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alumina interfaces
Applied Surface Science 182 (2001) 133±141
Effect of argon etching on alumina surfaces and on Pt/alumina interfaces P. Jonnarda,*, P. Kayserb a
Laboratoire de Chimie Physique Ð MatieÁre et Rayonnement, Universite Pierre et Marie Curie, UMR-CNRS 7614, 11 rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France b Of®ce National d'Etudes et de Recherches AeÂrospatiales, DMPH/CMT, 29, avenue de la Division Leclerc, BP 72, F-92322 ChaÃtillon Cedex, France Received 3 April 2001; accepted 6 August 2001
Abstract The effect of argon etching on a- and g-alumina surfaces as well as at Pt/alumina interfaces has been studied by electroninduced X-ray emission spectroscopy. After etching, the super®cial zones of both alumina present similar Al 3p spectral densities, showing that the environment around the Al atoms is similar. This effect is limited to the ®rst 10 nm because of the energy of the argon ions. The Al 3p valence states observed at the Pt/alumina interfaces are similar for both substrates. This is in agreement with adhesion force measurements, which do not show variation of adhesion between the two kinds of samples. It is suggested that the metal/ceramic adhesion observed in these conditions is due to an interaction between the Al and Pt valence states. # 2001 Elsevier Science B.V. All rights reserved. PACS: 61.80.-x (physical radiation effects, radiation damage); 68.35.-p (solid surfaces and solid±solid interfaces); 71.55.-i (impurity and defect levels); 73.20.-r (surface and interface electron states); 78.70.En (X-ray emission spectra and ¯uorescence) Keywords: Alumina; Platinum; Metal/ceramic interface; Adhesion; X-ray emission; Valence density of states
1. Introduction Ceramics are widely used for industrial applications. For the electrical applications, such as, an insulating coating carrying a thin ®lm sensor on a metallic part, it is necessary to achieve a good ohmic contact between the ceramic and the metallic ®lm. Joining a metallic ®lm onto a ceramic substrate is generally not straightforward [1]: it can be necessary to clean or to prepare the surface of the ceramic by *
Corresponding author. Tel.: 33-1-44-27-62-58; fax: 33-1-44-27-62-26. E-mail address: [email protected] (P. Jonnard).
etching (sputter cleaning) or heating before the metal deposition in order to strengthen the adhesion at high temperature under oxidising atmosphere or to process the joining at high pressure. The joining conditions are generally obtained empirically, little being known on the physics and chemistry at these interfaces. For example, in the case of the Pt/alumina system joined by hot-pressing, the formation of an Al(Pt) solid solution or the AlO2 compound at the interface has been evidenced, depending on the working atmosphere ([2] and references therein). In this paper, we present our results on the Pt/ alumina system. Two different phases of polycrystalline alumina have been studied: commercial a-Al2O3
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 4 7 0 - 6
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P. Jonnard, P. Kayser / Applied Surface Science 182 (2001) 133±141
samples and g-Al2O3 samples prepared by cathodic sputtering. The Al 3p valence states were probed by using EXES (Electron-induced X-ray Emission Spectroscopy) either in the bulk or in the super®cial zone of the bare alumina substrates and at the buried Pt/ alumina interfaces. The effect of etching by argon ions on the Al valence states is discussed. The results obtained at the interface are correlated with adhesion force measurements performed by the pull-off test method. 2. Sample preparation The a-Al2O3 samples were 0.6 mm thick plates Ê ) made of 99.6% pure (10 mm 10 mm, Ra < 750 A sintered powder, purchased from Coors Ceramics Electronics Ltd. (Superstrate 996 Hirel grade). The 8 mm thick g-Al2O3 coatings were deposited by cathodic sputtering (diode, rf 13.56 MHz) on NiCoCrAlY primer layers [3,4]. Substrates were nickel-base superalloy disks (diameter, 13 mm; thickness, 2 mm). The bare alumina samples were treated with the following etchings: in situ, in the preparation chamber of the analysis apparatus, with Ar, 2 kV, 0.1 mA, 30 min, for aAl2O3 and g-Al2O3 samples: etching (1); ex situ, in Ar, 380 V, 200 W, 3 min or 580 V, 200 W, 3 min or 800 V, 400 W, 20 min, for a-Al2O3 only: etching (2); in an Ar±O2 mixture (90±10 mol.%), 390 V, 200 W, 1 min for a-Al2O3 and g-Al2O3: etching (3). The ex situ etchings were performed in the cathodic sputtering system used to sputter NiCoCrAlY and gAl2O3. For etchings (2) and (3), the voltage and power conditions were applied to the substrate-holder table (diameter, 300 mm). A 20 nm thick Pt layer, was deposited on the bare a-Al2O3 and g-Al2O3 samples in the cathodic sputtering system used to produce the g-Al2O3 coatings and etchings (2) and (3). First, the substrate was Ar etched (640 V, 400 W, 20 min) then the platinum was sputtered, the etching being maintained during the metal deposition (duration: 1 min). These conditions lead to an effective thickness of the Pt layer much <20 nm.
