Evolution of step-terrace (vicinal or stepped) structure of cleaned c-plane sapphire substrates irradiated with low-pressure air plasma (18 W/12 MHz) was studied with atomic force microscopy (AFM). Depending on plasma irradiation time and post-annealing treatment, original structure with uniform terrace width and sharp steps undergoes distinct morphology changes. With longer plasma irradiation up to 30 mm, we observed pairing of neighboring terraces into alternating wider and narrower terraces, steps roughening and terrace etching, and "step-terrace free" morphology with etched pits which are stable against annealing. These phenomena are discussed in terms of surface diffusion and chamber temperature effects. The findings reported here will have important implications for plasma modification and contamination control of sapphire substrates. (C) 2013 Elsevier B.V. All rights reserved.