There are times when processes drift, and the best course of action is to make an adjustment. This situation occurs when overlay correctable parameters or focus offsets are changed. Every lithography operation should have controls to detect process drift. As discussed in Chapter 1, the Â±3Ï rule is not particularly sensitive to shifts in the process mean, compared to other Western Electric Rules. Consequently, it is advisable to include at least one additional Western Electric Rule to supplement the Â±3Ï rule to increase sensitivity to process drift. An alternative to the Shewhart control chart, with greater sensitivity to shifts in the process mean, is the exponentially-weighted moving average (EWMA) chart, discussed in the next section.
Online access to SPIE eBooks is limited to subscribing institutions.