Summer00 matching automated

Lithography S

P

E

C

I

A

L

F

O

C

U

S

Matching Automated CD SEMs in Multiple Manufacturing Environments

by John Allgair and Dustin Ruehle, Motorola, John Miller and Richard Elliott, KLA-Tencor Corporation

As critical dimension (CD) design rules for semiconductor manufacturing become increasingly stringent, manufacturers of automated CD SEMs are developing systems with improved linewidth measurement repeatability and reproducibility 1. The ultimate technical performance of CD SEMs, however, is very much dependent on consistent and tight operational controls.

This is especially true in multiple tool manufacturing environments where system matching is required to preserve proper operation.

The matching and repeatability of CD SEMs can be evaluated using a standard daily monitor wafer that tracks the major system components that impact performance.

By using a method of statistical analysis on the data, matching can be verified immediately. This control procedure tracks tool stability, provides a common CD SEM length reference, and enables the seamless use of multiple CD SEMs within a single manufacturing environment or between separate manufacturing environments, without significantly increasing tool qualification time.

Shrinking linewidths and manufacturing challenges

The latest production devices have critical dimensions well below 0.25 Âľm, and future generations are targeted to have transistor gate structures at or below 100 nm. The value of tight dimensional control at the gate level is well understood, with the dollar value estimated to be as much as $7.50 in average selling price (ASP) per nanometer of difference in gate CD2.

An automated CD SEM can demonstrate sufficient repeatability for effective process control of these leading edge technologies3.

50

Spring 2000 Yield Management Solutions

Equally important to process control is tracking basic tool performance once it is integrated into a production line, which ensures that the expected precision is in fact realized from the metrology tool on a daily basis4.

Even when the performance of an individual tool is verified, it is also necessary to ensure that multiple metrology tools in the production line will deliver the same results.

Matching multiple CD SEMs in one or more manufacturing locations becomes even more important in large manufacturing facilities. Many fabs have numerous CD SEMs that are operated by separate groups within the facility, and production lots can be directed to various areas with available SEM capacity. Additionally, process development facilities must transfer new devices and processes to production environments, which requires the measurement of established devices on different CD SEMs. In both cases, there is a strong requirement for all CD SEMs to consistently match to within a predetermined specification.

Method: Setting up the match study

Once basic tool performance has been established in the manufacturing facility, effective matching programs must be relatively simple and not require specific personnel. Realistic verification techniques require minimizing time and effort, whether on start up, integrating new production layers, technology families or new CD

S

SEMs; or when re-qualifying systems after significant maintenance activity. Fortunately, a short daily qualification procedure that monitors CD stability on a single wafer is sufficient to ensure matching across all tools.

First, it is necessary to verify that each individual system is operating to specification, and that the resolution of each SEM is within specification and the imaging of each system is comparable. Next, a common set of wafers to which all of the SEMs can be calibrated is needed.

In the following study, the standards were based on the pitch of a nested line structure on a series of etched poly wafers. Using the average pitch of a series of sites from these wafers is a practical, easy-to-implement calibration option in the absence of a traceable length standard for SEMs. Wafer-to-wafer variation in the pitch structure used for calibration was measured to be less than 1 nm. Comparison of these standard wafers to an early version of a proposed NIST CD SEM pitch standard showed the absolute calibration to be off by less than 1 percent.

Using an etched wafer for calibration also has the advantage that closely matched wafer standards can be kept in close proximity to each SEM to simplify daily tool qualification and stability monitoring. The wafers can be easily transferred between fabs, serving as a portable length standard to further check CD matching between facilities. The wafers used in this study should have a useful lifetime of 40 months if care is taken to rotate measurement sites to avoid CD growth due to repeated measurements5. The six CD SEMs in this study were located in four physically distinct bays in two different manufacturing facilities. As a result, four separate calibration/tool qualification wafers were used for the calibration of the tools.

Measurements were taken on two KLA-Tencor 8100XP CD SEMs and four KLA-Tencor 8100 CD SEMs. To ensure operational consistency, all layers used in this study were measured with a single beam setting of 600eV landing energy, with a consistent beam current setting for each of the six tools.

P

E

C

I

A

F

L

O

C

U

S

The eight layers used in this study are specified in the following table: Type

CD(nm)

Feature Type

Etched Oxide

335

Dense Line

Resist on Nitride

325

Dense Line

Etched Nitride

380

Dense Line

Resist on Poly

<250

Dense Line

Etched Poly

<250

Dense Line

Resist on Metal

475

Dense Line

Etched Metal

475

Dense Line

Etched Oxide

280

Contact

All wafers were measured once on each tool, with the mean value calculated from nine sites measured on each wafer. In addition to these eight layers, a resist contact layer with nominal features of 330 nm in diameter was analyzed to evaluate contact hole imaging capabilities.

