The velocity distribution of sputtered atoms

Nuclear Instruments and Methods in Physics Research B18 (1987) 411-429 North-Holland, Amsterdam

411

Section 11. Sputtering studies using lasers or post-ionisation THE VELOCITY DISTRIBUTION OF SPUTTERED ATOMS M.W. T H O M P S O N

University of East Anglia, Norwich NR4 7TJ, England

Experimental methods for determining the velocity distribution of sputtered particles are reviewed. A critical comparison is made of the main classes: calorimetric, dynamometric, electromagnetic, kinematic and photonic. The main features of the observed distributions are described in the cases of polycrystalline metals, monocrystalline metals and nonmetallic compounds. The principal mechanisms of sputtering that operate in these systems are identified.

1. I n t r o d u c t i o n

The velocity, energy and mass distributions of sputtered particles are of decisive importance in testing theoretical models of the sputtering process. This has been recognised since sputtering became a branch of solid state physics [1,2]. But the measurements are formidable because the majority of the sputtered particles from metals are electrically neutral and their energies span many decades. Furthermore their detection must be accomplished in the confusing presence of many other forms of radiation, such as scattered ions, emitted electrons and photons. In the early investigations the velocities were deduced from the mean energy of sputtered atoms [3,4], and primitive time-of-flight experiments [5]. The early experiments produced some conflicting results and with hindsight it is easy to see why, for fig. 1 shows the enormous range of energies covered by sputtered atoms, from 10 -2 to 10 4 eV, and the vastly different intensities that may encounted in ion-induced sputtering ranging over six decades. On the other hand very high resolution is required to pick out the fine structure in the velocity distributions from single crystal targets, vital to the testing of mechauistic theories. Fig. 2 shows this in a comprehensive set of data where intensity is plotted against the time-offlight over 95 cm (inversely proportional to speed) for a variety of emission directions relative to the crystal

sputtered atom E. Referring to fig. 3 the direction of the sputtered atom makes an angle 0 to be incident ion's direction which itself makes an angle 01 to the normal n from the target surface. Azimuthal angles may also be defined but most experiments have been conducted with vt, v and n coplanar. The angular distribution of sputtered atoms is the

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The basic quantity that describes sputtering is the yield S defined as the average number of target atoms sputtered by each incident projectile characterised by Z 1, M 1 and E l, the atomic number, mass ratio and kinetic energy respectively. The elemental target may be characterised by Z 2, M2 and the kinetic energy of 0168-583X/87/$03.50 9 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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Fig. 1. The energy distributions of atoms sputtered from a polycrystal]ine gold target by various ions (Shoaib Ahmad et al. [531).

II. SPUTFERING WITH LASERS OR POST-IONISATION

M. IV. Thompson / The velocitydistribution of sputtered atoms

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This transformation by a factor 1 / t 3 accentuates high energy features in the energy spectrum which might appear as insignificant changes in slope in graphs like those in fig. 1 but are peaks with fine structure in TOF spectra, exemplified in fig. 2. Integrated quantities that experiments often aim to measure are as follows: angular distribution: dS faE~--Eb d2S d--~ = J0 d Id d E d E '

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distribution function d S / d i d , with d S the number sputtered per projectile into an dement of solid angle did. This is the quantity which, following Wehner [1], has been frequently measured in experiments to show the anisotropic emission of atoms sputtered from single crystal [6-10]. The quantity referred to as the energy spectrum is the distribution function d2S/diddE and the number of atoms sputtered per projectile into did at 0 in the energy interval d E at E is (d2S/diddE)diddE. From

S=

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where A is the maximum fraction of the projectile's energy that can be transferred to a target atom in a single collision. For nonrelativistic projectiles:

A = 4 M 1 M : / ( M 1 + M2) 2, and E b is the potential energy binding the target atom to the surface. Obviously the upper limit of the sputtered atoms' energy must be ( A E ~ - Eb). There may be several values of E b is a given solid depending on the ejection site labelled Ebl, Eb2 etc. The average energy per atom is:

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M. IV. Thompson / The velocity distribution o[ sputtered atoms This is related to the energy reflection coefficient y, or sputtering efficiency, which takes account of the energy of reflected ions Elr:

~, = ( S E + g ~ , ) l e , . In cases where M 1 >> M 2 the backscattered ions' contribution may be neglected and y = S/~/E1 but some early experiments failed to allow for reflected ions, secondary electrons etc. and gave erroneous values o f / ~

3. A survey of techniques 9Information about velocity distributions has come from four main classes of experiments: (1) Calorimetric or dynamometric measurement of the average energy or momentum carried by sputtered particles. (2) Electromagnetic analysis of the mass velocity of any partides sputtered in a charged state, or any that can be ionised in flight. (3) Kinematic analysis of the sputtered particles' velocity according to their time of flight over a known distance. Combinations of (2) and (3) add mass analysis of the TOF. (4) Photonic techniques using the Doppler broadening of an optical emission or absorption spectrum to deduce the velocity distribution of those sputtered atoms that are either emitted in an excited state, decaying to produce a photon in flight, or those which interact in flight with a photon.

413

exerted on a microbalance by the radiation pressure of the sputtered particles. He also used a rotating vane system [4]. Kopitski and Stier [13] used similar dynamometric. More recent experiments have anticipated that the reflected ion together with electrons and photons can carry significant amounts of energy, also that the atom may not stick to the collector and could cause resputtering of material collected already. Andersen [14] designed a calorimeter illustrated in fig. 4 in which the collector enclosed all but a small fraction of the solid angle of emission and it was estimated that only 2.5% of the atoms and < 1% of the reflected ions escaped. On the other hand he arranged an axial magnetic field to steer all the target electrons out of the collection zone. Unlike the earlier calorimeters which had very small thermal capacity and rapidly reached a steady state, Andersen used a large heat capacity well insulated from surroundings to integrate the energy inputs over long periods, minimising the effects of fluctuation in ion current. The average energy of sputtered atoms and backscattered ions ranged from 20 eV in the case of P b + - P b to 3000 eV in the case of Ne+-Ta. The angular distribution of the energy carried by sputtered and reflected heavy particles was determined by Almrn and Bruc6 [15]. Later Hildebrandt and Manns [16] used a differential calorimeter with two cups, one shielded from the particle radiation, the other open. The difference in temperature between them allowed the

3.1. Calorimetric and dynamometric techniques Alm~n and Bmc~ [11] reported in 1961 that they had placed a thin foil of copper suspended by thin wires of copper and constantan near a metal target being bombarded with 45 keV ions of Ar § Kr and Xe +. The temperature of the foil was measured by the thermocoupie emf and was found to increase with the ion beam on the target. This was attributed to the energy deposited by sputtered atoms and mean energies were deduced in the range 36-250 eV depending on the target element. It seems that no correction was applied for reflected ions and these values do seem high in comparison with modem data. The calorimetric technique was developed further by Weijsenfeld [12] who measured the average energy of Cu atoms sputtered by Kr + and Hg + ions with energies up to 2 keV. He estimated a correction for the energy input from reflected ions and found it to be small. The values of /~ ranged from 2 to 12 eV, and depended on both the ion energy and the orientation when a single crystal target was used. The earliest dynamometric measurements were reported by Wehner [3] in 1959, who determined the force

