Sputtering: survey of observations and derived principles papers/Baragiola [r] review sputtering...Sputtering: survey of observations and derived principles By Ra´ul A. Baragiola

10.1098/rsta.2003.1301

Sputtering: survey of observationsand derived principles

By Ra u l A. Baragiola

Laboratory for Atomic and Surface Physics, University of Virginia,Charlottesville, VA 22904, USA ([email protected])

Published online 25 November 2003

We review the most salient observations and physical principles of knock-on and elec-tronic sputtering and the role of sputtering in several astrophysical settings and appli-cations in research and technology. In addition, we emphasize some unsolved prob-lems, propose experiments and provide guides to representative literature reviewsand significant recent publications.

Keywords: sputtering; atomic collisions; surfaces; SIMS; depth profiling

1. Introduction

Sputtering is the removal of material from objects by energy transfer in collisions ofenergetic atomic projectiles. It occurs widely in nature, where it causes the erosionof the surface of airless astronomical bodies (interstellar dust particles, the Moon,etc.) subject to ambient energetic radiation. The first record of sputtering in thelaboratory (Grove 1852) indicated the formation of a deposit in the anode of agaseous discharge and its removal when the polarity of the electrodes was reversed.It is interesting to note that the discoverer of sputtering, William Robert Grove, isbest known for his role as a distinguished lawyer and for his invention of the fuel cell.For almost a century after Grove’s discovery, most of the observations on sputteringwere made using gas discharges; isolation and characterization of the process usingcontrolled ion beams in a vacuum and characterized materials started only a fewdecades ago.

Since sputtering has been reviewed so extensively, it would not be valuable tohave another review here in such a limited space. Rather, the aim of this paper is toprovide a useful guide to the review literature, an overview of the main phenomena,new views on some unsolved problems and suggestions for experiments. The task ismade easier because I am excused from dwelling on theory by the several theoreticalpapers in this issue, which allows me to concentrate on observations and their physicscontent. The reader should not expect exhaustive references or coverage of all topics.Many sub-topics of sputtering, such as chemical sputtering, reactive-ion etching, theeffect of sample temperature, phase and porosity, to name a few, are not treatedhere.

One contribution of 11 to a Theme ‘Sputtering: past, present and future. W. R. Grove 150th AnniversaryIssue’.

Phil. Trans. R. Soc. Lond. A (2004) 362, 29–5329

c© 2003 The Royal Society

30 R. A. Baragiola

In spite of 150 years of sputtering, and the impression that we may get from thereview literature, most of the field is poorly understood. Notable examples are thesputtering of ceramics, polymers and composite materials, the ejection of molecules,ions and excited atoms and the whole area of electronic sputtering. The interestinghistory of the topic can be seen in the reviews by Massey & Burhop (1956) andCarter & Colligon (1968). Other historically important reviews are those by Kamin-sky (1965) and the volume series Sputtering by particle bombardment (Behrisch 1981,1983; Behrisch & Wittmaack 1991).

Sputtering is a particular case of radiation damage. Displacement of atoms fromtheir equilibrium lattice positions is produced either by a single collision with theprojectile or, more generally, as a result of a collision cascade in the material. Therepulsive forces required to dislodge atoms or molecules from the lattice can occurduring close ‘knock-on’ collisions initiated by a projectile with sufficient momentumor, indirectly, by electronic excitations that lead to antibonding states. These casesresult in knock-on sputtering and electronic sputtering, respectively. The relatedcase of electronic sputtering by energetic electron or photon impact, which gener-ally only involves a single surface collision, will not be treated here. The interestedreader should look into the following reviews, which have individual flavours: Knotek(1984), Baragiola & Madey (1991), Ramsier & Yates (1991), Baragiola (1992) andAgeev (1994). This type of electronic sputtering is often named in the literatureas electronic desorption and photodesorption or, more generally, DIET (desorptioninduced by electronic transitions). For readers who wish an expanded horizon, it isworth mentioning that photodesorption is directly related to surface photochemistry(Zhou et al . 1991), a topic of considerable fundamental and practical importance.

The physics of sputtering can be grasped most easily by studying analytical theo-ries even though they have limited accuracy compared with the most accurate com-puter simulations. Analytical theories divide the sputtering process into three steps:(i) initial collisions with the projectile that generate a recoiling target atom thatmay be ejected directly; (ii) a cascade of collisions in the solid involving fast recoils;and (iii) the escape of recoils through a surface barrier. The calculation of step (i) isstraightforward for knock-on sputtering (but extremely hard for electronic sputter-ing), since interatomic potentials for close collisions are relatively well known afterdecades of heated debates. The collision cascade of step (ii) can be simulated simplywith a computer, except for poorly known interatomic potentials and electronic-energy losses relevant in slow collisions. Escape through the surface barrier has beentreated in an ad hoc fashion in analytical theories and most computer simulations;however, for molecular-dynamics simulations, escape is just a specific instance ofstep (ii).

A schematic of the manifestations of sputtering is shown in figure 1. Typically,an ion beam hits a target surface, eroding it; most of the ion beam is implanted inthe target and a small fraction is reflected after suffering a range of energy losses.In the area of the sample hit by the ion beam, a crater forms that contains micro-scopic cratelets produced by each ion impact. With ion-bombardment time, the craterbecomes deeper and its bottom rougher. The ejecta, which have a wide angular andenergy distribution, originate mostly from the top surface layer, with a small per-centage of atoms coming from the second layer and a negligible contribution fromdeeper layers. The ejecta can be intercepted by another surface (a collector), on whicha deposit forms. This process, called sputter deposition, is the primary method of

Phil. Trans. R. Soc. Lond. A (2004)

Sputtering: observations and derived principles 31

depositionsubstrate

projectiles

sputteredparticles

target

Figure 1. Schematic of the sputtering process.

producing thin films. Secondary effects include reflection of a small fraction of theejecta at the collector, and sputtering of the deposit by the most energetic sputteredatoms and by reflected projectiles. The ejecta can be analysed with mass spectrome-try or optical techniques to obtain information on the composition of the bombardedsurface. The sputtered particles will also coat other surfaces besides the purposelysituated collector, and become part of the background gas contaminating it.

