Demand-driven, iterative capacity allocation and cycle time estimation for re-entrant lines

Daily production target setting for each production stage of a semiconductor wafer fabrication factory (fab) is a challenging machine capacity allocation problem due to the complex and re-entrant process flows. This paper summarizes a methodology developed by the author and his colleagues for the design of a daily target setting system (TSS) over the past few years. The methodology realizes PULL-then-PUSH and proportional capacity allocation principles to meet production demands in a smooth way while maximizing machine utilization. As machine capacity allocation and available wafer flows are intertwined, the target setting problem can be viewed as a fixed-point iteration problem. A deterministic queueing analysis-based algorithm is designed to estimate cycle times and hence wafer flows. The methodology iterates between capacity allocation and cycle time estimation until a fixed-point capacity allocation is achieved. Recorded field implementation results of various versions of TSS based on the methodology include a decline of average total WIP by 8%, an increase of daily total wafer flows by nearly 20%, an increase in target achieving rate at the shop floor by more than 10%, the reduction of average cycle time per layer from 3.25 to 2.96 days, and the reduction of average +2 sigma of per-layer cycle time from 4.63 to 3.68.