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The application of surface analytical techniques to thin films and surface coatings
ThinSolidFibns,80(1981)213-220 METALLURGICAL
AND PROTECTIVE
213
COATINGS
THE APPLICATION OF SURFACE ANALYTICAL THIN FILMS AND SURFACE COATINGS*
TECHNIQUES
TO
J. M. WALLS Department of Physics, Loughborough University of Technology, Loughborough, Leics. LEll Britain)
3TU (Gt.
The provision of composition-depth profiles using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) is important in a number of commercially important applications. In this paper the techniques available to provide depth profiles over the depth range 1 nm to 1 mm are described. These include methods based on the variation in the escape depth, sputter-depth-profiling and taper-sectioning techniques (angle lapping and ball cratering). The areas of application of each technique are described and examples are given. The suitability of AES and XPS for each method is also discussed.
1. INTRODUCTION
Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) are well-established methods of performing quantitative analysis of the outermost atomic layers of solid surfaces’-3. Both techniques are also powerful methods of determining how composition changes with depth and this has made an important impact in thin film technology. The techniques are now beginning to be used in the study of much thicker surface coatings and surface treatments. Indeed, depth profiling is so important that use of the term “surface analysis” to describe the function of AES and XPS is now somewhat too narrow. In this paper the methods which can be used to obtain composition-depth profiles through thin films and surface coatings over the range of depths from 1 nm to 1 mm are described. The limitations of each technique are also discussed and some of their possible applications are reviewed. 2.
ESCAPE DEPTH VARIATIONS
In AES and XPS, surface analysis is achieved by monitoring the energy spectrum of Auger electrons or photoelectrons in the range 20-2000 eV. Only those electrons which do not suffer inelastic scattering during their escape contribute to *Paper presented at the 3rd International Conference on Ion and Plasma Assisted Techniques, Amsterdam, The Netherlands, June 30-July 2,198l. 0040-6090/81/0000-oooO/%02.50
0 Elsevier Sequoia/Printed in The Netherlands
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the intensity of the peaks. Hence the mean free path (or escape depth) ,I, determines the depth of analysis and this is dependent on electron energy4,5. Hence depth information is available in the range 0.4-4 nm by simply comparing the amplitudes of peaks at different energies against a standard. Alternatively, the surface sensitivity can be improved by tilting the specimen to high angles 8 of electron emission6*’ so that L(e) = & cos 6 An example of its use in XPS is shown in Fig. 1 where the spectra obtained from an oxidized silicon surface using Al KCLemission at 0” and 60” to the surface normal are shown. Comparison of the spectra in Figs. l(a) and l(b) show that the ratio of the intensities of silicon to SiOz changes markedly as the emission angle is changed from 0” to 60”. Since the escape depth at 60” is OS&, this implies that the oxide is very thin and sits directly on top of the underlying silicon. Another way of changing the escape depth is exclusive to XPS. Here the escape depth is altered by changing the kinetic energy (KE) of the photoelectron by using different excitation sources since KE = /W-BE-+ where hv is the energy of the X-ray photon, BE is the binding energy and 4 is the work function of the analyser (BE and rj are constant). The use of this technique is again illustrated in Fig. 1 where the silicon and SiO, peaks are shown in Figs. l(b) and
a
Sputterho
Time (minute)
Fig. 1. XPS spectra from an oxidized silicon surface: spectrum a, the relative intensities of the silicon and SiO, peaks (of binding energies 99.2 eV and 103.4 eV respectively) using Al Ku radiation; spectrum b, the same peaks when the sample is tilted 60” with respect to the analyzer; spectrum c, the relative intensities at 60” using Mg Ku radiation. The silicon photoelectron energy is 1384 eV using Al Ku and 1151 eV using Mg Ku X-rays. A higher surface sensitivity is obtained at 60” and at the lower photoelectron energy. Fig. 2. A sputter depth profile using AES through a contaminated chrome film (75 nm thick) on nickel. The Auger peak-to-peak heights (APPHs) of chromium, oxygen and nickel are given as functions of sputtering time. The thin film was deposited in three steps each approximately 25 nm thick and these positions correspond to increases in oxygen content.
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l(c), as obtained by using Al Ku (1487 eV) X-rays and Mg Kol(l254 eV) X-rays at the same emission angle (60”). Although the difference in energy is small (233 eV), it can be seen that the Mg Kct case shows an increased ratio of SiO, to silicon as would be expected from a shorter escape depth. This technique has been used in a number of studies, e.g. to investigate the initial oxidation of a Pb(l11) surface using a seriesof UV sources including He I(21.2 eV) and an X-ray source (Mg KcQa. Consideration of the escape depths showed that the oxide nucleates via islands and that a complete oxide layer is formed at an average thickness of two to three monolayers. Techniques based on the variation in escape depths are non-destructive and are valuable when information is required about the structure and composition of the outermost surface layers (less than 5 nm). Areas of application include the study of thin film nucleation, oxidation, adsorption, contamination and surface segregation. 3.
