Quantitative auger electron analysis of homogeneous binary alloys: Chromium in gold

Quantitative Auger electron analysis of Cr/Au alloys with up to 20% Cr has been accomplished. The surface composition of scribed areas were compared to bulk compositions and it was shown that corrections for variation of density, escape depth, and electron backscattering must be included; these corrections change the measured surface Cr concentrations by approximately 15%. Alloy sputter yield ratios have been calculated from surface concentrations after sputtering with Ar or Ne (0.5, 1.0, 1.5, and 2.0 keV). The sputter yield ratio of Cr to Au was 0.5 at 1% Cr (significant preferred sputtering) but was near unity at 20% Cr (no preferred sputtering). The sputter yield ratio was nearly independent of ion species and ion energy. The 2 keV argon ion sputter yields for pure Cr and Au were determined to be 2.0 and 7.9 atoms/ion, respectively. However, the 2 keV argon ion sputter yield for Au in the alloys drops rapidly from 7.9 atoms/ion for pure Au, to 5 atoms/ion at 10–20% Cr. The sputter yield for Cr in alloys (5 atoms/ion) is relatively independent of composition and is 2.5 times higher than the yield of pure Cr. No simple model is known by which pure elements sputter yields could be used to predict alloy sputtering behavior.     

Mechanisms of radiation-induced surface composition modification in VCr alloys at high and low temperature

The effect of 0.5 to 3 keV argon ion sputtering on the surface composition of several alloys of vanadium and chromium has been investigated at temperatures in the range 300 to 950 K. At room temperature, sputtering depletes the alloys in chromium and the relative V/Cr sputter rate constant ratio is found to be 1.20, consistent with the ratio of the pure element sputter yields. Annealing clean alloys at temperatures in the range 550 to 670°C further enriches the surface in vanadium and also oxygen segregates to the surface. This behaviour is attributed to co-segregation of oxygen and vanadium, an interpretation supported by the observation of vanadium chemical shifts analogous to those observed in vanadium oxides, the depth profiles of the segregated surfaces and the bond energies in the VCrO system. Sputtering the alloys at temperatures in the range 410 to 620°C enriches the surface in chromium to a depth which increases with sputter annealing time. Similar behaviour has been reported on sputtering a V/Cr alloy at these temperatures but with much higher energy V⁺ ions, and has been attributed to radiation-induced segregation of chromium. The results presented in this paper offer a different interpretation. Annealing enriches the surface in vanadium which co-segregates with oxygen. Thus on sputter- annealing the surface will become depleted in vanadium at a rate which is probably enhanced by an increased sputter yield of VO over either V or Cr. This interpretation is further supported by the observation that the alloy is still depleted in vanadium when the sputtering occurs at room temperature when performed in a cycle with intervening annealing intervals.     

Investigation on surface compositions of CuNi alloy under Ar+ ion bombardment by ISS and in situ AES measurements

Surface compositions of CuNi(50 wt%) alloy under 3 keV Ar⁺ ion bombardment were measured by Auger electron spectroscopy (AES) and ion scattering spectroscopy (ISS). In situ AES measurement of sputter-deposited layer and sputtered sample surface indicated that surface composition became Ni-rich due to Ar⁺ ion sputtering at steady state, in accord with previous reports of preferential sputtering. ISS measurements with 3 keV Ar⁺ ions, however, have suggested that the composition of the outermost atom layer is not Ni-rich but slightly copperrich. This leads to the conclusion that the degree of preferential sputtering is small and that the question of preferential sputtering, particularly, the ratio of the sputtering yields, $\text{S}{}_{\mathrm{<mtext>Cu</mtext>}}\text{S}{}_{\mathrm{<mtext>Ni</mtext>}}$, based on AES measurement requires further examination.     

Characterization of oxide layers on amorphous Zr-based alloys by Auger electron spectroscopy with sputter depth profiling

Amorphous Zr-Cu-Ni-Al-[Ti, Nb] ribbons prepared by melt spinning under argon atmosphere were subjected to electrochemical investigations. Passive films developed at potentiostatic anodic polarization in sulphuric acid solution were investigated by Auger electron spectroscopy (AES) and sputter depth profiling.Changes in the shape of the Auger peaks have been analyzed by factor analysis of the spectra obtained during depth profiling. Pronounced changes in shape and position occur for the Zr, Al, and Ti Auger transitions, but not for Cu and Ni. At least three different peak shapes for O(KVV) were found and attributed to different oxygen binding states. The alloy composition has no significant effect on the thickness and composition of the oxide layer.In multi-element alloys preferential sputtering is a common phenomenon. In the steady state of sputtering, a significant depletion in Cu is found. At the oxide/metal interface, a distinct enrichment of copper is found for all alloys and treatments. The degree of this Cu enrichment depends on the pretreatment. It is higher for the electrochemically-passivated samples than for samples with oxide layers grown during melt spinning.     

