Reactive etching mechanism of tungsten silicide in CF 4-O 2 plasma

Abstract

The WSi 2 etch rate contains contributions from the directional ion-enhanced etching mechanism which follows the nth power of the d.c. self-bias voltage across the plasma sheath ( n = 1.7±0.5) and from the isotropic chemical etching mechanism linearly proportional to the number density of fluorine atoms in the CF 4-O 2 r.f. plasma. The “apparent” reaction probability of chemical etching, defined as the chemical etching rate per fluorine atom, increases with the r.f. power because of additional heating by the eddy current induced by the r.f. magnetic field. The r.f. inductive heating in the silicide film makes the temperature of the underlying polycrystalline silicon (poly-Si) film increase, resulting in a poly-Si chemical etching rate higher than that of a blanket poly-Si film. Etch rates due to the chemical etching are proportional to the silicon content in the silicide film for a given plasma condition. At a fixed silicon content ( x = 2.6), a film prepared by the chemical vapor deposition technique exhibits a chemical etching rate higher by a factor of 2 than that of films prepared by either electron beam co-evaporation or co-sputter deposition.