SPUTTER DEPTH PROFILING
Sputter depth profiling is the most commonly used technique for obtaining composition-depth profiles through thin films. The profile is obtained by sequential ion beam etching and surface analysis. Ion beam energies are usually in the range from 500 eV to 10 keV with ion current densities up to 200 PA cme2. Inert gas species (usually argon) are normally used to preclude chemical interaction at the surface. 3.1. Auger electron spectroscopy In AES a hot-filament ion source is usually preferred and is used to produce a static ion beam with a gaussian ion current distribution (the full width at halfmaximum is typically 2 mm). The beam erodes a gaussian-shaped crater in the sample surface and the focused electron beam is centred in the crater to avoid edge effect contributions from the crater walls. Alternatively, the ion beam can be raster scanned to produce a square profile in the surface. When the depth distributions of only a few specified elements are required, the Auger peak from each element can be scanned selectively during simultaneous ion bombardment using a multiplexed or computer control unit. The amplitude of the selected Auger peaks can be plotted directly on a point plotter as a function of sputtering time. This speeds up the analysis considerably, but care must be exercised since some materials, notably aluminium, produce a high intensity of ion-induced Auger electron emission which causes uncertainty in quantitative analysis’. The applications of sputter depth profiling can be conveniently subdivided into two categories with increasing depth. On the finest scale (less than 5 nm) it facilitates comparison of the composition of the surface with that of the layers directly beneath it. This is important when identifying surface impurities or segregants or simply in surface cleaning. The distribution of the elements into the subsurface can be used to determine whether an impurity has its source extraneous to the material or whether it is a trace impurity in the bulk which has agglommerated to the surface. On a deeper scale, from 5 nm to 2 pm, sputter depth profiling is useful for the characterization of thin films or to investigate the interface between a thin film and its substrate. There is an enormous range of applications of this kind. For example,
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new information has been obtained on a wide range of protective oxide films and the technique has been applied to a range of thin films produced by vapour deposition, ion plating, r.f. sputtering and many other techniques l”-i3. It is also a powerful tool for failure analysis. A composition-depth profile through a vacuum-evaporated chromium film 74 nm thick on a nickel substrate is shown in Fig. 2. The chromium was evaporated in poor vacuum (lo-’ Torr) and shows a significant oxygen uptake. The interface is sharp as would be expected from a vapour-deposited thin film with low solid solubility with the substrate. 3.2. X-ray photoelectron spectroscopy The combination of sputter depth profiling and surface analysis by XPS is also valuable. A cold-cathode ion source is normally used in XPS since a much broader surface area must be etched because the minimum area of analysis is about 1 mm2. Such ion sources produce an ion beam with a wide distribution of ion energies and a high proportion of neutrals and electrons, these sources are generally less amenable to controlled sputter erosion than are the hot-filament sources used in AES. Also, since comparatively large areas (approximately 1 cm2) must be etched. the problem of non-uniform erosion is more acute than in AES and this restricts the depth over which useful information may be obtained. An example of the way in which XPS can be used with sputter depth profiling to characterize the chemical composition of thin films is shown in Fig. 3. One step in the manufacture of a cadmium mercury telluride (CMT) (Hg, _,Cd,Te) IR detector is the formation of an anodic layer on the surface approximately 100 nm thick to avoid surface leakage. It is important to characterize the oxide so that its electrical properties may be predicted. A detailed investigation of the tellurium peaks in the XPS spectra reveals that the oxidation state of the tellurium changes from Te4+ in
60
4 68 0.
I
I
I
I BINDIND
I
I
ENERDY
Fig. 3. XPS spectra showing the relative intensities of the Tel- and Te4+ peaks as functions of depth through an anodic layer on CMT. (By courtesy of R. F. C. Farrow and A. B. Christie.)
