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Implantation and ion beam mixing in thin film analysis
Nuclear Instruments and Methods 182/183 (1981) 15-24 North-Holland Publishing Company
IMPLANTATION AND ION BEAM MIXING IN THIN FILM ANALYSIS Peter WILLIAMS and Judith E. BAKER Materials Research Laboratory, University o f lllinois at Urbana-Champaign, Urbana, Illinois 61801, U.S.A. In thin film analyses obtained using sputtering techniques, primary ion implantation and ion beam mixing effects frequently produce significant alteration of the substrate before it can be sampled b)~ the sputtering front. The present state of qualitative and quantitative understanding of these effects is discussed, with particular reference to: ion yield matrix effects and interface transients in secondary ion mass spectrometry, reduction of overlayer sputter rates due to ion beam mixing, the role of ion beam mixing in preferential sputtering, and the use of ion beam mixing to allow quantitative analysis of interfacial layers. 1. Introduction
The combination of sputtering with surface-sensitive analysis techniques has, over the last decade, proved to be invaluable for thin film analysis. The principal surface analysis techniques used are Auger electron spectrometry (AES), secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS or ESCA) and low energ2¢ ion scattering (LEIS). The sputtering process in many practical materials is still not fully understood and analyses are subject to complications arising from sputtering "artifacts". A good early review of some of these problems was provided by Coburn [1]. It has long been apparrent that, because sputtering requires bombardment of a surface ~vith primary ions of appreciable energy (typically 1 - 2 0 keV) whose range in most solids exceeds the escape depth of the analytical signal (typically one to three monolayers), a zone of altered material (implanted, damaged)precedes the analytical zone through the sample. This article will present some recent experimental observations of the consequences of this sample alteration process. Most examples will be taken from SIMS measurements, in part because sputtered ion emission is exceptionally sensitive to surface chemical changes and also because primary ion energies are frequently higher than in the other techniques, due to the need to produce finely focussed ion beams. Thus the extent of the altered layer is greater, and the effects are most easily discerned.
o f oxygen ion sputtering as means of reproducibly introducing oxygen into a wide variety of materials [3]. The advantages of this appraoch are comparable to those of implantation, i.e. the initial sample chemistry is relatively unimportant, so that substances with low oxygen affinity or sticking coefficient e.g. GaAs can be analysed with high sensitivity. More recently, following the pioneering study of Krohn [4], cesium ion bombardment has been incorporated into ion microprobes with benefits similar to those conferred by the use of oxygen ion bombardment [5]. The use of reactive ion bombardment is responsible for the impressive sensitivity of state-of-the-art ion microprobe and ion microscope instrumentsdetection limits in the range 10El5 atom/cm 3 have been demonstrated in depth-profiling mode for many common dopants in semiconductors [6,7]. However, such performance is not bought without some sacrifice. Sputtered ion yields are found to vary strongly with the primary ion species surface concentration, [PJs, as: i ± cc [P]s',
where n is ~ 2 - 3 . A high, steady-state surface concentration of the primary ion species is not obtained immediately; instead [P]s builds up as the sputtering front moves into the initial surface over a distance of the order of the primary ion range Rp [8], and the steady-state level thus attained is inversely proportional to the substrate sputter yield [8,9] : [Pls = r/Sa.
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(approximately as the fifth power of S -1) and less strongly (approximately as the third power) with the surface cesium concentration, as measured by AES (fig. 2). The nonequivalent scaling with sputter yield and surface concentration was attributed to a systematic variation with substrate atomic mass of the preferential sputtering factor r in eq. (2). Such a variation had been observed by Liau et al. [12] and led them to propose a preferential sputtering model in which relative sputtering yields are proportional to escape depths for the different surface species. Escape depths are approximated in their model by low energy projected ranges. The deviations in surface cesium concentration from a simple S -~ dependence are qualitatively consistent with the model of Liau. The presence of steady-state matrix effects, although inconvenient, does not significantly hnpede quantitative SIMS analysis because the empirical use of standards, particularly ion-hnplanted standards, is routine [13]. More serious effects arise at interfaces, where standards are frequently impossible to make. These effects are of two types. The first, identified by Deline [14], is associated with sputter yield varia-
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[P]s with sputtered distance for three different sputter yields is simulated in fig. 1. The presence of a native surface oxide, together with the implantation effect, causes ion yields near the initi',d surface to vary strongly, precluding simple quantitative analysis. We will address the problem of quantitative surface layer analysis in section 4. As sputtering proceeds into the bulk, ion yields become stable, but are strongly dependent upon the substance sputter yield, and pronounced "matrix effects" occur. This problem was investigated by Deline et al. [10] and subsequently by Chelgren et al. [ 11 ]. They found that ion yields of Si- in a series of noble-metal silicides scaled very strongly with the substrate sputtering yield
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P. Williams, .I.E. Baker/Implantation and ion beam mixing
tions at interfaces. When a low sputter yield species (A) overlays a high sputter yield species (B), then, as the sputtering front approaches the interface, the concentration of the primary species in substrate B is initially determined by the sputter yield of A. When the sputtering front penetrates into B the initial ion yields reflect a primary species concentration transiently higher than the steady state level appropriate to substrate B and a transiently high ion signal will result. This simple model is modified somewhat because [P]s in A is governed not only by S a but also by the nuclear stopping power in B. However, the model qualitatively explains the significant ion yield transients ( 1 - 2 orders of magnitude above the steady-state level) observed by Storms for samples of Au, Sb, Te etc. (high sputter yield materials) when the sputtering front penetrates through an overlayer of carbon (low sputtering yield) [ 15]. The second type of transient is related to stopping power variations at interfaces. At steady-state, [P]s is independent of the sub-surface stopping power, depending only upon the sputter yield (the sputter yield is, of course, determined primarily by the stopping power and by surface binding energy effects). However, as the sputtering front approaches an interface with a substrate of higher mass, backscattering of P transiently increases, and [P]s and ion yields in the overlayer will increase, before the interface is reached. Conversely, if an interface to a lower mass substrate is approached, backscattering will decrease and [P]s and ion yields will show a down-
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17
ward transient (fig. 3). It should be stressed that this effect is a transient result of a change in stopping power, and if, for example, the sputtering yield of the underlying species were the same as that of the overlayer (and preferential sputtering effects were neglected), [P]s would recover to a steady-state level identical with that in the overlayer. In fig. 3 the "underlayer" is really an ion-beam-mixed "oxide" layer (containing as much C and H as O), and the reduced stopping power in this layer leads to a reduction in sputter yield and to an increase in [P]s (and in ion yields) when the sputtering front reaches the layer [ 16]. Understanding of these implantation-related matrix effects and transients is still at a qualitative level, in large part because of the lack of quantitative understanding of preferential sputtering. However, even a qualitative understanding is useful in that it provides guidelines for interpreting otherwise anomalous data and dispels concerns that the entire analysis may be suspect. In section 4 we will see how this understanding has been used to select situations where a judicious cancelling of artifacts can be used to facilitate quantitative analysis at interfaces.
