Unidirectional circular layout for overhead material handling systems

The reentrant flow type of the semiconductor wafer manufacturing process creates a large amount of overhead material flows in the interbay system. This paper proposes unidirectional circular layout for overhead multi-carrier systems. The layout configuration can potentially reduce the control complexity of heavy traffic flow, streamline the empty carrier supply, and meet the delivery requirements. The layout contains a central loop to reach every stocker at high speed and supply empty carriers, provide prompt delivery service to each loop addition in a dynamic and responsive manner. It is preferred to other track architectures, as minimal traffic control is required for the large amount of wafer flows in semiconductor fabs. The proposed construction procedure proceeds in two stages. In the first stage, four quantitative measures are provided to select good candidates of main loop layouts in order to minimize the total flow times distances, construction costs and service response time. Once the main loop is chosen, minimum carrier flow requirement and the critical segments can be identified. The required carrier flows depend on the minimum flow requirement on the critical segments. In the second stage, forward and backtrack reducible loops are found as loop additions to connect to exterior stockers (stockers outside the main loop) as well as to eliminate the flows on critical segments. A dynamic programming procedure is presented to minimize the total construction and operating costs in the hybrid loop addition process. This two-stage procedure identifies a number of good and feasible layouts in which multiple layouts are maintained throughout the execution process.