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Comparative study of thin film physical properties for TiNx deposited by DC magnetron sputtering under temperatures less than 100°C on monocrystalline silicon and polycrystalline iron substrates
Thin Solid Films 355±356 (1999) 357±362 www.elsevier.com/locate/tsf
Comparative study of thin ®lm physical properties for TiNx deposited by DC magnetron sputtering under temperatures less than 1008C on monocrystalline silicon and polycrystalline iron substrates Ph. Roquiny a,*, A. Poulet b, Y. Leys b, J.-C. Descamps b, F. Bodart a, P. VandenBrande c a
FaculteÂs Universitaires Notre-Dame de la Paix, Laboratoire d'Analyses par ReÂactions NucleÂaires, rue de Bruxelles 61, B-5000 Namur, Belgium b Faculte Polytechnique de Mons, Service Science des MateÂriaux, rue de l'Epargne 56, B-7000 Mons, Belgium c Cockerill Sambre Research & Development Centre, Boulevard de Colonster B67, B-4000 LieÁge, Belgium
Abstract In this research, titanium nitride thin ®lms were deposited by reactive DC magnetron sputtering with two original constraints imposed by the expected industrial application as roll to roll decorative coating of steel: the samples should be connected to earth and should not be heated during deposition. Previous work has shown that low N2 atmospheres should be maintained during sputtering processes in order to obtain a colour range from grey to gold. In this study, the nitrogen content measured by Resonant Nuclear Reaction Analysis and the crystal structure revealed by glancing angle X-ray diffraction, were used to determine the coating composition. The appearance presented in CIE L*a*b* colour coordinates and the micro-hardness obtained with a Berkovitch nano-indenter were also evaluated for all the ®lms. Results obtained with the TiNx layers deposited on monocrystalline polished silicon were then compared to coating physical properties measured on polycrystalline iron (a -Fe) substrates. Only the coating produced under the lowest N2 gas ¯ow exhibits a different nitrogen content on both substrates resulting in an hexagonal phase on iron. Colour differences appear and are probably due to different substrate roughness. Hardnesses between 12 and 22 GPa are obtained on both substrates. Such results tend to qualify this process for possible industrial application as a protective coating with a pleasant appearance. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Titanium nitride thin ®lm; DC magnetron sputtering; Nitrogen content; Color range; Microhardness
1. Introduction Both E.C. and U.S. environmental statutes and regulations require material surface ®nishers to recycle water and liquid-metal baths and to ban the use of hazardous solvents [1,2]. Due to the growing environmental concern of the legislators, dry coating processes tend now to become more competitive than wet coating methods because the tightening of the released liquid by-products standards generates critical additional cost. Furthermore, surface ®nishing technologies such as physical or chemical vapour deposition (PVD or CVD) present high degree of experimental ¯exibility and can be used with a wider variety of material coatings. PVD decorative coating has been used industrially for about 20 years but is mainly limited to small parts such as watches, writing instruments or spectacle frames. Nevertheless, it seems that recent developments in reactive sputtering allows high rate coating of larger parts [3]. Waste-free steel * Corresponding author. Tel.: 132-81-725479; fax: 132-81-725474. E-mail address: [email protected] (P. Roquiny)
strip coating plant is nowadays conceivable to replace some industrial polluting wet processes. Pilot plants have already been studied by some authors [4,5]. The purpose of this study is to evaluate DC reactive magnetron sputtering technology with two original constraints due to the expected industrial application as roll to roll decorative coating of low carbon steel sold as pre-painted coil. The samples should be connected to earth and should not be heated during deposition. In other words, the layers should be deposited without atom mobility induced by either temperature (i.e. the heating) or by intensive ion bombardment (i.e. the sample biasing). As stated in the Sproul review [6], there is a need for work in this domain. The Group IV B transition metal nitrides offer an unusual combination of physical properties such as metallic electrical conductivity, extreme hardness, and chemical inertness. These valuable properties have been exploited for sputtered TiNx in many industrial ®elds. Examples include tool coatings for tribological wear resistance, anti-diffusion ®lms in micro-electronics, and decorative layers for their metallic appearance. It is, therefore, not surprising that a considerable amount of work has been concentrated on this material
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but generally using the biasing and heating facilities [7]. For this reason, titanium nitride coatings were chosen as a wellknown representative model system to begin this research [8]. To start investigations of thin ®lm physical properties, monocrystalline polished silicon wafers were used as easy to handle substrates. Previous work has shown that reproducible TiNx colour from metallic grey to gold and ®nally brownish red can be obtained on silicon. Nitrogen content in the gas discharge appears also to be the key parameter in colour control for TiNx sputtered ®lms especially under low N2 atmosphere [9]. In this study, the physical properties of titanium nitride prepared on monocrystalline Si(100) under different low nitrogen mass ¯ows are compared to the properties obtained on more realistic polycrystalline iron (a -Fe) substrates. The comparison between both substrates is then discussed in order to evaluate the application ®eld of these low temperature TiNx thin ®lms as decorative coating. 