3. EXES analysis In this non-destructive method, a nlj core hole is created in a Z atom by an incident electron. This inner core hole can relax by radiative transition involving an outer electron (Fig. 1). If the outer electron comes from the valence band, an emission band is observed. In this case, the observed spectral density describes the local (given Z) and partial (given symmetry: s, p, etc.) occupied valence densities of states [5]. The local character comes from the spatial localisation of the wave function of the inner shell involved in the transition. The partial character comes from the dipole selection rules. Because the valence states are sensitive to the physico-chemical environment, bonds between the various atoms present in the material can be characterised from the spectral densities. Transitions from occupied defect states in the band gap of insulating or semiconducting materials can also be observed (Fig. 1). Because EXES uses charged incident particles (electrons) which gradually loose their energy in the matter, this method is depth selective [5±12]. In contrast, detected particles are photons, which are nondepth selective and have a greater range in the matter than electrons. The analysed depth of the sample is that where the incident electrons have an energy higher than the ionisation threshold Enlj(Z) of the element Z under study. In the case of a substrate or a buried ®lm (alumina) covered by a ®lm of another material (Pt), the energy Ei of the incident electrons is set so that a chosen thickness is analysed. With Ei suf®ciently close to Enlj(Z), the method is convenient to characterise the bonds in thin layers located at abrupt [6±8] or diffuse [10±12] interfaces as deep as a few tens of nanometers. In the best cases, a one monolayer sensitivity can be achieved [9]. The emissive thicknesses are calculated by using the semi-empirical model Intrix of the production of characteristic X-rays generated by low-energy electrons [13,14]. This model takes into account of the parameters describing the electron±matter interaction: the transmission and backscattering coef®cients, the energy and angular distributions of the transmitted and backscattered electrons and the ionisation cross-sections. It also takes into account of the X-ray absorption within the sample. Another spectroscopic technique used to probe the occupied valence states is the XPS (X-ray
P. Jonnard, P. Kayser / Applied Surface Science 182 (2001) 133±141
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Fig. 1. Principle of the EXES, in the case of a compound, like alumina. The observed spectral DOS are local (transitions from the occupied states of the valence band to O or Al core levels fall at different photon energies) and partial (if the core level has an s symmetry, the transition comes from occupied valence states having a p symmetry). Transition from occupied defect states located in the band gap can also be observed.
Photoemission Spectroscopy). In this method, the observed spectral density gives the total (sum over all the elements and all the symmetries, weighted by the photoionisation cross-sections) density of valence
states. Moreover, because of the short escape depth of the photoelectrons, XPS is used to study super®cial zones and native interfaces (i.e. a substrate covered by a few monolayers or a fraction of monolayer of a
Fig. 2. Al 3p spectral densities of untreated a-Al2O3 (dots) and g-Al2O3 (solid line). The bulk is analysed.
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super®cial ®lm) and in this way is complementary with the EXES which can probe buried interfaces. The X-ray analysis was performed in a multipletechnique ultra-high vacuum apparatus [15]. It is
equipped with a high-resolution X-ray spectrometer giving a relative resolution <10 3. The electron irradiation takes place over almost all the surface of the sample (1 cm2) and is performed by a 0±10 kV,
Fig. 3. Al 3p spectral densities of (a) a-Al2O3 and (b) g-Al2O3. Solid line: bulk analysed, before etching (1); dots: super®cial zone analysed, after etching (1).
P. Jonnard, P. Kayser / Applied Surface Science 182 (2001) 133±141
0±10 mA Pierce electron gun. The current density applied to the sample is about 1 mA/cm2. The basic pressure is a few 10 10 mbar and increases to about 10 9 mbar when the electron gun is on. In order to study the Al 3p valence states of the alumina we used the Al Kb (3p ! 1 s) emission band, which is very sensitive to the physicochemical environment [16,17]. This emission was analysed with a (1 0 1 0) quartz crystal at the ®rst re¯ection order. The bare alumina samples were probed with 2 and 4 keV electrons, leading to analysed thicknesses of 11 and 85 nm, respectively. At low energy, the analysis (acquisition time of about 10±15 h) involves a super®cial zone, whereas at high energy (acquisition time of a few hours) it involves the bulk. The current density and the resulting electron dose are much lower than the dose necessary to produce some holes in alumina ®lms [18]. In the case of the Pt/alumina samples, the Al 3p states at the interface were probed with 2.1 keV electrons. Because the real deposited Pt thickness is not known, the analysed thickness at the interface cannot be calculated. However, it is less than the analysed thickness of the bare super®cial zone.
