Simulation models are useful to predict and understand the impact of changes to a manufacturing system. Typical factory simulation models include the parts being manufactured in the factory and the people and resources processing and handling the parts. However, these models do not include equipment or process details, which can affect operational performance such as cycle time and inventory. Separate models are used to evaluate processes and equipment. Thus, it is difficult to evaluate the operational impact of equipment or process changes. However, this information could help factory managers and manufacturing process engineers make better decisions when changing processes or selecting equipment configurations. This paper describes a heterogeneous simulation environment for understanding how equipment and process changes affect the performance of a wafer fabrication facility. This integrated tool incorporates response surface models that describe process behavior, operational and optimization models of equipment behavior, and a discrete-event simulation model of factory operations. Thus, the tool can measure how process changes and equipment configuration changes change the system performance. We have applied this tool to a specific wafer fab problem.Maryland College Park in May 1999. His current research interest is focusing on cluster tool scheduling. GARY W. RUBLOFF is currently technology; ultraclean, integrated processing and advanced equipment; and advanced diagnostic techniques. He has published well over 100 papers and holds 15 U.S. patents. He is the founding chairman of the AVS Manufacturing Science and Technology Group, a Fellow of APS and AVS, and active in semiconductor technology roadmapping activities. His current research efforts are aimed at real-time process sensing, dynamic simulation for design optimization, process control, environmentallyconscious manufacturing, manufacturing operations, and manufacturing education and training. His email and web addresses are and .