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Sputtering in the high-voltage electron microscope
Surface Science 90 (1979) 339-356 0 North-Holland Publishing Company
SPUTTERING IN THE HIGH-VOLTAGE
ELECTRON MICROSCOPE
D. CHERNS Oxford University, Department UK
of Metallurgy and Science of Materials, Parks Road, Oxford,
Received 23 October 1978; manuscript received in final form 5 December 1978
The 1 .O MeV electron microscope has been used to observe and analyse transmission sputtering caused by the electrons in the incident beam. The method, reviewed here, is particularly suitable for investigating low energy collision events. Total sputtering yields and angular distributions of sputtered atoms have been measured for (111) gold films to within a few eV of the sputtering threshold energy. It is shown that the results can be explained by the sputtering of surface atoms either directly by electrons, or indirectly by collision sequences generated down ( 110) directions. The necessity of using a many-body collision model to interpret the results is stressed. High resolution electron microscopy has been used to study the surface structure of (111) gold films during sputtering on a near-atomic level. It is shown how the results confirm a model where surface roughness develops due to the migration and agglomeration of surface vacancies produced during sputtering. The future scope of the 1 .O MeV electron microscope as an analytical tool for sputtering is also discussed. It is suggested that the r61e of long range focussed collision sequences in sputtering may be determined for materials of medium atomic number. A need for further high resolution studies of sputtered surfaces is identified; such studies are seen as complementary to those by other surface analysis techniques.
1. Introduction
The study of sputtering has greatly contributed to our understanding of atomic collision processes that occur within solids. Work on sputtering has been extensive, particularly in the case of ion-sputtering which has been investigated over a wide range of targets, incident ions and incident ion energies from several hundred eV upwards [l-4]. It is now accepted that sputtering yields for a wide range of conditions can be approximately explained by the collision cascade model developed by Sigmund [3,5,6]. In Sigmund’s model, collision cascades initiated by the incoming particles are treated by assuming that recoil atoms undergo a series of binary collisions in a random target. At the intersection of the cascade with the surface, recoil atoms with energy greater than the surface binding energy are sputtered. However, the assumptions of Sigmund’s model are least reliable at low recoil energies, below about 10 eV [6], which dominate the cascade energy spectrum. Indeed most sputtered atoms are found to have energies of a few eV or less [7,8]. 339
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At these low energies the crystal structure may be expected to have an important influence in determining recoil trajectories. It was first shown by Wehner 193 and subsequently by others [4,10] that angular distributions of sputtered atoms from single crystals show maxima close to prominent crystallographic directions. This has been interpreted in terms of focussed collision sequences which transfer momentum down close-packed rows of atoms in the lattice. However it has been suggested that the knock-on of surface atoms by essentially random sub-surface recoils can also contribute substantially to the spot patterns [ 1 I]. Recent work [ 12-141 has demonstrated that the 1 .O MeV electron microscope can be used to study electron sputtering caused by collisions between incident beam electrons and target nuclei. In the past electron sputtering of this type has been little used as an experimental tool owing to the experimental difficulties associated with low yields. However the yield problem is overcome in the electron microscope by the very high electron beam densities that can be achieved; these exceed beam densities in conventional electron accelerators by factors of up to 10’. It is the purpose of this paper to explain how the high voltage electron microscope (HVEM) can be used to study sputtering and, in particular, the low energy collision events that are so important in the collision cascade. The principles of the method are explained in section 2. In sections 3 and 4 the main achievements of the HVEM method are summarised. In section 3 it is shown how sputtering yields from (111) gold films may be obtained to within a few eV of the sputtering threshold energy; yields can be explained by transmission sputtering of surface atoms either directly by electrons or indirectly by collision sequences generated down (110) directions. The necessity of using a many-body rather than a binary collision theory is stressed. In section 4 the application of high resolution techniques of electron microscopy to the sputtered (111) gold surface is considered. It is outlined how a surface diffusion model explains the roughness of the sputtered surface on the atomic level. The general application of the HVEM techniques to sputtering studies is discussed in section 5. It is suggested that the HVEM may be specifically used to investigate the contribution of long range focussed collision sequences to sputtering. The need for more extensive studies of surface roughness, and their particular relevance to the use of other surface analysis techniques such as low energy electron diffraction (LEED), is explained.
2. Concept of sputtering in the high voltage electron microscope The scattering situation in the electron microscope is illustrated in fig. 1. In the 1 .O MeV electron microscope specimens are prepared for viewing in transmission and have thicknesses generally in the range 0.01-l pm. The electron beam passes through specimens of this thickness range virtually undiminished in energy or intensity, and undeviated except for substantial low angle elastic and inelastic scattering at angles comparable to the Bragg angle (-low2 rad). This scattering is well under-
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electrons
/ sputtered
la1
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Fig. 1. (a) Sputtering in the electron microscope. Incident electrons pass through thin electron microscope specimens predominantly unabsorbed and undeviated, except for small angle (-10e2 rad) scattering, to form the normal transmission images. A small fraction (-lOA) of incident electrons in the 1.0 MeV electron microscope undergo sufficiently violent, highangle collisions with target atoms to cause sputtering. (b) The sputtering is mainly in transmission causing foil thinning as shown. (After ref. [ 121.)
stood in terms of the dynamical theory of electron diffraction [15]. High angle electron-atom nucleus scattering, leading to substantial energy transfer is a comparatively rare event. In a direct collision between an electron of energy E, and an atom nucleus of mass M, the energy E of the recoil atom varies with the angle 0 of the recoil atom trajectory to the electron beam direction by E = E,,,
cos28 ,
where Em,
(1)
is given by
E max = 2E,(E, + 2moc2)/Mc2
,
(2)
where m. is the rest mass of the electron and c is the velocity of light. Values of E max are given in table 1 for various atoms in the atomic number range Z = 13 to 79 (aluminium to gold), and for electron energies of 0.1, 1 .O MeV. As the minimum energy for sputtering these materials is only a few eV, it is clear that electron-atom collisions in the 1.0 MeV electron microscope can transfer sufficient energy to sputter most materials. In contrast sputtering in the conventional 0.1 MeV microscope is only possible for the lightest elements. Since differential crosssections for back-sputtering are also negligible for a flat specimen compared to the cross-sections for forward sputtering, for the essentially unabsorbed electron beam, sputtering should occur mainly in transmission as illustrated in fig. lb. Scattering angles are large, typically up to 1 rad, so that the sputtered atoms will be
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Table 1 Values of Emax at 0.1, 1.O MeV from eq. (2) Element
Aluminium Copper Silver Gold
Atomic number Z
13 29 47 79
Emax (eV) E, = 0.1 MeV
Ee=l.OMeV
8.9 3.8 2.2 1.2
161 68 40 22
ejected outside the imaging system of the microscope, and may be collected on a suitable thin film substrate as shown later. The differential scattering cross-sections, do/da, for electron-atom nucleus collisions are accurately known from the theory given by Mott [16]. Tables of cross-sections for various elements have been compiled by Oen [17]. In principle, therefore, as electron scattering by the crystal is well-understood, primary recoil cross-sections may be reliably calculated. In practice the calculation is greatly simplified as electron densities in the crystal may usually be assumed to be isotropic. The main exceptions are for very thin crystals at orientations exciting strong Bragg reflections and (particularly) orientations of high symmetry [ 181.
Fig. 2. Foil thinning caused by sputtering with 1.0 MeV electrons. (a) A 400 A (001) gold foil irradiated for about 50 min to a,dose of up to 1.7 X 1O23 electrons cmM2. (b) A nickel foil irradiated at 530°C to a dose of up to 3.6 X 1O24 electrons cme2, equivalent to a radiation damage dose of approximately 140 displacements atom- 1. A void layer in the most heavily irradiated central region has been partially destroyed by foil thinning. (Courtesy of F.J. Min-
ter, unpublished.)
