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19 July 1989 Optical Overlay Versus Electric Probe Measurement
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The predominant method used in the past for the measurement of overlay has been manual reading of the "optical vernier." This method can be reasonably precise and has been sufficient for most semiconductor products made up until a few short years ago. The ever-increasing number of masking levels below 1.5-micron Minimum Feature Size (MFS) requires large statistical bases of overlay measurements with quick turnaround. No longer are 4 to 10 sites per wafer sufficient to accurately judge overlay, nor can we afford to wait 20 minutes for an operator to manually read these verniers. For years, Perkin-Elmer has used a unique and proprietary electrical probe system custom-built by Perkin-Elmer prior to the introduction of the Micralign Model 500. Capable of gathering large amounts of data and performing statistical analysis, it became a standard for overlay evaluation within Perkin-Elmer. An alternative to electrical probe is automated optical measurement. One such system is the Perkin-Elmer OMSTM. This system has the advantage of being "non destructive" and can be used to measure actual product wafers in process. This paper will provide a performance comparison of both techniques, optical and electrical. Using a mask with both optical and electrical probe patterns, a series of wafers was exposed. The evaluation compares accuracy, precision, speed, and statistical capabilities.
© (1989) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Keith Y. Mortensen and Betty Ann Blachowicz "Optical Overlay Versus Electric Probe Measurement", Proc. SPIE 1087, Integrated Circuit Metrology, Inspection, and Process Control III, (19 July 1989);

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