AES studies of surface composition of AgCu alloys

The influence of pretreatments like sputtering, annealing and cleaving on the surface composition is important in quantitative Auger electron spectroscopy. The present paper deals with this influence for the binary alloy system AgCu with 100, 95, 80, 60, 50, 40, 20, 5, and 0 at% Ag. Sputtering causes an enrichment of Cu on the surface due to the higher sputter coefficient of Ag. In contrast cleaving yields an enrichment of Ag due to the lower tensile strength of Ag. The surface composition obtained by scribing is used as standard for the bulk composition because this technique is independent of parameters like sputter coefficient or tensile strength. The results are compared with previous measurements of homogeneous alloy systems.     

Quantitative auger electron spectroscopy analysis of AgPd and NiPd alloys

The relationship between peak amplitude in derivative Auger spectra and alloy composition was investigated for AgPd and NiPd alloys. Measurements were performed on polished and sputter etched samples as well as on samples fractured in vacuo. For clean alloy surfaces and in the absence of sputtering, a linear relationship between peak amplitude of a given alloy component and atomic concentration was observed. Relative Auger peak amplitudes measured on sputter cleaned surfaces were different from those measured on fractured surfaces. Results were interpreted in terms of a simple model considering surface enrichment of the alloy component with the lower relative sputtering yield.     

Backscattering correction for quantitative Auger analysis: II. Verifications of the backscattering factors through quantification by AES

The validity and utility of the backscattering correction factors obtained from Monte Carlo calculations for quantitative analysis by Auger electron spectroscopy (AES) were examined through practical quantification of surface concentrations of binary alloys. Quantifications were attempted, first, to access the surface composition of a sputter-deposited NiPt layer, which is probably the most appropriate test-sample with known surface composition for surface analysis. The quantification by AES has led to the result that the surface composition of the layer agrees well with the bulk composition of the sputtered NiPt alloy, as expected. The composition of a sputtered AuCu alloy surface was, then, examined according to the same correction procedure as for the NiPt layer, leading to the confirmation that no preferential sputtering is observed for AuCu alloys by AES as Färber et al. reported.     

The backscattering factor for systems with a non-uniform surface region: Definition and calculations

It has been recently shown that the backscattering factor (BF) in Auger electron spectroscopy (AES) noticeably depends on the in-depth structure of the surface region. This is a particularly important problem in sputter depth profiling monitored by AES since the signal intensity cannot be described with a single BF value. The BF depends on the removed amount of material and thus varies with sputtering time.In the present work, the definition of the BF is generalized to extend its applicability to systems with an in-depth composition profile. The generalized definition of the BF was applied to the special case of a depth profile, i.e. a buried thin layer. It has been shown that the BF for this case is expressed by the excitation depth distribution function (EXDDF) which is equivalent to the “Phi-Rho-Z” function used in electron probe microanalysis (EPMA). Different algorithms for calculating the BF are discussed. Calculations of the BF for buried layer were performed for the Ag M₄N₄₅N₄₅ Auger transition in a thin layer of silver located at different depths in three matrix materials: Si, Cu, and Au. It was found that, indeed, the BF noticeably varies with the depth of the layer in the analyzed volume, although the extent of this variation depends on the matrix material. For Si, the variation is observed for the lowest primary beam energies considered, i.e., 1 and 2 keV. For Cu, a distinct depth dependence of the BF is visible at 10 keV and lower energies, while for Au, the BF varies with depth even at the highest considered energy, i.e. 30 keV.     

Auger electron spectroscopy investigation of sputter induced altered layers in SiC by low energy sputter depth profiling and factor analysis

The thickness of the altered layer created by ion bombardment of the 6H–SiC single crystal was determined by means of Auger electron spectroscopy (AES) depth profiling in conjunction with factor analysis. After pre-bombardment of the surface by argon ions with energies 1, 2 and 4 keV until the steady state, the depth profiles of the induced altered layers were recorded by sputtering with low energy argon ions of 300 eV. Since the position and shape of the carbon Auger peak depend on the perfection of the crystalline structure, they were used for depth profile evaluation by factor analysis. In this way the depth profiles of the damaged surface region could be estimated in dependence on the ion energy. As a result, the thickness of the altered layer of SiC bombarded with 1, 2 and 4 keV Ar ions using an incident angle of 80° as well as the corresponding argon implantation profile could be measured.     

