SHEWMAC: an end-of-line SPC scheme for joint monitoring of process mean and variance

Under the effects of multiple-stream and sequence-disorder, process change caused by one machine at an in-line step may result in changes in both the mean and variance of end-of-line wafer acceptance test (WAT) data sequence. To speed up trend detection of WAT data without resorting to an intensive computing power, an end-of line SHEWMAC scheme is proposed, which combines a Shewhart, an exponentially weighted moving average (EWMA), and an exponentially weighted moving Cpk (EWMC) charts for jointly monitoring the mean and variance of wafer lot average sequence from WAT data. In view of the wide ranges of process conditions and low volume of each product in a foundry fab, a data normalization technique is adopted to aggregate data of similar products and a new design method is developed to generate a robust set of scheme parameters. Simulation and field data validation show that SHEWMAC is superior to the combined Shewhart-EWMA scheme in shift detection speed and is complementary to the in-line SPC.