Performing the matching test

Previous matching analysis methods have compared measurements collected over an extended period of time. Typically one or two wafers are used. The wafers are measured on each system every day for a period of days, and the differences between wafer means for each day are compared to estimate system matching.6,7 The time and effort required to gather several days worth of measurements from each SEM using an extensive sampling plan is often prohibitive in a manufacturing environment. In addition, when evaluating matching between facilities, the technique becomes completely impractical.

A more practical strategy for estimating system matching is to apply past knowledge of within-system performance to the matching analysis. This can alleviate the need to run tests on multiple tools over multiple days.

The wafers used in the study were taken from a production device and represent a variety of layers, including resist and etched features. Matching performance was evaluated on eight layers including one of the calibration wafers.

Summer 2000

Yield Management Solutions

51

S

P

E

C

I

A

F

L

O

C

U

S

In order to test for a statistically significant difference between two populations of data at the level of the matching specification, we need estimates of the major sources of CD variation in the tools. The total measurement variation can be written as: σT2 = σM2 + σL2 + σD2

measuring the mean of the measurements on a particular wafer, the dynamic component of the variance of a wafer mean is reduced by a factor of 1/√N when N points are averaged to obtain a wafer mean. The subsequent estimate for the variance in the estimate of the wafer mean, µ, measured on a single system is: σµ2 = σD2/√N + σL2.

where, σM2 = the variation due to matching between systems σL2 = the variation within a single system over time (long-term component) σD2 = the variation within a single system between multiple measurements and wafer loading (dynamic precision) The combination of terms σM2 and σL2 correspond to “reproducibility” while dynamic precision (σD2) corresponds to “repeatability”8.

Within a single system, only the long-term and dynamic components are relevant. For this study, a standard tool acceptance test was used to quantify the dynamic precision of a single measurement on the system for each particular layer. To measure σD , a single job was run several times in succession with the wafer fully unloaded from the system between each run. When 1.96σ L

1.96σ L

SEM A

SEM B

α/2 = 0.025

95% confidence interval for ∆

With sufficient sampling per wafer (30 points per wafer for example), the contribution of the dynamic precision term becomes negligible and we can focus on the long-term variation as the major source of uncertainty in the wafer mean (Figure 1).

The uncertainty due to long term variation cannot, however, be reduced by increased sampling on a single run or by multiple runs in a short period of time.

Multiple measurements are required over an extended period of time. These measurements can be performed on the wafers to be matched or, alternatively, on a separate wafer used to track the CD stability of the tool.

The second approach has the advantage that the long term variation for each system can be monitored as part of the daily tool qualification. This is the approach adopted in this study.

Because many points are sampled on a wafer, the variation in the estimation of a particular wafer mean should be roughly equal to the long-term precision specification of a single system. For example, for the 8100XP system, the long term specification is 5 nm 3σ, or about 1.7 nm 1σ. Analysis of data obtained from the daily qualification of the SEMs verifies that the systems do, indeed, operate within this long term specification.

With this understanding of the variance in the mean for a single system, the most common test used for the comparison of two populations were examined, the student’s t-test9. For two systems, we wish to test the hypothesis that the measurements do not differ by more than the matching specification. (The null hypothesis is that the means do not differ by the predetermined amount.) For this example, we will use the matching specification for the 8100XP of 5 nm, mean to mean (∆M).

The test statistic for significant differences between wafer means measured on two different systems, ∆µ is thus: ∆µ > σL*t + ∆M

Figure 1. Typical distribution of CD measurements due to long-term system variation of two SEMs. The means of these two distributions can be considered significantly different when the overlap between the distributions is smaller than a predetermined value. The width of each distribution is determined by σL.

52

Summer 2000

Yield Management Solutions

or ∆µ > 1.7 * 1.96 + 5.0

S

P

E

C

I

A

F

L

O

C

U

S

370 365 360

30 20

355 350 345

10 340

0

1

2

3

4

5

6

7nm

335

Figure 2. Histogram of tool-to-tool wafer mean differences, all layers, all possible pairs. Differences are in nm. Total number of pairs for all 8 layers = 120. The number of differences for each layer is given by

nm 1

2

3

4

5

6

SEM #

N(N-1)/2 where N is the number of tools.