2. Experimental 2.1. Sample preparation TiNx coatings were produced by DC reactive magnetron sputtering. The vacuum chamber (ù 0:25 m 3) was equipped with a 19.6 cm 2 (50 mm diameter) magnetron cathode and a 200 l/min turbo molecular pump. The base pressure was less than 5 £ 10 24 Pa and the deposition working pressure and plasma power density were maintained respectively at 0.20 Pa and 11.7 W/cm 2. In these conditions, the sample's temperature measured with a thermocouple ®xed on the rear, did not exceed 1008C on grounded substrates located at 95 mm from the sputtering target. A study of the deposition rate versus nitrogen partial pressure of the incoming gas mass ¯ow is presented elsewhere [10]. Thicknesses are easily calculated with these calibrated deposition rates multiplied by the time. Firstly, conductive Sb n-doped polished monocrystalline silicon wafers (100) were coated to start the examination with an easy-to-handle and plane substrate. Secondly, TiNx layers were deposited on more realistic unpolished polycrystalline iron substrates (a -Fe). Both substrates were rinsed in acetone, then plasma etched for approximately 1 min at 0.4 Pa in pure argon atmosphere prior to nitride deposition. The nitrogen partial pressures were ®xed between 0.01 and 0.03 Pa during TiNx deposition. 2.2. Composition and structure The nitrogen content in the ®lms was determined using the well-known 15N(p,a g ) 12C nuclear reaction. At a proton energy of 429 keV, a sharp and very intense resonance occurs and can be revealed by the speci®c gamma emission. An automatic energy scan system mounted on a Van de Graaff accelerator is used to give the nitrogen concentration
depth pro®les by comparison with a standard sample [11± 13]. The structure and crystalline parameters of the layers were also analysed by glancing angle X-ray diffraction with Ê ) at an incidence angle of 28 Cu Ka radiation (1.5406 A selected on a Siemens D5000 u ±2u diffractometer [14]. 2.3. Colour Previous transmittance measurements of TiNx deposited onto glass substrates have shown that the coating appearance is governed by the nitride ®lm only and not by the substrate when the thickness of the nitride layer is greater than 100 nm [9]. Colour and gloss, which are the most important properties of decorative coatings, were studied by spectral re¯ectance spectroscopy with a Micro Color tristimulus colorimeter equipped with an Ulbricht globe coupled to a xenon ¯ash lamp for diffuse illumination of the sample. The light diffuse re¯ection from the sample was measured at an angle of 88 in accordance with the German industrial standard DIN5033 [15±18]. Results are presented in the physiologically relevant CIE L*a*b* colorimetric system: where the human colour perception is described by three colour coordinates: the lightness (L*), the redgreen value (a*) and the yellow-blue value (b*). 2.4. Micro-hardness Micro-hardness were also evaluated for all the ®lms. These results have been obtained with a depth-sensing Berkovitch diamond nano-indenter furnished by NanoInstruments. In this type of apparatus, the elastic properties are determined from continuous depth sensing load-displacement curves and not calculated on the basis of evaluation of the residual indentation area dimension. This kind of measurement allows more accurate micro-hardness calculations [19,20]. 3. Results 3.1. Elemental composition The nitrogen depth pro®les obtained by resonant nuclear reaction analysis (RNRA) present a fairly stable concentration plateau with sharp edges, and are proof of very good homogeneity within the layers. Since previous work have shown that the oxygen contamination (around 2 at.%) in the nitride does not vary with the N2 gas ¯ow (f N2) [9], the measurement of the nitrogen concentration is suf®cient to characterise the composition of the entire layer. Table 1 presents the nitrogen atomic concentration within the thin ®lms deposited under various f N2 on silicon and iron. The nitrogen content in the ®lm deposited under the poorest N2 atmosphere on iron substrates is signi®cantly lower than in the layers produced on silicon. Nevertheless, the nitrogen atomic fraction evolution versus f N2 increase is roughly the same on both substrates. Under fN2 0:5
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Table 1 Nitrogen atomic concentration measured by 15N(p,ag ) 12C RNRA within titanium nitride layer reactively sputtered under various low nitrogen mass ¯ow (f N2) on Si and Fe substrates
f N2 (sccm)
N content in TiNx/Si (at.%)
N content in TiNx/Fe (at.%)
0.5 1.5 2.5 3.5
22 39 38 48
15 42 39 44
sccm, the nitrogen content is very poor. For the largest f N2 values, the concentration increases and approaches TiN stoichiometry. 3.2. Structure Four 0.5-mm TiNx layers were been prepared with fN2 0:5, 1.5, 2.5 and 3.5 sccm on both substrates in order to determine their structure by glancing angle X-ray diffraction (GXRD). The GXRD spectra measured from the nitride ®lms deposited on monocrystalline Si(100) exhibit all the usual peaks of the NaCl-type cubic face centred lattice of the stoichiometric d -TiN [21] as shown in Fig. 1 for the sample prepared with fN2 2:5 sccm. The texture coef®cients [22] have been computed but they are not remarkable (i.e. included between 0.8 and 1.2) except when fN2 0:5 sccm. In this case, the (111) texture coef®cient reaches 2.6, indicating that the titanium nitride ®lm exhibited a (111) preferential orientation. For TiNx layers deposited on polycrystalline iron we observe that the sample produced with fN2 0:5 sccm gives a surprising diffracted signature of the hexagonal a Ti-N0.30 phase [23] as presented in Fig. 2a. The other ®lms produced under richer N2 atmospheres exhibit the same diffracted spectra as on Si(100) from the usual stoichiometric d -TiN cubic lattice without texture. Fig. 2b shows
Fig. 2. GXRD spectrum for 0.50 mm TiNx deposited on polycrystalline a Fe (p 0:20 Pa, P 11:7 W/cm 2) with fN2 0:5 sccm (a) and 2.5 sccm (b).
the GXRD spectrum obtained from the layer prepared on iron under fN2 2:5 sccm. 3.3. Colour
Fig. 1. GXRD spectrum for 0.50 mm TiNx (fN2 2:5 sccm N2, p 0:20 Pa, P 11:7 W/cm 2) deposited on monocrystalline Si(100).