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4. Results 4.1. Bare alumina We present in Fig. 2 the Al 3p spectral densities of a-Al2O3 and g-Al2O3 when the bulk is analysed (4 keV electrons). They present a main peak centred at about 1553 eV, about 6 eV wide and a second peak around 1538 eV. With the help of the Al 3s [19±21] and O 2p [16] spectral densities, photoelectron spectra [16,22±24] and theoretical valence density of states [25±28], these peaks have been related to features in the valence band of alumina [16]: the main peak (the upper valence band: UVB) is due to pure Al 3p states or hybridised with Al 3s and O 2p states; the second peak (the lower valence band: LVB) is due to the Al 3p±O 2s hybridisation. Beyond the UVB is the optical gap. With respect to g-Al2O3, the main peak of a-Al2O3 is wider and its maximum is more structured. This has been explained by the presence of different environments around the aluminium atoms in a-Al2O3 (Al in octahedral sites) and in g-Al2O3 (Al in octahedral and tetrahedral sites). It results a change in the
Fig. 4. Al 3p spectral densities of a-Al2O3. Dots: before etching (2); dashed line: after etching (2) with the most severe conditions (800 V, 400 W, 20 min). The super®cial zone is analysed.
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hybridisation of the valence states. Weakly hybridised states are more numerous in a-Al2O3 and it corresponds to the states located toward the low photon energy of the UVB (around 1552 eV) [16]. Whatever the etching treatment, when the bulk is analysed, there is no signi®cant evolution of the Al 3p
spectral densities before and after etching. This means that the Ar ions do not disturb the bulk of the alumina samples and that the effect of the etching is limited to the ®rst nanometers of the sample. This is con®rmed when looking at the Al 3p spectral densities obtained with 2 keV electrons.
Fig. 5. Al 3p spectral densities obtained at the interfaces of the Pt/alumina samples (dots) for a-Al2O3 (a) and g-Al2O3 (b). They are compared with the Al 3p spectral densities obtained in the super®cial zone of the bare etched alumina (etching (1)) (solid line).
P. Jonnard, P. Kayser / Applied Surface Science 182 (2001) 133±141
We present in Fig. 3 the spectra obtained after in situ etching (1) when a super®cial zone is analysed, in comparison with the spectra obtained when the bulk is analysed. It is clearly seen that densities of states at the surfaces are modi®ed under such an Ar bombardment. We present in Fig. 4, the comparison of the Al 3p spectral densities obtained with 2 keV electrons on non-etched a-alumina and samples etched in the most severe ex situ conditions (etching (2) at 400 W, 20 min). Whatever the treatment, there is no signi®cant difference between the non etched sample and the samples etched at low power (etching (2) at 200 W, 3 min) and their spectra have been omitted in the Fig. 4 for sake of clarity. Small differences are observed in the super®cial zone when the energy of the ions, their power and the etching time are increased (400 W, 20 min). These differences are the appearance of some extra states at the bottoms of the UVB, the slight modi®cation of the shape of the maximum of the UVB and a decrease of the LVB intensity. Concerning the etching applied in an Ar±O2 gas mixture (etching (3)), no signi®cant difference of the Al 3p spectral densities is seen on the super®cial zones between etched and non-etched samples.
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4.2. Pt coated alumina We present in Fig. 5 the Al 3p spectral distributions obtained at the interfaces of the Pt/alumina samples. Because of the poor statistics in the LVB regions, only the UVB regions are presented. They are compared with the corresponding spectra obtained when the super®cial zone of the bare etched alumina (etching (1)) samples is analysed. No signi®cant change of the shape at the maximum of the UVB is observed between a-Al2O3 and g-Al2O3 in the interfacial zones. Small modi®cations with respect to the super®cial zone of the bare alumina are the appearance of some extra states in the optical gap and between the UVB and LVB. 5. Discussion Upon various argon etchings, small differences in the Al 3p spectral density are observed only under severe treatments and in the super®cial zone. This con®rms the well-known stability of alumina under irradiation conditions [29]. When comparing the effect of etching (1) on super®cial zones of a- and g-alumina (Fig. 6), it is seen, for
Fig. 6. Al 3p spectral densities after etching (1) of a-Al2O3 (dots) and g-Al2O3 (solid line). The super®cial zone is analysed.