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the oxide to Te’- in the CMT. Prom this it can be concluded that there are no intermediate oxidation states. A further experiment using a standard TeO, sample showed that this oxide does not reduce under ion bombardment. The determination of the oxidation state with depth is a major advantage of using XPS in depth profiling. 3.3. Factors aficting the integrity of composition-depth profiles 3.3.1. Depth resolution Ideally, the depth resolution of a sputter depth profile should be constant and should be given simply by the escape depth; in practice it is found that the depth resolution deteriorates with increasing depth I4 . Many mechanisms contribute to the deterioration of the depth resolution including geometrical factors, the movement of material to different depths by cascade mixing and radiation-enhanced diffusion, and non-uniformity in the ion current distribution. However, in most analyses the most important factor is the formation of ion-induced surface topography’ 5. Surface topography is formed during ion bombardment whenever there is a local variation in sputtering yield and this can be caused by a number of mechanisms. The most serious cause is the occurrence of impurities or second-phase particles which have a different sputtering yield to the host material. Another important cause is the formation of ion-induced dislocations and other extended lattice defects which cause local lattice stress and result in an increased sputtering yield; on further ion bombardment this leads to uneven topography around the defect16*’ 7. This is important since, even if the sample is smooth, pure, homogeneous and isotropic, there still exists a mechanism for non-uniform etching. This nonuniform etching mechanism explains why the depth profiles of metals and alloys tend to lead to worse depth resolution than those of semiconductors and insulators. Insulators and semiconductors tend to become amorphous under ion bombardment; this results in a much more even etch17. Since the sputtering yield is a sensitive function S(0) of the ion incidence angle 8, this offers a means of reducing the effects of ion-induced topography. Topography could be suppressed effectively by rotating the sample15. This leads to technical difficulties with the ultrahigh vacuum systems and the sample-mounting systems at present adopted. However, a considerable improvement may be obtained by using two ion guns symmetrically inclined to the sample surface”. This partially overcomes the S(0) dependence and also undercuts the impurities which lead to severe cone formation. 3.3.2. Composition artefacts Since different elements have different sputtering yields, prolonged ion bombardment of a surface changes the surface composition. Preferential sputtering of the high sputtering yield material occurs, leaving a surface enriched in the low sputtering yield elements. In homogeneous alloys a new equilibrium composition is established which can be predicted for a wide range of elementslg. Another serious problem concerns the change in composition brought about by ion-induced chemical decomposition. It is known, for example, that a range of metallic and non-metallic oxides are reduced, leaving the surface depleted in oxygen.
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Most polymers are reduced to elemental carbon 2o. Although much work needs to be done, it seems that ion-induced reactions may be related to the free energy of formation AG,“, and this may eventually offer a means of predicting the reactions that occur21*22. In all cases it is necessary to perform experiments on standard compounds to ensure that composition changes are real and not just ion-induced artefacts. 4.
TAPER-SECTIONING
TECHNIQUES
For depth profiles greater than 2 pm the process of sputter etching leads to considerable loss of depth resolution; it is also slow and wasteful of instrument time. At these depths, recourse is taken to mechanical polishing techniques such as angle lapping and ball cratering. These techniques demand fine spatial resolution with AES and so are unsuitable for use with XPS. 4.1. Angle lapping Angle lapping is a well-established technique for use with the electron probe microanalyser, but it has only recently been applied to depth profiling with AES23-26. A taper is polished in the surface and the angle-lapped region exposes the entire depth to be analysed. For depths in the range l-10 pm it is necessary to produce angles c1in the range 0.1 “-1”. The technique has the advantage of providing a precise measurement of the depth scale and a well-defined depth resolution. On the assumption of a perfectly flat finish, the depth AZanalysed using an electron beam of diameter b is simply given by Az=btanu Although this technique is useful, the provision of an angle of less than 1” is difficult and tedious; the technique is also limited to flat surfaces. 4.2. Ball cratering
Ball cratering has been developed recently and overcomes many of the difficulties associated with angle lapping 27,28. In this technique a rotating steel ball coated with fine diamond paste (0.1-l pm) is used to fashion a well-defined spherical crater in the sample surface. The geometry of the crater is illustrated schematically in Fig. 4. Since the radius R of the ball is known (and is typically l-3 cm), the depth d of the crater is given by D22/8R, where D, is the crater diameter. The lateral position of the electron beam on the spherical crater can be related to depth by simple geometry. On the assumption that the crater is perfectly smooth the maximum vertical depth AZ analysed by an electron beam of diameter b is given by AZ = ;{ZR(d - y)} ‘jz where y is the depth at the point of analysis. The depth resolution improves with increasing depth and approaches the escape depth at the bottom of the crater. In practice the depth resolution is limited by the surface roughness generated by the wear process and the finite diameter of the electron beam. The appearance of a crater through a hard chro’me coating 6 pm thick on mild steel is shown in Fig. 5. An example of the use of ball cratering to obtain a compositiondepth profile through an electrodeposited zinc coating on a mild steel substrate is shown in Fig. 6. A
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surface chromate treatment had been applied to the sample to improve paint adhesion and the profile shows that the treatment penetrates to at least 5 pm beneath the surface. The profile also shows that a sharp interface exists between the zinc coating and the substrate, as would be expected from electrodeposition. In addition the technique also allows the coating-substrate interface to be located and investigated in fine detail. For example, the point marked A in Fig. 5 locates the interface within about 100 nm and a conventional sputter depth profile can be conducted at this point to determine the interface composition with high depth resolution.
Fig. 4. Schematic diagram of a ball crater through a surface coating. The coating thickness t is given by (D,2-D,2)/8R. Fig. 5. An optical micrograph of a ball crater through a hard chrome coating on mild steel. Point A locates a position close to the coating-substrate interface.
