Modeling and scheduling stepper operations in the photolithography process in wafer fabrication

Abstract

The photolithography process, as a single stage, multi-product batch process with non-identical, parallel units, used in wafer fabrication is described. In the process, the wafer is exposed to a light source that passes through the reticle which holds the pattern of the circuitry for a particular layer with the use of steppers. A mathematical model has been developed to schedule the batches of integral lots using slot-based continuous-time representation. The model allows the scheduling of multiple slots of the same type of product/device in a single slot and hence is able to handle larger problems and hence solve faster.