832437

Photoresist Qualification using Scatterometry CD Roie Volkovich*a, Yosef Avrahamova, Guy Cohena, Patricia Fallonb, Wenyan Yinb, a

KLA-Tencor Corporation Israel, Halavian St., P.O.Box 143, Migdal Haemek 23100, Israel.

b

The Dow Chemical Company, Dow Electronic Materials, 455 Forest St., Marlborough, MA 01752, USA.

ABSTRACT As the semiconductor industry advances to smaller design rules, Photoresist performance is critical for the tight lithography process. Critical Dimension (CD), Side Wall Angle (SWA) and Photoresist height, which are critical for the final semiconductor patterning, depend on the Photoresist chemistry. Each Photoresist batch has to be qualified to verify that it can achieve the required quality specifications.

Photoresist qualification is done by exposing Photoresist and monitoring outcome after developing.

In this work, Archer 300LCM scatterometry-based Optical CD (OCD) was evaluated using Dow 193 Immersion Top Coat Free Photoresist and Anti Reflection Layers (ARL). As part of the sensitivity analysis, changes in Photoresist thickness, ARL thickness and Photoresist formulation were evaluated.

Results were compared to CD-SEM measurements. The CD sensitivity was evaluated on two grating dense line and space features with nominal Middle CD (MCD) values of 37nm and 75nm. Sensitivity of the OCD for Photoresist parameters was demonstrated.

Keywords: CD measurements, Photoresist, Scatterometry, resist qualification, pitch, Reflectometry, wavelength metrology. *

Electronic mail: Roie.Volkovich@KLA-Tencor.com Metrology, Inspection, and Process Control for Microlithography XXVI, edited by Alexander Starikov, Proc. of SPIE Vol. 8324, 832437 路 漏 2012 SPIE 路 CCC code: 0277-786X/12/$18 路 doi: 10.1117/12.918392

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1.

Introduction

For several years, OCD measurements through reflectometry technique contributed in an effective way to increase the degree of knowledge of wafer process outputs by giving the user valuable information about key pattern profile parameters that previously required costly and time consuming cross sections for a proper evaluation1. Process control became more efficient and faster given the measurement time of an optical CD system. Taking advantage of the excellent repeatability and the high measurement throughput, ideal for high-sampling applications, this technique has been largely adopted in photolithography manufacturing area2-6.

This paper further explores OCD’s application into the resist qualification process. During the Photoresist qualification process, wafers are patterned through a litho cluster using defined process conditions and CD is currently monitored primarily using CD-SEM7-8. CD is an important monitored characteristic for resist manufacturing and the precision and accuracy of the CD measurement is critical. In this study, we demonstrated Dow Photoresist CD qualification with Archer 300LCM and compared it to CD-SEM. The analysis includes MCD uniformity along with tolerance and sensitivity to changes in the wafer process and resist formulation. In section 2, the measurements technique is described. Section 3 specifies the results and discussion, and the conclusions are given in section 4.

2.

Measurement Technique

2.1 The stack process condition and the proposed model The process conditions for the examined stack are detailed in Table 1. The Photoresist (PR) was coated over two Anti Reflection Layers (ARL) for two feature size: 37nm Middle Critical Dimension (MCD) and 75nm MCD where the line space ratio was 1:1. A die size of 85x85μm was used for this study.

Stack ARL 1 ARL 2 Photo Resist

80 nm Dow AR™40A Antireflectant 40 nm Dow AR™104A Antireflectant 85 nm

Target Feature Target Feature

37 nm L:S 1:1 75 nm L:S 1:1

Table 1: Process conditions for the examined stack.

A model was generated for the scatterometry measurements, as illustrated in Fig. 1 (a).

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(a)

(b)

Fig.1: (a) Schematic illustration of the examined model, (b) Cross section image of Dow PR.

In this study, process changes and formulation changes were made to the stack and resist for the purpose of studying the robustness of the OCD measurement as the actual measured parameters move away from the optimized model.

