The relationship between vacuum and atomic collisions in solids

The relationship between collisions in solids+ received

10 October

vacuum

and atomic

1979

G Carter and D G Armour,

Department

of Electrical

Engineering,

University

of Salford,

Salford

M5

4WT.

UK

Atomic collision events in solids are frequently stimulated by external irradiation with energetic heavy ions. This requires production, acceleration and manipulation of ion beams in vacuum system with ensuing problems arising in perturbations to ion beam quality from gas phase collisions. In addition the dynamic interaction between the gas phase and any surfaces at which atomic collisions are under investigation can lead to perturbation to the collision events by adsorbed contaminant. This review discusses both gas phase requirements for ion accelerators to minimize deleterious effects and outlines some of the processes which occur in atomic collisions due to the presence of adsorbed impurities. Finally it is shown how certain atomic collision processes involving elastic scattering may be employed to investigate surface adsorption and related effects.

Introduction

Readersof this journal will need no definition of the word ‘vacuum’ in the title but elaboration of ‘atomic collisions in solids’may be helpful. This term is frequently usednowadays to describecollision phenomenain solidsin which someatoms have beenset in kinematic (as opposedto thermodynamicsuch as occurs in solid state diffusion) motion. Such atomic motion may be initiated internally in the solid following nuclearfission in which the atomic fragmentsof individual fissioneventsmove through and collidewith atomsof the solid.Collisionsmay result in displacementof atomsfrom their lattice sitesand thesedisplaced atoms may generate further displacementsand a collision cascadewill ensue. Atomic displacementmay also result from interaction of external radiation with a solid suchas photons, electrons, nucleons and energetic atoms. In the presentreview we will restrict discussionto the atomic collision processes generatedin solidsby external irradiation with energetic atoms generally with initial energiesgreater than about 1 keV and up to initial energiesof severalMeV. The reasonfor the choice of this energy range considerationis that although atomic collision processesoccur for both lower and higher energiesthan in this regime,it is essentiallythe most important technologically for modifying the properties of solids by the controlled introduction of atomic impurities and for composition and structural analysis of solids employing external energeticatom irradiation. It should be noted, however, that the most convenient manner of acceleratingatomsto energiesin the above range is to first ionize them, then acceleratethem through an electrostatic potential difference,probably then effect a massanalysis to ensure atomic projectile uniquenessand finally allow the

energetic ions to be focused to strike the solid substrateof interest. In order that the ion-solid interaction processes should be well defined, however, it is important that the ion flux to the target shouldbe asspatially and temporally uniform as possible,that the ion speciesshould be unique (possiblyof isotopic purity) and that the ions should be as monoenergetic as possible.If, in transit from the ion generation(or source) region to the target, ionsmay collide with gasphaseatomsthen all of these important parametersof the ion beam may be perturbed to someextent, In order to minimize this perturbation it is clear that adequately low gas densitiesmust prevail betweensourceand target, i.e. the vacuumconditions in the ion acceleratormust be adequatelyand reproducibly defined.It is equally clear, however,that sincethe gasphaseand any exposed surfaces in a vacuum system are mutually interactive, the surfaceconditions may also be important in atomic collisions in solidsstudies.Moreover, just as ion-atom collisionsin the gasphasemay modify the projectile characteristicssoalso may the presenceof surface contaminants modify near surface atomic collisions processesin the solid. If such is the case, however, then it may be further anticipated that careful studies of near surfaceatomic collision processes could provide useful information on the stateof the surface.In the following review, therefore, we will divide discussioninto three topics. (1) Vacuum (or gasphase)requirementsin accelerators. (2) The influence of surface contaminants on some specific atomic collisionsphenomena. (3) The use of ion-solid interactions to study surface conditions.

1. Vacuum requirements

* Paper first presentedat a 2-day conference‘Developmentsin Vacuum Scienceand Technologyfor GeneralApplication’ held 5-6 April 1979at The Polytechnicof Central London. The paper presented hereis a considerable enlargement of the onepresented at that Conference. Vacuum/volume30/number1. Pergamon

Press

LtdlPrinted

in Great

in accelerators

In one of the earliest applications of ion-solids interaction which is still of profound technological significance, the bombardmentof and entrapmentin solidsof energeticionswas utilized in ion pumping. In suchdevices(including ionization Britain

1

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G Armour:

The relationship between vacuum and atomic collisions in solids

gauges) the ion flux to the trapping surfaces or electrodes is not totally controlled and certainly varies with system total and partial pressures. In more recent applications of the controlled introduction of atomic impurities into solids it is necessary to form the ion flux into a well specified beam so that known quantities of known impurity atoms, of carefully defined energy are introduced into the solid in order to modify, in a precisely controlled manner, some physical or chemical property of the solid. Perhaps one of the best known of such contemporary applications is the controlled doping of semiconductor substrates’ to effect conductivity variation, by ion implantation, but particularly equally important studies have been commenced relatively recently aimed at property modification of metal surfaces’ (e.g. corrosion inhibition, wear resistance, catalytic activation) and insulator surfaces (e.g. refractive index) employing ion implantation. The general requirements for these types of application are relatively large area (> 20 cmz) uniform fluence implants with, at least elementally, resolved single mass ions of all types (from H+ + Ur’) with energies in the tens to hundreds of keV range. Accelerators employed to fultil such requirements are frequently similar to isotope separator employing magnetic sector field mass analysis rather than resonant path mass dispersion filters since focused ion beams are required. In situations, to be described in further detail in Section 3, where ion beams are employed as surface (and near surface) analytic probes the same beam uniformity, or even more stringent, requirements must generally be met and although a narrower range of ions (e.g. He+ and inert gas ions generally) is usually required, ion energies in the lower keV range on the one hand or hundreds of keV to several MeV on the other hand, depending upon the probe technique.employed, are demanded. Common to both types of accelerator system generally, however, are four sequential elements. (I) the ion source, extraction and acceleration region, (2) a field free region before mass analysis, (3) mass analysis region and (4) the post analysis beam transport and manipulation region and the target environment. Vacuum requirements in these regions may be substantially different and ion-gas atom collision processes in the different region may be of various importance depending upon beam quality and intensity requirements. Detailed assessment of requirements in region (I) have been given by Camp1an2, region (3) by Menat and regions (2) and (4) by Freeman4. More detailed consideration to the target environment section of region (4) will be given in Section 2 of this review, and in view of the earlier comprehensive evaluations of the other regions only brief attention will be paid to regions (l), (2) and (3). A more lengthy account of requirement in region (4) will be given here since some illuminating studies of processes occurring in this region have been undertaken in the authors’ laboratories. (1) Vacuum requirements in ion sources are generally not stringent since, with mass analysis facilities, any undesirable ion species generated in background gases resulting from real leaks or outgassing may generally be filtered. However, unless very high resolution is employed hetero-contamination of an ion beam such as %+ (which is becoming of increasing importance for self ion pre-damaging of Si substrates before chemical doping) with ions of Co+ and N,+ can occur, whilst similar contamination of 3*S+ by O,+ or Zn+ + can also obtain. In order to minimize such effects ion sources, usually constructed of refractory materials so that outgassing is generally 2

of little difficulty, shouldbeasleak tight aspossible.An estimate of tolerable leak rates may be obtained as follows. If the ion production processfor both the required ion beam and for impurities is similar (e.g. single collision electron impact) which might be expectedfor contaminant of the beamby equal massimpurities, then the ratio of impurity beam current to desiredbeam current It/Z, should be approximately equal to ai+/a,+ . PIP,, where a+ is the ionization production rate in the ion sourceand p the pressure.If a 1% impurity content of the beam is tolerable, and the ionization production rates of the two speciesare approximately equal then the equivalent pressureof impurities in the sourceshould be only 1% of the pressurerequired of the desiredion species.Most ion sources operatewith vapour or gaspressures of the beamspeciesin the approximate range lo-‘-lo4 torr, whilst the ion beam exit aperturesgenerally limit pumping of the sourceswith a conductancebetweenIO-’ and 10 I s-t. Thus minimum tolerable leak ratesmay be aslow aslo-’ torr 1s-l bur rangeup to 10m4 tot-r 1 s-t. Such real leak rates are generally not difficult to determineor control by good vacuum practice. The throughput of gasfrom the sourceto later sectionsof the accelerator may have further consequencesfor ion beam quality, however. Thus if the next sectionof pumping occursin the field free region before the massanalyserand the general cross-sectionof most acceleratorbeam transport systemsis of order 100 cm’, it is unlikely that a pump speedgreater than 2-300 1 s-t will be attained in this region. Thus in high gas throughput from sourceconditions the ultimate total pressure in the field free region is unlikely to fall below 3-5 x 10m5torr even without account for outgassing.Possiblehigh pressures of this magnitudecan leadto a form of beamquality contamination known as autocontamination which may arise via collisions betweenthe required beam and residual gas atoms in region (2) of the accelerator, when multiply charged and/or polyatomic ions are employed and change ionic charge or atomicity in collisions. Thus it is readily shown that if a polyatomic ion of massM (containing a, atoms) and charge state b,q (4 is the electronic charge) undergoesa collision in the field free region (2) and continuesas an ion of atomicity u2 and chargestateb2q. then it is analysedby the magnetto focus at a position as if it had beenan ion of initial massM’ given by4

Thus Freeman4hasclearly shownthat the recordedion current due to, ostensibly,P4+ in an isotopeseparator,deriveslargely from the ion P+ stripping to P++ by collision in region (2) where the pressurewasaslow as 5 x lO-‘j torr. Sincemultiply charged ions are often consideredfor use in increasingthe equivalent ion energy (i.e. an ion of charge 114,accelerated through a potential difference V volts possesses an energy nqV equivalent to a singly charged ion acceleratedthrough nV volts) in implantation it is clearthat suchcharge(and atomicity) changingprocesses may be important in modifying the energy spectrumof an elementallypure beamif vacuum conditionsare poor. An estimate of desirable pressure requirements in this region may be made by demanding,as earlier, a 1y0 level of autocontamination of the desired beam. To a first approximation the ion current I,+ of ionsof chargestateb resultingfrom charge changing collisions without change in atomicity in

G Carter and

D

G Armour:

The relationship between vacuum and atomic collisions in solids

region (2) will be related to the initial ion current I,,+ of ions of charge state a, by the equation I*+ = 3.3 x 1ol6 .p,. 1. CT.;. Ia+, where py is the effective total ultimate pressure in region (2) in torr, 1 is the ion flight path in this region in cm and D is the charge changing cross-section in cm’ from charge state a to charge state b. The ratio of autocontaminant impurity : desired beam current is thus Ib+ a,+ II+=cII+*

3.3 x 10’6p”lu.

f .

