SIZE DEPENDENCE OF BOMBARDMENT CHARACTERISTICS PRODUCED BY CLUSTER ION BEAMS T. SEKI, M. TANOMURA, T. AOKI, J. MATSUO and I. YAMADA Ion Beam Engineering Experimental Laboratory, Kyoto University, Sakyo Kyoto, Japan ABSTRACT Cluster ion beam processes provide new surface modification techniques, such as surface smoothing, high rate sputtering and very shallow implantation, because of the unique interactions between cluster and surface atoms. To understand interactions with cluster and surface, Scanning Tunneling Microscope (STM) observations have been done for single impact traces. Highly Oriented Pyrolitic Graphite (HOPG) surfaces were bombarded by carbon cluster ions (Va_<300kV), and large ridges and craters have been observed as a result of single cluster ion impact. The impact site diameters are proportional to the cluster size up to 10 atoms, and increase drastically for cluster sizes above 10. This indicates that non-linear multiple collisions occur only when a local area is bombarded by more than 10 atoms at the same time. INTRODUCTION A cluster is an aggregate of a few to several thousands atoms. We have been accelerating cluster ions to various targets. Because a local area is bombarded by many atoms constituting a cluster ion, high-density energy deposition and multiple- collision are realized. Because of the interactions, cluster ion beam processes can produce unusual new surface modification effects, such as surface smoothing, high rate sputtering and very shallow implantation [1-4]. Large impact ridges and craters have been observed on solid surfaces [5,6], when Ar cluster ion beams with a size of more than 100 atoms were irradiated on solid surfaces. Such a large crater, which has not been observed on monomer ion bombardments, is caused by high-density energy deposition to the surface and multiple-collision between cluster and surface atoms. In this work, we investigated the threshold size of clusters to find such a difference between monomer impacts and cluster impacts by means of STM. We believe that the threshold size gives us information to reveal the mechanism of the cluster-surface interaction. Fullerenes were frequently used as a source of carbon cluster beams. [7-9]. Smaller sizes of Ar clusters, less than 100, can't be generated, but smaller sizes of carbon clusters, up to 70, can be generated as a consequence of cracking of the fullerene. These small clusters are ionized and subsequently accelerated to high energy. In order to observe cluster-surface interactions, it is important to investigate a single trace formed by a cluster impact on a solid surface. Especially the size dependence of the trace diameter is most important, because the cluster size, which is a unique parameter of the cluster ion beam, causes unique interactions which occur between cluster atoms and surface atoms. We have obtained clear images of a single trace formed by a Cn ion impact and investigated the mechanism of the cluster-surface interaction. 93 Mat. Res. Soc. Symp. Proc. Vol. 504 01998 Materials Research Society