Cyclic Scheduling of Sequentially Connected Cluster Tools with a Single I/O Module

A cluster tool has been widely used for most wafer or flat panel fabrication processes. We examine cyclic scheduling problems of sequentially connected cluster tools with a single input and output module, which includes multi-cluster tools as well as linear cluster tools. We first compute workloads for each process step and each robot, of which the maximum provide a lower bound of the tool cycle time. We then identify an assignment rule of assigning the chambers to the process steps that makes the tool cycle time independent of the order of using the parallel chambers. We also propose a robot task sequence for each component cluster tool, which is modified from the well-known swap sequence. We prove that the modified swap sequence is optimal when one of the process steps, not a robot, is the bottleneck. Finally, we present a scheduling strategy which controls robot task timings to deal with interference of wafer flows between each pair of adjacent component cluster tools, which is complicated by blocking due to limited buffer capacity between them and backward wafer flows.