Hierarchical modeling and control of re-entrant semiconductor manufacturing facilities

This paper addresses hierarchical modeling and control issues within a modem semiconductor fabrication facility. It is well known that fabs are appropriately modeled via discrete event systems. Such models, however, are difficult to analyze and even more difficult to use for design purposes. In this paper a new method for modeling the fab is presented. The method leads to models which approximate the fab over small time scales. These models provide synchronous discrete-time approximations which may be useful for analysis and control design. They also provide a natural tool for systematically addressing aggregation/de-aggregation issues. It is also shown how the fab can be approximated by a high-level flow model. Such model. is useful for making high-level long term decisions, determining realistic commands for low-level tracking policies, and for assessing achievable performance. A low-level tracking policy is presented and integrated with a high-level state variable feedback policy. The low-level tracking policy is shown to track low-frequency commands generated by the high-level controller.