Scanning Low Energy Electron Loss Microscopy (SLEELM): Au on Si

320

Applied

SCANNING LOW ENERGY ELECTRON (SLEELM): Au ON Si

LOSS MICROSCOPY

Surface

Science 32 (1988) 320-331 North-Holland, Amsterdam

M.M. EL GOMATI Electronics

Department,

University

of York, Heslington,

York YOl SOD, UK

and J.A.D.

MATTHEW

Physics Department, Received

6 November

University of York, Hedington, 1987; accepted

for publication

York YOl SOD, UK 16 February

1988

High resolution scanning low energy electron loss imaging of a well characterised system of Au on Si is demonstrated in reflection geometry at a primary electron energy E, of 680 eV and spatial resolution of 3 pm. Characteristic losses below 30 eV are chosen to examine contrast between two regions (gold and silicon dominated) respectively. By differencing between a maximum in a loss intensity and an adjacent trough, high quality Si images are obtained with contrast superior to L,,,W Auger electron images produced in comparable times. It should be noted that, in contrast to scanning Auger microscopy, the spatial resolution of SLEELM is limited only by the profile of the incident beam. An advantage of low electron energy loss imaging is that the depth resolution may be varied by altering the primary electron energy, with the possibility of bridging the gap between Auger microscopy and the X-ray microprobe analyser. Disadvantages of the technique lie in the weakness of the elastic backscattering which is essential to observe the losses in reflection geometry, and the possibility of overlapping low energy characteristic loss peaks, an effect which is illustrated here for the Au losses. It is shown that electron probes with characteristics rather different from those currently used in Auger microscopes are required to optimize the technique, but that SLEELM has the potential to be a very useful form of surface microscopy which will complement other surface imaging techniques.

1. Introduction Scanning Auger electron microscopy (SAM) is the dominant technique for providing spatially resolved surface chemical analysis, but electron energy loss spectroscopy in reflection geometry has recently been used in imaging mode to give complementary information. As long ago as 1974 Wells [l] obtained scanning electron images using a broad band (- 200 eV) of loss electrons, but Le Gressus [2], Boiziau et al. [3] and Bevolo [4] have produced chemically specific images of surface using characteristic plasmon losses under ultra-high vacuum conditions. 0169-4332/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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El Gomati and Matthew [5] termed the loss imaging technique scanning low energy electron loss microscopy (SLEELM) and compared core-core-core (CCC) Auger images with loss images for a test metal/semiconductor device structure of W on Si. Images were obtained for KLL Si and NNO W Auger transitions under conventional primary energy conditions (E, = 20 keV), and for Si losses (single (18 eV) and double (36 eV) plasmon) and W losses (main plasmon 26 eV and core loss/double plasmon at 54 eV) at E, = 730 eV. For the same electron dose the loss images showed signal-to-noise characteristics superior to CCC Auger images. Although this represents a convincing demonstration of the potential of the technique, it is possible to argue that it was not a totally fair competition! At the high primary energies used to obtain Auger images electron ionisation cross sections are significantly reduced, while the choice of CCC transitions with relatively high initial state binding energies also reduces the ionisation cross section and electron emission signal. Possibly a more relevant comparison is of SLEELM images with low energy CW Auger images using the same exciting electron beam. Such a study is presented here for a Au pattern deposited on a Si substrate. The results obtained are perplexing at first glance, but the insights they give carry important messages for imaging strategies in surface analysis.

2. Experiment The experimental set-up is basically that used previously for a wide range of Auger analytical studies [6]. The computer controlled scanning Auger microscope (SAM) has an electrostatic field emission column designed to focus with a resolution of order 0.2 pm at 17-22 keV (normal Auger mode), but has a secondary focus regime in the range 0.5-0.8 keV with resolution 2-3 pm. It is this “bonus” operating regime which is exploited in the SLEELM mode indeed the apparatus is far from ideal for SLEELM studies - and one of the messages of this paper is that SAM’s should be designed more flexibly to accommodate a range of imaging techniques. Beam currents are typically of order l-10 nA, and the column is characterised by a relatively long working distance of 55 mm. Auger/loss electron analysis is carried out with a wellcharacterised concentric hemispherical analyser (CHA) capable of 1 eV energy resolution, but operated for improved signal-to-noise ratio in the imaging mode with energy resolution of 3-6 eV.