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The cross-sections that lead to sputtering events are small, typically about 10d6 atoms per incident electron. However, at the very high beam densities -10 A cme2 achieved in the focussed electron beam in the HVEM, electron sputtering can be observed at practicable rates. Fig. 2 shows that observable rates are indeed achieved. Fig. 2a shows an evaporated (001) gold foil of initial thickness 400 A which was sputtered completely through by 1 .O MeV electrons in a 50 min irradiation. Stereo microscopy confirmed that thinning was from the exit surface of the foil. Fig. 2b shows a rather different example in which a nickel foil was irradiated by 1 .O MeV electrons under conditions where radiation damage by the electron beam caused the growth of high densities of voids. Substantial thinning indicated by absorption contrast in the most heavily-irradiated central area has caused a partial destruction of the void layer. The HVEM has been applied quantitatively to the study of sputtering from (111) gold films. Sputtering yields and the surface structure of the sputtered films have been investigated. The main techniques and results are summarised in sections 3 and 4. All experiments to date have been carried out on the Harwell AEI EM7 electron microscope operating at a variable voltage in the range 0.1-l .2 MeV and at a focussed beam density of about 10 A cme2. The specimen stage in the Harwell instrument has additional pumping facilities to enable working background pressures of 5 X lo-’ Torr or better to be achieved in the specimen chamber.
3. Sputtering yields Total sputtering yields and the angular distribution of sputtered atoms have been measured for 200-1000 A thick evaporated (111) gold foils irradiated at near-normal incidence. Total sputtering yields were measured from the time taken to sputter through films of known thickness [ 121. Fig. 3 illustrates the total sputtering yield measured as a function of electron energy for films at ambient temperatures. The results indicate a sputtering threshold energy of 0.3-0.4 MeV, or about 5 eV using eq. (2). Angular distributions of sputtered atoms were obtained using the experimental arrangement in fig. 4 [ 131. Evaporated carbon films about 200 A thick were mounted in the specimen holder about 5-15 pm below the irradiated gold foil. The sputtered gold atoms issuing from an irradiated area of l-2 pm across were condensed on to the carbon substrate to form discrete gold islands. The relative orientation and separation of the foils were determined by standard electron microscope techniques. After irradiations the foils were removed and separated, and the carbon foil examined in the combined electron microscope and microanalysis equipment EMMA3 [ 191. The average thickness of the gold deposit was measured at individually selected points (0, @) by counting AuM, X-rays backscattered from an electron microprobe of width about 0.2 pm. The X-ray detector was of the wavelength dispersive type with a collection solid angle of about 10m2 sterad. The sensitivity limit for detection of gold was about 0.2 A average thickness com-
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Au ill11 I
Fig. 3. The total sputtering yield, o, for (111) gold films as a function of electron energy, results are given by circles. Curves (a) and (b) given the yield os from eq. (3) for Es = UO/cos20, Uo respectively with UO = 5 eV.
Ek. Experimental
Fig. 4. A technique for measuring the angular distribution of sputtered atoms from (111) gold foils. The incident electron beam focussed to a spot size of X-2 pm transmission-sputters gold atoms in directions (8, #) on to an ~Iectron-transparent carbon fiim at distance d - 5-15 Frn below the gold. The distribution of the gold deposit is subsequently determ~ed by X-ray microanalysis. (After ref. [ 131.)
pared with a typical gold thickness in the range I-10 8. From the thickness measurements the differential cross-section for sputtering J(Q) was determined a a function of B and #J. The variation of f(G) with 8 is illustrated in fig. 5. The orientations II include the projection of (110) directions from the gold on to the carbon foil while the positions I bisect these directions. Experimental means are indicated by the dotted lines. The results at 0.8 and 1 .I MeV indicate pronounced maxima in the sputtering cross-sections close to (1 IO) directions. At 0.5 and 0.6 MeV the crosssections are approximately isotropic azimuth~y . The results in figs. 3 and 5 can be qu~itativeIy explained by the model proposed by Cherns et al. [ 121. At low enough energies the sputtering yield is due to direct electron-sputtering of atoms on the exit gold surface. Thus the sputtering crosssection is given by @max us =
s 0
(do/dS2) 2n sin @d@ ,
(3)
where emax is the angle for which E = EB(13),the binding energy of a surface atom to the crystal in direction 6. As the electron energy increases there should be an additional contribution from sub-surface recoils which generate collision sequences
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Pig. 5. The differential sputtering cross-section for (111) gold films 1(n) (arbitrary units) as a function of 0 for different electron energies. The orientations II include the projection of (110) directions from the gold (i.e. those inclined at 35” 16’ to the [ 1111 film normal) on to the carbon foil. The orientations I bisect these directions. The solid lines are theoretical curves determined by computer simulations (see text). (Courtesy_of the author and the Philosophical Magazine.)
down (110) directions. At the recoil energies involved for gold in these experiments (<25 eV) such collision sequences are expected to be strongly focussed collision sequences involving transfer of momentum but not mass [4]. Thus an additional sputtering of surface atoms is expected by this mechanism. The surface contribu-
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tion u, depends sensitively on the surface potential. The solid curves in fig. 3 show calculations of o, from eq. (3) for two extreme models of surface potential Curves (a) and (b) are for surface binding energies En = U,,/cos*B, Ue respectively with Ue = 5 eV chosen to match theoretical and experimental threshold energies. The electron density has been assumed to be uniform within the crystal (section 2). The surface contribution can explain sputtering up to 0.6-0.8 MeV. At higher energies an additional sub-surface contribution is required. The results in fig. 5 confirm this interpretation. The near-isotropic distributions at 0.5, 0.6 MeV are consistent with a surface layer contribution to sputtering. At higher energies the maxima near (110) directions indicate the additional (110) collision sequence contribution to the sputtering yield. Although the (110) contribution may be investigated by considering energy loss and focussing angle as adjustable parameters [ 12 1, a more detailed understanding of the results in fig. 5 has been obtained by computer simulation methods [13]. These results are worth some discussion. Theoretical simulations were derived using a model of the molecular dynamics type in which the gold crystal was simulated by an array of 324 atoms held together by a simple short range Morse potential. The potential @(r) was given by f$(r) =D{exp[-2o(r
- re)]} ,
- re)] - 2 exp[-cr(r
(4)
where r. = 2.88 A; D = 0.638 eV and a0 = 1 S81 A-’ were determined from the cohesive energy and bulk modulus of gold. Atom trajectories were calculated by integrating the equations of motion allowing all the atoms in the block to interact.
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Fig. 6. Energy loss to final energies, Eiitid direction (for a static The dotted line gives unpublished.)
the gold lattice at low recoil energies. The solid line gives the initial and and Ecd, of a surface atom sputtered following a recoil along a (110) lattice). The arrow indicates a recoil forming an adatom on the surface. the asymptote for recoils of high energy. (Courtesy of Dr. M.W. Finnis,
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The treatment differs from binary collision models, like the “Marlowe” type [203, which treat nearest neighbour collisions only in an otherwise frozen crystal. The essential difference is that the molecular dynamics program allows recoil energy to diffuse away in collective motions of the lattice. We may expect the energy loss to be significant for recoils with speeds comparable to, or less than, the speed of sound. This corresponds to a recoil atom energy of 10.7 eV in gold, compared to the maximum recoil energy of about 2.5 eV in these experiments. The energy loss for low energy recoils is, indeed, appreciable and can even be observed in the direct ejection of a surface atom as shown in fig. 6. This plots the final energy, Erinal, of a surface atom given an initial recoil energy, Einitial, along a (110) direction. The energy loss in sputtering near the threshold energy, about 7.5 eV, is nearly 2 eV higher than the asymptote for a primary recoil of high energy indicated by the dotted line. The theoretical simulations are in agreement with the qualitative picture of sputtering outlined earlier, namely that surface atoms are either directly sputtered, or indirectly sputtered by focussed collision sequences down (110) directions. No other types of sputtering event were recorded at these energies. The essential results are illustrated in fig. 7 which compares the differential scattering cross-section for various primary recoils with the differential sputtering cross-section with and without thermal vibrations. The results are for $ corresponding to position II i.e., 0 = 3.5’16’ corresponds to a (110) direction. Consider the results at 0.8 MeV where the sputtering is due to.a surface atom contribution with a sub-surface contribution from an atom recoil in the first sublayer into the crystal. For the surface contribution the differential scattering crosssection rises to a maximum at the cut-off value emax = 43’. The sputtering crosssection without thermal vibrations is modified by refraction as the escaping atom overcomes the surface binding energy combined with a glancing collision with a nearest neighbour atom in the surface steering the atom towards the foil normal. A cusp is produced in the sputtering cross-section at 0 - 28’ together with a tail at 0 >~max. When thermal vibrations are included the cusp is broadened to a half-width -12”. The sub-surface recoil contributes to the total sputtering yield for primary recoils in the range 13” < 0 < 36”. Similar effects to those for the surface atom are observed in the sputtering cross-section results. Consider now the results in fig. 5. The theoretical results are indicated by solid lines. The curves for position II represent the total sputtering yield with theoretical maxima normalised to the experimental results. The results for position I represent the corresponding surface atom yield. There is broad agreement between theoretical and experimental results and various features are worth noting. At 0.5 MeV, emax = 20” whereas sputteri& occurs at angles 0 up to 50” indicating the effect of refraction. In contrast the results at 0.8 and 1.I MeV shows theoretical maxima for position II between 25’ and 30”. The positions of these maxima reflect the “steering” of focussed collision sequences by the collision with the neighbouring surface atom towards rather than away from the [I 111 normal. The onset of the sub-surface
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yield is first apparent experimentally at 0.6 MeV. Theoretically a somewhat higher energy is predicted although the appearance of (110) maxima at 0.8, 1.1 MeV is predicted theoretically. It is worth examining the length of focussed collision sequences. Theoretically, even at 1.1 MeV, only 3-4 layers contribute to the sputtering yield reflecting an energy loss of 2-3 eV per atom in the sequence to the lattice (see earlier). The contribution of sequences initiated in the first sub-layer is larger (25% of the total yield) than the combined contributions from sequences of greater length (-15% of total yield). Qualitatively we might expect that the half-width of the (110) contribution reflects the degree of focussing and becomes a function of the sequence length. However, the theoretical results show that the spot width depends mainly on thermal broadening. Thus the form of the spots is no guide to the length of collision sequences. In order to ‘differentiate between contributions from (110) sequences of different lengths we need careful simulations of the total (110) spot yield at different energies. This requires a careful assessment of the interatomic potential, an area not yet fully studied. This point is discussed more fully in section 5.