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both phases and after etching, that the shapes of Al 3p UVB are very similar to that of g-alumina. This means that the environment of the Al atoms present at the surface of the etched a-alumina is disturbed. Etching moves the Al atoms randomly in octahedral and tetrahedral sites at the surfaces, whatever the initial surface arrangements. This is con®rmed by X-ray diffraction measurements which shows the presence of y-alumina on some etched samples. This alumina can be viewed as a distorted g-alumina [30,31] and should give a spectrum very similar to that of galumina. This is also supported by a recent study by transmission electron microscopy and electron energy loss spectroscopy on the morphology and microstructure of a-alumina surfaces. This study has evidenced the formation of a g-alumina super®cial layer upon Ar sputtering at 1 keV [32]. The changes observed on a-alumina upon etching (2) at 400 W, 20 min (see Fig. 4) can be related to the creation of oxygen vacancies. Indeed, the extra states observed in the optical gap and between the UVB and LVB have been ascribed to this kind of defects [16]. Moreover, the intensity of the LVB due to the hybridisation with the O 2s states, decreases. This implies a decrease of the Al p±O s hybridisation, which can be due to a decrease of the number of O atoms around the Al atoms. On the other hand, the hybridisation of Al p states with the O p states is also modi®ed by the oxygen vacancies. This produces the noted change at the maximum of the UVB. This is in agreement with XPS experiments [33] which show that the O to Al atomic ratio of an a-Al2O3 (0 0 0 1) is sensitive to the Ar bombardment: at 1 keV during 6 min, etching does not modify this ratio; at 2 and 5 keV during 10 min, a small decrease of this ratio is observed, showing a loss of oxygen atoms at the alumina surface [33]. By analogy with III±V ionic semiconductor, the extra states observed between the UVB and LVB could also be ascribed to point defects, such as cation±cation wrong bonds and antisites [34±36]. It is interesting to note that the largest modi®cations are observed after in situ etching (1), i.e. made inside the analysis apparatus. In this case, because of the vacuum environment, the defects created upon etching cannot recover. In contrast, the samples etched ex situ, the travel in air provides oxygen atoms, which partly ®ll the etching-induced vacancies. Then, only small modi®cations with respect to untreated samples are
observed. This has been veri®ed by looking at the in situ etched sample after stored in air during a few weeks. In this case, a spectrum characteristic of a nonetched sample is obtained. No clear evolution of the Al 3p valence states is seen at the Pt/alumina interfaces between a- and g-alumina. This is in agreement with adhesion force measurements of the Pt ®lm on the substrate, performed by the pull-off test method. The measurement results do not show signi®cant difference between the adhesion of Pt layers on a- and g-alumina substrates, >790 and >640 kg/cm2, respectively. On the other hand, scanning electron microscopy images have shown that after argon etching the roughness of the surface was higher for a-alumina than for g-alumina. Then, a mechanical anchoring cannot be the main responsible of the adhesion between the Pt ®lm and the alumina substrate. In contrast, whatever the crystal structure, a higher density of states is observed at the top of the UVB at the interfaces with respect to the etched super®cial zones. This could be ascribed to defect states as in the case of the bare etched alumina, suggesting that the adhesion could take place via these defects. But this is not supported by the adhesion force measurements, which do not evidence an evolution between Pt/alumina samples prepared with and without etching. In fact, one expects the presence of the Pt valence states in this low binding energy region, as shown by theoretical local density of states calculations [37,38]. Thus, we suggest that the states observed at the top of the UVB are due to an hybridisation between the Pt valence states and the Al 3p states. This interaction should be responsible of the metal/ceramic adhesion observed in our experimental conditions. 6. Conclusion After etching by argon ions, the super®cial zones of a- and g-alumina present similar Al 3p spectral densities, showing that the Al atoms are in a similar environment (Al in both octahedral and tetrahedral sites). It results that both alumina present the same adhesion behaviour when coated with a Pt ®lm. The supplementary states observed at the top of the UVB of the Pt/alumina interfaces, with respect to the bare etched super®cial zones, reveals the interaction
P. Jonnard, P. Kayser / Applied Surface Science 182 (2001) 133±141
between the Al and Pt valence states. We suggest that this interaction is responsible of the metal/ceramic adhesion.
[16] [17]
Acknowledgements
[18]
The authors thank Prof. C. Bonnelle for helpful discussions.
