Two-stage lot scheduling with waiting time constraints and due dates

We examine a two-stage lot scheduling problem with waiting time constraints and distinct due dates. Wafer lots in diffusion or etch processes generally have due dates specified for each process stage. Some lots even have more strict time constraints that their waiting times between two or multiple stages should not exceed specified limits. We also wish to minimize the variation of the waiting times at the intermediate buffer, which is detrimental to wafer quality variability. To solve such a scheduling problem, we develop a mixed integer programming model for small problems. Also, we suggest an efficient solution procedure for large problems by adopting the earliest due date policy and propose a timing control strategy.