The physiologically relevant CIE L*a*b* measurements from gold and from the layers deposited on both substrates are listed separately in Table 2. The sample produced with the poorest N2 atmosphere again appears different from the others, it has the highest lightness (L*). Fig. 3 shows the colour evolution in the a* (green-red) versus b* (blueyellow) axis. Older data [9] taken from samples prepared with a broader range of nitrogen gas ¯ow during TiNx deposition (light grey ®lled circles) and some interesting references (open grey triangles) are also presented in this Fig. 3. It can be seen from Fig. 3 that the new data for the coatings deposited on the polished silicon substrates describe, as do the older ones, a loop-curve in the a*, b* space as the nitrogen ¯ow increases. Starting from a grey metallic appearance, the ®lm colour becomes slightly yellow and
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Table 2 CIE L*a*b* colour coordinates measurement on 250-nm thick titanium nitride ®lms reactively sputtered under various low nitrogen mass ¯ow (f N2) on Si and Fe substrates; the L*a*b* values of gold are also indicated
f N2 (sccm)
TiNx/Si
TiNx/Fe
L*
a*
b*
L*
a*
b*
0.5 1.5 2.5 3.5
77.8 76.6 74.4 64.7
3.5 4.5 6.6 16.8
1.2 15.9 30.8 27.5
78.0 67.3 68.0 71.5
1.1 1.2 13.6 10.7
4.1 11.1 31.2 39.7
Gold
81.9
14.5
47.7
more copper-like as f N2 increases. A contrasting trend is observed for the titanium nitride deposited on non-polished polycrystalline iron: the colour jumps from grey towards a gold-like appearance when f N2 reaches 2.5 sccm. 3.4. Micro-hardness Fig. 4 shows the hardness as a function of the nano-indention depth measured for 250-nm thick TiNx samples prepared with different f N2 on monocrystalline Si(100). It is well known that this type of graph is representative of a thin hard coating and can be divided schematically into three sections [24]. On the left, typically under about a 50-nm sensing depth, the Berkovitch indentation diamond only begins to establish contact with the upper layer. When the diamond fully probes the thin ®lm (around 100 nm), the curve reaches its maximum value and this level characterises the micro-hardness of the ®lm. After this coating
Fig. 3. Chromatic diagram in the physiologically relevant a* (green-red axis), b* (blue-yellow) system (CIE L*a*b*) for 0.25 mm TiNx ®lms deposited on polished silicon wafers and on non polished iron substrates with f N2 between 0.5 and 3.5 sccm N2 (p 0:20 Pa, P 11:7 W/cm 2). Each point is labelled with the nitrogen mass ¯ow used during the deposition. The arrows illustrate the loop-curve evolution as f N2 increases already observed on older data (light grey ®lled circles [9]). Twenty-four carat gold, silicon, copper, steel and iron colour measurements are also plotted as reference data.
Fig. 4. Hardness versus Berkovitch indentation probe depth for 0.25 mm TiNx (p 0:20 Pa, P 11:7 W/cm 2) deposited on Si with various f N2.
sensing zone, the measured hardness decreases as the indentation probe enters within the layer and deforms the hard coating. The measured value then depends on the substrate. Afterwards, the indentation curve ®nally arrives at the substrate hardness: in this case, the hardness of silicon. The curves are similar for titanium nitride deposited under the same conditions on silicon or iron substrates. From this set of curves, the comparison on the TiNx hardness between both substrates is possible. Fig. 5 compares the titanium nitride ®lm hardness versus the nitrogen gas ¯ow used during the deposition on the silicon and the iron substrates. It can be observed on either Si or Fe that the hardness describes rather the same evolution if f N2 increases during the nitride deposition. The micro-hardness is maximised if the titanium nitride is produced with fN2 1:5 sccm. The ®lm hardness measured on polished silicon is between 14 and 18 GPa and between 12 and 22 GPa on non-polished iron.
Fig. 5. TiNx hardness measured with a Berkovitch nanoindenter as a function of f N2 for 0.25 mm ®lm on polished Si wafer and for 0.50 mm ®lm on non-polished Fe (p 0:20 Pa, P 11:7 W/cm 2).
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4. Discussion The most striking difference between the titanium nitride layers deposited on monocrystalline silicon wafers and on non polished iron occurs when f N2 0.5 sccm. A signi®cant nitrogen content loss is revealed within the layers deposited on Fe (15 at.% vs. 22 at.%) that leads to a phase distinct from the usual cubic d -TiN lattice. For the ®rst time, during this work on sputtered TiNx deposition at temperatures below 1008C and without negative substrate bias, the hexagonal a -Ti-N0.30 phase has been found (Fig. 2). According to the literature [25,26], some authors have distinguished that another hexagonal nitride phase 1 can be formed by reactive sputtering. Another group have also produced this 1 -Ti2N nitride by ion assisted reactive evaporation but the deposition conditions are very precise and dif®cult to reach [27]. Furthermore, this phase appears to be dif®cult to produce without heating and biasing the substrate. It is possible that the substrate and the small nitrogen content differences lead to this different phase under the poorest N2 atmosphere during the deposition. These results also exhibit the dif®culties of generation the hybrid metallic titanium phase with some nitrogen inclusion. This particular metallic distorted Ti lattice seems to be limited to very poor nitrogen concentration in the case where the substrate is not heated or biased. Surprisingly, except if fN2 0:5 sccm, the thin ®lms are not textured on both substrates. From the literature, a strong TiN (111) texture can appear at low substrate temperature either on silicon or on steel [28,29]. On the basis of the Quaeyhaegens model [30] and some other results [22±31] demonstrating that the texture evolves from random towards (111) with increasing layer thickness between 0.9 and 3.0 mm, the lack of a dominant plane re¯ection can be explained by the relatively thin thickness (0.50 mm) of the TiNx ®lms analysed by GXRD. The new colour data obtained on TiNx deposited on Si and presented in Fig. 3 con®rms the loop evolution as f N2 increases by reaching the intermediate colour between grey and gold as predicted in a previous publication on the basis of the Drude model [9]. However, it can be seen that 22 at.