Controlled Sputtering of Metals by Low-Energy Hg Ions

Abstract

Sputtering has been studied under well controlled conditions, i.e., low gas pressure (1 micron), defined energy (up to 300 ev) and angle of incidence of the bombarding ions, and high ion-current density (10 ma/${\mathrm{cm}}^{2}$), by immersing the target in a low-pressure Hg plasma of high density created in a pool-type vacuum arc.It turned out that threshold energies (minimum ion energy for sputtering to ensue) are considerably lower under oblique than under normal incidence, and the atoms are ejected away from the direction of incidence. Thresholds were measured for twenty-three metals under normal incident Hg-ion bombardment. They follow, without exception a simple law: The product of the momentum transferred at threshold from ion to target surface atom and the sound velocity of the target material is proportional to the heat of sublimation of the target material. The threshold energies to be expected accordingly in the case of rare gas ion bombardment under normal incidence are tabulated. The yield (atoms per ion) $\mathrm{vs}$ ion energy was carefully measured for the case of polycrystalline Pt. When metal single crystals were sputtered, it was discovered that atoms are preferentially ejected in the directions of closest packing, that is [110] in fcc, [111] in bcc and diamond lattice. Deposits sputtered from plane low-index surfaces of single crystals therefore form characteristic patterns. Their study reveals more details: At low ion energy, only such atoms are sputtered which have no obstructing neighbors in the way of a close-packed direction. At higher ion energy, additional atoms are set free from positions where neighbor atoms interfere. Such atoms deviate in a characteristic way from the directions of close packing. Most atoms sputtered in close-packed directions which are parallel to the surface are trapped again, causing growth of oriented hillocks. The possibility for crystal growth by sputtering was demonstrated in another experiment. Some characteristic features of the etch effects caused by sputtering are described.The basic process in sputtering at low ion energies is one of momentum transfer. The important parameters with respect to the gas discharge are ion energy, angle of incidence, and atomic weight of ion; on the target side they are the atomic weight, the elastic constants, crystal structure and orientation, the heat of sublimation, dislocations, and surface roughness. Many details need durther clarification, and the studies are being continued.