Wafer lot release policies based on the continuous and periodic review of WIP levels

This paper evaluates the impact of different lot release policies on the performance of semiconductor wafer fabrication facilities (wafer fabs). Lot release policies presented in this work are based on continuous review and periodic review inventory control policies. Proposed policies can be considered as modifications to the well known CONstant Work-In-Process (CONWIP) method. Simulation of the Intel Mini-Fab model is used to compare the performance of the fab using the CONWIP method to using these proposed policies. Additionally, two different dispatching rules are used at the bottleneck station to evaluate its impact on the fab performance when combined with the release policies. Performance is measured using an overall efficiency measure to represent the changes that occur in both cycle time and throughput rate. The work shows that periodic WIP review can improve the performance of the fab; while, continuous WIP review will have the same effect of the CONWIP method.