Modeling the effect of hot lots in semiconductor manufacturing systems

The presence of hot lots or high-priority jobs in semiconductor manufacturing systems is known to significantly affect the cycle time and throughput of the regular lots since the hot lots get priority at all stages of processing. In this paper, we present an efficient analytical model based on re-entrant lines and use an efficient, approximate analysis methodology for this model in order to predict the performance of a semiconductor manufacturing line in the presence of hot lots. The proposed method explicitly models scheduling policies and can be used for rapid performance analysis. Using the analytical method and also simulation, we analyze two re-entrant lines, including a full-scale model of a wafer fab, under various buffer priority scheduling policies. The numerical results show the severe effects hot lots can have on the performance characteristics of regular lots.