Practical queueing models for preventive maintenance plan optimization: Multiple maintenance types and numerical studies

Preventive Maintenance (PM) activities in semiconductor manufacturing are important for equipment availability and reliability. Since PMs are down events that remove a tool from service, when to conduct a PM and how to group multiple PM tasks into shared PM events are key issues. In this paper, multiple types of PMs with different intervals are considered in G/G/m failure-prone queueing systems. PMs are classified into two categories: time-based preemptive and time-based non-preemptive. For each category of PM, this paper provides PM planning optimization models to seek optimal PM intervals that minimize the mean cycle time of a wafer lot. Simulation studies are conducted to explore the performance of the optimization model and the mean cycle time approximations. Moreover, sensitivity analysis is conducted and reveals several properties of the PM planning model. Implementation results are briefly reported for a toolset in semiconductor manufacturing that achieved a 12% reduction in mean cycle time using PM plans based on these modelling approaches.