[19]
References [1] P. Vincenzini (Ed.), Ceramics: getting into the 2000, advances in science and technology, in: Proceedings of the 9th CIMTEC-World Ceramics Congress and Forum on New Materials, Vol. 15, Part C, Techna Srl, 1999. [2] Y.-C. Lu, R. Dieckmann, S.L. Sass, Acta Metall. Mater. 42 (1994) 1125. [3] J.C. Godefroy, C. Gageant, D. FrancËois, M. Portat, J. Vac. Sci. Technol. A5 (1987) 2917. [4] J.C. Godefroy, M. CleÂry, C. Gageant, D. FrancËois, Y. Servouze, Thin Solid Films 193/194 (1990) 924. [5] C. Bonnelle, Annual Report C, Royal Society of Chemistry, London, 1987, p. 201. [6] D. Fargues, F. Vergand, C. Bonnelle, Surf. Sci. 156 (1985) 590. [7] B. Iraqi, F. Vergand, D. Fargues, C. Bonnelle, Surf. Sci. 162 (1985) 871. [8] D. Fargues, F. Vergand, C. Bonnelle, Surf. Sci. 163 (1985) 489. [9] P. Jonnard, C. Bonnelle, Surf. Sci. 436 (1999) L724. [10] E.Z. Kurmaev, V.R. Galakhov, S.N. Shamin, Crit. Rev. Solid State Mater. Sci. 23 (1998) 65. [11] M. Iwami, M. Kusaka, M. Hirai, H. Nakamura, K. Shibahara, H. Matsunami, Surf. Sci. 199 (1988) 467. [12] P. Jonnard, C. Bonnelle, A. Bosseboeuf, K. Danaie, E. Beauprez, Surf. Interf. Anal. 29 (2000) 255. [13] P.F. Staub, X-Ray Spectrom. 27 (1998) 43. [14] P.F. Staub, P. Jonnard, F. Vergand, J. Thirion, C. Bonnelle, XRay Spectrom. 27 (1998) 58. [15] C. Bonnelle, F. Vergand, P. Jonnard, J.-M. AndreÂ, P.F. Staub, P. Avila, P. ChargeleÁgue, M.-F. Fontaine, D. Laporte, P.
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Effect of argon etching on alumina surfaces and on Pt/alumina interfaces P. Jonnarda,*, P. Kayserb a
Laboratoire de Chimie Physique Ð MatieÁre et Rayonnement, Universite Pierre et Marie Curie, UMR-CNRS 7614, 11 rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France b Of®ce National d'Etudes et de Recherches AeÂrospatiales, DMPH/CMT, 29, avenue de la Division Leclerc, BP 72, F-92322 ChaÃtillon Cedex, France Received 3 April 2001; accepted 6 August 2001
Abstract The effect of argon etching on a- and g-alumina surfaces as well as at Pt/alumina interfaces has been studied by electroninduced X-ray emission spectroscopy. After etching, the super®cial zones of both alumina present similar Al 3p spectral densities, showing that the environment around the Al atoms is similar. This effect is limited to the ®rst 10 nm because of the energy of the argon ions. The Al 3p valence states observed at the Pt/alumina interfaces are similar for both substrates. This is in agreement with adhesion force measurements, which do not show variation of adhesion between the two kinds of samples. It is suggested that the metal/ceramic adhesion observed in these conditions is due to an interaction between the Al and Pt valence states. # 2001 Elsevier Science B.V. All rights reserved. PACS: 61.80.-x (physical radiation effects, radiation damage); 68.35.-p (solid surfaces and solid±solid interfaces); 71.55.-i (impurity and defect levels); 73.20.-r (surface and interface electron states); 78.70.En (X-ray emission spectra and ¯uorescence) Keywords: Alumina; Platinum; Metal/ceramic interface; Adhesion; X-ray emission; Valence density of states
1. Introduction Ceramics are widely used for industrial applications. For the electrical applications, such as, an insulating coating carrying a thin ®lm sensor on a metallic part, it is necessary to achieve a good ohmic contact between the ceramic and the metallic ®lm. Joining a metallic ®lm onto a ceramic substrate is generally not straightforward [1]: it can be necessary to clean or to prepare the surface of the ceramic by *
Corresponding author. Tel.: 33-1-44-27-62-58; fax: 33-1-44-27-62-26. E-mail address: [email protected] (P. Jonnard).