Atomic diameters are of order 10-a cm and thus total collision cross-sections are of 10-16-10-‘5 cm2 and charge changing cross-sections are of similar magnitude as will be demonstrated later. Assuming that a,+ M a,+ and that charge states a and b only differ by a small ratio then for a region (2) and typical path iength of order 1 m, the ultimate pressure in this region must be less than 10T4 torr in order to satisfy the 1% contamination criterion. When multiply charged ions form the desired beam for higher beam energy requirements (e.g. P4+) the ion production cross-section may be orders of magnitude lower than the production cross-section for lower charge state ions which strip by collision to higher charge states (e.g. P+ + PZ+ )and thus the ratio a,+/a,+ may be substantially greater than unity. Consequently the ultimate pressure requirements in region (2) may be orders of magnitude lower than 10e4 torr as was demonstrated by Freeman4 to be necessary to ensure low contamination of the P4+ beam by P+ - P2+ collisions. A further example of the potential seriousness of such a process revealed by equation (I), in a situation where beams are used for solids analysis employing the technique of MeV energy light ion scattering from solids and outlined in Section 3, is the charge changing process t60+ + r602+ which is mass analysed upon the target as equivalent to 4He+. Such processes have been examined by Picraux et al5 and Hemment et al” in Van der Graaff accelerators where it was found that at a pressure of 5 x 10T6 torr in the pre-analysis region, and an initial t60+ ion beam of about 15% intensity of the 4He+ leaving the ion source, the raOz+ beam upon target (as deduced from the backscattering of both the 1602+ and 4He+ from an Au film on a C substrate) contribute about 1% of the 4He+ beam. This corresponds to a charge stripping cross-section of order IO-r6 cm2 as suggested earlier. Charge exchange processes are equally important in the field free post mass analysis region (4) of accelerators since they can lead to ambiguities in measurements of total number of particles incident upon a target, i.e. dosimetry. Thus if an ion beam contains a number of different charge states ,rq (when n = 0, 1, 2, . . .), the measured beam current will be equal to &I,. n whereas the number of incident discrete ions will be proportional to X,1, . r&n. The most serious charge exchange process generally in ion implantation machines is that of neutralization from the single charge state with a cross-section or0 and whilst many measurements of this parameter exis: for light ion projectiles in a range of target gases, relatively few are available for the wide range of heavy ion projectiles employed in atomic collisions studies.

A significant problem in basic studies of these cross-sections is precise specification of the state of excitation of projectile ions since this can vary dramatically with ion source conditions. Consequently a more technological approach has been adopted recently at Salford where neutralization cross-sections have been measured’ for singly charged, but unspecified excitation, ions emerging from the ion source of a mass separator used routinely for ion implantation and other atomic collisions studies. Although this does not give unequivocal physical measurements of o,~, it does provide information on neutralization of the ion flux employed in an operational accelerator. Values of or,, were measured for a range of projectile ions and energies from IO-40 keV, in a target of Ar (since this is the most common support gas used in the ion source and generally provides the major gas phase constituent in the remainder of the accelerator) by determining beam attenuation after traversal of a known gas target thickness at a carefully measured pressure. The results of these measures are shown in Figure 1 from which it

I

I

I

I

,

I

I

20

0 ion

onorgy

I

I 40

IkeV)

Figure 1. Energy dependencesof a range of different ions in argon. is clear that (a) all neutralization cross-sections are of order IO-r6 cm’ and above for all projectiles at all energies studied, and (b) that the variation of cross-section with energy is very projectile species dependent. Some of these results correspond reasonably to theoretical predictions and to other limited experimental data, whilst in other cases (e.g. Cu+ + Ar, Cd+ --t Ar and Cl+ + Ar) correspondence with theory is poor, probably because of unknown initial excitation states. Nevertheless, the data does reveal, as outlined before that since cross-sections exceed IO-r6 cm2, ion beams transported over a distance of about 1 m may contain of the order of 1% neutral atoms which would not be recorded upon the target ion current 3

G Carter

end D G Armour:

The relationship between vacuum and atomic collisions in solids

integrator and thus lead to dosimetry errors of this magnitude. Indeed if the curve for B+ + Ar continues rising to higher energies, the neutral component in such a beam (which is of very considerable importance in semiconductors ion implantation) could be unacceptably large. HemmenP’ has also studied neutralization of heavy ion beams in the range 50-400 keV in N, by measuring the energy deposited by both the (ion + neutral) and neutral components (with electrostatic removal of the ions) in a calorimeter. This work did not measure 0, ,, values precisely but leads essentially to the same conclusions as our own works that neutral fractions amounting to several per cent will be produced in a 1 m flight path at 10ms torr. In the preceding discussion the problems associated with pressure requirements in accelerators designed to provide a wide range of ion species and energies, largely arising from gas throughput from the ion source have been outlined. When very low pressures are required in the final target environment of such systems it is usual to pass the ion beam through a series of defining apertures with differential pumping between apertures to provide a pressure gradient through the system. The target environment may then be fabricated according to conventional uhv requirements or simulated uhv conditions, if not clean target surface conditions can be achieved by surrounding the target with a cryoshield’ cooled to 30-40 K. In accelerator systems which operate over a restricted energy range (usually s IO keV) and for a limited range of ion species (e.g. inert gases) for specific atomic collisions in solids studies, the complete accelerator-target system may be uhv compatible with ion source, analysis section, etc. fabricated from stainless steel, with metal gasket sealing, well trapped oil diffusion and getter pumps or turbomolecular pumps, etc.“. In some systems of this type which employ reactive gas ion beams (e.g. H +, D+) where it is necessary to provide a target environment free of neutral species of the ion beam and thus a pressure differential from source to target of six or seven orders of magnitude, differential pumping has been achieved by means of combined diffusion pump-Ti sublimation pump arrangements”. Finally, although in the preceding discussion the deleterious effects to ion beam conditions arising from gas phase collisions have been outlined, it should be noted that such collision effects may be also used to advantage. In addition to electron removal and transfer in ion-atom collisions, electron excitation in both ion and target gas atoms may occur with the subsequent emission of characteristic photons of both species during radiative decay. Doorn et al I2 have used this effect, by passing ion beams through gas collision chambers of known length containing gas of known species at known pressure and recording the emission intensities of photons from the ion species. The photon production rate obeys a quite analogous defining equation to equation (2), with o being the cross-section for photon generation. Once the photon detection system has been calibrated measured photon intensities can be interpreted in terms of ion currents and since each ion species gives rise to its own characteristic optical spectram any impurities in the ion beam may potentially be detected. Using this method Doorn et ai” indicated that beam currents as low as equivalent to 10” particles s-’ could be detected using a collision chamber of 5 cm length at a pressure of He or Ne gas at -10e3 torr and that in a %i+ beam C,H,+, Nz+ and Co+ contaminant components could be recorded as less than 2, 2 and 6%, respectively. 4

2. The influence of surface contaminants atomic collision phenomena

on some specific

In any interatomic collision there are two major energysharing processes’*‘3.At all atomic separationdistancesthe nuclear chargesand extra nuclear electronsinteract to produce forces of attraction and repulsion, the latter dominating at closer approach distances. During a collision the mutual forces betweenatomsact continuously to definethe spatialtrajectories of the collision partners and there is a continuous interchange between the kinetic energiesof the atoms and the potential energy of the systemdetermined by the interaction force. If atoms commenceand finish interaction at infinite separation then the differencesin initial and final kinetic energiesof both partners representthe elasfic component of energy exchange during the interaction. This elasticenergytransfer is a function of the initial kinetic energy of the collision, the massesof the particles,the nature of the interatomic force law and the initial directionsof motion of the colliding atoms. During any such collision, however, the mutual forces of interaction may also perturb the energy statesof electrons in theseatomsand both electron and photon emissionmay result. This emission,together with any residual excitation of the collision partnersconstitutesan itrelasfic energysharingprocess and dependsupon the sameparametersas the elastic transfer processesand upon the electronic structuresof the interacting atoms. Thesetypes of processare responsiblefor the charge exchange,charge stripping and electron excitation phenomena discussedin the precedingsection. Both componentsexist in all collisions but the major energy transfer processmay be taken, asa very rough guide, to be the elasticprocesswhen the initial kinetic energy of the collision is less than about MR keV’*‘3, where MR is the effective mass(measuredin the centre of masssystem)of the colliding partners. Even under such circumstances,however, the inelasticenergy transfer will be real and although contributing little to perturbing the collision trajectories, and hencethe elastic energy transfer or changesin kinetic energy, will still be manifestedin electron excitational and radiation emissionprocesses.In a dilute gas collision betweenan atomic projectile and target atoms will be well separatedspatially but in a solid collisions will occur at very small interatomic spacingand in somecircumstancesa simultaneouscollision betweena projectile and severalatoms will also occur. It is frequently acceptable,however, to treat collisions in a solid as a linear sequenceof binary isolated collisions(i.e. quasidilute gasdynamics)and in each of these collisionsboth elastic and inelastic processeswill occur, with relative importance depending upon mainly the mass and energy of the projectile. When a projectile ion flux strikes a solid surface two immediate processescan happen. First a proportion of the ion flux can be backscatteredor reflectedfrom surfaceatomswhilst the remainder forward scatters into the solid, undergoing further collisionswith atoms in the bulk. Of this latter fraction somewill suffer sufficient momentumreversalto causethem to also reflect out from the solid whilst others will slowly lose energyto sucha point wherethey can no longer move dynamically in the solid (a thresholdenergy of =25 eV for cessationof such motion is frequently assumed)‘*13and they either trap in some favoured location in the solid or diffuse thermodynamically until trapped or releasedfrom the solid. During the slowing down history of the projectile, elastic

G Carter

and D G Armour:

The relationship between vacuum and atomic collisions in solids

energy may be imparted to the atoms of the solid and if sufficiently large may generate atomic recoils or displacements. The energy threshold for such displacements is atom type, solid structure and orientation dependent but is often taken as about 25 eV also’* 13. These recoils may in turn generate further recoils and an atomic cascade of moving atoms will ensue. If distances between atomic displacements are much larger than atomic spacings in the solid then the cascade is dilute and can be treated as a quasi gas or linear cascade process employing Boltzmann transport equations i4-16. If displacement free paths are short, however, the process more resembles the formation of a superheated liquid zone and is described qualitatively as an energy spike”* la or a quasi shockig. Quantitative modelling in this regime which corresponds to high energy transfer density conditions is currently very difficult. Whatever the dominant cascade energy dispersal process, however, the presence of atomic impurities within a solid will modify the magnitude and extent of the recoil cascades and of particular significance to vacuum physics is the presence of surface adsorbed impurities. The effects of such contaminants upon elastic collision processes will be several. Firstly, surface contaminant atoms may shield the underlying solid from part or all of the incident flux, thus modifying the surface backscattering process, a feature which will be discussed in more detail in Section 3. Secondly, contaminant atoms may be directly recoiled away from the surface by the incident ions or by recoiling substrate atoms and thereby constitute a contaminant sputtering process, again to be discussed in detail in Section 3. Moreover, the recoiling surface impurity atoms will generally be directed forward into the solid (rather than backwards to constitute sputtering) and this can create a source of impurity implantation into the solid. Generally the mean energy of such recoils will be much lower than that of the incident projectiles and as recoiled impurities will slow down to rest rather close to the surface. Studies of this elastic process will be further considered shortly. Thirdly, even though minor contamination may not significantly modify the energy transfer and cascade generation process in the solid, it may significantly alter the binding energy of atoms of the solid at the surface. In general the sputtering of a solid may be described as resulting” from the interaction of the dynamic recoil cascade with surface atoms such that these latter receive energy in excess of a surface binding energy. Thus, the sputtering yield (number of atoms of the substrate ejected per incident ion) may be approximated by the relation Yield

a

near surface elastic energy transfer surface binding energy ’

