A real-time scheduling approach for two-cluster tools with wafer revisiting

In semiconductor manufacturing, there are revisiting processes where some processing steps are required to be visited by a wafer for multiple times. In many cases, the production of such revisiting processes is implemented in a multi-cluster tool composed of multiple individual cluster tools. Due to the complex wafer flow patterns and the coordination requirement of multiple robots, it is very challenging to schedule such a tool and there is no research report on this issue. This work deals with the scheduling problem of a two-cluster tool with wafer revisiting. To cope with the huge system state space, a simulation-based method is proposed. Based on this method, a real-time scheduling approach is proposed to efficiently find a good feasible solution. To implement the system simulation, a simulation model is developed based on object-oriented programming, and it can describe various wafer flow patterns and varied tool configurations. Furthermore, in this way, a schedule by optimally configuring process modules (PMs) for the operations can be efficiently found. Experiments are presented to demonstrate the proposed approach.