OPTIMIZATION VERSUS CONSTRUCTION OF TRANSPORT SCHEDULES TO REDUCE TRAVEL TIME VARIABILITY AND AVOID CONGESTIONS IN CONVEYOR-BASED AMHS FOR WAFER FABS

摘要

Advanced transport scheduling for conveyor-based automated material handling systems (AMHS) in semiconductor fabricating facilities (wafer fabs) can reduce transport times and thus cycle times. Commonly, transport operations of arriving wafer lots at conveyor junctions, such as rotary tables, are sequenced ad-hoc by myopic first-come-first-serve policies. In contrast, improved transport schedules for the transport operations can be produced ahead of the time in conjunction with the overall production schedule for process operations. More precisely, such a transport schedule can either be sequentially constructed by fixing one transport after another, or it can be optimized by simultaneously fixing several conflicting transports. Hence, two conceptually different transport scheduling methods, which both avoid congestions by enforcing a no-wait constraint, are compared with special regard to transport-related variability. Furthermore, three different AMHS models that exhibit the typical interbay-intrabay layout are used for computational experiments.