2. Experimental methods

The purpose of this section is to give the reader some criteria that can be used toevaluate experimental reports. The important quantities in sputtering are the sput-tering yield Y (number of atoms ejected per projectile), the composition of the ejectawith respect to species (atoms, molecules, ions), the angle and the ejection velocity.Progress made in the first 120 years of sputtering led to refinement in experimentaltechniques, as methods producing errors or unreliable results were discarded. Forinstance, sputtering was discovered in electrical discharges in gases and, for nearlya century, it was mostly studied under those circumstances. Although the study ofsputtering in a discharge is important per se, a more clear control of the process isachieved with measurements using ion beams in a vacuum. The more ideal condi-tions are more suitable for testing theoretical models. Even under those conditions,several problems became apparent over the years. For instance, prolonged ion irra-diation leads to accumulation of implanted projectiles in the target and with it atime-dependence of sputtering, particularly in cases of low sputtering yield. At mod-erate pressures (0.1–10 µbar), gases adsorbed from the ambient atmosphere (usuallywater vapour but even vacuum pump oil) form a contaminant layer that alters sput-tering. The use of dynamic conditions, where the sputtering process itself preventsaccumulation of adsorbed layers, leads often to the problem of contamination ofthe surface by implantation of recoil impurity atoms. In addition, dynamic cleaningusually causes the development of topographical features (roughness) on the sur-face, which in turn may affect sputtering. A large roughness affects sputtering yieldsby locally varying the angle of incidence. It also alters the angular distribution ofthe ejecta as sputtered particles become trapped in surfaces adjacent to the emissionpoint. The same consideration applies to sputtering of naturally rough surfaces, frompowdered solids to the Moon’s regolith (Hapke 2001).

Another potential problem, usually not discussed, affects measurements using poly-crystalline targets. A reduced-energy deposition by channelling effects near the sur-face will occur if there is a preferred orientation of the grains (texturing) on the


32 R. A. Baragiola

surface: a common occurrence in foil or ribbon targets, where it is caused by thelamination process. This problem and that of surface contamination in insufficientvacuum are probably at the root of the large spread in experimental results thatcan be discerned in compilations of experimental sputtering yields (Andersen & Bay1981; Yamamura & Tawara 1996).

The excellent review by Andersen & Bay (1981) gives many details of importantexperimental issues that need to be addressed for accurate measurements of totalsputtering yields but does not stress enough the need for very low pressures. Thecrucial criteria for good measurements are the purity of the ion beam (single mass,absence of multiply charged ions), the accurate measurement of the incident ionflux, the characterization of the target, and ultrahigh vacuum (less than 0.01 µbar).Unfortunately, most of the studies reported in the literature do not fulfil these cri-teria. To study the fundamental aspects of sputtering it is better not to modify thetarget material significantly during irradiation; this implies very low bombardmentdoses (projectiles per unit area). On the other hand, for specific applications suchas depth profiling or ion-beam machining, the quantity of interest is the amount ofmaterial removed after irradiation with a large dose that produces significant erosion,roughening and contamination of the target by implanted ions (Lehrer et al . 2001).To understand those cases, experiments must be done under those conditions.

For low-dose sputtering, two experimental methods are notable and are mentionedbriefly here: the quartz-crystal microbalance (QCM) for absolute measurements oftotal sputtering yields and angular distributions, and sensitive laser techniques thatare capable of measuring yields and the angular, mass and velocity distributions ofthe ejecta. The QCM is a simple and sensitive technique for measuring mass loss perunit area due to sputtering, allowing the use of a very small fluence of projectiles. Thesensitivity can be better than 1% of a monolayer/second under irradiation (Balajiet al . 1990; Westley et al . 1995). The technique has some limitations, the mostimportant being that the material must be deposited as a thin film onto the QCM sothat bulk samples cannot be studied. Such films show an initial decrease in sputteringyield with dose as a few monolayers are sputtered away, most likely due to the removalof weakly bound adatoms (Oliva-Florio et al . 1983). The QCM can also be used tocollect the ejecta on a plate to measure the amount of material sputtered. As withother collection techniques, it is important to quantify the sticking coefficient versusdeposit thickness and to evaluate re-sputtering of the deposit by energetic reflectedprojectiles. Sensitive alternatives to the QCM technique for quantifying sputtereddeposits are Auger or X-ray electron spectroscopies, Rutherford backscattering andradioisotope techniques.

Using pulsed ion beams, it is possible to measure the velocity distribution of sput-tered particles from their time of flight over a defined path. The elegant mechanicalmethod developed by Thompson (1987) has now given way to laser techniques (Betz& Wien 1994). Depending on the specific set-up, the velocity distribution can be mea-sured by the Doppler shift of laser-induced fluorescence, or by time-of-flight analysisof ions produced by laser ionization of the sputtered species. Examples of mod-ern laser-based instrumentation can be found in Husinsky (1985), Wahl & Wucher(1994), Ma et al . (1995) and Pacholski & Winograd (1999). The laser techniques aremuch more complex (thus prone to error) and costly than the QCM method, but arepreferable because not only are they at least as sensitive as the QCM for total yieldsbut also they provide data on velocity distributions.



2.0

1.6

1.2

0.8

0.4

0 20 40 60 80 100

Be

C

Si

Al

Cu

Cr NiCo

FeGe

Ag

Pd

Rh

Ru

Nb

MoZr

Au

Pt

IrOs

WTaHf

ReU

Th

atomic number

sput

teri

ng y

ield

(1

+

)−1γ

VTi

Figure 2. Sputtering versus the atomic number of the target for 400 eV Ne+ projectiles (fromLaegried & Wehner (1961)). γ is the secondary electron yield, which should be � 0.5 (Baragiola1994b).

The next two sections survey observables for the cases of knock-on and electronicsputtering.

