Simplified Cusum Model for Automated Control of Fab Processes

摘要

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.