0
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IO
15
20 Depth
Fig. 6. Composition-depth ball cratering and AES.
25
30
35
I
0
(,um)
profile through an electroplated zinc coating on mild steel (33 pm thick) using
5. DISCUSSION In this paper we reviewed the various methods by which composition-depth profiles may be obtained using AES and XPS. On the finest scale over the first few atomic layers the variations in escape depth may be used. In this regime XPS has the advantages that the escape depth may be altered by using different excitation sources and that the concentric hemispherical analyser is better suited to angular measurements. Sputter depth profiling is the most commonly used technique and is ideal for thin film analysis. In general the advantages of XPS over AES are that it provides
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chemical information and that it can be used with delicate materials. Unfortunately, ion beam etching can cause ion-induced chemical decomposition of delicate surfaces and for these the advantages of XPS are lost. The lack of spatial resolution imposes a further limitation since uniform sputter etching must be obtained over a much larger area than with AES and the maximum recommended depth using XPS is about 500 nm. AES can be used to much greater depths, especially with semiconductors, but beyond 2 urn it is usually more fruitful to use a taper-sectioning technique. The lack of spatial resolution in XPS also precludes its use with sectioning methods which are suitable for use with AES using focused electron beams. Angle lapping is well established and can be used to section depths of 1 urn and greater. However, the ball-cratering method is more convenient to use and is suitable for depths of about 2 pm and above. The use of the ball-cratering technique is comparatively recent and this technique opens up whole new areas of application for AES including the study of surface coatings, surface treatments and diffusion profiles. These types of applications have usually been approached using electron microprobe analysis; however, AES has important advantages as it has a much better depth sensitivity which can be exploited in interface analysis and it also possesses a more even range of detection sensitivities through the elements. In general, AES is the most appropriate technique for composition-depth profiling and now offers the possibility of quantitative three-dimensional materials analysis. REFERENCES
1 A. W. Czanderna (ed.), Methods of Surface Analysis, Elsevier, Amsterdam, 1975. 2 P. F. Kane and G. B. Larabee (eds.), Characferisation of SolidSurfaces, Plenum, New York, 1974. 3 D. Briggs (ed.), Ha&book of X-ray and VItraviolet Photoelectron Spectroscopy, Heyden, London, 1977. 4 D. R. Penn, Phys. Rev. B, 13 (1976) 5248. 5 M. P. Seah and W. A. Dench, Surf. Interface Anal., I(l979) 2. 6 C. S. Fadley and S. A. L. Bergstrom, Phys. Lett. A, 35 (1971) 375. 7 W. A. Fraser, J. V. Florio, W. N. Delgass and W. D. Robertson, Surf. Sci., 36 (1973) 661. 8 D. Chadwick and A. B. Christie, J. Chem. Sot., Faraday Trans. IZ, 76 (1980) 267. 9 R. A. Powell, J. Vat. Sci. Z’echnol., 15 (1978) 125. 10 P. W. Palmberg, J. Vat. Sci. Tech&, 9 (1971) 160. 11 D. M. Holloway, J. Vat. Sci. Techno!., 12 (1975) 392. 12 J. M. Walls, D. D. Hall, D. G. Teer and B. L. Delcea, Thin Solid Films, 54 (1978) 303. 13 S. Hofmann, Talanta, 36 (1979) 665. 14 S. Hofinann, Appl. Phys., 13 (1977) 205. 15 R. Smith and J. M. Walls, Surf. Sci., 80 (1979) 557. 16 N. Hermanne and A. Art, Radiat. Eff., 19(1973) 161. 17 R. D. Webber and J. M. Walls, Thin SolidFilms, 57 (1979) 201. 18 D. E. Sykes, D. D. Hall, R. E. Thurstans and J. M. Walls, Appl. Surf. Sci., 5 (1980) 103. 19 P. M. Hall and J. M. Morabito, Surf. Sci., 83 (1979) 391. 20 S. Storp and R. Holm, J. Electron Spectrosc., 16 (1979) 183. 21 K. S. Kim and N. Winograd, Surf. Sci., 43 (1974) 625. 22 A. B. Christie, I. Sutherland and J. M. Walls, Vacuum, in the press. 23 M. L. Tamg and D. G. Fisher, J. Vat. Sci. Technol., I5 (1978) 50. 24 J. P. Chubb, J. Billingham, D. D. Hall and J. M. Walls, Mef. Technol., 7 (1980) 293. 25 C. Lea and M. P. Seah, Thin SolidFilms, 75 (1981) 67. 26 J. F. Monlder, D. G. Jean and W. C. Johnson, Thin Solid Films, 64 (1979) 427. 27 V. Thompson, H. E. Hintermann and L. Chollet, Surf. Technol., 8 (1979) 421. 28 J. M. Walls, D. D. Hall and D. E. Sykes, Sur$ Interface Anal., 1(1979) 204.
