Petri Net-Based Scheduling Analysis And Simulation Of Single-Arm Cluster Tools With Wafer Revisiting Process

With the increase of wafer size in semiconductor manufacturing, wafer fabrication becomes more and more complicated. In order to increase productivity, reduce cost, and improve quality, there is an increasing use of cluster tools for most wafer fabrication processes.Some wafer fabrication processes performed by cluster tools contain a revisiting process. With wafer revisiting, there are a large number of possible schedules for the revisiting process, such that a cluster tool is very difficult to be scheduled. Atomic layer deposition (ALD) is a typical revisiting process in wafer fabrication performed by cluster tools. It is very important to obtain an optimal schedule for such a process.This paper addresses the scheduling problem of single-arm cluster tools for the ALD revisiting process. In scheduling such a system, the most difficult part is to schedule the revisiting process. In the ALD process, it may require to revisit up to more than five times. The larger the revisiting times are the more complex the scheduling problem is. The revisiting process can be denoted as (a, b)k,where a and b are number of processing modules for the two revisiting steps, k is the revisiting times. Because (1,2)k is the typical process, this paper discusses the scheduling problem for the ALD revisiting process (1,2)k with revisiting times k= 3,4. and 5.To obtain an optimal schedule, a Petri net model is developed to describe the behavior of single-arm cluster tool for this process. Based on this model, the scheduling problem of the ALD revisiting process (1,2)k is studied. It is known that the optimal schedule can be obtained by comaring the cycle time of k possible schedules for a given k. Furthermore, analytical expressions are derived to calculate the cycle time for each candidate schedule. Then, the schedule with the minimal cycle time is the optimal one. In this way, the scheduling problem of such a revisiting process becomes very simple and this is a significant improvement in scheduling cluster tools with wafer revisiting.With Petri net model and the scheduling analysis, an eM-Plant-based simulation system is developed to make the method visionized. By using this system, cycle time of an optimal schedule for the ALD revisiting process (1,2)k can be found to verify the proposed method. Examples are given to show the feasibility and correction of the proposed method.Finally, a production management system for ALD revisiting process is developed. All the analytical expressions have been coded in this software system, when the process parameters are known, we can simply enter the parameters to return an optimal steady-state schedule and the minimal cycle time.