Predicting cycle time distributions with aggregate modelling of work areas in a real-world wafer fab

In a semiconductor wafer fabrication facility (wafer fab) it is important to accurately predict wafer outs, i.e.the remaining cycle time of the wafers in process. A wafer fab consists of multiple work areas, each containinga specific process technology, for example, photolithography, metal deposition or etching. Therefore, toaccurately predict the wafer outs, an accurate prediction of the cycle time distribution at each work areais essential. This paper proposes an aggregate model to simulate each of these work areas. The aggregatemodel is a single server with an aggregate process time distribution and an overtaking distribution. Bothdistributions are WIP-dependent, but an additional layer-type dependency is introduced for the overtakingdistribution. Application on a real-world wafer fabrication facility of a semiconductor manufacturer ispresented for the work areas of photolithography, oxidation and dry etch. These experiments show that theaggregate model can, under certain circumstances, accurately predict the cycle time distributions in workareas by layer-type.