This paper presents a new method for scheduling cluster tools in semiconductor fabrication. A cluster tool consists of a group of single-wafer chambers organized around a wafer transport device, or robot. The cluster fabrication considered consists of serial cluster tools. Due to constraints imposed by multiple routes of each type of wafer and machines with no buffer, it is very difficult to find an optimal or near-optimal schedule. The proposed method uses a job requirement table with random keys as a solution representation, and simulated annealing to find an optimal or near-optimal schedule. To determine which operation can be scheduled, a candidate list is used. To show the effectiveness of the proposed method, a scheduling example of cluster fabrication is presented. From the experimental results, it is shown that the proposed method is promising.

A scheduling method that uses Petri net modeling and heuristic search is extended to handle the tool setup. The machine setup is to attach a tool to a particular machine and detach the tool from the machine, and this is a time-consuming activity. The processing time of operations varies according to the attached tool and the machine used. The method proposed in this paper uses timed-transition Petri net to model these features and applies a search algorithm to the reachability of the Petri net model to generate an optimal or near-optimal schedule. In addition, this method can determine how many tools are needed in order to reduce the makespan. Search algorithm employs an evaluation function to guide the search procedure. New heuristic functions are developed for efficient search. The experimental results that show the effectiveness of the proposed method are presented.