D;two or three wafers per lot--and fed directly into a process control scheme. Most fabs perform automated disposition inspection on every lot since the problems can happen on a lot-to-lot basis and resultin expensive loss. Because it is done on (almost) every lot, it is important that the inspection also be fast to keep production moving.

Most material will pass. The rejected material may go into one of several paths, depending on the process module and type and severity of problem:

Disposition inspection criteria

The disposition needs to be accurate and consistent; it should neither reject good material (often called the Alpha risk), nor should it pass bad material (Beta risk).

Both errors can be costly. The limitations of manual inspection are known to give it a high Beta risk, and this shows up in higher downstream scrap and yield loss. Automated inspectionâ&#x20AC;&#x2122;s higher capture rate of all defect types improves this, but it should not do this at the cost of higher Alpha risk, which may impact fab productivity. It is important that the disposition inspection be immune to normal process variation which could result in false rejects. Along these lines, it is also useful for the inspection to be able to bin defects according to their level of criticality to yield (i.e., nuisance vs. killer defects). Such a capability can further speed up disposition decision-making. The sample size is also important to both of these risks; the sampling should be statistically valid for the nature of the problems that the inspections target.

The inspection needs to be fast, so that it does not hold up production material, and it must carry a low operational cost to be feasible in a production environment.

Yet, enough material must be sampled to have good discrimination and accurate lot disposition.

JOTQFDUFE BSFB

â&#x20AC;˘ Rework â&#x20AC;&#x201C; often handled by the operator

NN XBGFST TBNQMFE 9 WJTVBM mFMET BU 9 #BDLTJEF JOTQFDUJPO NJOVUFTĂ&#x160; MPU JOTQFDUJPO

â&#x20AC;˘ Scrap â&#x20AC;˘ Waive â&#x20AC;&#x201C; for captures not meet- ing rework or scrap criteria

Spring 2006

"VUPNBUFE EJTQPTJUJPO

NN XBGFST TBNQMFE PG XBGFS

#BDLTJEF JOTQFDUJPO NJOVUFTĂ&#x160; MPU JOTQFDUJPO

'BCT NBZ VOEFSDVU JOTQFDUJPO BDDVSBDZ CZ JOTQFDUJOH GFXFS UIBO XBGFST MPU

Yield Management Solutions

Figure 2. Manual and automated Viper 243X inspection time comparison.

66

.BOVBM EJTQPTJUJPO

I

n

s

p

e

c

t

i

o

n

BACKSIDE DEFECT INSPECTION

1BSUJDMF JNBHF

'4 %FGPDVT

#4 1BSUJDMF

Numerous process steps can leave particles on wafer backsides, due to the process, maintenance or cleaning problems, or handling. When these are carried on to the next step, the backside particles can result in process problems at these subsequent steps. The most common example is in lithography, where a particle causes a bump on the wafer frontside as the wafer is pulled onto the scanner chuck. The scanner can then have a focus error at that point, resulting in a CD error or pattern failure.

Wafer backside particle (right side) caused a defocus hot spot on the wafer

In some cases, the particle will stay with the wafer, but in other cases, it can transfer to the chuck. The high force of the vacuum pull-down often causes it to almost fuse to the scanner chuck. This results in a hot spot at the same location on each wafer. Once the problem is identified, the scanner must be taken down to clean off the particle or replace the chuck, and a requalification is always required. This scanner can incur significant downtime.

frontside. The captured image of the particle is seen at the far right.

Finally, the disposition inspection needs to be compatible with the level of factory automation, especially for advanced 300 mm fabs.

This is a challenge for manual inspection, since fabs minimize the number of operators on the production floor.1 Material movement to and from the tool, host control, and results integration are all important considerations.

In addition to product disposition, fabs use similar inspection strategies to requalify production tools and cells. After a production tool has been down for maintenance or to correct a problem, it must be requalified before being released back into production. This requalification, depending on the specific process step, typically includes measurement of the appropriate parameters (overlay and CD for litho, film thickness for films and CMP etc.), microdefect qualification, and gross process qualification. The requirements described for disposition inspection apply to this tool qualification inspection as well.

These requirements will now be explored in greater detail for manual and automated disposition inspection.

The manufacturing challenge

Device manufacture requires fast, accurate decisions to keep good product moving.

Integral to this is high productivity from process tools, so rapid decisions are needed to requalify them periodically or after maintenance. In most cases, fabs want to verify that

A backside product disposition inspection is typically done at process steps prior to litho (and other sensitive process steps) to avoid the rework, yield loss, and the hit to scanner productivity. Because the litho process itself can also create backside defects (through splashback, for example), a backside inspection is also typically done as part of the develop inspect disposition.

Based on these issues and customer inputs, optional backside inspection has been added to the KLA-Tencor Viper 2435 automated disposition system. An additional backside stage is joined with the primary scanning stage; the scanning motion of the frontside inspection also scans the backside stage. After a wafer is inspected on the first stage, it is transferred to the second stage where it is scanned simultaneously with the following frontside wafer scan. The backside stage has its own darkfield illumination and camera to perform this inspection. Because the backside inspection adds a step to the sequence, the system throughput is slightly reduced.

