Petri net-based cycle time analysis of dual-arm cluster tools with wafer revisiting and swapping strategy

There are wafer fabrication processes in cluster tools that require revisiting. It is shown that swapping is efficient in operating a dual-arm cluster tool. For dual-arm cluster tools with wafer revisiting, if a swap strategy is applied, it forms a three wafer periodical process with three wafers completed in each period. Such a period contains three cycles in a revisiting process and another three cycles in non-revisiting process. Hence, analysis and scheduling of dual-arm cluster tools with wafer revisiting become very complicated. In this work, a Petri net model is developed to describe the operations of such tools. Based on it, it is found that if a swap strategy is applied to a dual-arm cluster tool with wafer revisiting, it is always in a transient state. A systematic method is presented to analyze its performance.