Autumn98 p13

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Identifying Process Drift with CD SEMs Getting more than critical dimensions from SEM linescans and images with correlation scoring by David M. Goodstein, Applications Engineer

In situations where CD-only-based monitoring of process integrity proves inadequate, the CD SEM continues to provide essential monitoring capability through the use of linescan and image correlation metrics. This additional information is extracted from the same line scans and images acquired in the course of standard automated CD measurement. Consequently, there is almost no impact on throughput. The high sensitivity of these metrics to even small degrees of process variability suggests they will play an important role in all demanding CD SEM-based process monitoring and control applications. CD SEMs play an essential role in photolithography and etch process characterization and monitoring. Post-develop (DI) and post-etch (FI) monitoring of device critical dimensions (CD) as well as fast, thorough characterization of processes and process equipment are routinely handled by advanced high-throughput CD SEMs. Realtime process monitoring is especially important to catch yield-compromising process variations as soon as possible, before a significant and costly fraction of in-process wafers are affected. While CD is the standard metric on which CD SEM monitoring is based, additional and potentially more sensitive metrics are available from the very same tool. To see why more sensitive monitoring metrics might be necessary even at large design rules, consider the CD variation of an i-line resist isolated line on metal as a function of stepper defocus and exposure (figure 1). With a nominal CD of approximately 540 nm (at 0 Âľm defocus, 190 mJ exposure), a DI monitor that flagged resist-line CD deviations greater than five percent would still allow significant process variation (figure 2). Process drift of this magnitude might well impact post-etch metal line CD and overall line integrity, compromising device performance and total yield. This can be avoided

with tighter process monitoring at the post-develop stage, using non-CD-based metrics. Limitations of CD-only monitoring

It should not be surprising that not all process drift can be identified by changes in critical dimension. Interconnects, gates, contacts and vias are fundamentally three-dimensional structures, and characterizing them solely in terms of CD (which is itself a function of the measurement algorithm applied to the CD SEM image or linescan) is necessarily a simplification. If a device feature simply scales uniformly with process drift, then CD is an adequate metric. However, this is often not the case. The limitations of CD-only characterization and monitoring are evident in figure 3, where isolated end-of-line structures are imaged by a CD SEM at 75kX magnification. Measurement linescans, acquired near the middle of the line, are also shown. The resist line printed at 1.2 Âľm defocus and 200 mJ exposure (bottom image) measures at 535 nm, a CD deviation of less than two percent from nominal (543 nm). Such a line would pass most CD-only monitors, where process windows are typically 10 percent of nominal. However, when compared to the line printed at optimal focus and exposure (top image), the non-optimal line clearly suffers from reduced sidewall steepness (wider, but lower intensity edges) and variable resist thickness (bright fringes across the line). These changes can lead to an appreciably Autumn 1998

Yield Management Solutions

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