Surface diffusion model accounting for the temperature dependence of tungsten etching characteristics in a SF6 magnetoplasma

To explain the influence of the substrate temperature T(s) on the etching characteristics of tungsten in a SF(6) magnetoplasma, we have extended the surface diffusion model originally developed for the etching of the W-F system at constant (ambient) temperature. It allows us to understand our experimental observations which include the influence of T(s) on the anisotropy and the fact that the lateral (spontaneous) etch rate of W as a function of 1/T(s) does not follow an Arrhenius law. The model is valid as long as the pressure is sufficiently low (less than or equal to 0.5 mTorr) to neglect the influence of coadsorption and passivation effects, likely related to oxygen contamination of the gas phase coming from the fused silica discharge tube interacting with fluorine atoms. Consistency of the model is well demonstrated by observing that the lateral to vertical etch rate ratio as a function of 1/T(s), under different plasma conditions, leads to a unique value of the activation energy (R = 0.65 eV) for the associative desorption of WF(6), the volatile reaction product of tungsten with fluorine adatoms. (C) 1998 American Vacuum Society.