Topographic phase boundary shifts and saturation for anisotropic ion straggle during sputter etching

Abstract

Surfaces sputtered by ion beam bombardment have been known to exhibit patterns whose behavior is modeled with stochastic partial differential equations. A widely accepted model is the Cuerno-Barabasi model which is robust in its predictions of sputtered surface morphologies. An understanding of the factors responsible for such surface topographies can be achieved by using scaling arguments on the stochastic model. For such explanations, knowledge of the coefficients is crucial. The more so since these vary with different materials, the sputtering process itself generates non-equilibrium surfaces within some finite timescale, and the implication of recent results of surface topographies unexplained by the continuum theory. We calculate and study these coefficients as functions of the sputtering parameters for yet unreported cases of anisotropic ion energy distribution within the sputtered material. Consequently, we present phase diagrams for the significant case of anisotropic ion straggle. We observe shifts in the phase boundaries when the collision cascade geometry rotates, and we also found saturation behavior in the diagrams; in which case the boundaries become independent of the penetration depths. Our results indicate a possible origin of yet unexplained nanodot topographies arising from oblique incidence ion etching of amorphized surfaces.