Makespan computation of lot switching period in single-armed cluster tools

In semiconductor manufacturing, wafer cassettes often contain two to three different wafer types due to the larger wafer size, the continual circuit width reduction and the increasing demand for customized products. Hence, the lot switching operation in which the last few wafers of the preceding lot and the first few wafers of the next lot are processed together in cluster tools occurs frequently. In this paper, we analyze an efficient robot task sequence proposed by the previous work for the lot switching operation in single-armed cluster tools and derive closed-form expressions of the makespan of the lot switching period for the first time. With this research, the completion time of a wafer lot can be easily estimated, and then idle times of tools and turnaround times of wafers can be reduced by sending automated material handling systems in advance to unload completed cassettes.