Hans Henrik Andersen

Sputtering-yield studies on silicon and silver targets

Abstract The sputtering yield of vacuum-deposited silicon and silver targets has been measured with 15 different 45-keV ions throughout the periodic system. The dependence of the yield on the projectile atomic number follows rather closely Sigmunds prediction, especially for silicon. The self-sputtering yield of silver was measured in the 30–500 keV energy range. Similar to what is known to be the case for other heavy particles on silver, the maximum of the yield was found to be much more pronounced than predicted by Sigmunds theory. This effect, together with small systematic deviations in the Z1 dependence, is explained as being caused by non-linear effects in very dense collision cascades. This point of View is strongly supported by a comparison of the sputtering yield per atom for irradiation with atomic and molecular ions.

The Z1 dependence of heavy-ion sputtering yield in copper

Abstract The sputtering yield of copper has been measured with 21 different 45-keV ions as projectiles. The measurements were performed by means of an oscillating quartz micro-balance. This allows the sputtering yield to be accurately measured even if only 40–50 A of the target material was sputtered away. It was found that the measured sputtering yield depended sensitively on the previous irradiation of the target. We therefore preferred to measure all sputtering yields relative to copper self-sputtering. This ratio of sputtering yields was found to be insensitive to previous bombardments and to agree well with Sigmunds theory, although small systematic disagreements are found. The overall agreement implies firstly that the modifications of the target during bombardment only affect the ejection conditions at the surface, and secondly that any Z1 oscillations of the nuclear stopping power are less than 10 per cent.

Angular distribution of particles sputtered from Cu, Pt and Ge targets by keV Ar+ ion bombardment☆

Abstract Measurements of angular distributions of material sputtered from polycrystalline copper and platinum and amorphous germanium targets by irradiation with argon ions are reported. The beam energy was varied from 1.25 keV to 320 keV (for Cu and Pt upwards from 10 keV only). All targets yielded an angular distribution more outward peaked than the cosine predicted by collision-cascade theory. The germanium result were well-fitted by cos n distributions with n varying from 1.3 at the lowest to 1.6 at higher energies. The copper and platinum targets yielded distributions of the same general shape superimposed with distinct shoulders indicating preferential ejection along nonrandomly distributed close packed directions typical for a polycrystalline textured target. The measured distributions for germanium are in excellent agreement with the results from recent computer simulations of sputtering.

Surface segregation during alloy sputtering and implantation

Abstract The angular distribution of material sputtered from a two-component system carries information on concentration gradients close to the target surface. The surface layer will preferentially reduce that part of the flux from deeper layers, which exits from the target at angles far away from the surface normal. If a concentration gradient exists the element being depleted from the very surface will hence be emitted with a more forward-pointed angular distribution than that of the component in which the surface is enriched. An earlier setup for measurements of differential angular distributions has been improved to give higher sensitivity and reproducibility of measurement. The sputtered material is collected on cylindrically mounted thin carbon collectors and analysed with Rutherford backscattering. The setup has been used to investigate surface segregation in sputtered and ion-implanted alloys. Copper targets implanted to saturation with 45 keV Bi + at 77 K are found to have weak copper segregation at the surface. Alloy samples sputtered with argon at energies higher than 20 keV are found to have the weaker-bound component segregated to the surface (Ag from AgAu, Cu from CuPt, Au from Cu 3 Au, Pd from Ni 5 Pd, and Ni from NiPt) even at 77 K, where thermal segregation is usually prohibited. The segregated component is exactly the one in which the surfaces are usually assumed to be depleted of due to preferential sputtering. Chemical driving forces may be utilized to invert the segregation. For example oxygen will drive Ni to the surface instead of Pd from a Ni 5 Pd sample.

Computer simulations of atomic collisions in solids with special emphasis on sputtering

Computer simulations of atomic collisions in solids are traditionally divided into fully interacting or molecular dynamics (MD) simulations on the one side and simulations based on the binary collision approximation (BCA) on the other. The historical development of both branches is followed and other dichotomies, viz. between static and dynamic target models, and between models using crystalline and amorphous targets, are introduced. The influence of the main input parameters, viz. interatomic potentials, surface-and bulk-binding energies and inelasticity, is discussed before selected results are treated. Here, results for nonlinear effects, clusters, fluctuations and for angular distributions are presented. The review is concluded with a discussion of the influence of computer developments on future simulations.

Measurements of angular distributions of sputtered material as a new tool for surface-segregation studies: Segregation in CuPt alloys

Abstract The angular distribution of copper and platinum sputtered by argon ions from polycrystalline CuPt targets at 77 K has been measured by a collection technique. The argon-ion energy has been varied from 1.25 to 320 keV. For bombardment energies above 20 keV, differences in the angular distributions for the two elements indicate a strong copper segregation to the surface during bombardment.

Sputtering from atomic-collision cascades

Abstract Yield data are reviewed for metal, semiconductor and insulator targets and found to be in good agreement with the predictions of transport-theory-based analytical results provided the non-linear (spike) regime and electronic sputtering are excluded. The surface-binding energies to be used in connection with yield calculations are discussed. Angular distributions from elemental targets, isotopic mixtures and binary alloys are analysed and the “missing-plane” surface-scattering model discussed and found to give a possible explanation of the data.

The dose dependence of 45 keV V+ and Bi+Ion sputtering yield of copper

Abstract The sputtering yield of 45 keV V+ ions on copper is found to decrease with increasing dose even for very large doses, whilst the yield of 45 keV Bi+ ions on copper increases with increasing dose until approximately ten times the projected range of the Bi+ ions has been sputtered away. The fate of the incoming V+ and Bi+ ions was investigated. A radiotracer technique showed that vanadium accumulated on the target surface, whilst bismuth diffused into the target, as was seen by means of Rutherford-scattering of light ions.

The dependence of sputtering efficiency on ion energy and angle of incidence

Abstract Earlier measurements of sputtering efficiency of polycrystalline targets (fraction of impinging ion energy leaving the target through sputtering and backscattering) have been extended to higher energies. Lead and copper targets were bombarded with several different projectiles with energies between 80 and 1200 keV. The sputtering efficiency decreases with increasing energy. This decrease is ascribed to the combined influence of changes in the scattering cross section with energy, and to electronic stopping. The results may be described as a function of the mass ratio M 2/M 1 and the reduced energy ∊ only. The sputtering efficiency was measured as a function of angle of incidence of the bombarding ions. To ensure complete collection of sputtered and backscattered particles, it was possible to cover only the region of incidence angle from 0° to 45°. Targets of copper, silver, and lead were investigated with 17 different ion-target combinations. The sputtering efficiency increased with angle of inci...

A survey of sputtering-yield data for plasma-wall interaction calculations

Abstract An extensive survey of the literature has been conducted with the aim of presenting experimental sputtering-yield data for elemental target materials in a standardized way. The result of the survey encompasses some 7000 data points and will, together with a review of the data, be published as part of a forthcoming monograph. This collection contains numerous data for controlled-fusion devices. Some of these data sets will be discussed here and compared to the results of current theory and semiempirical yield predictions. As a conclusion, broad guidelines for designers of plasma devices as to which sputtering-yield data should be used for specific applications will be indicated.