4. Surface structure studies It has been possible using the high resolution facilities of the HVEM to investigate the development of surface structure on (111) gold during sputtering [143. In general, tr~smission electron microscopy enables us to obtain lateral point-to-point
Fig. 8. Steps on the surface of a 400 A thick evaporated resolution using a “forbidden” reflection (see text).
(111) gold film imaged to atomic
3.50
D. Cherns /Sputtering in high-voltageEM
resolutions of down to 2-3 a in the best available instruments. However, more significantly for applications to surface topology, special techniques have been developed to resolve, depthwise, steps of atomic and fear-atone height on foil surfaces [21-241. The imaging of single atomic steps on the (11 I) gold surface, using the technique reported by Cherns [Zl], is illustrated in fig. 8. This shows a micrograph imaged in a “forbidden” reflection. Kinematically such an image should show no significant intensity in regions of crystal which are an integral number of unit cells thick (3n atomic layers thick where n is an integer). However, weak but detectable intensity is possible in areas of crystals which are a non-integr~ number of unit cells thick (3n + 1 layers thick). The boundaries between dark and light contours in fig. 8 should, therefore, reveal surface steps of atomic height and greater. This micrograph, therefore, enables us to both identify individual atomic steps and to characterise surface roughness. In this case areas up to -500 a across appear to be atomically smooth. For a further explanation of the features in fig. 8, see ref. [Zl]. The surface of (111) gold films sputtered at room temperature in the HVEM was found to develop a high density of pits and cones [12]. At an early stage of development the sputtered surface was as illustrated in fig. 9a. The small brightly transmitting patches are surface pits seen by absorption contrast. The pits do not cover the irradiated area uniformly. Using the forbidden reflection technique it was shown f 141 that initia~y perfect (atomica~y smooth) areas of (111) gold coincided
Fig. 9. (a) An uneven distribution. of surface pits formed on an evaporated (111) gold film during electron-sputtering at room temperature. Traces, Y, which cross the pitted regions, are bands relatively denuded of pits. (b) An overlay identifying some of the surface steps present on the original (Ill) gold surface.
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Fig. 10. Density of surface pits on uniformly pitted regions of (111) gold as a function of foil temperature. Results are for sputtering by 1.0 MeV electrons at a dose rate of 7 X 1019 electrons cmv2 set-t The straight line is a theoretical fit to the results for Em,, = 0.45 eV. (Courtesy of the author and the Philosophical Magazine.)
with the uniformly pitted areas in subsequent sputtering. Heavily stepped areas on the original foil surface subsequently showed few pits. The line traces, Y, across the pitted areas in fig. 9a, which are regions relatively denuded of pits, were found to be closely associated with individual steps on the unsputtered surface. Fig. 9b illustrates this with an overlay on the micrograph in (a) of steps identified from a preirradiation micrograph, although in this case steps were identified from a conventional bright field micrograph and are probably at least several atom spacings high. The high resolution studies strongly support the model proposed by Cherns [14] where surface vacancies produced by sputtering migrate in the surface layer until they annihilate at steps or agglomerate to form the nuclei of the pits eventually observed. The pit density that develops on an atomically smooth area during sputtering is given by a nucleation and growth theory analogous to that for the nu-
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cleation and growth of islands on a substrate following adatom deposition, studied case. The saturation pit density is given by N, = k [J exp(E,,/kfl]
r/3 ,
a well
(5
where k is a calculable constant, J is the surface vacancy production rate (i.e. the sputtering rate), T is the foil temperature and Emvs is the migration energy for surface vacancies. In an area containing an initial surface step, the pit density is reduced within a distance L from the step given by N;‘f3 approximately. The experimental pit density N, agrees quite well with eq. (5). The variation of N, with I/kT is illustrated in fig. 10. Pit densities range up to values in excess of 10” cme2 at temperatures below O’C. The results over the temperature range 0-200°C have been approximately fitted by a straight line whose slope gives E,, = 0.45 eV in eq. (5). For a more detailed analysis of these results the reader is referred to ref. ]141.
5. Discussion The work described in section 3 shows how the angular distribution of sputtered atoms from (111) gold films may be interpreted in terms of individual low energy collision events. At low enough energies (0.4-0.6 MeV) sputtering may be explained by direct ejection of surface atoms. At higher energies (X.6 MeV) sputtering yields indicate indirect ejection of surface atoms by collision sequences down (110) directions. Theoretical features of atom trajectories such as refraction and “steering” towards the foil normal, energy dissipation and the influence of thermal vibrations may be seen in the experimental results. Thus by the angular distribution technique we have the ability to resolve sputtering contributions from individual collision events. A logical extension of the technique is to examine the role of long range focussed collision sequences to sputtering. This experiment is an important potential application of the high voltage electron microscope which will now be considered. The first point is that long range focussed collision sequences are difficult to study in gold, at least for the 1 .O MeV electron microscope, since only 3-4 atomic layers can contribute to the sputtering yield. The situation is better for materials of lower atomic number where larger recoil energies are possible (eq. (2)) and permanent displacements may occur. Under these conditions focussed collision sequences of a hundred atoms of more in length may be generated [25]. However the sputtering yield increases with atomic number and we must consider the effect of this on the experimental measurements. Fig. 11 compares sputtering cross-sections for materials of differing atomic number 2 ranging from aluminium (2 = 13) to gold (2 = 79). Sputtering cross-sections are a sensitive function of binding energy as previously noted. For purposes of comparison, cross-sections have been derived from tables by Oen [17] for the sputtering of a single atom with an isotropic binding
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Al 0.5
E,(MeVl
1.0
Fig. 11. Hypothetical sputtering cross-sections for atoms whose binding energy to the lattice is assumed to be 4 eV. Results are taken from tables by Oen 1171. :
energy of 4 eV. This appears the most useful comparison as we require to detect sputtering from an individual sputtering event. The cross-sections decrease with decreasing atomic number for the same value of E,,,. For example, for Em,, = 8 eV (indicated by arrows) the decrease is a factor “3, from gold to aluminium, and is larger for larger Em,,. The decrease in sputtering rate from gold to aluminium may probably be accommodated using the existing experimental method. Specifically, sensitivity may be improved by optimising values of d and the halfwidth of the electron beam in fig. 4. The total electron dose may also,be increased (but not greatly - gold irradiations in fig. 5 required irradiation times between i and 2: hours [ 131). For microanalysis, although we might expect a slight improvement in X-ray yield over the range 2 = 78-13 [26], measurements by Ecker [27] suggest a nearly constant sensitivity in practice. The method can probably be improved substantially, however, by using energy dispersive rather than wavelength dispersive analysis. In principle this should enable X-ray count rates to be increased by several orders of magnitude. However the question of increased background is non-trivial [28,29] and further work is evidently required here. Theoretically, the contribution to sputtering from short range collision sequences in the spot patterns is probably important at all energies compared to the long range focussed collision contribution [ll]. Thus in order to resolve the focussed collision contribution we need detailed comparisons of experimental and theoretical yields as a function of recoil energy. This in turn requires a careful assessment of the interatomic potential for each material examined. Indeed the main limitation to the investigation would appear to lie with the accuracy of the interatomic potential. The potential is fortunately the only serious approximation in the theoretical simulations here. The Morse potential for gold in eq. (4) is appropriate for uniform compressions s-1 0% of the gold lattice. Although interatomic displacements in the collisions considered here do not generally exceed this value, elec-
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Fig. 12. Stacking fault tetrahkdra
in a quenched gold foil shrinking as a result of irradiation by 1.0 MeV electrons. The foil temperature was 140°C and the dose rate 7 X 1019 electrons -2 set-l (a) before irradiation (b) After 3 min irradiation and (c) after 6 min irradiation. Re foil orientation is close to [ 001].