Defect results are shown as an additional channel which may be reviewed just like any other defect, and may be transferred to KLA-Tencor Klarity Defect software. Fabs have used this new capability to identify the cause of hot spots. The above graphic shows an example of a hot spot which was caused by a confirmed backside particle.

This backside inspection is a convenient capability to add to an existing frontside disposition inspection. An alternative to consider where no frontside inspection is being done is to use a KLA-Tencor Surfscan SP1 unpatterned wafer inspection tool with a backside inspection module3.

Spring 2006

www.kla-tencor.com/magazine

67

I

n

s

p

1

.BOVBM

e

2

t

i

o

3

1

9

c

4

3

n

4

5

6

%JTQPTJUJPO

%JTQPTJUJPO

.JOVUFT

9

.BOVBM

t 'JOE EFGPDVT JO TDSFFO HBMMFSZ t *OGPSN QSPDFTT FOHJOFFS t 1& DIFDL TDSFFO HBMMFSZ EJSFDUMZ t 1& DIFDL XBGFS JO $% 4&.

3FBE ,-"3'

t %JTQPTJUJPO

t 'JOE EFGPDVT JO NBDSP t *OTQFDUJPO JO NJDSPTDPQF t 3FDPSE EFGFDUT QPTJUJPO t *OGPSN QSPDFTT FOHJOFFS t 1& DIFDL XBGFS JO ."%* t 1& DIFDL XBGFS JO $% 4&. t %JTQPTJUJPO

Figure 3. A comparison of manual and automated (Viper 243X) lot disposition time with 300-mm wafers inspected at a frequency of six wafers per lot. Source: Powerchip, from YMS Europa poster paper.

every production lot is good, paying particular attention to problems which could result in significant yield hits. Because the vast majority of lots are good, the inspection is actually very tedious. The operator expects product to be good, and becomes complacent (or bored), often missing the improperly processed wafers or lot.

While a manual inspector must hunt (often unsuccessfully) for defects, an automated system can spend more productive time actually viewing the problems found, and making clear and documented disposition decisions.

The following bar graph compares the complete cycle time for a manual inspector and an automated disposition tool for a representative case. The result is that a

22

7JQFS .BOVBM

21

19

%FGFDU $PVOUT

9

6

3

1

2

4

1

3

1

3

0

3

0

3

0

3

1

0

1

0

1

0

DVT GPDVT FGPDVT FGP F E F E EHF E FEH F

O O U I MF PO F E PS PU WPJ QBSUJD DPNF JTDPM QPTVS TDSBUD USJBUJP JOBUJP GBMM TQ E T IPU FY UBN OP DPO

%FGFDU 5ZQF

Figure 4. Manual and automated (Viper 243X) defect capture for all types (frontside inspection only). In every case, manual inspection missed defects, and in some cases, complete defect types. Note that dispositions based on the low manual capture rates would result in downstream scrap and yield loss.

68

Spring 2006

Yield Management Solutions

problem can be identified more quickly with automated disposition, putting less production material at risk.

One operator can run and manage multiple automated disposition tools at the same time, which s/he obviously cannot do for manual inspection. Consequently, from a cost of ownership (CoO) standpoint, operation costs with an automated inspector are approximately 1/3 of those of a manual inspection operator. In addition, the disposition results are much more consistent than those produced by multiple inspection operators.

Any automated inspection system requires recipes to run. Fabs state that it is necessary to minimize any overhead associated with this disposition activity. Using automated recipe creation (ARC), which draws on recipes already created, most new devices and layers should be completed in well under 10 minutes by an average production operator. Derivative layer recipes are automatically created by the tool at first run.

Automated inspection operation should not require an engineer or highly trained technician.

Limitations of manual inspection

Multiple fab studies have shown that manual inspection finds only one-tenth to one-quarter of what automated disposition inspection uncovers. These errors come from spinners, exposure tools, developers, etch tools, polishers, other process equipment, and handling. They can be visible to the naked eye or with moderate magnification.

But manual inspection completely misses most defects, often because the wafer and inspection conditions are challenging, the operator looks in the wrong place, or the operator is not paying sufficient attention. In many cases, inspection operators have missed the fact that some wafers had no pattern — on the whole lot, even on multiple lots — or the material is not sampled. As automated inspections at ADI (after develop inspect), AEI (after-etch inspect), and after polish have become more routine, it is clear that manual inspection — although relatively low in initial cost — leaves the door wide open for revenue and profitability loss due to increased scrapped wafers and lower yield.

