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Surface Science 292 (1993) 121-129 North-Holland

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XPS investigation

of the a-C : H/Al

interface

R. Hauert, J. Patscheider SW&

Federal Laboratories

for Materials

Testing and Research (EMPA),

Ueberlandstrasse

129, CH-8600 Diibendorj

Switzerland

M. Tobler Berna AG, Industriestrasse

36, CH-4600 Olten, Switzerland

and R. Zehringer Almaden

Research Center, IBM Research Division, 650 Harry Road, San Jose, CA 95120-6099,

USA

Received 21 December 1992; accepted for publication 8 April 1993

The original state of the interface between ultrahard amorphous hydrogenated carbon (a-C : H) and aluminum was analyzed by non-destructive in-situ direct ion beam deposition (CH,, -400 V) as well as angle resolved XPS (X-ray photoelectron spectroscopy) analysis through a thin a-C: H coating. Depending on the deposition conditions a 0.6 to 1.9 nm thick Al& interlayer, held responsible for the good adhesion between a-C:H and Al, could clearly be resolved. Furthermore, an interaction between a-C: H and Al& at the a-C: H/Al& interface was detected. The ability of C and Al to form a reactive Al& interface as well as an interaction of a-C : H with Al& has also been confirmed by XPS and AES sputter depth profile analysis.

1. Introduction Thin films of ultrahard amorphous hydrogenated carbon (a-C: H), also named “diamondlike carbon” (DLC) are used for a large variety of applications due to their outstanding properties such as high hardness (up to Hv 5000), high chemical inertness (corrosion resistance), infrared transparency, low friction coefficient, low wear and high thermal conductivity [l-3]. These unique properties as well as the possibility of a-C: H to be deposited at temperatures as low as room temperature make them well suited for the use in many different industrial applications. a-C : H coatings are used to increase the lifetime of parts suffering wear, as optical coatings, and to lower friction [3-51. a-C : H films can be prepared by different ionassisted processes. Plasma-assisted chemical va-

por deposition (PACVD) is a widespread technique which employs the decomposition of hydrocarbons in a low pressure discharge. In order to obtain hard and dense films a substrate bias exceeding about 1.50 V is required [6,7]. Another method for the synthesis of a-C : H films is sputter deposition using carbon targets [2,7]. Direct ion beam deposition techniques allow a direct control of ion energy of the incoming hydrocarbons. In industrial scale radio frequency (RF)PACVD deposition is used to obtain a uniform and three-dimensional coating of large work pieces. The average ion energy can be controlled by the sample bias which is depending on the total pressure, gas composition, deposition chamber geometry and the wave form of the radio frequency power [8,91. Depending on the deposition conditions the film properties may continuously be varied from soft polymer-like films (low

0039~6028/93/%06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

122

R. Hauert et al. / XPS investigation of the a-C: H/Al

ion energies) to ultrahard diamond like coatings. a-C : H films deposited at high ion energies exhibit, depending on the deposition conditions, high internal stress (compressive) sometimes exceeding 1 GPa [7,10,11]. For good adherence of the film to the substrate the adhesion forces at the film/substrate interface have to withstand the forces arising from the internal stress in the film as well as the additional external forces applied to the coating in use. Applications of thin films, especially ultrahard coatings, are often limited by poor adhesion of the coatings to their substrates and, therefore, great efforts have been made in order to understand and, thus, to improve adhesion. The mechanical adhesion of thin films is mainly controlled by the chemistry of the film/substrate interface [12-161. A very thin reactive interface which is usually only a few atomic layers thick is held responsible for good adhesion. It has been shown previously that good adherence of a-C : H films may be expected on carbide forming materials such as W, MO, Ti, Zr and Si. On these substrates the good adhesion is usually correlated with the presence of a reactive interface, i.e. the build-up of a carbidic compound between the substrate and the a-C: H coating [12-161. Even on materials as germanium, which does not form a thermodynamically stable carbide, a metastable compound is formed at the interface to which good adhesion is attributed [14]. On the other hand poor adherence is observed when no interfacial carbidic phase is present (Cu, Au) [15] or when phase transformations in the substrate underneath the interface occur as in the case of GaAs [12,13]. However, an interface carbide is not necessarily a guarantee for good adhesion as in the case of tungsten [15]. Aluminum, aluminum alloys and other light metals are well suited for a-C : H coating. The combination of an ultrahard coating on a light and relatively soft material opens new possibilities in construction. The relevant parameters (weight, costs, mechanical properties, wear corrosion, fretting) may be optimized independently for the base materials and the active surface. Good adhesion of the coating to the substrate is usually achieved. It is the aim of this work to elucidate