Preferred sputtering on binary alloy surfaces of the AlPdSi system

Auger spectroscopy has been used to investigate the behavior of preferred sputtering on surfaces of homogeneous AlPd, SiPd and AlSi alloy films. These combinations of alloys were chosen for studying the effects of mass and bonding differences on preferred sputtering. Experiments have been carried out using a 1 keV Ar ion beam over a range of alloy compositions. Our results can be summarized as follows: (a) The preferred sputtering of these binary alloys cannot be predicted according to the sputter yields of individual elements, e.g. both Al and Si have been observed to be removed preferentially relative to Pd although pure Pd has a higher sputter yield, (b) In the alloys studied, mass difference appears to dominate over bonding difference in controlling the preferred sputtering behavior since the extent of preferred sputtering of Al and Si relative to Pd is about the same. This observation is interpreted on the basis of the binary alloy sputtering theory formulated by Andersen and Sigmund. (c) Judging from the composition change of the sputtered surface, there is no evidence for formation of compounds with specific compositions as a result of preferred sputtering in the AlPd and SiPd alloys investigated.     

Angular distributions of Au and Cu atoms sputtered from Au-Cu alloys by keV Ar+ ion bombardment

Angular distributions of Au and Cu atoms sputtered from Au-Cu alloys under 3 keV AR⁺ ion bombardment were measured to understand the preferential sputtering. The surface composition of sputter-deposited Au-Cu films on substrates mounted at different ejection angles was analyzed by Auger electron spectroscopy and electron probe microanalysis. Although the result indicated that the proportion of sputtered Cu atoms to the Au atoms in the Au-Cu alloy depends on the ejection angle, marked enhancement of the lighter component in the direction normal to the surface has not been observed in spite of the larger mass ratio of the constituent atoms of the Au-Cu alloy.     

The surface composition of the gold-palladium binary alloy system

The surface composition of the AuPd alloy system has been determined by Auger electron spectroscopy. Measurements were performed on polished polycrystalline alloy foils. After cleaning, the intensities of the 71 eV and 2024 eV gold Auger transitions, and the intensity of the 330 eV palladium transition were measured, and then converted to atom concentrations in the surface layer. The surfaces of the annealed samples were found to be significantly enriched in gold with respect to the bulk. This result disagrees with the regular solution theory prediction. After extensive sputtering of the AuPd alloys with 1.5 keV Ar⁺ ions, a slight surface enrichment with palladium was found, as predicted by the simple theoretical model for sputtering.     

三元合金Cu76Ni15Sn9的离子溅射研究

在２７ｋｅＶ Ａｒ⁺离子轰击时,用收集膜技术结合俄歇谱仪（ＡＥＳ）,研究了三元合金Cu₇₆Ni₁₅Sn₉系统的择优溅射行为。同时使用扫描电子显微镜（ＳＥＭ）与电子探针微分析（ＥＰＭＡ）．观察了靶点表面形貌变化并测定了形貌特征微区的合金组份原子的相对百分浓度。结果表明,Ｃｕ原子较Ｎｉ原子、Ｎｉ原子较Ｓｎ原子,在所测定范围（０─６０°）内择优发射。最后讨论了靶点表面形貌特征和“元素局域富集”现象对择优溅射过程的影响。 关键词：     

Effects of enhanced diffusion on preferred sputtering of homogeneous alloy surfaces

The effects of enhanced diffusion caused by sputter damage on the kinetics of preferred sputtering of alloy surfaces have been analyzed. The diffusion flux is specifically included in considering the mass balance within the altered layer and on the sputtered surface. An analytical solution has been derived by solving the two mass-balance equations simultaneously under certain conditions. The solution is used as the basis of an interative method to obtain exact numerical solutions as a function of sputtering time. The analysis enables one to define an altered layer with respect to the steady state and to express its thickness in terms of the sputter yields and the diffusivity. Illustrative results showing the evolution of the altered layer during sputtering are given for a typical binary alloy surface. Implications of this analysis and possible future experiments are discussed.     

The effect of particle size on the surface composition of microcrystalline alloys

Thin films a Ni?1at%Pd alloy, ranging in thickness from 10 to 100 Å were deposited onto alumina substrates by RF sputtering. The films were subsequently heat treated to produce small particles of the alloy with average sizes in the range 300 to 1000 Å. The surface composition of aggregates of small particles was determined by Auger electron spectroscopy after equilibration of the samples at 650°C. Average particle sizes were determined by analysis of photomicrographs taken by electron microscopy techniques. The results show a strong dependence of surface composition on particle size. A decrease of a factor of five in surface concentration of palladium is observed over the range of particle sizes studied. The dependence of equilibrium surface composition on particle size has been estimated by means of a mass balance model. While this model accounts qualitatively for the effects of particle size on surface composition, discrepancies between model predictions and the experimental results suggest the intervention of other phenomena, possibly related to capillarity effects.     