Fig. 4. The calorimeter of Andersen for measuring the energy reflection coefficient [14]. The target block is tightly hatched. To the right of the target is the collector with calibration heaters. Thermocouples are seen on both target and collector with shielded reference points. The magnet which steers electrons from the target through the hole in the collector is shown cross-hatched. II. SPUTI'ERING WITH LASERS OR POST-IONISATION

414

M. IV. Thompson / The velocity distribution of sputtered atoms

sputtered energy to be deduced. The collector had a solid angle d ~ of 5 • 10 -3 sr, an~ a good resolution was possible. The angular distribution approximated roughly to a cosine law. The measurements of sputtered energy have an historic significance in indicating that sputtering was not solely an evaporation process from hot spots around the point of ion impact with temperatures of a few thousand degrees, as had been postulated [17]. They have also enabled some important results of the transport theory of sputtering to be confirmed [18]. For the technology of fusion reactors the energy reflection coefficients will be important design parameters. But for disentangling the mechanisms of sputtering the actual spectrum of sputtered energies is indispensable.

3.2. Electromagnetic analysis of charged particles The momentum and electrical charge of a particle determine the deflection of its trajectory in a magnetic field. The deflection in an electric field depends on the charge and the kinetic energy. Any apparatus in which both types of deflection occur can, in principle, simultaneously determine the energy and momentum of a stream of charged particles and, by varying the fields, their mass and velocity distributions. Many instruments have been developed on this principle, some especially adapted to analysing the sputtering process. Unfortunately, a very small proportion ( < 1%) of the atoms ejected from a clean metal surface are in a charged stated. This charged fraction may be increased by adsorping atoms like oxygen on the surface which have a large cross section for charge exchanging collisions. Alternatively it may be increased by passing the particles through an ionising region filled with plasma or streams of low energy electrons. If any of these devices is employed g r e a ~ a r e is needed to be satisfied that the momentum of the particles is not significantly altered in the ionising process. The dependence of the probability of ionisation on the speed of the particle must also be known and a correction applied to the measured spectra. Two extreme conditions can be postulated for ionisers using plasma or electron beams. That in which the efficiency is so great that every atom passing through is singly ionised and that in which the probability of ionisation is small. In the first conditions no instrumental correction is needed, in the second some allowance must be made for the dependence of the ionisation probability on the time spent by the atom with speed or in passage through the ionising region. This latter case gives a detection efficiency proportional to v - 1 giving the instrument a greater sensitivity for low energy partides. Thus the number of atoms arriving at the ioniser in d/2 and do is (d2S/dl'2dv)dl2do; but the number leaving as ions is proportional to v- l(d2S/d~2dv)dfldv.

If the ionisation occurs in a deliberately adsorped layer of atoms on the sputtering surface and conditions are such that the momentum distribution is not significantly perturbed (e.g. heavy target atoms with light adsorbate atoms) the instrumental correction is found as follows. The ionisation probability depends on the adsorpate thickness and the ionisation cross section, and will generally be a function of v or E, say R(v). The detector records a quantity R(v)d2S/dl2dt, and since R(v) usually increases with v the instrumental correction increases the sensitivity in the high energy part of the spectrum. This point is discussed further in section 5.1 and extended to cover the case of self-ionisation at a clean sputtering surface. Whether or not an ionising device is used, a further instrumental correction may be needed if the solid angle within which a particle must be emitted to pass through to the detector varies with either mass or energy. One of the earliest electromagnetic measurements was by Honig [19] who in 1958 reported on a mass spectrometer of the type where the particles are first formed into a beam, then accelerated by a known electric field and then deflected magnetically through 180 ~ onto an electrode which detects the particle current. A retarding potential V between the target and the entrance of the spectrometer removes all particles with kinetic energy less than Vne from the detector (ne is the charge). Measuring the detector current as a function of V allows the energy distribution to be determined for that particular mass number and charge state n. Honig showed that the energy spectrum of Ge + from Ge bombarded with 400 eV Kr + ions had a peak at about 2 eV and a tail extending beyond 20 eV. By ionising a small fraction of the neutrals in an electron beam before the acceleration stage he estimated that only 1% of the sputtered particles were Ge + and 0.02% were Ge~. Molecular ions were also seen from a silver target, and many impurities coming from residual gases adsorped on the surface. Honig's work emphasised the importance of working in ultrahigh vacuum and the preponderance of neutral atoms. He laid the foundation for secondary ion mass spectrometery (SIMS) as a tool for the physico-chemical analysis of solids. Others who used this type of instrument early on for energy analysis of sputtered ions include Jurela and Perovic [20] and Veksler et al. [21]. The invention of the quadrupole mass spectrometer QMS [22] with its compactness and simplicity, encouraged a new class of instruments exemplified by that of MacDonald, Bayly and Dennis [23-25], and shown in fig. 5. These used an electrostatic analyser to pass ions of any mass within an energy band of say (10 +_ 1) eV into the QMS which determined the mass spectrum in the energy band. The electrostatic analyser was preceded by an electrode system which accelerated all ions leaving the target in a well defined 2 ~ sector by a fixed energy, and then

M. 14/. Thompson / The t,elocity distribution of sputtered atoms

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decelerated by a known variable amount. Tuning the QMS to a particular mass number the decelerating potential was used to identify the energy band of the ions being detected. Hence the energy spectrum was deduced. An ionising stage was introduced [24] to study the neutral atoms and they were seen to have a similar energy spectrum to the ions in the case where 10 keV Ar + bombarded a copper target and Cu § was detected. MacDonald et al. showed that the charged fraction and the apparent energy spectrum was strongly dependent on minute quantifies of absorbed oxygen [25]. Both MacDonald et at. [24] and Yurasova's group [26], used this type of system to compare the energy spectra of Cu + ions emitted from a crystal along the (110) axis and between such axes. Each group obtained results consistent with the findings of my own group [42] using a kinematic system to be described later. Oechsner et at. [27] developed a system with a plasma in front of the target to ionise a fraction of the neutrals which were then passed into a retarding field and detected by a QMS. They showed that the ratio Ag2/Ag increased in the energy spectra at low energies - 1 eV per atom. A similar instrument, but without the ionising stage, was developed by K.rauss and Gruen [28]. They took considerable care in calculating the probability of ion transmission through the retarding and accelerating fields but the angle of acceptance ( + 15 ~ was so large

415

that precise determination of d2S/d~2dE was impossible. Neutral atom emission does not predominate in sputtering from ionic crystals, and these energy distributions have been measured by Bayly and MacDonald [25], and extensively studied by the Amsterdam group [60-69] using a combination of kinematic and electromagnetic techniques to be described in the next section.

