AN ANALYSIS OF DISPATCHING RULES IN A SEMICONDUCTOR WAFER FABRICATION ENVIRONMENT

Certain manufacturing processes are characterized by reentrant product flows: notable among these is semiconductor wafer fabrication. Wafers repeatedly undergo processing at hub operations (photolithography), and between successive sets of hub operations the wafers are processed elsewhere. Reentrant how shops are notoriously difficult to model analytically, especially for due-date performance measures. In this research five dispatching rules are described; where appropriate the priority assignments are plotted in three dimensions. A simple simulation model of a wafer fab is used to compare the rules under a variety of conditions. The effectiveness of the rules is compared in terms of mean flow time, number of tardy jobs, mean tardiness and maximum tardiness. Overall, Critical Ratio and Dispatch Priority outperform Earliest Due Date, Slack and First-Come First-Served.