Electron Rutherford back-scattering case study: oxidation and ion implantation of aluminium foil

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1 SURFAE AND INTERFAE ANALYSIS Surf. Interface Anal. 27; 39: Publishe online 5 October 27 in Wiley InterScience ( Electron Rutherfor back-scattering case stuy: oxiation an ion implantation of aluminium foil M. R. Went an M. Vos Atomic an Molecular Physics Laboratories, Research School of Physical Sciences an Engineering, Australian National University, anberra, AT 2, Australia Receive 12 June 27; Revise 19 August 27; Accepte 3 September 27 Electron Rutherfor back scattering (ERBS) is a new spectroscopy for etermining the composition of surfaces. In this work the surface sensitivity of ERBS was investigate by changing the entrance an exit angle of the electron beam while keeping the scattering angle constant. It was foun that in this way the surface sensitivity of the technique can be varie consierably. We use aluminium as a test case for ERBS, as it is well stuie. The technique has been use to investigate the oxie film of aluminium foil as manufacture an the native oxie ( 2 O 3 ) film forme on a clean aluminium surface expose to air. We have also use ERBS to investigate the presence of, implante uring the sputter cleaning process, at a variety of epths within an aluminium matrix. opyright 27 John Wiley & Sons, Lt. KEYWORDS: aluminium; aluminium oxie; electron Rutherfor back scattering; Rutherfor back scattering INTRODUTION Electron Rutherfor back scattering (ERBS) is a relatively new surface analysis technique. 1 4 Analogous to (ion) Rutherfor back scattering (RBS), it relies on the elastic scattering of particles from the surface an near-surface region at high-momentum transfer. However, in ERBS the scattere particles are electrons rather than ions. Electrons, which scatter elastically from materials, are often naïvely thought to have the same incient an exit energy because of the relatively large mass ratio between the target an projectile. In reality, only the total kinetic energy (electron an target) is conserve as the electron transfers momentum an thus a small, but measurable, amount of energy to the scattering atom. This energy transfer (the recoil energy) results in an energy loss of the scattere electron. The amount of energy loss of the electron scattering from a particle at rest is given by E r D q 2 /2M (where E r is the energy loss, q is the magnitue of the momentum transfer q an M is the mass of the atom). Further, even at K the atoms are vibrating in the lattice. This prouces a Doppler broaening of the elastic peak such that the energy loss of an electron scattering from a target with momentum p is given by: E r D q2 2M qðp 1 M Thus, for high-momentum transfer the elastic peak will split into peaks iniviual for each of the constituent elements (with atomic mass M) withapeakwithwhichis relate to the vibration motion of that nucleus in the lattice. Ł orresponence to: M. R. Went, Atomic an Molecular Physics Laboratories, Research School of Physical Sciences an Engineering, Australian National University, anberra, AT 2, Australia. michael.went@anu.eu.au The intensity of electrons scattere from each constituent element is proportional to the elastic cross-section at the chosen scattering angle which, to a first approximation, can be escribe by the Rutherfor cross-section an is thus proportional to Z 2,withZ being the atomic number. Only electrons that have not scattere inelastically will contribute to the elastic peak. The probability (I l )thatan electron has not scattere inelastically ecreases with l, the path length insie the material as: I l D e l/ where is the inelastic mean free path (IMFP). Thus, for a reflection ERBS experiment of a homogeneous system the elastic signal on average comes from electrons which have a total path length of 1 1 le l/ l e l/ l D 2 D In the work presente here, the sample can be rotate aroun the vertical axis which in turn changes the incient ( in ) an outgoing ( out ) angle that the electron beam makes with the surface normal. In this way, the average signal epth (υ) at which the electrons scattere elastically can be varie. Figure 1 shows a schematic representation of the measurement, it follows from this figure that the average signal epth is relate to in, out an the IMFP by υ D cos in cos out cos in cos out Assuming a single scattering approach, ie. contributions from multiple elastic scattering are minor, then the count opyright 27 John Wiley & Sons, Lt.