3.3. Kinematic analysis of sputtered particles The history of kinematic systems of shutters for determining the speed of fast particles began with the classic work of Fizeau in 1849 measuring the speed of light travelling from Montmartre to Suresne and back. This was followed by Foucault's determination using rotating mirrors in 1850. Similar principles may be applied to the measurement of atomic speeds but hecause sputtered atoms travel very much slower than light the apparatus condenses conveniently into the space of a laboratory. Such techniques have great advantage in that they do not depend on the state of charge or excitation of the atoms. They have the disadvantage of being unable to distinguish between atoms travelling singly from those in molecular clusters, without some additional mass analysis. This history with atoms began with tests of the kinetic theory of gases. Throughout the 1920s, experiments were being made to test the Maxwell-Boltzmann law which relates the temperature of a gas to the velocity distribution of its molecules. Stern [29] conducted a classic experiment in 1920 with an atomic beam from an oven of hot vapour passing along a tube which was rotating about an axis perpendicular to the beam. The deposit collected from the beam on the end of the tube was smeared out by the velocity distribution. Better techniques were developed, with other groups joining in, and by 1931 the Maxwell-Boltzmann distribution had been confirmed [30-32]. Fig. 6a shows the kinematic spectrometer used by Eldridge [30]. A series of slotted discs rotated on a single shaft above a source of Cd atoms effusing from hot vapour. With the discs rotating very slowly the only paths available for the atoms were in the plane defined by the source and the shaft. With the shaft turning at - 102 rps an atom with kinetic energy - kT could only pass through the successive slots if its path was inclined at an angle to the source-shaft plane. A deposit of Cd collected after the last slot was smeared out because of the distribution of speeds. The graph of fig. 6b shows how the density of the deposit varied across the collector. Two peaks are seen, the one on the left was collected with the discs turning slowly, that the right with the discs spinning fast. The scale underneath shows the speed in m s-1. It had to be assumed, with some justification in this case, that the II. SPUTTERING WITH LASERS OR POST-IONISATION

M. IV. Thompson / The velocity distribution of sputtered atoms

416 a

COLLECTOR COOLED "BY LI~'UID AIR

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CADMIUM TWO PHASE STATOR ROTOR

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i i 300

i 400

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Fig. 6. (a) The apparatus used by Eldridge et al. [30] to measure the speed of atoms from a hot vapour source. (b) The distribution of material on the collector.

distribution in speed does not vary with the direction of emission from the source. This device is strictly a velocity selector through which atoms of a particular speed and direction can pass to a particular point on the collector. Because the distributions can be highly anisotropic, as in fig. 2, a velocity filter of this type is not ideal for our purposes since we need to select a direction and then measure the energy distribution associated with it. A time-of-flight spectrometer was therefore designed. In its first version, shown in fig. 7, the shaft and one slotted disc were carried over from Eldridge's design but the collector also rotated on the same shaft. The atomic beam direction was defined by having a small sputtering target and a collimating slit. The beam was chopped into short pulses by the slit in the first disc (slit width - 1% of circumference) and the atoms then dispersed along their flight path in inverse proportion to their speed. They were collected at a position on the second disc determined by their time of flight, hence their speed and energy. The density of the deposit as a function of angular position gave the TOF spectrum directly [5,33]. A sensitive means of detecting the deposit was devised because the intensity is reduced first by the inverse square law over the flight path and second by the 1~ duty cycle of the chopping slit. A radioactive gold target was prepared by previously irradiating it with neutrons to convert some of the stable ~95Au to active t96Au. This isotope decays with a convenient half-life of 2.7 d emitting (fl, 7) radiation. Thus the collected deposit could be detected and measured by 7 ray spectrometry or (fl, 7)-autoradiography after the sputtering was done. An irradiation of 3 d in a reactor neutron flux of 10 ~3 n cm -2 s-1 activates the target to a level of about 100 Ci g - l , in which approximately 0.01~ of the atoms are 196Au. There is no reason to suppose that the

196Au is sputtered much differently to the 195Au and hence the TOF spectrum of all gold atoms may be obtained by densitometry of the autoradiograph film. The density may be calibrated by preparing several standard autoradiographs with different doses of the (fl, y) radiation. The technique is capable of detecting deposits as thin as 10 l~ Au atom cm-2, equivalent to 1 0 - 5 monolayers. A similar technique has been applied to Cu whose isotope ~ C u decays with a half-life of 12.9 h, but the sensitivity is an order of magnitude less. The speeds achievable with the apparatus of fig. 7,

FLIGHT PATH

,

CHOPPER

CO ATOMIC B E A M LLIMATING SLIT

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_

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ATOMS~ ~ . ~ . ~ ' ~ - . " '% \

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Fig. 7. The time-of-flight spectrometer used by Thompson in 1961 for sputtered atoms [5,33].

M.W. Thompson / The velocity distribution of sputtered atoms however, were not high enough to resolve energies much greater than 1 eV, but were sufficient to show that in the ease of 66 keV Xe + on Au the energy spectrum has a peak near 0.15 eV and an unresolved tall extending to higher energies, much more intense than a MaxwellBoltzmann distribution would predict. This was a puzzling result at the time since the indications from electromagnetic analysis of sputtered ions were of a spectral peak at much higher energies. However, it is now clear from fig. 2 that the position of the peak is strongly dependent on the mass and energy of the bombarding ions. The conditions in Honig's experiments [19] were different. There was, and is, no inconsistency. . The speed of such a rotor system with mechanical bearings in a vacuum is limited to some 150 rps, insufficient to cover the energy range of sputtered atoms. Another system based on the same principles was therefore designed [34]. The beam of ions was chopped electrically and this resulted in a pulsed source of sputtered atoms. The rotating collector was in the form of a flat cylinder suspended magnetically by a servo system and spun to speeds as high as 3000 rps by a system of magnetic field coils. The pulsing of the ion gun was synchronised to the rotor by a light beam reflected from a mirror on the rotor onto a photocell (see fig. 8). With a flight path of 94 cm and a collimating slit of 1 mm, the TOF resolution was down to a few microseconds. The target, often a crystal, was mounted in a goniometer to control its orientation. Several rotors would be exposed to a single target, spun at different speeds in order to cover the entire range of energies from 10 -2 to 104 eV. The deposit on the low speed rotor would be used to correct the high speed rotors for slow particles which arrived after it had turned through more than one revolution since they started. The first apparatus was built and used at Harwell from 1960 to 1963. In 1965 it moved with me to Sussex University and was further developed over the next

PAl-fERNS

Fig. 8. The Sussex time-of-flight spectrometer in its most advanced form circa 1980 [34-38].

