Analysis of Backward Sequence in Lot Switching Periods for Single-Armed Cluster Tools

Jeongsun Ahn, Hyun-Jung Kim

Abstract: Due to the increasing diameter of wafers and diverse customer demands, cluster tools frequently encounter lot switching periods, where wafer lots with different recipes are processed consecutively. Many previous studies have often used the backward sequence, which is considered optimal for cyclic robot operations with identical lots. This practice persists even when addressing lot switching period scheduling. Nevertheless, it remains uncertain whether the backward sequence is the optimal choice or can guarantee a specific level of performance during the transient periods. Therefore, we analyze the performance of the backward sequence in lot switching periods of single-armed cluster tools, both theoretically and experimentally for the first time. We first conduct an analysis of the dominance relations among backward operations, then propose a workload concept for the lot switching period and establish the conditions under which the backward sequence provides an optimal schedule. We finally empirically verify the effectiveness and performance of the backward sequence.