Pattern dependencies in plasma etching processes have been widely noted, but only a limited understanding of the fundamental cause exists. Typical pattern dependencies are variations of the etching rate and sidewall profile with line spacing or other changes of the local topography. The dispersion of the ion impingement angle (caused by scattering within the plasma sheath), scattering of ions (by glancing collisions with the profile features), and redeposition of material emitted in the etching process (which blocks the sidewall etching) are thought to be the primary factors in pattern dependencies. In this work, a string-and-point model has been extended to describe both the wafer surface and the interface between the redeposited layer and the underlying material. The etching rate is modeled as an ion-enhanced etching accompanied by isotropic etching which proceeds at a rate which is inversely related to the redeposited film thickness. The model successfully describes phenomena such as "mouse bites" in which the redeposition layer, which blocks the etching, becomes too thin and cavernous etching proceeds from a point midway along the sidewall.
Scott C. Jackson,
Timothy J. Dalton,
"A Profile Evolution Model With Redeposition", Proc. SPIE 1185, Dry Processing for Submicrometer Lithography, (30 January 1990); doi: 10.1117/12.978062; https://doi.org/10.1117/12.978062