417

twenty years by a succession of colleagues and research students: B.W. Farmery, R. England, D.W. Palmer, I.H. Wilson, C. Foster, G.E. Chapman, I. Reid, D. Hole, Shoaib Ahmad, P.D. Townsend and F. Lama. During its life it made five doctoral theses [35-39] and eighteen papers [40-57] possible. Figs. I and 2 are examples of spectra obtained. Fig. 8 shows it in the most advanced form with capability to: heat or cool the target [35], vary the target orientation [35,37], vary the direction of emission [37], meausure the absolute d2S/dl2dt by adding 3' spectrometry of the rotor to autoradiography [37]. By introducing gas into the flight chamber the scattering cross section of noble gas atoms for gold atoms as a function of energy was measured [36]. A cylindrical collector could be placed around the target to obtain angular distributions [37]. The radioactivation technique was adapted to unirradiated uranium targets using neutron irradiation of the exposed rotor surface to generate fission tracks in a mica film. Thus followed the idea of Weller and Tombrello who in 1978 devised their kinematic spectrometer for measuring the TOF spectrum of U particles sputtered from uranium metal or oxide [58,59]. Their ion beam was first chopped electromagnetically into pulses which were then trimmed by passing through a rotating disc with two slits in it whose passage synchronised the electromagnetic pulsing. The ion beam then passed through a collimating slit onto the target. Sputtered uranium deposit was collected on the surface of the disc facing the target. The direction of emission thus always coincided with the direction of ion incidence. The geometry limits the type of experiment that can be performed, making its unsuitable for experiments where anisotropic effects are sought. But the technique for analysing the deposit using neutron activated fission was a major advance making it possible to study the sputtering of nuclear fuel materials. The rotor speed was limited by the mechanical bearings to 500 rps but this was sufficient to resolve the TOF of a uranium atom having 1 keV of kinetic energy. The Amsterdam group at the FOM laboratory have been active in the sputtering field since the 1950s. They developed a hybrid technique combining kinematic and electromagnetic analysis of the sputtered atoms. In earl~ versions it was for detecting alkali atoms or ions using surface-ionisation detectors [60-62]. It was brought into the advanced form by Overeijnder [63], than by Harin~ [64] and is illustrated in fig. 9. The ion beam was pulsed electromagnetically onto a target, usually an alkali. halide, with steps taken to avoid surface charge accu. mulating and affecting the energy of sputtered ions. The atomic/ionic beam was collimated and passec through a surface ionisation detector intercepting abou 1% of the beam to monitor the neutrals. An ioniseJ II. SPUTTERING WITH LASERS OR POST-IONISATIOb

M. IV. Thompson / The veloci(v distribution of sputtered atoms

418

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using an oscillating electron beam ionised a fraction (10-3-10 -4 ) of the neutrals after which ions were accelerated into a quadrupole mass filter and detected singly in an electron multiplier device, giving great sensitivity. The delay time between the pulse hitting the target and the ions detected was mainly due to their TOF over the 1.43 m section before the ioniser. A correction to the TOF distribution was needed because the efficiency of the ioniser depended on the passage time through it, hence approximately on v - i . Noise levels in the detecting system were reduced by applying a random sequence of pulses to the target and imposing a time-delayed correlation on the detector signal. The system was used both for sputtering with ions [65-68] and with electrons [69]. It has the advantage of being able to determine the mass of the sputtered particles, essential for the sputtering of compounds and desirable for some metals which emit significant numbers of atom dusters. A rather similar hybrid system was designed and developed by Vietzke et al. at K.F.A. Jfilich [70,71] but it used a rotating disc with a slit in it to chop the sputtered beam. After a flight path of 15 cm a sequence: ioniser/accelerater/mass filter preceded the multiplier-detector. It has mainly been used to study sputtering from graphite at high temperature. An earlier version was purely kinematic with a velocity selector of the Eldridge type [72] and used neutron activation analysis to examine the deposit. Light emission from sputtered particles was developed into a fine technique for measuring the TOF distribution by Whener's group [73]. Ions were pulsed onto the target in a short burst ( - 1 /~s). A dense plasma region was confined some distance away in which some atoms were excited in flight. The plasma was kept under observation by a monochromator tuned to characteristic radiation of target atoms.

The light detector was gated to operate during an interval delayed in time after the ion pulse. Photons were observed with a distribution of delay times from which the TOF spectrum wad deduced. Obviously this method is only applicable when the lifetime of the excited state responsible for the emission is very short compared with the time of flight, and one should confirm that the plasma does not change the atom's momentum. Unless the plasma is so dense that every sputtered atom is excited, the probability of detection depends on the passage time through the region under observation. When the probability is small the instrumental correction is proportional to v-1 as sho~l~ln in section 3.2 for the case of ionisers.

3.4. Photonic techniques The photon emission from atoms excited in the sputtering process can give some information about the velocity of sputtered atoms by measuring the light intensity as a function of distance from the target [74-80]. Indeed, this was one of the earliest methods of estimating sputtered velocities, used by Spore in 1939 [81]. But because the dependence of excitation probability on velocity must be known, also the lifetime of the excited state and the light intensity is obtained by integrating over all velocities, the technique lacks the precision required. Photonic techniques in a strict definition use the photon as a probe to determine the velocity of an atom in flight. So far this has employed the Doppler effect by which the observed wavelength h of a photon is shifted by Zik according to the relative motion of the emitter and the observer: Ak k

- -

~

v c

--COS

Ol,

419

M. IV. Thompson / The velocity distribution of sputtered atoms

and later to samarium atoms from Sm [88] and chromium atoms from Cr [89]. The wavelength of the laser light was varied whilst the intensity of the fluorescence light was measured. A distribution of wavelengths was found rather than a sharp line, as for stationary atoms, and from this data the velocity distribution was deduced. Other groups, at KFA Jiilich [90-96] and Argonne National Laboratory [97,98] also published results from such experimenta and the technique known as Doppler shifted laser fluorescence spectroscopy, DSLFS, was launched. Its obvious advantages are its suitability as a remote probe for sputtered atoms in plasma fusion devices and its specificity to atoms of a particular element either in the ground state or in a particular state of excitation. In its earliest forms however a rather large volume was needed to excite enough atoms, leading to imprecision in the direction of atom emission from the target. Fig. 10 illustrates a recent version of the experiment from the Vienna group [89]. The ion beam bombarded a Cr target. The rate of sputtering was monitored by collecting some of the sputtered atoms on a quartz microbalance which measured their rate of arrival. Other sputtered atoms were collimated into a beam which crossed an intense cw laser beam of 100 mW cm -2 tuneable to +10 GHz. The unshifted wavelengths of adsorption and emission were 425.4 rim, corresponding

with v the speed of relative motion and a the angle between the photon's direction and the path of the emitter. In principle photonic techniques could be based upon other effects such as the scattering of photons with change of frequency, as in the Compton effect, but the technology does not yet exist. The Doppler shift was first used for sputtered atoms by Stuart, Wehner and Andersen [82] in their time of flight apparatus described above [73]. The optical spectrum of the light exhibited an asymmetric line broadening due to the Doppler effect from which the velocity of the atoms was deduced. The magnitude confirmed the findings of the more precise TOF measurements [73,82]. Such excitation methods are open to the objection that the slow atom's velocity may be changed by the collisions in the plasma. The advent of the tuneable lasers as intense sources of monochromatic light opened up a range of new possibilities and progress in the last decade has been recently reviewed by Husinsky [83]. It was shown in 1976 by Hammer, Benes, Blum and Husinsky [84], working in Vienna, that a beam of sodium atoms from a thermal source could be made to fluoresce in flight through an intense laser beam tuned to the frequency of a D line. The experiment was extended to a beam of sodium atoms sputtered from NaI, NaC1 and Na [85-88]

Laser Beam

Ion Beam

Deflection plates

Quartz microbalance Diaphragm Particle Beam

-

T

Wavemeter DSLFS

Light Guide

Fig. 10. DSppler shifted laser fluorescencespectroscopy; the Vienna apparatus used by Husinsky et al. [89]. II. SPUTTERING WITH LASERS OR POST-IONISATION

