Lowering Variability in Transport Times by Scheduling Conveyor-Based AMHS in Wafer Fabs

Abstract

Variability increases cycle times and inventory levels in semiconductor fabricating facilities (wafer fabs). Variability in wafer fabs originates from many sources such as reentrant material flows, batching, lot release patterns or varying processing times. In contrast, there is also variability caused by the automated material handling system (AMHS). Thus, optimized schedules for process operations as well as for transport operations aim to compensate a wafer fab’s intrinsic variability in order to reduce cycle times. This paper investigates novel transport scheduling methods for conveyor-based AMHS and validates their effects on reducing cycle times. Most importantly, the resulting variabilities in transport-related delays are quantified. As a result, the hypothesis that lower variability in transport times contributes to a decrease in overall cycle times is confirmed.