Cascade Sputtering Research Articles

Excited atom emission under ion bombardment of metal surfaces covered by alkaline films

The photon yield, Y, emitted from excited Al and Cu atoms sputtered and scattered under ion bombardment of clean and Cs-covered surfaces has been measured. It is shown that the value of Y 1 for excited sputtered atoms is independent of Cs-coverage degree. By contrast, the value of Y 2 for excited scattered atoms drastically increases (by a factor of 10) with Cs coating. Such sharp differences in the behavior of Y 1 and Y 2 values may be explained by the different characteristic times for the cascade sputtering (ρ <≈ 10 −13 s) and scattering (ρ ≈ 10 −15 s) processes. We propose that the incident ion generating a sputtering cascade induces the desorption of adsorbated Cs atoms during the period characteristic for the scattering time. Therefore, the excited state of sputtered atoms unlike that of the scattered ones is formed by electron exchange with the local clean surface area.

Fundamental Concepts of Ion-Solid Interactions: Single Ions, 10−12 Seconds

An energetic ion undergoes two major mechanisms of energy loss: (1) screened Coulomb collisions with target atoms (nuclear stopping), and (2) interactions with bound or free electrons in the solid (electronic stopping). This article discusses how these two energy-loss processes—and related quantities such as the ion range, collision cascade behavior, sputtering, and spike effects—depend on the experimental parameters of beam energy, atomic number, and target density. We restrict ourselves to an individual incident ion and a time scale (~10−12 s) sufficient for the cascade to fully develop (Figure 1), but short enough for subsequent quenching or diffusion processes to be ignored. What happens at longer times and higher ion f luences, due to diffusion and cascade overlap phenomena, is treated in the article by Brown and Ourmazd in this issue of the MRS Bulletin.Energy-Loss ConceptsFigure 2 summarizes the velocity (ε1/2) dependence of the two energy-loss processes, nuclear stopping (dε/dρ)n and electronic stopping (dε/dρ)e, in terms of the dimensionless Thomas-Fermi (TF) energy, ε, and length, ρ, units derived by Lindhard. The energy unit ε is defined as the ratio a/b where a is the TF screening length (typically, ~0.01 nm) and b is the distance of closest approach in an unscreened head on collision. The following simple relationship exists between TF and lab energies.where M1, Z1, M2, and Z2 are the masses and atomic numbers of projectile and target atoms.

AES study of amorphous Fe 100− xB x ( x = 14–25 at.%) alloys

Auger electron spectroscopy in conjunction with argon ion sputtering was used to study the sputtering behaviour of melt-spun amorphous Fe 100− x B x ( x = 14–25 at.%) alloys. Measurement of equilibrium surface composition using the low energy peak of iron Fe(47 eV, MVV) indicated an enrichment of boron in the sputter altered layer in all alloys for argon ion energies of 1 keV and 3 keV. The corresponding sputtering yield ration ( Y Fe Y B ) had a value between 1.3 and 1.6. The surface composition, calculated by using the high energy iron peak (651 eV, LMM), indicated depletion of boron in the sub-surface region in comparison to that measured by using the low energy peak. It appears that binding effects rather than mass play a significant role in determining the preferential sputtering behaviour of these alloys and the possible sputtering mechanisms could involve cascade sputtering and/or surface segregation. The plot of surface composition or the sputter yield ration versus boron atom fraction in the bulk showed a break at 18 at.% boron. This is attributed to the known structural differences in FeB amorphous alloys. Analysis of native oxide films on these alloys showed enrichment of boron near the oxide-alloy interface and the film thicknesses were approximately in the range 4–6 nm. It was inferred from an analysis of the low energy Auger peaks that the oxide films consisted of Fe 3+, Fe 2+ and B 3+ species.

