A Simulation Study of Automated Material Handling Systems in Semiconductor Fabs

A critical aspect of semiconductor manufacturing is the design and analysis of material handling and production control polices to optimize fab performance. As wafer sizes have increased, semiconductor fabs have moved toward the use of automated material handling systems (AMHS). However, the behavior of AMHS and the effects of AMHS on fab productivity are not well understood. The first aspect of the research involves the development of a design and analysis methodology for evaluating the throughput capacity ofAMHS. A set of simulation experiments is used to evaluate the throughput capacity of an AMHS and the effects on fab performance measures. This research utilizes two simulations of SEMATECH fab data of actual production fabs. The AMHS vehicle utilization point at which fab performance is degraded is studied. Results show a large increase in lot cycle time at a vehicle utilization of 75%, far below the maximum 100% utilization. These results stress the importance of using a performance indicator that takes into account the performance of the entire fab and not only the AMHS. The second aspect of this research involves the study of AMHS and tool dispatching rules. The hypothesis of this study is that fab performance is affected by both the choice of AMHS and tool dispatching rules as well as their interaction. A full factorial design experiment is conducted to test this hypothesis. Results show that for each fab tested the vehicle rules, machine rules, and their interactions are significant using an ANOVA test on average lot cycle time and other fab performance measures. Additional analyses are conducted to identify robust combinations of AMHS and tool dispatching rules among those tested. The overall results of this study indicate that AMHS and tool dispatching rules effect fab performance and must be considered together when trying to optimize fab performance.