3. Ion beam mixing effects in sputtering The possibility of deleterious effects of energetic primary ion bombardment on subsurface elemental distributions has been recognized for some time. A particularly careful early study was that done by McHugh [17], who investigated the effects of primary ion energy on the measured profile of a shallow phosphorus-rich layer (approximately a delta function) in a Ta20s film. McHugh found that increasing primary ion energy significantly degraded the sharpness of the measured profile and showed that, when the primary ion energy was such that a significant fraction of the incident ions had ranges in excess of the layer depth, the layer prof'lle was significantly skewed by recoil effects. Partly as a consequence of work such as this, the rule-of-thumb was established that depth resolution was limited by ion beam mixing effects to a depth on the order of the primary ion range. However, other results appearing in the literature were cause for concern, because they appeared to indicate that, in some instances, depth resolution could be much worse. The most striking result was that of Hart et al. [18] who found that when a copper layer, initially present on the surface I. CASCADES, RECOIL PHENOMENAAND RANGES
18
P. Williams, J.E. Baker / Implantation and ion beam mixing
of a silicon wafer, was bombarded by 20 keV Ne ~ to a dose sufficient to remove some 90 A of silicon, almost none of the copper was removed. Instead, the copper was found to be redistributed into the bulk over a depth corresponding roughly with the Ne range. The existence of efficient material transport processes during sputtering is now generally accepted following the work of Liau et al. [ 12]. Three different processes can be identified: cascade mixing, recoil mixing and radiation-enhanced diffusion. Cascade mixing is distinguished from the other radiationinduced transport processes in that it is not thermally activated (as distinct from radiation-enhanced diffusion) and it is isotropic (as distinct from recoil implantation). The relative efficiencies of these various transport processes will depend to some extent upon the material studied. However, experimental results of Liau et al. [19], and calculations by Anderson [20] and by Littmark and Hofer [21] suggest that cascade mixing is the dominant material transport process accompanying sputtering. We will examine here some of the consequences of this phenomenon.
3.1. Anomalous sputter yieMs during layer removal Fig. 4 shows typical SIMS depth profiles obtained when sputtering through an interfacial layer containing the impurities H, C, and F. The sample was made by evaporating ~1300 A of silicon onto a
silicon wafer which had been cleaned by various chemical processing steps terminating in an HF etch [22]. The characteristic skewed profile shape is observed for all samples of this type and is qualitatively consistent with the p r o n e predicted by the calculations of Littmark and Hofer [21]. The exponential decay of the profiles beyond the interface extends some 600 A into the sample, well beyond the projected range of the primary ions (~150A) or of recoils. It can be shown [23], by assuming that the exponential tail represents a dilute regime (so that ion yields are constant and the ion signals are proportional to concentration), that data such as that of fig. 4 can be analysed to extract partial sputter yields, S', for the impurity species. S' is defined for a species A such that the removal rate of A (atoms sputtered per primary ion) is S'a" [A], where [A] is the bulk concentration ofA. The results of such measurements for 20 keV Cs÷ bombardment, with a variety of species present either as interfacial impurities (H, C, O, F) or in overlayers (C, N, Ni, Pd, Pt, and Au) on silicon, are shown in fig. 5. The partial sputter yield of silicon under these conditions was determined to be (6 +- 1) atoms/ion, so that the results shown in fig. 5 appear to represent an extreme example of preferential sputtering. These results can be understood, however, by considering that after the sputtering front has passed through the interface, the remaining impurity atoms have been dispersed by cascade mixing over a depth of the order of the primary ion range (150 ?t or ~50 atom layers). Spat-
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P. Williams,J.E. Baker/hnplantation and ion beam mixing
tering, on the other hand, extracts atoms only from an escape depth of the order of 1 - 3 atom layers. Thus, only a few percent of the total impurity layer is accessible during the sputtering of any given Si layer. Liau et al. found that the creation of the mixed zone over a depth Rp occurs significantly faster than the sputtering front can advance over the same distance [19]. This should be true whenever the primary ion energy is such that the energy deposited into nuclear motion is a maximum at a depth beyond the sputtered atom escape depth, which is generally the case in analytical systems which use sputtering. Thus it can be seen that the effect of cascade mixing is to continuously push the mixed impurity layer into the substrate ahead of the sputtering front. For every layer of Si removed (together with the impurity atoms within it) the mixed layer is diluted by one additional layer of clean silicon. Obviously the mixed zone does not terminate sharply but the argument is identical in either case and we can define an effective mixing depth as that depth into which a surface monolayer would need to be uniformly mixed in order to produce the obselwed concentration at the surface. If, as the results of ref. 19 suggest, this effective depth is ~/¢p (~50 atom layers for the system studied here), the surface concentration of impurity species decreases at a rate which reflects an effective sputter yield just 1/50 that of silicon, or 0.12 (line 1 in fig. 5). The deviations of the data points from the calculation reflect either preferential sputtering effects or different effective mixing depths. Line 2 in fig. 5 shows the consequence of including mass-dependent preferential sputtering in the model, using the model of Liau [12], and estimating low energy projected ranges using the low energy extrapolations of Schi~btt [24]. Clearly the correlation with the data is much improved, but this result should be viewed with caution. The concept of escape depth may not have much meaning and may not be related to a projected range, if the majority of sputtered atoms originate in the surface layer. It would seem inarguable that a mass-related preferential sputtering effect must occur, simply because light atoms can backscatter from heavier atoms and thus have a larger number of trajectories which can result in escape from the surface; it remains to be seen whether the model of Liau estimates these differences correctly. In addition, at least some of the results of fig. 5 must reflect different effective mixing depths. Blank and Wittmaack [25] have shown that gold, which is known to diffuse
19
readily in silicon, can be mobilised at depths several times the projected range of the primary species (20 keV and 100 keV Ar ÷ were used to sputter gold overlayers on silicon). Effective mixing depths in excess of Rp due perhaps to thermally activated radiation (or defect) enhanced diffusion at room temperature may well account for the extremely low removal rates ofNi, Pd, Au and Pt. The analytical consequences of the reduced removal rates discussed here are potentially severe, particularly if the effect is not recognized. In a multielement depth profile through an interface, such as is routinely obtained in SIMS or AES instruments, there is a natural tendency to assume that the elemental profile which falls most steeply at the interface reflects the instrumental depth resolution (or at least gives an upper limit to this quantity). Profiles which fall less sharply may then be taken to be diffused. The fact that depth resolution, when limited by cascade mixing effects, can vary for different elements makes this assumption untenable. Exponentially decaying profiles should be taken as indicative of cascade mixing, in the absence of evidence to the contrary. The effects of cascade mixing are most severe in SIMS analyses, particularly in those instruments where high primary ion and secondary ion accelerating potentials are used to increase sensitivity and where the analyst is frequently looking for species present at very low concentrations immediately beyond an interface. 3.2. Ion beam mixing and preferential sputtering
We have referred already to the work of Liau et al. [12] in which surface composition changes resulting from preferential sputtering were extended into the bulk, through cascade mixing, to a depth sufficient to make the effects readily detectable using a technique with limited depth resolution and surface sensitivity (Rutherford backscattering). Many more studies exist in which preferential sputtering has been probed with more surface sensitive techniques, principally AES. The literature has been reviewed by Kelly [26] to determine whether chemical bonding effects or mass effects are the factors controlling preferential sputtering. Kelly f'mds that the literature data is generally consistent with the idea that chemical bonding effects are important, in that it is generally the alloy component with the weaker binding (in the pure element) which is preferentially sputtered, although he concludes that the effects calculated by incorporating I. CASCADES, RECOIL PIIENOMENAAND RANGES
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differing bond energies into a variant of Sigmund's sputtering model [27] are somewhat too small to account for the observed magnitude of preferential sputtering effects. It is the purpose of this section to point out that cascade mixing can be significant in preferential sputtering in two ways, first by complicating the interpretation of experimental results, and second by providing a mechanism - cascadedriven segregation - which could result in a correlation with bond energies. We consider, for simplicity, the case of a binary alloy AB. Partial sputter yields for the two components will be S'a and S~. Preferential sputtering occurs if S'a ~ S~. If, for example, S'a > S~ then the surface composition will change to a new composition A'B' determined by the requirement that the sputtered flux composition be identical to that of the bulk, i.e.: S~" [A] ' = S ~ " [B]'
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where a probable composition dependence of the partial sputter yields is indicated by the use of S" and [A]' < [A]. In almost all experiments, the effect of preferential sputtering is determined, not by directly measuring S', but by monitoring changes in [A] and [B]. However, any process which alters the surface composition can lead to effects which are difficult to distinguish from preferential sputtering. Two possible cases are considered in fig. 6. Consider first an experiment with a sample AB in which the surface preparation has involved an annealing step. In general alloys exhibit segregation. For example, a N i 5% Cu alloy has been shown to have a surface layer composition ~5~% Cu [28,29]. When sputtering begins, the combined effects of recoil and cascade mixing will act to reverse the segregation of, say, A leading to the conclusion that A is preferentially sputtered (fig. 6a). Alternatively, consider that the sample