% nitrogen is not suf®cient to quit the metallic grey appearance. The colour control by way of N2 atmosphere seems to be more dif®cult when TiNx is produced on non polished polycrystalline iron. This difference is probably due to the substrate roughness. Nevertheless, the reference values measured from non polished gold are almost reached with fN2 2:5 or 3.5 sccm. To avoid the dif®culties encountered with the TiNx/Fe samples if f N2 varies, a new colour control study based on the thickness variation of one gold-like titanium nitride layer is currently in progress. As [N] increases when f N2 grows, then the same microhardness evolution shape as on Fig. 5 can be seen in the Sundgren reviews [25,26]: starting from approximately 15 GPa, the hardness reaches a maximum around 20 GPa and ®nally decreases when [N] grows. Values within the same
361
range have been reported in the literature, 20 GPa or 14 GPa, respectively, if the TiNx coatings are produced under 1008C [32] or without bias [33]. The titanium nitride ®lm produced can then be quali®ed as hard coating on either substrates. To be decorative, the thin ®lm should not only have a pleasant appearance, but should also be protective. It is obvious that the high nano-hardness will prevent scratching, but to complete this study and qualify the TiNx as a decorative coating, the layer should also resist the stamping of the steel strip. Further tribological properties analyses are in progress. On the other hand, the ®rst corrosion results that will be published later indicate that these thin ®lms are unfortunately not chemically protective. A corrosion barrier should then be prepared between the steel substrate and the decorative TiNx deposited below 1008C and without bias. 5. Conclusion In this work, TiNx thin ®lms has been deposited by reactive magnetron sputtering under different N2 atmosphere without heating or bias inducing additional atom mobility on monocrystalline Si(100) wafers and on polycrystalline iron. An effort has been made to analyse differences in composition, structure, colour and nano-hardness in order to reach the properties expected for decorative and protective coatings. No signi®cant differences were measured between the two substrates except for the samples produced with fN2 0:5 sccm on iron where the presence of the hexagonal a -TiN0.30 phase instead of cubic d -TiN can be correlated with a small loss of nitrogen concentration. The 1 -Ti2N nitride phase seems to be impossible to obtain below 1008C and without bias. The colour and the high nano-hardness (between 12 and 22 GPa) are roughly the same on both substrates. Nevertheless, the transition between grey and gold colour as f N2 increases is more critical on iron than on silicon. The colour control on iron should be achieved with a parameter other than f N2, such as the thickness of the nitride coating. Further mechanical and electrochemical tests are in progress to qualify this low temperature titanium nitride ®lm as an interesting decorative layer. This work has also reinforced the role of monocrystalline silicon wafer as an easy-to-handle and advantageous model substrate to start the characterisation of dry coated thin ®lm whenever possible. In the near future, the characterisation of coatings with other colours will start on this substrate to wider the available colour palette. Acknowledgements This work is supported by the MinisteÁre de la ReÂgion Wallonne (Belgium) within the FISRT Program (Programme de Formation et d'Impulsion aÁ la Recherche
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Scienti®que et Technologique) and by the Research and Development Centre of Cockerill Sambre. References [1] D.J. Smukowski, in: C.M. Cotell, J.A. Sprague, F.A. Smith Jr. (Eds.), Surface Engineering ASM Handbook, Vol. 5, ASM International, Materials Park, OH, 1994, p. 911. [2] Commission of the European Communities, Proposal for a council directive establishing the framework for Community action in the ®eld of water policy, in: Of®cial Journal of the European Communities C series 184/97, Strasbourg, 1997, p. 20. [3] M. Maidhof, D. Hofmann, S. Hunkel, H. Schluessler, in: 39th Annual Technical Conference Proceedings, Society of Vacuum Coaters, Albuquerque, 1996, p. 109. [4] Y. Miyamoto, Y. Kubo, N. Ono, M. Hasimoto, T. Takahashi, I. Ito, F. Arrezzo, P. Gimondo, Thin Solid Films 270 (1995) 253. [5] M. Hasimoto, Y. Miyamoto, Y. Kubo, S. Tokomaru, N. Ono, T. Takahashi, I. Ito, Mater. Sci. Eng. A198 (1995) 75. [6] W.D. Sproul, J. Vac. Sci. Technol. A 12 (1994) 1595. [7] C.G. Ribbing, A. Roos, in: R.L. Hall (Ed.), Proceedings of SPIEE, 3133, Belligham, 1997, pp. 148. [8] R. Roth, J. Schubert, E. Fromm, Surf. Coat. Technol. 74±75 (1995) 461. [9] Ph. Roquiny, F. Bodart, G. Terwagne, in: 6th Int. Conf. On Plasma Surface Engineering Proceedings, Garmish-Partenkirchen, 1998 (accepted). [10] Ph. Roquiny, F. Bodart, S. Lucas, G. Terwagne, in: 11th Int. Colloquium on Plasma Processes CIP97 Proceedings, Le Mans, 1997, p. 28. [11] B. Maurel, G. Amsel, Nucl. Instrum. Methods 218 (1983) 159. [12] G. Terwagne, M. Piette, F. Bodart, Nucl. Instrum. Methods B 19±20 (1987) 145. [13] G. Terwagne, S. Lucas, F. Bodart, Nucl. Instrum. Methods B 66 (1992) 262.