I

Automated inspection has the advantage of providing real data rather than anecdotal descriptions from operators. The real data can then be used to understand trends and isolate sources, similar to what is done with micro defect data, allowing quantification and prioritization of problem severity using Klarity Defect or alternative analysis systems. In almost all cases, fabs implementing automated inspection of macro defects have been surprised by the extent of defects and process issues, and their newfound ability to respond quickly to fix them.

n

s

p

e

c

t

i

Manual Disposition

Viper 243X Disposition

Inspection area

5 points (< 10%) (operator dependent)

Whole wafer (100%)

Within-lot sampling

3 wafers/lot (10%)

Whole lot (100%)

Lot sampling

100% (desired)

100%

Throughput

23-26 wph*

100 wph*

Inspection time/wafer

1-7 to 2.7 minutes* (3 wafers only)*

0.6 minutes* (25 wafers)*

Lot inspection time

5-8 minutes

15 minutes

Frontside illumination

Brightfield

Brightfield and Darkfield

Device (chip) inspection

Yes

Yes

Scribe line inspection

Limited

Yes

Unpatterned area inspection

Yes (although often ignored)

Yes

Wafer edge exclusion

0 mm

0 mm

Edge bead removal inspection

Inconsistent

Yes

Backside inspection*

Optional

Optional

o

n

Very good While the operational costs of both the manual and the autoReport detail & archive Limited operator note Lot summary, wafer report, gallery mated disposition inspections report, recipe report are relatively low, they are not Image save No Wafer images, defect clips the most important cost. The main goal of the disposition Factory automation No Yes inspection is to prevent bad Table 1. A comparison of manual and Viper 2345 disposition inspections reveals that automated inspection is more material from continuing down comprehensive and is faster in delivering go/no-go decisions. * Backside inspection requires more time per lot. the line, where it will either be scrapped or result in yield loss.

Since these types of process errors can be very large in scope, • Cost due to missed defects is higher for manual these costs have been known to be very high. When inspection in losses through scrap and decreased yield. rework is possible, such as in litho or at some CMP steps, the value of the wafers can even be rescued. The • Risk of zero-yield lots is higher with manual other major goal for disposition inspection is to identify inspection; this can affect a fab’s relationship with a process tool which is unfit for production. This may be its customer. a result of the product wafer inspection, or through a periodic cell monitor.

• The value of information is greater from the automated disposition inspection.

As seen in Figure 4, manual inspection was found to be inferior in both its sensitivity across all defect types and Table 1 summarizes some additional details of comparison. its ability to even see certain defect types. Each missed problem will move downstream, only to be scrapped later, or rejected as a yield loss.

Accurate disposition decisions Ideally, whatever method is used for reaching a disposition, the decision should be as close to the specification Cost and benefit of ownership: as possible. It is sometimes hard to quantify the cormanual vs. automated inspection rectness of decisions. One fab performed an evaluation In summary, the overall cost of manual inspection is of two automated inspection tools to see which one best higher than that of automated disposition inspection: matched the engineer responsible for the process module. The evaluation was done on product wafers with • Manual CoO operational cost is higher for a normal process variation. statistically valid sample size Consistency between operators/ tools, and over time

Very poor

Spring 2006

www.kla-tencor.com/magazine

69

I

n

s

p

e

c

t

i

o

n

Figure 5 shows the deviation of the two inspectors from the disposition decision made by the responsible engineer. In the cases where the other tool found too many defects, it was found that these were not real defects, so the tool would be rejecting a good lot (Alpha risk), or incorrectly placing it on hold. Where the tool found far fewer defects, it was failing to reject a bad lot (Beta risk); this would result in downstream scrap or yield loss. The 243X disposition decisions closely matched the engineerâ&#x20AC;&#x2122;s decisions.

"CPWF o UPP NBOZ EJF SFKFDUFE

7JQFS 9 0UIFS BVUPNBUFE UPPM

%JGGFSFODF OVNCFS PG %JF 'BJMFE

Fabs require the capability to fine-tune go/no go disposition rules based on defect distribution, both by size and location. Rules can include zones on the wafer, as well as overall lot defectivity.

This helps to optimize both the alpha and beta risks of the disposition decisions. All results must be available for review, and should be able to be sent to the fab defect analysis system for correlation and historical analysis.

#FMPX o UPP GFX EJF SFKFDUFE

-PU OVNCFS XBGFS OVNCFS

Figure 5. Field comparison of two automated inspectors in a competitive head-to-head, deviation from engineer decision.

Litho defocus disposition â&#x20AC;&#x201D;â&#x20AC;&#x201D; case studies

- -

- -

-

-

- -

-

-

-

-

- -

-

-

- -

-

-

-

-

-

-

.BOVBM JOTQFDUJPO

JOTQFDUJPO

%FUFDUJPO UISFTIPME Ăł Â&#x2DC;N EFGPDVT DBQUVSF

%FUFDUJPO UISFTIPME Ăą Â&#x2DC;N EFGPDVT DBQUVSF

%FUFDUJPO UISFTIPME Ăł Â&#x2DC;N EFGPDVT DBQUVSF

1SPCBCJMJUZ PG EFUFDUJPO

.BOVBM

%FGPDVT Â&#x2DC;N

Figure 6. Defocus detection threshold -- the Viper 2435 is demonstrated to be significantly more sensitive.

Programmed defocus settings are indicated on the wafer maps.