2 872 M. R. Went an M. Vos a+b=l -α α= α +α sample b Electron energy analyzer 45 electron gun θ in θ out a Figure 2. Schematic representation of the experiment. The sample can be rotate aroun the vertical axis to change the epth sensitivity. If the sample surface normal is in the vertical plane through the gun, D. The experiment is in its most bulk sensitive geometry when the sample is rotate by '2 towars the analyzer ( D 2 ) see Fig. (3). Figure 1. Geometry use to calculate the average signal epth from the incient an outgoing beam angles at the average signal epth υ, a b D. rate (I hom. ) for the elastic peak of a homogeneous layer is: I hom. ( ) D ( ) D N 1 δ ( l 1 e ) 1 cos in cos out cos in cos out N cos in cos out With, a constant, etermine by the experimental conitions (beam current, etector opening angle, etc). For an overlayer-substrate system, this becomes slightly more complicate. onsier the special case of an 2 O 3 overlayer (with thickness a) on aluminium. The substrate contribution is attenuate from the value given by Eqn (5) as electrons scattere elastically from the substrate may scatter inelastically in the overlayer. I subst. D I hom. e ( ) a O cos 3 in cos out The intensity of electrons scattere from oxygen is given by ( ) 3 I O D O 5 N 2 O 3 a e ( ) l O cos 3 in cos out For aluminium, the observe intensity is the sum from the contribution of in the overlayer an substrate: ( ) 2 I D 5 N 2 O 3 ( ) In the previous a ( l 1 2 O cos 3 in cos e out )l I subst. 8 are the ifferential elastic scattering cross-sections, N,2 O 3 O, the atomic ensities of an 1 l l O 3,an,2 O 3 the respective IMFP. The elastic crosssections an IMFPs are available from relativistic partial wave calculations 5 an TPP-2M calculations, 6 respectively. EXPERIMENT Figure 2 shows the experimental configuration use for the ERBS measurements. An electron gun (Kimball Physics ELG- 2 with BaO filament for reuce energy sprea ³3 mev) prouces a beam of electrons at 55 ev, the electrons are accelerate by the sample which is hel at kv. Thus, the 4 kev electrons hit the sample. This energy allows the, an O elastic peaks to be separate within our experimental resolution. The choice of such a high energy also allows the sample to be investigate over a significant epth. The electron gun is mounte below the horizontal plane at an angle of 45. The analyzer is place such that the scattering angle is 12. At this angle, the mean recoil energy accoring to Eqn (1) is then 4.3 ev (O), 2.5 ev () an.5 ev (), an uner these conitions the calculate ifferential elastic cross-section is 9.2 ð 1 23 cm 2 /Sr for O, 2.44 ð 1 22 cm 2 /Sr for an, 6.33 ð 1 21 cm 2 /Sr for ). 5 For an O, the cross-sections increases proportional to Z 2 (as expecte for the Rutherfor cross-section), but for the increase in cross-section is less than expecte from the Rutherfor formula. This is ue to screening of the nucleus by boun electrons inclue in the calculations but absent in the Rutherfor cross-section. At 4 kev, the IMFP as etermine by the TPP-2M formula is 544 Åfor 2 O 3 an 528 Åfor. 6 Electrons, which scatter from the sample into the entrance slit of the analyzer, are ecelerate to 2 ev an focusse at the entrance of a hemispherical electron-energy analyzer. The scattere electrons are energy isperse as they pass through the analyzer. Electrons that scatter elastically an with small energy loss (<4 ev loss) are etecte simultaneously by the two-imensional etector at the exit of the electronenergy analyzer. Electron-energy loss spectra can be obtaine opyright 27 John Wiley & Sons, Lt. Surf. Interface Anal. 27; 39:

3 Electron Rutherfor back-scattering case stuy 873 over a wier energy range by scanning the potential of the analyzers. Scanning also corrects for inhomogeneities in the channel plate efficiency. The electron-energy analyzer is mounte in the horizontal plane at 45 to the plane forme by the gun an the vertical irection. The electron-energy analyzer has been escribe in more etail elsewhere. 7 Beam currents for these measurements are typically 5 na. Measurements generally take 1.5 h an consist of a number of scans across the region of interest (elastic peak). With the elastic peak centere on the etector count rates are of the orer of 2 Hz. Signal-to-noise ratio is etermine from the ark counts in the region of negative energy loss an the total counts in the elastic peak an is greater than 3 : 1. By rotating the sample aroun the vertical axis, the angle that the incient an outgoing electrons make with the surface changes, an thus the length over which the electrons, originating from a certain epth, travel is either increase or ecrease. This changes the sensitivity of the technique from epth sensitive when the sample normal is towar the analyzer to more surface