[14] C. Quaeyhaegens, M. Van Stappen, L.M. Stals, F. Bodart, G. Terwagne, R. Vlaeminck, Surf. Coat. Technol. 54-55 (1992) 279. [15] G. Reiners, H. Hantsche, H.A. Jehn, U. Kopacz, A. Rack, Surf. Coat. Technol. 54±55 (1992) 273. [16] H.A. Jehn, in: Y. Pauleau (Ed.), Materials and Processes for Surface and Interface Engineering, Kluwer Academic, Dordrecht, 1995, pp. 359. [17] ISO 7724/1,2,3 ± (1984) E. [18] Y. Dordet, La Colorimetrie, Principes et Applications, Eyrolles, Paris, 1990. [19] D.L. Joslin, W.C. Oliver, J. Mater. Res. 5 (1990) 123. [20] W.C. Oliver, G.M. Phar, J. Mater. Res. 7 (1992) 1564. [21] J.C.P.D.S. Powder Diffraction File, number 38-1420, International Centre for Diffraction Data, Swarthmore, 1995. [22] D.S. Rickerby, A.M. Jones, B.A. Bellamy, Surf. Coat. Technol. 37 (1989) 111. [23] J.C.P.D.S. Powder Diffraction File, number 41-1352, International Centre for Diffraction Data, Swarthmore, 1995. [24] A.M. Korsunsky, M.R. McGursk, S.J. Bull, T.F. Page, Surf. Coat. Technol. 99 (1998) 171. [25] J.-E. Sundgren, Thin Solid Films 128 (1985) 21. [26] J.-E. Sundgren, T.G. Hentzell, J. Vac. Sci. Technol. A 4 (1986) 2259. [27] M. Van Stappen, K. De Bruyn, C. Quaeyhaegens, L.M. Stals, V. Poulek, Surf. Coat. Technol. 74-75 (1995) 143. [28] P. Panjan, B. Navinsek, A. Zabkar, V. Marinkovic, D. Mandrino, J. Fiser, Thin Solid Films 228 (1993) 233. [29] H. Wegenmair, J.W. Gerlach, U. Preckwinkel, B. Stritzker, B. Rauschenbach, Nucl. Instr. Methods B 113 (1996) 196. [30] C. Quaeyhaegens, G. Knuyt, J. D'Haen, L.M. Stals, Thin Solid Films 258 (1995) 170. [31] I. Grimberg, B. Boua®, U. Draugelates, K. Soifer, B.Z. Weiss, Surf. Coat. Technol. 68±69 (1994) 166. [32] R. Gahlin, M. Bromark, P. Hedenqvist, S. Hogmark, G. Hakasson, Surf. Coat. Technol. 76±77 (1995) 174. [33] W.D. Sproul, P.J. Rudnik, M.E. Graham, Surf. Coat. Technol. 39±40 (1989) 355.
Comparative study of thin ®lm physical properties for TiNx deposited by DC magnetron sputtering under temperatures less than 1008C on monocrystalline silicon and polycrystalline iron substrates Ph. Roquiny a,*, A. Poulet b, Y. Leys b, J.-C. Descamps b, F. Bodart a, P. VandenBrande c a
FaculteÂs Universitaires Notre-Dame de la Paix, Laboratoire d'Analyses par ReÂactions NucleÂaires, rue de Bruxelles 61, B-5000 Namur, Belgium b Faculte Polytechnique de Mons, Service Science des MateÂriaux, rue de l'Epargne 56, B-7000 Mons, Belgium c Cockerill Sambre Research & Development Centre, Boulevard de Colonster B67, B-4000 LieÁge, Belgium
Abstract In this research, titanium nitride thin ®lms were deposited by reactive DC magnetron sputtering with two original constraints imposed by the expected industrial application as roll to roll decorative coating of steel: the samples should be connected to earth and should not be heated during deposition. Previous work has shown that low N2 atmospheres should be maintained during sputtering processes in order to obtain a colour range from grey to gold. In this study, the nitrogen content measured by Resonant Nuclear Reaction Analysis and the crystal structure revealed by glancing angle X-ray diffraction, were used to determine the coating composition. The appearance presented in CIE L*a*b* colour coordinates and the micro-hardness obtained with a Berkovitch nano-indenter were also evaluated for all the ®lms. Results obtained with the TiNx layers deposited on monocrystalline polished silicon were then compared to coating physical properties measured on polycrystalline iron (a -Fe) substrates. Only the coating produced under the lowest N2 gas ¯ow exhibits a different nitrogen content on both substrates resulting in an hexagonal phase on iron. Colour differences appear and are probably due to different substrate roughness. Hardnesses between 12 and 22 GPa are obtained on both substrates. Such results tend to qualify this process for possible industrial application as a protective coating with a pleasant appearance. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Titanium nitride thin ®lm; DC magnetron sputtering; Nitrogen content; Color range; Microhardness
1. Introduction Both E.C. and U.S. environmental statutes and regulations require material surface ®nishers to recycle water and liquid-metal baths and to ban the use of hazardous solvents [1,2]. Due to the growing environmental concern of the legislators, dry coating processes tend now to become more competitive than wet coating methods because the tightening of the released liquid by-products standards generates critical additional cost. Furthermore, surface ®nishing technologies such as physical or chemical vapour deposition (PVD or CVD) present high degree of experimental ¯exibility and can be used with a wider variety of material coatings. PVD decorative coating has been used industrially for about 20 years but is mainly limited to small parts such as watches, writing instruments or spectacle frames. Nevertheless, it seems that recent developments in reactive sputtering allows high rate coating of larger parts [3]. Waste-free steel * Corresponding author. Tel.: 132-81-725479; fax: 132-81-725474. E-mail address: [email protected] (P. Roquiny)
strip coating plant is nowadays conceivable to replace some industrial polluting wet processes. Pilot plants have already been studied by some authors [4,5]. The purpose of this study is to evaluate DC reactive magnetron sputtering technology with two original constraints due to the expected industrial application as roll to roll decorative coating of low carbon steel sold as pre-painted coil. The samples should be connected to earth and should not be heated during deposition. In other words, the layers should be deposited without atom mobility induced by either temperature (i.e. the heating) or by intensive ion bombardment (i.e. the sample biasing). As stated in the Sproul review [6], there is a need for work in this domain. The Group IV B transition metal nitrides offer an unusual combination of physical properties such as metallic electrical conductivity, extreme hardness, and chemical inertness. These valuable properties have been exploited for sputtered TiNx in many industrial ®elds. Examples include tool coatings for tribological wear resistance, anti-diffusion ®lms in micro-electronics, and decorative layers for their metallic appearance. It is, therefore, not surprising that a considerable amount of work has been concentrated on this material