tronic exchange interactions must act to reduce the local asymmetry in wavefunction and, therefore, the interaction energy during a collision. It is probable therefore that the Morse potential overestimates the ‘%lardness” of the interatomic potential. This may be seen in the calculated displacement energy Ed. From simulations following (110) recoils the Morse potential gives Ed above 45 eV. Resistivity mea~rements suggest .!?d - 36 eV [30]. Recent experiments by the author [31] to derive the threshold energy from the rate of growth of loops under irradiation suggest that much lower values of Ed are possible. An example is shown in fig. 12. This shows the shrinkage of vacancy stacking fault tetrahedra irradiated at 14O’C by 1 .O MeV electrons along a direction within a few degrees of [OOl ] . The inference is that 1 .O MeV electrons can create permanent damage in bulk gold leading to a net absorption of interstitials at tetrahedra which proceed to shrink. However the interpretation of these results is complicated by sub-threshold effects [32,33] and more work is required. A similar study already completed for copper has also revealed threshold energies much lower than previously accepted [34] _ It is, therefore, felt that substantial effort is required to obtain a satisfactory interatomic potential at low energies. A second important application of the HVEM is in the study of surface structure. Surface roughness on a macroscopic level is well known and has been explained as a consequence of the sputtering yield versus scattering angle relationship (see review by Carter, Colligon and Nobes [35]). The surface may be treated as a continuum. However, the results in fig. 10 indicate that the sputtered surface
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of (I 11) gold is rough at the atomic level. The explanation of roughness at the atomic level is quite different from that at the macroscopic level requiring a surface diffusion model where the atomic structure of the surface is an essential feature. It is anticipated that diffusion-controlled surface roughness may, indeed, be a general microscopic feature of sputtering, and one usually unresolved by, for example, scanning microscopy methods. The question of microscopic surface rou~ness is particularly relevant to the application of various surface analysis techniques during sputtering. For instance, in low energy electron diffraction (LEED), degradation of diffraction patterns from a single crystal surface is expected when the surface step separation become less than the beam coherence width - several hundred A 136-383 _ We may, therefore, expect marked degradation to occur in gold sputtered at room temperature. degradation of LEED patterns during sputtering has already been noted (see review by FUread [39]). Another technique expected to be highly sensitive to surface roughness is ion-beam scattering [40]. Thus it is suggested that a general high resolution study of surface roughness generated by sputtering is desirable both from a theoretical viewpoint and from the ~ew~int of other surface analysis techniques. This can readily be done in the HVEM.
6. Summary The use and scope of the high voltage electron microscope for the study of sputtering has been explained with reference to work carried out on (111) gold. The two most important areas of future study would appear to be: (1) Evaluation of the role of long range focussed collision sequences in sputtering, for materials of medium atomic number. (2) Extension of high resolution studies of microscopic surface roughness caused by sputtering to other materials and crystal orientations.
References [I] M. Kaminsky, Atomic and ionic Impact Phenomena on Metal Surfaces (Springer, Berlin, 1965). [2] G. Carter and J.S. Colligon, Ion Bombardment of Solids (Elsevier, New York, 1968). [3] P. Sigmund, Phys. Rev. 184 (1969) 383. [4] R.S. Nelson, The Observation of Atomic Collisions in Crystalline Solids (North-Holland, Amsterdam 1968). [S] P. Sigmund, Rev. Roumaine Phys. 17 (1972) 1079. (61 P. Sigmund, in: international Workship on Inelastic Ion-Surface Collisions, Eds. N.H. Tolk et al. (AcademicPress, New York, 1977) p. 121. [7] M.W. ~ompson,Ph8. Mag. 18 (1968) 377. [8] B.W. Farmery and M.W. Thompson,Phil. Mag. 18 (1968) 415. 191 G.K. Wehner, Phys. Rev. 102 (1955) 690.
356
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1101 R;S. Nelson and R. von Jan, Can. J. Phys. 46 (1968) 747. [ 111 C. Lehmann and P. Sigmund, Phys. Status Solidi 16 (1966) 507. [12] D.Cherns, F.J. Minter and R.S. Nelson, Nucl. Instr. Methods 132 (1976) 369. [ 131 D. Cherns, M.W. Finnis and M.D. Matthews, Phil. Mag. 35 (1977) 693. [14] D. Cherns, Phil. Mag. 36 (1977) 1429. [ 151 P.B. Hirsch, A. Howie, R.B. Nicholson, D.W. Pashley and M.J. Whelan, Electron Microscopy of Thin Crystals (Butterworths, London, 1965). [16] N.F. Mott, Proc. Roy. Sot. (London) Al24 (1929) 426; Al35 (1932) 429. [ 171 OS. Oen, ORNL 4897. [18] D. Cherns, A. Howie and M.H. Jacobs, 2. Naturforsch. 28a (1973) 56.5. [19] C.J. Cooke and I.K. Openshaw, in: Proc. 4th Natl. Conf. on Electron Microprobe Analysis, Pasadena, 1969 (Electron Probe Analysis Society of America) paper 64. (201 M.T. Robinson and I.M. Torrens, Phys. Rev. B9 (1974) 5008. 1211 D. Cherns, Phil. Mag. 30 (1974) 549. [22] R.L. Hines, Thin Solid Films 35 (1975) 229. [23] A.F. Moodie and C.E. Warble, Phil. Mag. 16 (1967) 891. [24] K. Kambe and G. Lehmpfuhl, Optik 42 (1975) 187. 1251 J.A. Venables and GJ. Thomas, in: Vacancies and Interstitials in Metals, Eds. A. Seeger et al. (North-Holland, Amsterdam 1970) p. 531. 1261 J.I. Goldstein, J.L. Costley, G.W. Lorimer and S.J.B. Reed, in: Scanning Electron Microscopy/1977, Vol. 1, Ed. 0. Johari (Univ. of Chicago Press) p. 315. [27] K.H. Ecker, J. Phys. D6 (1973) 2150. [28] J.D. Geller, Scanning Electron Microscopy/l977, Vol. 1, Ed. 0. Johari (Univ. of Chicago Press) p, 281. [29] W.A.P. Nicholson, B.W. Robertson and J.N. Chapman, in: Developments in Electron Microscopy and Analysis 1977, Ed. D.L. Misell (Inst. Physics, London) p. 373. [30] W. Bauer, A.I. Anderman and A. Sosin, Phys. Rev. 185 (1969) 870. [31] D. Cherns, to be published. 1321 W. Bauer and A. Sosin, J. Appl. Phys. 35 (1964) 703. [ 331 D. Cherns, F.J. Minter and R.S. Nelson, in: Proc. Intern. Conf. On Fundamental Aspects of Radiation Damage in Metals, Gatlinburg, Vol. II, Eds. M.T. Robinson and F.W. Young (1975) p. 178. [34] N. Yoshida and K. Urban, in: Proc. 5th Intern. Conf. on High Voltage Electron Microscopy, Kyoto, 1977. [35] G. Carter, J.S. Colligon and M. Nobes, Radiation Effects 31 (1977) 65. (361 J.E. Houston and R.L. Park, Surface Sci. 34 (1973) 477. [37] G.E. Laramore, J.E. Houston and R.L. Park, J. Vacuum Sci. Technol. 10 (1973) 196. [38] J.M. Cowley and H. Shuman, Surface Sci. 38 (1973) 53. [39] G.E. Rhead, Surface Sci. 68 (1977) 20. 1401 S.H.A. Begemann and A.L. Boers, Surface Sci. 30 (1972) 134.