etching (sputter cleaning) or heating before the metal deposition in order to strengthen the adhesion at high temperature under oxidising atmosphere or to process the joining at high pressure. The joining conditions are generally obtained empirically, little being known on the physics and chemistry at these interfaces. For example, in the case of the Pt/alumina system joined by hot-pressing, the formation of an Al(Pt) solid solution or the AlO2 compound at the interface has been evidenced, depending on the working atmosphere ([2] and references therein). In this paper, we present our results on the Pt/ alumina system. Two different phases of polycrystalline alumina have been studied: commercial a-Al2O3
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 4 7 0 - 6
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samples and g-Al2O3 samples prepared by cathodic sputtering. The Al 3p valence states were probed by using EXES (Electron-induced X-ray Emission Spectroscopy) either in the bulk or in the super®cial zone of the bare alumina substrates and at the buried Pt/ alumina interfaces. The effect of etching by argon ions on the Al valence states is discussed. The results obtained at the interface are correlated with adhesion force measurements performed by the pull-off test method. 2. Sample preparation The a-Al2O3 samples were 0.6 mm thick plates Ê ) made of 99.6% pure (10 mm 10 mm, Ra < 750 A sintered powder, purchased from Coors Ceramics Electronics Ltd. (Superstrate 996 Hirel grade). The 8 mm thick g-Al2O3 coatings were deposited by cathodic sputtering (diode, rf 13.56 MHz) on NiCoCrAlY primer layers [3,4]. Substrates were nickel-base superalloy disks (diameter, 13 mm; thickness, 2 mm). The bare alumina samples were treated with the following etchings: in situ, in the preparation chamber of the analysis apparatus, with Ar, 2 kV, 0.1 mA, 30 min, for aAl2O3 and g-Al2O3 samples: etching (1); ex situ, in Ar, 380 V, 200 W, 3 min or 580 V, 200 W, 3 min or 800 V, 400 W, 20 min, for a-Al2O3 only: etching (2); in an Ar±O2 mixture (90±10 mol.%), 390 V, 200 W, 1 min for a-Al2O3 and g-Al2O3: etching (3). The ex situ etchings were performed in the cathodic sputtering system used to sputter NiCoCrAlY and gAl2O3. For etchings (2) and (3), the voltage and power conditions were applied to the substrate-holder table (diameter, 300 mm). A 20 nm thick Pt layer, was deposited on the bare a-Al2O3 and g-Al2O3 samples in the cathodic sputtering system used to produce the g-Al2O3 coatings and etchings (2) and (3). First, the substrate was Ar etched (640 V, 400 W, 20 min) then the platinum was sputtered, the etching being maintained during the metal deposition (duration: 1 min). These conditions lead to an effective thickness of the Pt layer much <20 nm.
3. EXES analysis In this non-destructive method, a nlj core hole is created in a Z atom by an incident electron. This inner core hole can relax by radiative transition involving an outer electron (Fig. 1). If the outer electron comes from the valence band, an emission band is observed. In this case, the observed spectral density describes the local (given Z) and partial (given symmetry: s, p, etc.) occupied valence densities of states [5]. The local character comes from the spatial localisation of the wave function of the inner shell involved in the transition. The partial character comes from the dipole selection rules. Because the valence states are sensitive to the physico-chemical environment, bonds between the various atoms present in the material can be characterised from the spectral densities. Transitions from occupied defect states in the band gap of insulating or semiconducting materials can also be observed (Fig. 1). Because EXES uses charged incident particles (electrons) which gradually loose their energy in the matter, this method is depth selective [5±12]. In contrast, detected particles are photons, which are nondepth selective and have a greater range in the matter than electrons. The analysed depth of the sample is that where the incident electrons have an energy higher than the ionisation threshold Enlj(Z) of the element Z under study. In the case of a substrate or a buried ®lm (alumina) covered by a ®lm of another material (Pt), the energy Ei of the incident electrons is set so that a chosen thickness is analysed. With Ei suf®ciently close to Enlj(Z), the method is convenient to characterise the bonds in thin layers located at abrupt [6±8] or diffuse [10±12] interfaces as deep as a few tens of nanometers. In the best cases, a one monolayer sensitivity can be achieved [9]. The emissive thicknesses are calculated by using the semi-empirical model Intrix of the production of characteristic X-rays generated by low-energy electrons [13,14]. This model takes into account of the parameters describing the electron±matter interaction: the transmission and backscattering coef®cients, the energy and angular distributions of the transmitted and backscattered electrons and the ionisation cross-sections. It also takes into account of the X-ray absorption within the sample. Another spectroscopic technique used to probe the occupied valence states is the XPS (X-ray
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Fig. 1. Principle of the EXES, in the case of a compound, like alumina. The observed spectral DOS are local (transitions from the occupied states of the valence band to O or Al core levels fall at different photon energies) and partial (if the core level has an s symmetry, the transition comes from occupied valence states having a p symmetry). Transition from occupied defect states located in the band gap can also be observed.
Photoemission Spectroscopy). In this method, the observed spectral density gives the total (sum over all the elements and all the symmetries, weighted by the photoionisation cross-sections) density of valence
states. Moreover, because of the short escape depth of the photoelectrons, XPS is used to study super®cial zones and native interfaces (i.e. a substrate covered by a few monolayers or a fraction of monolayer of a
Fig. 2. Al 3p spectral densities of untreated a-Al2O3 (dots) and g-Al2O3 (solid line). The bulk is analysed.
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super®cial ®lm) and in this way is complementary with the EXES which can probe buried interfaces. The X-ray analysis was performed in a multipletechnique ultra-high vacuum apparatus [15]. It is
equipped with a high-resolution X-ray spectrometer giving a relative resolution <10 3. The electron irradiation takes place over almost all the surface of the sample (1 cm2) and is performed by a 0±10 kV,
Fig. 3. Al 3p spectral densities of (a) a-Al2O3 and (b) g-Al2O3. Solid line: bulk analysed, before etching (1); dots: super®cial zone analysed, after etching (1).