Study of amorphous and crystallised Fe 75B 25 by Auger electron spectroscopy and X-ray photoelectron spectroscopy

Auger electron spectroscopy (AES) in conjunction with argon ion sputtering was used to study the sputtering behaviours of an amorphous Fe 75B 25 alloy and its corresponding crystallized alloy forms, namely the single-phase metastable Fe 3B and a two-phase mixture of α-Fe and Fe 2B. Bombardment with the energetic argon ions of energy 1 and 3 keV resulted in the establishment of a boron-rich sputter altered layer in all the alloy forms. The magnitude of enrichment of boron was relatively much higher and similar in the amorphous and the metastable Fe 3B alloys than in the alloy consisting of a two-phase mixture of α-Fe and Fe 2B. The sputter-induced compositional differences in these alloys have been rationalized in terms of their different structural states and are suggestive of a similar local atomic order in the structures of the amorphous and the corresponding crystallized Fe 3B. The binding effects appeared to play a significant role in determining the sputtering behaviours of these alloys and the probable sputtering mechanisms would include cascade sputtering and/or surface segregation. The evidence for the occurrence of bombardment-induced surface segregation is provided by a simple model calculation based on the values of the concentration ratio of iron to boron obtained by using the low energy (47 eV) and the high energy (651 eV) Auger peaks of iron. Native oxide films formed on these structurally different alloy forms were also characterized using AES and X-ray photoelectron spectroscopy (XPS). The oxide films were enriched in boron, the enrichment being maximum near the oxide/alloy interfacial region and incorporated Fe 3+, Fe 2+, and B 3+ species. The oxide films on amorphous Fe 75B 25 and Fe 3B were similar in thickness (3.6 and 3.3 nm) while a relatively thicker film (9.7 nm) was formed on the alloy consisting of α-Fe and Fe 2B.

Fast atom and beam etching assisted by a flourine-containing radical flow

A novel technique of dry etching single crystal Si and thermally grown SiO 2 films is investigated. It consists in the simultaneous irradiation of samples by independently controlled flows of flourine-containing radicals and inert gas ions or fast atoms. The presence of a flourine-containing flow leads to a significant increase of the silicon chemical sputtering yield as compared to physical inert gas ion sputtering. The obtained dependences of the etch rate on ion energy and mass cannot be completely accounted for within the scope of the model of collision cascade sputtering of loosely bound silicon fluorination products.

Current views on electronic and cascade sputtering of alkali halides

Contrary to elemental materials, sputtering of alkali halides occurs due to a few distinct mechanisms which, in general, could be classified as electronic- and cascade-type processes. In recent years, these processes have been the subject of numerous experimental studies. In particular, the development of such refined experimental techniques as laser-induced fluorescence spectroscopy and mass-selected time of flight spectroscopy has provided new data on the relative balance between electronic and cascade sputtering due to ion bombardment and on the characteristic features of sputtering with electrons. In this paper the results from various laboratories will be reviewed with particular emphasis on clarifying the following questions and controversies: — anticorrelation between electronic sputtering and the luminescence of alkali halides; — surface structural and compositional changes due to electronic sputtering; — the temperature dependence of the sputtering yield; — directional emission of halogen atoms sputtered with electrons: — electronic versus cascade contribution to sputtering due to ion bombardment. A short summary will also be given of the theoretical model describing the excitonic mechanism of defect production and electronic sputtering, elucidating the theory with experimental observations.

Stochastic modeling of cascade damage — collisional processes

A stochastic method of modeling was developed for the processes containing cascade collisions, and applied to the problems of the diffusion of impurity atoms and the evolution of precipitates under irradiation producing cascades. Calculations are presented for the first and second moments of the fluctuating loss of impurity atoms constituting precipitates which is produced by cascade sputtering and subsequent thermal migrations. These moments, together with those related to the cascade disordering of precipitates and the transport of impurity atoms to precipitates by diffusion associated with neighboring cascades, provided Fokker-Planck equations which describe the evolution of precipitates.

