Basic Questions Related to Electron-Induced Sputtering

Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms, 1986

Morphological changes of a solid that occur under ion and atom bombardment due to erosion by sputtering have been used to produce a conical end-cap, of prescribed cone half angle, to an emitter wire. First order erosion theory, that predicts the development of this conical shape, associates morphological changes with the variation in sputter efficiency due to variation in the angle of incidence between the ion flux and target surface. Transmission Electron Microscopy (TEM) of the cone apex region reveals details associated with the spatial distribution of the sputter process on a length scale on the order of the ion penetration depth. This second order effect links the events taking place on an atomic scale within the atomic collision cascade with those features that can be predicted from the macroscopic first order theory. A competing process, such as surface smoothing due to atomic surface and volume migration, that can be radiation enhanced, limits the scale of surface roughness which develops due to the second order mechanism at an equilibrium temperature for the sputtering process.

Physical Review B, 2007

Measurements of the sputtering yield of solid O 2 by 25-240-keV H + show that it is double valued in its dependence on electronic stopping power. We propose that this is because the electronic sputtering yield is dominated by repulsion of ions in the ionization track of the projectile which, at low velocities, is augmented near the surface due to the additional ionization resulting from electron captures. This process may also be responsible for enhanced radiation damage in insulators, in particular in the production of fission tracks. Ionization effects in insulators by fast ions are evident in a wide range of areas, including sputtering, dating by nuclear fission tracks, destruction of interstellar matter by cosmic rays, and medical therapy using ion beams. Some of the most basic information on these effects has come from the study of the sputtering of condensed gases 1 and fission tracks, 2 both occurring through the conversion of the electronic energy stored in electronic excitations and electron-hole pairs into repulsion between lattice atoms or molecules. Atoms set in motion when repulsive states evolve can be ejected directly or initiate a collision cascade of recoil atoms in the solid leading to sputtering or the amorphization of the ion track. The transfer of electronic to recoils depends strongly on specific details of the material, and its description can only be done in very few cases, due to the lack of knowledge of the excited electronic states in the solid state. For this reason, the understanding of ionization effects is usually limited to their correlation to the linear energy loss or electronic stopping cross section S e = dE / Ndx as the ion enters the solid. Here, dE / dx is the energy loss per unit path length and N is the atomic density. In the case of sputtering, it is often assumed that the yield Y ͑molecules ejected per incident ion͒ is proportional to the energy deposited near the surface ͑several monolayers, depending on the material͒, which can be considered to be proportional to the energy lost by the ion in that region. However, there is evidence that parameters other than the stopping power affect the sputtering yields for a given material. Since S e increases at low E, passes through a maximum, and then falls at high E, there are two ion energies for a given value of S e and one would expect the same sputtering yield in both cases if the deposited energy was the only ion property of importance. Instead, what is found is that there is a "loop" in the Y͑S e ͒ dependence where, for the same S e , Y is double valued, being larger at low than at high ion energies, as well documented for Ar ͑Ref. 1͒ and H 2 O. In addition, Y is often not proportional to S e but depends on a higher power n ͑Y ϰ S e n ͒, with n up to 4, depending on the type of condensed gas and the type and energy of the projectile. 1 For instance, n = 1 for Ar, and n = 2 for water ice, a case of great astrophysical importance because of the role of sputtering in the erosion of surfaces and formation of atmospheres around icy bodies, such as the Galilean satellites. The quadratic dependence has been thought to signal that sputtering is dominated by the interaction of pairs of excitations, such as the screened Coulomb repulsion from ionized molecules. 2 It has also been suggested that the quadratic behavior is caused by "thermal spikes," 5 temporary hot regions caused by the energy deposition of the projectile, from which evaporation can transiently occur.

