Surface morphology development during ion sputtering: Roughening or smoothing?

We report on STM studies of ion-sputtered surfaces, applying sputtering conditions which were shown to produce a relatively smooth surface. The height correlation function was calculated for the nickel layer in both the as-received and sputtered conditions. The large-scale roughness of the as-received specimen was reduced by ion sputtering according to expectations derived from Auger depth profiling. On the other hand, the small-scale roughness was increased due to sputtering. Self-affine scaling regions are identified, and the exponents are compared to theoretical and numerical results.