Effects of ion incidence angle on the angular distributions of sputtered Zr atoms

We report results on the angular distributions of Zr sputtered from a clean polycrystalline surface for ion incidence angles of 30, 45, and 60 degrees with respect to the surface normal. The atoms are detected state-specifically using laser-induced fluorescence in a detector with a three degree angular acceptance in both the azimuthai and polar directions. The experimental angular distributions are explained by direct and cascade sputtering producing a forward peaking in a underlying cosine distribution.

Effects of ion incidence angle on the angular distributions of sputtered Zr atoms

We report results on the angular distributions of Zr sputtered from a clean polycrystalline surface for ion incidence angles of 30, 45, and 60 degrees with respect to the surface normal. The atoms are detected state-specifically using laser-induced fluorescence in a detector with a three degree angular acceptance in both the azimuthal and polar directions. The experimental angular distributions are explained by direct and cascade sputtering producing a forward peaking in a underlying cosine distribution.

Sputtering effects in Auger depth profiles of TiN thin films

TiN coatings have gained significant technological importance because of their unique properties. Quantitative determination of their composition by Auger analysis may be impeded by sputtering effects. In this paper we study to what extent such artefacts are to be expected for TiN. Several independent experiments show that the N:Ti atomic ratio is reduced at the surface by about 10%–20% with respect to the bulk. A similar magnitude is predicted by a linear cascade sputtering model. The observed selective removal of nitrogen depends mainly on the ion angle of incidence, whereas the composition and microstructure of the TiN films, as well as the ion energy and ion species (Ar + or Xe +), seem to be less important.

Sputtering of insulators

Ion and electron induced particle emission from insulators will be discussed. Ion bombardment leads to nontransient particle emission for all insulators. For oxides and low to medium ion energies cascade sputtering (momentum transfer collisions) dominates particle emission, while for alkali halides even under ion bombardment non-cascade processes dominate. However, even for oxides pronounced differences in yield, energy distribution and especially in the composition of the sputtered flux can be observed as compared to the results for pure metals. Experimental techniques such as laser-induced fluorescence spectroscopy (LIF), bombardment induced light emission (BLE), secondary ion mass spectrometry (SIMS) and quartz microbalance measurements have been employed to study these effects for oxides as well as alkali halides. For electron bombardment oxides do not show any nontransient particle emission in contrast to alkali halides, where emission is due to electronic excitation induced by the incident particles producing stable defects, which lead to the ejection of the halogen atoms either via diffusion to the surface or through focused collision sequences. In addition, thermal evaporation of the neutralized metal atoms occurs depending on target temperature and will be the rate limiting process. We will discuss differences in the energy distribution of ejected ground-state and excited atoms under electron and ion bombardment and compare the results for two exemplary representatives, i.e. Nad and CaF 2 with the case of the pure metals. In addition, the yield dependence for ground-state and excited atoms will be discussed as a function of the target temperature, which yields information on the origin of excited atoms under electron bombardment.

Analytical expressions for the internal and kinetic energy distributions of sputtered clusters and molecules

Analytical expressions are presented for the internal and kinetic energy distributions of sputtered molecules and clusters. The molecules and clusters are treated as systems built up of two identical sub-units which are activated independently in the sputter process. First general expressions are obtained from approximate momentum space integrations. Then explicit functions are given for collision cascade and thermal sputtering. The quality of the analytical expressions for the collision cascade model is evaluated by comparison with a numerical calculation of the momentum space integrals.

On the chemical sputtering of oxygen-exposed molybdenum

The sputtering of oxygen-exposed molybdenum was studied by means of mass analysis of emitted neutral and charged particles. The irradiation was performed with 8 keV Ar+ ions at temperatures of 25° and 485°C. It was found that the enhanced sputtering yield at elevated temperature during oxygen exposure is due to beam-induced desorption of MoO2 and cascade sputtering of MoO. At this temperature considerable oxygen incorporation also takes place owing to recoil mixing and diffusion.