AND PROTECTIVE
213
COATINGS
THE APPLICATION OF SURFACE ANALYTICAL THIN FILMS AND SURFACE COATINGS*
TECHNIQUES
TO
J. M. WALLS Department of Physics, Loughborough University of Technology, Loughborough, Leics. LEll Britain)
3TU (Gt.
The provision of composition-depth profiles using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) is important in a number of commercially important applications. In this paper the techniques available to provide depth profiles over the depth range 1 nm to 1 mm are described. These include methods based on the variation in the escape depth, sputter-depth-profiling and taper-sectioning techniques (angle lapping and ball cratering). The areas of application of each technique are described and examples are given. The suitability of AES and XPS for each method is also discussed.
1. INTRODUCTION
Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) are well-established methods of performing quantitative analysis of the outermost atomic layers of solid surfaces’-3. Both techniques are also powerful methods of determining how composition changes with depth and this has made an important impact in thin film technology. The techniques are now beginning to be used in the study of much thicker surface coatings and surface treatments. Indeed, depth profiling is so important that use of the term “surface analysis” to describe the function of AES and XPS is now somewhat too narrow. In this paper the methods which can be used to obtain composition-depth profiles through thin films and surface coatings over the range of depths from 1 nm to 1 mm are described. The limitations of each technique are also discussed and some of their possible applications are reviewed. 2.
ESCAPE DEPTH VARIATIONS
In AES and XPS, surface analysis is achieved by monitoring the energy spectrum of Auger electrons or photoelectrons in the range 20-2000 eV. Only those electrons which do not suffer inelastic scattering during their escape contribute to *Paper presented at the 3rd International Conference on Ion and Plasma Assisted Techniques, Amsterdam, The Netherlands, June 30-July 2,198l. 0040-6090/81/0000-oooO/%02.50
0 Elsevier Sequoia/Printed in The Netherlands
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the intensity of the peaks. Hence the mean free path (or escape depth) ,I, determines the depth of analysis and this is dependent on electron energy4,5. Hence depth information is available in the range 0.4-4 nm by simply comparing the amplitudes of peaks at different energies against a standard. Alternatively, the surface sensitivity can be improved by tilting the specimen to high angles 8 of electron emission6*’ so that L(e) = & cos 6 An example of its use in XPS is shown in Fig. 1 where the spectra obtained from an oxidized silicon surface using Al KCLemission at 0” and 60” to the surface normal are shown. Comparison of the spectra in Figs. l(a) and l(b) show that the ratio of the intensities of silicon to SiOz changes markedly as the emission angle is changed from 0” to 60”. Since the escape depth at 60” is OS&, this implies that the oxide is very thin and sits directly on top of the underlying silicon. Another way of changing the escape depth is exclusive to XPS. Here the escape depth is altered by changing the kinetic energy (KE) of the photoelectron by using different excitation sources since KE = /W-BE-+ where hv is the energy of the X-ray photon, BE is the binding energy and 4 is the work function of the analyser (BE and rj are constant). The use of this technique is again illustrated in Fig. 1 where the silicon and SiO, peaks are shown in Figs. l(b) and
a
Sputterho
Time (minute)
Fig. 1. XPS spectra from an oxidized silicon surface: spectrum a, the relative intensities of the silicon and SiO, peaks (of binding energies 99.2 eV and 103.4 eV respectively) using Al Ku radiation; spectrum b, the same peaks when the sample is tilted 60” with respect to the analyzer; spectrum c, the relative intensities at 60” using Mg Ku radiation. The silicon photoelectron energy is 1384 eV using Al Ku and 1151 eV using Mg Ku X-rays. A higher surface sensitivity is obtained at 60” and at the lower photoelectron energy. Fig. 2. A sputter depth profile using AES through a contaminated chrome film (75 nm thick) on nickel. The Auger peak-to-peak heights (APPHs) of chromium, oxygen and nickel are given as functions of sputtering time. The thin film was deposited in three steps each approximately 25 nm thick and these positions correspond to increases in oxygen content.
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l(c), as obtained by using Al Ku (1487 eV) X-rays and Mg Kol(l254 eV) X-rays at the same emission angle (60”). Although the difference in energy is small (233 eV), it can be seen that the Mg Kct case shows an increased ratio of SiO, to silicon as would be expected from a shorter escape depth. This technique has been used in a number of studies, e.g. to investigate the initial oxidation of a Pb(l11) surface using a seriesof UV sources including He I(21.2 eV) and an X-ray source (Mg KcQa. Consideration of the escape depths showed that the oxide nucleates via islands and that a complete oxide layer is formed at an average thickness of two to three monolayers. Techniques based on the variation in escape depths are non-destructive and are valuable when information is required about the structure and composition of the outermost surface layers (less than 5 nm). Areas of application include the study of thin film nucleation, oxidation, adsorption, contamination and surface segregation. 3.