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00460-5
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but generally using the biasing and heating facilities [7]. For this reason, titanium nitride coatings were chosen as a wellknown representative model system to begin this research [8]. To start investigations of thin ®lm physical properties, monocrystalline polished silicon wafers were used as easy to handle substrates. Previous work has shown that reproducible TiNx colour from metallic grey to gold and ®nally brownish red can be obtained on silicon. Nitrogen content in the gas discharge appears also to be the key parameter in colour control for TiNx sputtered ®lms especially under low N2 atmosphere [9]. In this study, the physical properties of titanium nitride prepared on monocrystalline Si(100) under different low nitrogen mass ¯ows are compared to the properties obtained on more realistic polycrystalline iron (a -Fe) substrates. The comparison between both substrates is then discussed in order to evaluate the application ®eld of these low temperature TiNx thin ®lms as decorative coating. 2. Experimental 2.1. Sample preparation TiNx coatings were produced by DC reactive magnetron sputtering. The vacuum chamber (ù 0:25 m 3) was equipped with a 19.6 cm 2 (50 mm diameter) magnetron cathode and a 200 l/min turbo molecular pump. The base pressure was less than 5 £ 10 24 Pa and the deposition working pressure and plasma power density were maintained respectively at 0.20 Pa and 11.7 W/cm 2. In these conditions, the sample's temperature measured with a thermocouple ®xed on the rear, did not exceed 1008C on grounded substrates located at 95 mm from the sputtering target. A study of the deposition rate versus nitrogen partial pressure of the incoming gas mass ¯ow is presented elsewhere [10]. Thicknesses are easily calculated with these calibrated deposition rates multiplied by the time. Firstly, conductive Sb n-doped polished monocrystalline silicon wafers (100) were coated to start the examination with an easy-to-handle and plane substrate. Secondly, TiNx layers were deposited on more realistic unpolished polycrystalline iron substrates (a -Fe). Both substrates were rinsed in acetone, then plasma etched for approximately 1 min at 0.4 Pa in pure argon atmosphere prior to nitride deposition. The nitrogen partial pressures were ®xed between 0.01 and 0.03 Pa during TiNx deposition. 2.2. Composition and structure The nitrogen content in the ®lms was determined using the well-known 15N(p,a g ) 12C nuclear reaction. At a proton energy of 429 keV, a sharp and very intense resonance occurs and can be revealed by the speci®c gamma emission. An automatic energy scan system mounted on a Van de Graaff accelerator is used to give the nitrogen concentration
depth pro®les by comparison with a standard sample [11± 13]. The structure and crystalline parameters of the layers were also analysed by glancing angle X-ray diffraction with Ê ) at an incidence angle of 28 Cu Ka radiation (1.5406 A selected on a Siemens D5000 u ±2u diffractometer [14]. 2.3. Colour Previous transmittance measurements of TiNx deposited onto glass substrates have shown that the coating appearance is governed by the nitride ®lm only and not by the substrate when the thickness of the nitride layer is greater than 100 nm [9]. Colour and gloss, which are the most important properties of decorative coatings, were studied by spectral re¯ectance spectroscopy with a Micro Color tristimulus colorimeter equipped with an Ulbricht globe coupled to a xenon ¯ash lamp for diffuse illumination of the sample. The light diffuse re¯ection from the sample was measured at an angle of 88 in accordance with the German industrial standard DIN5033 [15±18]. Results are presented in the physiologically relevant CIE L*a*b* colorimetric system: where the human colour perception is described by three colour coordinates: the lightness (L*), the redgreen value (a*) and the yellow-blue value (b*). 2.4. Micro-hardness Micro-hardness were also evaluated for all the ®lms. These results have been obtained with a depth-sensing Berkovitch diamond nano-indenter furnished by NanoInstruments. In this type of apparatus, the elastic properties are determined from continuous depth sensing load-displacement curves and not calculated on the basis of evaluation of the residual indentation area dimension. This kind of measurement allows more accurate micro-hardness calculations [19,20]. 3. Results 3.1. Elemental composition The nitrogen depth pro®les obtained by resonant nuclear reaction analysis (RNRA) present a fairly stable concentration plateau with sharp edges, and are proof of very good homogeneity within the layers. Since previous work have shown that the oxygen contamination (around 2 at.%) in the nitride does not vary with the N2 gas ¯ow (f N2) [9], the measurement of the nitrogen concentration is suf®cient to characterise the composition of the entire layer. Table 1 presents the nitrogen atomic concentration within the thin ®lms deposited under various f N2 on silicon and iron. The nitrogen content in the ®lm deposited under the poorest N2 atmosphere on iron substrates is signi®cantly lower than in the layers produced on silicon. Nevertheless, the nitrogen atomic fraction evolution versus f N2 increase is roughly the same on both substrates. Under fN2 0:5