is prepared with no surface segregation (perhaps by co-evaporation onto a cooled substrate). When sputtering begins, the near-surface atoms are mobilized by the cascade mixing process and there must be some tendency for segregation to occur (fig. 6b). The final surface layer composition will result from a balance between the extent to wlfich segregation can occur during the short lifetime of the cascade, and the tendency of recoil implantation to oppose segregation. However, it seems difficult to argue that, at steadystate, the surface layer composition of a segregationprone alloy should be identical to that of the bulk, even in the absence of "true" preferential sputtering effects (differences in S'), and it therefore seems probable that cascade-driven segregation and de-segregation processes should accompany sputtering and result in surface composition changes similar to those produced by preferential sputtering. If the compositional change results from segregation alone, the equality of S'a and S~ and conservation of matter ensure that the surface layer composition is identical to that of the bulk, and it is the sub-surface composition which is altered, as suggested in fig. 6c. Thus the sampling depth of the analytical technique used critically determines the result obtained. Because segregation can be correlated with binding energies (the component with the weaker bulk bonds tending to segregate to the surface [30]), an experiment such as that shown in fig. 6b would lead to the conclusion that preferential sputtering of the more weakly bonded component occurred. To distinguish between the two possibilities, the most straightforward experimental approach would be that of Liau et al. [12]. As indicated in fig. 6c, the in-depth elemental distribution in either case should be the same at steady state, with the preferentially sputtered element being depleted, relative to the bulk, over a
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depth comparable to the primary ion range. It is obviously desirable to establish the profile by high energy sputtering and to analyse this composition profile with a non-sputtering technique, or if sputtering is used, to analyse using a low energy sputtering beam. In a recent experiment, Williams and Baker have investigated the effects of segregation in the sputtering cascade by using adsorption of a reactive gas to change the driving force for segregation in the same two-component system [31]. The experiment was performed in a SIMS system, and the sample geometry was similar to that of fig. 3. It consisted of a thin evaporated layer of silver buried beneath ~1000 A of evaporated silicon on a silicon substrate. Depth proFries of the silver were obtained, and, as discussed in section 2, the slope of the exponential decay of the silver signal beyond the interface was used as a measure of the relative partial sputtering yields of silver and silicon. The absolute value of the silver yield is of little significance in the present context, reflecting as it does the combined effects of cascade mixing and preferential sputtering. The significant finding was that the relative partial sputter yield of Ar-sputtered silver could be reduced by a factor of five by continuously flooding the sample surface with oxygen during sputtering as shown in fig. 7. Due to the much greater affinity of silicon for oxygen, strong segrega-
21
tion of silicon to the sample surface is presumed to have occurred [30], and the diminished surface concentration of silver resulted in the observed reduction in the removal rate of the silver. The experiment described above represents perhaps an extreme case of segregation, in which the surface binding energy differences may be greater than occur in the majority of alloys. However, the experiment is felt to be significant in that it demonstrates that chemical effects can play a role in cascade mixing; the possibility that apparent preferential sputtering in other systems may, at least in part, result from cascade-driven segregation should seriously be considered in theoretical discussion. In addition, the result provides a serious warning that depth proffries can be significantly altered by an injudicious choice of analytical conditions. The use of oxygen flooding to enhance sputtered ion yields under argon ion bombardment is seen to be contraindicated, and the use of reactive ion bombardment to enhance depth resolution in AES [32] should be approached with caution in systems where the ion gun is not differentiaUy pumped. Oxygen ion bombardment produces far less distortion of the depth profile [31] presumably because the near surface distribution of oxygen is an error function and therefore relatively flat. It should be noted that Kelly [26] considered the possibility of segregation in sputtering, noting that diffusion processes associated with a flux of bombardment-induced point defects [33] can lead to segregation. The analytical difficulty of demonstrating the effect has not been discussed previously. Webb et al. considered the case where diffusion rates for two components in the cascade differ but did not consider the possibi]ity of surface segregation effects in such a system [34]. It is worth noting that segregation effects may account for effects other than preferential sputtering. In an early review, Andersen [35] reported an observation that the presence of a few percent of aluminum in gold could reduce the sputtering yield of gold by an order of magnitude. Such an effect would, as he pointed out, be of great concern in all sputterprofiling applications because it would totally negate the usual assumption that the sputtering rates remain constant in the absence of a major change in matrix composition. (Such an assumption is necessary in order to be able to establish depth scales by postanalysis measurement of sputtered crater depths). Because no obvious mechanism for such a gross effect is apparent, Williams used thin film gold samples, ionI. CASCADES, RECOIL PHENOMENA AND RANGES
22
P. Williams, J.E. Baker/Implantation and ion beam mixing
implanted with A1 to a peak concentration of ~3%, to detemline the effect of A1 on the gold sputter yield [36]. By measuring the time required to sputter through the gold f'tlm to the silicon substrate it was determined that the sputter yield of gold was influenced by no more than ~5% by a 3% peak concentration of implanted aluminum. Andersen's observation originated in a transmission sputtering experiment on gold, in which a thin film of A1203 was used on the upstream side of the sample to suppress backsputtering of gold. Aluminum from this fdm was observed to be recoiled into the gold film, causing the downstream side of the sample to appear silver [37]. It seems reasonable to suppose that this recoiled aluminum would segregate to the surface of the gold firm forming a thin oxide with the oxygen and water present in most accelerator vacuum systems. A thin surface layer of aluminum (oxide) would indeed be expected to significantly reduce the gold sputtering yield and could account for Andersen's observations.
4. Quantitative analysis of surface and interfacial layers using cascade mixing As pointed out in section 2, the effects of primary ion implantation and ion beam mixing upon SIMS analyses are generally cause for concern only near interfaces, but in such cases they can be disastrous. Because the relationship between ion yields and sample corriposition is poorly understood [38], quantitative analysis relies on the use of standards, and it is almost impossible to prepare well-characterized and stable standard surfaces and interfaces. Such a limitation is distressing because the high sensitivities achieved by SLMS for dopants and impurities in bulk and thin film samples extend also to interfacial impurities, and chararcterization of interfaces is of increasing importance. The realization that cascade mixing was a general consequence of sputtering offered a means to remedy this situation. The calculations of Littmark and Hofer [21] clearly show that an initially sharp subsurface layer is appreciably mixed into the surrounding, initially clean, material before being sampled by the advancing sputtering front. If, once again, the effective extent of the mixed zone is taken to be ~Rp, and Rp is chosen to be ~50 atom layers, then an interfacial layer containing one monolayer of impurities (near the limits of detection for nuclear reaction analysis or RBS for light impurities) would be diluted, by mixing, to an
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Fig. 8. Depth proffie of a silicon sample containing a "native oxide" interfacial layer ~ one monolayer thick. Primary species: Cs÷ at 20 keV impact energy. effective concentration ~2%. This is low enough for quantitative analysis by SIMS, because such a concentration would be expected to perturb the sputtering yield, and therefore the primary species concentration by no more than about 2%, depending principally oll the relative masses of the impurity and matrix species. Ion yields in the interface region should therefore vary by no more than about 5%. Such a situation is quite conducive to quantitative analysis. The samples used by Williams and Baker [16] to test this idea came from a study in which the effects of different cleaning procedures on interfacial oxide thickness in silicon had been evaluated [22]. The oxide thicknesses had been determined using nuclear reaction analysis (NRA) techniques and were thought to range from one to several monolayers. After cleaning, the samples were immediately inserted into an evaporation system where ~ 1 0 0 0 A of Si was deposited on top on the "native oxide" layer by electron-beam evaporation. The cleanest sample gave a depth profile shown in fig. 8. The matrix Si-signal varied by no more than ~5% through the interface region, indicating that the interracial oxygen could be analysed quantitatively. This was clone by comparing the integrated, background-subtracted O- signal (the background arose from oxygen adsorption from the residual vacuum in the ion microprobe) with the integrated signal from a silicon sample containing an oxygen ion implant of accurately known dose. The surprising, and at first dismaying, result was that this sample contained less than one monolayer of oxygen in the native oxide ([0] = 1.9×1014 atom/cm2). However, this result was confirmed when NRA measurements, at the limit of sensitivity tbr the technique, found [0] ~ ( 2 - 3 ) × 1014 atom/cm 2. Analysis
P. Williams, J.E. Baker / Implantation and ion beam mixing
for the other expected impurities at the interface gave [C] = 1 . 3 X 1 0 is atom/era 2 and [It] = 2.3 X1014 atom/era 2. Evidently, thc presence o f carbon protccted the surface from oxidation during the short exposure to the atmosphere prior to loading in the evaporator. This analytical technique, which was termed by the authors "cascade dilution" extends the quantitative analytical capabilities of SIMS to an extremely important new area. In addition to buried interfaces, surface layers could be quantitatively analysed, if facilities are available to bury them in situ beneath a layer o f material (either the substrate material or another species ~vith the same sputter yield as the substrate) of sufficient thickness that a steady-state level of the primary species is attained before the interface is sampled. The SIMS measurements ha this study gave results which were consistently ~ 3 0 % lower than the NRA measurements. It is not yet clear whether this reflects systematic errors in the SIMS or NRA measurements or in both. Because of the uncertainties associated with cross-section determinations for NRA measurements, samples have been prepared which will allow quantitative comparison with Rutherford backscattering measurements. The statistical uncertainty in the SIMS measurement can be <2% (standard deviation of the mean o f 13 repeat runs was 1.7%) and there is therefore reason to hope that the cascade dilution technique will become a valuable multielement quantitative analytical technique which may even be capable of characterizing standards for NRA cross-section measurements.