(4)

Thus even though surface contamination may not materially vary the numerator on the right-hand side of this equation, possible modifications of the binding energy of self atoms at the surface of a solid due to the presence of contaminant may modify the denominator and hence the yield. This effect will be discussed later but it should be noted that it is unlikely to be a very large effect since the presence of adsorbed contaminants will generally perturb surface binding energies by less than an order of magnitude. However, when atoms are sputtered or ejected from solids they may acquire excitation in any inelastic process which occurs in the final atomic collision event which results in their ejection or during their exit from the surface where their

electron energy states change from those associated with the solid state to those of the ‘free’ atom. The presence of surface impurities may seriously modify the band structure of the solid at and near the surface and via chemical bonding to individual substrate atoms may modify the energy states of such individual atoms. AS a result of these perturbations there may be very significant partial changes in the excitation and ionization states of sputtered atoms, much in excess of any perturbations of total sputtered atom yield due to binding energy modifications. The latter part of this section will, therefore, be devoted to a discussion of observed perturbations to such inelastic processes following a brief outline of effects due to binding energy variations and recoil implantation. The discussion of sputtering yield variations will be relatively brief since, to the authors’ present knowledge there is only one group who have made definitive measurementZ1* 22oftheeffect of speciticcontaminants at known or estimated surface coverages upon substrate sputtering yields. This largely arises from difficulties in measuring differentially the sputtering yield of all sputtered atoms of each species (i.e. substrate and contaminant separately) since techniques such as weight loss, etc. measure the rota1 sputtering yield of all species whilst measurements of sputtered ions or excited atoms do not record neutral ground state atoms. The main evidence for yield perturbations due to contamination is thus inferential although it is well known that23B24 the partial sputtering yield of the metallic component from a metallic oxide is substantially lower than from the pure metal due probably to both binding energy and cascade modifications. Colligon and his colleagues25~‘6 studied the sputtering yield variations as a function of incident ion fluence for both low energyz5 (600 eV N+z and Ar+) ions and high energyZ6 (-40 keV Ar+) ions of polycrystalline Au targets. In these studies the Au was activated by neutron irradiation before ion bombardment and the sputtered Au was collected and subjected to radioactive assay following each ion fluence increment with a sensitivity of about one-tenth of a sputtered monolayer. These investigations were carried out in a uhv system with base pressure = 10eg torr and with the Au targets both chemically cleaned before insertion into the system and then baked with the system to about 600 K. It was observed, in all studies, that during about the first lOI ions cm --z irradiation the sputtering yield of Au atoms increases rapidly by a factor of about 5 and then stabilized and this was interpreted as indicating that it was necessary to remove some unknown surface contaminant before the clean surface sputtering yield of Au was obtained. Similar, but less marked, variations in yield were observed if the Au target was maintained for long periods (16 h) at lo-’ torr without ion bombardment. Other, even more inferential, evidence of the effects of contamination upon substrate sputtering yield may be perceived in studies of resistance changes in Ag27*28 Ti, Au and W2* films during sputtering where it has been observed that, initially at low ion fluences, the resistance decreased rather than the expected increase due to film thinning by sputter erosion. Again this has been interpreted” in a requirement to remove contaminant before full target sputtering can occur. Measurements by Bay and Andersenzg of the sputtering of Ag with a range of different ion species indicated a ‘memory effect’ when ion species were suddenly changed, SO that substantial further irradiation was required after the species change was made before an equilibrium yield was achieved. Careful studies of this type, using a quartz oscillator system to detect 5

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The relationship between vacuum and atomic collisions in solids

the sputtered products, enabled Bay and Andersen” to show that the variation of the true, effectively zero fluence, sputtering yield of a substrate was a smoothly varying function of the incident ion atomic number Z, as predicted theoreticallyzO. Earlier studies by Almen and Bruce3’ had revealed a (theoretically unexpected) periodic variation of yield with Zi, when large ion fluences were employed to effect the yield measurement. The current understanding is thus that large ion fluences lead to substantial implant incorporation which may drastically modify the surface binding energy of substrate atoms and, to some extent, the collision cascade. In the only rather precisely defined study of the effects of contaminants on substrate sputtering so far published, Hofer et a/ 22 have recently measured the sputtering yields of Ti and V, exposed to dynamically controlled, constant pressure gas environments of 02, N, and Hz bombarded with 11 keV Ar+ and N,+ ions. Yields were determined from measurements of the ion fluence required to erode completely through a thin film of the substrate of precisely defined initial thickness. A sudden reduction by a factor of three in substrate sputtering yield was determined in all cases of Art bombardment where the ratio of ion bombardment rate:contaminant gas molecule arrival rate at the substrate was of order unity (only a minor change in yield occurred for Ti bombarded with N2+ ions in N2 atmospheres). Moreover, if ion bombardment, gas exposure was performed sequentially2’ rather than simultaneously much higher effective gas exposures were required to effect similar sputter yield variations. The relative independence of yield perturbation to contaminant gas species but the substantial dependence upon exposure conditions (i.e. dynamic or sequentially static) strongly suggests that in addition to gas adsorption occurring on the target surface, recoil implantation of adsorbed gas also occurs, moderating the near surface composition of the solid and thus binding energies and cascade development. This process is also confirmed by the insensitivity to N2 gas pressure of the Ti sputtering yield under Nz+ irradiation since this particle flux modified the near surface solid composition much more extensively than the adsorbed and recoil implanted N 2. The above investigations indicate, therefore, that the presence of contaminants either on or near to the surface of a solid may substantially modify the sputtering yield of the solid, but the detailed mechanisms are currently unclear. Thus not only may the contaminant potentially modify the self binding energy of surface substrate atoms but it may also act as an overlayer to prevent exit of potentially sputterable substrate atoms, i.e. the target surface now becomes an effectively deeper layer in a composite target of substrate and contaminant. It is suggested that useful experiments could be conducted in this area employing well defined target surfaces with known contaminant species and coverages. The process of recoil implantation of undesirable surface contaminants, although potentially deleterious in, for example, ion implantation of semiconductors has also not been studied in great detail although investigations of the forward recoiling of desirable impurities from a thin deposited source layer on different solids have been performed3’-34 whilst similar studies have been carried out on implantation induced recoiling of 0 from SiOz layers on Si35*36. In these latter areas both experimental and theoretical studies3’-40 employing different models, of the depth profile of impurities and the total impurity density recoiled into the substrate have been made and reasonable agreement between experiment and theory achieved. In the area 6

of recoil implantation of undesirable surface contaminants few definitive studies are available since, excepting the studies of Si02 on Si. well controlled surface condition experiments are absent. Studies of oxygen recoil implantation from ‘*O enriched Si02 layers on Si, using nuclear reactions stimulated by energetic protons to detect recoil ‘*O, have indicated that cross-sections for recoil implantation by 24 keV Kr+ ionsjs are of the order 5 x 10” cm2 (with backsputtering yields about 10 times this value) whilst the cross-section for 30 keV P+ ions were about an order of magnitude lower. Results of this nature indicate that an initial surface monolayer coverage by oxygen on Si would result in a one-tenth monolayer implantation of oxygen to depths between 100 and 1000 A following an incident fluence of ions in excess of lOI cm-‘. Some measurement by Grotzchel ef a/36 on the recoil implantation of C into Si resulting from B+ ion irradiation have recently been reported also. These authors, using the Rutherford backscattering channelling technique to be outlined in Section 3 measured the depth profile of disorder (defects) created by B+ ion implantation and observed a near surface peak in the defect density which was attributed to C recoil atoms from hydrocarbon layers on the surface, creating nearer surface damage than the more energetic B+ ions. The density of this disorder was observed to decrease with increasing B+ ion flux density indicating cleaner surface conditions induced by contaminant sputtering removal at higher beam fluxes. Similar near surface defect production was observed when layers of C (250-400 A thick) were deposited upon the Si surface before B+ ion irradiation, thus confirming the probability of this contaminant as a major source of trouble. Estimates of the recoil fraction from these films indicated that about 3 x IO” C atoms would recoil into the Si for a fluence of 4 x 1016,30 keV B+ ions. cm-‘, suggesting a recoil implantation cross-section for this combination of the order of IO-i9 cm’, much smaller than values of oxygen on Si induced by heavier incident ions. In the absence of detailed experimental studies it is, therefore, necessary to estimate recoil implantation effects theoretically. Since, generally, surface contaminants will be present in the submonolayer to several monolayers coverage regimes it is possible to theoretically predict the forward recoil of contaminant atoms by assuming discrete single, binary collisions between incident ions and contaminant atoms as if these latter were present in thin or only moderately thick source configuration. Probably the best and most extensive estimates of recoil implantation effects have been made by Kelly and Sanders4’ and Dzioba and Kelly42 who calculated the ratio of the probabilities for forward recoil implantation and backwards sputter removal for a range of ions incident upon different mass films themselves located upon different mass substrates. The more important results of these studies were that for light impurity films recoil implantation and back sputtering were of similar importance whereas for heavy impurity films forward recoil implantation was dominant by a factor of IO or more. Thus if ion bombardment sputter etching is used, as it frequently is, in an attempt to produce clean surface conditions for a variety of surface analytical studies, it should be clearly remembered that even from a single initial monolayer of contaminant, a very substantial fraction may be more deeply implanted into the solid during removal of this layer. If many layers are sputter etched the problem becomes of reducing importance but fractional percentages of impurity may be anticipated in the near surface layers even after substantial

G Carter

and D G Armour:

The relationship

between

vacuum

and atomic

erosion. The problem is even more severe if the ambient partial pressures of contaminant are high enough (e.g. >lOes torr for 1 mA . cm-’ sputtering ion currents) during etching to allow continuing contaminant to accumulate and be driven into the solid. Turning now to inelastic process modification by surface contaminants we state first our belief that no theories presently describe fully the processes of emission of secondary excited and ionized particles 43 . We are firmly convinced, however, that such particles are merely a portion of the total sputtered particle flux which have been excited or ionized either in collision or during exit from the surface. Thus physical models which describe sputtering generally, therefore, must be applicable as the bases to describe excited and ionized particle emission. Theories44-46 based upon the assumption of, for example, local thermodynamic equilibrium in plasma like zones created by each ion appear to bear little similarity to the linear cascade modelZo of atomic sputtering which is well known to account extremely well for the great majority of sputtering phenomena. Consequently although such excitation/ionization theories44-46 appear to be able to predict quantitatively the yields of ions and excited atoms from solids bombarded with 0+ ions or 0 contaminated solids, it is here suggested that the agreement between experiment and ‘theory’ is fortuitous and that this agreement indicates rro more than that experimental data can be explained in terms of an empirical matching equation.

