Autocorrelation of cycle times in semiconductor manufacturing systems

Due to the high complexity of semiconductor manufacturing systems simulation has become a useful tool for the performance analysis of these systems. However, there are some serious problems in analyzing simulation output data. For example, the quality of confidence intervals for certain performance measures is affected by correlations among the data. Furthermore, analyzing the correlation of simulation output data might give valuable hints for understanding the behavior of complex systems such as semiconductor manufacturing systems. In this paper we present examples where we have detected remarkable correlation phenomena, in the simulation output of semiconductor fabrication facilities caused by exo- or endogenous factors. Based on real specifications we examine several semiconductor manufacturing systems in steady state. First we calculate the autocorrelation function of the sequence of successive cycle times and then compute the corresponding frequency spectrum. By this means, periodicities in the simulation output data can efficiently be determined.