ACHIEVING UNIFORMITY IN A SEMICONDUCTOR FABRICATION PROCESS USING SPATIAL MODELING

Material is deposited onto the wafer surface during several steps of w afer fabrication. This material must be deposited evenly across the en tire wafer surface, close to the targeted thickness, and with little w afer-to-wafer variability. But unequal variances across the wafer and under different process conditions, as well as nonstationary correlati on across a wafer, make these goals difficult to achieve, because trad itional methods for optimizing deposition processes assume homogeneity and independence. We avoid these assumptions and determine the best s ettings of process variables using physically motivated statistical mo dels for the mean response. unequal variances, and nonstationary spati al correlation structure. Data from a rapid thermal chemical vapor dep osition process is used to illustrate the approach. A simulation exerc ise demonstrates the advantages of fitting flexible variance models an d using appropriate performance measures.