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Proceedings Paper

Simplified cusum model for automated control of fab processes
Author(s): John Todd Downing; Tracey Sorenson
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Paper Abstract

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.

Paper Details

Date Published: 16 July 2002
PDF: 7 pages
Proc. SPIE 4689, Metrology, Inspection, and Process Control for Microlithography XVI, (16 July 2002); doi: 10.1117/12.473522
Show Author Affiliations
John Todd Downing, Micron Technology, Inc. (United States)
Tracey Sorenson, Micron Technology, Inc. (United States)

Published in SPIE Proceedings Vol. 4689:
Metrology, Inspection, and Process Control for Microlithography XVI
Daniel J. C. Herr, Editor(s)

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