M. 14/. Thompson / The velocity distribution of sputtered atoms

420

mA O + --

~oz~z~

'9~

f

~ B C+.~Cr

B ; o~lA/x

":'CAr+

\ f--

"

3

6

9

~~176

~

~z~

Km/sec

9

Fig. 11. Velocity distributions determined by Husinsky et al. [89].

to ground state resonance fluorescence of Cr. The luminous region was imaged by a lens into the entrance of a monochromator and detector system tuned to receive the fluorescence radiation. This was not Doppler shifted because it was viewed perpendicular to the sputtered beam. The laser wavelength was calibrated at the v = 0 value by deflecting a proportion of a laser beam into a photo-galvanic detector detector (H.C.L. in fig. 10) based on a chromium hollow cathode sensitive only to light at the exact v = 0 Cr resonance fluorescence wavelength. In this case the fluorescence light was emitted by a transition between the same two energy levels as were used in the resonant excitation process. Other systems have been used in which three levels are involved: the excitation goes from 1 to 2, the emission from 2 to 3 [90-96]. Typical is the case of Zr where adsorption is at X = 593.52 nm and emission is at X = 614.52 nm [93]. The DSLFS technique has been successfully applied to N a [85-88], Sm [88] and Cr [89] as two level systems, and to Fe [93], Zr [95] and Ti [91] as three level systems. Absolute measurements of d2S/d~2dv are difficult and only relative measurements have been reported so far. The exprimental geometry of the K F A Jiilich group means that d2S/dt2doz is measured, o~ being the velocity component perpendicular to the target surface. The intensity of the light detected should depend on the density of atoms in the measuring volume; hence on the product of d2S/dl2dv and the residence time of an atom in that volume (co v-1), and also upon the power density of the laser beam. If a single atom were in the fluorescence region and the excitation probability is small the intensity would be simply proportional to v-ld2S/dI~dv, but since there will always be an ensemble of atoms coherent effects may lose the simple pro-

9 portionality. Berres et al. [94,95] have investigated the conditions needed to avoid difficult calibrations and Husinsky [83] has reviewed these and other sources of error in DSLFS emphasising the need for care to avoid misinterpretation of results. In the future laser beams may reach such high intensity that every atom passing through will be excited. Then the first condition postulated in section 3.2 will apply and no instrumental correction for the passage time wiU be needed. Fig. 11 shows some velocity spectra obtained with the resonance fluorescence apparatus of fig. 10 [89]. using three different types of ions. Good reproducibility is evident and the solid line shows the theoretical prediction [41] 2 3

d2S/dl2dv = v 3 / ( v 2 + Oh) , with 02 = 2 E b / / M 2 a n d E b = 2 eV. Husinsky et al. [89] also investigated the effect of adsorped oxygen on the target surface. As might be expected from the experience with the ionised fraction of particles in electrom'agnetic experiments, the proportion of ground state neutrals in the sputtered beam falls by 98% with adsorption and the apparent shape of the velocity distribution is much affected. DSLFS is a powerful photonic technique, but not without difficulty. Care is needed to establish conditions where treacherous calibration can be avoided, the solid angles within which atoms are ejected have to be relatively large, vacuum must be very good and it best detects ground-state neutrals, not atoms in clusters, molecules, nor in excited states, although it could be specifically set up to do so. However, ground-state neutrals are the predominant species from a clean metal surface and it is their spectrum that tests many predictions of the sputtering theory.

M. IV. Thompson / The velocity distribution of sputtered atoms

4. C r i t i q u e Calorimetric and dynamometric methods do not give detailed spectra and are mainly of historical interest. The same must be said of optical emission from atoms sputtered in an excited state. Calorimetry however did provide a valuable test of transport theory in sputtering [18] and may be useful in the technology of fusion devices. The three remaining classes of technique will be considered against a set of criteria listed in table 1. The kinematic group is subdivided according to the means of detection, nuclear or mass spectrometric, and according .to whether the bombarding particle beam is chopped to give a starting pulse or the sputtered beam. The spectrum d 2 S / d l 2 d x has as its primary kinetic variable x the energy, in the case of electromagnetic methods, TOF in the case of kinematic, and velocity in the case of the photonic DSLFS. The relationship in section 2, because of a t -3 factor, gives kinematic techniques an advantage in sensitivity over the others at high energy. This is especially true of the nuclear kinematic method since those requiring an ioniser and some versions of the DSEFS method have a detection efficiency that falls off like v -z, when d2S/d~2do is being measured, as shown in section 3.2. By examining the logarithmic scales in fig. 1 and 2 in relation to the areas under the curves it will be seen that very few sputtered atoms have energies above 100 eV. For high energy processes, 100 eV and upwards, nuclear kinematic techniques have clear advantages. For processes below 1 eV only kinematic or photonic techniques have been successful. Electromagnetic instruments have not been used for energies much below 1 eV because of the difficulty of establishing retarding fields of order 1 V cm -z with precision. Kinematic and

421

photonic methods cover a wider energy range. Initially the upper energy limit of kinematic systems was set by mechanical bearings but with magnetic suspension [34] the range was extended beyond 104 eV. The techniques that readily give spectra in absolute units are electromagnetic (but only for sputtered ions) and nuclear kinematic. Others quote results in arbitary units. The energy resolution of electromagnetic instruments is limited to about + 1 eV in energy for the same reason as above. The resolution in TOF is related to the ratio of slit width to rotor circumference, usually about 1%, and is independent of energy. The highest resolution is achieved by photonic DSLFS, which resolves speeds down to 3 m s -z, equivalent to about 0.1% in the worst case of thermal velocities. Directional resolution, on the other hand, has been limited in DSLFS by the large size of the fluorescent volume needed to achieve sensitivity. The narrow slits intrinsic to kinematic methods give them the best angular resolution, down to 10 -6 sr in one version [37]. Consequently research on collisions in single crystals has used either kinematic or electromagnetic spectrometers. The detection efficiency for ions entering an electromagnetic system can be 100%. For certain cases like Na it can be almost as high in DSLFS. But when neutrals have to be ionised the efficiency is much reduced and depends on o-z. Under conditions when the excitation probability is small the detection efficiency of photonic methods is also related to o-z, but this simple relationship is not intrinsic to DSLFS which requires considerable care in setting up the excitation conditions. The nuclear kinematic technique presents the fewest problems in calibration but even in the optimum case of gold only 0.01% of the sputtered atoms are detected.

Table 1 Criteria

Techniques Electromagnetic

Kinematic

Photonic

E 1 "-* 10 4 eV Absolute (ions) Arbitary (neutrals) , 4 E - 1 eV

t

0

10 -2 ~ 10 4 eV

Abs. (nuclear) Arb. (mass spec.) ,4 t / t - 10- 2

10- 2 __,104 eV Arbitary

10 -2 " ' 1 0 -3 S

10 -4 -"10 -6 S 10 -4 (nuclear) v - l 10-2 ~ 10-5 (mass spec.)