3. Results Following Ar ion bombardment (a few hours at l-2 PA cm-*) small traces of carbon and oxygen are still detectable using Auger electron spectroscopy

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Fig. 1. Electron energy loss spectra of an Ar ion bombarded Si surface (full line), 0.65 pm thick Au overlayer on a Si surface (dotted line). The spectra were collected under the following conditions: primary electron beam energy E, = 632 eV, electron beam current I~ = 3 nA, analyser constant pass energy window of 6 eV, dwell time 100 ms per energy point, and 0.5 eV energy step. The spectra are taken in specular geometry with angle of incidence 45O and have also been smoothed using a fast Fourier transform algorithm.

(AES) at 20 keV, but the smoothed loss spectra from Si and Au dominated regions show bulk elemental features (fig. 1). It should be emphasised that these are not intended to be the best Au and Si loss spectra ever published, though they are consistent with spectra in the literature. To distinguish successfully between Au and Si regions it may be sufficient to operate at modest energy resolution exploiting distinctive differences in the loss spectra in the main bulk plasmon loss region, and not emphasising other weaker but sharper features at lower energy. Semiconductors (Si, Ge, GaAs, InSb, etc.) tend to have bulk plasmons in the 15-20 eV region, while transition/noble metals have strong plasmon losses modified by interband effects at around 25 eV. It is this simple difference that is used to map distinct chemical regions at the surface. Fig. 1 shows smoothed secondary electron loss spectra within 200 eV of the primary energy. On Si the bulk plasmon at AE = 18 eV is sharper than the main Au loss feature and is accompanied by a deeper trough below the elastic peak; double and triple losses are clearly identifiable. On Au a very broad loss peaking at AE = 24 eV is most prominent. It is interesting to note that the multiple loss background 100-200 eV below the elastic peak is comparable in count rate in the two regions in contrast to the case of W on Si [5], where the Si loss region showed the higher multiple loss intensity. These results reflect the fact that elastic scattering in the sub-keV region does not vary in any simple way with atomic number, and that the relative elastic intensities of different regions may depend in a complicated way on scattering geometry ~

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Fig. 2. Si 18 eV bulk plasmon loss image of a test pattern consisting of Au overlayer on a Si substrate. This is obtained by imaging the count difference between the counts N, at the Si plasmon loss energy at AE = 18 eV and N2 at the dip in the Si spectrum of fig. 1 at A E = 7 eV. Marker represents 30 pm.

see later discussion. Fig. 2 shows the Si loss image obtained by imaging the count difference P between the count N, at AE = 18 eV (plasmon peak) and N2 at AE = 7 eV (dip). The imaged regions are divided into 128 X 128 pixels with a dwell time of 20 ms per pixel and an image collection time of 54 min. The loss count rates are, of course, much less (- 5’610%) than those for the elastic channel in which the detector would saturate. The intensity histogram on the right of the image gives the number of pixels having a particular count rate (horizontal scale). The well defined dip in the distribution indicates the presence of two chemically distinct regions in the surface. To each count rate region a colour is assigned and each pixel is then displayed on a graphic monitor with a colour appropriate to its count rate. Such colour images enhance the visual impact, and should where possible be used in SAM and SLEELM; however, for publication purposes the rainbow of colours has been transformed to an 8-level grey scale. The bright regions on the Si loss image correctly identify the Si exposed surface and the dark region the Au overlayer - the image is of good quality with the edges between the Au and semiconductor well defined. Using the same count differencing technique on the Au loss (fig. 3a) using Nt at AE = 25 eV (main plasmon) and N2 at AE = 11 eV (the dip in the Au loss spectrum) a poor noisy image is obtained: furthermore it is dark rather than bright on the Au overlayers: The problem is that although A E = 25 eV and 11 eV do not correspond to positions of a peak and trough in the Si region, the difference Nt - N, is nevertheless bigger than for Au. This arises because the peal-trough count difference is much greater in the case of

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Fig. 3. (a) Au 24 eV loss image obtained by differencing between NI at A E = 24 eV and N:L at AE=l Ll eV. (b) Au N6,,W Auger image of the same region as in (a) above collected under the same c:onditions. N, at E, = 67 eV and N2 at E, = 78 eV. (c) Si L2,3W Auger image of the s:Ime region as (a) and (b) obtained by differencing between N, at E, = 88 eV and N2 at E, = 100 eV. Markers represent 30 pm.

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Fig. 3 (continued).