70

Spring 2006

Yield Management Solutions

Collapsing lithographic process windows mean that there is increasing likelihood of areas of the wafer being out of focus. Defocus can result from scanner errors for a field or within a field, and due to particles on the wafer backside or on the scanner chuck, resulting in a â&#x20AC;&#x153;hot spot.â&#x20AC;? These areas of defocus may result in complete pattern failure or a significant change in CD. In most cases, these are very hard to detect on real product wafers, but they result in yield loss. In the cases where it is due to a persistent particle on a chuck, every wafer may have yield loss at that location.

An experiment was run to determine the ability of automated and manual inspectors to identify defocus. A metal 1 logic product wafer was created with fields with known amounts of defocus, and then

I

n

s

p

e

c

t

i

o

n

material out of specification. Such a low-level defect may be isolated to its source by stacking the results in the defect analysis system across wafers and even across lots.

CMP disposition – case studies

Figure 7. The same defocus hot spot detected on the wafers as indicated2.

inspected by both methods. The results are shown in Figure 6.

The automated tool was found to be significantly more sensitive than the inspection operator. The fab’s process window specification for this layer is 0.1 µm defocus; the automated inspector found 50% of the defective fields at this limit. The inspection operator did not find defocused fields until they were well beyond the spec.

In this case, the defocus covers the full field. Hot spots may be smaller, and the 2435 has been shown to be effective at finding them. Advanced processes are quite sensitive to even small particles on the backside.

CMP is a process module which is still seeing ongoing challenges and rapid process development. Many fabs have implemented automated disposition inspection of macro defects because of the relatively high rate of such defects and because of process instability. As an example, a fab sampled a portion of a Cu CMP lot and found one wafer with underpolish defects, as shown in the following figure. The cassette map shows that wafer 22 was rejected according to the disposition rules, and the wafer map clearly shows the problem, as does the saved wafer image.

In Figure 8, the CMP polish tool did not flag that there was anything wrong with either the wafer or the lot. The inspection result triggered an inspection of the whole lot, where it was found that four additional wafers were bad.

In Figure 9, it was possible to rework the wafers by sending them for additional polishing. This saved scrapping high-value, near-end-of-line wafers.

In some cases, the backside event is not a particle on a specific wafer, but on the scanner chuck. Automated disposition results very quickly indicate the problem, as can be seen in the following lot review screen. In this case, a scanner with twin stages produced a hot spot on every second wafer. This can easily be seen in the cassette map, and is highlighted on the individual wafer thumbnail results.

Such an inspection result may be used to drive a CD SEM to the specific location and measure the pattern to determine the CD deviation.

While Figure 7 clearly shows the nature of the problem, not all such cases are as clear. A small stage problem may be right on the threshold of the process window, and not always produce

Figure 8. Cu CMP lot sample with residual Cu detected. Disposition inspection thumbnails are shown above, and the sampled cassette map is to the left.

Spring 2006

www.kla-tencor.com/magazine

71

I

n

s

p

e

c

t

i

o

n

Process flow

Automated disposition inspection fits into the normal process flow, taking the place of the manual inspection. Because the system is fully factory automation compliant, it likely fits into the flow more easily than the manual step.

Figure 11 shows an example of how automated disposition inspection can be fit into a litho process flow.

Figure 9. Cu CMP lot with residual Cu detected on five wafers. The cassette map clearly shows the rejected wafers.

Of course, the disposition inspection may be performed within any sequence, but the abovedescribed flow is the most common BKM (bestknown method), following the expected yield and cost cascade. In advanced litho processes, overlay is typically the greatest yield detractor, so it makes sense to do this first. The CD SEM is often the busiest, so any material which can be routed away from it (through rejection at a prior disposition) saves its capacity.

Note that this shows that all three inspections and measurements are performed, but there are layers where neither overlay nor CD measurement are done. These non-critical steps can still have process problems which result in gross process errors (such as no resist or no develop), and should still have a disposition inspection performed.

For each wafer or lot which is rejected, an operator will review the result to determine the course of action. In many fabs, for layers where the cause and fix are clear, the operator may directly take the corrective action, particularly when it involves rework. Usually, however, before a lot is scrapped, it will go to an engineer for disposition.

Figure 10. Post CMP disposition inspection identified eight lots of wafers with edge damage.

Wafer maps of the failed lots are shown above.

In another case (see Figure 10), an excursion was highlighted during a disposition inspection at metal CMP. Eight full lots (200 wafers) were found to have a similar type of wafer edge defects. Investigation showed that one ECP tool was causing the problem across all the lots.

While the wafers had to be scrapped, the quick recognition of the problem allowed the ECP tool to be shut down, minimizing damages to additional lots.

72

Spring 2006

Yield Management Solutions

Similar process flows have been established for other process modules, with attention to the most cost-effective sequence.

