Hierarchical Decision-Making for Qualification Management in Wafer Fabs: A Simulation Study

A qualification management problem arising in semiconductor wafer fabrication facilities (wafer fabs) is discussed. The stepper equipment needs to be qualified to process lots that belong to different families. Stepper- and family dependent qualification time windows exist. Time windows can be reinitialized if required and can be extended by on-time processing of lots from qualified families. A hierarchical approach is proposed. The base-level provides a dispatching strategy that takes into account qualification decisions, while the mid-level consists of a mixed integer linear program (MILP) for making qualification decisions. The top-level comprises a linear program (LP) that computes target quantities for the families on the steppers in each period of the planning window taking into account fab-wide objectives. In addition to an LP formulation with conventional capacity constraints, we provide a formulation where clearing functions are used to represent the congestion of the wafer fab. We also discuss how to anticipate the behavior of the mid-level on the top-level. Results of simulation experiments where the hierarchical approach is applied in a rolling horizon manner demonstrate that the LP-based approaches outperform a heuristic to determine the target quantities. Among the LP formulations, the ones with clearing functions perform best. We also demonstrate the value of anticipation.