Plasma etch endpoint via interferometric imaging

We describe a full wafer interferometric imaging technique to provide spatial and time resolved etch rate and endpoint information in plasma etch processes. We discuss models and simulations of key effects and non-idealities that complicate the estimation of etch rate and endpoint via interferometry signals. These include multilayer structures, patterned areas, steps in underlying films, transition layers between films, index of refraction dependencies, and etched film uniformity. A series of full wafer images for the etching of 5100 Å thick polysilicon test patterns over 1100 Å, 450 Å, 250 Å, and 120 Å oxide were collected using a high resolution CCD camera operating at 1.5 Hz. We demonstrate that, with adequate models of these effects, etch rate and time-to-endpoint as a function of spatial position can indeed be extracted. Full wafer interferometric imaging enables the control of plasma endpoint based on new criteria, including clearing at sets of specific locations or percentage wafer cleared. Future applications include in-situ control of plasma etch rate uniformity.