Thus in the frequently observed relationships for contaminated solids sputtered ion yield c1atomicconcentration

x exp -@/kT

(5a)

and photon yield from sputtered excited atoms a

atomic concentration

Eli x exp - kT

(5b)

where CDis the work function of the substrate and E, is the upper excitation level of atoms from which photons are emitted, it is suggested that the quantity T is no more than an empirical fitting parameter and it certainly not an ‘equivalent plasma temperature’. One of the major problems in determining applicable physical models for the excited/ionized atom emission process is the lack of definitive data on ion and excited atom sputtering from in the first instance clean and well characterized solids and under controlled contamination conditions. Thus whilst there may be little doubt that, for many solids, the use of 0+ ion beams as primary irradiation or of inert gas ion beams in the presence of a high O2 over pressure leads to enhanced and stable secondary ion or excited atom yields of substrate atoms by perhaps several orders of magnitude compared to inert gas ion irradiation without the presence of continuous contamination, the physical or chemical state of the surface has rarely been properly defined. In technological application terms it may not, at first sight, appear to matter that since yields are both enhanced and stabilized by the presence of a defined contaminant (e.g. 0, or other electro-negative gas species) thus rendering the techniques more sensitive and apparently more reproducible, that the composition and structure are being drastically modified by the irradiation or environmental conditions. From the point of view of physical understanding, however, this situation is most unsatisfactory whilst even in technological applications the

collisions

in solids

perturbations introduced by the analysis technique may be such that the solid near surface region apparently being analysed bears little resemblance to the initial substrate. This is evident from the studies of Kelly and his colleaguesZ3* 24 who have shown that O+ irradiation of certain metallic oxides leads to both a gradual change in the oxide stoichiometry and the structures of the oxide until new stable states are produced. Acknowledging the realities of such, frequently extreme, changes in excited/ionized sputtered atom yield under conditions where either or both the ion irradiation and the gaseous environment around the target lead to large changes in the near surface physics and chemistry of the solid, we now review some, few, studies where attempts have been made to at least partially control contaminant effects. These studies, therefore, refer to conditions where inert gas ion beams were used as projectiles. The quite dramatic effects of variation of surface contaminant level upon an inelastic emission process are well demonstrated in studies of secondary ion emission induced by 43 keV Ne+, Ar+, or Kr+ bombardment of polycrystalline Al, Si, Ti, V, Cr, Fe, Ni, Cu. Hf. W and W by Snowdon and MacDonald47*48 and Snowdon43b. Using a sophisticated time of flight analysis system and minimizing all experimental artifacts these authors were able to record the velocity or energy distributions of secondary ions rather than total energy (andsometimesspatially) integrated ion yields. Although it was not possible to accurately specify the surface conditions and the degree and type of contamination in these investigations it was legitimate to infer that under continuously maintained vacuum conditions (IO-*-IO-’ torr total pressures) and continued ion irradiation that the metal surfaces would be gradually cleaned of contaminant by sputtering processes. It is, therefore, important to note that as irradiation fluence increased for any given substrate the energy spectra of sputtered ions suffered quite extreme characteristic modifications as did the total yield (the integral of the energy spectra) which usually decreased quite significantly. A quite typical sequence of observations is illustrated for a Cu substrate, where Cu+ ions were analysed, in Figure 2. This figure displays, quite clearly that for the initially most contaminated surface (run I), the spectrum is composed of the convolution of a number of peaks and as the surface is increasingly

*

.-p 40 0I. 0 5 w z -I

xx ++ xY +,x”

I

8 5 ‘1llll

l+ 40

Lo + lo

0 run4

f + x

2

^x

xx.

+ run 2 x run 3

I

20

I

00 Y

,I,,111 SO

400

I 2ocl

Figure 2. Secondary

ion energy spectra obtained from MRC Ltd research grade polycrystalline Cu at base pressures in the low IO-’ torr region bombarded by 43 keV Ar+. For comparison, spectra have all been normalized at 200 eV. The different spectra obtained front a particular target represent different, but undefined surface contamination conditions. 7

G Carter

and D G Armour:

The relationship

between

vacuum

and atomic

decontaminated (runs 2-3) the lower energy components are eliminated. In carefully argued discussion of such data SnowdotP has shown that these forms of energy spectra cannot be accounted for on the basis of ‘plasma’ models but require the existence of both linear cascade and direct surface atom recoil process as sputtering components each with different ionization production and possibly survival probabilities. Snowdon and MacDonald4’ also pointed out that other authors4g. So have observed somewhat different detailed changes in energy spectra as a function of Ot partial pressure above similar metallic substrates and suggested, that these differences arose from differing, but unknown, levels and types of contaminant. Studies of this type reveal the clear necessity of studying inelastic processes under better defined surface conditions in which some attempt is made to measure, at least, surface coverage densities of contaminants. Ideally the atomic positions occupied by such contaminants should also be investigated employing single crystal substrates and studies of this nature are currently in progress in our laboratories. At present, however, there is only one report of the influence of defined surface contaminant nature and coverage upon secondary ion yield by Taglauer et UP’. In those studies Co was adsorbed upon polycrystalline Ni and the Co coverage assessed using several low energy ion beam induced processes. These techniques included observations of (1) backscattering of 2 keV He+ ions from 0 atoms (to be discussed in full in Section 3), (2) the more energetic Ni+ secondary ions sputtered from the surface by the 2 keV He+ ions and (3) the photons emitted with 352.2 nm wavelength from Ni atoms sputtered in excited states and with 388.8 nm wavelength emitted from scattered excited He atoms. It was demonstrated that the backscattered ion signal was linearly proportional to Co coverage” and thus the Ni+ and Ni excited atom populations could be interpreted in terms of Co coverage. The results of these studies indicated that both the Ni+ and excited Ni atom yields were linearly increasilrg functions of Co coverage at submonolayer levels, whether determined at different initial coverages or during Co decontamination from the surface during sputtering. Such results are important since they illustrate that although the fraction of clean Ni surface decreases with increasing Co contamination the emission of excited or ionized Ni atoms increases. The authors thus suggested that, for submonolayer Co coverage, the enhanced Ni+ ion and excited Ni atom yields were proportional to the number of substrate metal-adsorbed molecule bonds formed and thus, in this case, that yield enhancements resulted from some form of, but unidentifiable bond dissociation mechanisms. It is clear that further careful work of this type and energy spectra measurements as commenced by Snowdon and MacDonald, is vitally important to a more complete basic understanding of inelastic emission processes and only when this is achieved can, for example, secondary ion emission be considered to be a predictable and quantifiable surface analytical technique. AS already indicated above sputtered excited atoms are also sensitive to surface contamination and to incident ion species which may modify the near surface composition and structure. Other evidence for such sensitivity of excited atom emission to surface conditions may be found in studies by Tolk and his colleagues53*54 Kerkdijk and Kelly5s-s7 and Rausch et UP*. In excited atom sputtering investigations a fraction of the sputtered flux of such atoms relax to their ground states via photon emission contiguously with competing non-radiative de-excitation processes (e.g. resonance or Auger de-excitation similar to reson8

collisions

in solids

ante and Auger neutralization which can cause neutralization of emitted secondary ions). The photon emission is frequently as a line spectrum, with lines typical of most near surface components of the sputtered solid excepting, in the visible region, optical lines of O2 and other electronegative species. The reason for such absence is not understood. In many circumstances the photon intensity of a given line appears to be linearly related to the atomic density of the elemental constituent, of which the line is typical in the solid at least for low fractional atomic concentrations. Since optical spectroscopy is both sensitive and elemental discriminating it appears that ion bombardment induced photon emission may be a powerful technique for solids composition analysis and several investigators 54*5g*60 have reported analytic applications. These applications have been generally restricted to systems where either composition assay could be confirmed by alternative methods or where calibration against standards was possible. The reason for such restriction is that, just like secondary ion emission, production cross-sections or yields of emitted excited atoms cannot yet be predicted analytically whilst yields are matrix and environment dependent as outlined for the Co-Ni system discussed earlier. In Tolk et al’ss3* 54 studies it was shown for example, that photon yields in different lines of a Mg spectrum for identical irradiation-conditions was substantially dependent upon whether Mg was present in a MgO or a MgF2 matrix, and that for a wide variety of metals and metal oxides the photon yields per incident ion (energy s 5 keV) was of order 10m4 + 10m3 for metals and two orders of magnitude higher for the metallic component of the oxide, clearly showing that the bulk environment is of substantial importance in determining excited atom yield. That adsorbed oxygen could also influence photon yields was inferred by Van der Weg and Lugujjo61 and proved more conclusively in studies of photon emission from 10 keV Ne+ ion bombarded Al by Kelly and Kerkdijk55*s6. These authorss5* 56 observed that for a given incident ion flux density, the photon yield, in a number of typical Al lines, increased from low values at the background system pressure (~7 x lo-’ torr) increased rapidly with increasing O2 partial pressure, saturating at pressures in the region of 10e4 torr. It was also found that at a given O2 pressure there was a dependence of photon yield upon incident ion flux density.at low values of flux but independence at higher values. Finally, it was observed that transients occurred in the photon yield either upon interrupting bombardment or following changes in O2 pressure or ion flux density the time constant of the transient being much smaller for beam interruption conditions than for step function parameter changes. Kelly and Kerkdijkss*s6 showed that these results could be largely interpreted on the assumption that 0 adsorption caused an enhancement in excited Al atom emission (but not in total sputtered Al atom yield) which was linearly proportional to 0 coverage, but that some recoil implantation of 0 occurred in addition to the ion bombardment induced sputtering of adsorbed 0. It was suggested that the enhancement mechanism resulted from adsorbed 0 locally perturbing the band structure of the solid surface increasingly towards that of aluminium oxide with increasing 0 coverage, which inhibited non-radiative decay of the emitted excited Al atoms. Somewhat similar results were subsequently observeds7 with Mg in O2 environments but in this case the majority of Mg atomic spectral lines increased in intensity with increasing (assumed) 0 coverage, some were

G Carter and

D

G Armour:

The relationshio between vacuum and atomic collisions in solids

coverage independent whilst some lines from excited Mg+ ions decreased with coverage. The band structure modification model thus appeared to be applicable for sputtered atoms but not for sputtered excited ions. A further failure of the band structure modification induced by adsorption model was observed in studies of photon emission from Al and Cu surfaces with adsorbed layers of Cs by Thomas et UP where the adsorption, which leads to substantial work function depression resulted in no enhancement in photon yield which should have occurred if the band structure model had been appropriate. Investigations of the effects of O2 adsorption on photon and secondary ion emission from Cr by MacDonald and MartuP also showed different behaviour from Al in that, for fixed ion flux density, the yield of photon from sputtered Cr atoms first increased with increasing O2 pressure, reached a maximum at pressures between 10e6 and 10m5 torr and then decreased. The O2 pressure at which the yield maximized was observed to be a linear function of ion flux density. Congruent with these variations in photon yield, changes in the secondary ion intensity of Cr + , polyatomic Cr. + and molecular chrome oxide Cr,O,+ ions also occurred with changing O2 environment and it was suggested that this behaviour was better understood in terms of the formation and sputtering of different excited and ionized molecular species with varying oxygen adsorption than in terms of band structure modifications. On the other hand further studies by Martin and MacDonald64 comparing photon and secondary ion emission from Ar+ and 02+ ion bombarded Si and Ar+ bombarded Si02 suggested that the band theory modification model was reasonably appropriate to this system. Clearly, therefore, the effects of contaminant on excited atom emission are substrate (and certainly one would expect contaminant) species dependent and no one model seems to currently fit all observations. A particularly interesting probable effect of contaminant adsorption is in the production of continuum spectra from some solids. As indicated earlier the major photon emission is generally observed in the line spectra of sputtered excited atoms but a number of authors5**65*66 have also observed continuum spectra from certain sputtered transition metals. Such observations are of substantial importance since the continua represent a background to the line spectra which are employed for surface composition analysis and, therefore, construct a noise or sensitivity limitation. Several hypotheses have been advanced for their occurrence but the most plausible current model is that by Rausch ef aIs* who attribute the process to surface contamination effects. This model is based upon one of the very few clean or well defined studies made in the excited atom sputtering field. Rausch et al58 investigated the optical emission from a MO surface bombarded by -24 keV Ar + ions at a base pressure of 3 x 1Omg torr with an 02 partial pressure of 5 x lo-” torr and observed almost exclusively the MO line spectrum. Upon increasing O2 partial pressure the continuum emerged with the line spectrum superimposed and studies were made of the continuum intensity as a function of O2 pressure and ion flux density. It was shown that the observations were consistent with oxygen adsorption leading to sputtering of excited MOO molecules which give rise to the continuum spectrum. As a final example of contaminant problems it may be noted that not only may certain contaminants not give rise to excited atom emission (e.g. 0 and electronegative atoms) whereas others do, White ef af54 reporting on CH associated spectra arising from surface

contamination and Thomas et UP observing OH, CH and NH related spectra and substrate atoms-hydrogen molecular spectra but in certain circumstances the ion beam used for excitation can generate molecular species from surface atomic species. Thus Bazhin et UP have observed spectra associated with sputtered CN molecules from alkali halide substrates formed from contaminants of carbon and nitrogen and excited by exciton (electron-hole pair) recombination in the solid, the excitons themselves being generated by the incident ions. It is thus abundantly clear that surface contaminant adsorption can grossly modify, at present often in poorly understood ways, the sputtering of excited substrate atoms and the probability of these giving rise to photon emission. Many more definitive studies of the type indicated by Taglauer et al for Co on Ni are necessarybefore fundamental processesare better quantified. Indeed suchstudiesare vital if excited and ionized atom emissiontechniquesare to be developedfor quantitative solids analysis purposes since as this review illustrates the magnitudeand nature of sputteredexcited atoms and ions is often crucially dependentupon, often poorly defined,environmentalconditions. An order of magnitudeestimateof contamination problems can be deducedby consideringthe dynamic surfaceequilibrium coverageof a surfaceat constant ambient pressure,when ion bombardmentinducedsputtering of adsorbedimpurities is the dominant processdictating adatom removal. From the studies of transient photon emission changes by Kerkdijk and Kelly55*56 and Rauschet aIS and from direct adsorbedatom recoil sputtering studiesusing low energy ion scatteringto be discussedin the next section, it is found that the contaminant atom sputtering cross-sectionfor low keV range heavy ions (e.g. Ne+, Ar+) is of the order of lo-r5 cm’. Thus the effective gassputteringcoefficient for monolayer coverageof contaminant is of order unity. In equilibrium betweengas sputtering induced by a projectile ion flux densityj A . cm-’ and adsorption at constant pressurep torr, assumingunity sticking coefficient, the fractional surfacecontaminant coverage 0 will be given, approximately by j6x

10’s.~=1015

’ ‘10-6

(64

or e-

lO?p/j.

(6W

In most investigationsthe ion flux density is unlikely to exceed 1 mA . cm-’ so at an ambient pressureof contaminant of 10m6torr the equilibrium surfacefraction will be one-tenthof a monolayer. Rememberingthat Talk et uP3 suggestedthat the photon yield from metal atoms in a metal oxide was two orders of magnitudehigher than from the pure metal and inferring that monolayer coverageis approximately similar to the oxide and also noting that the Co on Ni data of Taglauer et al” suggest a two order of magnitudeincreaseof Ni excited atom ejection for monolayer coverage as compared to a clean surface, the presenceof a fractional surfacecoverageof 0 would lead to a yield enhancementof order 10’ . 0 comparedto the cleanmetal. Thus at ambient of low6 torr contaminant the photon yield would be 10timesthat for the cleansurfaceyield achievableby heating the systemand substrateand maintaining uhv. Small partial pressurefluctuationsAp torr would thus leadto constant 9

G Carter

and D G Armour:

The relationship between vacuum and atomic collisions in solids

yield fluctuations Ay/uO relative to the clean surface value yo, for a 1 mA . cme2 ion flux density of order 10’ Ap, i.e. tenfold changes in the yield for partial pressure fluctuations of the order of 10m6torr or 10% changes in yield for pressure fluctuations of 1Om8torr. Varying environment is thus seen to be a non-trivial problem if inelastic processes induced by ion impact on surfaces is to be used for analysis purposes as well as for understanding the physical processes involved. Of course use of contaminant beams of ions (e.g. O+) will minimize such problems but themselves must introduce considerable perturbations in both the composition and structure of the solid under analysis.

mass ratio of the target and incident particles, A(= M2/ M1 2 1) by the relationship:

El =E,,

cos 0 4 (A2 - sin2 19)~‘~

(7)

Since Eo, El and 0 can be precisely specified, an ion yield at a particular energy gives an immediate indication of the mass and hence identity of the target atoms, and, through a knowledge of the differential scattering cross-section, a measure of its abundance. In its simplest form, the relevant yield equation for scattering from a target species i with surface concentration N, atoms. crns2 is:

3. The use of ion-solid interactions to study surface conditions The use of ion-solid interactions to study surface conditions has been based not only on the measurement of secondary ion and photon yields but also on the study of the energy distributions of scattered primary ions. The development of scattering techniques employing a wide range of primary ion energies has made it possible to carry out compositional and structural analysis of the outermost atomic layers of solid materials and has provided a means of probing these regions of the solid, the properties of which are dependent on both the material and its environment. Because of their high surface specificity (low energy ion scattering, for example, is sensitive only to the outermost layer of atoms) and their ability to yield information concerning composition and structure simultaneously, these techniques have found increasing application in the study of adsorption processes, particularly those in which accommodation of gas atoms is accompanied by surface relaxation or reconstruction and, perhaps of more relevance in the present context, beam induced desorption, which has led to a greater understanding of contamination problems in large vacuum devices such as fusion machines and storage rings. Throughout the entire energy range covered by ion scattering studies, a few hundred eV up to several MeV, the techniques are based on the energy analysis and detection of particles scattered through a specified angle during impingement of a mono-energetic, highly collimated beam of ions onto the solid surface. In some cases the overall energy and hence mass, resolution of the system may be limited by the quality, in terms of the energy and angular spreads, of the primary beam and in desorption studies the beam intensity profile also becomes an important parameter. The way in which information concerning the mass, number density and atomic arrangement of the target atoms is deduced from experimentally measured energy distributions and in fact the limitations of the technique in terms of mass and depth resolution, sensitivity and quantitative accuracy, depend on the nature of the fundamental scattering processes occurring during the ion-solid interaction and three comparatively distinct scattering techniques, low energy ion scattering (ISS), medium energy ion scattering (MEIS) and Rutherford backscattering (RBS) have been developed. Although the physical processes characterizing these techniques differ considerably in detail, their use in the study of surface composition is based on a common description of the motion of the particles using classical mechanical trajectory equations. Hence, in all cases, assuming only a single collision between the projectile ion and a surface atom is responsible for the scattering, the energy of the reflected particle, E,, is given in terms of the primary energy, Eo, the scattering angle 0, and the 10

(8) where IO is the incident ion current, A the bombarded area, do/da the differential cross-section for scattering into unit solid angle, An the acceptance solid angle of the analysis and T, the overall transmission factor of the analysis and detection system. Because of the dependence of the scattering cross-sections, energy loss processes and inelastic effects such as neutralization on the details of the particular collision situation under consideration, straightforward interpretation of experimental data in terms of these equations is not always possible, and in fact, the situation is so complex that demarcation of the different scattering regimes simply on the basis of primary energy, is more or less meaningless. The low, medium and high energy techniques are thus more appropriately categorized in terms of the dimensionless energy parameter, Pg*‘O which is given by the expression aE &=y

zlz2e

M2

M, + M, ’

(9

where z, and z2, and M, and M2 are the atomic numbers and masses of the projectile and target atoms, respectively, E is the projectile energy and a is the Thomas-Fermi screening length. In terms of this classification, ISS corresponds to e values below about 0.3 and is characterized, in its simplest form, by scattering via a single elastic collision event at the surface, MEIS is associated with E values between 0.3 and about 10 for which significant multiple scattering occurs and elastic and inelastic energy losses are similar in magnitude and RBS conditions apply when e is greater than about 20. In this region, as for ISS, the deflection occurs in a single collision, but in this case, this may occur not only at the surface but at a depth of up to several microns beneath it. The projectile suffers quasicontinuous loss of energy to the electrons in the target during its inward and outward trajectories and consequently the measured energy spectra contain quantitative information concerning variations in target composition as a function of distance from the surface. In the case of ISS, rare gas ions in the energy range between about 500 eV and 10 keV are typically employed as the probe particles and the technique has been developed as a powerful method of analysing both the composition and structure (in terms of the relative positions of the surface atoms) of the outermost surface layer. The choice of probe ion species and energy depends on the nature of the surface and process under study, the resolution required and the amount of perturbation due to

G Carter

and D

G Armour:

The relationship between vacuum and atomic collisions in solids

the primary beam that can be tolerated. The lower mass ions, helium and neon, have thus been widely used to study adsorption processes at sub-monolayer coverages where the ability to identify and locate light impurities on a comparatively heavy substrate is important and beam induced damage and desorption must be kept to a minimum. In the medium energy regime, even though the use of heavy probe ions does result in spectra which contain information concerning the composition and, under appropriate circumstances, structure of the target, the data are difficult to interpret and substantial perturbation of the surface region during analysis is unavoidable. Consequently the MEIS technique has been developed as a means of studying overlayer and substrate structures using light ions, H+, and He+, in the energy range between about 50 and 300 keV as the probe. It has proved to be of particular value when used in the so-called double alignment mode in which both the incident and outgoing ion trajectories are aligned with major crystal axes such that, by suitable angular scans, the existence and magnitude of any relaxation of the surface layer, can be determined. For studies of the surface and near surface characteristics of single crystals, the RBS technique, which typically makes use of l-2 meV H+ or He+, has also been combined with channelling for improved depth resolution. Since, for the scattering of these high energy, light particles, the differential scattering crosssection is known precisely and the conventionally employed solid state detectors are equally sensitive to both ionized and neutral particles, this technique provides accurate quantitative information and, from the point of view of surface condition monitoring, is uniquely capable of providing absolute coverage data and allowing determination of thin film and surface oxide stoichiometry. The ion scattering techniques are thus capable, collectively, of providing information on essentially all the factors that characterize the condition of a surface. Their additional ability, to provide this compositional and structural information simultaneously without, from the point of view of structure studies, the requirements of long range order, is of particular value in the investigation of the early stages of adsorption. Historically, because of the extreme surface specificity of the technique and the need to employ low probe ion fluxes to minimize perturbation, ISS systems have employed ultra-high vacuum target chambers and have been widely used to study adsorption and beam induced desorption processes. The light ion, medium energy technique, which utilizes similar accelerator technology to that employed in ISS, has also been specifically developed for study of the surface and near surface regions of solids and target chambers operating in the 10e9 torr region are conventionally employed. In contrast to these methods RBS has only recently been used to make detailed studies of surfaces under well controlled conditions. The failure to fully capitalize on the capabilities of the technique in this application at an earlier stage probably stems from its use in the routine analysis of ion implanted materials where the quality of vacuum in the scattering chamber is not critical and, the need to change targets quickly renders maintenance of a uhv target environment either impractical or extremely expensive. The characteristics of the low energy technique which make it particularly useful for surface studies are its ability to detect light impurities adsorbed on heavy substrates and, by virtue of the pronounced shadowing effects which result from the large scattering cross-sections for low energy particles, to indicate