SPUTTER DEPTH PROFILING
Sputter depth profiling is the most commonly used technique for obtaining composition-depth profiles through thin films. The profile is obtained by sequential ion beam etching and surface analysis. Ion beam energies are usually in the range from 500 eV to 10 keV with ion current densities up to 200 PA cme2. Inert gas species (usually argon) are normally used to preclude chemical interaction at the surface. 3.1. Auger electron spectroscopy In AES a hot-filament ion source is usually preferred and is used to produce a static ion beam with a gaussian ion current distribution (the full width at halfmaximum is typically 2 mm). The beam erodes a gaussian-shaped crater in the sample surface and the focused electron beam is centred in the crater to avoid edge effect contributions from the crater walls. Alternatively, the ion beam can be raster scanned to produce a square profile in the surface. When the depth distributions of only a few specified elements are required, the Auger peak from each element can be scanned selectively during simultaneous ion bombardment using a multiplexed or computer control unit. The amplitude of the selected Auger peaks can be plotted directly on a point plotter as a function of sputtering time. This speeds up the analysis considerably, but care must be exercised since some materials, notably aluminium, produce a high intensity of ion-induced Auger electron emission which causes uncertainty in quantitative analysis’. The applications of sputter depth profiling can be conveniently subdivided into two categories with increasing depth. On the finest scale (less than 5 nm) it facilitates comparison of the composition of the surface with that of the layers directly beneath it. This is important when identifying surface impurities or segregants or simply in surface cleaning. The distribution of the elements into the subsurface can be used to determine whether an impurity has its source extraneous to the material or whether it is a trace impurity in the bulk which has agglommerated to the surface. On a deeper scale, from 5 nm to 2 pm, sputter depth profiling is useful for the characterization of thin films or to investigate the interface between a thin film and its substrate. There is an enormous range of applications of this kind. For example,
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new information has been obtained on a wide range of protective oxide films and the technique has been applied to a range of thin films produced by vapour deposition, ion plating, r.f. sputtering and many other techniques l”-i3. It is also a powerful tool for failure analysis. A composition-depth profile through a vacuum-evaporated chromium film 74 nm thick on a nickel substrate is shown in Fig. 2. The chromium was evaporated in poor vacuum (lo-’ Torr) and shows a significant oxygen uptake. The interface is sharp as would be expected from a vapour-deposited thin film with low solid solubility with the substrate. 3.2. X-ray photoelectron spectroscopy The combination of sputter depth profiling and surface analysis by XPS is also valuable. A cold-cathode ion source is normally used in XPS since a much broader surface area must be etched because the minimum area of analysis is about 1 mm2. Such ion sources produce an ion beam with a wide distribution of ion energies and a high proportion of neutrals and electrons, these sources are generally less amenable to controlled sputter erosion than are the hot-filament sources used in AES. Also, since comparatively large areas (approximately 1 cm2) must be etched. the problem of non-uniform erosion is more acute than in AES and this restricts the depth over which useful information may be obtained. An example of the way in which XPS can be used with sputter depth profiling to characterize the chemical composition of thin films is shown in Fig. 3. One step in the manufacture of a cadmium mercury telluride (CMT) (Hg, _,Cd,Te) IR detector is the formation of an anodic layer on the surface approximately 100 nm thick to avoid surface leakage. It is important to characterize the oxide so that its electrical properties may be predicted. A detailed investigation of the tellurium peaks in the XPS spectra reveals that the oxidation state of the tellurium changes from Te4+ in
60
4 68 0.
I
I
I
I BINDIND
I
I
ENERDY
Fig. 3. XPS spectra showing the relative intensities of the Tel- and Te4+ peaks as functions of depth through an anodic layer on CMT. (By courtesy of R. F. C. Farrow and A. B. Christie.)
