Growth characteristics of tungsten-carbon films deposited by magnetron sputtering E. Harry a,*, Y. Pauleau b, M. Adamik ‘, P.B. Barna ‘, A. Sulyok ‘, M. Menyhard
a CEA-Grenoble, CEREM-DEM-SGM, 17, rue des Murtyrs, 38054 Grenoble, Cedex 9, France b Nutional Polytechnic Institute oj’Grenoble, ENSEEG, B. P. 75, 38402 Sain Martin JHeres. Cedex. France ’ Research Institute for Technical Physics, H-1325 Budapest, PO Box 76, Hungary
Abstract Tungsten and tungsten-carbon films have been deposited by direct and reactive magnetron sputtering in pure Ar and Ar-CH, mixtures, respectively. The structural properties of W and W-C/substrate samples were investigated by cross-sectional transmission electron microscopy, concentrating on the structure of the interface regions. Both pure tungsten and tungsten-carbon films exhibit a regular columnar structure, but in the case of carbon-doped films, growth begins by the formation of a nanocrystalline cc-W structure. At a certain film thickness, depending on the amount of the incorporated carbon, the growth of the films changes to the formation of a polycrystalline columnar structure. The carbon content is higher in the nanocrystalline part of the film, which is always present independently of the applied substrate material. 0 1998 Published by Elsevier Science S.A. Keywords:
Growth characteristics:Magnetron sputtering;Tungsten-carbon
1. Introduction Tungsten-carbon protective coatings have been intensively studied in the last few years [l-5] and shown to have remarkable erosion-resistant properties [ 51,making them suitable for industrial applications. The former investigations concentrated on physical properties like microhardness, residual stresses,electrical resistivity etc., and on the structure of film determined by X-ray diffraction technique and other analytical methods. Theseinvestigations provided a detailed and comprehensive understanding of the properties and growth of these films at various carbon concentrations and deposition conditions. The aim of the present work was to complete this knowledge using transmission electron microscopic inves-tigations of cross-sectional samples( X-TEM ). In order to reveal the homogeneity of the carbon concentration along the thickness of the films, in-depth Auger electron spectroscopic analyseshave also beenperformed.
2. Experimental Tungsten and tungsten-carbon thin films were deposited by reactive d.c. magnetron sputtering using a planar * Corresponding
0257-8972/98/$19.00 0 1998 Published PII SO257-8972(97)00635-X
target (21 x 9) cm2 and a pure argon or an argon-methane mixture as the sputtering gas. The films were deposited on Si( 100) single crystal substrates as well as stainless steel and titanium alloy (TA6V) substrates polished by a fine diamond paste. The deposition chamber was pumped down to a pressure of 2 x 10S6mbar, using a turbomolecular pump. Prior to deposition, the substrate was polarised at - 1600V and was ion-etched for 40 min, and the target was cleaned for a few minutes. A thermocouple was bonded to the stainless steel substrate for temperature measurement. During sputter deposition, the substrate temperature could rise up to 250 “C in a few minutes. The pressure of the sputtering gas was maintained at 2 Pa for pure argon and 2.12 Pa for the argon-methane mixtures. The argon and methane flow rates were fixed at 90 and 12 cm3 min- ‘, rates were respectively. Typical deposition 150 nm min-’ for pure W and 140 nm min-’ for W-C films at a d.c. power density of 5.3 W cme2. The structure of the films was investigated in crosssection using transmission electron microscopy, applying bright field and dark imaging as well as selected area electron diffraction. The cross-sectional samples were prepared by glancing incidence ion beam thinning. The Auger depth profiles were taken using our dedicated depth-profiling device . The Auger peaks ( W
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peak at 179 eV and C peak at 272 eV ) were in derivative mode. Ion-sputtering was carried out on rotated samples (at a speed of 10 r.p.m.). The incidence angle of At-+ ions was 84 ” relative to the surface normal, the incidence energy was 2000 eV. The measured concentration is an average value on the analysed spot of 0.04 mm and depth corresponding to the escape depth of Auger electrons (it is below 1 nm in our case). These conditions enable us to observe a depth profile with a depth resolution better than 5 nm . The global carbon concentration of W-C thin films sputter-deposited on Si substrate was measured by nuclear reaction analysis on the basis of the “C(d p)13C nuclear reaction with a deuteron beam energy of 1 MeV. The backscattered particles were detected at a scattering angle of 165 ‘. The residual stresses in films deposited on Si substrates were determined by measuring the changes in the radius of curvature of substrates, using the Stoney formula [ 81.