3. Knock-on sputtering

(a) Total sputtering yields

Figure 2 gives an example of sputtering yields induced by 400 eV Ne+ ions for awide range of elements across the periodic table (Laegried & Wehner 1961). Theoscillations with the atomic number are mainly due to variations in the surfacebinding energy U . Figure 3 shows the energy dependence of sputtering of Ni by avariety of ions at normal incidence (Biersack & Eckstein 1984). It can be seen thatthe sputtering yields rise from a threshold energy below which sputtering does notoccur, passing through a maximum and then falling at high energies. For energiesmuch larger than the threshold, sputtering follows the energy dependence of Sn =∫

σ(T ) dT , the nuclear stopping cross-section (σ(T ) is the cross-section for energytransfer T ).

The threshold is not at the surface binding energy U but at a substantially higherenergy. Assuming that all collisions are binary (often a good approximation), it isrequired that the maximum value of the energy transfer T (the centre-of-mass energy)is larger than U . Moreover, sputtering also requires the motion of other atoms besidesthe one sputtered and the reversal of momentum—the projectile moves initiallytowards the surface, while the ejected atom must move outward. Typical values ofthreshold energies are 15–40 eV (Malherbe 1994). Unfortunately, most measurementsat low energies with heavy ions have not used ion beams analysed in mass/chargeand therefore they may have been contaminated with multiply charged ions which,because of their higher energy, dominate threshold behaviour, as shown by Baragiolaet al . (1991) for the sputtering of inner-shell excited atoms. The threshold behaviour


34 R. A. Baragiola

10

1

10−1

10−2

10−3

sput

teri

ng y

ield

Y

Xe

Ar

Ne

4He

D

H

ion calc. meas.

HD

4HeNeArXe

10 102 103 104 105

incident energy E0 (eV)

Figure 3. Compilation of sputtering yields of Ni by different projectiles versus projectile energy.Also shown are values calculated using the transport of recoils and ions in matter (TRIM) MonteCarlo simulation program. (Reproduced with permission from Biersack & Eckstein (1984).)

of sputtering, which is important in astrophysics and in many applications, is gen-erally unexplained for heavy ions, especially on multi-component targets.

(b) Effect of impact angle and channelling in single crystals

When the projectile is incident at an oblique angle θ to the surface normal, thesputtering yield first increases with θ, as more of the projectile energy is deposited inthe thin layer responsible for sputtering. However, increasing θ also means that moreprojectiles are reflected from the surface, thereby depositing substantially less energy.The competition of these two factors explains why the dependence of sputtering yieldwith angle of incidence Y (θ) peaks at some incidence angle θm. For incidents veryclose to 90◦ on a flat surface, Y should become zero, as the projectile is reflecteddue to a succession of very soft collisions (surface channelling) with energy andmomentum transfer insufficient to eject a surface atom. Both θm and Y (θm) increasewith the energy of the projectile, as shown in figure 4. The study of Y (θ) requiresflat surfaces and low doses, since surface roughness averages the angular dependencedue to a distribution of microscopic incidence angles. Most published measurementsof Y (θ) are affected by this problem at large incidence angles.

In single crystals, Y (θ) is strongly modulated close to major crystallographic direc-tions due to channelling, where atomic potentials steer the projectiles away from



30 keV10 keV6 keV3 keVarccosθ

Ar + → Au

S(

)/S

(0)

θ3

2

10 20 40 60 80

θ (deg)

S(

)/S

(0)

θ

2.5

1.00 20 40 60 80

θ (deg)

1.5

2.0

50 keV30 keV10 keV5 keVarccosθ

Xe + → Cu

Figure 4. Sputtering yield as a function of angle of incidence for Ar+ and Xe+ projectiles onpolycrystalline films evaporated onto optically flat substrates and measured at low doses toprevent development of surface roughness. (Reproduced from Oliva-Florio et al . (1987).)

small-impact-parameter collisions, thus decreasing the energy deposition and there-fore the sputtering yield (Roosendal 1981). As mentioned above, channelling canalso cause low sputtering at normal incidence for polycrystalline targets if they aretextured with grains that have a preferential surface orientation.

(c) Molecular effect and nonlinear effects

The linear dependence of the sputtering yield with nuclear stopping cross-sectionSn (Sigmund 1969) is not obeyed for high densities of energy deposition, as may befound in the superposition of collision cascades produced by the atomic componentsof molecular projectiles. The molecular effect is the difference in sputtering yieldproduced by a molecular projectile and the sum of the yields of each one of the atomsin the molecule, impinging independently in the solid. This effect usually appears asan enhancement of the yields for fast molecular projectiles, which is largest at theenergy for maximum Sn. At low projectile velocities there is a depression of the yields,as shown in figure 5 (Oliva-Florio et al . 1979; Andersen 1993). However, a recentstudy (Yao et al . 1998) gave an unexpected enhancement in sputtering yield of Aufor N+

2 compared with N+ below 500 eV, calling for further experimentation. Sincethe review of Andersen (1993) on nonlinear effects in sputtering, huge deviations fromlinearity were reported for MeV heavy molecular ions (Bouneau et al . 2002). Earlycomparisons with the linear theory of Sigmund (1969) suggested that nonlinearity isdue to the appearance of collisions between moving atoms in the cascade. However,molecular-dynamics simulations show that collisions between moving atoms occureven when yields are linear with energy deposition and that the reason for the hugeyields is shock waves, hydrodynamic flow and near thermal evaporation in the densecascades (Jakas et al . 2002). Figure 6 shows that deviations from linear behaviour


36 R. A. Baragiola

0.01 0.10.001

2.5

2.0

1.5

1.0

0.60

0.45

0.30

0.15

0

mol

ecul

ar to

ato

mic

yie

ld r

atio

energy per atom ( units)

nuclear stopping cross-section Sn

(reduced units)

ε

Figure 5. Molecular effects on the sputtering yield of Au by Xe+ and Xe+2 ions versus reduced

energy ε (Sigmund 1969). The line is the result from Sigmund’s analytical theory. (Reproducedfrom Oliva-Florio et al . (1979).)

