Deadlock-free assignment of wafer processing in photolithography equipment - by using a CPN model

Cluster tools are widely used in wafer fabrication nowadays. Photolithography equipment, the most expensive equipment in wafer fabrication, is comprised of a serial of cluster tools. So far, there has been much research on the assignment of wafers processing in photolithography equipment. Since a production lot may include several kinds of wafer with different routes and the sequence order of wafers in a lot is stochastic, deadlock may occur at any time. Hence, we are not able to determine a schedule before the production batch starts; instead, we need a deadlock-free wafer dispatching method that can not only complete the requested wafer routine procedures but also reduce the total makespan. Here, we employ a transition digraph to check the occurring deadlock and a coloured Petri net (CPN) approach to model the photolithography equipment to ascertain clearly the system state via identification of the kind of wafers situated on respective positions and obtain a smooth and satisfactory control for the executed process. The employed CPN model is a timed high-level Petri net. Also, a well known CPN Tool (http://wiki.daimi.au.dk/cpntools/cpntools.wiki) is used for model editing, simulating and analysing. Accompanied by the photolithography equipment model built, a deadlock-avoidance supervisor is designed to ensure wafer deadlock-free dispatching at each step. This graphic model is easy to understand and can be extended to more complex cases.