Semiconductor FAB OHT track analysis using queueing network model

Abstract

We address the analysis of the distribution of vehicles on the OHT system in semiconductor FAB. As semiconductor wafer production has grown in recent year, the number of OHT vehicles to transport wafers also is increasing. Therefore, hundreds of OHT vehicles are simultaneously operating in the AMHS. Since the tools between heavy flow interactions are adjacent to each other, and OHT vehicles usually travel the shortest distance, they are often concentrated on OHT track at those tools. These concentration of vehicles on OHT track causes waiting and blocking among vehicles. The waiting and blocking among OHT vehicles increase the delivery time of wafers including transport time and waiting time, and it leads to decline the productivity of the entire semiconductor FAB. We first verify the given semiconductor FAB layout and OHT track design using simulation as the flow volume and the number of vehicles increasing, and identify the congestions by OHT vehicles. We show that the congestion can be alleviated when the distance between two tool groups with heavy flow interaction is modified away by re-allocation two tool groups or re-design the OHT track using simulation experiment. We find that the distance between tool and the number of vehicle effect the performance of OHT system. We propose a finite buffer closed queueing network model to analyze the distribution of vehicles and the number of vehicles on the track based on the insight from simulation experiments. First, we propose the model for a single closed loop OHT track without shortcuts. The entire OHT track is decomposed into unit systems, and decomposition method is applied to analyze the finite buffer closed queueing network model, and the distribution of OHT vehicles is obtained. The trend of the distribution of OHT vehicles is similar to that of simulation. The result of model shows that the distance between tools effect the performance of OHT system. In an actual semiconductor FAB, the configuration of OHT track is a multiple loop type including several shortcuts to reduce the travel distance. We propose the method to decompose the multiple loop OHT track into several sub-single loop. Flow-interaction data is also divide into sub-data corresponding sub-single loop. The distribution of vehicles for the multiple loop is obtained by using the results from sub-single loops. We analytically identify that the distribution of vehicles is changed by re-allocation two tool groups or re-design the OHT track as shown in the simulation experiments. From the result of model, we reconsider the traditional design method based on the minimum distance and the shortest path. While the simulation based FAB and OHT system design requires significant cost and efforts to verify all alternatives, we provide the guideline for design them considering the distribution of OHT vehicle though the analytical approach.