Open-loop predictive control of plasma etching of tungsten using an in-situ film thickness sensor

摘要

Abstract: Automated control schemes would greatly improve the reproducibility of plasma-assisted etching processes. In this paper we report on the application of an in situ metal film thickness sensor to control a plasma tungsten etch process. The process consists of an anisotropic step to control line profile and remove as much tungsten as possible, followed by an isotropic step with etches through to the underlying layer. In typical operations, a pilot wafer is measured off-line to determine the initial tungsten thickness. An etch time for the first step is then calculated before processing the entire lot. Single wafer lots require the elimination of a pilot wafer. Recently, we integrated a metal film thickness sensor (based on the technology of eddy currents) into a single-wafer plasma tungsten etch module. Our control strategy uses the sensor in a feedforward manner. A measurement of the tungsten film thickness is made in situ prior to processing. Process control software adjusts the etch time for the wafer based on the measured thickness and the predicted etch rate for the equipment settings. The etch rate is calculated from an empirical model obtained using response- surface methodology. A three-fold decrease in wafer-to-wafer variability in final thickness after the etch step was realized compared to that for the deposited thickness.!16