SPUTTERING IN THE HIGH-VOLTAGE
ELECTRON MICROSCOPE
D. CHERNS Oxford University, Department UK
of Metallurgy and Science of Materials, Parks Road, Oxford,
Received 23 October 1978; manuscript received in final form 5 December 1978
The 1 .O MeV electron microscope has been used to observe and analyse transmission sputtering caused by the electrons in the incident beam. The method, reviewed here, is particularly suitable for investigating low energy collision events. Total sputtering yields and angular distributions of sputtered atoms have been measured for (111) gold films to within a few eV of the sputtering threshold energy. It is shown that the results can be explained by the sputtering of surface atoms either directly by electrons, or indirectly by collision sequences generated down ( 110) directions. The necessity of using a many-body collision model to interpret the results is stressed. High resolution electron microscopy has been used to study the surface structure of (111) gold films during sputtering on a near-atomic level. It is shown how the results confirm a model where surface roughness develops due to the migration and agglomeration of surface vacancies produced during sputtering. The future scope of the 1 .O MeV electron microscope as an analytical tool for sputtering is also discussed. It is suggested that the r61e of long range focussed collision sequences in sputtering may be determined for materials of medium atomic number. A need for further high resolution studies of sputtered surfaces is identified; such studies are seen as complementary to those by other surface analysis techniques.
1. Introduction
The study of sputtering has greatly contributed to our understanding of atomic collision processes that occur within solids. Work on sputtering has been extensive, particularly in the case of ion-sputtering which has been investigated over a wide range of targets, incident ions and incident ion energies from several hundred eV upwards [l-4]. It is now accepted that sputtering yields for a wide range of conditions can be approximately explained by the collision cascade model developed by Sigmund [3,5,6]. In Sigmund’s model, collision cascades initiated by the incoming particles are treated by assuming that recoil atoms undergo a series of binary collisions in a random target. At the intersection of the cascade with the surface, recoil atoms with energy greater than the surface binding energy are sputtered. However, the assumptions of Sigmund’s model are least reliable at low recoil energies, below about 10 eV [6], which dominate the cascade energy spectrum. Indeed most sputtered atoms are found to have energies of a few eV or less [7,8]. 339
340
D. Chems /Sputtering in high-~o~tugeEM
At these low energies the crystal structure may be expected to have an important influence in determining recoil trajectories. It was first shown by Wehner 193 and subsequently by others [4,10] that angular distributions of sputtered atoms from single crystals show maxima close to prominent crystallographic directions. This has been interpreted in terms of focussed collision sequences which transfer momentum down close-packed rows of atoms in the lattice. However it has been suggested that the knock-on of surface atoms by essentially random sub-surface recoils can also contribute substantially to the spot patterns [ 1 I]. Recent work [ 12-141 has demonstrated that the 1 .O MeV electron microscope can be used to study electron sputtering caused by collisions between incident beam electrons and target nuclei. In the past electron sputtering of this type has been little used as an experimental tool owing to the experimental difficulties associated with low yields. However the yield problem is overcome in the electron microscope by the very high electron beam densities that can be achieved; these exceed beam densities in conventional electron accelerators by factors of up to 10’. It is the purpose of this paper to explain how the high voltage electron microscope (HVEM) can be used to study sputtering and, in particular, the low energy collision events that are so important in the collision cascade. The principles of the method are explained in section 2. In sections 3 and 4 the main achievements of the HVEM method are summarised. In section 3 it is shown how sputtering yields from (111) gold films may be obtained to within a few eV of the sputtering threshold energy; yields can be explained by transmission sputtering of surface atoms either directly by electrons or indirectly by collision sequences generated down (110) directions. The necessity of using a many-body rather than a binary collision theory is stressed. In section 4 the application of high resolution techniques of electron microscopy to the sputtered (111) gold surface is considered. It is outlined how a surface diffusion model explains the roughness of the sputtered surface on the atomic level. The general application of the HVEM techniques to sputtering studies is discussed in section 5. It is suggested that the HVEM may be specifically used to investigate the contribution of long range focussed collision sequences to sputtering. The need for more extensive studies of surface roughness, and their particular relevance to the use of other surface analysis techniques such as low energy electron diffraction (LEED), is explained.
2. Concept of sputtering in the high voltage electron microscope The scattering situation in the electron microscope is illustrated in fig. 1. In the 1 .O MeV electron microscope specimens are prepared for viewing in transmission and have thicknesses generally in the range 0.01-l pm. The electron beam passes through specimens of this thickness range virtually undiminished in energy or intensity, and undeviated except for substantial low angle elastic and inelastic scattering at angles comparable to the Bragg angle (-low2 rad). This scattering is well under-
D. Cherns /Sputtering
in high-voltage EM
341
electrons
/ sputtered
la1
lb)
Fig. 1. (a) Sputtering in the electron microscope. Incident electrons pass through thin electron microscope specimens predominantly unabsorbed and undeviated, except for small angle (-10e2 rad) scattering, to form the normal transmission images. A small fraction (-lOA) of incident electrons in the 1.0 MeV electron microscope undergo sufficiently violent, highangle collisions with target atoms to cause sputtering. (b) The sputtering is mainly in transmission causing foil thinning as shown. (After ref. [ 121.)
stood in terms of the dynamical theory of electron diffraction [15]. High angle electron-atom nucleus scattering, leading to substantial energy transfer is a comparatively rare event. In a direct collision between an electron of energy E, and an atom nucleus of mass M, the energy E of the recoil atom varies with the angle 0 of the recoil atom trajectory to the electron beam direction by E = E,,,
cos28 ,
where Em,
(1)
is given by
E max = 2E,(E, + 2moc2)/Mc2
,
(2)
where m. is the rest mass of the electron and c is the velocity of light. Values of E max are given in table 1 for various atoms in the atomic number range Z = 13 to 79 (aluminium to gold), and for electron energies of 0.1, 1 .O MeV. As the minimum energy for sputtering these materials is only a few eV, it is clear that electron-atom collisions in the 1.0 MeV electron microscope can transfer sufficient energy to sputter most materials. In contrast sputtering in the conventional 0.1 MeV microscope is only possible for the lightest elements. Since differential crosssections for back-sputtering are also negligible for a flat specimen compared to the cross-sections for forward sputtering, for the essentially unabsorbed electron beam, sputtering should occur mainly in transmission as illustrated in fig. lb. Scattering angles are large, typically up to 1 rad, so that the sputtered atoms will be
342
D. Cherns /Sputtering in high-voltage EM
Table 1 Values of Emax at 0.1, 1.O MeV from eq. (2) Element
Aluminium Copper Silver Gold
Atomic number Z
13 29 47 79
Emax (eV) E, = 0.1 MeV
Ee=l.OMeV
8.9 3.8 2.2 1.2
161 68 40 22
ejected outside the imaging system of the microscope, and may be collected on a suitable thin film substrate as shown later. The differential scattering cross-sections, do/da, for electron-atom nucleus collisions are accurately known from the theory given by Mott [16]. Tables of cross-sections for various elements have been compiled by Oen [17]. In principle, therefore, as electron scattering by the crystal is well-understood, primary recoil cross-sections may be reliably calculated. In practice the calculation is greatly simplified as electron densities in the crystal may usually be assumed to be isotropic. The main exceptions are for very thin crystals at orientations exciting strong Bragg reflections and (particularly) orientations of high symmetry [ 181.
Fig. 2. Foil thinning caused by sputtering with 1.0 MeV electrons. (a) A 400 A (001) gold foil irradiated for about 50 min to a,dose of up to 1.7 X 1O23 electrons cmM2. (b) A nickel foil irradiated at 530°C to a dose of up to 3.6 X 1O24 electrons cme2, equivalent to a radiation damage dose of approximately 140 displacements atom- 1. A void layer in the most heavily irradiated central region has been partially destroyed by foil thinning. (Courtesy of F.J. Min-
ter, unpublished.)
D. Cherns /Sputtering in high-voltage EM
343
The cross-sections that lead to sputtering events are small, typically about 10d6 atoms per incident electron. However, at the very high beam densities -10 A cme2 achieved in the focussed electron beam in the HVEM, electron sputtering can be observed at practicable rates. Fig. 2 shows that observable rates are indeed achieved. Fig. 2a shows an evaporated (001) gold foil of initial thickness 400 A which was sputtered completely through by 1 .O MeV electrons in a 50 min irradiation. Stereo microscopy confirmed that thinning was from the exit surface of the foil. Fig. 2b shows a rather different example in which a nickel foil was irradiated by 1 .O MeV electrons under conditions where radiation damage by the electron beam caused the growth of high densities of voids. Substantial thinning indicated by absorption contrast in the most heavily-irradiated central area has caused a partial destruction of the void layer. The HVEM has been applied quantitatively to the study of sputtering from (111) gold films. Sputtering yields and the surface structure of the sputtered films have been investigated. The main techniques and results are summarised in sections 3 and 4. All experiments to date have been carried out on the Harwell AEI EM7 electron microscope operating at a variable voltage in the range 0.1-l .2 MeV and at a focussed beam density of about 10 A cme2. The specimen stage in the Harwell instrument has additional pumping facilities to enable working background pressures of 5 X lo-’ Torr or better to be achieved in the specimen chamber.
