In-situ spectroscopic ellipsometry for the real time process control of plasma etching of silicon nitride

Spectroscopic ellipsometry (SE) is a commonly used non-destructive, non-invasive in-situ sensor for dry etching. SE measures the change in the polarization state of light reflected from a surface. Sample thickness is obtained by fitting a model to the experimental ellipsometry data. In this paper we describe the design, testing and evaluation of an SE based adaptive real time Feedback controller for etch rate regulation in CF4/O-2 plasma etching of silicon nitride films. The feedback variable is the current etch rate as determined from the insitu SE measurements of the film's thickness. The controller compensates for drifts in etch rate which occur during a given etch, and adaptively adjusts for the run-to-run variability inherent to plasma processing. Experimental results are presented and discussed.