Magazine spring06 focusing drifts

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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

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(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.

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Monitoring ADI and ACI processes

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

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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),

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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.

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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

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

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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.

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

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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.

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APC control with SCD

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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

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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

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Comparing SCD and CD SEM performance

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

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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.

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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.

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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)

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