interface

the situation present at the a-C : H/Al interface responsible for good adhesion. In the present paper different approaches have been undertaken to investigate the a-C: H/Al interface. The very small extension of the interface region of only a few atomic layers necessitates the use of surface sensitive techniques such as XPS, AES, SIMS, etc. To analyze the original state of the interface two techniques have been used. Angle resolved photoelectron spectroscopy (ARXPS) analysis through a thin a-C : H coating and in-situ a-C: H deposition to directly observe the build-up of the interface as a true in-situ characterization. Additionally XPS measurements in combination with sputter depth profiling were carried out. A limiting factor in depth profiling is the possible impact on triggering interface reactions by the high energetic argon ions and, hence, the build-up of interfaces which were originally not present [17,18]. The interface reaction products generated by sputter depth profiling are expected to be the same as generated by the a-C: H deposition from hydrocarbon ions. Only the spatial distribution of these products (i.e. layer thickness) is expected to be enlarged to several nm by depth profiling at these conditions [17,18].

2. Experimental 2.1. Analysis

Several surface sensitive techniques were used to analyze the interface between a-C : H and aluminum. XPS sputter depth profiles, in-situ ion beam deposition profiles as well as ARXPS studies were undertaken. The latter uses the dependence of the information depth on the observation angle as the basic concept for non-destructive depth profiling. XPS measurements were carried out on a Perkin-Elmer PHI 5400 ESCA system using Mg Ka excitation (hv = 1253.6 eV). The electron energy analyzer was operated at a constant pass energy of 35.36 eV giving a total energy resolution of 0.89 eV (Ag3d,,,). The electron take off angle was 43”, the analyzed area 1.5 mm*. The binding energy scale was calibrated

R. Hauert et al. / XPS investigation of the a-C: H/Al

based on the Au4f,,, signal at 83.8 f 0.1 eV. The angles given in the ARXPS spectra (fig. 3) refer to the electron takeoff angle between the surface plane and the analyzer. Sputter depth profiling was performed using Ar+ at 3 keV. The sputter rate was calibrated with reference films to 3.0 nm/min for SiO,, 2.5 nm/min for Al,C, and 1.0 nm/m~ for a-C : H, The depth profiles were taken as a set of discrete sputtering cycles intercepted by XPS measurements of the C Is, 0 1s and Al2p core level lines.

2.2.2. PACK5 under clean conditions An a-C : H film has been prepared under clean conditions in an UHV preparation chamber equipped with a magnetron sputter source and a RF plasma system operating at 13.6 MHz. The substrate holder is connected to the powered electrode of the RF circuit. The deposition system is directly connected to the XPS spectrometer chamber for contamination-free sample transfer. Since ARXPS requires surfaces as flat as possible, 50 nm aluminum has been sputter deposited on polished (111) silicon wafers prior to the a-C : H coating. a-C : H was deposited from a C,H,-discharge operated at a gas pressure of 1.4 X 10e2 mbar and a feed gas flow of 3.6 seem (standard cubic centimeters per minute). The sample self bias was -400 V and the deposition time 90 s. After deposition the system was

2.2.1. In-situ ion beam deposition In order to analyze the chemical states at the interface in a non-destructive way a-C: H was deposited in the XPS spectrometer by ion beam deposition. The substrate used here was single crystalline silicon sputter coated with 50 nm high purity aluminum (target purity > 99.99). The sputter ion gun of the system was used as the source of monoenergetic hydrocarbon ions. The ion gun was operated with methane (CH,,

75

74

73

72

71

123

99.995% purity) at an acceleration voltage of 400 V as described in refs. 112-153. After each deposition cycle of 15 min XPS spectra of the, Al 2p, C 1s and 0 1s region were recorded in order to obtain growth profiles.

2.2. Preparation of a-C: H on aluminum

76

interface

70

25%

286

254

282

280

binding energy [ev] Fig. 1. (a) AI 2p and (b) C 1s photoelectron spectra acquired after each of the consecutive 15 min ion beam (CH,, 400 V) deposition cycIeons on aluminum substrate. Tbe reference spectra taken from Al, AI&, and a-C: H are included.