Elemental specificity

100% (ions) v-1, 10 -3 -,10 -5 (neutrals) All elements

Mass resolution Sensitivity to adsorpate

,4 M2 < 1 amu Strong

Sensitivity to stray fields Sensitivity to delayed emission

Strong None

Primary kinetic variable Range of energies U n i t s of m e a s u r e m e n t

Kinetic resolution Directional resolution Detector response

Cu, Au, U (nuclear) All (mass spec.) Weak (nuclear) Strong (mass spec.) Strong Possible

A v - 3 m s -t 10-t --.10 -2 s V-1

Conditional Na, Sm, Cr, Fe, Zr, Ti

V. Strong Weak None

II. SPUTTERING WITH LASERS OR POST-IONISATION

422

M. W. Thompson / The t,elocity distribution of sputtered atoms

The combination of neutron activation cross section and nuclear half-life decides which target elements can be used in a nuclear kinematic experiment. Cu, Au and U are successful cases. Photonic techniques are in principle applicable to almost all elements. Favourable cases that have so far been found include Na, Sin, Cr, Fe, Zr and Cr with large fluorescence yields and sharp spectral lines. The mass spectrometric detector works with virtually every element. It also resolves mass, which enables atoms, molecules and dusters to be distinguished. Nuclear kinematic techniques cannot do this and DSLFS only detects atoms in one state, usually single atoms in the ground state. For studying reactive sputtering or clustering detection by a mass spectrometer is best. The techniques which rely on the sputtered atom being excited or ionised are all very sensitive to the presence of adsorpate on the target and this may explain why their development had to wait for ultrahigh vacuum. Nuclear kinematic methods are not so susceptible to adsorpate. However it is possible to study the excitation of sputtered atoms to particular states by collision with adsorped species using this sensitivity of DSLFS. Strong pulsed electromagnetic fields can prevent many instruments from working. Certainly magnetically suspended rotors and sensitive mass spectrometers are vulnerable and for this reason the laser-based photonic methods seem best suited to application in plasma fusion devices. The time taken to record a spectrum can also be an important consideration, especially in a pulsed fusion device. Here again the photonic method has the advantage. The delayed emission of sputtered atoms due to the time taken for some solid state process to operate will not affect the spectrum recorded by any instrument but those where the ion beam is pulsed [37,63,64]. Tests are needed, especially in nonmetallic targets, to establish whether or not delayed emission is occurring, which requires the use of a pulsed ion beam and a phase-locked detection system. It is then possible to study solid state or surface reaction kinetics. There is no simple answer to the question "which technique is best?". It depends on the kind of experimental information that is most important to the questioner on the environment in which the instrument is to operate and on what can be afforded. Electromagnetic systems are cheapest, photonic the most expensive. Time of flight has the greatest sensitivity to high energy features of the spectrum, photonic techniques can extend with greatest sensitivity to low energies. Being the newest photonic systems offer the greatest potential for future development.

5. Main features of the distributions

Recent review articles give a comprehensive bibliography that include sputtered velocity distributions [83,99-101]. It is not intended to repeat these, only to draw out the main results of the experiments and to summa_rise the theory making reference to selected publications. The section is divided between polycrystalline and single crystal metallic targets, and nonmetals, including the alkali halides, oxides and graphite. 5.1. Polycrystalline metallic targets The early observations [19,40-42,73] showed the energy distribution to be a skewed peak with a high-energy tail approximating to E-". Neutral atoms predominate, with n = 2, as clearly seen in fig. 1. For positive ions n ranges from 0.5 to 1.5, [20,21,24,26,27] as seen in fig. 12 [102]. The most probable energy for neutrals is a property of target material related to the sublimation energy, typically in the range 1-10 eV.

10 0

Z3

z_~

. ~

W§

10 -1

10 .=

o

10 -4 _ _

10 .5 10

1 O0

1000eV

Fig. 12. Energy distributions of sputtered ions and cluster-ions observed by Staudenmaier [103].

M. IV. Thompson / The velociO, distribution of sputtered atoms

423

20 KeY Ar poly. Au ,

10-'

,

~

"

\

....

900~ 950 700 30

10 -;~

t~ I"

lO-a

tic r

I 0 -4

~'o 10 "~

10 -~

10 -7

10 -2

I

I

I

I

I

10-'

10 0

10'

10 2

10 a

I

10 4

10 s

E eV

Fig. 13. The effect of ambient target temperature on the energy distribution of sputtered atoms from gold measured by Chapman et al. [43].

Exceptional cases where the spectral peak is at much lower energies associate either with targets near their melting temperature [35,43] or ions with a large linear rate of energy loss and hence a large sputtering yield (S > 20) [33,53,99]. Examples can be seen in fig. 1 with 20 keV Kr +, Xe + bombarding gold [53], and in fig. 13 with 20 keV Ar + bombarding gold at high temperature [43]. Although neutral atoms are the most numerous, neutral dimers, trimers and larger clusters and also present, but are increasingly rare the larger they are [102,103]. Sputtered positive ions follow a similar pattern, but their energy distribution has a peak which shifts to lower pattern, but their energy distribution has a peak which shifts to lower energies the larger the cluster, and the exponent n increases too. Thus the probability of a sputtered atom being in a cluster is highest at the lowest energies. Staudenmaier's observations [103] exemplify these points in fig. 13. The theory of atomic collision cascades in radiation damage preceded its application to sputtering [121]. From every version E-2 approximates the high energy tail of the energy spectrum [41,99,104-07]. This reflects the flux of atoms across any plane intersecting a collision cascade. In transforming this flux through a model

target surface in the form of a planar potential barrier Eb a refraction of trajectories leads to an emerging flux in the approximate from E / ( E + Eb) 3 [41]. E b is related to the sublimation energy. The flux has a peak at Eb/2 and for E >> E b behaves like E -2. At higher energies still, the flux is cut off at A Et, the maximum recoil energy. This theory has now been tested in many laboratories and seems to fit the neutral atom spectrum very well in cases where collision cascades are the dominant mechanism. If the probability of an atom emerging as an ion is R ( E ) then the energy spectrum of ions will approximate R ( E ) E l ( E + Eb) 3. Theories of inelastic collisions between similar atoms with speeds much less than valence electrons approximate R ( E) (x E 1/2 [108-110], suggesting that for E >> Eb the ions' flux should behave like E - " with n = 3/2. But in cases where charge-exchanging collisions at the surface are likely, for example with adsorped oxygen as discussed in section 3.1, R (E) could be a stronger function of E resulting in a smaller value of n and a shift of the peak to lower energy, which is consistent with some observations [25]. Clustering phenomena have been reviewed by Hofer [101]. Cascade theory has been extended to include the probability that neighbouring atoms are ejected simultaII. SPUTTERING WITH LASERS OR POST-IONISATION