sensitive when it is rotate away (Fig. (3)). RESULTS AND DISUSSION First, an aluminium foil sample (99.99% purity: metal content only, supplie by fa Aesar) was cleane with ethanol an place in the vacuum. In the ERBS measurement, the elastic peak showe a small shouler at the positive energy loss sie of the main aluminium peak (Fig. (4)). As the absolute energy scale of our apparatus is unknown within ³š.5eV, the peak positions cannot be measure. As such, the main elastic peak is aligne with the expecte recoil energy for (2.53 ev). The shouler appears then at an energy that correspons to the recoil energy for oxygen (4.21 š.3 ev) in goo agreement with 4.27 ev as preicte by Eqn (1). For each spectra, the peak areas were etermine by nonlinear fitting using two Gaussians, one for each constituent element. Doppler broaening is clearly resolve as the full with at half maximum (FWHM) of the aluminium an oxygen Figure 3. Average signal epth for homogeneous materials as a fraction of the IMFP for accessible sample angles as calculate using Eqn (4). Squares-inicate angles measure in this work. are 1. š.1 an1.5 š.1 ev respectively, larger than the experimental resolution of '.6eV. The relative peak areas Fig. (4) obtaine from Gaussian fits of the elastic peaks are then compare with values obtaine from Eqns (5) (8). These calculations use the accepte values for the bulk ensity an stoichiometry of 2 O 3.Forthe D sample orientation, goo agreement with the measurement was foun for a layer thickness of ³19 Å. This thickness is more than the oxie thickness that grows on clean when expose to atmosphere (referre to as the native oxie). It is presumably cause by a combination of the native oxie layer an oxygen introuce into the surface by the rolling proceure use in forming the foil. This value is in goo agreement with an oxie layer prouce at ³3 8 a surface temperature which woul likely be attaine uring col rolling. By rotating the sample to the more surface sensitive position an performing the same measurements, one fins that the relative intensity of the O-peak increases, but not as much as calculate base on Eqns (5) (8). This woul be inicative of a inhomogeneous oxie layer covering the surface. At the 4 position, the average signal epth is.8 times the IMFP an so the peak-area ratio : O shoul approach N / O N O D 1.83 ³ 13 2 ð 2 / 8 2 ð 3 D 1.76 for a thick oxie film. The measure value is 2.5, which woul support the hypothesis that the film is inee inhomogeneous as the beam woul only nee to see a small amount of substrate for this to occur. Le et al. 9 investigate the fracturing of the oxie layer in col rolle aluminium an foun that in the col rolling process, pure aluminium coul be extrue through cracks in the oxie layer. This process will increase the amount of aluminium observe on the surface layer, thus the : O ratio coul approach that measure. By changing the angle we change the fraction of the signal that originates from the substrate. For D, the majority comes from the substrate; for D 4 the majority comes from the overlayer. As the boning of in metal an in 2 O 3 is ifferent, its vibrational properties (an hence Doppler broaening) are not necessarily the same. However, we o not observe a clear tren in the peak with in Fig. 4. The sample was then cleane of the oxie layer by sputtering at 3 kev for ³3han2µA/cm 2 current ensity with the ion beam perpenicular to the sample surface. In this case, the oxygen peak is remove but a surprisingly large aitional peak appears at the low energy loss sie of the main aluminium peak Fig. (5). Again by aligning the peak with the position etermine from Eqn (1) this new peak is centere at.56 š.2 ev in goo agreement with the calculate energy loss for of.52 ev. The peak is therefore attribute to which is implante into the aluminium surface. The large intensity of the peak can be explaine by its large elastic scattering cross-section. The with of the peak (FWHM.6 š.1 ev) is less than that of either the aluminium or oxygen peaks an is probably etermine mainly by the combine energy resolution of analyzer (<5 mev) an energy sprea of the gun (³3 mev). Measurements of -ion eposition at higher energies have foun that these implante ions precipitate to form bubbles opyright 27 John Wiley & Sons, Lt. Surf. Interface Anal. 27; 39:

4 874 M. R. Went an M. Vos Figure 4. omparison of the native oxie layer from the raw foil in each of the three measurement geometries. Soli line D, ashe line D25 an otte line D4. The ratios of - to O-peak area as etermine by curve fitting are given in the legen. This figure is available in colour online at below the surface Our measurements at this lower energy have foun the ions to become similarly trappe in the matrix, however the form of the is unknown. A SRIM 13 simulation of the sputtering inicates that these ions are implante at a epth up to ³6 Å. However, keep in min that the sputtering process also eroes the surface. By measuring the sample in the, 25 an 25 positions the elastic peak appeare largest in the most surface sensitive measurement. This woul suggest that the istribution was centere at a epth smaller than υ, the average signal epth. In orer to test our unerstaning aluminium was evaporate on the -cleane film, in three steps: 1, 15 an 15 Å. Hence, assuming that the -epth istribution after implantation is centere at 6 Å, after evaporation the layer appears at a total epth of 16, 31 an 46 Å for the respective aluminium epositions. The thickness of eposite aluminium was calibrate by conventional ion RBS in a separate evaporation on molybenum. Table 1 shows the incient an outgoing beam angles for the three sample positions an the average signal epth as a function of the IMFP. These calculations inicate that for an implantation epth of ³.3ð the IMFP that to a first approximation rotating the sample will have only a small effect on the observe intensity. The energy loss spectra of the 6, 16, an 46 Å ( epth) measurements in each of the three geometries is presente in Fig. (5). These measurements emonstrate three possible cases; where the is above the average signal epth; where the is at the average signal epth an when the is below the average signal epth. It can be clearly seen that in the as implante film, when is close to the surface, rotating the sample to the surface sensitive position increases the -peak intensity. ternatively, for that is at a epth of 46 Å rotating the sample to the bulk sensitive position increases the peak. However, when the is near the average signal epth of the, rotating the sample to the three positions causes only small changes in the intensity. In Fig. (6), we present calculations for the -to- ratio for each of the three measurement angles for epths up to 6 Å for three ifferent values of the IMFP compare to the measurements of the : ratio from the epths of 6, 16, 31 an 46 Å. It can be seen that the average signal epth (where the three lines are closest) changes as the IMFP varies. The measurement is in agreement with the TPP-2M calculation 14 of the IMFP 528 Å. Finally, we prepare a clean aluminium surface by evaporating 15 Å of (99.98%) over an aluminium Intensity (Arbitrary Units) α= α= α= Energy Loss (ev) Figure 5. Measurements of the elastic peak for -implante for three epths an the three sample angles. 6 Å 16 Å 46 Å opyright 27 John Wiley & Sons, Lt. Surf. Interface Anal. 27; 39:

5 Electron Rutherfor back-scattering case stuy 875 Figure 6. alculate peak ratios for each of the three geometries for impurities in uminium at ifferent epth, compare to the measure peak ratios (top left). alculations were one assuming ifferent values for the inelastic mean free path, as inicate. Soli line D, ashe line D 25, otte line D25. This figure is available in colour online at journal/sia. Table 1. Angular values for each of the three sample positions an the associate average signal epth Description Sample angle ( ) Incient angle in outgoing angle out average signal epth υ Bulk sensitive Normal Surface sensitive Extremely surface sensitive in comparison to 1.76 : 1 (assuming bulk oxie) etermine from the simple ratio N / O N O. One explanation may be the rough nature of the evaporate film, an the spectrum is mainly ue to those surfaces for which the outgoing angle is less than 85. SUMMARY AND ONLUSION Measurements using the ERBS technique have been use to etermine the thickness of the oxie layer cause by col substrate. The cleanliness was confirme with ERBS; no sign of oxygen was present in the measurement (Fig. (7)) an the elastic peak consiste of only one component. The sample was then remove from the vacuum an expose to air at 21 for approximately 2 ð 1 5 s before being returne to vacuum an measure again with the ERBS technique at the 25, an 25 orientations. The native oxie layer was etermine to be 55 š 1 Å thick in reasonable agreement with measurements by other groups 15,16 who etermine the native oxie layer to be ³4 Å thick. Figure (7) also shows that the oxygen peak is inee larger for the rolle film than for the film oxiize in air. By changing to 4, the outgoing electrons become very glacing (85 ). Now, even at such large glancing angles, the substrate still contributes (as the oxie overlayer is quite thin) an given the ifference in cross-section between an O if the beam sees only a small amount of substrate it will affect the observe ratio consierably. The : O peakarea ratio is foun to be 8.7 : 1 while that calculate from Eqns (5) (8) (assuming a oxie film thickness of 55 Å) is 2.9 : 1 Figure 7. omparison of ERBS measurements from clean aluminium, the oxie layer from col rolling an the native oxie layer forme by exposure of clean aluminium to air. l measurements taken with D. Soli line, native aluminium oxie; ashe line, raw aluminium foil; otte line evaporate aluminium foil. This figure is available in colour online at opyright 27 John Wiley & Sons, Lt. Surf. Interface Anal. 27; 39:

6 876 M. R. Went an M. Vos rolling. From measurements in a bulk sensitive geometry, we obtain an oxie thickness of ³19 Å, which is goo agreement with other measurements of an oxie layer create in air at 3. 8 At large glancing angles, the : O peak-intensity ratio oes not approach the ratio calculate for 2 O 3 which suggests that the oxie film is inhomogeneous. The oxie layer create by exposing a freshly eposite aluminium layer to air was etermine to be 55 š 1 Å in comparison to a value of ³4 Å as etermine by other groups. 15,16 Measuring at large glancing angles (85 between surface normal an analyzer) again oes not lea to the peak-intensity istribution as calculate for an 2 O 3 compoun. In this configuration, we see more than expecte. This coul be explaine by the rough nature of the evaporate films. However, there is an alternate explanation for the eviation observe at large glancing angles. A similar problem has been observe before for other stoichiometric compouns 2 an was then attribute to errors in the calculate cross-sections. The same coul be true for the iscrepancy seen in the large angle measurement of the col rolle aluminium foil an oxiize evaporate films. This technique has also been use to examine ion implantation of into an matrix. When the implante layer is centere at smaller epth than the average signal epth, the signal from the layer can be increase by making the measurement more surface sensitive an vice versa. As the average signal epth is relate to the IMFP of electrons in the material, we have foun goo agreement between our measurement an a TPP-2M calculation of the IMFP of aluminium at 4 kev. Doppler broaening is clearly resolve for an O as their observe with (1. š.1ev an 1.5 š.1ev respectively) is larger than the with of the peak (.6 š.1 ev) The FWHM of the peaks is clearly relate to its mass (as expecte from Eqn (1)), with lighter elements exhibiting a larger Doppler broaening. In this work, a strong epenence on Z for etection in an ERBS spectrum has been shown. For heavy impurities in a light substrate, the ERBS technique oes very well an only a small amount of the heavy element ( in this case) is require for a strong elastic peak to be observe. In cases where light elements are in the presence of heavy elements (oxygen into aluminium), the large ifference in cross-section (/ Z 2 ) means that even for cases where the light element constitutes the majority, as in the case of 2 O 3, their contribution to the elastic peak is small. Thus small amounts of a light element will be ifficult to etect in a heavy element matrix. In the case where the two elements are of similar mass, the Doppler broaening of the iniviual peaks means that their peaks can not be resolve. In general, ERBS is an electron spectroscopy with a rather unique set of characteristics. We think that it has the potential to be evelope into a quantitative surface analysis technique, especially for stuy of materials on a somewhat larger epth-scale than can be stuie by conventional electron spectroscopy using energies of '1 kev an less. Acknowlegements This research was mae possible by a grant of the Australian Research ouncil. The Authors woul like to acknowlege the assistance of R.G. Elliman in performing the convention ion RBS calibration. REFERENES 1. Went M, Vos M, Elliman R. J. Electron Spectrosc. Relat. Phenom. 27; : Went M, Vos M. Appl. Phys. Lett. 27; 9(2): Vos M, Went M. J. Electron Spectrosc. Relat. Phenom. 27; 155(1 3): Went M, Vos M. Surf. Sci. 26; 6: Salvat F, Jablonski A, Powell. omput. Phys. ommun. 25; 165: Tanuma S, Powell, Penn D. Surf. Interface Anal. 1993; 21: Vos M, ornish GP, Weigol E. Rev. Sci. Instrum. 2; 71: Smeltzer WW. Journal of the Electrochemical Society 1956; 13(4): Le H,Sutcliffe M,Wang P,Burnstein G.Acta Mater. 24; 52: Potter D, Rossouw. J. Nucl. Mater. 1989; 161: Ishikawa N, Awaji M, Furuya K, Birtcher R, len. Nucl. Instrum. Methos Phys. Res., Sect. B 1997; 127/128: vom Fele A, Fink J, Müller-Heinzerling T, Pflüger J, Scheerer B, Linker G. Phys. Rev. Lett. 1984; 53(9): Ziegler J, Biersack J, Littmark U. The Stopping an Range of Ions in Solis. Pergamon Press: New York, Tanuma S, Powell, Penn D. Surf. Interface Anal. 1993; 2: Feliu S Jr, Bartolomé M.Surf. Interface Anal. 27; 39: abrera N, Mott N. Rep. Prog. Phys. 1949; 12: 163. opyright 27 John Wiley & Sons, Lt. Surf. Interface Anal. 27; 39:

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