124

R. Hauert et al. / XPS investigation of the a-C: H/Al

pumped to a pressure below lo-’ mbar within 1 min and the sample was transferred into the spectrometer chamber. 2.2.3. Industrial standard deposition The a-C: H films have been prepared in a capacitively coupled RF plasma chamber (13.6 MHz) using C,H, as process gas. The polished aluminum “anticorrodal” substrates (99 wt% Al, 0.5 wt% Mg and 0.5 wt% Si) were argon sputter cleaned prior to a-C: H deposition. Immediately after sputter cleaning the feed gas was directly switched to C,H,. During a-C: H deposition the bias was kept at -700 V with a RF power input of 100 W and a feed gas pressure of 1.2 X lop2 mbar. Further details concerning the deposition process can be found elsewhere [19]. 2.2.4. Al 4C, reference film deposition The Al,C, reference film was produced by co-evaporation from two magnetrons with a carbon and an aluminum sputter target, respectively. The deposition rates have been calibrated separately for each magnetron using a quartz microbalance to obtain the correct stoichiometry in the Al,C, film. The atomic concentration of the sputter cleaned film was verified by XPS yielding 57% Al, 41% C and 2% 0, which is within 2% the expected stoichiometry. Since only one C 1s and one Al2p photoelectron peak have been observed, aluminum carbide in the Al,C, stoichiometry is assumed to be formed. Because Al&, is hygroscopic 20 nm aluminum was deposited as a protective coating against atmospheric contamination. Additionally an Al,C, film with slightly enhanced carbon content has been produced.

3. Results 3.1. In-situ ion beam deposition To analyze the chemical situation at the aC: H/aluminum interface an in-situ ion beam (CH,, 400 V) deposition profile was acquired. The Al2p and C 1s photoelectron spectra acquired between each of the 15 min deposition

interface

cycles are shown in fig. 1. Also enclosed in fig. 1 are the reference spectra acquired under the same conditions from argon sputter cleaned aC : H (deposited by PACVD at a bias voltage of - 800 V; C 1s: 284.5 eV), metallic aluminum (Al 2p: 72.6 eV) and magnetron sputter deposited Al&, (C 1s: 282.2 eV, Al 2p: 73.4 eV). Clearly, the positions of the C 1s and the Al 2p photoelectron signals in the first stages of deposition indicate that aluminum carbide is formed. The C 1s signal at the a-C : H position (284.5 eV) shows the growth of the a-C : H film. The chemical information contained in this set of spectra can be extracted by several methods, e.g. by fitting with a linear combination of appropriate peak shapes, by spectral fitting, or by a target factor analysis (TFA) [20,21]. TFA is a mathematical procedure where a set of data (e.g. a profile) is treated as a numerical matrix in which each column contains the data points of one spectrum. The target factor analysis was performed with the software package PHI-MATLAB, version 3.OA, provided by Perkin-Elmer Corporation, Physical Electronics Division (PHI). A TFA performed on the Al2p photoelectron signals yields two principal components (linearly independent spectra) necessary to fully describe the Al2p profile. This indicates that there are only two different chemical states of Al present in the deposition profile. In the next stage the Al and Al& reference spectra are chosen as a base for the reproduction of the acquired data. The use of any external spectrum as a base (Al&, in this case) requires that this spectrum can also be described as a linear combination of the two principal components. The fact that this is the case for the recorded spectrum of Al&, indicates that Al& is an existing component encountered in the deposition profile. The chemically resolved description of the Al2p signals acquired in the ion beam deposition profile is plotted in fig. 2a (solid lines) as a function of deposition time. Starting from clean aluminum, the build-up of aluminum carbide accompanied by a decrease of the metallic aluminum contribution can clearly be seen. This process is followed by a decreasing Al&, intensity due to the overgrowing a-C : H coating. The results obtained from

R Hauert et al. / XPS investigation of the a-C: H/Al integace

a)

0

20

40

60

60

100

120

140

160

deposition time [min]

Fig. 2. (a) Relative intensity (I/I,,) as compared to the pure materials of Al, Al&, and a-C:H taken from the spectra in fig. 1. Results calculated from Al2p-spectra are given with solid lines, those from Cls with dashed lines. (0): Al, (W ): Al&,, (0): a-C:H. (b) Thickness of the Al,C, and a-C:H layers as calculated from the relative intensity as a function of deposition time.