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0±10 mA Pierce electron gun. The current density applied to the sample is about 1 mA/cm2. The basic pressure is a few 10 10 mbar and increases to about 10 9 mbar when the electron gun is on. In order to study the Al 3p valence states of the alumina we used the Al Kb (3p ! 1 s) emission band, which is very sensitive to the physicochemical environment [16,17]. This emission was analysed with a (1 0 1 0) quartz crystal at the ®rst re¯ection order. The bare alumina samples were probed with 2 and 4 keV electrons, leading to analysed thicknesses of 11 and 85 nm, respectively. At low energy, the analysis (acquisition time of about 10±15 h) involves a super®cial zone, whereas at high energy (acquisition time of a few hours) it involves the bulk. The current density and the resulting electron dose are much lower than the dose necessary to produce some holes in alumina ®lms [18]. In the case of the Pt/alumina samples, the Al 3p states at the interface were probed with 2.1 keV electrons. Because the real deposited Pt thickness is not known, the analysed thickness at the interface cannot be calculated. However, it is less than the analysed thickness of the bare super®cial zone.
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4. Results 4.1. Bare alumina We present in Fig. 2 the Al 3p spectral densities of a-Al2O3 and g-Al2O3 when the bulk is analysed (4 keV electrons). They present a main peak centred at about 1553 eV, about 6 eV wide and a second peak around 1538 eV. With the help of the Al 3s [19±21] and O 2p [16] spectral densities, photoelectron spectra [16,22±24] and theoretical valence density of states [25±28], these peaks have been related to features in the valence band of alumina [16]: the main peak (the upper valence band: UVB) is due to pure Al 3p states or hybridised with Al 3s and O 2p states; the second peak (the lower valence band: LVB) is due to the Al 3p±O 2s hybridisation. Beyond the UVB is the optical gap. With respect to g-Al2O3, the main peak of a-Al2O3 is wider and its maximum is more structured. This has been explained by the presence of different environments around the aluminium atoms in a-Al2O3 (Al in octahedral sites) and in g-Al2O3 (Al in octahedral and tetrahedral sites). It results a change in the
Fig. 4. Al 3p spectral densities of a-Al2O3. Dots: before etching (2); dashed line: after etching (2) with the most severe conditions (800 V, 400 W, 20 min). The super®cial zone is analysed.
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hybridisation of the valence states. Weakly hybridised states are more numerous in a-Al2O3 and it corresponds to the states located toward the low photon energy of the UVB (around 1552 eV) [16]. Whatever the etching treatment, when the bulk is analysed, there is no signi®cant evolution of the Al 3p
spectral densities before and after etching. This means that the Ar ions do not disturb the bulk of the alumina samples and that the effect of the etching is limited to the ®rst nanometers of the sample. This is con®rmed when looking at the Al 3p spectral densities obtained with 2 keV electrons.
Fig. 5. Al 3p spectral densities obtained at the interfaces of the Pt/alumina samples (dots) for a-Al2O3 (a) and g-Al2O3 (b). They are compared with the Al 3p spectral densities obtained in the super®cial zone of the bare etched alumina (etching (1)) (solid line).
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We present in Fig. 3 the spectra obtained after in situ etching (1) when a super®cial zone is analysed, in comparison with the spectra obtained when the bulk is analysed. It is clearly seen that densities of states at the surfaces are modi®ed under such an Ar bombardment. We present in Fig. 4, the comparison of the Al 3p spectral densities obtained with 2 keV electrons on non-etched a-alumina and samples etched in the most severe ex situ conditions (etching (2) at 400 W, 20 min). Whatever the treatment, there is no signi®cant difference between the non etched sample and the samples etched at low power (etching (2) at 200 W, 3 min) and their spectra have been omitted in the Fig. 4 for sake of clarity. Small differences are observed in the super®cial zone when the energy of the ions, their power and the etching time are increased (400 W, 20 min). These differences are the appearance of some extra states at the bottoms of the UVB, the slight modi®cation of the shape of the maximum of the UVB and a decrease of the LVB intensity. Concerning the etching applied in an Ar±O2 gas mixture (etching (3)), no signi®cant difference of the Al 3p spectral densities is seen on the super®cial zones between etched and non-etched samples.
139
4.2. Pt coated alumina We present in Fig. 5 the Al 3p spectral distributions obtained at the interfaces of the Pt/alumina samples. Because of the poor statistics in the LVB regions, only the UVB regions are presented. They are compared with the corresponding spectra obtained when the super®cial zone of the bare etched alumina (etching (1)) samples is analysed. No signi®cant change of the shape at the maximum of the UVB is observed between a-Al2O3 and g-Al2O3 in the interfacial zones. Small modi®cations with respect to the super®cial zone of the bare alumina are the appearance of some extra states in the optical gap and between the UVB and LVB. 5. Discussion Upon various argon etchings, small differences in the Al 3p spectral density are observed only under severe treatments and in the super®cial zone. This con®rms the well-known stability of alumina under irradiation conditions [29]. When comparing the effect of etching (1) on super®cial zones of a- and g-alumina (Fig. 6), it is seen, for
Fig. 6. Al 3p spectral densities after etching (1) of a-Al2O3 (dots) and g-Al2O3 (solid line). The super®cial zone is analysed.
