ION CLUSTER BEAM DEPOSITION OF THIN-FILMS

The idea of using energized ionized clusters of atoms to control the quality of thin film deposition is due to Takagi and his associates at Kyoto University. They have worked to develop and demonstrate the benefits of bringing selected energy and momentum to the surface of a growing solid film without involving any other atomic species than that desired in the film. This technique is related to other ion assisted deposition methods, many of which involve bombardment of a growing film with rare gas ions of selectable energy and current density to improve film density and texture. The ionized cluster beam (ICB) concept involves the supersonic expansion of the atomic vapor of the material of interest from a crucible with a small exit aperture. Species from the expansion (atoms, dimers and larger atomic clusters which may form) are subsequently electron impact ionized and accelerated to a selected energy to bombard the surface of a growing film. ICB has been thought to involve clusters as large as 500 to 1000 atoms. Very high quality films, some of them epitaxial metals on silicon, have been formed with an ICB apparatus. This article reviews the present status of ICB. Based on recent measurements, the number of large clusters is so small that it seems very unlikely they have a dominant influence on the properties of deposited films. The high quality of ICB films seems instead to be due to ion assisted processes involving atomic ions or perhaps ions of quite small clusters.