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Table 1 Nitrogen atomic concentration measured by 15N(p,ag ) 12C RNRA within titanium nitride layer reactively sputtered under various low nitrogen mass ¯ow (f N2) on Si and Fe substrates
f N2 (sccm)
N content in TiNx/Si (at.%)
N content in TiNx/Fe (at.%)
0.5 1.5 2.5 3.5
22 39 38 48
15 42 39 44
sccm, the nitrogen content is very poor. For the largest f N2 values, the concentration increases and approaches TiN stoichiometry. 3.2. Structure Four 0.5-mm TiNx layers were been prepared with fN2 0:5, 1.5, 2.5 and 3.5 sccm on both substrates in order to determine their structure by glancing angle X-ray diffraction (GXRD). The GXRD spectra measured from the nitride ®lms deposited on monocrystalline Si(100) exhibit all the usual peaks of the NaCl-type cubic face centred lattice of the stoichiometric d -TiN [21] as shown in Fig. 1 for the sample prepared with fN2 2:5 sccm. The texture coef®cients [22] have been computed but they are not remarkable (i.e. included between 0.8 and 1.2) except when fN2 0:5 sccm. In this case, the (111) texture coef®cient reaches 2.6, indicating that the titanium nitride ®lm exhibited a (111) preferential orientation. For TiNx layers deposited on polycrystalline iron we observe that the sample produced with fN2 0:5 sccm gives a surprising diffracted signature of the hexagonal a Ti-N0.30 phase [23] as presented in Fig. 2a. The other ®lms produced under richer N2 atmospheres exhibit the same diffracted spectra as on Si(100) from the usual stoichiometric d -TiN cubic lattice without texture. Fig. 2b shows
Fig. 2. GXRD spectrum for 0.50 mm TiNx deposited on polycrystalline a Fe (p 0:20 Pa, P 11:7 W/cm 2) with fN2 0:5 sccm (a) and 2.5 sccm (b).
the GXRD spectrum obtained from the layer prepared on iron under fN2 2:5 sccm. 3.3. Colour
Fig. 1. GXRD spectrum for 0.50 mm TiNx (fN2 2:5 sccm N2, p 0:20 Pa, P 11:7 W/cm 2) deposited on monocrystalline Si(100).
The physiologically relevant CIE L*a*b* measurements from gold and from the layers deposited on both substrates are listed separately in Table 2. The sample produced with the poorest N2 atmosphere again appears different from the others, it has the highest lightness (L*). Fig. 3 shows the colour evolution in the a* (green-red) versus b* (blueyellow) axis. Older data [9] taken from samples prepared with a broader range of nitrogen gas ¯ow during TiNx deposition (light grey ®lled circles) and some interesting references (open grey triangles) are also presented in this Fig. 3. It can be seen from Fig. 3 that the new data for the coatings deposited on the polished silicon substrates describe, as do the older ones, a loop-curve in the a*, b* space as the nitrogen ¯ow increases. Starting from a grey metallic appearance, the ®lm colour becomes slightly yellow and
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Table 2 CIE L*a*b* colour coordinates measurement on 250-nm thick titanium nitride ®lms reactively sputtered under various low nitrogen mass ¯ow (f N2) on Si and Fe substrates; the L*a*b* values of gold are also indicated
f N2 (sccm)
TiNx/Si
TiNx/Fe
L*
a*
b*
L*
a*
b*
0.5 1.5 2.5 3.5
77.8 76.6 74.4 64.7
3.5 4.5 6.6 16.8
1.2 15.9 30.8 27.5
78.0 67.3 68.0 71.5
1.1 1.2 13.6 10.7
4.1 11.1 31.2 39.7
Gold
81.9
14.5
47.7
more copper-like as f N2 increases. A contrasting trend is observed for the titanium nitride deposited on non-polished polycrystalline iron: the colour jumps from grey towards a gold-like appearance when f N2 reaches 2.5 sccm. 3.4. Micro-hardness Fig. 4 shows the hardness as a function of the nano-indention depth measured for 250-nm thick TiNx samples prepared with different f N2 on monocrystalline Si(100). It is well known that this type of graph is representative of a thin hard coating and can be divided schematically into three sections [24]. On the left, typically under about a 50-nm sensing depth, the Berkovitch indentation diamond only begins to establish contact with the upper layer. When the diamond fully probes the thin ®lm (around 100 nm), the curve reaches its maximum value and this level characterises the micro-hardness of the ®lm. After this coating
Fig. 3. Chromatic diagram in the physiologically relevant a* (green-red axis), b* (blue-yellow) system (CIE L*a*b*) for 0.25 mm TiNx ®lms deposited on polished silicon wafers and on non polished iron substrates with f N2 between 0.5 and 3.5 sccm N2 (p 0:20 Pa, P 11:7 W/cm 2). Each point is labelled with the nitrogen mass ¯ow used during the deposition. The arrows illustrate the loop-curve evolution as f N2 increases already observed on older data (light grey ®lled circles [9]). Twenty-four carat gold, silicon, copper, steel and iron colour measurements are also plotted as reference data.
Fig. 4. Hardness versus Berkovitch indentation probe depth for 0.25 mm TiNx (p 0:20 Pa, P 11:7 W/cm 2) deposited on Si with various f N2.
sensing zone, the measured hardness decreases as the indentation probe enters within the layer and deforms the hard coating. The measured value then depends on the substrate. Afterwards, the indentation curve ®nally arrives at the substrate hardness: in this case, the hardness of silicon. The curves are similar for titanium nitride deposited under the same conditions on silicon or iron substrates. From this set of curves, the comparison on the TiNx hardness between both substrates is possible. Fig. 5 compares the titanium nitride ®lm hardness versus the nitrogen gas ¯ow used during the deposition on the silicon and the iron substrates. It can be observed on either Si or Fe that the hardness describes rather the same evolution if f N2 increases during the nitride deposition. The micro-hardness is maximised if the titanium nitride is produced with fN2 1:5 sccm. The ®lm hardness measured on polished silicon is between 14 and 18 GPa and between 12 and 22 GPa on non-polished iron.