5. Conclusion It cannot be stressed too strongly that sputtering is n o t simply a technique for peeling away successive layers of the sample, thereby allowing in-depth analysis. Instead, sputtering inevitably alters the subsurface layers of the sample, in most cases before the sputtering front reaches these layers. Thus, the analyst who wishes to know the composition or structure o f a sample unaffected b y the sputtering process has the choice o f finding another technique, or o f attempting to understand, in detail, the mechanisms by which sputtering alters a solid in order to be able, qualitatively or quantitatively, to compensate for or eliminate these effects. Because the high sensitivity of secondary ion mass spectrometry and the hiNl lateral resolution of Auger electron spectrometry are essen-
23
tial in many areas of materials science, resort to nonsputtering techniques is not an option. This article has indicated some areas in which a (semi-)quantitative understanding of sputtering and ion yield "artifacts" is beginning to emerge. This understanding is allowing sputtering techniques, and particularly SIMS, to be applied with some degree of confidence to problems of analysis at interfaces where hitherto their use has been suspect. We thank D.A. Reed for critical comments on the manuscript. Work at the University o f Illinois was supported by the National Science Foundation under the MIlL Grant DMR-77-23999 and, in part, by the Office o f Nav',d Research (L.R. Cooper).
References [ 1 ] J.W. Cobum, J. Vac. Sci. Tech. 13 (1974) 1037. [2] G. Slodzian and J-F. Ilennequin, Compt. Rend. (Paris) 263 (1966) 1264. [3] C.A. Anderson, Int. J. Mass Speclr. Ion Plays. 3 (1970) 413. [4] V. Krohn, J. Appl. Phys. 33 (1962) 3523. [5] P. Williams, R.K. Lewis, C.A. Evans and P.R. ttanley, An~. Chem. 49 (1977) 1399. [6] P. Williams and C.A. Evans, Appl. Phys. Lett. 30 (1977) 559. [7] J-M. Gourgout. Proc. 2nd Int. Conf. on Secondary ion mass spectrometry (ed. A. Benninghoven; Springer, NY, 1979) p. 286. [8] J.M. Morabito, R.K. Lewis and J.C.C. Tsai, Appl. Phys. Lett. 23 (1973) 260. [9] 1.'.Schulz and K. Wittmaack, Rad. Eff. 29 (1976) 31. [ 10] V.R. Define, W. Katz, C.A. Evans and P. Williams, Appl. Plays. Lett. 33 (1978) 832. [11] J.E. Chelgren, W. Katz, V.R. Deline, C.A. Evans, R.J. Blattner and P. Williams, J. Vacuum Sci. Technol. 16 (1979) 324. [12] Z.I.. Liau, J.W. Mayer, W.L. Brown and J.M. Poate, J. Appl. Phys. 49 (1978) 5295. [13] P. Williams, IEEE Trans. Nucl. Sci. NS-26 (1979) 1807. [14] V.R. Deline, Thesis (University of Illinois, 1978) unpublished. [15] H. Storms, K.F. Brown and J.D. Stein, Anal. Chem. 40 (1977) 2023. [16] P. Williams and J.E. Baker, Appl. Phys. Lett. 36 (1980) 842. []7] J. McHugh, Rad. Eft. 21 (1974) 209. [18] R.R. Hare H.L. Dunlap and O.J. Marsh, J. Appl. Phys. 46 (1975) 1947. [ 19] Z.L. Liau, B.Y. Tsaur and J.W. Mayer, J. Vac. Sci. Tech. 16 (1979) 121. [201 H.H. Andersen, Appl. Phys. 18 (1979) 131. [21] U. Littmark and W.O. llofer, Nucl. Instr. and Meth. 168 (1980) 329. I. CASCADES, RECOIL PIIENOMENA AND RANGES
24
P. Williams, J.E. Baker /Implantation attd ion beam mixing
[22] M. von Allmen, S.S. Lau, D.M. Scott, J.W. Mayer, W.F. Tseng, T.T. Sheng, P. Williams and J.E. Baker, in Thin film interfaces and interactions (eds. J.E.E. Baglin and J.M. Poate; The Electrochemical Society, Princeton, t980) p. 195. [23] P. WiUiams, Appl. Phys. Lett. 36 (1980) 758. [24] II.E. Schi~btt, in Ion implantation (eds. F. Eisen and L.T. Chatterton; Gordon and Breach, London, 1971) p. 197. [25] P. Blank and K. Wittmaack, Rad. Eft. Lett. 43 (1979) 105. [26] R. Kelly, Surf. Sci., in press. [27] P. Sigmund, Phys. Rev. 184 (1969) 383. [28] Y.S. Ng, T.T. Tsong and S.B. McLane, Phys. Rev. Lett. 42 (1979) 588. [29] H.H. Brongersma, M.J. Sparnaay and T.M. Buck, Surf. Sci. 71 (1978) 657.
[30] F.L. Williams and D. Nason, Surf. Sci. 45 (1974) 377. [31] P. Williams and J.E. Baker, unpublished. [32] R.J. Blattner, S. Nadel, C.A. Evans, A.J. Braundmeier and C.W. Magee, Surf. Interface Anal. 1 (1979) 32. [33] R.C. Piller and A.D. Marwick, J. Nucl. Mat. 71 (1978) 309. [34] R. Webb, G. Carter and R. Collins, Rad. Eft. 39 (1978) 129. [35] H.H. Andersen, in Physics of ionized gases (ed. V. Vujnovic; Institute of Physics of the University of Zagreb, 1974) p. 361. [36] P. Williams, unpublished. [37] H.H. Andersen, University of Aarhus, private communication. [38] P. Williams, Surf. Sci. 90 (1979) 588.
