Dependence of surface smoothing, sputtering and etching phenomena on cluster ion dosage

The dependence of surface smoothing and sputtering phenomena of Si (1 0 0) solid surfaces irradiated by CO2 cluster ions on cluster-ion dosage was investigated using an atomic force microscope. The flux and total ion dosage of impinging cluster ions at the acceleration voltage of 50 kV were fixed at 109 ions/cm2 s and were scanned from 5x1010 to 5x1013 ions/cm2, respectively. The density of hillocks induced by cluster ion impact was gradually increased with the dosage up to 5x1011 ions/cm2, which caused that the irradiated surface became rough from 0.4 to 1.24 nm in root-mean-square roughness (σrms). At the boundary of the ion dosage of 1012 ions/cm2, the density of the induced hillocks was decreased and σrms was about 1.21 nm, not being deteriorated further. At the dosage of 5x1013 ions/cm2, the induced hillocks completely disappeared and the surface became very flat as much as σrms=0.7 nm. In addition, the irradiated region was sputtered as deep as 5.4 nm. From the experiment of isolated cluster ion impact on the Si surfaces, it was observed that the induced hillocks nm high had the surfaces embossed at the lower ion dosages. The surface roughness was slightly increased with the hillock density and the ion dosage. At higher than a critical ion dosage, the induced hillocks were sputtered and the sputtered particles migrated from the side of the hillocks to the valleys among the hillocks in order to fill. After prolonged irradiation of cluster ions, it was observed that the irradiated region was very flat and etched. A simple phenomenological model in terms of dosage-dependent sequential surface embossment, sputtering, smoothing and etching processes was suggested to explain these cluster-ion impact interactions with the Si surfaces at various dosages