Process tool requalification takes a similar approach, although it may vary depending on whether the qualification is done on product or test wafers. In litho, for example, the requalification is typically done with PCM4 (Photo Cell Monitoring) on resist test wafers; these same wafers may be used for the gross error requalification

I

n

s

p

e

c

t

i

o

n

CAPTURING ALL DEFECT TYPES The goal of disposition inspections is to capture all of the types of errors which can happen. There are several issues which make this very challenging:

such as color due to film errors, spin or develop problems, residual pattern, arcing, striations, and backsplash.

•

Defects may have very different reflectivity or scattering profiles. Missing resist, hot spots, copper residual, particles, scratches, and etch errors all look very different from one another.

–

•

Some defects may be uniform, either covering the whole wafer or consistently appearing on a specific location. Manual and some automated inspection may miss these because there is no fixed reference.

– Backside darkfield for backside particles and damage

Problems to address include:

– No resist on a wafer

– No exposure

– No develop

– No etch

These channels are inspected concurrently, maximizing throughput. The frontside inspections are done through two channels, and the backside is inspected while the next wafer is on the frontside stage (discussed in sidebar 1).

• Algorithms – High speed image processing extracts defects from each of the image channels, while suppressing noise from process variations:

– Process window failure which occurs on a particular pattern (in litho, etch, and CMP)

– Die-to-die – Detects random defects such as scratches, striations, and others which vary across the wafer. This will also capture defects which repeat from exposure field to exposure field.

* A wafer (especially 300 mm) is a very large surface to look at for subtle errors. %BSLmFME $BNFSB

#SJHIUmFME $BNFSB

– Field-to-field – Detects additional random defects, including scribe line problems

$BNFSB $POUSPMMFS

#BDLTJEF .PEVMF 0QUJPOBM

4DBO

Frontside darkfield (scattered light imaging) for defects which result in changes in scatter- ing, such as scratches, defocus, particles, peeling, pattern collapse, and under etch.

*MMVNJOBUPST

*NBHF 1SPDFTTPS %FGFDU .BQ

8BGFS : 4UBHF

$POUSPM $PNQVUFS

Concurrent brightfield and darkfield channels feed frontside wafer images to the image processor to capture all defect types.

KLA-Tencor’s Viper 2435 automated disposition system (and its predecessor) is designed to specifically target a wide range of defect types. This is done through several aspects of the design: • Hardware – optical channels are designed to detect defects with different characteristics:

– Frontside brightfield (direct reflection imaging) for defects which change the appearance,

– Wafer-to-wafer – Wafer reference identifies missing film, uncoated, undeveloped, unetched wafers where the entire wafer processing is incorrect

– EBR – Detects errors in edge bead removal

– Edge Damage Check – Detects problems with the wafer edge and bevel

– Unpatterned or partially patterned area outside the main device array – Detects random and process errors outside the main patterned area The specialized hardware and algorithms combine to cover a wide range of defect types, giving good sensitivity to the types of problems which fabs need to capture at disposition inspections at litho, etch, CMP, films, and other process modules.

Spring 2006

www.kla-tencor.com/magazine

73

I

n

s

p

Stepper / Track

e

c

t

Next step

Rework or feedback

i

o

n

Overlay

IN or OUT Spec?

Viper 243X

Pass

Pass or Fail Spec?

Hold

CD SEM

Pass

IN or OUT Spec?

Pass

Wafer Out

Rework or feedback

Operator review

PL

for first filter out is Previous / Current Layer?

Pass Notes on Host SW

CL Rework or feedback

Engineer disposition

Monitor 100% wafer Alert

For final decision or stop excursion

Waive

Scrap

Figure 11. Litho disposition process flow with 2435 performing defect disposition.

as well. This quickly and accurately qualifies the resist coat, exposure, and develop process. If PWQ5 (Process Window Qualification) is used for the qualification, then additional test or product wafers are processed for the gross defect portion of the requalification.

Conclusion

Fast, accurate disposition decisions are needed in today’s 300 mm fabs. The disposition decisions require the ability to consistently detect a broad range of defects and process failures on the wafer, at the wafer edge, and even on the backside, at as low a cost as possible.

That low cost should not, however, come at the cost of broad defect type capture or a low false count rate, not to mention accuracy. Manual inspection approaches have been shown to be inadequate for both product disposition and process tool requalification, due to their limitations in sensitivity to wide-ranging defect types, in repeatability, and in agreement to engineering decisions. Manual inspection has also been shown to not meet the demands of factory automation and good documentation of the results.

Automated gross production disposition and tool qualification have been in use in many wafer fabs, and have been shown to be capable of meeting the requirements of advanced 300 mm fabs. Today’s systems benefit from the learning and development gleaned from prior generations while offering new innovations such as backside inspection. Automated inspection has been shown to be a consistent and reliable tool for fast disposition in litho, CMP, etch, and other process modules. Furthermore, automated disposition inspection in fabs has been shown to result in cost savings from reduced scrap and improved yield, and through increased fab productivity.

Acknowledgement

The authors would like to thank Scott Ashkenaz for his contributions to this article.

