Modeling and implementing a real-time scheduler for dual-armed cluster tools

As application of cluster tools is being extended to more wafer fabrication processes including chemical vapor deposition processes as well as etching processes, cluster tool configurations and wafer flow patterns tend to be more complicated. Frequent recipe changes require prompt adaptation of the scheduling logic to cope with changed configurations and wafer flow patterns. The scheduler should be responsive to occurrences of various events within the tool and should be able to coordinate and control complicated activities and tasks of the process modules and the wafer-handling robot. Moreover, the scheduler should meet a critical timing constraint that a wafer should leave the process module within a specified time limit after processing. Therefore, it is essential to have a systematic, flexible and agile method of developing a robust real-time scheduler. To do this, we propose a model-based approach based on finite state machines (FSMs) to develop a real-time scheduler for dual-armed cluster tools. We propose the scheduling decisions for normal operation. We then develop a scheduler model based on FSMs that specifies the scheduling decisions for normal operation and encodes real-time control decisions to cope with disturbances. We then verify the scheduler model and transform the model into an executable control code.