IMPLANTATION AND ION BEAM MIXING IN THIN FILM ANALYSIS Peter WILLIAMS and Judith E. BAKER Materials Research Laboratory, University o f lllinois at Urbana-Champaign, Urbana, Illinois 61801, U.S.A. In thin film analyses obtained using sputtering techniques, primary ion implantation and ion beam mixing effects frequently produce significant alteration of the substrate before it can be sampled b)~ the sputtering front. The present state of qualitative and quantitative understanding of these effects is discussed, with particular reference to: ion yield matrix effects and interface transients in secondary ion mass spectrometry, reduction of overlayer sputter rates due to ion beam mixing, the role of ion beam mixing in preferential sputtering, and the use of ion beam mixing to allow quantitative analysis of interfacial layers. 1. Introduction
The combination of sputtering with surface-sensitive analysis techniques has, over the last decade, proved to be invaluable for thin film analysis. The principal surface analysis techniques used are Auger electron spectrometry (AES), secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS or ESCA) and low energ2¢ ion scattering (LEIS). The sputtering process in many practical materials is still not fully understood and analyses are subject to complications arising from sputtering "artifacts". A good early review of some of these problems was provided by Coburn [1]. It has long been apparrent that, because sputtering requires bombardment of a surface ~vith primary ions of appreciable energy (typically 1 - 2 0 keV) whose range in most solids exceeds the escape depth of the analytical signal (typically one to three monolayers), a zone of altered material (implanted, damaged)precedes the analytical zone through the sample. This article will present some recent experimental observations of the consequences of this sample alteration process. Most examples will be taken from SIMS measurements, in part because sputtered ion emission is exceptionally sensitive to surface chemical changes and also because primary ion energies are frequently higher than in the other techniques, due to the need to produce finely focussed ion beams. Thus the extent of the altered layer is greater, and the effects are most easily discerned.
o f oxygen ion sputtering as means of reproducibly introducing oxygen into a wide variety of materials [3]. The advantages of this appraoch are comparable to those of implantation, i.e. the initial sample chemistry is relatively unimportant, so that substances with low oxygen affinity or sticking coefficient e.g. GaAs can be analysed with high sensitivity. More recently, following the pioneering study of Krohn [4], cesium ion bombardment has been incorporated into ion microprobes with benefits similar to those conferred by the use of oxygen ion bombardment [5]. The use of reactive ion bombardment is responsible for the impressive sensitivity of state-of-the-art ion microprobe and ion microscope instrumentsdetection limits in the range 10El5 atom/cm 3 have been demonstrated in depth-profiling mode for many common dopants in semiconductors [6,7]. However, such performance is not bought without some sacrifice. Sputtered ion yields are found to vary strongly with the primary ion species surface concentration, [PJs, as: i ± cc [P]s',
where n is ~ 2 - 3 . A high, steady-state surface concentration of the primary ion species is not obtained immediately; instead [P]s builds up as the sputtering front moves into the initial surface over a distance of the order of the primary ion range Rp [8], and the steady-state level thus attained is inversely proportional to the substrate sputter yield [8,9] : [Pls = r/Sa.
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16
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(approximately as the fifth power of S -1) and less strongly (approximately as the third power) with the surface cesium concentration, as measured by AES (fig. 2). The nonequivalent scaling with sputter yield and surface concentration was attributed to a systematic variation with substrate atomic mass of the preferential sputtering factor r in eq. (2). Such a variation had been observed by Liau et al. [12] and led them to propose a preferential sputtering model in which relative sputtering yields are proportional to escape depths for the different surface species. Escape depths are approximated in their model by low energy projected ranges. The deviations in surface cesium concentration from a simple S -~ dependence are qualitatively consistent with the model of Liau. The presence of steady-state matrix effects, although inconvenient, does not significantly hnpede quantitative SIMS analysis because the empirical use of standards, particularly ion-hnplanted standards, is routine [13]. More serious effects arise at interfaces, where standards are frequently impossible to make. These effects are of two types. The first, identified by Deline [14], is associated with sputter yield varia-
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[P]s with sputtered distance for three different sputter yields is simulated in fig. 1. The presence of a native surface oxide, together with the implantation effect, causes ion yields near the initi',d surface to vary strongly, precluding simple quantitative analysis. We will address the problem of quantitative surface layer analysis in section 4. As sputtering proceeds into the bulk, ion yields become stable, but are strongly dependent upon the substance sputter yield, and pronounced "matrix effects" occur. This problem was investigated by Deline et al. [10] and subsequently by Chelgren et al. [ 11 ]. They found that ion yields of Si- in a series of noble-metal silicides scaled very strongly with the substrate sputtering yield
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P. Williams, .I.E. Baker/Implantation and ion beam mixing
tions at interfaces. When a low sputter yield species (A) overlays a high sputter yield species (B), then, as the sputtering front approaches the interface, the concentration of the primary species in substrate B is initially determined by the sputter yield of A. When the sputtering front penetrates into B the initial ion yields reflect a primary species concentration transiently higher than the steady state level appropriate to substrate B and a transiently high ion signal will result. This simple model is modified somewhat because [P]s in A is governed not only by S a but also by the nuclear stopping power in B. However, the model qualitatively explains the significant ion yield transients ( 1 - 2 orders of magnitude above the steady-state level) observed by Storms for samples of Au, Sb, Te etc. (high sputter yield materials) when the sputtering front penetrates through an overlayer of carbon (low sputtering yield) [ 15]. The second type of transient is related to stopping power variations at interfaces. At steady-state, [P]s is independent of the sub-surface stopping power, depending only upon the sputter yield (the sputter yield is, of course, determined primarily by the stopping power and by surface binding energy effects). However, as the sputtering front approaches an interface with a substrate of higher mass, backscattering of P transiently increases, and [P]s and ion yields in the overlayer will increase, before the interface is reached. Conversely, if an interface to a lower mass substrate is approached, backscattering will decrease and [P]s and ion yields will show a down-
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Fig. 3. Transients in matrix ion yields due to transient changes in primary species concentration near interfaces. The interfacial layer is ~7 monolayers of contamination (O, C, and H) beneath 1000 A of evaporated Si on a Si substrate. The position of the interface is indicated by the oxygen proFile. Primary species: Cs÷ at 20 keV impact energy.
17
ward transient (fig. 3). It should be stressed that this effect is a transient result of a change in stopping power, and if, for example, the sputtering yield of the underlying species were the same as that of the overlayer (and preferential sputtering effects were neglected), [P]s would recover to a steady-state level identical with that in the overlayer. In fig. 3 the "underlayer" is really an ion-beam-mixed "oxide" layer (containing as much C and H as O), and the reduced stopping power in this layer leads to a reduction in sputter yield and to an increase in [P]s (and in ion yields) when the sputtering front reaches the layer [ 16]. Understanding of these implantation-related matrix effects and transients is still at a qualitative level, in large part because of the lack of quantitative understanding of preferential sputtering. However, even a qualitative understanding is useful in that it provides guidelines for interpreting otherwise anomalous data and dispels concerns that the entire analysis may be suspect. In section 4 we will see how this understanding has been used to select situations where a judicious cancelling of artifacts can be used to facilitate quantitative analysis at interfaces.
