Lithography
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Spectroscopic Critical Dimension (SCD) Metrology for CD Control and Stepper Characterization Authors: John Allgair, Motorola, APRDL & Pedro Her rera, KLA-Tencor Corporation Co-authors: R. Hershey, L. Litt, D. Benoit; Motorola, APRDL, A. Levy, U. Whitney; KLA-Tencor Corporation
Smaller device dimensions and tighter process-control windows have created a need for CD metrology tools to detect and measure changes in feature profiles that are becoming critical to inline process control and stepper evaluation for sub-0.18 µm technology. Spectroscopic CD is an optical metrology technique that can address these needs. This work describes the use of a spectroscopic CD metrology tool to measure a sub-0.18 µm gate level focus and exposure matrix in order to characterize the lithography process window. The results include comparison to the established inline CD SEM, as well as profiles from a cross-section SEM. Repeatability, long-term stability, and matching data from a gate-level nominal process are also presented.
Introduction
Smaller device dimensions and tighter process control windows have created a need for metrology tools that measure more than just one-dimensional critical dimension (CD) features. The need to easily detect, identify, and measure changes in feature profiles is becoming critical to controlling current and future semiconductor lithography and etch processes. Measuring changes in sidewall angle and resist height, as well as detecting subtle phenomena such as line-rounding, t-topping, and resist footing, is now as important as the traditional CD line-width measurement. This additional profile information can be used to enhance process-control mechanisms and can also be used to evaluate and characterize the performance of a stepper/track module. Traditional CD metrology techniques give no indication of a measured feature’s sidewall angle or height. Spectroscopic CD is an optical metrology technique that can address these needs. SCD is based on spectroscopic ellipsometry (SE), an accepted and 50
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widely used optical technique for measuring film thickness and film properties. This work describes the SCD measurement technique and its use in measuring a sub-0.18 µm gate-level focus and exposure matrix to characterize the lithography process window. SCD results are compared to results from a CD SEM and a cross-section SEM to determine if SCD is able to measure accurately the feature behavior through changes in focus and exposure. Furthermore, SCD is used to monitor features outside the process window to determine if it can detect and identify out-of-control process conditions. Repeatability, long-term stability, and matching data from a gate-level nominal process are also presented. These repeatability and stability tests were performed to verify SCD meets the roadmap requirements for current and future semiconductor processes. SCD Measurement
The SCD measurement technique is summarized in Figures 1 and 2. Gratings on the production wafers
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F i g u r e 1. Schematic of SCD measure m e n t .
have polarized light directed onto them and the spectrum of the reflected light is recorded. The gratings are repeating line/space features of uniform period. The line size and period of the grating are designed to represent the in-die feature that is being controlled. A model of the grating geometry and underlying film stack is created and incorporates such parameters as the grating height, the line width, the sidewall angle, and the film properties (thickness, n, and k). Varying the grating parameters and calculating theoretical spectra construct the library. This library is linked to the recipe on the metrology tool. As the wafer is measured the data are compared to the library. The best match between the measured spectra and the modeled spectra determines the parameter values that best describe the physical grating.
of polysilicon, and 30Å of oxide. One wafer type was printed as a focus-and-exposure matrix with focus varied from -0.35 µm to +0.05 µm (0.05 µm steps) and exposure varied from 18.6 mJ/cm2 to 21.8 mJ/cm2 (0.4 mJ/cm2 steps). The second wafer type was printed with nominal settings for focus and exposure. The gratings on these wafers are 50 µm x 50 µm in size and have a line/space ratio of 150/210 nm (360 nm pitch). Measurements from the focus and exposure matrix wafers were used to generate Bossung curves, determine the correlation between SCD and CD SEM
Procedure
Metrology Tools The metrology tools used in this work were a KLA-Tencor SpectraCD SCD measurement system, a KLA-Tencor 8100XP CD SEM, and a Leopold 982 Cross-Sectional SEM. SpectraCD allowed for full spectrum (240-780 nm wavelength) matching with its broadband light source. Spectra collected on the SpectraCD were matched in real-time to a library generated with KLA-Tencor’s Library Generation Service (LGS) modeling software. The 8100XP is a top-down (electron beam normal to the sample) CD SEM used for inline process control and engineering development. The CD SEM was calibrated to a known pitch standard and production recipes were used in automated mode for all the measurements described herein.