3. Sputtering yields Total sputtering yields and the angular distribution of sputtered atoms have been measured for 200-1000 A thick evaporated (111) gold foils irradiated at near-normal incidence. Total sputtering yields were measured from the time taken to sputter through films of known thickness [ 121. Fig. 3 illustrates the total sputtering yield measured as a function of electron energy for films at ambient temperatures. The results indicate a sputtering threshold energy of 0.3-0.4 MeV, or about 5 eV using eq. (2). Angular distributions of sputtered atoms were obtained using the experimental arrangement in fig. 4 [ 131. Evaporated carbon films about 200 A thick were mounted in the specimen holder about 5-15 pm below the irradiated gold foil. The sputtered gold atoms issuing from an irradiated area of l-2 pm across were condensed on to the carbon substrate to form discrete gold islands. The relative orientation and separation of the foils were determined by standard electron microscope techniques. After irradiations the foils were removed and separated, and the carbon foil examined in the combined electron microscope and microanalysis equipment EMMA3 [ 191. The average thickness of the gold deposit was measured at individually selected points (0, @) by counting AuM, X-rays backscattered from an electron microprobe of width about 0.2 pm. The X-ray detector was of the wavelength dispersive type with a collection solid angle of about 10m2 sterad. The sensitivity limit for detection of gold was about 0.2 A average thickness com-
344
D. Cherns /Sputtering
in high-voltage EM
Au ill11 I
Fig. 3. The total sputtering yield, o, for (111) gold films as a function of electron energy, results are given by circles. Curves (a) and (b) given the yield os from eq. (3) for Es = UO/cos20, Uo respectively with UO = 5 eV.
Ek. Experimental
Fig. 4. A technique for measuring the angular distribution of sputtered atoms from (111) gold foils. The incident electron beam focussed to a spot size of X-2 pm transmission-sputters gold atoms in directions (8, #) on to an ~Iectron-transparent carbon fiim at distance d - 5-15 Frn below the gold. The distribution of the gold deposit is subsequently determ~ed by X-ray microanalysis. (After ref. [ 131.)
pared with a typical gold thickness in the range I-10 8. From the thickness measurements the differential cross-section for sputtering J(Q) was determined a a function of B and #J. The variation of f(G) with 8 is illustrated in fig. 5. The orientations II include the projection of (110) directions from the gold on to the carbon foil while the positions I bisect these directions. Experimental means are indicated by the dotted lines. The results at 0.8 and 1 .I MeV indicate pronounced maxima in the sputtering cross-sections close to (1 IO) directions. At 0.5 and 0.6 MeV the crosssections are approximately isotropic azimuth~y . The results in figs. 3 and 5 can be qu~itativeIy explained by the model proposed by Cherns et al. [ 121. At low enough energies the sputtering yield is due to direct electron-sputtering of atoms on the exit gold surface. Thus the sputtering crosssection is given by @max us =
s 0
(do/dS2) 2n sin @d@ ,
(3)
where emax is the angle for which E = EB(13),the binding energy of a surface atom to the crystal in direction 6. As the electron energy increases there should be an additional contribution from sub-surface recoils which generate collision sequences
D. Cherns /Sputtering
70-
0 *
60-
C
0
10
20
1900
+ 6
rl600
.+
.270“
30
LO
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60
01 0
1
10
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20
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1
10
20
30 LO 0 (degrees1
50
60
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30 LO 0 (degrees1
L
50
I
Id1 1lOOkeV
90
0
345
in high-voltage EM
0
10
20
30 0
LO
50
(degrees1
Pig. 5. The differential sputtering cross-section for (111) gold films 1(n) (arbitrary units) as a function of 0 for different electron energies. The orientations II include the projection of (110) directions from the gold (i.e. those inclined at 35” 16’ to the [ 1111 film normal) on to the carbon foil. The orientations I bisect these directions. The solid lines are theoretical curves determined by computer simulations (see text). (Courtesy_of the author and the Philosophical Magazine.)
down (110) directions. At the recoil energies involved for gold in these experiments (<25 eV) such collision sequences are expected to be strongly focussed collision sequences involving transfer of momentum but not mass [4]. Thus an additional sputtering of surface atoms is expected by this mechanism. The surface contribu-
346
D. Cherns /Sputtering
in high-voltage
EM
tion u, depends sensitively on the surface potential. The solid curves in fig. 3 show calculations of o, from eq. (3) for two extreme models of surface potential Curves (a) and (b) are for surface binding energies En = U,,/cos*B, Ue respectively with Ue = 5 eV chosen to match theoretical and experimental threshold energies. The electron density has been assumed to be uniform within the crystal (section 2). The surface contribution can explain sputtering up to 0.6-0.8 MeV. At higher energies an additional sub-surface contribution is required. The results in fig. 5 confirm this interpretation. The near-isotropic distributions at 0.5, 0.6 MeV are consistent with a surface layer contribution to sputtering. At higher energies the maxima near (110) directions indicate the additional (110) collision sequence contribution to the sputtering yield. Although the (110) contribution may be investigated by considering energy loss and focussing angle as adjustable parameters [ 12 1, a more detailed understanding of the results in fig. 5 has been obtained by computer simulation methods [13]. These results are worth some discussion. Theoretical simulations were derived using a model of the molecular dynamics type in which the gold crystal was simulated by an array of 324 atoms held together by a simple short range Morse potential. The potential @(r) was given by f$(r) =D{exp[-2o(r
- re)]} ,
- re)] - 2 exp[-cr(r
(4)
where r. = 2.88 A; D = 0.638 eV and a0 = 1 S81 A-’ were determined from the cohesive energy and bulk modulus of gold. Atom trajectories were calculated by integrating the equations of motion allowing all the atoms in the block to interact.
/
30-
_
Eflnal (eV1 2.0 -
l.O-
0 5.0
/I
/ 6.0
/
/
/
/
/
/
/
/
I
/
I
7.0
I
_
9.0 E
Fig. 6. Energy loss to final energies, Eiitid direction (for a static The dotted line gives unpublished.)