Figure 3. Average diameter by SEM for resist contact layer. Middle of

using the t-value appropriate for a 95 percent confidence interval10. Therefore differences in wafers means ∆µ that satisfy

diamond indicates wafer mean, points indicate individual measurements. A clear difference is evident between SEMs #4 and #5 (8100XP), and SEMs #1, 2, 3 and 6 (8100).

∆µ > 8.3 nm can be considered significantly different at the level of the 5 nm matching specification.

It is important to consider the application of this test value for the wafer means. Each individual pair of systems is evaluated at each layer to this maximum measured matching value. This ensures that no two systems differ with statistical significance by more than 5 nm.

Understanding the results

Figure 2 depicts the distribution of the pair-wise differences between the wafer means measured on six SEMs for all eight layers. The graph shows that all layers measured pass the specification of 5 nm with a significance limit of 8.3 nm. The largest difference between SEM#

3

6

2

5

4

1

3

0

1.5

1.9

2.8

3.7

3.9

0

0.4

1.3

2.2

2.4

0

0.9

1.8

2.0

0

0.9

1.1

0

0.2

6 2 5 4 1

0

Table 1. Pair wise SEM differences (nm) for resist on poly layer.

any pair of SEMs on any layer was less than 7 nm. The average difference was ( 3 nm for all layers individually (Table 1).

Figure 3 shows the results of measurements on a resist contact layer with nominal features of 330 nm in diameter. With this layer, a distinct difference between the two populations of SEM was identified. Within the subgroup of the four KLA-Tencor 8100 systems, the maximum delta between any two systems was 3.5 nm.

Between the two 8100XP systems, the delta was 1.1 nm. The difference between the two subgroups was 8 nm. This difference is attributable to the improved contact hole imaging of the 8100XP. To determine if the use of an etched wafer to establish CD SEM matching has an impact on the matching of resist features, we looked at the correlation between the average pitch value for the dense line structures measured on the two critical layers in this study and the average feature size for these wafers by tool. This was compared to the correlation between the average pitch value on a calibration wafer measured on the tools. In both cases, for the etched polysilicon wafer and the resist on polysilicon layer, excellent correlation (>90 percent) was found between the pitch value of the product wafer and the pitch value measured on the calibration wafer. This is a clear indication that differences in the wafer means for the two critical layers are not dominated by the choice of the calibration standard.

Summer 2000

Yield Management Solutions

53

S

P

E

C

I

A

L

F

O

C

U

S

Summary

References

It is possible to match multiple SEMs in more than one manufacturing environment using a straightforward, simple daily qualification procedure and a calibration standard based on the pitch of a nested line structure. Using a simple statistical method to test the matching between all pairs of tools allows matching compliance to be established with a minimum amount of time and effort. In addition, a wide variety of production layers can be measured on CD SEMs without the need to distinguish between the tools on which the measurements were made.

1. J. Allgair, et. al. “Towards a Unified Advanced CD SEM Specification for Sub-0.18 um Technology,” in Proceedings SPIE 3332, 1998, pp.138-149.

2. J. Sturtevant, et. al. “Implementation of a closed loop automatic CD and overlay controller for sub 0.25 micron patterning,” in Proceedings SPIE 3332, 1998.

3. K. Monahan, et. al. “Subnanometer-precision metrology for 100-nm gate linewidth control,” in Proceedings SPIE 3332, 1998, pp.110-123.

4. J. Allgair, et. al. “SPC Tracking and Run Monitoring of a CD SEM,” in Proceedings SPIE 3332, 1998, pp.243-251.

5. Ibid.

6. R.R. Bowley, et. al. “Matching analysis on seven manufacturing CD SEMs,” in Proceedings SPIE 3331, 1998, pp.94-99.

7. D. Erickson, et. al. “Statistical verification of multiple CD SEM matching,” in Proc. SPIE 3050, 1998, pp.93-100.

8. J. Allgair, et. al. “Towards a Unified Advanced CD SEM Specification for Sub-0.18 um Technology,” in Proceedings SPIE 3332, 1998, pp.138-149.

9. Box, Hunter & Hunter, “Statistics for Experimenters”, Wiley, 1978, p111.

10. Ibid., p630.

All of these results demonstrate that it is possible to ensure CD SEM system matching within a fab and between fabs by employing a simple daily method. The process control gained offers clear benefits to ensure the reliable, repeatable performance of automated CD SEMs in a production environment.

circle RS#009

Mark Your Calendar for. . .

KLA-Tencor’s 7th Annual Data Storage Technical Information Session and Reception Wednesday, September 20th, 2000 6:30 p.m. — 10:00 p.m.

San Jose Hilton and Towers

Limited seating available.

To reserve your space today, please contact Tavis Szeto at tavis.szeto@kla-tencor.com