3. Ion beam mixing effects in sputtering The possibility of deleterious effects of energetic primary ion bombardment on subsurface elemental distributions has been recognized for some time. A particularly careful early study was that done by McHugh [17], who investigated the effects of primary ion energy on the measured profile of a shallow phosphorus-rich layer (approximately a delta function) in a Ta20s film. McHugh found that increasing primary ion energy significantly degraded the sharpness of the measured profile and showed that, when the primary ion energy was such that a significant fraction of the incident ions had ranges in excess of the layer depth, the layer prof'lle was significantly skewed by recoil effects. Partly as a consequence of work such as this, the rule-of-thumb was established that depth resolution was limited by ion beam mixing effects to a depth on the order of the primary ion range. However, other results appearing in the literature were cause for concern, because they appeared to indicate that, in some instances, depth resolution could be much worse. The most striking result was that of Hart et al. [18] who found that when a copper layer, initially present on the surface I. CASCADES, RECOIL PHENOMENAAND RANGES
18
P. Williams, J.E. Baker / Implantation and ion beam mixing
of a silicon wafer, was bombarded by 20 keV Ne ~ to a dose sufficient to remove some 90 A of silicon, almost none of the copper was removed. Instead, the copper was found to be redistributed into the bulk over a depth corresponding roughly with the Ne range. The existence of efficient material transport processes during sputtering is now generally accepted following the work of Liau et al. [ 12]. Three different processes can be identified: cascade mixing, recoil mixing and radiation-enhanced diffusion. Cascade mixing is distinguished from the other radiationinduced transport processes in that it is not thermally activated (as distinct from radiation-enhanced diffusion) and it is isotropic (as distinct from recoil implantation). The relative efficiencies of these various transport processes will depend to some extent upon the material studied. However, experimental results of Liau et al. [19], and calculations by Anderson [20] and by Littmark and Hofer [21] suggest that cascade mixing is the dominant material transport process accompanying sputtering. We will examine here some of the consequences of this phenomenon.
3.1. Anomalous sputter yieMs during layer removal Fig. 4 shows typical SIMS depth profiles obtained when sputtering through an interfacial layer containing the impurities H, C, and F. The sample was made by evaporating ~1300 A of silicon onto a
silicon wafer which had been cleaned by various chemical processing steps terminating in an HF etch [22]. The characteristic skewed profile shape is observed for all samples of this type and is qualitatively consistent with the p r o n e predicted by the calculations of Littmark and Hofer [21]. The exponential decay of the profiles beyond the interface extends some 600 A into the sample, well beyond the projected range of the primary ions (~150A) or of recoils. It can be shown [23], by assuming that the exponential tail represents a dilute regime (so that ion yields are constant and the ion signals are proportional to concentration), that data such as that of fig. 4 can be analysed to extract partial sputter yields, S', for the impurity species. S' is defined for a species A such that the removal rate of A (atoms sputtered per primary ion) is S'a" [A], where [A] is the bulk concentration ofA. The results of such measurements for 20 keV Cs÷ bombardment, with a variety of species present either as interfacial impurities (H, C, O, F) or in overlayers (C, N, Ni, Pd, Pt, and Au) on silicon, are shown in fig. 5. The partial sputter yield of silicon under these conditions was determined to be (6 +- 1) atoms/ion, so that the results shown in fig. 5 appear to represent an extreme example of preferential sputtering. These results can be understood, however, by considering that after the sputtering front has passed through the interface, the remaining impurity atoms have been dispersed by cascade mixing over a depth of the order of the primary ion range (150 ?t or ~50 atom layers). Spat-
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P. Williams,J.E. Baker/hnplantation and ion beam mixing
tering, on the other hand, extracts atoms only from an escape depth of the order of 1 - 3 atom layers. Thus, only a few percent of the total impurity layer is accessible during the sputtering of any given Si layer. Liau et al. found that the creation of the mixed zone over a depth Rp occurs significantly faster than the sputtering front can advance over the same distance [19]. This should be true whenever the primary ion energy is such that the energy deposited into nuclear motion is a maximum at a depth beyond the sputtered atom escape depth, which is generally the case in analytical systems which use sputtering. Thus it can be seen that the effect of cascade mixing is to continuously push the mixed impurity layer into the substrate ahead of the sputtering front. For every layer of Si removed (together with the impurity atoms within it) the mixed layer is diluted by one additional layer of clean silicon. Obviously the mixed zone does not terminate sharply but the argument is identical in either case and we can define an effective mixing depth as that depth into which a surface monolayer would need to be uniformly mixed in order to produce the obselwed concentration at the surface. If, as the results of ref. 19 suggest, this effective depth is ~/¢p (~50 atom layers for the system studied here), the surface concentration of impurity species decreases at a rate which reflects an effective sputter yield just 1/50 that of silicon, or 0.12 (line 1 in fig. 5). The deviations of the data points from the calculation reflect either preferential sputtering effects or different effective mixing depths. Line 2 in fig. 5 shows the consequence of including mass-dependent preferential sputtering in the model, using the model of Liau [12], and estimating low energy projected ranges using the low energy extrapolations of Schi~btt [24]. Clearly the correlation with the data is much improved, but this result should be viewed with caution. The concept of escape depth may not have much meaning and may not be related to a projected range, if the majority of sputtered atoms originate in the surface layer. It would seem inarguable that a mass-related preferential sputtering effect must occur, simply because light atoms can backscatter from heavier atoms and thus have a larger number of trajectories which can result in escape from the surface; it remains to be seen whether the model of Liau estimates these differences correctly. In addition, at least some of the results of fig. 5 must reflect different effective mixing depths. Blank and Wittmaack [25] have shown that gold, which is known to diffuse
19
readily in silicon, can be mobilised at depths several times the projected range of the primary species (20 keV and 100 keV Ar ÷ were used to sputter gold overlayers on silicon). Effective mixing depths in excess of Rp due perhaps to thermally activated radiation (or defect) enhanced diffusion at room temperature may well account for the extremely low removal rates ofNi, Pd, Au and Pt. The analytical consequences of the reduced removal rates discussed here are potentially severe, particularly if the effect is not recognized. In a multielement depth profile through an interface, such as is routinely obtained in SIMS or AES instruments, there is a natural tendency to assume that the elemental profile which falls most steeply at the interface reflects the instrumental depth resolution (or at least gives an upper limit to this quantity). Profiles which fall less sharply may then be taken to be diffused. The fact that depth resolution, when limited by cascade mixing effects, can vary for different elements makes this assumption untenable. Exponentially decaying profiles should be taken as indicative of cascade mixing, in the absence of evidence to the contrary. The effects of cascade mixing are most severe in SIMS analyses, particularly in those instruments where high primary ion and secondary ion accelerating potentials are used to increase sensitivity and where the analyst is frequently looking for species present at very low concentrations immediately beyond an interface. 3.2. Ion beam mixing and preferential sputtering
We have referred already to the work of Liau et al. [12] in which surface composition changes resulting from preferential sputtering were extended into the bulk, through cascade mixing, to a depth sufficient to make the effects readily detectable using a technique with limited depth resolution and surface sensitivity (Rutherford backscattering). Many more studies exist in which preferential sputtering has been probed with more surface sensitive techniques, principally AES. The literature has been reviewed by Kelly [26] to determine whether chemical bonding effects or mass effects are the factors controlling preferential sputtering. Kelly f'mds that the literature data is generally consistent with the idea that chemical bonding effects are important, in that it is generally the alloy component with the weaker binding (in the pure element) which is preferentially sputtered, although he concludes that the effects calculated by incorporating I. CASCADES, RECOIL PIIENOMENAAND RANGES
