Reducing the Variance of Cycle times in Semiconductor Manufacturing Systems

In semiconductor manufacturing due to rework and re entrant ow overtaking of wafers can occur The e ect of overtaking is that cycle times at successive service centers are not mutually independent As far as the distribution of cycle times is concerned only higher moments are a ected the mean cycle time remaining unchanged by the in uence of overtaking Further in the literature it is conjectured that variance of cycle times increases when overtaking increases Taking into account this conjecture we attempt at reducing the variability of cy cle times by diminishing the magnitude of overtaking This can be done by reversing the overtaking through appropriate sequencing rules In order to achieve this goal we examine several sequencing rules by means of simulation studies based on real data sampled at four di erent semiconductor manufacturing facilities Our results elucidate that there is no general correlation between the magnitude of overtaking and the variance of cycle times We show that the performance measures under consideration depend very much on the type of the facility and the product mix