Formulation was altered by changing polymer ratio to affect optical properties of material and to move CD away from target to analyze sensitivity of CD variation with OCD. The film stack was modified to test for the measurement’s response to changes in ARL and resist thickness.

2.2 OCD measurement method The OCD measurements were carried out using KLA-Tencor’s Archer 300 LCM. This tool is a Spectroscopic Reflectometer based scatterometry system. The operating wavelength range is 240-750nm. 2-Dimensional profiles of targets are used to generate the library. Two libraries were generated, for the two different MCD bases. Optical spectra were collected on target 50 μm x 50 μm. The scatterometry libraries required to perform the measurements were generated using KLA-Tencor Acushape™ software versions 2.0. The input parameters for library generation are the optical dispersion properties of resist, ARL and underlying film stack; the grating pitch, Photoresist height and SWA, and the range of possible MCD values. The optical dispersion properties of the materials were modeled from blanket wafers made previous to the grating measurements.

For generating the library spectra, each line profile that could potentially be obtained through the technology process on the wafer was simulated using Rigorous Coupled Wave Analysis9. The associated optical signals are stored in a library. The library can be optimized so as to be scanned with a full set of parameters, as efficiently as possible. The

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optimization reduces convergence errors and enhances the accuracy of the measurement. A schematic illustration of the measurement procedure is illustrated in Fig. 2.

Fig. 2: Schematic illustration of OCD measurement procedure.

3.

Results and Discussion

Archer 300LCM was evaluated in 4 parameters; Bossung curve for the MCD, sensitivity to changes in Photoresist composition, across wafer to wafer uniformity in MCD, and sensitivity to ARL and PR thickness changes. In the next sub-sections, we discussed each evaluated parameter.

3.1 Bossung curves 37nm Focus-exposure matrix (FEM) wafers were generated with POR resist and a variation of the formulation by polymer ratio change. The wafers were evaluated using the OCD tool. KLA-Tencor’s KT Analyzer™ software versions 7.2 generated Bossung curves to assess CD sensitivity to focus and exposure variations. The result plots (Fig. 3 and 4) demonstrated that OCD and CD-SEM give similar process window.

This measurement was done to characterize the robustness of the OCD model to variations in optical properties of the resist as conditions moved from best focus and energy. The Bossung curves generated by OCD measurements (Fig. 3 (a), and Fig.4 (b)) demonstrated better agreement to the model than the CD-SEM Bossung curves due to less measurements noise.

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(a)

(b)

Fig.3 : Bossung curves (MCD as function of focus for different exposure values) for 37nm feature size. POR resist measured using (a) CD-SEM and (b) OCD .

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(a)

(b)

Fig.4 : Bossung curves (MCD as function of focus for different exposure values) for 37nm feature size. Resist with formulation changes measured using (a) CD-SEM and (b) OCD.

3.2 Sensitivity to resist composition For the PR composition evaluation, the PR formulation was altered to move the MCD away from target. The average MCD of the two wafers was plotted to compare the sensitivity of OCD and CD-SEM to MCD changes.

Fig. 5 (a) demonstrates that both tools have the same sensitivity to MCD changes with a slope of 1.2.

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(a)

A

(b)

B

C

A

B 2SWbIE

C

Fig.5: 37nm MCD feature size (a) and 75nm MCD feature size (b) are plotted vs. different sample composition, comparing OCD and CD-SEM.

Similar sensitivity was obtained when the three samples were measured at the larger feature size of 75nm for OCD and CD-SEM.

3.3 MCD variation across wafer The variation across wafer of MCD on POR wafers for 37nm feature size is demonstrated in Fig. 6. The variation (difference between minimum and maximum values) was around 1nm for the CD-SEM and OCD measurements. An expected radial signature with center to edge behavior was obtained.

(a)

(b)

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Fig.6: Contour map for MCD values in 37 nm features size, measured with CD-SEM (a) and OCD (b). The values in the contour bar are in nm.