TFA can be confirmed by peak fitting using two peaks at the Al (72.6 eV> and the Al& (73.4 eV) positions. Performing a TFA as described above on the C 1s data was somewhat problematic. The principal component analysis required, besides the expected a-C: H and Al& components, an additional third component to fully describe the spectra acquired at the Al&/ a-C : H interface. The C Is profile was analyzed by TFA using the a-C : H and Al,C, reference spectra as a base for describing the C 1s data. The chemically resolved deposition profile (a-C : H and Al&) obtained from the C 1s data is displayed in fig. 2a as dashed lines. The C 1s spectra acquired between 0 and 45 min deposition time could entirely be described as a linear combination of the two base spectra confirming the results derived from the

125

Al 2p spectra. The C 1s spectra acquired between 60 and 165 min deposition time were somewhat disturbed by the additional photoelectron intensity around 283.4 eV and an absolute error of 0.16 is caused. It was possible to properly reproduce the spectra by introducing an additional C 1s state centered around 283.4 eV. The possible nature of this additional C 1s signal will be discussed below. The intensity of this 283.4 eV C 1s state was rather low and yields a maximum relative contribution of 0.16 at the Al,C,/a-C: H interface. The thickness of the Al& interface as well as the one of the a-C : H overlayer can be calculated from the relative Al, Al&, and a-C : H intensities in fig. 2a, using formulas (1) and (2) [22]: d = -A

sin cz ln(Z,d/Z:),

(1)

d = -A

sin a ln(1 -Z,d/Z,“),

(2)

Z”*d,” denotes to the signal intensities from the s:bstrate or the overlayer at an overlayer thickness of 0, d or w measured under the exact same conditions. With an electron takeoff angle (a> of 43” and an attenuation length (A) of 1.5 nm for Al2p photoelectrons [22-241 the thickness (d) of the a-C : H overlayer can be calculated from the substrate signal which is the sum of the relative contributions of Al and Al&. The total thickness of the a-C : H + Al&, layers has been calculated using the signal intensity from the metallic Al substrate. The difference yields the thickness of the Al& interface. The results as a function of deposition time is illustrated in fig. 2b. The thicknesses of the Al& and a-C : H layers can analogously be calculated from the relative C 1s contributions of Al&, and a-C : H using formula (2) and an inelastic mean free path (attenuation length) of 1.4 nm for Cls photoelectrons [23,24]. The results are displayed in fig. 2b. As mentioned, the presence of the additional C 1s intensity between Al& and a-C : H causes an error margin of k 0.4 nm for the a-C : H overlayer and of f0.8 nm for the A14C3 interlayer at deposition times between 60 and 165 min. It can, clearly be seen how the approximately 1.9 nm thick aluminum carbide interface (value obtained from the Al2p signals) is rapidly built

R. Haueri et al. / XPS investigation of the a-C: H/Al

interface

interlayer calculated from these relative intensities are displayed in fig. 4b. A thickness around 0.65 nm for the Al,C, and 0.5 nm for the a-C: H overlayer is obtained for all the angles. Analysis of the C 1s spectra was problematic for all spectra due to the same reasons as described above and error margins exceeding the value of the results have been obtained.

Al 2~

3.3. Industrial a-C: H films

Fig. 3. Angle resolved Al2p photoelectron spectra at different electron takeoff angles a taken from an a-C:H/Al film deposited in the UHV preparation chamber (for reference spectra see fig. 1).

up in the first stage of deposition which, after, is slowly covered by the continuously ing a-C : H overlayer. The slow growth rate a-C: H film (compared to the build-up aluminum carbide) becomes apparent.

The 6-10 nm thick a-C: H film deposited on aluminum in the industrial a-C: H coater was analyzed for comparison with the previously described samples which were deposited under quasi-ideal conditions. Since the a-C : H coating was too thick for analysis by non-destructive

theregrowof the of the

3.2. PACED UHV preparation chamber deposition The other non-destructive method used for investigating the interface was ARXPS of a sample with an approximately 1 nm thick a-C: H layer on aluminum. The Al2p photoelectron spectra obtained at the angles between 23” an 90 are displayed in fig. 3. At high electron takeoff angles the signal from metallic aluminum is markedly enhanced due to the increased depth analyzed. The 0 Is signal, measured for contamination control, never exceeded 3 at%. Analysis of the Al2p signals for the different angles was performed analogous to the way described above. TFA performed on the Al2p photoelectron signals again revealed that there are only two principal components (linearly independent spectra) necessary to describe the full set of A12p spectra. The relative intensities of the Al and Al&, reference spectra necessary to describe the A12p spectra acquired at the different angles are displayed in fig. 4a as a function of sin (Y. The thicknesses of the a-C : H overlayer and the Al&,

-.-

I

0.4

I

0.5

I

0.6

I

0.7

sin (a

/

0.9

I

0.9

I

1.0

)

Fig. 4. (a) Relative intensity (I/I,) of Al and AI,C, to the A12p spectra in fig. 3 as a function of sin a (same symbol notation as in fig. 2). (b) Thickness of the AI,C, and a-C:H layers as calculated from the relative contributions (f/I,,) as a function of sin (Y.