their position relative to the substrate atoms. When small total scattering angles are considered, these shadowing effects lead to characteristic multiple scattering processes70~71 the contribution of which to the observed energy distributions provide information concerning the spacing between surface atoms. The ISS technique, owes its surface specificity to the highly efficient neutralization of those particles which penetrate below the surface layer combined with the use in conventional systems of electrostatic analysers which allow analysis and hence detection of only those particles which are reflected in the ionized state. This highly desirable feature of the measurements is only obtained, however, at the expense of sensitivity (only between 0. I and IO % of the total reflected yield is ionized) and quantitative reliability since adequate theoretical models describing the charge exchange processes occurring during the interaction of the projectile with the surface are not available at the present time. The combination of neutralization and shadowing processes thus modifies the general yield equation which, for the low energy regime must be written in the form: Ii - (Ni - aN,)A

doi

. I, . do AR. Ti . Pi,

whereN. is the surfacedensity of the substrate,a a shadowing or geometricalfactor and P1 the ion escapeprobability. The shadowingis determinedby the cross-sectionof the species,a, which blocks the accessof the impinging ions to the speciesi suchthat the impact parametersrequiresfor deflectioninto the chosenscatteringangleare not available. This equation clearly illustratesthe problemassociatedwith the quantification of ISS. The differential scattering cross-sectionis not known precisely in the relevant energy range, the ion escapeprobability is not well defined and may be dependenton chemicalenvironment or coverageand the shadowingfactor may also change with coverage,particularly if the surfacereconstructs. Despite theselimitations, which can be overcomein certain casesby calibration againststandardtargets,the ISS technique has proved to be a valuable tool in surface analysisand its extreme surfacespecificity has beendemonstratedby the complete suppressionof substrate yields at single monolayer coveragesin a number of different cases72-74. In most cases wheregasadsorption is studied,neonor helium ionshave been usedas the probe and with the latter, even the adsorption of hydrogen on tungsten can be monitored by observing the variations in scattering yield from the substrate due to the shadowingaction of the adsorbedgas75. In all cases,however, straightforward useof the techniqueto study adsorptionkinetics or to deducethe relative positionsof surface atoms on the basisof shadowingeffects relieson the existenceof a linear relationshipbetweenion yield andcoverage and in many casesthis linearity, which dependson P, remaining constant, has beencheckedby comparisonwith measurements using alternative techniquessuch as neutron activation analysis76,photon emission” and Auger electron spectroscopy”. Since none of thesealternatives share the samesurface layer specificity as ISS, the comparisonsare only valid at submonolayer coverages.Under thesecircumstances,for the case of a nickel surface containing oxygen or sulphur78*79the scattering yield from the adsorbedspecieshas beenshown to dependlinearly on coverageand detection limits in the range 5 x 10e2 to 10e3 of a monolayer deduced. In fact, this linearity appearsto hold in most casesand has allowed the 11

G Carter

and D G Armour:

The relationship between vacuum and atomic collisions in solids

technique to be used in a wide range of applications including the investigation of the molecular orientation of carbon monoxide adsorbed on tungsten (100) and nickel (100) and (110) and (111)) surfaces 80*81, the measurement of the copper enrichment of the surfaces of copper-nickel and copperplatinum alloyss2 and the study of the equilibrium surface composition of binary materials” and oxide?’ (i.e. the study of preferential sputtering effects). A second major problem associated with the use of ISS for adsorption studies concerns the possibility that the probe beam may damage the surface and create adsorption sites which would not normally be available or remove adsorbed gas atoms during the analysis. From the point of view of damage production, adequate annealing rates can generally be obtained by operating at elevated target temperatures and for nickel and copper targets a temperature of about 200°C has been found to be sufficient*5P86. The removal of adsorbed gas by ion impact desorption is important not only from the point of view of measurement accuracy but also in its own right as an important process in vacuum technology since gas discharge and ion beam cleaning techniques are now widely used. The linear relationship between coverage and yield allows, in principle, desorption cross-sections to be deduced from ion scattering measurements and the available experimental data indicate values between about lo-l4 and lo-l5 cm’ depending on the system under study and the energy and mass of the primary beam particles*7-8g. The reliability of this type of measurement, however, depends strongly on the degree of uniformity of the primary beam and on the validity of the assumption that the drop in signal for scattering from the adsorbed species is due to sputtering from the surface rather than removal from the bombarded area to an alternative position either on the surface or inside the solid. The beam uniformity problem is clearly demonstrated in Figure 3 where experimental data for the desorption of oxygen from a nickel (110) surface are compared with computed curves for various beam intensity profiles.

r

3. The dependence of the oxygen peak height on the ion fluence (dots). Full curves represent the predicted behaviour for different ratios of c/b where c and b are indicated in the inset, which shows an intensity profile of the circular beam on target. Figure

12

In contrast to the compositional analysis or coverage measurement application of ISS in which scattering geometries and probe ion species and energy are simply chosen to obtain the best compromise between resolution (enhanced using heavy ions and large scattering angles) and sensitivity, the use of low energy ion scattering for the study of surface structures has led to the development of two distinct types of analysis, the binary scattering-shadowing and low angle-multiple scattering techniques. Both methods are based on the same fundamental shadowing effect, and the accuracy with which the atoms can be located in the surface is limited by the difficulty in calculating the precise incident particle trajectories since the scattering equation cannot be solved analytically for the interatomic potentials involved, the inability to take full, quantitative account of the role of thermal lattice vibrations and the inadequacy in some cases, of the two-dimensional models used in the computer simulation analysis. Owing to these uncertainties, it is generally accepted that the atomic positions cannot be specified to better than kO.2 A using low energy techniques. The binary scattering-shadowing method of analysing surface structures makes direct use of the potential masking effect and relies on observation of the extent to which the scattering yield from one atomic species is suppressed by the presence of atoms of another. The basic principle of the technique is to vary the scattering geometry (in general the angle of incidence and target azimuth are changed while the scattering angle is kept constant) in order to determine the conditions under which atoms of one species are located within the shadow cones of others. The analysis is simplified by using light primary ions, usually helium, and large scattering angles such that beam induced damage and shadowing effects on the outward trajectory are minimized. The small cross-section for multiple scattering for the light ions is generally assumed to allow the data to be analysed in terms of a simple, two atom binary scattering model. Since, under these circumstances, the masking is essentially a straightforward geometrical effect, qualitative information about the surface atomic arrangement, for example whether or not the surface is reconstructed or whether adsorbed atoms take up sites in or on top of the surface, can be obtained without detailed calculations involving consideration of interatomic potentials or thermal lattice vibrations. Quantitative analysis, however, does involve these factors since the screening function determines the effective size of the atoms and the thermal vibrations affect the extent to which masking occurs. The major application of this technique has been in the study of the adsorption of gases and other substances onto single crystal surfaces and, for example, the 0-Ni (1 1O)g’ and (100)92, the O-W (110)93 and (100)g4 and the 0-Ag (110)95 system have been investigated. In those cases where ISS has been combined with low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) studies, good agreement between the measurements has been obtained. The combination of ion scattering with LEED is particularly useful since a knowledge of the crystallography of the surface obtained from electron diffraction measurements often allows the most effective azimuthal directions for ion scattering to be selected. The basic simplicity of this type of measurement is evident in the study of oxygen adsorption onto the silver (I 10) surface carried out by Heiland et ~1~~. On this particular silver surface a (2 x 1) LEED pattern is formed after adsorption of half a monolayer of oxygen. Ion scattering spectra for different target azimuths, obtained using a 600 eV He+ beam, and constant

G Carter and

D

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The relationship between vacuum and atomic collisions in solids

angles of incidence and scattering, immediately suggest that the oxygen atoms adsorb onto sites between the top
Quasi

0.8

Single

/

,

Quasi Single

0.9

I

I

1

titi)

EIEo

Figure 4. Energy spectraof 6 keV Ar+ ions,scatteredspecularly

through30” from a Ni (1IO)surfaceshowingthe dependence of the multiplescatteringprocesses on interatomicspacing.

employed. The overall analytical procedureis to measurethe scatteredion energy distributions for different target azimuths and to comparethe data with the resultsof calculationsbased on assumedgeometricalsurfacearrangements.In commonwith the binary scattering-shadowingmodel,qualitative information can be obtained relatively easily but quantification requires more complex calculationswhich, for the multiple scattering situations, invariably involves the useof computer simulation. The way in which multiple scatteringenergy spectradepend on interatomic spacing can be seenin Figure 4 where the scattering of 6 keV Ar+ through 30” from the four closest packed atomic rows in a Ni (I 10) surface is considered. The dependenceof the QS and QD peakson interatomic spacingis obvious and hencethe spectracontain information not only on the identity of the atoms in the chain from which scattering occursbut alsoon their relative positions.It is this aspectof the technique that makesit particularly useful for the study of adsorptionprocesses which leadto changesin surfacecomposition and, in somecases,structure. Its potential in this area is clearly demonstratedby the changeswhich occur in the above spectrawhen the surfaceis exposedto oxygenQ7.The presence of oxygen atoms on the surfacewas seento affect the spectra obtained for scatteringalong the various atomic rows in very different ways. The major changesoccur in the two closest packed directions, namely the (110) and spectra from the oxidized spectra suggest,if anything, a decreasein the interatomic spacingin this direction and a possibleexplanation of theseresultsis that the adsorption of oxygen causesthe surfaceto reconstructin sucha way that the atomic spacing in the (i10) direction is doubled, i.e. every alternate (001) row of atomsis missing,and the oxygen atoms residein, or closeto, the remaining(OOi) rows leadingto an effective reduction in the atomic spacingin this direction. Useof Ne+ asthe probe speciesallowsthe feasibility of this surface model to be further investigated since, for this ion. scattering through 30” from the oxygen atoms themselvesis energetically possibleand the azimuthal dependenceof the height of the oxygen peak givesan indication of the location of the oxygen atoms. The observation of considerablemasking of the oxygen atoms in the (001) direction confirm that they probably lie in the (OOi) rows. In fact, by carrying out careful measurements asa function of oxygenexposure,the kineticsof the adsorptionprocesscan be studiedand, the observationof a changein visibility of the oxygen atomson the (007) direction after an exposure of torr . s further suggeststhat during the early stagesof chemisorption,a two-stage process,involving initial oxygen adsorption onto sites situated in the four-fold surface hollows followed by a reconstruction processduring which the adatoms stabilize at positions in the rows, occursoB. The marked dependenceof the smallangle scatteringof the heavier rare gasions from singlecrystal surfaceson the identity of the relative positionsof the surfaceatoms hasthus allowed ISS to make a valuable contribution to the overall study of processes associatedwith exposureof the surfaceto adsorbing gasatomsor bombardmentwith energeticparticles.Useof this 13

G Carter

and D G Armour:

The relationship

between vacuum and atomic collisions in solids

technique does not, however, yield information on the immediate subsurface layers and in order to study phenomena such as surface relaxation or, in fact, to accurately quantify adsorption measurements, the medium (MEIS) or high (RBS) energy techniques must be employed. In this particular application these methods, as already indicated, make use of the comparatively large angle scattering of light ions, H+ and He+, at energies in the XI-500 keV and few MeV regions and their ability to yield quantitative information is based on the description of the scattering process by an unscreened or bare nucleus Coulomb potential. Under circumstances where such a description is valid (departures in the case of He+ on high Z target atoms have been observedgg) the differential scattering crosssection in the laboratory frame of reference is given by the Rutherford formula ? \ 312 / 1 do -= cm

(Z,Z2e2)2 16ECM2

sin-4o

,2

’ + $

-I- : ’ ‘OS ‘CM

CM

>

Ibl

Figure 5. (a) Schematic energy spectrum for Rutherford scattering from a random target. (b) Diagram illustrating the processes leading to the type of spectrum shown in Figure 5a).