Wafers and Gratings Two types of wafers were used for this work. Both types had 3800Å tall resist lines over 200Å of ARC, 1500Å F i g u re 2. SCD measurement summar y.
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generation also incorporated such features as line rounding, t-topping, resist footing, and asymmetry between the left and right sidewalls. Figure 3 shows the range of profile shapes and illustrates how the line width (CD), height (HT), and sidewall angle (SWA) are defined. The shaded area between the inner and outer profiles represents the range of modeled profiles from the smallest, rounded profile to the largest, t-topped profile. Results
Analysis of a Focus-and-Exposure Matrix for Stepper Characterization
F i g u re 3. Libr ar y profil e ra nge wit h defi nition of CD, HT, and SWA .
measurements, and compare SCD to cross-section. The nominal wafers were measured to characterize the precision, stability, and matching performance of SCD.
SCD Library The library created to measure these wafers was developed to encompass the range of CD, height, and sidewall angle found in the focus and exposure matrix described above. CD was varied from 100-200 nm, sidewall angle from 81-93 degrees, and height from 200-400 nm. These are very large parameter ranges; a library for a normal, production layer would use much smaller ranges. The profile shapes used in library
F i g u re 4. CD-SEM B ossung cur v e s .
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Bossung Curves and Measurement Correlation: When measured on the CD SEM, the Bossung curves are as drawn in Figure 4. As shown, focus is centered near -0.15 Âľm. CD is stable through focus at the center exposures, but shows more variation at the minimum and maximum exposures. Overall, there is symmetry in the curves from left to right and top to bottom. Comparing these to the Bossung curves generated with SCD, see Figure 5, it is clear there is good agreement between the two measurement techniques at negative defocus. Both techniques show an upward trend at small exposure and a downward trend at large exposure as focus becomes more negative. However, the two techniques do not agree at positive defocus. In this region, the SCD measurements show a consistent downward trend. This is very noticeable on the 19.8 and 20.2 mJ/cm2 exposure trend lines where CD values are very low at positive defocus. The SCD Bossung curves do not demonstrate the same left to right and top to bottom symmetry found in the CD SEM Bossung curves.
F i g u re 5. SCD Bossung curv e s .
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F i g u re 8. Cross-sect ions at center and po sitive focus.
(positive defocus). The SEM measurements will key on the bright line edges and report similar measurements for both lines. The SCD measurements will detect the change in sidewall angle and give measurements to reflect the different midpoint CDs.
F i g u re 6. Sidewall a ngle a nd heigh t through focus and exposure .
Sidewall Angle and Height Measurements: The SCD measurements for sidewall angle and height explain why the Bossung curves do not agree. In Figure 6 the line height decreases with positive defocus; this is also the case for the sidewall angle. The line profile is shorter and getting wider at the bottom than at the top. Figure 7 shows the difference between a well-defined vertical line (center of focus) and a shorter rounded line
SCD Comparison to Cross-Section: Cross-sections taken on wafers with a larger focus range agree with the SCD height and sidewall angle measurements. As shown in Figure 8, the cross-section at center focus shows vertical edges and good uniformity from line to line. The cross-section at positive defocus shows the resist height loss and less vertical edges described in the SCD measurements. Furthermore, a comparison between SCD and cross-section reveals SCD gives accurate (referenced to cross-section) measurements of CD, height, and sidewall angle. Table 1 shows the SCD and cross-section results from the same grating. Given the amount of error in the cross-section (~2%), the two methods give identical CD measurements. SCD can, therefore, be used to accurately and non-destructively measure linewidth CD and characterize a feature’s profile real-time on product wafers. Fault Detection: At process extremes, the need to measure changes in CD or sidewall angle is replaced by the need to detect if the features are completely outside of the process window. The edge of the focus-and-exposure matrix yielded the opportunity to test SCD against out-of-control, beyond-process-window conditions. One such condition is when lines are over-exposed to the point where they are either faint smears or missing
CD (nm) Height (nm) Sidewall Angle (deg)
Cross Section (nm) SCD (nm) 117 113.7 344* 364 N/A 89.0
* Cros s-section height measurement taken at cent er, s horter l ine F i g u re 7. Grating line at cen ter an d positive focus (150kX images ).
