Petri net-based scheduling of time constrained single-arm cluster tools with wafer revisiting

It is very difficult to schedule a single-arm cluster tool with wafer revisiting such that wafer residency time constraints are satisfied. This article conducts a study on this challenging problem for a single-arm cluster tool with atomic layer deposition process. With a so-called p-backward strategy being applied, a Petri net model is developed to describe the dynamic behavior of the system. Based on the model, the existence of a feasible schedule is analyzed, schedulability conditions are derived, and scheduling algorithms are presented if there is a schedule. A schedule is obtained by simply setting the robot waiting time if schedulable, and it is very computationally efficient. The obtained schedule is shown to be optimal. Illustrative examples are given to demonstrate the proposed approach.