Supervisory run-to-run control of polysilicon gate etch using in situ ellipsometry

Polysilicon gate etch is a critical manufacturing step in the manufacturing of MOS devices because it determines the tolerance limits on MOS circuit performance. The etch used in the current study suffers from machine aging, which causes processing results to drift with time. Performing the etch for the same time with fixed process setpoints (recipe) for all wafers would produce unsatisfactory results. Thus, an in situ ellipsometer was employed with a new run-to-run supervisory controller, termed predictor corrector control (PCC), to eliminate the impact of machine and process drift. A novel modeling technique was used to predict uniformity from the ellipsometry data collected at a single site on the wafer. Predictive models are employed by the PCC supervisory controller to generate optimal settings (recipe) for every wafer which will achieve a target mean etch rate, while maintaining a spatially uniform etch. A 200 wafer experiment was conducted to demonstrate the benefits of process control. Implementation of PCC resulted in a 36% decrease in standard deviation from target for the mean etch rate. In addition, the data indicates that controlling etch rate may improve the control and uniformity of the line width change