R. Hauert et al. / XPS investigation of the a-C: H/Al

1 A

interface

121

ing the chemical depth profile from the CWLL) Auger spectra was again difficult due to an additional Auger signal revealing an additional chemical state of carbon between the a-C: H and the Al & 3 layers. 3.4. Adhesion

0

200

400

600

600

sputter time [set] Fig. 5. Relative intensity (I/I,,) of Al, Al&, and a-C:H to the spectra (not shown) aquired during an argon sputter depth profile on a industrially coated a-C:H/Al sample as a function of sputter time (same symbol notation as in fig. 2).

ARXPS and furthermore C contaminations are expected to be present on the surface due to air exposure, the a-C: H/Al interface could only be accessed by ion etching. Therefore, an argon sputtering depth profile was acquired. Analysis of the acquired data was performed again by TFA. The relative contribution of the reference spectra necessary to describe the Al 2p and C 1s spectra acquired in the depth profile are displayed in fig. 5. Describing the C 1s spectra acquired between 100 and 400 s sputter time (between the a-C : H and the Al,C, interlayer) was again problematic and an absolute error of 0.2 is caused by the additional photoelectron intensity around 283.4 eV (cf. section 3.1). As the sputter time increases the consecutive a-C : H/Al&/Al structure can be seen from the relative contributions of the corresponding reference spectra. The Al,C, interlayer was determined to be about 10 nm thick. The 0 1s signal indicates a small oxygen contamination with a maximum oxygen concentration of 4 at% at the Al&, interlayer. The same sputter depth profile was also acquired in an Auger spectrometer. Describing the Al(LMM) signals as a function of the Al and Al,C, reference spectra revealed the same compositional depth profile as shown in fig. 5. Deduc-

The adhesion of coatings can qualitatively be characterized by observing a-C : H ablation caused by substrate deformation around a Rockwell indentation deliberately destroying the coating. Excellent adhesion was observed for a 3 pm thick a-C: H coating on aluminum prepared by the industrial standard deposition conditions as described in the experimental section. Furthermore, no ablation problems have been reported by customers using a-C : H coated aluminum parts for different technical applications. Aluminum and its alloys are very ductile usually having yield strengths between 50 and and 500 MPa (at room temperature). Forces exceeding the yield strengths cause plastic deformation in the substrate. So the maximum load acting on the interface is determined by the yield strengths of the substrate which was 200 MPa for the aluminum substrate used.

4. Discussion Different approaches have been made to characterize the situation at the a-C : H/Al interface. All three approaches revealed the same chemical behavior of the a-C : H/Al system with the buildup of a stoichiometric Al,C, interlayer, but with different interface thicknesses. The energy necessary to enable interdiffusion, or ion beam mixing, of Al and C is delivered by the incoming hydrocarbon ions. The Al& interlayer thickness of about 0.65 nm thickness measured on the film produced by PACVD in the UHV preparation chamber is less than the 1.9 nm value obtained by ion beam (CH,, 400 V) deposition. The lower Ai,C, interlayer thickness originates from collisional losses of kinetic energy of the hydrocarbon ions impinging on the growing surface as they cross the