50 10(dE/dx)n (100 eV Å−1)

1

0.1

S theo

r / S

expt

Au

Figure 6. Ratio of sputtering yields calculated by linear theory (Sigmund 1969) to experimentsmeasured at low doses versus NSn. The arrow indicates measurements at energies above themaximum of the stopping cross-section. (Reproduced from Oliva-Florio et al . (1987).)

already occur at relatively low values of NSe ca. 100 eV A−1, where N is the targetatomic density (Oliva-Florio et al . 1987).

If the experiments with MeV clusters were to be extended to higher projectilemasses, the results should merge with those from measurements of cratering inducedby dust grains with diameter greater than 0.1 µm accelerated to several km s−1, usedto simulate micrometeorite impact in space (McDonnell 1992). These fast grains‘vaporize’ the surface into a high-temperature plasma that leaves a crater at theimpact point. The collection of the charge in the sputtered plasma is used to detectand identify dust particles by space probes (see Baragiola 1994a). A quantitative



1.0

0.8

0.6

0.4

0.2

0 4 8 12 16

Ti

NiCu

Al

E (eV)

N(E

)/N

(Ew

)

Figure 7. Energy distributions of sputtered particles from several targets bombarded with 900 eVAr+ ions, measured for emission close to the surface normal. (Reproduced with permission fromOechsner (1970).)

understanding of the factors governing sputtering in the transition from atomic tonanoparticle impacts is one of the unsolved problems in sputtering.

(d) Ejecta

Extensive reviews of the angular and energy distribution of sputtered particlesare given by Hofer (1991), Winograd (1993) and Betz & Wien (1994). In general,the angular distribution of sputtered particles varies as the cosine of the angle ofemission with respect to the surface normal. The cosine distribution, which alsopertains to sublimation, is a consequence of an isotropic distribution of moving recoilsjust below the surface. At glancing incidence or at low projectile energies, where fewrecoil collisions are important, the collision cascade is not isotropic. As a result,emission near normal incidence is depressed and the direction of incidence of theprojectile becomes important. For multi-component targets, the angular distributionmay depend on the specific atom being sputtered, affecting the characterization ofsputtering by mass spectrometry along a specific emission angle (Wucher et al . 1988).

In crystals there is a preferential ejection of atoms along close-packed directionsthat results in spotty deposits (Hofer 1991). These spot patterns are due to chan-nelling and focusing in collision sequences (Thompson 1981, 2002; Hofer 1991). It issignificant that directional effects due to the crystalline structure are not washed outby the apparently chaotic motion in the collision cascade. Spot patterns should loseprominence under conditions that give nonlinear sputtering yields.

Studies of angular distributions have so far been done in conditions that producesurface roughness, which means that emission angles were not well defined. Observa-tions of angular and energy distributions from atomically smooth surfaces at grazingemission angles should provide a sensitive test of models of surface barriers actingon sputtered atoms.


38 R. A. Baragiola

In addition to the observation of spot patterns in deposits, the measurements ofenergy distribution of sputtered atoms Y (E) have historical importance, since theyconfirmed that sputtering is due to non-equilibrated collision cascades and not tothermal effects. For low Y the distribution follows the expression

dY

dE∝ E

(E + U)3,

derived with the assumption of a planar surface barrier for sputtered particles(Thompson 1981, 2002), and generally reproduces experiments. Thompson’s expres-sion gives a peak in dY/dE at E = U/2, which is of the order of what is observed(see figure 7). For large Y many atoms are emitted simultaneously from a microscop-ically small region, and the model for a planar surface barrier becomes unrealistic.A spherical surface barrier appears more adequate at not-too-large emission anglesand gives a shift in the peak to lower energies. This effect has often been interpretedas an indication of thermal components to sputtering (thermal spikes). To clarifythis problem, it should be useful to measure the ejecta at large emission angles, tomaximize the difference between a cosine distribution proper of thermal evaporationand a broad distribution resulting from a quasi-spherical surface barrier.

The expression for dY/dE given above holds only away from the maximum recoilenergy given by energy and momentum conservation in binary collisions at the spe-cific ejection angle being measured. Near the sputtering threshold, therefore, theenergy distribution should have a different shape and be very narrow.

(e) Sputtering of ions and excited atoms

Except for the alkali metals and some ionic targets, ground-state neutrals consti-tute the vast majority of the ejecta with very weak emission of ions and excited atoms.The processes responsible for efficient neutralization at surfaces and the survival ofions and excited atoms have been reviewed by Yu (1991). Although the mechanismsof electron transfer between sputtered particles and surfaces are in principle under-stood, no theory is currently able to predict yields of excited and ionized speciesaccurately. This is probably the most fundamental unsolved problem in sputtering.

Although the positive-ion fraction can be as low as 10−5 and is extremely depen-dent on the matrix, it is of great value, since ions can readily be mass analysed andsensitively detected to give information about the composition of the sample. Thisis the basis of the technique of secondary-ion mass spectrometry (SIMS; see below).Yields of negative ions can be zero, since many elements do not have stable nega-tive ion states. In addition, a fraction of neutrals similar to that of positive ions isejected in an electronically excited state. A good reference for modern techniques forstudying sputtering of excited atoms by photon emission is Cortona et al . (1999).The sputtering of inner-shell excited atoms that decay by Auger electron emissionhas been reviewed by Valeri (1993) and Baragiola (1994b).

(f ) Sputtering of clusters

The ejecta contain simple molecules and large clusters in addition to the generallypredominant atomic species. Cluster emission has been reviewed by Hofer (1991) andBetz & Wien (1994). Recent relevant papers include those of Birtcher et al . (2000),



Rehn et al . (2001) and Staudt & Wucher (2002). The formation of small moleculeshas been explained in some cases by association of atoms ejected very closely in phasespace. This mechanism is insufficient for the emission of very large molecules thatcan only be explained by the simultaneous ejection of already associated atoms byshock waves or similar mechanisms. Whether an observed molecule existed alreadyon the surface or was instead synthesized by sputtering has important implicationsfor methods of surface analysis based on sputtering. In the case of biomaterials thereis sufficient evidence of ejection of intact molecules, which has enabled a power-ful method for biological analysis (see Sundqvist 1991; Ens 1993; Reimann 1993;Hakansson 1993). In extremely dense cascades, such as those resulting from excita-tion tracks produced by fission fragments, molecules can be synthesized at an earliertime and then subsequently ejected in the cascade—an example is the synthesis ofC60 molecules from PVDF polymers by swift heavy ions, a phenomenon which is farfrom understood (Hakansson 1993).