P. Williams, J.E. Baker /Implantation and ion beam mixing
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differing bond energies into a variant of Sigmund's sputtering model [27] are somewhat too small to account for the observed magnitude of preferential sputtering effects. It is the purpose of this section to point out that cascade mixing can be significant in preferential sputtering in two ways, first by complicating the interpretation of experimental results, and second by providing a mechanism - cascadedriven segregation - which could result in a correlation with bond energies. We consider, for simplicity, the case of a binary alloy AB. Partial sputter yields for the two components will be S'a and S~. Preferential sputtering occurs if S'a ~ S~. If, for example, S'a > S~ then the surface composition will change to a new composition A'B' determined by the requirement that the sputtered flux composition be identical to that of the bulk, i.e.: S~" [A] ' = S ~ " [B]'
(4)
where a probable composition dependence of the partial sputter yields is indicated by the use of S" and [A]' < [A]. In almost all experiments, the effect of preferential sputtering is determined, not by directly measuring S', but by monitoring changes in [A] and [B]. However, any process which alters the surface composition can lead to effects which are difficult to distinguish from preferential sputtering. Two possible cases are considered in fig. 6. Consider first an experiment with a sample AB in which the surface preparation has involved an annealing step. In general alloys exhibit segregation. For example, a N i 5% Cu alloy has been shown to have a surface layer composition ~5~% Cu [28,29]. When sputtering begins, the combined effects of recoil and cascade mixing will act to reverse the segregation of, say, A leading to the conclusion that A is preferentially sputtered (fig. 6a). Alternatively, consider that the sample
is prepared with no surface segregation (perhaps by co-evaporation onto a cooled substrate). When sputtering begins, the near-surface atoms are mobilized by the cascade mixing process and there must be some tendency for segregation to occur (fig. 6b). The final surface layer composition will result from a balance between the extent to wlfich segregation can occur during the short lifetime of the cascade, and the tendency of recoil implantation to oppose segregation. However, it seems difficult to argue that, at steadystate, the surface layer composition of a segregationprone alloy should be identical to that of the bulk, even in the absence of "true" preferential sputtering effects (differences in S'), and it therefore seems probable that cascade-driven segregation and de-segregation processes should accompany sputtering and result in surface composition changes similar to those produced by preferential sputtering. If the compositional change results from segregation alone, the equality of S'a and S~ and conservation of matter ensure that the surface layer composition is identical to that of the bulk, and it is the sub-surface composition which is altered, as suggested in fig. 6c. Thus the sampling depth of the analytical technique used critically determines the result obtained. Because segregation can be correlated with binding energies (the component with the weaker bulk bonds tending to segregate to the surface [30]), an experiment such as that shown in fig. 6b would lead to the conclusion that preferential sputtering of the more weakly bonded component occurred. To distinguish between the two possibilities, the most straightforward experimental approach would be that of Liau et al. [12]. As indicated in fig. 6c, the in-depth elemental distribution in either case should be the same at steady state, with the preferentially sputtered element being depleted, relative to the bulk, over a
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depth comparable to the primary ion range. It is obviously desirable to establish the profile by high energy sputtering and to analyse this composition profile with a non-sputtering technique, or if sputtering is used, to analyse using a low energy sputtering beam. In a recent experiment, Williams and Baker have investigated the effects of segregation in the sputtering cascade by using adsorption of a reactive gas to change the driving force for segregation in the same two-component system [31]. The experiment was performed in a SIMS system, and the sample geometry was similar to that of fig. 3. It consisted of a thin evaporated layer of silver buried beneath ~1000 A of evaporated silicon on a silicon substrate. Depth proFries of the silver were obtained, and, as discussed in section 2, the slope of the exponential decay of the silver signal beyond the interface was used as a measure of the relative partial sputtering yields of silver and silicon. The absolute value of the silver yield is of little significance in the present context, reflecting as it does the combined effects of cascade mixing and preferential sputtering. The significant finding was that the relative partial sputter yield of Ar-sputtered silver could be reduced by a factor of five by continuously flooding the sample surface with oxygen during sputtering as shown in fig. 7. Due to the much greater affinity of silicon for oxygen, strong segrega-
21
tion of silicon to the sample surface is presumed to have occurred [30], and the diminished surface concentration of silver resulted in the observed reduction in the removal rate of the silver. The experiment described above represents perhaps an extreme case of segregation, in which the surface binding energy differences may be greater than occur in the majority of alloys. However, the experiment is felt to be significant in that it demonstrates that chemical effects can play a role in cascade mixing; the possibility that apparent preferential sputtering in other systems may, at least in part, result from cascade-driven segregation should seriously be considered in theoretical discussion. In addition, the result provides a serious warning that depth proffries can be significantly altered by an injudicious choice of analytical conditions. The use of oxygen flooding to enhance sputtered ion yields under argon ion bombardment is seen to be contraindicated, and the use of reactive ion bombardment to enhance depth resolution in AES [32] should be approached with caution in systems where the ion gun is not differentiaUy pumped. Oxygen ion bombardment produces far less distortion of the depth profile [31] presumably because the near surface distribution of oxygen is an error function and therefore relatively flat. It should be noted that Kelly [26] considered the possibility of segregation in sputtering, noting that diffusion processes associated with a flux of bombardment-induced point defects [33] can lead to segregation. The analytical difficulty of demonstrating the effect has not been discussed previously. Webb et al. considered the case where diffusion rates for two components in the cascade differ but did not consider the possibi]ity of surface segregation effects in such a system [34]. It is worth noting that segregation effects may account for effects other than preferential sputtering. In an early review, Andersen [35] reported an observation that the presence of a few percent of aluminum in gold could reduce the sputtering yield of gold by an order of magnitude. Such an effect would, as he pointed out, be of great concern in all sputterprofiling applications because it would totally negate the usual assumption that the sputtering rates remain constant in the absence of a major change in matrix composition. (Such an assumption is necessary in order to be able to establish depth scales by postanalysis measurement of sputtered crater depths). Because no obvious mechanism for such a gross effect is apparent, Williams used thin film gold samples, ionI. CASCADES, RECOIL PHENOMENA AND RANGES
22
P. Williams, J.E. Baker/Implantation and ion beam mixing
implanted with A1 to a peak concentration of ~3%, to detemline the effect of A1 on the gold sputter yield [36]. By measuring the time required to sputter through the gold f'tlm to the silicon substrate it was determined that the sputter yield of gold was influenced by no more than ~5% by a 3% peak concentration of implanted aluminum. Andersen's observation originated in a transmission sputtering experiment on gold, in which a thin film of A1203 was used on the upstream side of the sample to suppress backsputtering of gold. Aluminum from this fdm was observed to be recoiled into the gold film, causing the downstream side of the sample to appear silver [37]. It seems reasonable to suppose that this recoiled aluminum would segregate to the surface of the gold firm forming a thin oxide with the oxygen and water present in most accelerator vacuum systems. A thin surface layer of aluminum (oxide) would indeed be expected to significantly reduce the gold sputtering yield and could account for Andersen's observations.