Si. In the previous SLEELM study of W on Si the W plasmon loss image was poorer than the Si image, but did give the right contrast. It is of interest to compare the loss patterns with low energy CW Auger images of Au and Si excited by an incident beam at the same primary energy (Er = 736 eV) and electron dose. Fig. 3b also shows the Au Nh,W image, which differences signals at 67 eV (peak energy) and 78 eV: this is of excellent quality, comparable to that of the SEM image, with the edges of the metal overlayers now very well defined. In contrast, the L2,3W Si image (fig. 3c) obtained by differencing counts at 88 and 100 eV is of very poor definition. The reason for this anomaly is illustrated in fig. 4, which shows the secondary electron emission spectrum of Au in the range 50-150 eV. Although there are no strong Auger peaks in the range 80-120 eV, the secondary background is already rising steeply in the Au region so that Ni - N, is still large. On the Si region L2,3W Auger emission is clearly visible in the N(E) spectrum, but Ni - N2 is little different from what is observed in the Au region. Sampling N(E) at a number of additional energies leads to better isolation of the Auger peak as shown elsewhere [7], but that does imply larger imaging times. The paradox of the results presented here is that SLEELM is good for Si, bad for Au; SAM is good for Au, poor for Si. Insights into this empirical pattern will be given in the next section. It is possible to take account of topographical effects to first order by dividing the (Ni - N,) signal by (Ni + N2) in SLEELM just as in SAM [8]. In this semiconductor system (as opposed to most metallurgical systems) topo-

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COUNTS 32000

I

0 ENERGY

Fig. 4. Electron

(eV)

energy spectra of (a) Au and (b) Si regions collected under the same condition that of fig. 1 but with spectrometer window of 2 eV.

as

graphical effects are not great, and (Ni - N2)/( Ni + N2) images are here very similar to corresponding Ni - N2 images, but slightly noisier. Topographical corrections certainly do not account for the anomalies outlined above.

4. Discussion In comparing SLEELM and SAM signal levels it is important to understand the differences in the nature of the processes that lead to electrons reaching the spectrometer in the two cases. The Auger signal is proportional to the total inner shell ionisation cross section a( E,, E,), where E, is the binding energy for the level i; following ionisation a substantial fraction fA de-excites by the Auger channel being monitored. Auger emission is approximately isotropic so that the spectrometer with acceptance angle dD samples approximately dQ/4a of the emitted electrons. The fraction of the Auger electrons k, that are sampled depend on the size of the spectrometer window SE and on spectrometer/detector efficiency factors. In addition, the signal is proportional to the concentration of atoms NA that we wish to investigate and to the effective attenuation depth X, for Auger electrons of energy EA. For characteristic losses AE with E, >>AE Born-Bethe ideas [9] may be applied. Losses are concentrated into small scattering angles comparable to 19,= AE/(2E,). Emission into the spectrometer requires a large angle (- 90” in this case) elastic scattering to precede or follow the loss. To lowest order the

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loss signal depends on the product of two cross sections - the total inelastic loss cross section ur (Et,, AE) and the differential cross section da/dQ for elastic scattering - multiplied by an effective attenuation depth X,. Following the ideas of Nassiopoulous and Cazaux [lo] and the analysis of El Gomati and Matthew [5] the ratio of the loss signal i, to the Auger signal Ia will be of order

where rE d0 is the probability of elastic scattering into da. This expression is only accurate within a factor 2-5 or so, and could be improved by detailed analysis of the spectrometer response function, breadths of Auger/loss features in relation to SE, etc. It nevertheless gives a good indication of the way in which the relative signals depend on E,, AE and E,. Within Born-Bethe

Il.

I,-

E, AI_ ln(yU) ~AE X, ln(aU’)

where U constants Let us E,/AE = ing a=y

4

r r E’

and U’ are Ep/AE and E,/AE respectively, while (Y and y are of order unity. now compare the Si plasmon loss and L2,3W Auger electron signals; 6 and X,/X, = 4 if it is assumed that X CCE” with x = 0.7. Assum= 1 the logarithmic factor gives a ratio of around 2, and so

IL/I‘4 = 6

x lO*r,.