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both phases and after etching, that the shapes of Al 3p UVB are very similar to that of g-alumina. This means that the environment of the Al atoms present at the surface of the etched a-alumina is disturbed. Etching moves the Al atoms randomly in octahedral and tetrahedral sites at the surfaces, whatever the initial surface arrangements. This is con®rmed by X-ray diffraction measurements which shows the presence of y-alumina on some etched samples. This alumina can be viewed as a distorted g-alumina [30,31] and should give a spectrum very similar to that of galumina. This is also supported by a recent study by transmission electron microscopy and electron energy loss spectroscopy on the morphology and microstructure of a-alumina surfaces. This study has evidenced the formation of a g-alumina super®cial layer upon Ar sputtering at 1 keV [32]. The changes observed on a-alumina upon etching (2) at 400 W, 20 min (see Fig. 4) can be related to the creation of oxygen vacancies. Indeed, the extra states observed in the optical gap and between the UVB and LVB have been ascribed to this kind of defects [16]. Moreover, the intensity of the LVB due to the hybridisation with the O 2s states, decreases. This implies a decrease of the Al p±O s hybridisation, which can be due to a decrease of the number of O atoms around the Al atoms. On the other hand, the hybridisation of Al p states with the O p states is also modi®ed by the oxygen vacancies. This produces the noted change at the maximum of the UVB. This is in agreement with XPS experiments [33] which show that the O to Al atomic ratio of an a-Al2O3 (0 0 0 1) is sensitive to the Ar bombardment: at 1 keV during 6 min, etching does not modify this ratio; at 2 and 5 keV during 10 min, a small decrease of this ratio is observed, showing a loss of oxygen atoms at the alumina surface [33]. By analogy with III±V ionic semiconductor, the extra states observed between the UVB and LVB could also be ascribed to point defects, such as cation±cation wrong bonds and antisites [34±36]. It is interesting to note that the largest modi®cations are observed after in situ etching (1), i.e. made inside the analysis apparatus. In this case, because of the vacuum environment, the defects created upon etching cannot recover. In contrast, the samples etched ex situ, the travel in air provides oxygen atoms, which partly ®ll the etching-induced vacancies. Then, only small modi®cations with respect to untreated samples are
observed. This has been veri®ed by looking at the in situ etched sample after stored in air during a few weeks. In this case, a spectrum characteristic of a nonetched sample is obtained. No clear evolution of the Al 3p valence states is seen at the Pt/alumina interfaces between a- and g-alumina. This is in agreement with adhesion force measurements of the Pt ®lm on the substrate, performed by the pull-off test method. The measurement results do not show signi®cant difference between the adhesion of Pt layers on a- and g-alumina substrates, >790 and >640 kg/cm2, respectively. On the other hand, scanning electron microscopy images have shown that after argon etching the roughness of the surface was higher for a-alumina than for g-alumina. Then, a mechanical anchoring cannot be the main responsible of the adhesion between the Pt ®lm and the alumina substrate. In contrast, whatever the crystal structure, a higher density of states is observed at the top of the UVB at the interfaces with respect to the etched super®cial zones. This could be ascribed to defect states as in the case of the bare etched alumina, suggesting that the adhesion could take place via these defects. But this is not supported by the adhesion force measurements, which do not evidence an evolution between Pt/alumina samples prepared with and without etching. In fact, one expects the presence of the Pt valence states in this low binding energy region, as shown by theoretical local density of states calculations [37,38]. Thus, we suggest that the states observed at the top of the UVB are due to an hybridisation between the Pt valence states and the Al 3p states. This interaction should be responsible of the metal/ceramic adhesion observed in our experimental conditions. 6. Conclusion After etching by argon ions, the super®cial zones of a- and g-alumina present similar Al 3p spectral densities, showing that the Al atoms are in a similar environment (Al in both octahedral and tetrahedral sites). It results that both alumina present the same adhesion behaviour when coated with a Pt ®lm. The supplementary states observed at the top of the UVB of the Pt/alumina interfaces, with respect to the bare etched super®cial zones, reveals the interaction
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between the Al and Pt valence states. We suggest that this interaction is responsible of the metal/ceramic adhesion.
[16] [17]
Acknowledgements
[18]
The authors thank Prof. C. Bonnelle for helpful discussions.
[19]
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