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the oxide to Te’- in the CMT. Prom this it can be concluded that there are no intermediate oxidation states. A further experiment using a standard TeO, sample showed that this oxide does not reduce under ion bombardment. The determination of the oxidation state with depth is a major advantage of using XPS in depth profiling. 3.3. Factors aficting the integrity of composition-depth profiles 3.3.1. Depth resolution Ideally, the depth resolution of a sputter depth profile should be constant and should be given simply by the escape depth; in practice it is found that the depth resolution deteriorates with increasing depth I4 . Many mechanisms contribute to the deterioration of the depth resolution including geometrical factors, the movement of material to different depths by cascade mixing and radiation-enhanced diffusion, and non-uniformity in the ion current distribution. However, in most analyses the most important factor is the formation of ion-induced surface topography’ 5. Surface topography is formed during ion bombardment whenever there is a local variation in sputtering yield and this can be caused by a number of mechanisms. The most serious cause is the occurrence of impurities or second-phase particles which have a different sputtering yield to the host material. Another important cause is the formation of ion-induced dislocations and other extended lattice defects which cause local lattice stress and result in an increased sputtering yield; on further ion bombardment this leads to uneven topography around the defect16*’ 7. This is important since, even if the sample is smooth, pure, homogeneous and isotropic, there still exists a mechanism for non-uniform etching. This nonuniform etching mechanism explains why the depth profiles of metals and alloys tend to lead to worse depth resolution than those of semiconductors and insulators. Insulators and semiconductors tend to become amorphous under ion bombardment; this results in a much more even etch17. Since the sputtering yield is a sensitive function S(0) of the ion incidence angle 8, this offers a means of reducing the effects of ion-induced topography. Topography could be suppressed effectively by rotating the sample15. This leads to technical difficulties with the ultrahigh vacuum systems and the sample-mounting systems at present adopted. However, a considerable improvement may be obtained by using two ion guns symmetrically inclined to the sample surface”. This partially overcomes the S(0) dependence and also undercuts the impurities which lead to severe cone formation. 3.3.2. Composition artefacts Since different elements have different sputtering yields, prolonged ion bombardment of a surface changes the surface composition. Preferential sputtering of the high sputtering yield material occurs, leaving a surface enriched in the low sputtering yield elements. In homogeneous alloys a new equilibrium composition is established which can be predicted for a wide range of elementslg. Another serious problem concerns the change in composition brought about by ion-induced chemical decomposition. It is known, for example, that a range of metallic and non-metallic oxides are reduced, leaving the surface depleted in oxygen.
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Most polymers are reduced to elemental carbon 2o. Although much work needs to be done, it seems that ion-induced reactions may be related to the free energy of formation AG,“, and this may eventually offer a means of predicting the reactions that occur21*22. In all cases it is necessary to perform experiments on standard compounds to ensure that composition changes are real and not just ion-induced artefacts. 4.
TAPER-SECTIONING
TECHNIQUES
For depth profiles greater than 2 pm the process of sputter etching leads to considerable loss of depth resolution; it is also slow and wasteful of instrument time. At these depths, recourse is taken to mechanical polishing techniques such as angle lapping and ball cratering. These techniques demand fine spatial resolution with AES and so are unsuitable for use with XPS. 4.1. Angle lapping Angle lapping is a well-established technique for use with the electron probe microanalyser, but it has only recently been applied to depth profiling with AES23-26. A taper is polished in the surface and the angle-lapped region exposes the entire depth to be analysed. For depths in the range l-10 pm it is necessary to produce angles c1in the range 0.1 “-1”. The technique has the advantage of providing a precise measurement of the depth scale and a well-defined depth resolution. On the assumption of a perfectly flat finish, the depth AZanalysed using an electron beam of diameter b is simply given by Az=btanu Although this technique is useful, the provision of an angle of less than 1” is difficult and tedious; the technique is also limited to flat surfaces. 4.2. Ball cratering
Ball cratering has been developed recently and overcomes many of the difficulties associated with angle lapping 27,28. In this technique a rotating steel ball coated with fine diamond paste (0.1-l pm) is used to fashion a well-defined spherical crater in the sample surface. The geometry of the crater is illustrated schematically in Fig. 4. Since the radius R of the ball is known (and is typically l-3 cm), the depth d of the crater is given by D22/8R, where D, is the crater diameter. The lateral position of the electron beam on the spherical crater can be related to depth by simple geometry. On the assumption that the crater is perfectly smooth the maximum vertical depth AZ analysed by an electron beam of diameter b is given by AZ = ;{ZR(d - y)} ‘jz where y is the depth at the point of analysis. The depth resolution improves with increasing depth and approaches the escape depth at the bottom of the crater. In practice the depth resolution is limited by the surface roughness generated by the wear process and the finite diameter of the electron beam. The appearance of a crater through a hard chro’me coating 6 pm thick on mild steel is shown in Fig. 5. An example of the use of ball cratering to obtain a compositiondepth profile through an electrodeposited zinc coating on a mild steel substrate is shown in Fig. 6. A
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SURFACE ANALYTICAL TECHNIQUES AND THIN FILMS
surface chromate treatment had been applied to the sample to improve paint adhesion and the profile shows that the treatment penetrates to at least 5 pm beneath the surface. The profile also shows that a sharp interface exists between the zinc coating and the substrate, as would be expected from electrodeposition. In addition the technique also allows the coating-substrate interface to be located and investigated in fine detail. For example, the point marked A in Fig. 5 locates the interface within about 100 nm and a conventional sputter depth profile can be conducted at this point to determine the interface composition with high depth resolution.
Fig. 4. Schematic diagram of a ball crater through a surface coating. The coating thickness t is given by (D,2-D,2)/8R. Fig. 5. An optical micrograph of a ball crater through a hard chrome coating on mild steel. Point A locates a position close to the coating-substrate interface.