Sputtering of frozen Xenon by keV heavy ions

Sputtering measurements of frozen Xe have been made using 20–80 keV heavy ions (N, Ar, Sb and Bi), i.e. where nuclear stopping is the dominant energy loss process. Sputtering yields ( Y) of up to 1.6 × 10 4 Xe atoms ion have been obtained. The dependence of Y on the surface deposited (nuclear) energy F D(0) was found to exhibit two distinct regimes: at low F D(0) values, Yincreases linearly with deposited energy, as predicted by cascade sputtering theory, but above a threshold value of ≈500 eV A ̊ 2/ atom the sputtering yield exhibits a much stronger (approximately cubic) dependence on F D(0). Comparison of the sputtering yields obtained by monatomic and diatomic equal velocity heavy ion bombardments (e.g. Sb/Sb 2) further confirmed this non-linear behaviour at large F D(0). Similar monatomic and diatomic light ion bombardments (e.g. N/N 2) showed no molecular enhancement, hence a linear behaviour at lower F D(0). The current results, which are obtained in the energy regime where nuclear stopping dominates, are compared to earlier measurements at MeV energies in which electronic stopping dominates. This comparison suggests that the observed sputtering behaviour of frozen Xe is complex, with significant contributions from both nuclear and electronic processes.

A review of compositional changes of multicomponent materials (especially biomaterials) induced by ion sputtering

It is well known that the ion sputtering process may modify the surface chemical composition of multicomponent materials as a result of preferential sputtering. Ion bombardment leads generally to surface enrichment of the component having a lower sputtering yield. Although the sputtering yield studies are very important, they do not contribute to understanding the mechanism of the preferential sputtering. The main reason is that the sputtering yield is the phenomenological coefficient and it depends on many factors, particularly on ion energy, its angle of incidence, target atomic mass to ion mass ratio, and target surface binding energy. For multicomponent materials, the effects of component mass difference and binding energy seem to be of fundamental importance for understanding the nature of the preferential sputtering mechanism. Another interesting problem is the influence of ion mixing and diffusion on the compositional changes of multicomponent materials. The main purpose of the article is to discuss the mechanism of preferential sputtering of multicomponent materials and to present the role of such phenomena as recoil and cascade sputtering, recoil implantation, mixing, diffusion, and segregation. Another issue worthy of discussion is the practical aspect of preferential sputtering, i.e., the influence of ion sputtering on compositional changes of biological implant materials.

Preferential sputtering of PtSi, NiSi2, and AgAu

Auger electron spectroscopy has been used to study ’’preferential sputtering’’ of PtSi, NiSi2, and 0.55 Ag–0.45 Au. The data show that Pt, Ni, and Au are enriched on the surface by either Ar+ or Xe+, but enrichment is larger for Ar+. Furthermore, the enrichment is much larger for ions of 500 vs 2000 eV energy. Various mechanisms which could lead to surface enrichment were considered, including direct knock‐off, recoil sputtering and implantation, energy partitioning, and binding energy effects in cascade sputtering. Because of discrepancies in the predicted sputter yield ratios vs ion mass and energy, these mechanisms were not considered to be correct. Rather, it was postulated that surface segregation assisted by irradiation‐enhanced diffusion was dominant and led to the surface enrichment.

On the problem of whether mass or chemical bonding is more important to bombardment-induced compositional changes in alloys and oxides

Abstract The bombardment of alloys, oxides, and halides often leads to marked compositional changes at the surface, and these changes have been attributed to an interplay of mass-dependent effects, chemical bonding, electronic processes, and diffusion. We attempt here to answer the limited question of whether, considering only alloys and oxides, mass or bonding is normally more important. The relevant theory is reviewed and extended, with mass effects being shown to be associated most explicitly with recoil sputtering and bonding effects being shown to be associated with all three of cascade sputtering, thermal sputtering, and surface segregation. As far as experimental examples are concerned, mass correlations are found to be quite unsuccessful, whereas most observations can be understood rather well in terms of bonding. Nevertheless, there is a basic problem in that the cascade component of sputtering, normally judged to be predominant, should give significantly less compositional change than is observed. Thermal sputtering would lead to more significant changes, but there is a new problem that, at least with alloys, the absolute yields are probably rather small. A combination of surface segregation with sputtering would also lead to more significant changes, but it is unclear whether segregation is rapid enough to be important in room-temperature bombardments.

