Continuous flow transport scheduling for conveyor-based AMHS in wafer fabs

Abstract

Automated material handling systems (AMHS) can greatly impact the manufacturing performance of a semiconductor fabricating facility (fab). High traffic loads within an AMHS can impede individual wafer lots so that they arrive late at their destination machines. Thus, corresponding process operations as well as dependent succeeding operations will be delayed due to the fab schedule's precedence constraints. Consequently, such transport-related delays can widely propagate throughout the overall fab schedule. In order to reduce transport-related delays before time-critical operations, novel ways of planning wafer transports have been investigated in this study. For validation, a well-known realistic representative wafer fab model has been extended with conveyor elements constituting a typical AMHS for continuous flow transport (CFT). As a result, improvements of the overall fab performance due to advanced transport scheduling methods are demonstrated and compared. Finally, the practicality of the suggested methods is discussed in the dynamic scheduling context of real fabs.