In the absence of any specular enhancement the probability beam being elastically scattered into da will be of order rE d0 - N,(du/dS2 1,,,o)X,

of the electron

dun,

where NA is the appropriate atom density and da/d9 the differential elastic scattering cross section. Interpolating the cross sections of Fink and Ingram [ll] and Riley et al. [12] suggests du/dS2 - lop2 A2 sr-l. With plausible values of NA and X,, IL/IA will be of order 4. This does not take account of possible differences in the spectrometer response at 100 and 660 eV, but is in reasonable accord with observed areas under loss and Auger peaks respectively. The loss signal gains on the roundabouts of a superior inelastic scattering cross section, but loses on the swings of a relatively modest elastic reflectivity. The result is that moderately comparable signals are to be expected from low energy characteristic losses and strong CW Auger channels. However, gross signal is only one indicator of a good imaging channel. It is crucial that the Auger or loss signal is distinctive to a particular region of the surface. This is well satisfied by the Au N,,,W Auger signal and the Si 18 eV plasmon, but for different reasons neither the SiL,,3W nor the Au 25 eV loss

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are good for imaging. In the former case it is a signal-to-background problem with the steeper secondary electron background in Au not clearly distinguishable from the Si Auger signal when simple two energy count sampling techniques are applied. In the latter the Au loss profile is too broad to give an appropriate count difference to fingerprint the Au region. Imaging is a dangerous game: to get reasonable frame times the secondary emission spectrum must be sampled at a limited number of energies, and artefacts may emerge whether SAM or SLEELM is used. Of course CCC Auger transitions avoid the worst of the background problems and are generally very element specific; however, this is achieved at great cost in cross section reduction as discussed by El Gomati and Matthew [5]. In the move towards electronic devices of smaller and smaller dimensions the challenge to scanning electron spectroscopies is one of defining edges chemically with greater and greater spatial precision. In this regard SAM suffers two distinct problems: ionisation by backscattered electrons tends to smear chemical edges [13], and, although deconvolution of this effect is possible, the process is not trivial and far from easy to apply routinely. In addition overlayers of height h lead to a variety of edge artefacts which limit resolution to the order of this height dimension [14]. These difficulties all occur because Auger signals can arise from ionisation by electrons that have lost substantial amounts of energy and have complex pathways through the material. SLEELM does not have this problem - once the incident electron has travelled more than a few inelastic mean free paths at E, it will typically have suffered a number of losses and will cease to carry an imaging flag. SLEELM is therefore much less prone to edge artefact. There are difficulties, however, SAM at high spatial resolution has usually been carried out at high E, (> 10 keV) to optimise spatial resolution and convenience of column design. SLEELM can only be carried out at much more modest primary energy (2 2-3 key, and, even if this were not so for instrumental reasons, the elastic reflectivity eventually falls off very rapidly with primary energy so that the high signal advantages of SLEELM will rapidly decline. At high E, the reflectivity factor rE will become strongly atomic number dependent so that SLEELM will largely provide atomic number and topographical contrast as in the work of Wells [l]. In the regime investigated here this is not so; the elastic reflectivities of Si and Au are closely comparable, and this is one of the reasons for the difficulties in the Au loss image, i.e. reverse contrast. Also there are marked differences observed for the reflectivities of W and Au, a result consistent with differences in the 90 o differential elastic scattering cross sections predicted by Fink and Yates [15], Fink and Ingram [ll] and Riley et al. [12]. For SLEELM to reach its full potential electron- guns giving high spatial resolution and reasonable current over a range of energies (say 200 < E, < 300 eV) are needed. Some commercial La$ electron sources do begin to satisfy these requirements as is illustrated by the work of Bevolo [4] using the

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PHI model 600 SAM with a La$ gun coaxial to a CMA (resolution 0.4 pm as low as E, = 250 eV). However, the future design of electron sources must keep in mind the diversity in imaging techniques available and their complementarity. Variation of primary energy may have two important functions. Firstly the depth sampled varies with E, - down to 30-40 A at 3 keV - and the technique might begin to complement information from electron probe microanalysis (EPMA) and various low primary energy variants of it. Investigation of insulators covered by very thin conducting layers should be possible. Secondly variations of primary energy will vary elastic scattering probabilities for different regions, and it should be possible to accentuate contrast between regions exploiting the effect. The ability to vary experimental geometry would also enhance contrast and control of spectrometer energy resolution may be important in some systems. SLEELM identifies a distinct chemical phase rather than the presence of particular elements at the surface. It is in no sense a replacement for SAM, but a complement to it. Its difficulty is in finding a suitable energy window to highlight a particular component of the surface. In important metal-semiconductor systems the semiconductor, with the lower plasmon energy, will generally be the easier to image. In applying the technique it is always essential to examine loss spectra at a number of points at the surface before developing an imaging strategy - in this respect it is much less routine than Auger spectroscopy - but the ultimate bonus will be in its ability to characterise edges chemically without many of the difficulties encountered in SAM. The intensity of the loss signal depends both on inelastic and elastic scattering cross sections, and so does not bear any simple quantitative relation to an atomic surface concentration; in practice sharp spatial discrimination between distinct surface phases is often what is required, and SLEELM has a useful role to play within a flexible multi-imaging philosophy, which is the key to improved spatial resolution in electron spectroscopy. Finally it is relevant to note the relationship between this technique and LEED microscopy as developed by Telieps and Bauer [16] and Ichinokawa et al. [17]. If the SLEELM beam has a diameter small compared to those of oriented crystallites on the surface it may be possible to examine SLEELM images either in the specular beam, in the vicinity of a LEED beam or away from diffraction features. In the first two cases the technique corresponds to ILEED microscopy, while in the latter diffraction effects may be de-emphasised; the respective images then combine complementary information. 5. Summary

