Angular distributions of the particles sputtered with Ar cluster ions

This paper discusses angular distributions of sputtered particles from the target in cluster ion beam irradiation. For a copper target, the angular distribution of Cu particles with Ar cluster ion beams bombarding at a normal incidence was found to follow under-cosine law, whereas for Ar monomer ions the distribution follows the cosine law, as predicted from the linear cascade collision theory. From our molecular dynamics simulations, the sputtering mechanism with Ar cluster ions is as follows. At the impact of a cluster on a target a crater is formed. Numerous atoms acquire high lateral momentum and are ejected in the lateral direction. Subsequently, these particles cause an under-cosine distribution, and thus the behavior is drastically different from that with monomer ion bombardment. Experimental results show that the angular particle distribution is not strongly dependent on the incident angle above 10 degrees, but is very sensitive to the incidence angle below that. Microscopic images of a trace observed with scanning tunneling microscopy (STM) reinforce these results. (C) 1998 Elsevier Science S.A. All rights reserved.