The emission of clusters Xn from a solid composed of X atoms is very weak butsufficient to make sputtering a useful method for producing clusters for subsequentstudies of the transition between the atomic and the condensed states. The sput-tering yield follows a power law distribution Y (n) ∝ n−δ; the exponent δ is largefor small clusters but the distribution falls more slowly (δ ≈ 2) for clusters withn > 500 (Rehn et al . 2001; Staudt & Wucher 2002). The description of sputter-ing of molecules containing hundreds of atoms is beyond the current capabilitiesof molecular-dynamics simulations, because of the difficulty and computer powerneeded for sufficiently accurate quantum-mechanical descriptions of the interactionsand internal excitations.

(g) Preferential sputtering of multi-component solids

By multi-component solids we mean solids such as compounds, alloys, nanocom-posites, etc., that are heterogeneous in composition on a scale smaller than the sizeof the collision cascade or the sputtered depth. Sputtering of such solids occurs com-monly in astrophysics (see § 5) and in depth-profiling applications. In these cases itis found that different component atoms are removed at a different rate. This areaof sputtering, which has been reviewed by Betz & Wehner (1983), Sigmund & Lam(1993), Malherbe (1994) and Gnaser (1996), is very undeveloped, since most extantexperiments and theories have concentrated on elemental metals and semiconductors.

Causes for preferential sputtering are differences in energy transfer with atomicmass, differences in the binding energies of each component, and chemical alterationwith the formation and out-diffusion of volatile species. Due to preferential sputter-ing the target surface is enriched in the component that sputters less; this change incomposition causes a dose dependence of the sputtering yield. The simplest materialsto understand are metallic and semiconductor alloys. In general, it is not possibleto calculate the surface composition from partial sputtering yields due to segrega-tion, chemical changes, radiation-induced diffusion, recoil implantation and cascademixing. Instances where complex phenomena arise are in oxides, polymers, miner-als, etc.—the main reason being the changes in atomic composition, in molecularstructure and in binding energies induced by ion impact. Due to the ubiquity ofmulti-component solids, the full understanding of preferential sputtering is one ofthe most important needs in the field of sputtering.


40 R. A. Baragiola

(h) Bombardment-induced topography

Stochastic processes in sputtering, the relaxation of the material to the genera-tion of point defects, inhomogeneities in the sample (e.g. pores, impurities) and theaccumulation of implanted projectiles led to the development of topography in thesurface being bombarded. This is one of the most complex problems in the field ofsputtering. The effect of implanted gas on topography development, with implicationfor fusion reactors, was discussed by Scherzer (1983). Cone formation on surfaces,often attributed to impurities, has been reviewed in the book by Auciello & Kelly(1984). Recent reviews of topography induced by low-energy ion–atom collisionsinclude those of Smentkowski (2000), Carter (2001), Murty (2002) and Valbusa etal . (2002). Scanning force microscopy of initial cratering has been discussed recentlyby Chey & Cahill (1997), Malherbe & Odendaal (1999) and Kim et al . (2003). Onlyrecently has it generally been realized that microscopic craters with rims may becreated more readily in events that do not produce sputtering, by the migration tothe surface of vacancies and interstitials generated in the collision cascade in thebulk.

The competition of material removal with bombardment-induced surface diffusionleads to the formation of ripples on the surface, with characteristics that dependprincipally on ion energy and incidence angle. Induced diffusion can also lead tosmoothing of the surface (Chason et al . 1994; Mayr & Averback 2001). The area ofion-beam-induced topography is one of the most active sputtering topics at present,partly due to potential applications in nanotechnology (Chason et al . 1994; Jiang &Alkemade 1998; Rusponi et al . 1998; Facsko et al . 1999; Batzill et al . 2000; Frost etal . 2000; Costantini et al . 2001; Rost et al . 2001).

4. Sputtering by electronic excitations

The initiation of sputtering by the conversion of electronic energy to atomic motionhas been studied mostly in rare gas and molecular gas solids (Johnson & Schou1993) and in alkali halides (Townsend 1983; Szymonski 1993). The particular caseof electronic-energy deposition by slow multiply charged ions has been discussed,for example, by Varga & Diebold (1994), Schenkel et al . (2000) and Hayderer et al .(2001). For sputtering by MeV heavy ions and cosmic rays see Betz & Wien (1994)and Toulemonde et al . (2002).

Electronic sputtering is of great importance because it provides a rare windowinto non-radiative relaxation of electronic excitations in insulators. Unlike the caseof knock-on sputtering, the yields and energy distributions of the ejecta in electronicsputtering are extremely dependent on target properties. For this reason, we willdivide the discussion according to the type of material. Unlike knock-on sputtering,which can occur on all materials, electronic sputtering only occurs on good insulators,where electronic excitations are not degraded quickly by excitation of electrons tothe conduction band. In extreme conditions of electronic-energy deposition by fastheavy ions, the decay of the electronic relaxation can be sufficiently slow in metals(especially alloys) to produce radiation damage and sputtering.

Experimentally, electronic sputtering is separated from knock-on sputtering by itsdifferent energy dependence: electronic sputtering is related to the electronic stoppingcross-section Se rather than Sn. The dependence on Se is different for different solidsand can vary from linear to cubic in the same material. Sputtering yields depend on



2 3 4 5internuclear separation (Å)

2Ar + Ar

initialexitation

event

M-bandluminescence

initialionization

event

Ar2*

1Ar* + Ar

0

2

4

6

8

10

12

14

16

pote

ntia

l ene

rgy

(eV

)

Ar2+ Ar+ + Ar + (e− + K.E.)