Laser fluorescence spectroscopy of sputtered uranium atoms

Laser induced fluorescence (LIF) spectroscopy was used to study the sputtering of 99.8% 238U metal foil when bombarded by normally incident 500–3000 eV Ne +, Ar +, Kr + and O 2 +. A three-level atom model of the LIF processes is developed to interpret the observed fluorescent emission from the sputtered species. The model shows that close attention must be paid to the conditions under which the experiment is carried out as well as to details of the collision cascade theory of sputtering. Rigorous analysis shows that when properly applied, LIF can be used to investigate the predictions of sputtering theory as regards energy distributions of sputtered particles and for the determination of sputtering yields. The possibility that thermal emission may occur during sputtering can also be tested using the proposed model. It is shown that the velocity distribution (either the number density or flux density distribution, depending upon the experimental conditions) of the sputtered particles can be determined using the LIF technique and that this information can be used to obtain a description of the basis sputtering mechanisms. These matters are discussed using the U-atom fluorescence measurements as a basis. Both the v-parallel (the exciting laser beam is parallel to target surface) and the v-perpendicular (the laser beam is perpendicular to the target surface) number density velocity distributions were measured using the LIF Doppler shifted absorption frequencies of the sputtered ground state U(1) atoms. The results are compared with the collision cascade sputtering theory using the LIF model presented here. The relative sputtering yields for various incident ions on uranium were also measured for the first time using the LIF technique. A surprisingly high fraction of the sputtered uranium atoms were found to occupy the low lying metastable energy levels of U(I). The population of the sputtered metastable atoms were found approximately to obey a Boltzmann distribution with an effective temperature of (920 ± 100) K.

Secondary-ion emission from clean and oxygen-covered beryllium surfaces II. energy dependence

Abstract The secondary-ion energy distribution obtained by sputtering clean and oxygen-covered Be has been analyzed in terms of competing processes in secondary ion emission. The ion energy distribution N+(E) has been separated into an ionization coefficient R+(E) and a total energy distribution, N(E), i.e. N+(E) = R+(E) N(E). Experimentally, the dependence of R+(E) on both energy and oxygen coverage indicated a linear superposition of adiabatic tunneling and resonanance ionization processes from clean and oxygen-covered portions of the surface with no contributions to the secondary-ion yield from regions of intermediate coverage. Total energy distributions of sputtered Be atoms have been deduced and the principal features agree with the predictions of the collision cascade sputtering model. Variations of the energy distributions with oxygen coverage are in accord with small changes expected in the surface binding energy as a result of surface oxidation.

Secondary-ion emission from clean and oxygen-covered beryllium surfaces: II. Energy dependence

The secondary-ion energy distribution obtained by sputtering clean and oxygen-covered Be has been analyzed in terms of competing processes in secondary ion emission. The ion energy distribution N +( E) has been separated into an ionization coefficient R +( E) and a total energy distribution, N( E), i.e. N +( E) = R +( E) N( E). Experimentally, the dependence of R +( E) on both energy and oxygen coverage indicated a linear superposition of adiabatic tunneling and resonanance ionization processes from clean and oxygen-covered portions of the surface with no contributions to the secondary-ion yield from regions of intermediate coverage. Total energy distributions of sputtered Be atoms have been deduced and the principal features agree with the predictions of the collision cascade sputtering model. Variations of the energy distributions with oxygen coverage are in accord with small changes expected in the surface binding energy as a result of surface oxidation.