the gold lattice at low recoil energies. The solid line gives the initial and and Ecd, of a surface atom sputtered following a recoil along a (110) lattice). The arrow indicates a recoil forming an adatom on the surface. the asymptote for recoils of high energy. (Courtesy of Dr. M.W. Finnis,
D. Chews
J’ Sputtering
..~_I_““‘__-_‘
___---___-_-_-
in high-voltage
EM
348
D. Cherns /Sputtering
in high-voltage
EM
The treatment differs from binary collision models, like the “Marlowe” type [203, which treat nearest neighbour collisions only in an otherwise frozen crystal. The essential difference is that the molecular dynamics program allows recoil energy to diffuse away in collective motions of the lattice. We may expect the energy loss to be significant for recoils with speeds comparable to, or less than, the speed of sound. This corresponds to a recoil atom energy of 10.7 eV in gold, compared to the maximum recoil energy of about 2.5 eV in these experiments. The energy loss for low energy recoils is, indeed, appreciable and can even be observed in the direct ejection of a surface atom as shown in fig. 6. This plots the final energy, Erinal, of a surface atom given an initial recoil energy, Einitial, along a (110) direction. The energy loss in sputtering near the threshold energy, about 7.5 eV, is nearly 2 eV higher than the asymptote for a primary recoil of high energy indicated by the dotted line. The theoretical simulations are in agreement with the qualitative picture of sputtering outlined earlier, namely that surface atoms are either directly sputtered, or indirectly sputtered by focussed collision sequences down (110) directions. No other types of sputtering event were recorded at these energies. The essential results are illustrated in fig. 7 which compares the differential scattering cross-section for various primary recoils with the differential sputtering cross-section with and without thermal vibrations. The results are for $ corresponding to position II i.e., 0 = 3.5’16’ corresponds to a (110) direction. Consider the results at 0.8 MeV where the sputtering is due to.a surface atom contribution with a sub-surface contribution from an atom recoil in the first sublayer into the crystal. For the surface contribution the differential scattering crosssection rises to a maximum at the cut-off value emax = 43’. The sputtering crosssection without thermal vibrations is modified by refraction as the escaping atom overcomes the surface binding energy combined with a glancing collision with a nearest neighbour atom in the surface steering the atom towards the foil normal. A cusp is produced in the sputtering cross-section at 0 - 28’ together with a tail at 0 >~max. When thermal vibrations are included the cusp is broadened to a half-width -12”. The sub-surface recoil contributes to the total sputtering yield for primary recoils in the range 13” < 0 < 36”. Similar effects to those for the surface atom are observed in the sputtering cross-section results. Consider now the results in fig. 5. The theoretical results are indicated by solid lines. The curves for position II represent the total sputtering yield with theoretical maxima normalised to the experimental results. The results for position I represent the corresponding surface atom yield. There is broad agreement between theoretical and experimental results and various features are worth noting. At 0.5 MeV, emax = 20” whereas sputteri& occurs at angles 0 up to 50” indicating the effect of refraction. In contrast the results at 0.8 and 1.I MeV shows theoretical maxima for position II between 25’ and 30”. The positions of these maxima reflect the “steering” of focussed collision sequences by the collision with the neighbouring surface atom towards rather than away from the [I 111 normal. The onset of the sub-surface
D. Cherns /Sputtering
in high-voltage EM
349
yield is first apparent experimentally at 0.6 MeV. Theoretically a somewhat higher energy is predicted although the appearance of (110) maxima at 0.8, 1.1 MeV is predicted theoretically. It is worth examining the length of focussed collision sequences. Theoretically, even at 1.1 MeV, only 3-4 layers contribute to the sputtering yield reflecting an energy loss of 2-3 eV per atom in the sequence to the lattice (see earlier). The contribution of sequences initiated in the first sub-layer is larger (25% of the total yield) than the combined contributions from sequences of greater length (-15% of total yield). Qualitatively we might expect that the half-width of the (110) contribution reflects the degree of focussing and becomes a function of the sequence length. However, the theoretical results show that the spot width depends mainly on thermal broadening. Thus the form of the spots is no guide to the length of collision sequences. In order to ‘differentiate between contributions from (110) sequences of different lengths we need careful simulations of the total (110) spot yield at different energies. This requires a careful assessment of the interatomic potential, an area not yet fully studied. This point is discussed more fully in section 5.
4. Surface structure studies It has been possible using the high resolution facilities of the HVEM to investigate the development of surface structure on (111) gold during sputtering [143. In general, tr~smission electron microscopy enables us to obtain lateral point-to-point
Fig. 8. Steps on the surface of a 400 A thick evaporated resolution using a “forbidden” reflection (see text).
(111) gold film imaged to atomic
3.50
D. Cherns /Sputtering in high-voltageEM
resolutions of down to 2-3 a in the best available instruments. However, more significantly for applications to surface topology, special techniques have been developed to resolve, depthwise, steps of atomic and fear-atone height on foil surfaces [21-241. The imaging of single atomic steps on the (11 I) gold surface, using the technique reported by Cherns [Zl], is illustrated in fig. 8. This shows a micrograph imaged in a “forbidden” reflection. Kinematically such an image should show no significant intensity in regions of crystal which are an integral number of unit cells thick (3n atomic layers thick where n is an integer). However, weak but detectable intensity is possible in areas of crystals which are a non-integr~ number of unit cells thick (3n + 1 layers thick). The boundaries between dark and light contours in fig. 8 should, therefore, reveal surface steps of atomic height and greater. This micrograph, therefore, enables us to both identify individual atomic steps and to characterise surface roughness. In this case areas up to -500 a across appear to be atomically smooth. For a further explanation of the features in fig. 8, see ref. [Zl]. The surface of (111) gold films sputtered at room temperature in the HVEM was found to develop a high density of pits and cones [12]. At an early stage of development the sputtered surface was as illustrated in fig. 9a. The small brightly transmitting patches are surface pits seen by absorption contrast. The pits do not cover the irradiated area uniformly. Using the forbidden reflection technique it was shown f 141 that initia~y perfect (atomica~y smooth) areas of (111) gold coincided
Fig. 9. (a) An uneven distribution. of surface pits formed on an evaporated (111) gold film during electron-sputtering at room temperature. Traces, Y, which cross the pitted regions, are bands relatively denuded of pits. (b) An overlay identifying some of the surface steps present on the original (Ill) gold surface.
D. Chews /Sputtering
cTI°C) 100
3KlMo
351
in high-voltage EM
0
-100
10'3
10’2
I ? 5 i
/*’ II
10”
10’0
ol
6
10
I
20
30 ‘/kT
LO leV”)
SO
I
60
70
-)
Fig. 10. Density of surface pits on uniformly pitted regions of (111) gold as a function of foil temperature. Results are for sputtering by 1.0 MeV electrons at a dose rate of 7 X 1019 electrons cmv2 set-t The straight line is a theoretical fit to the results for Em,, = 0.45 eV. (Courtesy of the author and the Philosophical Magazine.)
with the uniformly pitted areas in subsequent sputtering. Heavily stepped areas on the original foil surface subsequently showed few pits. The line traces, Y, across the pitted areas in fig. 9a, which are regions relatively denuded of pits, were found to be closely associated with individual steps on the unsputtered surface. Fig. 9b illustrates this with an overlay on the micrograph in (a) of steps identified from a preirradiation micrograph, although in this case steps were identified from a conventional bright field micrograph and are probably at least several atom spacings high. The high resolution studies strongly support the model proposed by Cherns [14] where surface vacancies produced by sputtering migrate in the surface layer until they annihilate at steps or agglomerate to form the nuclei of the pits eventually observed. The pit density that develops on an atomically smooth area during sputtering is given by a nucleation and growth theory analogous to that for the nu-
352
D. Cherns /Sputtering in high-voltage EM
cleation and growth of islands on a substrate following adatom deposition, studied case. The saturation pit density is given by N, = k [J exp(E,,/kfl]
r/3 ,
a well
(5
where k is a calculable constant, J is the surface vacancy production rate (i.e. the sputtering rate), T is the foil temperature and Emvs is the migration energy for surface vacancies. In an area containing an initial surface step, the pit density is reduced within a distance L from the step given by N;‘f3 approximately. The experimental pit density N, agrees quite well with eq. (5). The variation of N, with I/kT is illustrated in fig. 10. Pit densities range up to values in excess of 10” cme2 at temperatures below O’C. The results over the temperature range 0-200°C have been approximately fitted by a straight line whose slope gives E,, = 0.45 eV in eq. (5). For a more detailed analysis of these results the reader is referred to ref. ]141.