Fig. 5. TiNx hardness measured with a Berkovitch nanoindenter as a function of f N2 for 0.25 mm ®lm on polished Si wafer and for 0.50 mm ®lm on non-polished Fe (p 0:20 Pa, P 11:7 W/cm 2).
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4. Discussion The most striking difference between the titanium nitride layers deposited on monocrystalline silicon wafers and on non polished iron occurs when f N2 0.5 sccm. A signi®cant nitrogen content loss is revealed within the layers deposited on Fe (15 at.% vs. 22 at.%) that leads to a phase distinct from the usual cubic d -TiN lattice. For the ®rst time, during this work on sputtered TiNx deposition at temperatures below 1008C and without negative substrate bias, the hexagonal a -Ti-N0.30 phase has been found (Fig. 2). According to the literature [25,26], some authors have distinguished that another hexagonal nitride phase 1 can be formed by reactive sputtering. Another group have also produced this 1 -Ti2N nitride by ion assisted reactive evaporation but the deposition conditions are very precise and dif®cult to reach [27]. Furthermore, this phase appears to be dif®cult to produce without heating and biasing the substrate. It is possible that the substrate and the small nitrogen content differences lead to this different phase under the poorest N2 atmosphere during the deposition. These results also exhibit the dif®culties of generation the hybrid metallic titanium phase with some nitrogen inclusion. This particular metallic distorted Ti lattice seems to be limited to very poor nitrogen concentration in the case where the substrate is not heated or biased. Surprisingly, except if fN2 0:5 sccm, the thin ®lms are not textured on both substrates. From the literature, a strong TiN (111) texture can appear at low substrate temperature either on silicon or on steel [28,29]. On the basis of the Quaeyhaegens model [30] and some other results [22±31] demonstrating that the texture evolves from random towards (111) with increasing layer thickness between 0.9 and 3.0 mm, the lack of a dominant plane re¯ection can be explained by the relatively thin thickness (0.50 mm) of the TiNx ®lms analysed by GXRD. The new colour data obtained on TiNx deposited on Si and presented in Fig. 3 con®rms the loop evolution as f N2 increases by reaching the intermediate colour between grey and gold as predicted in a previous publication on the basis of the Drude model [9]. However, it can be seen that 22 at.% nitrogen is not suf®cient to quit the metallic grey appearance. The colour control by way of N2 atmosphere seems to be more dif®cult when TiNx is produced on non polished polycrystalline iron. This difference is probably due to the substrate roughness. Nevertheless, the reference values measured from non polished gold are almost reached with fN2 2:5 or 3.5 sccm. To avoid the dif®culties encountered with the TiNx/Fe samples if f N2 varies, a new colour control study based on the thickness variation of one gold-like titanium nitride layer is currently in progress. As [N] increases when f N2 grows, then the same microhardness evolution shape as on Fig. 5 can be seen in the Sundgren reviews [25,26]: starting from approximately 15 GPa, the hardness reaches a maximum around 20 GPa and ®nally decreases when [N] grows. Values within the same
361
range have been reported in the literature, 20 GPa or 14 GPa, respectively, if the TiNx coatings are produced under 1008C [32] or without bias [33]. The titanium nitride ®lm produced can then be quali®ed as hard coating on either substrates. To be decorative, the thin ®lm should not only have a pleasant appearance, but should also be protective. It is obvious that the high nano-hardness will prevent scratching, but to complete this study and qualify the TiNx as a decorative coating, the layer should also resist the stamping of the steel strip. Further tribological properties analyses are in progress. On the other hand, the ®rst corrosion results that will be published later indicate that these thin ®lms are unfortunately not chemically protective. A corrosion barrier should then be prepared between the steel substrate and the decorative TiNx deposited below 1008C and without bias. 5. Conclusion In this work, TiNx thin ®lms has been deposited by reactive magnetron sputtering under different N2 atmosphere without heating or bias inducing additional atom mobility on monocrystalline Si(100) wafers and on polycrystalline iron. An effort has been made to analyse differences in composition, structure, colour and nano-hardness in order to reach the properties expected for decorative and protective coatings. No signi®cant differences were measured between the two substrates except for the samples produced with fN2 0:5 sccm on iron where the presence of the hexagonal a -TiN0.30 phase instead of cubic d -TiN can be correlated with a small loss of nitrogen concentration. The 1 -Ti2N nitride phase seems to be impossible to obtain below 1008C and without bias. The colour and the high nano-hardness (between 12 and 22 GPa) are roughly the same on both substrates. Nevertheless, the transition between grey and gold colour as f N2 increases is more critical on iron than on silicon. The colour control on iron should be achieved with a parameter other than f N2, such as the thickness of the nitride coating. Further mechanical and electrochemical tests are in progress to qualify this low temperature titanium nitride ®lm as an interesting decorative layer. This work has also reinforced the role of monocrystalline silicon wafer as an easy-to-handle and advantageous model substrate to start the characterisation of dry coated thin ®lm whenever possible. In the near future, the characterisation of coatings with other colours will start on this substrate to wider the available colour palette. Acknowledgements This work is supported by the MinisteÁre de la ReÂgion Wallonne (Belgium) within the FISRT Program (Programme de Formation et d'Impulsion aÁ la Recherche
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