References 1. Electronic News, “Automation to Proliferate in Intel Manufacturing,” December 2005 2. KLA-Tencor, Think Shrink, 2004 3. KLA-Tencor, Start Yield Enhancement from the Wafer Backside, 2005

74

Spring 2006

Yield Management Solutions

➤ 0MXLS 'LEPPIRKI

;LIR SYV '(W [IRX SYX SJ GSRXVSP [I HMHR X NYWX RIIH XS ORS[ XLEX WSQIXLMRK [EW [VSRK ;I RIIHIH XS ORS[ [L] ERH LS[ XS ´\ MX

( %8%

- 2 * 3 6 1 %8 - 3 2

/ 2 3; 0 ) ( + )

()'-7-327

2S[ MX W RSX E TVSFPIQ

➤ 0MXLS 7SPYXMSR 0MRI[MHXL 'SRXVSP -R XSHE]«W GSRWXERXP] GLERKMRK QERYJEGXYVMRK IRZMVSRQIRX TVIHMGXEFPI PMRI[MHXL GSRXVSP MW I\XVIQIP] HMJ´GYPX =IX PMXLS IRKMRIIVW LEZI JSYRH XLEX F] YWMRK XLI VMKLX QIEWYVIQIRX XSSPW [MXL XLI VMKLX HEXE EREP]WMW XLI] GER WMKRM´GERXP] VIHYGI EGVSWW GLMT PMRI[MHXL ZEVMEXMSRW -RGVIEWMRK XVERWMWXSV WTIIH -QTVSZMRK PIEOEKI GYVVIRX %RH QE\MQM^MRK ]MIPH ERH IPIGXVMGEP HIZMGI TIVJSVQERGI =SY XSS GER QEOI VETMH ORS[PIHKIEFPI HIGMWMSRW VIKEVHMRK '( IVVSVW ERH LS[ XS ´\ XLIQ 8SKIXLIV [I GER EGLMIZI E LMKL ]MIPHMRK VIPMEFPI TEXXIVRMRK TVSGIWW

I'( ¦ -RPMRI '( 7)1 QIXVSPSK] JSV MR HIZMGI '( GSRXVSP ERH HIWMKR ZIVM´GEXMSR

¦ -RPMRI WTIGXVSWGSTMG '( QIXVSPSK] JSV '( ERH PMRI[MHXL TVS´PI %4' 7TIGXVE'(

¦ %YXSQEXIH PMXLS GIPP GSVVIGXMFPIW JSV '( ERH SZIVPE] VIKMWXVEXMSR /8 %REP]^IV

¦ 0MXLSKVETL] WMQYPEXMSR JSV TVSGIWW STXMQM^EXMSR 4630-8,

%TTPMGEXMSRW I\TIVXMWI

=MIPH GSRWYPXMRK

81

81

81

81

*SV QSVI PMRI[MHXL GSRXVSP WXVEXIKMIW ZMWMX

[ [ [ O P E X I R G S V G S Q P M X L S =SYV SRPMRI PMXLSKVETL] VIWSYVGI GIRXIV

/0% 8IRGSV 'SVTSVEXMSR

Product News SpectraCD-XT

Cost-effective Optical CD Metrology

SpectraCD-XT provides high-performance 2D/3D CD metrology for the 90-nm and 65-nm nodes at the lowest cost per yield-relevant measurement. Optimized to meet chipmakers’ increased sampling requirements without compromising sensitivity, the SpectraCD-XT features sub-two-second move-acquire-measure (MAM) time and a two-fold increase in throughput (to more than 100 wph) compared to previous-generation platforms. Leveraging spectroscopic ellipsometry technology, the system provides leading measurement precision for advanced applications where device structures are highly complex and require multiple types of measurements, such as shallow trench isolation (STI), gate, and spacer. Oblique illumination optics enable the identification of smaller structural anomalies—such as notching and footing of gate profiles—that more accurately correlate to end-of-line device performance and yield. Improved model setup and analysis tools available with the SpectraCD-XT speeds library generation by 30 to 60x, providing significant savings in time and engineering resources.

SpectraCD-XT

TeraScan STARlight-2

Cost-effective Reticle Contamination Inspection

The TeraScan STARlight-2 provides cost-effective inspection of contamination and electrostatic discharge on advanced 65-nm node photomasks at high resolution and throughput. Its enhanced capabilities are uniquely suited for inspecting mainstream XRET (extreme resolution enhancement technique) photomasks, as well as detecting crystal growth and other progressive defects, which lead to gradual yield roll-off. Smaller pixel sizes (125 nm and 90 nm) provide the resolution and sensitivity needed to detect extremely small mask contaminants well before they impact the process window. Improved algorithms enable contamination detection in high-density patterned areas. TeraScan STARlight-2’s full-field contamination inspection capability allows detection in scribes and borders — where progressive defects generally emerge—as well as in high density patterned areas for both single-die and multi-die reticles. After defects are captured, a ReviewSmart option on the TeraScan STARlight-2 dramatically reduces the number of defects that require review by grouping similar defects into common bins to facilitate disposition and speed corrective measures.