Figure 8. Potential-energy diagram appropriate to the sputtering of solid argon (see text).Arrows labelled ‘1’ and ‘2’ symbolize repulsive paths that may lead to sputtering.

many sample properties besides surface binding energies, such as lifetimes of electron-ically excited states, hole mobilities, the presence of minute amounts of impuritiesand, importantly, on the precise shape of the intervening intermolecular potentialcurves. In addition, the dependence on energy deposition is different for projectilevelocities below and above the maximum of Se. For these reasons, the understandingof electronic sputtering is still rather uncertain.

(a) Rare-gas solids

The electronic sputtering of rare-gas solids (Johnson & Schou 1993) has beenwell studied because of its fundamental simplicity, as these materials are elementalsolids and do not undergo chemical modification during irradiation. As an example,consider the case of solid argon, where sputtering can be explained with the aid of theinteratomic energy curves shown in figure 8. The impact of a fast ion produces a track


42 R. A. Baragiola

of ionizations (electron–ion pairs) and excitations (excitons) in the solid, followingwhich the atomic ions and excitons diffuse primarily by resonant electron transferwith the lattice atoms. A diffusing Ar+ ion can strongly attract ground-state atomsforming an Ar+2 dimer (or an Ar+3 , Ar+4 multimer) that becomes trapped in 1–10 psby a structural relaxation assisted by lattice vibrations. After electrons have sloweddown sufficiently (in ca. 0.1 ns), they can undergo dissociative recombination, withAr+2 producing excited Ar atoms (Ar*), ground-state Ar atoms, and kinetic energy(pathway ‘1’ in figure 8). If this recombination occurs near the surface, it can produceimmediate sputtering of the Ar or Ar* involved or even of neighbouring atoms struckby the separating pair. Excitons can also be produced directly by the projectile, by itsassociated electronic-collision cascade, or by Auger recombination of an ion and anelectron. Regardless of how an Ar* is formed, it can pair with a neighbouring ground-state atom in an attractive or repulsive state. If the interaction is repulsive and atthe surface, the excited Ar* can desorb by a process called cavity ejection. In theattractive state, Ar* combines with a neighbour, again assisted by lattice vibrations,to form the Ar2* excimer. The vibrationally relaxed Ar2* excimer will then decayby emission of a 9.8 eV M-band photon to the ground state of Ar2 in ca. 3 ns (1.4 µs)for singlet (triplet) excimer states. Since the interatomic spacing of the excimer issignificantly smaller than that of the ground-state atoms, the decay is to the repulsivepart of the potential-energy curve. The kinetic energy released in this decay (path ‘2’in figure 8) can also produce sputtering if the decay occurs near the surface. The totalsputtering yield is proportional to Se, since the number of ionizations and excitationsproduced by the projectile per unit path length is proportional to Se. The productionof excited states by the ion is very efficient; half of the electronic energy deposited bythe ions is transformed into the 9.8 eV excimer luminescence (Grosjean et al . 1997).The simultaneous measurement of luminescence, charge collection and sputtering insolid Ar by Grosjean et al . (1995) allowed separation of the contribution of electron–ion pairs and direct excitations to sputtering. The study of the effect of the substrateand overlayers on the sputtering of thin Ar films (Grosjean et al . 2000) has producedfurther elucidation of the sputtering mechanisms and the role of diffusion and driftof excitons and holes.

(b) Alkali halides

A step up in complexity from the rare-gas solids is the alkali halides, which havealso been studied extensively under electron and photon impact. These materials offerthe advantage of easy preparation of crystalline samples. A distinguishing aspect isthat the targets are modified by the formation of molecular halides and metallicalkalis; the highly volatile halogen molecules can leave the sample even at moder-ately low temperatures, whereas the alkalis can evaporate at room temperature dueto their high vapour pressure. This leads to an interesting temperature dependenceof sputtering. The mechanisms for halogen emission have similarities with the caseof rare-gas solids (Townsend 1983; Szymonski 1993; Betz & Wien 1994) but thereis, in addition, a thermal component of sputtering. The process starts with ioniza-tion and excitations that lead to the formation of a trapped molecular exciton thatdecays into an electron trapped at a halogen site (F-centre) and an energetic halo-gen (H-centre). From there, there are two possible pathways, sputtering either by areplacement-collision sequence or by thermalization and diffusion out of the solid.



If the temperature of the sample is kept low, an alkali metal overlayer forms thatinhibits electronic sputtering. At temperatures where the alkalis can desorb ther-mally, sputtering proceeds stoichiometrically. Contrary to what might be expectedfrom such ionic solids, the vast majority of the sputtered particles are neutral.

(c) Other insulators

For condensed molecular gases, where binding energies are small and sputteringyields are large, it is often found that yields grow faster than linearly with depositedelectronic energy (Johnson & Schou 1993). In the astronomically important case ofice, which we have recently reviewed (Baragiola et al . 2003), sputtering is propor-tional to the square of Se and results in the emission of H2O and synthesized O2 andH2 molecules with a strong temperature dependence. The transition from the atomicto condensed phases is being investigated using large clusters (Bobbert et al . 2002).Other condensed gases, such as CO2, SO2, etc., show complex behaviour due to themultiplicity of chemical reactions that occur in the solid as a result of the radicalsproduced by the projectile. The sputtering of oxides by electronic excitations hasrecently been discussed by Itoh & Stoneham (2001) and Matsunami et al . (2002).This topic is important in astrophysical settings, the subject of § 5.