1 + f . cos ec*, >

(11)

where EC,,, = M,/(M, 4 h4,) . E,, and Bcnr is the centre of mass scattering angle, and accurate quantitative analysis of the composition of the target is possible. For scattering in these energy regimes, however, the measured energy spectra also contain depth information and the analysis provides the ability not only to distinguish between and measure the number per square centimetre of the different elements present in a target but also to indicate their distribution in depth below the surface. The inherent availability of depth information in RBS spectra stems from the fact that in the energy range of interest the elastic scattering cross-sections are small and particles which penetrate beyond the target surface lose energy predominantly in nondeflecting interactions with the electrons in the solid. They thus essentially straight line trajectories through the material until they undergo a hard collision with an atom core which may deflect them back out of the solid into the detector. A knowledge of the rate of loss of energy due to inelastic losses, dE/dx (the stopping power) and its dependence on energy allows the measured energy scale to be simply converted to a depth scale. As a result of this combination of elastic and inelastic energy loss processes the energy spectrum for scattering from an amorphous material would be of the form shown schematically in Figure 5 where the high energy edge of the spectrum corresponds to scattering from the surface layer atoms and the increase in yield at low energies is related to the E,,Ie2 dependence of the scattering cross-section. Although the technique is clearly sensitive to heavy impurities present on the surface of or inside light substrates (the yield from such materials is high due to the Z22 dependence of the cross-section and their contribution to the energy spectra falls outside the substrate yield), the problems associated with the study of light atoms adsorbed on heavier substrates are immediately apparent since the scattering yield from them falls within the plateau region of the substrate spectrum. While suppression of this bulk yield to give enhanced sensitivity to light impurities is a natural feature of conventional low energy scattering systems, it can only be achieved at medium and high energies for single crystal targets by employing channellingioO or channelling and blocking techniquesiol whereby the yield from subsurface atoms is reduced by virtue of geometrical shadowing effects. 14

hl

Referring to Figure 5b and on the inward and outward So(E) the energy difference depths I and I -t Ar is given

E,-E,=AE=

assuming that the stopping powers trajectories are given by S,(E) and between particles scattered from by:

k2 k2 -. Si(E) + S,(E) Al, cosq5 i cos 0 I (12)

where kZ is the ratio between the projectile energy before and after the hard collision. (See equation (7)) For small penetration depths or shallow range profiles, the stopping powers on the inward and outward trajectories are often considered to be constant at Si = (dE/dx)E, and So = (dE/dx)k2Eo. Equation (12) can be rewritten in terms of the backscattering energy loss parameter, IS1’02 such that

AE = (SI At = N I&JAt,

(13)

where N (atom cmw3) is the target density and 1~1(eV . cm2/10’s atom) is the stopping cross-section. This equation indicates that the depth resolution is directly related to the detector energy resolution and that it can be optimized by, for example, selecting the primary beam energy which gives the highest stopping power (100-200 keV H +)io3 or employing grazing incidence or emergence geometries (large values of B and 4 in equation (12)). From the point of view of surface and near surface analysis where very high depth resolution is required, the primary energy region below about 200 keV has the additional advantage that the scattered particle energies can be conveniently measured with an electrostatic analyser. Since these instruments, for example 90” or 127” cylindrical or hemispherical condensers having radius of curvature in the range 50-200 mm, can be designed to have small geometrical energy width, AE N 4-5 x 10m3 E, energy resolutions for 100 keV particles of 400-500 eV can be obtained. This compares favourably with for example, the values of 2.8 keV FWHM for 120 keV H+ lo4 and 5 keV for He+ + lo5 that have been achieved with cooled, thin window solid state detectors. For I50 keV H+ scattering from Ni at 0 = 4 = 60” (see Figure 5b) a depth resolution of about 7 8, is theoretically possible using an electrostatic analysis withAE/E =0.4%andassumingastoppingpowerof22eVA-’. In the high energy scattering regime, e.g. 2 MeV He*, where

G Carter

and 0 G Armour:

The relationship

between vacuum and atomic collisions in solids

solid state detectors are almost universally employed, depth resolutions of between 100 and 300 A are typically obtained using a 15 keV resolution detector while values of the order of 30 8, have been reported when using the glancing incidence techniqueto6. Use of an electrostatic analyser does, however, affect the analysis of scattering data since only ions can be detected and considerably higher probe fluences are required (typically about 25 times higher than when using a solid state detector and multichannel analyser) due to the inability to detect neutrals and to the single channel nature of the device. The form of a random target spectrum such as that shown in Figure 5a is also dependent on the type of energy analyser since the height of the plateau measured with an electrostatic analyser depends on the resolution while for a solid state detector it is simply a function of the amplifier gain, i.e. of the channel width, SE of the pulse height analyser. It is interesting to note that in the medium energy regime, in contrast to the situation at low energies, the introduction of an ion escape probability term, P,, into the yield equation, i.e.

I = IcN At(da/dQ)

AQ . T . xPi,

(14)

where N is the target density (atoms cm-‘) and the other terms have already been defined, does not seriously impair the quantitative capabilities of the technique since measurements of the ion fractions for backscattered He+ and H+ in the energy range 25-150 keV have indicated that the neutralization efficiency depends smoothly on velocity and is not a strong function of depth or target material”‘. The major problem with using electrostatic analysers thus concerns the need for increased probe fluences which leads some cases to significant perturbation of the targetlo8. From the above discussion it is clear that the application of medium and high energy ion scattering to the analysis of surface conditions in particular, exploitation of their unique quantitative capabilities in surface composition monitoring when light adsorbates are involved is restricted to the study of single crystals for which channelling techniques can be applied. In this context their ability to provide depth information has made it possible to extend surface structure measurements to study the displacement of substrate atoms at the surface due to relaxation, reconstruction or overlayer adsorption. The technique is thus of potential value for monitoring thin film reactionsro9 such as solid phase epitaxial growth. The experimental method used in surface relaxation and composition measurements using MEIS is based on the alignment of both the primary beam and detector with major crystal axes’“‘. For composition analysis, this procedure reduces the level of the background behind the surface peak in the energy spectrum to such an extent that surface coverages of carbon, oxygen and sulphur of 0.2, 0.1 and 0.03 monolayers, respectively, on a Ni (1 IO) surface can be detected using a 200 keV proton beam”‘. In the particular study referred to here, the influence of pre-treatment, target chamber residual gas pressure, target heating, sputtering and proton bombardment on the condition of the nickel surface were investigated. The marked build up of hydrocarbon and nickel oxide on the surface, induced by the proton bombardment at a base pressure of IO-’ torr, clearly illustrates the importance of the vacuum environment in this type of study. For relaxation studies, which have been carried out on the Pt(lll)“’ and Ni (1 1O)‘12 surfaces the experimental method is

to measure the angular positions of the surface and bulk blocking cones. A displacement of these cones with respect to one another indicates that some surface atomic relaxation has occurred. The experimental procedure for obtaining a surface blocking dip is to align the primary beam with a channelling direction in the crystal such that it can only hit atoms situated in the first and second layers (e.g. beam entering a Ni (I 10) surface along the (3i4) direction), and then to monitor the intensity of the surface peak as the detector is moved around directions where the scattering yield from the second layer atoms is blocked by those in the first layer (e.g. the (OOi) direction). The bulk blocking direction can be established by setting an energy window in the low energy region of the backscattered energy spectrum and recording its content during the same angular scan. Measurements on a clean Ni (I 10) surface using 100 keV H+ have been shown that the surface blocking dip is shifted towards smaller scattering angles by 0.9” f 0.2” indicating a contraction of the upper layer spacing4 3 1%. For the same surface containing one-thirdmonolayerof adsorbed oxygen an expansion of I * 1 ‘A was measured while for a sulphur covered surface an expansion of 6 * 3 y0 was observed. Relaxation effects on the Pt (I I I) surface have been studied using both the medium energy double alignment technique”’ and high energy channelling’L3~L14 and there is now general agreement that the clean surface plane relaxes outwards by about l-2%. This is also in agreement with the results of low energy electron diffraction studies ’ I5 . It is interesting, however, from a technique evaluation point of view to consider the application of high energy scattering to this type of measurement in more detail since preliminary measurements on this surface indicated an outward relaxation of 10%“6. The technique is based on the fact that the channelling phenomenon enables a clearly resolved, high energy peak to be observed in the RBS spectrum and that the area of this peak is an accurate quantitative measurement of the number of unshadowed atoms per square centimetre in the surface region. The major limitations of the technique are thus related to the extent to which the background yield behind the surface peak can be reduced, the errors invclved in its subtraction from the surface peak and the accuracy of the yield calibration. The problem of background yield subtraction emphasises once again. the need for high energy (depth) resolution for surface studies since the most efficient method of minimizing the subtraction errors is obviously to reduce the background level. This has been achieved using solid state detectors by means of the glancing exit technique and by employing a high resolution magnetic spectrometertoo* ’ 13. Even when adequate correction and calibration procedures are used, however, the measured surface peak area still depends on the precise condition of the surface and the data are inevitably dependent on the method of surface preparation, the quality of vacuum in the target chamber and on the damaging or perturbing influence of the probe beam. In fact, the reasons for the large relaxation effects observed in the preliminary studies of the Pt (I I I) surface at 40 K are considered to be related to the damaging influence of the 2 MeV H+ beam combined with the presence of adsorbed hydrogen atoms on the surface which may play some role in enhancing the sensitivity to beam induced displacements. The combination of high energy scattering and channelling has also been employed very effectively for studying the interaction of hydrogen with the W (001) surface”‘. Although this surface has been the object of numerous LEED studies”8*“g 15

