Cratering in PMMA induced by gold ions:: dependence on the projectile velocity

Surface tracks induced by individual ion impacts on the surface of poly(methyl methacrylate) thin films are investigated for Au ions of different velocities v (from 0.02 to 1 MeV/u). The incident ions hit the surface at 79 degrees to the surface normal. inducing particle ejection (a crater) and surface plastic deformation (a hillock) close to the zone of impact. Crater and hillock dimensions were measured using scanning probe microscopy in the tapping mode. Typical craters for 197 MeV impacts are 22 nm wide. 60 nm long, and 10 nm deep. For 20 MeV ions average dimensions are: 20 nm (width). 35 nm (length), and 2.5 nm (depth). Crater length and depth. as well as hillock length and height increase with projectile velocity up to I: around 0.7 cm/ns and tend to saturate for higher velocities. Crater width, however, varies very weakly with projectile velocity. The total mass of ejected particles per,MeV ion impact, Y. is estimated to be around 3 x 10(6) u for 197 MeV Au ions and of the order of 3 x 10(5) u for 20 MeV Au ions. A power fit to the data gives Y proportional to v(2), but for 1 > 1 cm/ns the trend is a saturation of the total sputtering yield. (C) 1999 Elsevier Science B.V. All rights reserved.