The standard deviation within wafer was compared for all wafers for the two measurement types. The pooled standard deviation for all 37nm wafers measured using OCD was 0.26nm and 0.64nm for CD-SEM. The standard deviation is smaller for the OCD measurement and can be related to a larger inspection area of the OCD tool vs.

CD-SEM.

3.4 Sensitivity to changes in ARL and PR thickness For the sensitivity analysis of changes in ARL and PR thickness, 37nm lines were patterned on wafers where variation was made to the POR stack. The ARL thickness was moved 2.5nm from target and the PR thickness was shifted 5nm. For each type process condition, two wafers were measured. For the samples where ARL thickness and PR thickness were varied, the exposure conditions for the two wafers were also varied. OCD’s ability to detect CD changes between the 2 wafers was compared to the CD-SEM to determine OCD’s sensitivity to MCD changes as the stack model moves away from the POR’s stack model. Table 2 shows that OCD’s sensitivity to the effect of exposure conditions when stack is changed is comparable to that of the CD-SEM.

Stack Change

Measurement Tool OCD

ARL Thickness CD-SEM

OCD Resist Thickness CD-SEM

Exposure Setting

CD (nm)

1

33.5

2

38.7

1

37.2

2

42.4

1

27.4

2

31.8

1

32.9

2

37.7

Wafer to Wafer Measured Change (nm) 5.3 5.3

4.4 4.8

Table 2: Summary of MCD measurements comparing OCD to CD-SEM with stack changes. The same trend is obtained across the two different measurements process.

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The same trend was obtained between the OCD and CD-SEM measurements, illustrating good sensitivity by OCD to the stack changes.

4. Conclusions In this study, we demonstrated a Dow resist qualification with Archer 300LCM. Two feature sizes were examined, 37nm and 75nm MCD. The MCD uniformity across wafers with different PR composition, or changes in PR and ARL thickness, were examined. The MCD uniformity measured by OCD demonstrated uniformity below 1nm, and has shown less measurement noise then CD-SEM. The OCD measurements demonstrated sensitivity to MCD changes comparable to CD-SEM measurement. This was also seen when stack and formulation were moved away from the designed model, showing robustness and good MCD sensitivity for the OCD model.

Bossung curves were generated for the FEM 37nm wafers, and showed focus and exposure control analysis comparable to that seen with CD-SEM.

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SPIE 6617, 661716-1 (2007).

[3] Gros H, Model R, Bar M, Wurm M, Bodermann B and Rathsfeld A, Mathematical modelling of indirect measurements in scatterometry, Measurement 39, 782-794 (2006).

[4] Gros H, Rathsfeld A, Scholze F, Bar M and Dersch U, Optimal sets of measurement data for profile reconstruction in scatterometry, Proc. SPIE 6617, 66171B-1− 66171B-12 (2007).

[5] Gros H, and Rathsfeld A, Sensitivity analysis for indirect measurement in scatterometry and the reconstruction of periodic grating structures, Waves in Random and Complex Media, 18:1, 129-149 (2008).

[6] Pomplun J, Burger S, Schmidt F, Zschiedrich L, Scholze F, Laubis C and Dersch U, Rigorous FEM simulation of EUV masks: influence of shape and material parameters, Proc. SPIE 6349, 63493D (2006).

[7] Dersch U, Korn A, Engelmann C, Frase C. G, Haessler-Grohne W, Bosse H, Letzkus F, and Butschke J, Impact of EUV mask pattern profile shape on CD measured by CD-SEM, Proc. SPIE 5752 , 632-645 (2005).

[8] Cho S, Yedur S, Kwon M and Tabet M, CD and profile metrology of EUV masks using scatterometry based optical digital profilometry, Proc. SPIE 6349, 63492I (2006).

[9] M. G. Moharam, Eric B. Grann, and Drew A. Pommet, T. K. Gaylord, Formulation for stable and efficient implementation of the rigorous coupled-wave analysis of binary gratings, J. Opt. Soc. Am. A,Vol. 12, No. 5, 1068 (1995).

™ AR is a trademark of The Dow Chemical Company (“Dow”)

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