Observation of surface segregation of sulphur in CuNi alloys at elevated temperature with scanning Auger electron microscope

Surface segregations of Cu and S in a CuNi (50wt%) alloy at elevated temperatures of ～600°C were studied under scanning Auger electron microscope, JAMP-3. Scanning images of S, Cu and Ni Auger signals have revealed that the surface segregations of both Cu and S take place preferentially depending on grains and the contrasts in S and Cu Auger images are complementary. This leads to another confirmation of the previous work, namely, that the surface segregation of Cu is considerably suppressed by the existence of S at the outermost atom layer.     

Sputtering of Cu and Zn atoms from elemental and alloy targets

The energy distributions of Cu and Zn atoms sputtered from elements and Cu_xZn_1-x alloys (x=0.80, 0.24) with a 6 keV Ar⁺ beam have been measured. It was found that the collision-cascade theory properly described the flux of sputtered atoms. From the spectra the binding energies of Cu and Zn atoms in the elemental and alloy surfaces were determined. The collision-cascade theory and the experimentally adjusted values of the binding energies allowed for calculation of the total and partial sputtering yields, and the equilibrium surface composition of the ion bombarded alloys. This work was carried out as a part of Research Project M.R. I/5.     

The simulation of nanoscale sputter depth profiles using molecular dynamics

The feasibility of using molecular dynamics (MD) for simulation of a nanoscale sputter depth profile experiment is examined for the idealised case of depth profiles of individual atomic layers in a Cu(1 0 0) target. Issues relating to the extraction of depth profile information from MD simulations are discussed in detail. The simulations examine the sputter erosion of static and azimuthally-rotated Cu(1 0 0) targets produced by 3 keV Ar projectiles incident at 25° from the surface. The simulated projectile fluence extends to 5 × 10¹⁵ cm⁻², and the mean value of the sputter depth, z, amounts to 8 Cu(1 0 0) monolayers (ML) or 15 Å. The simulations directly supply progressive layer erosion profiles (curves that depict the extent of sputter erosion of each atomic layer vs. total sputter depth). A fitting method is then used to extract smooth depth profiles for each atomic layer from these predicted erosion profiles. The depth profile characteristics (height, width, shift) for the first 10 layers of the target show a pronounced dependence on layer depth.     

Auger-spectrum simulation techniques for surface analysis of alloys with overlapping peaks: sputtered and annealed W—Mo alloys

The surface composition of a 50% W—Mo alloy has been investigated using Auger electron spectroscopy. Problems arising from carbon contamination and the strong overlap of the intense, surface-sensitive low-energy Auger peaks have been solved. Spectra for the alloy were collected digitally and could be simulated satisfactorily using pure-element spectra. The steady-state surface composition under 500 V Ar⁺ ion bombardment shows an enrichment of the surface in W, caused by preferential Mo sputtering. It is observed that the surface composition is uniform over the range of escape depths of the Auger electrons. This is consistent with enhanced diffusion and mixing over the penetration depth of the bombarding Ar⁺ ions. On annealing, the W-enriched sputtered surface becomes Mo-enriched, consistent with surface segregation of the more weakly bonded Mo atoms. However, in contrast with the sputtered surface, it is observed that a single simulation over the range of Auger-electron energies is not possible. The Mo enrichment indicated is largest for the 120 eV peak and decreases with increasing energy (escape depth), becoming a minimum at 220 eV. This is consistent with a monolayer segregation model, which is tested using published values for electron inelastic mean free paths. The calculated monolayer composition is x_Mo 0.94, in agreement with the predicted ideal-solution values of 0.98 ± 0.02 for different orientations.     

Changes in gold concentration at the surface of a AuCu alloy sputtered at low temperature

A series of depth profiles in Au_0.56Cu_0.44 produced by bombarding at ? 120°C with an argon ion beam of 2 keV energy and current densities of 4, 6, 12 and 24 μA/cm², respectively, are presented. Gold is enriched in the top layer but the concentration rapidly decreases to a minimum at an average depth of about 2 Å, then increases slowly and saturates at about 30 Å. The change in the composition at this point is called a dip. The magnitude of the dip depends on the ion current density, but the depth is nearly independent of it. Segregation and diffusion effects in establishing the surface composition are stressed.