4. Quantitative analysis of surface and interfacial layers using cascade mixing As pointed out in section 2, the effects of primary ion implantation and ion beam mixing upon SIMS analyses are generally cause for concern only near interfaces, but in such cases they can be disastrous. Because the relationship between ion yields and sample corriposition is poorly understood [38], quantitative analysis relies on the use of standards, and it is almost impossible to prepare well-characterized and stable standard surfaces and interfaces. Such a limitation is distressing because the high sensitivities achieved by SLMS for dopants and impurities in bulk and thin film samples extend also to interfacial impurities, and chararcterization of interfaces is of increasing importance. The realization that cascade mixing was a general consequence of sputtering offered a means to remedy this situation. The calculations of Littmark and Hofer [21] clearly show that an initially sharp subsurface layer is appreciably mixed into the surrounding, initially clean, material before being sampled by the advancing sputtering front. If, once again, the effective extent of the mixed zone is taken to be ~Rp, and Rp is chosen to be ~50 atom layers, then an interfacial layer containing one monolayer of impurities (near the limits of detection for nuclear reaction analysis or RBS for light impurities) would be diluted, by mixing, to an
SILICON
Z
z O
I
IO
20
30
40
50
60
70
8O
DATA POINT # (Sputlerinq Time)
Fig. 8. Depth proffie of a silicon sample containing a "native oxide" interfacial layer ~ one monolayer thick. Primary species: Cs÷ at 20 keV impact energy. effective concentration ~2%. This is low enough for quantitative analysis by SIMS, because such a concentration would be expected to perturb the sputtering yield, and therefore the primary species concentration by no more than about 2%, depending principally oll the relative masses of the impurity and matrix species. Ion yields in the interface region should therefore vary by no more than about 5%. Such a situation is quite conducive to quantitative analysis. The samples used by Williams and Baker [16] to test this idea came from a study in which the effects of different cleaning procedures on interfacial oxide thickness in silicon had been evaluated [22]. The oxide thicknesses had been determined using nuclear reaction analysis (NRA) techniques and were thought to range from one to several monolayers. After cleaning, the samples were immediately inserted into an evaporation system where ~ 1 0 0 0 A of Si was deposited on top on the "native oxide" layer by electron-beam evaporation. The cleanest sample gave a depth profile shown in fig. 8. The matrix Si-signal varied by no more than ~5% through the interface region, indicating that the interracial oxygen could be analysed quantitatively. This was clone by comparing the integrated, background-subtracted O- signal (the background arose from oxygen adsorption from the residual vacuum in the ion microprobe) with the integrated signal from a silicon sample containing an oxygen ion implant of accurately known dose. The surprising, and at first dismaying, result was that this sample contained less than one monolayer of oxygen in the native oxide ([0] = 1.9×1014 atom/cm2). However, this result was confirmed when NRA measurements, at the limit of sensitivity tbr the technique, found [0] ~ ( 2 - 3 ) × 1014 atom/cm 2. Analysis
P. Williams, J.E. Baker / Implantation and ion beam mixing
for the other expected impurities at the interface gave [C] = 1 . 3 X 1 0 is atom/era 2 and [It] = 2.3 X1014 atom/era 2. Evidently, thc presence o f carbon protccted the surface from oxidation during the short exposure to the atmosphere prior to loading in the evaporator. This analytical technique, which was termed by the authors "cascade dilution" extends the quantitative analytical capabilities of SIMS to an extremely important new area. In addition to buried interfaces, surface layers could be quantitatively analysed, if facilities are available to bury them in situ beneath a layer o f material (either the substrate material or another species ~vith the same sputter yield as the substrate) of sufficient thickness that a steady-state level of the primary species is attained before the interface is sampled. The SIMS measurements ha this study gave results which were consistently ~ 3 0 % lower than the NRA measurements. It is not yet clear whether this reflects systematic errors in the SIMS or NRA measurements or in both. Because of the uncertainties associated with cross-section determinations for NRA measurements, samples have been prepared which will allow quantitative comparison with Rutherford backscattering measurements. The statistical uncertainty in the SIMS measurement can be <2% (standard deviation of the mean o f 13 repeat runs was 1.7%) and there is therefore reason to hope that the cascade dilution technique will become a valuable multielement quantitative analytical technique which may even be capable of characterizing standards for NRA cross-section measurements.
5. Conclusion It cannot be stressed too strongly that sputtering is n o t simply a technique for peeling away successive layers of the sample, thereby allowing in-depth analysis. Instead, sputtering inevitably alters the subsurface layers of the sample, in most cases before the sputtering front reaches these layers. Thus, the analyst who wishes to know the composition or structure o f a sample unaffected b y the sputtering process has the choice o f finding another technique, or o f attempting to understand, in detail, the mechanisms by which sputtering alters a solid in order to be able, qualitatively or quantitatively, to compensate for or eliminate these effects. Because the high sensitivity of secondary ion mass spectrometry and the hiNl lateral resolution of Auger electron spectrometry are essen-
23
tial in many areas of materials science, resort to nonsputtering techniques is not an option. This article has indicated some areas in which a (semi-)quantitative understanding of sputtering and ion yield "artifacts" is beginning to emerge. This understanding is allowing sputtering techniques, and particularly SIMS, to be applied with some degree of confidence to problems of analysis at interfaces where hitherto their use has been suspect. We thank D.A. Reed for critical comments on the manuscript. Work at the University o f Illinois was supported by the National Science Foundation under the MIlL Grant DMR-77-23999 and, in part, by the Office o f Nav',d Research (L.R. Cooper).
References [ 1 ] J.W. Cobum, J. Vac. Sci. Tech. 13 (1974) 1037. [2] G. Slodzian and J-F. Ilennequin, Compt. Rend. (Paris) 263 (1966) 1264. [3] C.A. Anderson, Int. J. Mass Speclr. Ion Plays. 3 (1970) 413. [4] V. Krohn, J. Appl. Phys. 33 (1962) 3523. [5] P. Williams, R.K. Lewis, C.A. Evans and P.R. ttanley, An~. Chem. 49 (1977) 1399. [6] P. Williams and C.A. Evans, Appl. Phys. Lett. 30 (1977) 559. [7] J-M. Gourgout. Proc. 2nd Int. Conf. on Secondary ion mass spectrometry (ed. A. Benninghoven; Springer, NY, 1979) p. 286. [8] J.M. Morabito, R.K. Lewis and J.C.C. Tsai, Appl. Phys. Lett. 23 (1973) 260. [9] 1.'.Schulz and K. Wittmaack, Rad. Eff. 29 (1976) 31. [ 10] V.R. Define, W. Katz, C.A. Evans and P. Williams, Appl. Plays. Lett. 33 (1978) 832. [11] J.E. Chelgren, W. Katz, V.R. Deline, C.A. Evans, R.J. Blattner and P. Williams, J. Vacuum Sci. Technol. 16 (1979) 324. [12] Z.I.. Liau, J.W. Mayer, W.L. Brown and J.M. Poate, J. Appl. Phys. 49 (1978) 5295. [13] P. Williams, IEEE Trans. Nucl. Sci. NS-26 (1979) 1807. [14] V.R. Deline, Thesis (University of Illinois, 1978) unpublished. [15] H. Storms, K.F. Brown and J.D. Stein, Anal. Chem. 40 (1977) 2023. [16] P. Williams and J.E. Baker, Appl. Phys. Lett. 36 (1980) 842. []7] J. McHugh, Rad. Eft. 21 (1974) 209. [18] R.R. Hare H.L. Dunlap and O.J. Marsh, J. Appl. Phys. 46 (1975) 1947. [ 19] Z.L. Liau, B.Y. Tsaur and J.W. Mayer, J. Vac. Sci. Tech. 16 (1979) 121. [201 H.H. Andersen, Appl. Phys. 18 (1979) 131. [21] U. Littmark and W.O. llofer, Nucl. Instr. and Meth. 168 (1980) 329. I. CASCADES, RECOIL PIIENOMENA AND RANGES
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P. Williams, J.E. Baker /Implantation attd ion beam mixing
[22] M. von Allmen, S.S. Lau, D.M. Scott, J.W. Mayer, W.F. Tseng, T.T. Sheng, P. Williams and J.E. Baker, in Thin film interfaces and interactions (eds. J.E.E. Baglin and J.M. Poate; The Electrochemical Society, Princeton, t980) p. 195. [23] P. WiUiams, Appl. Phys. Lett. 36 (1980) 758. [24] II.E. Schi~btt, in Ion implantation (eds. F. Eisen and L.T. Chatterton; Gordon and Breach, London, 1971) p. 197. [25] P. Blank and K. Wittmaack, Rad. Eft. Lett. 43 (1979) 105. [26] R. Kelly, Surf. Sci., in press. [27] P. Sigmund, Phys. Rev. 184 (1969) 383. [28] Y.S. Ng, T.T. Tsong and S.B. McLane, Phys. Rev. Lett. 42 (1979) 588. [29] H.H. Brongersma, M.J. Sparnaay and T.M. Buck, Surf. Sci. 71 (1978) 657.
[30] F.L. Williams and D. Nason, Surf. Sci. 45 (1974) 377. [31] P. Williams and J.E. Baker, unpublished. [32] R.J. Blattner, S. Nadel, C.A. Evans, A.J. Braundmeier and C.W. Magee, Surf. Interface Anal. 1 (1979) 32. [33] R.C. Piller and A.D. Marwick, J. Nucl. Mat. 71 (1978) 309. [34] R. Webb, G. Carter and R. Collins, Rad. Eft. 39 (1978) 129. [35] H.H. Andersen, in Physics of ionized gases (ed. V. Vujnovic; Institute of Physics of the University of Zagreb, 1974) p. 361. [36] P. Williams, unpublished. [37] H.H. Andersen, University of Aarhus, private communication. [38] P. Williams, Surf. Sci. 90 (1979) 588.