424

M. W. Thompson / The velocity distribution of sputtered atoms

neously with small enough relative velocity in relation to molecular binding forces to be. detected as a cluster [111,112]. Obviously the more atoms the cluster contains the smaller the probability, and the greater the kinetic energy per atom the fewer clusters there will be. The most probable energy should decrease with increasing cluster size whilst the exponent n will increase. Combinations of ion and target that favour high density cascades and high yield also favour neighbouring atoms being sputtered together and are most likely to favour cluster emission. Such combinations are known to give total-atom spectra with peaks shifted below Eb/2. Two theories [111,112] give a fair qualitative description of the observations in fig. 13 but is must be remarked that the metallurgical and chemical states of the surface could also be important influences on cluster emission. I t is observed [113] that the cluster size distribution is not a smooth function of size and odd numbers of copper atoms axe favoured over even numbers. This seems to be of accord with valence theory which relates the molecular binding energy per atom to size in this way. One explanation of the shift of peak to lower energies for high density cascades, seen in fig. 1, is that the solid's binding energy E b is momentarily reduced, both increasing the yield and reducing the most probable energy [114]. In cases where the peak appears at very low energy - Eb/20 and the cascade density is not high, the ambient target temperature is always high [33,43]. The explanation appears to he evaporation from the transient hot spot (thermal spike) left behind by the subsiding cascade. Theories developed from this idea seem able to explain most observations [33,115,117]. The energy spectrum is the sum of two components, one due to collision cascades, the other to evaporation from thermal spikes. Where a low energy peak appears and the target temperature is not high, as in the case of 20 keV X e + - A u in fig. 1, the cascade density is invariably large and the sputtering yield high ( S > 20). Although evaporation may still be an apt description the detailed mechanisms could be too complex to put into a simple theory. Sigmund and Szymonski [116] have defined criteria which identify cases of this kind. The assumption is made in the simple theory that a MaxweU-Boltzmann (M-B) distribution describes the atoms in the thermal spike with a characteristic temperature T~, higher than the ambient To. Evaporation through the planar potential barrier E b gives an emerging flux E e x p ( - ( E + Eb)/kTs) which is again the M - B form. (Note the interesting contrast: the 1 / E 2 distribution develops a peak at Eh/2 upon this tranformation, the M - B does not, but is simply attenuated by a factor exp( - Eb/kTs). Evaporation at the ambient target temperature To is

possible for certain combinations of E b and T0. It would not contribute to an energy spectrum in a time of flight experiment like fig. 8, because there is a phase sensitive detection synchronised to the pulsed ion beam. It would show up in an experiment with a mechanical chopper like fig. 7, or in electromagnetic or photonic experiments. In addition to this normal sublimation a radiationinduced component might occur if defects formed by radiation damage in the bulk solid diffuse to the surface and release potential energy sufficient either to eject an atom [46] or to place it in a site with low binding energy E~,. In the latter case the flux would depend on E exp - ( E + E~,)/kT 0. The variation with temperature would involve both E~, and the defect's migration energy and the spectrum should peak near k To. An example in the Sputtering of metals would be the inert gas atom implanted as an ion which diffuses to the surface and is then thermally desorped. If the time taken for the defect to release its potential energy were comparable to the time of flight, an erroneous spectrum would be recorded in a system using a pulsed ion beam. If such a mechanism were conceivable, use of a mechanical chopper or variation of the target temperature should reveal it. In Section 5(c) examples will be given of alkali halides which behave like this. 5.2. Monocrystalline metallic targets

The classic discovery by Wehner [1] of anisotropic emission related to the closely-packed directions of a crystal was followed by many investigations of the phenomenon [2-10]. They led on inevitably to measurements of the velocity distribution of atoms and ions [25,26,40,41] aimed at confirming theories of lattice-correlated collision sequences in a cascade. Another classic discovery was the effect of the direction of ion incidence relative to the crystal axes on the sputtering yield [118,119]. This was one of the phenomena leading to the discovery of channelling, the effect which imposes limitations on the trajectories of ions in a crystal and on the trajectories of sputtered atoms. These effects show themselves as features in the velocity distribution. Careful and detailed measurements of the anisotropy of velocity distribution and the effects of varying ion incidence near a channelling direction reveal a fine structure that show the interrelation of channelling and focusing [48,50]. When a polycrystalline target is used many directional effects are smeared out, and distributions axe smooth curves fitted quite well by E l ( E + Eb) 3. Departures from this are best revealed by plotting data as time of flight spectra on linear/log scales, as in fig. 2. Because experiments require several rotors at different

M. 14/. Thompson / The t,elocity distribution of sputtered atoms

425 Energy

IONS 10'

''

10'

10

1'2~k~tTo~ ,_I

I. . . .

. . . .

~l

16

1[1 a

(eV)

I110l ~

10

o:,o/ '2~

/ /

%

\

\

r T \

4 ~

~'(o.8)

\

2

Energy in eV 10000 [ ,

,

1000 i

i

[ ,

IO 0 i

i

i

[

I

I0 I

l

l

I

I

1 I

I

I

,

I I

I

,

I

I

I

I

10

100 t

3t3

.~

b

I

I

I 1000

(psec.)

Fig. 15. The TOF spectra from the {110} Au surface at and near the (010) direction from Reid et al. [49]. This illustrates how the blocking effect around atomic rows the crystal surface robs the near (010) spectrum below 102 eV [54]. The bracketed figures below the 0 values are the corresponding values of dS/dl2 in atom/ion/sr.

2C

Eo

10

0

I 10

I 100

1000

t in JJsec,

Fig. 14. (a) Surface recoils, the blocking effect and deflected surface recoils. (b) Their apparance in a TOF spectrum from a {001} Au surface ejected along the (101) direction with 0 = 82 ~ The direct recoil gives a peak at 80 its, the once-deflected recoil appears near 50 #s (Reid et al. [48], Thompson [99]).

speeds to build up a single T O F spectrum, this method of plotting data does not distort the error bars so much as would a l o g / l o g energy plot like fig. 1. In fig. 14b direct recoils contribute to the sharp peaks near 103 eV on the curves for 8 = 75 ~ and 80 ~ Their position on the energy scale depends on 8 through the usual equations of binary collision mechanics. Fig. 14a shows how a second peak can occur due to recoils at a higher energy being deflected in a second collision [49,99].

The blocking, or shadowing, effect is also illustrated in fig. 14a, which can rob a spectrum of atoms below an energy limit directly related to the direction of observation [54]. This is seen in the spectrum of fig. 15 where the atomic row causing the blocking lies along the (110) direction in the crystal surface. In fig. 2 there is evidently a great difference between time of flight spectra along the (100) and (110) crystal axes. The first has two main peaks at 150 and 10 eV with some fine structure in the latter. The 150 eV peak has been carefully studied and attributed [49,120] to a lens focusing action shown in fig. 16. Here atoms coming from the second layer are focused by atoms in the first layer, provided their energies are close to 150 eV, for this is a strongly dispersive lens that acts like an energy filter. Emission in the (110) directions of fee crystals is the most intense, as is evident from the scaling in fig. 2 and the many observations of ejection patterns [1,3-8]. The most closely-packed rows of atoms lie in such directions and it is along these that simple focused collision sequences travel [121]. They propagate most efficiently with kinetic energies less than a limit E~ l~ By careful analysis and experimentation the values of E~ 1~ 170 eV for Au [41,47,50,53] and 50 eV for Cu [42,23,25,26] have been deduced. II. SPUTTERING WITH LASERS OR POST-IONISATION

426

M. IV. Thompson / The velocity distribution of sputtered atoms

I

,001

Kr~---~- Li I

T

,.,o..

