An intelligent model-based control system provides a framework for the efficient application Of in situ sensors to semiconductor manufacturing. This article discusses the components of such a system. The specific example presented in this article is a system utilizing in situ ellipsometry developed for a polysilicon gate etch. The ellipsometer was used for robust endpointing, process modeling, fault detection, and run-to-run supervisory control. The methodology for each sensor application is presented. The coordination of the various applications is also discussed and implementation issues are covered. The goal of the controller for polysilicon gate etch is to achieve a target mean etch rate, while maintaining a spatially uniform etch. This process suffers from machine aging, which degrades the etch rate and final film uniformity. Experimental results demonstrating that the controller compensates for process drift are shown. Implementation of this system resulted in a 36% decrease in standard deviation from the target for the mean etch rate. All of the wafers processed with feedback control met the specifications on nonuniformity, while some of the wafers processed without control did not. In addition, the data indicated that controlling the etch rate may improve the control and uniformity of the linewidth change.