128

R. Hauert et al. / XPS investigation of the a-C: H/AI interface

plasma sheath [91. The average impact energy may be up to an order of magnitude less than the sheath potential [25]. Also a smearing out of the energy distribution will occur [26]. In the case of sputter depth profiling the influence of the highly energetic argon ions on the interface has to be taken into account [17,181. The enlargement of the Al,C, interlayer to 10 nm by the 3 kV argon ions can be seen in the chemically resolved XPS depth profile as well as in an Auger depth profile acquired under the same sputter conditions. The sputter depth profile only demonstrates the capacity of Al and C to form a reactive interface under ion bombardment and does not represent the original state of the interface. The calculated layer thicknesses as illustrated in figs. 2, 3 and 5 have to be considered as estimates only. The assumptions of ideally flat surfaces, homogeneous layer thicknesses and a material independent electron attenuation length (A), justifying formulas (1) and (21, can never be ideally fulfilled. However, the different aC : H/Al interfaces can be compared among each other. The thickness of the a-C: H overlayer and the Al& interlayer displayed in fig. 4b is expected to be constant independent of the observation angle ((~1. The deviations of kO.1 nm are caused by applying formula (1) to a real and, therefore, non-ideal sample. The spectral interpretation of the Al 2p signals on the basis of the Al an Al&, reference spectra clearly revealed the formation of the Al&, interlayer. For all three approaches made, the interpretation of the C 1s signal in terms of the a-C : H and Al& reference spectra is intricate at the a-C : H/Al&, interface by the presence of additional C 1s intensity centered around 283.4 eV. An estimation of the layer thickness causing this signal using formula (2) yields about 0.25 nm, for the ion beam deposited film (CH,, 400 Vl which corresponds to one monolayer. The observed shift indicates an additional chemical state of C which is very likely a carbide of the known C:- type (acetylide) [27,281. Very interestingly an additional C 1s intensity at 283.4 eV could also be measured on Al&, films produced with a C excess. The C state at 283.4 eV may be attributed

to the first row of carbon in the a-C: H film interacting with the Al,C, interlayer.

5. Conclusions The reactive interface between a-C : H and Al held responsible for good adhesion could clearly be resolved. Furthermore, an interaction between carbon and the Al& interlayer could be detected. The conditions necessary for the build-up of the Al.&, interlayer are a clean and oxygenfree aluminum surface and sufficient energy of the incoming hydrocarbon ions in the first few nanometers of the deposition. Since Al&, is reacting with Hz0 (humidity) the coating as well as the aluminum substrate have to be impermeable for H,O which is usually the case. Good adhesion of a-C : H films could also be obtained on aluminum oxides (sapphire, aluminum covered with a native oxide layer, aluminum oxide ceramics) under certain deposition conditions. It is the aim of our future work to elucidate the properties on the a-C : H/Al,O, interfaces with special attention on the question how a-C : H interacts with chemically highly stable Al,O,.

Acknowledgements

Financial support by the Swiss National Science Foundation (NFP24) is gratefully acknowledged. Discussions with Dr. Karl-Heinz Ernst (EMPA) are also acknowledged.

References [l] A.H. Lettington, in: Amorphous Hydrogenated Carbon Films, Vol. 17 of Proc. Eur. Mater. Sot., Eds. P. Koidl and P. Oelhafen (Les Editions de Physique, Les Ulis, 1987) p. 359. (21 J. Franks, K. Enke and A. Richard, Met. Mater. 6 (1990) 695. [3] M. Rubin, C.B. Hopper, N.-H. Cho and B. Bhushan, J. Mater. Res. S (1990) 2538. [4] H.-C. Tsai and D.B. Bogy, J. Vat. Sci. Technol. A 5 ( 1987) 3287.

R. Hauert et al. / XPS investigation of the a-C: H/Al

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129

[16] R. Zehringer, R. Hauert and M. Tobler, Thin Solid Films 208 (1992) 38. [17] M. Kawasaki, G.J. Vandentop, M. Salmeron and G.A. Somotjai, Surf. Sci. 227 (1990) 261. 1181 R. Zehringer and R. Hauert, Surf. Sci. 262 (1992) 21. 1191R.S. Bonetti and M. Tobler, Metalloberfhiche 44 (1990) 209. [20] R. Hauert, J. Patscheider, R. Zehringer and M. Tobler, Thin Solid Films 206 (1991) 330. [21] E.R. Malinowski, Factor Analysis in Chemistry, 2nd ed. (Wiley, New York, 1991). [22] M.P. Seah, in: Practical Surface Analysis, Eds. D. Briggs and M.P. Seah (Wiley, New York, 1990) p. 135. [231 E. Cartier, P. Pfluger, J.-J. Pireaux and M. Rei Vilar, Appl. Phys. A 44 (1987) 43. [24] M.P. Seah and W A Dench, Surf. Interface Anal. 1 (1979) 2. [25] J.C. Angus, P. Koidl and S. Domitz, Plasma Deposited Thin Films, Eds. J. Mort and F. Jansen (CRC Press, Boca Raton, FL,, 1986) p. 89. [26] K. Kiihler, J.W. Coburn, D.E. Home and E. Kay, J. Appl. Phys. 57 (1985) 59. [271 F.A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry (Wiley, New York, 1975). [28] J.F. Durand, Bull. Sot. Chim. France (1924) 161, 1141.