5. Sputtering in space

The role of sputtering in astronomy has been reviewed by Johnson (1990), Tombrello(1993) and Hapke (2001). Energetic ions are ubiquitous in space and produce sput-tering on any surface not protected by a substantial atmosphere. For instance, solar-wind ions (protons, alpha particles and some heavier ions at ca. 1 keV/amu) sputterthe surface of Mercury, our Moon, asteroids, comets, interplanetary dust and thesatellites of the outer planets. The flux of the solar wind is low, ca. 2×108 ions cm−2 sat Earth, and decreases with the square of the distance to the Sun. However, in astro-nomical time-scales doses can be similar to the high doses achieved in the laboratory,i.e. ca. 6 × 1018 ions cm−2 in 1000 years. Laboratory data suggest that preferentialsputtering of volatile components occurs in silicate minerals typical of the Moon(Johnson & Baragiola 1991) and asteroids (Dukes et al . 1999). In these objects sput-tering competes with erosion by micrometeorite impact; the relative removal ratesare mostly unknown.

Higher fluxes of ions in the 1–1000 keV range exist in the planetary magnetospheresof Jupiter and Saturn and are responsible for surface erosion and modification ofthe icy satellites (Shi et al . 1995; Cooper et al . 2001). The substantial gravity ofthose satellites binds a large fraction of heavy sputtered species. Water moleculessputtered from the surface into the atmosphere can be dissociated by solar light orby magnetospheric particles. H and H2, being light and therefore fast, can escape thegravitational pull. Molecular oxygen forms an atmosphere that does not condenseat the satellite temperatures, which adds to the transient water vapour atmosphere(Shi et al . 1995).

Cosmic rays in the interplanetary and interstellar media sputter efficiently but onvery long time-scales, since fluxes are low. Sputtering of dust grains is an importantprocess for the balance of grain destruction and creation in interstellar space (Dwek etal . 1996), where the dust grains are typically a few hundred nanometres or smaller in


44 R. A. Baragiola

size. For these small grains sputtering yields are enhanced, since the collision cascadeintersects not only the entrance surface but, depending on the ion energy, the sideand exit surfaces as well (Jurac et al . 1998).

A symmetrical situation occurs when a high-velocity grain goes through a gas atrest. Such is the case for micrometre- or sub-micrometre-sized dust particles enter-ing the Earth’s atmosphere at geocentric velocities of 0.2–1×107 cm s−1, which aredestroyed by sputtering and evaporation in collisions with atmospheric molecules(Meisel et al . 2002).

6. Applications

(a) Surface preparation

A common application of sputtering is cleaning surfaces for basic science studies.For instance, one can remove the native oxide layer of a silicon sample by sputteringwith low-energy noble-gas ions (to avoid chemical effects of implantation) and thenremove the radiation damage by high-temperature annealing. Several complicationsaccompany sputter cleaning.

(i) Ion bombardment also produces recoils in the contaminant layer, which movetowards the bulk (recoil implantation) thereby slowing the cleaning process.

(ii) Ion implantation, damage and the production of topographical features occur,which may not be acceptable. These can be removed by thermal annealing butonly for those materials that can take high temperatures without decomposing,vaporizing or melting.

(iii) Sputter cleaning is generally not possible for polymers, compounds, etc., whichare chemically altered by preferential sputtering. These effects can be mini-mized by using the lowest projectile energy possible (e.g. 200 eV) at the expenseof cleaning time.

(iv) Impurity ions present in the sputtering beam, which originate in the ion source,are incorporated. These can be removed by mass analysis, a precaution notusually taken, since most ion guns in commercial surface-science equipmentlack mass analysis.

Sputtering can also be used to polish rough surfaces by using glancing incidenceaccompanied by azimuthal sample rotation (Wissing et al . 1996), while the angulardistribution of reflected ions gives an indication of surface smoothness. A new methodfor surface polishing is the use of low-energy cluster beams, which are also useful forshallow ion implantation (Brown & Sosnowski 1995; Yamada 1999). As mentionedabove, cluster beams produce effects that are nonlinear in the energy deposition. Forlow-energy cluster impact the effects include greatly enhanced sputtering yields anda distorted angular distribution of sputtered particles, since the cluster interceptspart of the ejecta.

In other cases it is preferable to have rough surfaces that can be produced bysputtering. For instance, texturing of surfaces by ion impact is used to improveadhesion in thin-film deposition and may be applied in biomedicine (Kowalski 2001).



(b) Depth profiling

This is one of the most important applications of sputtering. Sequential removalof surface layers exposes the bulk of the material, allowing the generation of depthprofiles (composition versus depth) by continuous elemental analysis using any ofthe standard surface-analysis techniques such as low-energy ion-scattering spec-troscopy, X-ray photoelectron spectroscopy or Auger electron spectroscopy. An excel-lent review of the topic is given by Wittmaack (1991).

The depth resolution of the sputter removal technique is limited by ion-beammixing and surface roughness. In ion-beam mixing, layers at different depths aremixed prior to their removal by the collision cascade generated by penetrating ions.This process is complex because it depends on momentum transfer not only bythe ions but by the (enhanced) diffusion that occurs during cooling of the collisioncascade. The limitation of depth resolution by the induced roughness increases withsputtered depth; this effect can be reduced to a large extent by rotating the sampleduring sputtering, as mentioned above. Both ion-beam mixing and surface roughnessare alleviated by using very low incident energies (e.g. 200 eV) at the expense ofremoval rate.

(c) Surface analysis

There are several surface-analysis techniques based on the mass spectrometry ofsputtered species. The most popular is SIMS, which relies on measuring the massspectra of the sputtered positive or negative ions. Description of the techniques isoutside the scope of this paper but it should be said that SIMS is the most sensitivesurface-analysis technique, that it can detect all elements (including hydrogen) andthat, unfortunately, it is very hard to quantify even with standards, due to theextraordinary sensitivity of the ion yield to the local electronic structure of thesputtered species at the time of sputtering (matrix effect). For extensive reviews seeBenninghoven et al . (1987), Wilson et al . (1989) and Stephan (2001). Secondaryneutral mass spectrometry, or SNMS (Oechsner 1995; Nicolussi et al . 1996; He etal . 1997; Gnaser et al . 1998; Higashi 1999), is related to SIMS. Here the sputteredneutrals are ionized by electrons or a laser beam and then mass analysed. Thistechnique has the potential to replace SIMS, since it largely circumvents the problemof the matrix effect in ion yields. Pacholski & Winograd (1999) review the use of massspectrometry of sputtered particles for chemical imaging of surfaces, a fast-developingtechnical area.