0
5
IO
15
20 Depth
Fig. 6. Composition-depth ball cratering and AES.
25
30
35
I
0
(,um)
profile through an electroplated zinc coating on mild steel (33 pm thick) using
5. DISCUSSION In this paper we reviewed the various methods by which composition-depth profiles may be obtained using AES and XPS. On the finest scale over the first few atomic layers the variations in escape depth may be used. In this regime XPS has the advantages that the escape depth may be altered by using different excitation sources and that the concentric hemispherical analyser is better suited to angular measurements. Sputter depth profiling is the most commonly used technique and is ideal for thin film analysis. In general the advantages of XPS over AES are that it provides
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chemical information and that it can be used with delicate materials. Unfortunately, ion beam etching can cause ion-induced chemical decomposition of delicate surfaces and for these the advantages of XPS are lost. The lack of spatial resolution imposes a further limitation since uniform sputter etching must be obtained over a much larger area than with AES and the maximum recommended depth using XPS is about 500 nm. AES can be used to much greater depths, especially with semiconductors, but beyond 2 urn it is usually more fruitful to use a taper-sectioning technique. The lack of spatial resolution in XPS also precludes its use with sectioning methods which are suitable for use with AES using focused electron beams. Angle lapping is well established and can be used to section depths of 1 urn and greater. However, the ball-cratering method is more convenient to use and is suitable for depths of about 2 pm and above. The use of the ball-cratering technique is comparatively recent and this technique opens up whole new areas of application for AES including the study of surface coatings, surface treatments and diffusion profiles. These types of applications have usually been approached using electron microprobe analysis; however, AES has important advantages as it has a much better depth sensitivity which can be exploited in interface analysis and it also possesses a more even range of detection sensitivities through the elements. In general, AES is the most appropriate technique for composition-depth profiling and now offers the possibility of quantitative three-dimensional materials analysis. REFERENCES
1 A. W. Czanderna (ed.), Methods of Surface Analysis, Elsevier, Amsterdam, 1975. 2 P. F. Kane and G. B. Larabee (eds.), Characferisation of SolidSurfaces, Plenum, New York, 1974. 3 D. Briggs (ed.), Ha&book of X-ray and VItraviolet Photoelectron Spectroscopy, Heyden, London, 1977. 4 D. R. Penn, Phys. Rev. B, 13 (1976) 5248. 5 M. P. Seah and W. A. Dench, Surf. Interface Anal., I(l979) 2. 6 C. S. Fadley and S. A. L. Bergstrom, Phys. Lett. A, 35 (1971) 375. 7 W. A. Fraser, J. V. Florio, W. N. Delgass and W. D. Robertson, Surf. Sci., 36 (1973) 661. 8 D. Chadwick and A. B. Christie, J. Chem. Sot., Faraday Trans. IZ, 76 (1980) 267. 9 R. A. Powell, J. Vat. Sci. Z’echnol., 15 (1978) 125. 10 P. W. Palmberg, J. Vat. Sci. Tech&, 9 (1971) 160. 11 D. M. Holloway, J. Vat. Sci. Techno!., 12 (1975) 392. 12 J. M. Walls, D. D. Hall, D. G. Teer and B. L. Delcea, Thin Solid Films, 54 (1978) 303. 13 S. Hofmann, Talanta, 36 (1979) 665. 14 S. Hofinann, Appl. Phys., 13 (1977) 205. 15 R. Smith and J. M. Walls, Surf. Sci., 80 (1979) 557. 16 N. Hermanne and A. Art, Radiat. Eff., 19(1973) 161. 17 R. D. Webber and J. M. Walls, Thin SolidFilms, 57 (1979) 201. 18 D. E. Sykes, D. D. Hall, R. E. Thurstans and J. M. Walls, Appl. Surf. Sci., 5 (1980) 103. 19 P. M. Hall and J. M. Morabito, Surf. Sci., 83 (1979) 391. 20 S. Storp and R. Holm, J. Electron Spectrosc., 16 (1979) 183. 21 K. S. Kim and N. Winograd, Surf. Sci., 43 (1974) 625. 22 A. B. Christie, I. Sutherland and J. M. Walls, Vacuum, in the press. 23 M. L. Tamg and D. G. Fisher, J. Vat. Sci. Technol., I5 (1978) 50. 24 J. P. Chubb, J. Billingham, D. D. Hall and J. M. Walls, Mef. Technol., 7 (1980) 293. 25 C. Lea and M. P. Seah, Thin SolidFilms, 75 (1981) 67. 26 J. F. Monlder, D. G. Jean and W. C. Johnson, Thin Solid Films, 64 (1979) 427. 27 V. Thompson, H. E. Hintermann and L. Chollet, Surf. Technol., 8 (1979) 421. 28 J. M. Walls, D. D. Hall and D. E. Sykes, Sur$ Interface Anal., 1(1979) 204.