Control of Plasma Etch Profiles with Plasma Sheath Electric Field and RF Power Density

Abstract

In this paper it is shown that the plasma sheath electric field in which the etch sample is immersed controls the anisotropy of the etching of polysilicon films. These films are on wafers supported by the grounded electrode of a diode plasma etch reactor. The sheath electric field at the wafer supporting electrode is modified either by changing the rf power delivered to the plasma, or, at constant rf power, by application of d‐c voltage to the rf driven electrode, with similar consequences to the etch profiles. The absence of mask undercut and vertical etch walls characteristic of an anisotropic etch process are observed above a certain threshold power density‐sheath electric field, while below this threshold, the profiles become progressively more undercut until they are equal to the etch depth, as expected of isotropic etch processes. The experimental results presented in this paper, demonstrate these effects for a containing gas mixture at a total pressure of 65 Pa (0.5 Torr). A similar power density and pressure dependence of the etch anisotropy has been reported previously in different plasma etch gas under comparable conditions. These results will be shown to be explicable in terms of the increase in the anisotropy of the transport of ions for increasing sheath field.