Downgrade decision for control/dummy wafers in a fab

Abstract

Control and dummy (C/D) wafers are indispensable materials used in a semiconductor fab. C/D wafers stored in a high-grade buffer can be downgraded to several low-grade buffers. The downgrade decision is to determine the amount to downgrade for each of these low-grade buffers. Previous literature solves the downgrade decision by considering only the instantaneous WIP information, which is a short-term approach and may not yield the optimum solution in the long run. This paper presents an LP model to solve the downgrade decision problem, which aims to minimize the long-term daily usage of brand-new C/D wafers in a fab. The formulated problem assumes that the storage cost of C/D wafers is much less than the usage cost. This assumption has been justified by analyzing the cost structure of C/D wafers in a typical fab site.