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26 May 1995 Detailed study of a phase-shift focus monitor
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The phase shift focus monitor technique is based on the fact that phase shift structures which utilize a non-180 degree(s) shift exhibit asymmetric imaging which is proportional to defocus. This asymmetry results in a lateral displacement of the printed features, which can be measured quickly at a number of locations across the field and across the wafer with a registration measurement tool. The monitor provides a means of evaluating focus effects such as lens tilt, chip leveling, astigmatism, and field and wafer flatness. This paper presents a detailed investigation of the focus monitor. Experiments were performed to evaluate the effects of linewidth and exposure dose on the sensitivity of the monitor. An alternative measurement structure was also evaluated. Monitor repeatability was assessed, indicating that wafer-to-wafer variation is the largest source of repeatability error. Unexpected variation in sensitivity and calibration curve offset across the field was observed. These effects make interpretation of the focus data problematic. Potential factors which might contribute to the variability of the monitor were analyzed. Partial coherence uniformity appears to have a significant impact on the monitor, but does not entirely explain the observed effects. Potential use of the monitor as a coherence mapping tool is proposed.
© (1995) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Graham M. Pugh, Brian R. DeWitt, Craig B. Sager, and Patrick Reynolds "Detailed study of a phase-shift focus monitor", Proc. SPIE 2440, Optical/Laser Microlithography VIII, (26 May 1995);

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