G Carter and 0 G Armour: The relationship

between vacuum and atomic collisions in solids

the exact nature of the cleansurfaceand the effect of hydrogen adsorption on the substrate cannot be specified with certainty from the measurements. In contrast, the ability to accurately measure the number of tungsten atoms exposed to the incoming

beamasa function of hydrogen coverageusingthe channelling techniques in conjunction with proper calibration and background subtraction proceduresrzO makes it possible to show conclusively that both the clean W (001) surface and the C(2 x 2)-H surface are reconstructed and that the surface reorders by H saturation. In these particular experiments, the ion scattering system also incorporated LEED apparatus and the results clearly demonstrate the complementary nature of the two techniques. While there is increasing evidence to suggest that no single analytical technique is capable of solving all the problems associated with the study of the solid-vacuum interface, there is little doubt that the use of ion-solid interactions in the form of low, medium and high energy scattering has made an important contribution to our understanding of the effects of adsorbed

gaseson the structure of solid surfaces.The scattering techniques have clearly demonstrated the extreme sensitivity of many surfaces to the presence of adsorbates and their combined ability to monitor the composition and structure of the surface and sub-surface layers has been shown to be of particular value in adsorption studies.

r G Carter and W A Grant, Ion Implantation of Semiconductors. Edward Arnold, London (I 976). 2 J Camplan, JPhys Radium, 22, 1961,91 A. ’ M Menat, Can J Phys, 42, 1964, 164. 4 J H Freeman, Applications of Ion Beams to Materials. (Edited by G Carter, J S Colligon and W A Grant) Inst Phys Conf Series, no 28, London, p 340 (1975-I 976). 5 S T Picraux, J A Borders and R A Langley, Thin Solid Films, 19, 1973,371. 6 P L F Hemment, J F Singleton and K G Stephens, Thin Solid Films, 28, 1975, I. ’ K Leyland, D G Armour, G Carter and J H Freeman, Low Energy Ion Beams. (Edited by K G Stephens, I H Wilson and J L Moruzzi) Inst Phys Conf Series, no 38, London, p 175 (1977-1978). 8 P L F Hemment.Low Ener.ay Ion Beams. (Edited by K G Stephens. I H Wilson and J L Moruzzi)-Inst Phys Conf Series; no 38, London; p 117 (1977-1978). 9 D A Thompson and R S Walker, Nucl Instrum, 132, 1976,28 I. lo J A Van den Berg, D G Armour and L K Verheij, Low Energy Ion Beams. (Edited by K G Stephens, I H Wilson and J L Moruzzi) Inst Phvs Conf Series. no 38. London. D 298 (1977-1978). ” C M Braganza, G.Carter and G Farrell, Nucl Instrum Methods, 132, 1976,679. I2 S Doorn, C Foster, T Hoogkamer, H Roubens and F Saris, Nucl Instrum Methods, 120, 1974, 17 I. ‘a G Carter and J S Colligon, Ion Bombardment o/Solids. Heinemann Educ Books, London (I 968). I4 J Lindhard, M Scharff and M E Schitt, Kgl Dan Vid Selsk Mat Fys Medd, 33, No 14. I6 P Sigmund, Rev Roum Phys, 17, 1972,823,969 and 1079. ” J A Brinkman, J appl Phys, 25, 1954, 961. I8 P Sigmund, Appl Phys L&t, 25, 1954, 169, and 27, 1974, 52. I9 G Carter, Radiat EffLetts. to be oublished. 2o P Sigmund, Phys Rev, 184; 1969,383. ” W 0 Hofer and H Liebl, Ion Beam Surface Layer Analysis. (Edited by 0 Meyer et al). Plenum Press, New York, p 659 (I 976). 22 W 0 Hofer, H L Bay and P J Martin, J Nucl Mat, 76/77, 1978, 156. 23 T E Parker and R Kelly, Physics Chem Solids, 36, 1975,377. s4 R Kelly, Nucl Instrum Methods, 149, 1978, 553. 2s J S Colligon, C M Hicks and A P Neokleous, Proc Int Conf on Ion Surface Interactions, Gordon and Breach, London, p 77 (1973). 16

z6 J S Colligon and M H Pate& Radiat Efl, 32, 1977, 193. 27 V Teodosic, Appl Phys Letts, 9, 1966,209. ‘a B Navinsek and G Carter, Can J Phys, 46, 1968. 719. 29 H H Andersen and H L Bay, Radio; Eff, i9, 1973, 139. Jo 0 Almen and G Bruce, Nucl Instrum Methods, 11, 1961, 257. 31 L E Collins, P A O’Connell, J G Perkins, F R Pontet and P T Stroud, Nucl Instrum Methods, 92, 1971, 455. ‘l P T Stroud. Thin Solid Films. 9. 1979. 273. 33 0 Christensen and H L Bay, .Appl Phys Letts, 28, 1976, 491. 34 H Nishi, T Sakurai, T Akatnatsu and T Furuya, Appl Phys Letts, 25, 1974, 337. 35 R A Moline G W Reutlinger and J C North, Atomic Collisions in Solids. (Edited by S Datz, B R Appleton and C D Moate), Vol. 1, Plenum Press, New York, p 159 (I 975). 36 R Grotzschel, R Klabes, U Kreissig and A Schmidt, Radiat Efl, 36, 1978, 129. 37 R S Nelson, Radiat Efl, 2, 1969, 47. 38 R Kelly and J B Sanders, SurfSci, 57, 1976, 143. 39 J M Shannon, Inst Phys Con/ Series, no 28, 1976, 37. 4o G Fischer, G Carter and R Webb, Radiat Eff, 38, 1978,41. 41 R Kelly and J B Sanders, Nucl Instrum Methods, 132, 1976, 335. 42 S Dzioba and R Kelly, J Nucl Materials, 76, 1978, 175. 43 For review of different models see: G Carter, D G Armour and K J Snowdon, Radiat EB; 35, 1978, 175; (b) K J Snowdon, Radial Eff, 35, 1978, 141. 44 C A Anderson and J R Hinthorne, Science, 197, 1972, 853. 45 Z Jurela, Radiat Efl, 19, 1973, 175. 46 P J Martin and R J MacDonald, Srrrf Sci, 62, 1977, 551. 47 K J Snowdon and R J MacDonald, Int J Mass Spec Ion Phys, 28, 1978,233. 48 K J Snowdon and R J MacDonald, Int J Mass Spec Ion Phys, 29, 1979,101. 49 H Dusterhoft and A Ihlenfeld, Phys Sratrts Solidi, 39a, 1977 K147. 5o A E Morgan and H W Werner, Surf Sci, 65, 1977, 687. 5’ E Taglauer, W Heiland and R J MacDonald, Proc 2nd Int Workshop on Inelastic Ion Surface Collisions (edited by R Kelly), North Holland, Amsterdam, to be published. 52 W Heiland, W Englert and E Taglauer, J Vat Sci Technol, 15, 1978,419. s3 N H Tolk, D L Simms, E B Foley and C W White, Radiat Eff, 18, 1973,221.

sJ C W White, D L Simms and N H Talk, Science, 177, 1972,481. 55 C B Kerkdijk and R Kelly, Surf Sci, 47, 1975,294. s6 R Kelly and C B Kerkdijk, Surf Sci, 46, 1974, 537. 57 C B Kerkdijk and R Kelly, Radial Efl, 38, 1978, 73. 58 E 0 Rausch, A I Bazhin and E W Thomas, J them Phys, 65, 1976, 4447. 59 I S T Tsong and A C McLaren, Nature, Lond, 248, 1974.43. 6o W F Van der Wee. Proc NATO Summer School of Materials Characterisation Using’Ion Beams Corsica, Plenum Press, New York (1976). 6’ W F Van der Weg and E Lugujjo, Atomic Collisions in Solids. (Edited by S Datz, B R Appleton and C D Moak), Vol 2, Plenum Press, New York, p 51 I (1975). 6* G E Thomas and E E de Kluizenaar, Nucl Instrum Methods, 132, 1976, 449. 63 R J MacDonald and P J Martin, Sur Sci, 67, 1977,237. 64 P J Martin, A R Bayly, R J MacDonald, N H Tolk, G J Clark and J C Kelly, Surf Sci, 60, 1976, 349. 65 C B Kerkdijk, K H Schartner, R Kelly and F W Saris, Nucl Instrum Methods, 132, 1976, 427. 66 T S Kiyan. V V Gritsyna and Ya M Fogel. Nucl Instrum Methods, 132, 1976,41i ” G E Thomas, E E de Kluizenaar and M Beerlage, Chem Phys, 7, 1975,303. 68 A.1 Bazhin, E 0 Rausch and E W Thomas, J them Phys, 65, 1976, 3897. 69 J Lindhard, M Scharff and H E Schiott, Kgl Dav Vid Selsk Mat Fys Medd, 33, 1963, 14. ‘O H Verbeek, Material Characterisation Using Ion Beams. (Edited by J P Thomas and A Cachard), Plenum Press, New York, p 303 (1978). ” E S Parilis, N Yu Turaev and V M Kivilis, Proc 8th Int Conf Phen in Ionised Gases, Vienna (I 967). ” H H Brongersma and P M Mul, Chem Phys Letts ,14,1972,380. 73 H H Brongersma and P M Mul, Chem Phys Letts, 19,1973,217.

G Carter and D G Armour: The relationship

between

vacuum

and atomic

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83 G C Nelson, J Vat Sci Technol, 13, 1976, 974. 84 E Taglauer and W Heiland. Appl Phys Letts, 33, 1978,950. 8s L K Verheij, B Poelsema and A L Boers, Radiaf Eff, 27, 1978, 47. 86 J A Van den Berg, D G Armour and L K Verheij, Inst Phys Conf Ser, no 38,1978,298. 87 E Taglauer and W Heiland, J Nucl Mat 76/77, 1978, 328. aa E Taglauer, U Beitat, G Marin and W Heiland, J Nucf Mat, 63, 1976,193. 8g E Taglauer, G Marin, W Heiland and U Beitat, Surf Sci, 63, 1977, 507.

9o F Goff and D P Smith, J Vat Sci Technol. 7, 1970, 1. g1 W Heiland and E Taglauer, J Vat Sci Technol, 9, 1970, 620. g2 H H Brongersma and J B Theeten, Surf Sci, 54,1976,5 19. g3 H Neihus and E Bauer, Surf Sci, 47, 1975, 222. g4 S Prigge, H Niehusand and E Bauer, Surf Sci, 65, 1977, 141. g5 W Heiland, F Iberl, E Taglauer and D Menzel, Surf Sci, 53, 1975, 383.

g6 E S Mashkova and V A Molchanov, Soviet Phys solid St, 8, 1966, 1206. g7 L K Verheij, J A Van den Berg and D G Armour, Surf Sci, 84, 1979,408. gB J A Van den Berg, L K Verheij and D G Armour, accepted for publication in Surf Sci. ” A Van Wijngaarden, E J Brimner and W E Baylis, Can J Phys, 48,1970,1835.

collisions

in solids

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lo4 W F Van der Weg, H E Roosendaal and W H Kool, Radiat Eff, 1973,91. lo5 R R Hart, D H Lee and 0 J Marsh, Appl Phys Letts, 20, 1972,

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