~ 100. ///// Fig. 16. Particle optics in the {100} surface layer of a fcc crystal. The lenses are highly dispersive and act as band-pass energy filters only allowing recoils of certain energies to pass through to the detector [54]

//

iF ~

~coll /~///'~/ ,

5.3. Nonmetallic targets In metals the energy transferred to the electron system by the bombarding ion does not cause sputtering because the conduction electrons receive most of this energy and they rapidly disperse it over a large volume. In nonmetals, by contrast, electrons are mostly excited into localised states which can decay in ways that include photon emission, electron emission, photon generation or 10calised atomic recoil. It is the last that can cause sputtering and in some nonmetallic systems bombarded with fast ions it can dominate. Halides are particularly susceptible to this kind of sputtering. The mechanism was first demonstrated in sputtering by Townsend and Elliot [122] who showed that low-energy electron or photon bombardment could induce sputtering in alkali halides even though the momentum available was insufficient to cause sputtering by recoil from a direct collision. A comprehensive review has recently been published by Townsend [123]. Chemical decomposition of a bombarded compound can be induced either by electron excitation or by atomic recoil. The products may be emitted from the solid either by thermal desorption, having diffused to the surface, or by collision events. Clearly there are very many possibilities and few have been studied by measuring the velocity distributions. Those that have are mostly oxides or halides. The emission can be delayed significantly after the moment of impact by, for example, the lifetime of an excited state or the time taken for an atom to diffuse to the surface. Consequently the tests mentioned in sections 3.3 and 5::1 must be applied if kinematic techniques are used. The apparatus of Overeijnder and Hating [63,64] is well suited to the study of compounds because mass analysis enables molecules to be identified. In studying velocity distributions from ion bombarded halides Szymonski et al. [66] have identified three possible

9

0.01

,

0.1

~ o 1 energy (eV)

10 ~_~

Fig. 17. The energy distribution of iodine atoms sputtered for Li I by 6 keV Kr + ions. r has the form E / ( E + Eb)3; '#th is E e x p ( - E / k T ) , with T = 307 K; essentially the target ambient temperature (Symonski and de Vries [65]).

components: the first typical of collision cascades E l ( E + Eb) 3, the second of M - B form with a high temperature characteristic of a transient thermal spike T~ and the third a M - B component characterised by the ambient temperature TO. Those present in any case depend on the ambient target temperature, the sublimation or desorption energies, the energy density of the colhsion cascade and the degree of electron excitation. Bayly and MacDonald [25] have shown that the Na § and CI- ions sputtered by 43 keV Ar + from NaC1 exhibit the spectrum E l ( E + Eb) 3. This is the simplest case with only one component. Fig. 17 shows the energy distribution of iodine atoms sputtered from LiI by 6 keV Xe + ions [65], a system chosen for having a volatile anion and a nonvolatile cation. There are only two components contributing: collision cascades and evaporation at the ambient target temperature. It is presumed that some iodine atoms are directly sputtered by collision cascades, others reach the surface as a result of decomposition and evaporate at the ambient temperature. Other cases have been examined in which all three components are needed to fit the data: collisional, transient thermal and ambient thermal, e.g. iodine from CdI 2 bombarded by 6 keV Xe + ions [66]. In others only the collisional component is seen, for example the case of NaC1 [25] or silver atoms from AgBr bombarded with Xe + ions at 6 keV [66]. In this latter case, however, Br z molecules are also detected and they have a distribution characteristic just of ambient thermal evaporation and with no collisional component. Clearly the

M. IV. Thompson / The velocity distribution of sputtered atoms molecules have formed on the surface after the collision cascade and subsequently evaporated. Sputtering of alkali halides by electron bombardment has also been studied by Overeijnder et al. [68]. These have shown that emission of both alkali and halogen atoms can be delayed by milliseconds after the electron pulse hits target. This is attributed to the lifetime of excitons before releasing their potential energy to atomic recoil, rather than to the diffusion time of a migrating atomic defect. The measurement of timeof-flight is a powerful tool for distinguishing between the prompt emission of atoms by collision cascades or thermal spikes, and slower processes. This subject has recently been reviewed by Kelly [100]. The TOF technique of Weller and Tombrello [58,59] has been applied to the interesting case of UF4, an ionic solid. It was found to sputter at an enormous rate, three orders of magnitude greater than UO 2 would have done or coMsional theories could explain. The energy spectrum has an exponent n -- - 5 . 1 in the high energy tail and a most probable energy less than 1 eV [124,125]. Lama and Townsend [126] interpret this as a result of electron excitation first changing the molecular structure at the surface into weakly bound U2Fs clusters which then desorp by further excitation. The sputtering of uranium dioxide has been studied by Weller and Tombrello [59] and by Lama, Townsend et al. [39,57] using TOF to measure the velocity distribution. It appears to be dominated by a collision cascade component with the typical E -2 tail but the peak cannot be fitted with an expression E l ( E + Eb) 3. A modified form [127] is needed in which two binding energies are present: one for the uranium sites on the surface one for uranium below the surface. The values for these are 1.4 eV and from 3 to 30 eV respectively. Graphite is an important case in which radiation damage has been extensively studied. From this it is known that carbon interstitials are highly mobile defects above a temperature of 450 K and that they migrate in two dimensions between the lamellar planes of the graphite crystal structure. Vacancies do not migrate until temperatures in the region of 1750 K. Platelet clusters of interstitials are common in the intermediate temperature range. Vietzke et al. have used a TOF system with a quadrupole mass filter as a detector to measure the velocity distribution of sputtered carbon atoms [71]. Although the resolving power of their apparatus was not sufficient to study the highest energies it seemed that at room temperature the carbon atoms were in a cascade spectrum E / ( E + Eb) 3. But with targets in the region of 2000 K a M - B distribution dominated the spectrum with a characteristic temperature about 20% above the target ambient and a binding energy considerably less than the normal sublimation energy of graphite.

427

It may be that the atoms originate as interstitials which are lightly bound to the crystal lattice when they emerge at the surface. If the ion beam were to be pulsed in advanced synchroinism with the chopper, delay times might prove or disprove this idea.

6. The future There is little doubt that a measurement of the quantity d 2 S / d f l d E is a most powerful means of testing theories of sputtering and resolving out competing processes. Unfortunately, the experiments are amongst the most difficult to perform. The addition of compact mass spectrometers and single particle detection to time of flight experiments has done much to extend the range of targets into the compounds where fascinating molecular dynamics will be revealed. The laser technology will no doubt advance further to reach its full potential both as a laboratory instrument to study sputtering or inelastic atomic collisions at surfaces as sputtered atoms are emitted and as a remote probe for plasma fusion. So far both lack ~ e ability of the Sussex TOF system to define and vary angles with the precision needed to resolve atomic collision mechanisms in crystals. Here there still remain many processes to study. The particle optics of a crystal surface from which atoms are being sputtered is one which could bring major advances in surface science. I am grateful to the Council of the University of East Angha for granting me study leave during the winter of 1986 and to Dr. J.D. Elsworth for acting as Vice-Chancellor in my stead which enabled me to write this review article.

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II. SPUTTERING WITH LASERS OR POST-IONISATION