TeraScan STARlight - 2

Viper 2435

Automated Wafer and Tool Dispositioning System

As KLA-Tencor’s newest automated 300-mm wafer and tool dispositioning system, the Viper 2435 delivers quick go/no-go decisions to help chipmakers avoid production delays and minimize yield loss. Able to capture the broadest range of defect types at high throughput, the Viper 2435 can be implemented inline quickly to monitor the lithography, chemical mechanical planarization (CMP), and etch process modules. Automated dispositioning with the Viper 2435 delivers a 67 percent lower cost of ownership than manual inspection. Process engineers are equipped to identify critical defects through the tool’s enhanced signal-to-noise ratio and adaptive thresholding techniques, which result in stronger nuisance suppression capabilities.

Engineers can then quickly determine which wafers need to be scrapped or sent for rework and to promptly flag problematic tools or process modules. Extendible and fully 300-mm factory automation-compatible, the Viper 2435 can help chipmakers accelerate time to decisions.

Viper 2435

76

Spring 2006

Yield Management Solutions

2367

UV Line Monitor for Rapid, Low Cost of Ownership Yield Ramp

Extending the widely adopted 2365, the 2367 is the latest-generation UV brightfield patterned wafer inspection solution from KLA-Tencor. With increased sensitivity and a 2X faster data rate, the 2367 is equipped to deliver higher sampling rates and more effective capture of yield-impacting defects on critical FEOL and BEOL layers such as gate etch and Cu CMP. The 2367 complements the 2800 DUV brightfield inspector in a mix-and-match inspection strategy that enables chipmakers to customize the illumination wavelength applied to each unique layer for more effective defect capture. For faster tool and yield learning as well as production ramps, the 2367 features a common UI with the 2800 as well as the Puma 9000 darkfield inspection solution and the eS32 e-beam inspection tool.

2367

eS32

e-Beam Inspection for Faster Decision-making

Providing the widest capture of systematic, yield-impacting electrical and physical defects in FEOL and BEOL applications, the eS32 is integral in an inspection strategy facilitating faster decision-making. With its unique ability to find and fix electrical defects earlier, before production delays or yield loss result, the eS32 enables chipmakers to continue innovating to maximize transistor performance.

Methodologies such as the proprietary, on-board µLoop along with algorithms for advanced binning facilitate faster time to root cause. Enhancements in sensitivity and throughput, along with its ease of setup and use, enable the eS32 to provide a low cost of ownership. Rather than spending time calibrating the tool, for example, yield engineers can focus on implementing applications that can help them improve yield.

eS32

DesignScan

Lithography-aware Design Inspection

DesignScan is the industry’s first full-chip process window inspection system for inline post-RET reticle design layout inspection. Using DesignScan, chipmakers can catch systematic design defects on critical layer mask designs at the 90 nm and below nodes, before building the reticles. The system was developed with KLA-Tencor’s high speed image computing platform, proprietary calibration process, highly accurate physics-based process models, and defect review and disposition methodologies.

Across the process window and at best focus and exposure conditions, the system delivers the fastest time to results for comprehensive design data defect detection.

The tool facilitates communication of design-related information between foundries and the fabless design community. Easy to use and available in different speed configurations, DesignScan provides the lowest cost per inspection while helping chipmakers reduce their frequency of mask respins.

Spring 2006

www.kla-tencor.com/magazine

DesignScan

77

Product News Candela CS20

High Brightness LED Production Monitor

The Candela CS20 is the first automated wafer inspection system from KLA-Tencor designed to address the defect management requirements of the rapidly growing high brightness light-emitting diode (HB-LED) market. Leveraging a proprietary, multi-channel detection architecture, the Candela CS20 can inspect transparent wafers and epi layers for micro-pits and other defects non-destructively at throughputs of up to 25 wafers per hour—enabling, for the first time, a true production line monitor for wafers used to produce HB-LED devices. During HB-LED production, contaminants such as particles and stains can alter film characteristics or cause adhesion problems for subsequent layers. Surface and sub-surface defects, crystal dislocations, and excessive roughness can impact subsequent processes and substantially degrade device performance and yield.

Traditional inspection methods rely on manual review by an operator—making them extremely slow, unreliable, and often destructive. In addition, these methods are not easily scaled up to meet increasing production volumes. The Candela CS20 offers the sensitivity, versatility, and throughput needed for both process development and epi-growth production control for HB-LED manufacturing. The Candela CS20 is the first new product to come out of KLA-Tencor’s recent acquisition of Candela Instruments, a leading supplier of inspection systems for the data storage, compound semiconductor, and digital imaging markets.

Candela CS20

P-16 and P-16OF

Contact Stylus Profilers

The P-16 and P-16OF (Open Frame) are new contact stylus profilers designed for automated step height, surface contour, and roughness measurements. Providing detailed 2D and 3D topography analysis for a variety of surfaces and materials, these programmable surface metrology tools are utilized in a wide range of applications and industries. The profilers’ new sequencing feature, included as a standard configuration, is key to capturing the needs of those customers who want the added convenience of automated wafer mapping.

