Wafer fabrication scheduling using flow rate control strategy

The authors present a strategy for shop-level scheduling in a semiconductor wafer fabrication facility, which reduces WIP (work in process) and improves on-time ship performance. Algorithms for lot-release and lot-dispatch are defined on the basis of dynamic programming techniques. The strategy is called flow rate control because it treats the production rates according to certain rules. The most important rule is the two-boundary rule that consists of a simple inventory control policy and a simple surplus control policy. Development of the strategy was motivated by three features of the wafer manufacturing process: job re-entry, the large number of machines and products, and random events such as machine failures and yield fluctuations. Two-boundary control works by first forming a virtual job shop corresponding to the manufacturing process flow. Inventory and surplus levels are calculated for each job step and compared with predetermined values. These predetermined values are calculated from the failure rates and yield fluctuations of wafer fabrication. The loading rate at each job step in the wafer fabrication process is determined by the result of the comparison. An event-driven simulator designed for job shop production simulation was used to compare the flow rate control strategy with other scheduling strategies. The results showed marked improvement in WIP levels and ship performance over these other approaches