Analysis of the nonstationary behavior of a wafer fab during product mix changes

This paper presents a series of experiments that were conducted to investigate in the nonstationary behavior of a wafer fab after changes in product mix. The experiments were performed using a simulation model of the front end area of an existing semiconductor fab. We observe how short-term increases in wafer starts of a product influence the cycle time and WIP of this product and of the other products. It is examined how the fab recovers from such production surges under different dispatch rules. We also investigate how different lot start mechanisms affect the short term fab performance. More specifically, we observe the effects of changing the mix of the lots started into the fab on a weekly basis. Finally, we compare two alternative ways of releasing lots into the fab: the first is to distribute lot starts evenly over a given period, e.g. a week, and the other is to start all lots at the beginning of the period