1974

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1996

We investigated the process of collision cascade propagation through the solid-vacuum boundary for two-component targets: WSi, MoSi and VSi. The surface mechanism of preferential sputtering of atoms of light component based on stronger deflection of light atoms towards surface normal when scattering by heavy neighbouring surface atoms during ejection was studied. Simulations for ejection of 450000 Si or W atoms from the surface of WSi for cos &,/Ei and l/E: initial distributions gave Si/W sputtering ratio equal to 1.29-1.55 (for I : 1 Si/W concentration ratio at the surface) giving necessary addition to the Andersen-Sigmund formula which underestimated that ratio in comparison with available experimental data. Analysis of integral energy distributions of atoms of the components gave Si/W ratio maximum equal to 3.18-5.00 for energy interval 0.0-0.4 eV. Maxima of integral energy distributions of sputtered atoms were observed at 1.8 eV for Si and 3.4 eV for W in calculations with equal binding energies for atoms of light and heavy components in good agreement with experiment. The surface mechanism was shown to be the alternative mechanism in formation of observed maxima difference with respect to nonidentity of binding energy values for atoms of components proposed by Szymonski [Phys. Lett. A 82 (1981) 2031. The two-cone structure of ejection vs. initial polar angle for Si atoms sputtered was revealed and explained. Results obtained gave the new approach to solve the inverse problem of reconstruction of surface composition in low dose SNMS and showed that the surface mechanism of preferential sputtering is to be accounted for in analytical calculations of surface composition. Ejection was simulated in the plane containing the neighbouring surface atom.

X-Ray Spectrometry, 2011

We report new measurements of the surface ionization (0) for Ge Kα and Lα X-rays on an Fe substrate, Ga Kα and Lα X-rays on an Ni substrate, and Au Mα X-rays on Si, W and Ta substrates. The measurements were performed using the tracer technique for electron incident energies ranging from the ionization threshold up to about 40 keV. Improvements in sample preparation and in the X-ray measurement technique enabled surface ionization values to be obtained with uncertainties less than 2%. Simulations of (0) for the studied cases were performed using the general-purpose Monte Carlo (MC) code PENELOPE. Experimental measurements are compared with simulation results and with the results of several predictive formulae widely used in electron probe microanalysis and Auger electron spectroscopy (AES). The accuracy of the reported measurements makes it possible to assess the reliability of the different calculations for incident electron energies below 10 keV.

Surface Science, 1987

It is shown that deviations of the dependence of sputtering yield on the incident angle, 0, from the standard (cos 0))' dependence can be indicative of a depth dependence in the net energy deposition in the surface region. Information about the sputter ejection mechanism is shown to be obtained only secondarily. We give expressions that allow one to relate the measured dependence of the yields on angle of incidence to the dependence of energy deposition on depth. This is used to analyze results for condensed gas sputtering and heavy ion desorption of organic molecular ions. This analysis indicates that the spatial distribution of excitations produced by the secondary electrons is important in determining these yields. This is also confirmed by observed differences in the yields for transmission and back-sputtering which is a closely related effect.

arXiv (Cornell University), 2021

Many computational models have been developed to predict the rates of atomic displacements in two-dimensional (2D) materials under electron beam irradiation. However, these models often drastically underestimate the displacement rates in 2D insulators, in which beam-induced electronic excitations can reduce the binding energies of the irradiated atoms. This bond softening leads to a qualitative disagreement between theory and experiment, in that substantial sputtering is experimentally observed at beam energies deemed far to small to drive atomic dislocation by many current models. To address these theoretical shortcomings, this paper develops a first-principles method to calculate the probability of beam-induced electronic excitations by coupling quantum electrodynamics (QED) scattering amplitudes to density functional theory (DFT) single-particle orbitals. The presented theory then explicitly considers the effect of these electronic excitations on the sputtering cross section. Applying this method to 2D hexagonal BN and MoS2 significantly increases their calculated sputtering cross sections and correctly yields appreciable sputtering rates at beam energies previously predicted to leave the crystals intact. The proposed QED-DFT approach can be easily extended to describe a rich variety of beam-driven phenomena in any crystalline material.

Surface Science, 1989

Relative yields for chemical sputtering are commonly correlated with combinations of work function, ionization potential and electron affinity values. Here, we show that they correlate equally well with electronegativity values, as is expected for the post-impact ionization of sputtered neutral molecular species. Further, such plots have slopes whose absolute values are independent of the sign of the sputtered ion and of experimental conditions, as required by theory.

Applied Surface Science, 1998

Vacuum, 2010

In this paper, we propose a new approach to determine the adjustable parameters of differential elastic cross-section of Bentabet et al. approximation. The resultant approximation is an analytical expression that can be used to study the interaction of electron beams with solid targets. This approximation is applied for the energy range up to 100 keV, to calculate some quantities such as: the mean penetration depths, the mean number of wide angle collisions and the backscattering coefficient (BSC) of Al, Cu, Ag and Au semi-infinite solid targets. BSC was calculated by using both the Monte Carlo method and the Vicanek and Urbassek analytical model. The obtained results are compared with the experiment and good agreement is remarked.