Scheduling In-Line Multiple Cluster Tools

A cluster tool which is widely used for wafer fabrication processes consists of several processing modules (PMs), a transport robot, loadlocks, and an equipment front-end module (EFEM) in which a wafer cassette is loaded and unloaded. A wafer cassette with 25 identical wafers is transported by an overhead hoist transfer (OHT) between cluster tools and stored in a stocker when the corresponding tool is not available. Due to a large number of tools and wafer cassettes in a fab, it is extremely complicated to operate OHTs so as to minimize the idle times of the tools. Therefore, fabs tend to reduce the transportation tasks by arranging tools in a line with a buffer. In this paper, we introduce a new system called in-line multiple cluster tools which have many challenging operation issues for researchers and propose simple and practical methods to operate the system. We first develop mixed integer programming models to assign wafers to parallel PMs and derive a priority rule-based scheduling policy for the EFEM operation while minimizing the makespan of producing 25 wafers. We also propose multimode operation in which a component tool in the system processes multiple wafer types at the same time to improve the utilization rate of the tool.