Real-time scheduling of wafer fabrication with multiple product types

This paper presents a real-time scheduling policy for the fabrication of multiple wafer types. The real-time scheduling policy provides a method of controlling the mean and the standard deviation of the flow time of the cassettes. We employ GSPN (Generalized Stochastic Petri Nets) to model the wafer fabrication and to evaluate the performance of the workstations in the wafer fabrication. From the performance analysis of the workstations, we compute the utilization index representing the status of the workstations, which is used in the control method. The control method consists of the sequence control and the input release control, where the former for selecting a cassette to be processed in an available machine, the latter for determining the type of a cassette and the time to be released into the fabrication. In this paper, OPNDD (Operation Due Date) Rule and Regulation of Buffer Length are developed for the sequence control, and Pre-Simulation and Feedback Control are developed for the input release control