( 1998) X-294
3. Results and discussion The X-TEM investigations of pure tungsten film show that the structure is cx-W, and the morphology is homogeneous along the thickness (Fig. 1). The film consists of columns 60-70 nm in medium diameter. The substrate temperature corresponds to the 0.1-O. 13 T, (where T, is the melting point of the bulk tungsten) at which the film should grow according to zone 1 of the structure zone model [ 91. The weak 110 texture, however, and the slight conical shape of the columns indicate that competitive growth is also present, which is characteristic of zone T [lo]. W-C films exhibit an inhomogeneous morphology and composition along the thickness. The upper part of the film is polycrystalline, whereas the lower part is nanocrystalline Y-W (Fig. 2a), indicated by the selected area electron diffraction. The thickness of the nanocrystalline part is smaller in the sample deposited with a Fig. 2. X-TEM bright field image of a W--C film with 14at.s C (a) sputter-deposited on Si substrate; the insert shows the SAED pattern of the nanocrystalline part (b) image of the nanocrystalline-TA6V interface.
Fig. 1. X-TEM dark field image of a W film sputter-deposited substrate. The insert shows the respective SAED pattern.
lower CH, flow rate, which suggests that its thickness depends on the carbon concentration that could be incorporated into the films. At the same average carbon concentration (14 at.% C). the thickness of the nanocrystallyne part is smaller in samples deposited on to Si substrates; namely, this thickness is 350, 640 and 620 nm on Si, steel and TA6V substrates, respectively. The interface between the nanocrystalline and polycrystalline part is rather rough, and shows a saw-toothed morphology in cross-section (Fig. 2a). The thickness of the interface is about 80 nm. The Auger in-depth profile
rt al. 1 Surjacr
"m L 0.6 F 8c 0.4 8 0.2
sputtering Fig. 3. Auger depth on Si substrate.
of a W-C
(7 at% C ) film sputter-deposited
shows that the carbon concentration is higher in the lower part of the film (Fig. 3). This result indicates that the formation of the nanocrystalline structure can be related to a greater amount of carbon incorporation into the film during the initial stage of film formation. This could be obtained by the process-induced segregation of carbon on the surface of tungsten crystallites. At high carbon concentrations, the carbon can form a continuous covering layer on the surfacesof the tungsten crystallites, thereby blocking their growth. This phenomenon is responsible for the formation of nanocrystalline films in the case of many other materials [lo]. The change from a nanocrystalline to a columnar polycrystalline structure can be related to a decreased concentration of incorporated carbon and/or to stresses developing in the film during growth. The residual stresses of pure tungsten and tungsten-carbon films deposited on to Si substrates are compressive, - 1.5 GPa and - 1.4 GPa for W and W-C films, respectively. Thermal stressesare insignificant in films deposited on Si substrates. The level of intrinsic stress in films deposited on stainless steel and TA6V substrates were calculated from the algebraic difference of the residual stressesdetermined previously and thermal stressof films deposited on stainlesssteel and TA6V substrates. Thermal stresseswere induced by the large difference in dilatation coefficients of the film and the substrate. The value of thermal stressoth is given by the following expression [ 111:
temperature, respectively. The values 400 GPa and 0.28 , 4.5 x 1O-6 K-l, 15 x 10e6 K-l and 8 x 10e6 K-r [ 121were assumedfor E,, vf, aw ustee,and mTA6v,respectively. The level of thermal stressesin coatings deposited on steel substrates was found to be - 1.3 GPa. On TA6V substrate, it was in the order of -0.5 GPa. The residual compressive stressesin the tungsten and tungsten-carbon coatings were in the order of -2.8 GPa and -2 GPa for films deposited on steel substrate and TA6V substrate, respectively; therefore, the intrinsic stressesare about - 1.5 GPa, similar to those in films deposited on Si substrates. These results have been confirmed by X-ray diffraction analysis. The intrinsic stress of the films can be the cumulative result of chemical and microstructural defects incorporated during the film condensation process. The presenceof a nanocrystalline or amorphous layer at the substrate-film interface is not unique for W-C coatings but has been found also in reactively sputterdeposited tungsten trioxide films [ 131. In the case of W-C films deposited on stainless steel and Ti alloy substrates, 40-nm-thick crystalline layers can be found at the substrate-nanocrystalline interface (Fig. 2b). This layer could be formed by recrystallisation of the nanocrystalline structure.
4. Conclusions ( 1) The growth of reactively sputtered tungsten-carbon films begins with the formation of a nanocrystalline phasethat always exists independently of the applied substrate material. (2) The nanocrystalline part of the film has a higher carbon concentration, and its thickness depends on the available concentration of carbon related to the CH, flow rate during deposition. ( 3) The thickness of the nanocrystalline part also depends on the substrate. On metallic substrates, a recrystallisation may take place, forming a thin polycrystalline layer at the substrate-nanocrystalline interface.
Ef CJ - (a, -af) (Tl-0-b), th - 1 -vf
where Ef and vf are the elastic modulus and Poisson coefficient of the film; a, and txf are the linear thermal expansion coefficients of the substrate and the film, respectively; and Td and T, are the deposition temperature and measurement temperature of stress, i.e. room
The authors’ thanks are due to Mrs G. Glazer and Mr M.B. Grand for their technical assistance. The present work was partially supported by NATO Technology Scientific Affairs Division, High Collaborative Research Grant.
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