A model-based technique for real-time estimation of absolute fluorine concentration in a CF4/Ar plasma

Abstract

A technique for quantitative interpretation of actinometric data to deduce bulk plasma fluorine concentration in a CF4/Ar plasma has been developed and tested on a commercial reactive ion etcher. This static, in situ measurement is useful for monitoring fluorine in a manufacturing environment and, in particular, for application of real-time feedback control to plasma etching. Based upon a model of CF4 chemistry reaction pathways and products, it improves upon current fluorine estimation techniques by accounting for varying levels of argon dilution resulting from CF4 dissociation. A simple experiment was also developed in order to obtain an estimate of the actinometric scaling factor without an independent measurement of fluorine. Performance of this model-based fluorine estimation technique was compared to that of a standard technique by using time resolved etch rate measurements as an independent indicator of fluorine concentration, while a feedback control scheme decoupled the effects of physical etching by stabilizing the induced dc bias. The model-based estimator significantly reduced perturbations in the etch compared to those seen when using the standard estimator.