SHEWMAC: an end-of-line SPC scheme via exponentially weighted moving statistics

Due to the multiple-stream and sequence-disorder effects, a 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 WAT data trend detection without resorting to intensive computing power, an end-of-line SHEWMAC scheme was proposed by Fan et al (Proc. Int. Symp. Semicond. Manuf., pp. 233-236, 1999). The SHEWMAC scheme consists of a Shewhart, an exponentially weighted moving average (EWMA), and exponentially weighted moving Cpk (EWMC) charts for jointly monitoring the mean and variance of WAT lot average sequence. This paper aims at the robust design of a SHEWMAC scheme and the analysis of its effectiveness. Simulation results show that the SHEWMAC scheme with robust parameters reduces the time for detecting WAT trends by about 15% as compared to either the exponentially weighted mean square (EWMS) or combined Shewhart-EWMA schemes generally used for trend detection. Field data validation also shows that the incorporation of SHEWMAC complements the existing end-of-line data monitoring system and in-line SPC schemes for process integration.