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16 July 2002 Simplified cusum model for automated control of fab processes
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Advanced process control can be achieved using basic math to create a simplified cumulative sum (cusum) chart. The method is simple: plot the difference between the measured value and the desired target (error value). Each new data point is summed with the last data point. A process that is only slightly off target will be evidenced by a gradual trend towards the control limits. The farther from target the process is the steeper the slope will be, and the sooner it will breach the control limits. Subtracting a noise threshold value from the error value can control the sensitivity of the cusum chart. For example, if the threshold is 5 and the measured value is 8, the plotted value would be 3. Measured values of less than the threshold considered on-target and plotted as zero. Substantial improvements in process capability were realized using an APC system built on this simple cusum model.
© (2002) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
John Todd Downing and Tracey Sorenson "Simplified cusum model for automated control of fab processes", Proc. SPIE 4689, Metrology, Inspection, and Process Control for Microlithography XVI, (16 July 2002);

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