Table 1. Cross-section a nd SCD measure m e n t s .
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Min Max Mean 3
Sidewall Angle (deg) Height (nm) 85.92 549.26 85.94 549.37 85.93 549.32 0.01 0.08
CD (nm) 217.92 218.00 217.96 0.06
Tabl e 2. Single site stat ic measurement pre c i s i o n . F i g u re 9. Cross-sect ion and SEM image (75kX) of over-exp osed, outof -focus lin es.
from the wafer altogether, (see Figure 9). The SCD model can be modified and expanded to account for this and other out-of-control conditions. Figure 10 shows modeled and measured spectra for two such cases—one where the lines are missing from the wafer, another where the lines were not developed or exposed and the grating is actually a pad of resist with no pattern.
Min Max Mean 3
Sidewall Angle (deg) Height (nm) 85.89 549.19 85.96 549.53 85.93 549.37 0.05 0.24
CD (nm) 217.69 218.11 217.80 0.27
Table 3. Single site dynamic measurement pre c i s i o n .
Measurement of a Nominal Gate Wafer Process for Precision, Stability, and Matching Estimates
recipe measuring five fields (top, center, bottom, left, and right) on the nominal wafer. Figure 11 shows the measurements at each site for 31 days.
SCD Measurement Precision: The SCD static precision and dynamic measurement precision (1 site) are shown in the Tables 2 and 3. The results are from 20 measurements taken on the same field, where static indicates the wafer was not unloaded and reloaded between each measurement. The sidewall angle, height, and CD show good repeatability for both tests. Measurements from 24 sites across the wafer support these single-site tests by giving an average CD dynamic repeatability of 0.28 nm (3σ).
Tool-to-Tool Matching: The nominal wafer was measured on four different SpectraCD systems to gauge the techniques-matching capability. Each tool took 15 measurements on the wafer. Results from those measurements are shown in Table 4. Each tool was within 0.5 nm of the total average.
SCD Measurement Stability: The SCD measurement stability over a month-long time period was 0.31 nm (3σ). This was measured by running a daily qualification
As shown, SCD technique based on spectroscopic ellipsometry is able to characterize a focus-and-exposure matrix and demonstrate excellent measurement precision.
Conclusion
F i g u r e 10. Modeled an d measured spectra for no exp osure/ devel op and over- e x p o s u re .
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System 1 2 3 4 Average Range
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CD (nm) 195.14 194.55 195.18 195.00 194.97 0.63
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Delta to Average (nm) -0.17 0.42 -0.22 -0.03
Table 4. SCD matchin g re s u l t s .
F i g u re 11. SCD l ine widt h m easurements over 31 d ays.
SCD shows good correlation to a top-down CD SEM and demonstrates good accuracy when compared to crosssection SEM. The SCD measurements of resist height and sidewall angle give a clear picture of how the grating target lines are affected by changes in focus and exposure. Furthermore, SCD is capable of detecting when a process is out of its control window and identifying the out-ofcontrol failure condition. The SCD technique, performed on the SpectraCD system, is therefore a metrology technique that can be used for real-time, inline control of an advanced semiconductor pattern transfer process.
References 1. J. Allgair, D. Benoit, R. Hershey, L. Litt, I. Abdulhalim, M. Faeyrman, J. Robinson, U. Whitney, Y. Xu, “Manufacturing Considerations for Implementation of Scattero m e t ry for Process Monitoring,” Proceedings of the SPIE, Vol. 3998, pp. 125-134, March 2000. 2. H. Tompkins, W. McGahan, Spectroscopic Ellipsometry and Reflectometry, John Wiley & Sons, 1999. A version of this article was originally published in SPIE Proceedings vol. 4344, paper 57, entitled “Implementation of spectroscopic critical dimension (SCD) for gate CD control and stepper characterization” by J.A. Allgair, D.C Benoit, R.R Hershey, L.C Litt (Motorola); B.Braymer, P.P Herrera, C.A Mack, J.C Robinson, U.K. Whitney, P. Zalicki (KLA-Tencor Corp.)