Roughening and smoothing behavior of single crystal Si by low energy Ar+ ion bombardment

We examined the dependence of smoothing or roughening behavior on the initial surface roughness of single crystal Si. An atomically flat cleaved Si surface of roughness 0.038 nm rms and a Si wafer substrate roughness of 0.12 nm rms were sputtered by 0.2-3 keV Ar+ ion beam at normal ion incidence. Surface morphology was observed by atomic force microscope (AFM) under DFM mode. Result shows that the rough surface (R = 0.12 nm rms) becomes smooth (R = 0.068 nm rms) and the smooth surface (R = 0.038 nm rms) becomes rough (R = 0.068 nm rms) due to low energy Ar+ ion beam sputtering process and both finally saturates at 0.068 nm rms. The saturated roughness Rsat depends on ion beam energy and increases with increasing beam energy. To understand the roughening mechanism, we studied the dynamic scaling theory and measured the roughness exponent α and growth exponent β. The values of α = 1.03 and β = 0.28 are in agreement with the Cuerno's one dimensional simulation of the Linear diffusion equation with noise, that corresponds to the initial stage of dynamic scaling of ion bombardment.