Scanning low energy electron loss microscopy (SLEELM) in reflection mode has been applied to a test pattern of Au and Si, and has been compared

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with CW Auger imaging on the same sample. It is shown that on such a system both SLEELM and SAM have their difficulties. The Si bulk plasmon and Au N6,7W Auger images are of high quality, but both the main Au loss and L2,3W Si give poor images. In the Au loss case it proves difficult to find appropriate loss energies to sample in order to highlight the Au region, while in the Si Auger case differences in the secondary electron background between the different regions diminish contrast. The results do, however, support the conclusions of Bevolo [4] that loss imaging may have signal-to-noise and contrast characteristics superior even to CW Auger images in favourable cases. CCC Auger maps avoid the worst of the background problems experienced in CW Auger, but at the expense of a dramatic loss of signal. If appropriate characteristic energy loss windows for particular chemical regions are available, SLEELM is a highly competitive scanning spectroscopy. The development of electron guns of high spatial resolution and modest primary energy coupled with greater flexibility in sample/spectrometer geometry will enhance contrast and resolution. The potential for high spatial resolution with a minimum of edge artefacts should be widely exploited within a multi-imaging philosophy that exploits pragmatically various different properties of complex surfaces.

Acknowledgements The authors would like to thank G. Thomas of British Telecom Res. Labs. Martlesham Heath, Ipswich for supplying the Au/Si samples, and Professor M. Prutton for his interest and encouragement of this work. M.M.G. would also like to thank the SERC for financial support through the Alvey project VLSI 029.

References [l] O.C. Wells, Scanning Electron Microscopy/l974 (SEM, AMF O’Hare, Chicago, IL, 1974) p. 1. [2] C. Le Gressus, JEOL News 20E (1982) 17. [3] C. Boiziau, J.P. Duraud, C. Le Gressus and D. Massignon, Scanning Electron Microscopy IV/1983 (SEM, AMF O’Hare, Chicago, IL, 1983) p. 1525. [4] A.J. Bevolo, Scanning Electron Microscopy IV/1985 (SEM, AMF O’Hare, Chicago, IL, 1985) p. 1449. [5] M.M. El Gomati and J.A.D. Matthew, J. Microsc. 147 (2) (1987) 137; Vacuum (1988). in press. [6] M. Prutton, R. Browning, M.M. El Gomati and D.C. Peacock, Vacuum TAIP 32 (1982) 351. [7] M.M. El Gomati and C.G.H. WaIker, Appl. Surface Sci., submitted. [8] M. Prutton, L.A. Larson and H. Poppa, J. Appl. Phys. 54 (1983) 374. [9] M. Inokuti, Rev. Mod. Phys. 43 (1971) 297.

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A. Nassiopoulous and J. Cazaux, Surface Sci. 149 (1985) 313. M. Fink and J. Ingram, At. Data 4 (1972) 129. M.E. Riley, C.J. MacCallum and F. B&s, At. Data Nucl. Data Tables 15 (1975) 443. M.M. El Gomati and M. Prutton, Surface Sci. 72 (1979) 485. M.M. El Gomati, M. Prutton, B. Lamb and C. Tuppen, Surface Interface Anal. 11 (1988) 251. [15] M. Fink and A.C. Yates, At. Data 1 (1970) 385. [16] W. Telieps and E. Bauer, Ultramicroscopy 17 (1985) 57; W. Telieps, Appl. Phys. A 44 (1987) 55. [17] T. Ichinokawa, Y. Ishikawa, M. Kemmochi, N. Ikeda, Y. Hosokawa and J. Kirschner, Surface Sci. 176 (1986) 397.