ABSTRACT
Nested variation sources are often characteristic of batch processes. Levinson (1994) describes a process that coats semiconductor wafers with a photoresist whose thickness is the quality characteristic. Four or five measurements from a single wafer do not constitute a rational subgroup because they do not account for variation between wafers. The reference also cites a metallization chamber that processes more than a dozen silicon wafers at a time. A sample of four wafers from a batch is not a rational subgroup because it does not account for the variation that comes from the slightly different processing conditions for each batch. Figure 4.1 illustrates the different variation sources for an individual chip on a silicon wafer.
