Quantifying variability of batching equipment using effective process times

Process time variability plays a key role in the cycle time of wafers. Several sources of variability can be distinguished. However, identification and measurement of all individual sources is almost impossible. Therefore, in previous work, a new method has been proposed to measure effective process times (EPT) for single-lot machines. The EPT incorporates the various sources of variability. From the measured EPT realizations, the mean and the corresponding coefficient of variation can be computed for queueing performance analysis. This paper follows up on previous work. The EPT quantification approach is generalized toward batching equipment. The batching types of operations are commonly present in the semiconductor industry. The paper proposes a transformation algorithm that transforms lot events into batch events. This enables one to use the previously developed single-lot algorithm also for batch machine workstations. An industry case illustrates the approach using operational data of furnace workstations.