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 containing
a specific process technology, for example, photolithography, metal deposition or etching. Therefore, to
accurately predict the wafer outs, an accurate prediction of the cycle time distribution at each work area
is essential. This paper proposes an aggregate model to simulate each of these work areas. The aggregate
model is a single server with an aggregate process time distribution and an overtaking distribution. Both
distributions are WIP-dependent, but an additional layer-type dependency is introduced for the overtaking
distribution. Application on a real-world wafer fabrication facility of a semiconductor manufacturer is
presented for the work areas of photolithography, oxidation and dry etch. These experiments show that the
aggregate model can, under certain circumstances, accurately predict the cycle time distributions in work
areas by layer-type.