Another use of sputtered particles for surface analysis is given by the energy analy-sis of recoil atoms directly emitted by the projectiles. This recoil spectroscopy is avery powerful technique for the study of adsorbates on surfaces (Bertrand & Rabalais1994).

Finally, another view of the surface is obtained by wavelength analysis of thelight emitted from radiative decay of excited sputtered species. Optical spectroscopyis a very powerful technique for identifying atoms and molecules, but it is not asstraightforward as mass analysis in SIMS or SNMS. If, instead of using ion beams,we place the sample in a gas discharge and analyse the light emitted by the sputteredatoms, the technique is called glow-discharge optical-emission spectroscopy (Paylinget al . 1997).


46 R. A. Baragiola

× 2 µm FOV 19 µm

Figure 9. Membrane made by FIBs of an integrated circuit. Notice the topographyinduced by sputtering outside the membrane. (Reproduced courtesy of Micrion Inc.)

3 nm

0 nm

3 nm

105 nm

(a) (b)

(c)

Figure 10. Scanning force microscopy (SFM) images of localization of human serum albumin(HSA) on a GaAs surface. (a) An array of pits milled by an In+ FIB. Small raised crater rimswith apparent heights ca. 0.4 nm surround each pit, 60 nm in diameter and greater than or equalto 0.8 nm deep. (b) HSA adsorbed from a solution is adsorbed preferentially to the inner portionof the rims of the pits. Sizes of (a) and (b) are 1 µm × 1 µm. (c) Close-up of HSA moleculesadsorbed on the rim of one pit. (Adapted from Bergman et al . (1998).)

(d) Sputtering with nanoscale focused ion beams (FIBs)

The production of microstructures by controlled ion beams (Hauffe 1991; Li et al .2001) has seen a dramatic development in the last decade or so due to the spreadingavailability of nanometre-sized ion beams from field emission liquid-metal ion sources.In the FIB accelerator, typically 20–60 keV Ga+ ion beams are focused on a spotthat can be a few nanometres in diameter. Computer control of the FIBs can be usedto machine complex structures; this application needs new modelling tools when thelateral dimensions of the ion-milled structures become of the order of, or smallerthan, the lateral extent of the collision cascade. This implies sputtering not only



from the surface on which the FIB is incident but also from side surfaces, a situationfalling outside the range of standard Monte Carlo simulations such as TRIM. Thissituation is common in the popular technique of specimen thinning for transmissionelectron microscopy (Ishitani & Yaguchi 1996). See figure 9 for an application tosemiconductor devices.

Patterns formed by FIBs have widespread applications. Shallow pits on GaAssurfaces sputtered by a computer-controlled FIB (see SFM images in figure 10) wereused by Bergman et al . (1998) to localize proteins on a surface. It must be notedthat writing surfaces with nanometre-sized ion beams is very slow and thereforethe method is not suitable for applications requiring mass production of features onthe scale of centimetres or larger. Finely focused beams can nevertheless be veryuseful in making prototypes or building blocks of structures that can be replicatedby other means. Such tiny structures can have unusual properties for applications inmicroelectronics, photonics, micromechanics and biomedicine.

(e) Sputter deposition

The collection of the ejecta on a substrate forms the basis for the method ofsputter deposition, one of the most widely used thin-film-deposition techniques inthe laboratory and in industry (McClanahan & Laegreid 1991; Wasa & Hayakawa1992; Wadley et al . 2001). Sputtered films have higher adhesion to the substratethan films produced by thermal evaporation, due to the hyperthermal energy of thesputtered particles (see figure 7) but sputter deposition is very difficult to use toproduce stoichiometric films of multiple components, such as superconductor oxides.Although sputter deposition with ion beams is used, it is much more common toemploy different types of gaseous-discharge apparatus. This brings us back to thebeginning of the story, the observation of sputter deposits in a discharge tube byGrove nearly 150 years ago.

7. Outlook

It is interesting and refreshing to note how an intriguing observation by Grove hasevolved for so many unanticipated and beneficial applications. While we shouldacknowledge the historical importance of Grove’s curiosity, it should be said thatmost of the advances in the field have happened in the last 40 years, energized bydiscoveries in the laboratories of Gottfried Wehner and M. W. Thompson. Activityin basic experimental research has decayed lately but it nevertheless has, as reviewedhere, produced many interesting new observations, particularly when using molecularand cluster ion beams or high-energy deposition densities. In these cases there is stillthe need to know the effect of incidence angle, changes in the angular and energydistribution of the ejecta and the threshold behaviour of the yields. Fundamentaladvances might result from experiments designed to map the transition from impactwith large clusters to impact with microscopic dust particles. Further experimenta-tion is needed in preferential sputtering of compounds, including chemical changesto ceramics, polymers and biomaterials and, in general, in sputtering by electronictransitions, where the high material specificity makes it difficult to reproduce exper-imental results.

The current understanding of sputtering has led to its applications in other sciencesand to engineering and industry. For instance, a great deal of progress is being made


48 R. A. Baragiola

in understanding the role of sputtering in space, which spans from the destruction ofinterstellar dust by fast ions in supernova shocks to the production of atmospheresaround icy satellites in our Solar System. Sputtering has greatly aided surface analy-sis by providing an easily understandable way to sequentially etch materials withextremely high resolution. Sputtered ions are being used routinely in SIMS, themost sensitive technique to identify trace elements on surfaces. The full applica-tion of this technique must wait for our understanding of the, as yet unpredictable,matrix effects. SIMS mapping with sub-micrometre resolution is proving invaluablefor research and diagnostics in microelectronics, and is likely to revolutionize thelife sciences if sufficient progress is made in understanding the sputtering of bio-logical materials. The fact that most of the applications we have mentioned weremade possible by basic research in sputtering since the time of Grove, points tothe need to reactivate fundamental studies, not only along the lines outlined here,but also furthering new serendipitous ideas that will result when pushing for deepunderstanding.

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