Scheduling semiconductor lines using a fluid network model

We present a novel method for “what´s next” scheduling of semiconductor manufacturing lines based on the deterministic fluid network model of Chen and Yao. By “what´s next” scheduling we mean specifying the order in which jobs are processed at each tool group. The approach we take here is first to determine how to allocate tool capacity among competing job types by solving a series of linear and quadratic programming problems related to the fluid model and then to specify a “what´s next” scheduling algorithm designed to track these capacity allocations. The primary advantage of our approach is that it gives rise to a dynamic schedule which is based on global rather than local state information and which is responsive to stochastic changes in the line including tool incapacitation events and operator unavailability. This is in contrast to many other scheduling rules, such as first come first served and closest to completion which are essentially static rules based on local information. We also present some theoretical results about the fluid model that have important implications in the context of semiconductor manufacturing as well as some preliminary simulation results comparing the fluid model to other scheduling policies