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Most cluster tool scheduling studies assume identical access times between chambers or do not discuss the impact of access times. However, the optimal scheduling rule and the cycle time can depend on the access times or physical configuration of parallel chambers. Therefore, we examine cyclic scheduling problems for cluster tools that have nonidentical access times. We first develop Petri net models for tool behaviors and analyze the cycle time by identifying the workloads of the process steps. We prove that the conventional backward and swap sequencing strategies are still optimal for single-armed and dual-armed cluster tools, respectively, when a process step is the bottleneck and the tool repeats a minimal cyclic work cycle. We also present a closed-form formula for the cycle time and identify a coprime condition on the number of parallel chambers for which the cycle time is independent of the order of using parallel chambers. Finally, we develop a mixed integer programming model for cases in which the coprime requirement is not satisfied.
Date of Publication: Aug. 2012