With their user friendly and powerful Apex software, the P-16 and P-16OF support the creation of custom reports; apply a variety of ISO standard filtering methods and unique leveling techniques; set tolerance limits for statistical process control metrics; and calculate stress, bearing ratio, distance, volume, density, flatness, peak count distribution as well as an extended list of parameters for step height, roughness and waviness measurements. Apex software is available in several languages. Common applications include CMP monitoring and bump metrology (semiconductor), HDD head and disk tribology characterization (data storage), etch rates and film stress analysis (MEMS), general thin films and chemical coating analysis as well as surface characterization of electronic components, opto electronics, flat panels, and biomedical devices.

P-16

P-16OF

78

Spring 2006

Yield Management Solutions

MRW3 Quasi-static Tester

Measurement System for MRAM and HDD Industries

The MRW3 quasi-static wafer tester measures the magneto-resistive characteristics of MRAM (magneto-resistive random access memory), magnetic recording heads, and other magneto-resistive devices. The MRW3 is used to test the devices while they are still in wafer form â&#x20AC;&#x201D; prior to the wafer being cut into individual magneto-resistive devices or chips.

The MRW3, configurable for both 200 mm and 300 mm wafers, delivers increased productivity compared with other quasi-static wafer testers, with industry-leading repeatability (<0.5 Oe) and resistance (1000Kohms). The tester speeds MRAM stack development and tightens the process control loop by enabling access to MRAM bitcell read/write performance without the need for CMOS integration. The MRW3 applies external magnetic fields in the plane of the wafer, and measures device or thin film electrical response. It obtains wafer-level transfer and enables early access to patterned Magnetic Tunnel Junction (MTJ) performance for control of magnetic processes. By reducing set-up time, the MRW3 system also offers dramatic improvements in the software. The system integrates with standard semiconductor probers, and is Class 10 compatible. These features, combined with its superior magneto resistive characterization and analysis accuracy, make the system ideal for both inline monitoring and engineering analysis.

MRW3 Quasi-static Tester

KT Analyzer

Parametric Analysis Solution

KT Analyzer is KLA-Tencorâ&#x20AC;&#x2122;s new family of parametric analysis tools that help chip manufacturers make faster and more accurate decisions to achieve a high yielding, reliable patterning process. The solution combines automated analysis capabilities embedded on KLA-Tencorâ&#x20AC;&#x2122;s latest-generation overlay, optical CD, and CD SEM metrology systems with off-line engineering analysis. KT Analyzer provides the enhanced information needed to achieve real-time process control, including lot and tool disposition, run time and preventative maintenance feed-back and feed-forward, root-cause analysis, and automated fault detection.

KT Analyzer

Spring 2006

www.kla-tencor.com/magazine

79

➤ 0MXLS 'LEPPIRKI

¨- LEH RS MHIE EX [LEX TSMRX TVSKVIWWMZI QEWO HIJIGXW [SYPH EJJIGX Q] ]MIPH 2S[ - ORS[ LS[

( %8%

- 2 * 3 6 1 %8 - 3 2

/ 2 3; 0 ) ( + )

()'-7-327

XS ´RH SYX ©

➤ 0MXLS 7SPYXMSR 6IXMGPI 5YEPMX] %WWYVERGI *MRHMRK TVSKVIWWMZI HIJIGXW ERH EWWIWWMRK [LIR XLI]«PP TVMRX MW E GLEPPIRKI JSV QER] JEFW XSHE] =IX PMXLS IRKMRIIVW LEZI PIEVRIH XLEX [MXL XLI VMKLX VIXMGPI VI UYEPM´GEXMSR WXVEXIK] XLI] GER HIXIGX ERH GPEWWMJ] ]MIPH VIPIZERX TVSKVIWWMZI HIJIGXW MR XMQI &IJSVI XLI] TVMRX &IJSVI XLI] LYVX ]MIPH %RH FIJSVI XLI] MQTEGX HIZMGI VIPMEFMPMX] =SY XSS GER QEOI VETMH ORS[PIHKIEFPI HIGMWMSRW EFSYX ]SYV VIXMGPI UYEPMX] 8SKIXLIV [I GER EGLMIZI E LMKL ]MIPHMRK VIPMEFPI TEXXIVRMRK TVSGIWW

8IVE7GER 78%6PMKLX § ,MKL WIRWMXMZMX] VIXMGPI MRWTIGXMSR XS RQ 8IVE7XEV § ,MKL XLVSYKLTYX VIXMGPI MRWTIGXMSR XS RQ (IWMKR7GER §0MXLS E[EVI HIWMKR MRWTIGXMSR WSPYXMSR %TTPMGEXMSRW I\TIVXMWI =MIPH GSRWYPXMRK *SV QSVI VIXMGPI UYEPMX] EWWYVERGI WXVEXIKMIW ZMWMX 81

81

81

81

[ [ [ O P E X I R G S V G S Q P M X L S =SYV SRPMRI PMXLSKVETL] VIWSYVGI GIRXIV

/0% 8IRGSV 'SVTSVEXMSR