5. Discussion The work described in section 3 shows how the angular distribution of sputtered atoms from (111) gold films may be interpreted in terms of individual low energy collision events. At low enough energies (0.4-0.6 MeV) sputtering may be explained by direct ejection of surface atoms. At higher energies (X.6 MeV) sputtering yields indicate indirect ejection of surface atoms by collision sequences down (110) directions. Theoretical features of atom trajectories such as refraction and “steering” towards the foil normal, energy dissipation and the influence of thermal vibrations may be seen in the experimental results. Thus by the angular distribution technique we have the ability to resolve sputtering contributions from individual collision events. A logical extension of the technique is to examine the role of long range focussed collision sequences to sputtering. This experiment is an important potential application of the high voltage electron microscope which will now be considered. The first point is that long range focussed collision sequences are difficult to study in gold, at least for the 1 .O MeV electron microscope, since only 3-4 atomic layers can contribute to the sputtering yield. The situation is better for materials of lower atomic number where larger recoil energies are possible (eq. (2)) and permanent displacements may occur. Under these conditions focussed collision sequences of a hundred atoms of more in length may be generated [25]. However the sputtering yield increases with atomic number and we must consider the effect of this on the experimental measurements. Fig. 11 compares sputtering cross-sections for materials of differing atomic number 2 ranging from aluminium (2 = 13) to gold (2 = 79). Sputtering cross-sections are a sensitive function of binding energy as previously noted. For purposes of comparison, cross-sections have been derived from tables by Oen [17] for the sputtering of a single atom with an isotropic binding
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Al 0.5
E,(MeVl
1.0
Fig. 11. Hypothetical sputtering cross-sections for atoms whose binding energy to the lattice is assumed to be 4 eV. Results are taken from tables by Oen 1171. :
energy of 4 eV. This appears the most useful comparison as we require to detect sputtering from an individual sputtering event. The cross-sections decrease with decreasing atomic number for the same value of E,,,. For example, for Em,, = 8 eV (indicated by arrows) the decrease is a factor “3, from gold to aluminium, and is larger for larger Em,,. The decrease in sputtering rate from gold to aluminium may probably be accommodated using the existing experimental method. Specifically, sensitivity may be improved by optimising values of d and the halfwidth of the electron beam in fig. 4. The total electron dose may also,be increased (but not greatly - gold irradiations in fig. 5 required irradiation times between i and 2: hours [ 131). For microanalysis, although we might expect a slight improvement in X-ray yield over the range 2 = 78-13 [26], measurements by Ecker [27] suggest a nearly constant sensitivity in practice. The method can probably be improved substantially, however, by using energy dispersive rather than wavelength dispersive analysis. In principle this should enable X-ray count rates to be increased by several orders of magnitude. However the question of increased background is non-trivial [28,29] and further work is evidently required here. Theoretically, the contribution to sputtering from short range collision sequences in the spot patterns is probably important at all energies compared to the long range focussed collision contribution [ll]. Thus in order to resolve the focussed collision contribution we need detailed comparisons of experimental and theoretical yields as a function of recoil energy. This in turn requires a careful assessment of the interatomic potential for each material examined. Indeed the main limitation to the investigation would appear to lie with the accuracy of the interatomic potential. The potential is fortunately the only serious approximation in the theoretical simulations here. The Morse potential for gold in eq. (4) is appropriate for uniform compressions s-1 0% of the gold lattice. Although interatomic displacements in the collisions considered here do not generally exceed this value, elec-
354
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Fig. 12. Stacking fault tetrahkdra
in a quenched gold foil shrinking as a result of irradiation by 1.0 MeV electrons. The foil temperature was 140°C and the dose rate 7 X 1019 electrons -2 set-l (a) before irradiation (b) After 3 min irradiation and (c) after 6 min irradiation. Re foil orientation is close to [ 001].
tronic exchange interactions must act to reduce the local asymmetry in wavefunction and, therefore, the interaction energy during a collision. It is probable therefore that the Morse potential overestimates the ‘%lardness” of the interatomic potential. This may be seen in the calculated displacement energy Ed. From simulations following (110) recoils the Morse potential gives Ed above 45 eV. Resistivity mea~rements suggest .!?d - 36 eV [30]. Recent experiments by the author [31] to derive the threshold energy from the rate of growth of loops under irradiation suggest that much lower values of Ed are possible. An example is shown in fig. 12. This shows the shrinkage of vacancy stacking fault tetrahedra irradiated at 14O’C by 1 .O MeV electrons along a direction within a few degrees of [OOl ] . The inference is that 1 .O MeV electrons can create permanent damage in bulk gold leading to a net absorption of interstitials at tetrahedra which proceed to shrink. However the interpretation of these results is complicated by sub-threshold effects [32,33] and more work is required. A similar study already completed for copper has also revealed threshold energies much lower than previously accepted [34] _ It is, therefore, felt that substantial effort is required to obtain a satisfactory interatomic potential at low energies. A second important application of the HVEM is in the study of surface structure. Surface roughness on a macroscopic level is well known and has been explained as a consequence of the sputtering yield versus scattering angle relationship (see review by Carter, Colligon and Nobes [35]). The surface may be treated as a continuum. However, the results in fig. 10 indicate that the sputtered surface
D. Cherns /Sputtering in high-voltageEM
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of (I 11) gold is rough at the atomic level. The explanation of roughness at the atomic level is quite different from that at the macroscopic level requiring a surface diffusion model where the atomic structure of the surface is an essential feature. It is anticipated that diffusion-controlled surface roughness may, indeed, be a general microscopic feature of sputtering, and one usually unresolved by, for example, scanning microscopy methods. The question of microscopic surface rou~ness is particularly relevant to the application of various surface analysis techniques during sputtering. For instance, in low energy electron diffraction (LEED), degradation of diffraction patterns from a single crystal surface is expected when the surface step separation become less than the beam coherence width - several hundred A 136-383 _ We may, therefore, expect marked degradation to occur in gold sputtered at room temperature. degradation of LEED patterns during sputtering has already been noted (see review by FUread [39]). Another technique expected to be highly sensitive to surface roughness is ion-beam scattering [40]. Thus it is suggested that a general high resolution study of surface roughness generated by sputtering is desirable both from a theoretical viewpoint and from the ~ew~int of other surface analysis techniques. This can readily be done in the HVEM.
6. Summary The use and scope of the high voltage electron microscope for the study of sputtering has been explained with reference to work carried out on (111) gold. The two most important areas of future study would appear to be: (1) Evaluation of the role of long range focussed collision sequences in sputtering, for materials of medium atomic number. (2) Extension of high resolution studies of microscopic surface roughness caused by sputtering to other materials and crystal orientations.
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1101 R;S. Nelson and R. von Jan, Can. J. Phys. 46 (1968) 747. [ 111 C. Lehmann and P. Sigmund, Phys. Status Solidi 16 (1966) 507. [12] D.Cherns, F.J. Minter and R.S. Nelson, Nucl. Instr. Methods 132 (1976) 369. [ 131 D. Cherns, M.W. Finnis and M.D. Matthews, Phil. Mag. 35 (1977) 693. [14] D. Cherns, Phil. Mag. 36 (1977) 1429. [ 151 P.B. Hirsch, A. Howie, R.B. Nicholson, D.W. Pashley and M.J. Whelan, Electron Microscopy of Thin Crystals (Butterworths, London, 1965). [16] N.F. Mott, Proc. Roy. Sot. (London) Al24 (1929) 426; Al35 (1932) 429. [ 171 OS. Oen, ORNL 4897. [18] D. Cherns, A. Howie and M.H. Jacobs, 2. Naturforsch. 28a (1973) 56.5. [19] C.J. Cooke and I.K. Openshaw, in: Proc. 4th Natl. Conf. on Electron Microprobe Analysis, Pasadena, 1969 (Electron Probe Analysis Society of America) paper 64. (201 M.T. Robinson and I.M. Torrens, Phys. Rev. B9 (1974) 5008. 1211 D. Cherns, Phil. Mag. 30 (1974) 549. [22] R.L. Hines, Thin Solid Films 35 (1975) 229. [23] A.F. Moodie and C.E. Warble, Phil. Mag. 16 (1967) 891. [24] K. Kambe and G. Lehmpfuhl, Optik 42 (1975) 187. 1251 J.A. Venables and GJ. Thomas, in: Vacancies and Interstitials in Metals, Eds. A. Seeger et al. (North-Holland, Amsterdam 1970) p. 531. 1261 J.I. Goldstein, J.L. Costley, G.W. Lorimer and S.J.B. Reed, in: Scanning Electron Microscopy/1977, Vol. 1, Ed. 0. Johari (Univ. of Chicago Press) p. 315. [27] K.H. Ecker, J. Phys. D6 (1973) 2150. [28] J.D. Geller, Scanning Electron Microscopy/l977, Vol. 1, Ed. 0. Johari (Univ. of Chicago Press) p, 281. [29] W.A.P. Nicholson, B.W. Robertson and J.N. Chapman, in: Developments in Electron Microscopy and Analysis 1977, Ed. D.L. Misell (Inst. Physics, London) p. 373. [30] W. Bauer, A.I. Anderman and A. Sosin, Phys. Rev. 185 (1969) 870. [31] D. Cherns, to be published. 1321 W. Bauer and A. Sosin, J. Appl. Phys. 35 (1964) 703. [ 331 D. Cherns, F.J. Minter and R.S. Nelson, in: Proc. Intern. Conf. On Fundamental Aspects of Radiation Damage in Metals, Gatlinburg, Vol. II, Eds. M.T. Robinson and F.W. Young (1975) p. 178. [34] N. Yoshida and K. Urban, in: Proc. 5th Intern. Conf. on High Voltage Electron Microscopy, Kyoto, 1977. [35] G. Carter, J.S. Colligon and M. Nobes, Radiation Effects 31 (1977) 65. (361 J.E. Houston and R.L. Park, Surface Sci. 34 (1973) 477. [37] G.E. Laramore, J.E. Houston and R.L. Park, J. Vacuum Sci. Technol. 10 (1973) 196. [38] J.M. Cowley and H. Shuman, Surface Sci. 38 (1973) 53. [39] G.E. Rhead, Surface Sci. 68 (1977) 20. 1401 S.H.A. Begemann and A.L. Boers, Surface Sci. 30 (1972) 134.