CARBON and tungsten sputtering by noble-gases ion bombardment

IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37, NO. 8, AUGUST 2009 1581 Carbon and Tungsten Sputtering in a Helium Magnetron Discharge Vasile Tiron, Codrin Andrei, Andrei V. Nastuta, George B. Rusu, Catalin Vitelaru, and Gheorghe Popa Abstract This paper reports on carbon and tungsten deposition on a heated silicon substrate under He + bombardment in a magnetron-sputtering device. The discharge was operated at constant pressure of 1.33 Pa for two discharge-current intensities (200 and 600 ma) and target power density up to 40 W cm 2. The deposited films were characterized by scanning electron microscopy, atomic force microscopy, and X-ray diffractometry. The topography and cross section revealed the influence of the target power density on the surface roughness, grains size, and thickness of the deposited films. Index Terms Carbon, magnetrons, sputtering, tungsten. I. INTRODUCTION CARBON and tungsten sputtering by noble-gases ion bombardment has received considerable attention in recent studies of plasma wall interaction [1] [3], thin-film deposition [4], and other surface-science applications [5]. Many studies have shown that the physical properties of deposited carbon and tungsten films strongly depend on the sputtering parameters, like gas pressure, energy and nature of sputtering gases [6], target density [7], substrate temperature, substrate bias voltage, target-to-substrate distances, and deposition angle [8]. The deposition flux and the energy of the atoms and ions on the substrate act on the deposition rate and the film characteristics, respectively. Sputtering gas-to-target material mass ratio is an important parameter for controlling the growing film bombardment. In the deposition technique, when the sputtering gas atoms are less massive than the target atom, the inert incorporated gases play an important role in the formation of a fine-grained structure [9]. Physical sputtering is one of the most serious processes of erosion of the innermost surfaces of fusion machines, where carbon and tungsten materials in the form of tiles covering the metal vessel interact with helium that is being obtained as a reaction product in the fusion process. Consequently, the study of the helium atoms /ions interaction with W and/or C surfaces, as e.g., physical sputtering process, has a great interest for ITER project. In this paper, the experimental results are presented on carbon and tungsten deposition on a heated silicon substrate under He + bombardment in a dc magnetronsputtering device. Manuscript received October 31, 2008; revised May 26, 2009. Current version published August 12, 2009. The authors are with the Plasma Physics Department, Faculty of Physics, Alexandru Ioan Cuza University, 700506 Iasi, Romania (e-mail: vtiron@ plasma.uaic.ro; Codrin.Andrei@ucdcconnect.ie; anastuta@plasma.uaic.ro; grusu@plasma.uaic.ro; cvitelaru@plasma.uaic.ro; ghpopa@uaic.ro). Digital Object Identifier 10.1109/TPS.2009.2024421 II. EXPERIMENTAL DETAILS Carbon and tungsten thin films were deposited on an oxidized silicon wafer by dc magnetron sputtering using planar targets and helium as sputtering gas. The deposition chamber was pumped down using a turbomolecular system to an ultimate pressure of 10 4 Pa, and then backfilled to absolute working pressures of 1.33 and 6.67 Pa, respectively. A current-regulated dc power supply limited at 1500 V/800 ma was used to provide a discharge-current intensity between 200 and 600 ma and a corresponding target power density in the range of 8 to 40 W cm 2. The helium gas pressure was changed from 1.33 to 6.67 Pa by controlling the pumping speed and keeping constant the gasflow rate. The substrate was fixed at 9 cm above the target and deposition time was 2 h for each deposited film. Prior to deposition, the substrate was heated up to 200 C, and this value was maintained constant during film deposition. The deposition temperature was measured by a thermocouple fixed on the substrate. The cross section, film thickness, and surface morphology were measured by scanning electron microscopy (SEM). The deposition rates are estimated from the mean-thickness measurements taken from the cross-sectional SEM images with known deposition time. The atomic force microscopy (AFM) technique was used in order to obtain information about surface topography, grain size, and roughness. The AFM images were performed in ambient conditions using standard silicone nitride tips (NSC21), with tip radius of 10 20 nm. The analysis was made in tapping mode with 0.1-nm resolution in z-direction. The crystalline structure of thin layers was measured by X-ray diffraction (XRD) technique. III. RESULTS AND DISCUSSION To investigate the effects of the target power density on the film characteristics, the discharge current was changed from 200 to 600 ma, whereas the working pressure was kept constant at 1.33 Pa. In Figs. 1 and 2, the 2-D and 3-D AFM images of the W thin film deposited in He atmosphere at 8 and 40 W cm 2, respectively, are presented. Both grain size and surface roughness increase with increasing target power density (see Table I). In the case of tungsten target sputtering in helium atmosphere, the inert incorporated gases involve formation of a fine-grained structure. The inert-gas atoms trapping in deposited film play an important role in limiting grain growth [10]. When the sputtering-gas atoms are less massive than the target atom, large amounts of sputtering ions are 0093-3813/$26.00 2009 IEEE

1582 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37, NO. 8, AUGUST 2009 Fig. 1. (a) Two-dimensional and (b) 3-D AFM images of the W thin film deposited in He atmosphere at 8 W cm 2. Fig. 2. (a) Two-dimensional and (b) 3-D AFM images of the W thin film deposited in He atmosphere at 40 W cm 2. backscattered charge neutralized, and a high energetic flux of reflected neutrals will interact with the growing film. The He reflected neutrals stuffed the grain boundaries limiting the W impinging atoms mobility on the surface before they migrate to the preferred sites for crystallization growth. Increasing the target power density increases the resputtering probability of inert incorporated gases in thin film and, therefore, the grain size. In case of the carbon deposition, the amount of the inert incorporated gases is less important due to the low mass ratio of the sputtered atoms/inert-gas atoms and, therefore, a larger grain size. Figs. 3 and 4 present the 3-D AFM images of the C thin film deposited in He atmosphere at 10 and 34 W cm 2, respectively. Both grain size and surface roughness increase with increasing target power density (see also Table I). The surface morphology of tungsten films, as observed by SEM top-view micrographs and cross-sectional images, are shown in Figs. 5 and 6. Both surface images exhibit grained structure. Fine grain size occurs at low target power density. The mean grain-size radius increases from 100 to 590 nm with increasing the target power density from 8 to 40 W cm 2. In addition, by increasing the target power density, the film thickness increases from 206 to 328 nm. The SEM surface images are in good correlation with the 2-D AFM images. During W deposition, the oxidized silicon is bombarded with reflected W neutrals and, therefore, the oxidized film may be sputtered. The cross-sectional images reveal that in the deposition process, the oxidized film was removed from silicon substrate. This phenomenon does not occur in C deposition case (Figs. 7 and 8) due to the lower energy of sputtered atoms. At higher helium pressure and the same target power density (40 W cm 2 ), both deposition rate of W and surface roughness decrease, whereas the mean grain size remains unchanged (see Table I). The excess He gas pressure induces a shorter mean free path of the He + ions with more frequent collisions, which induces a lower kinetic energy when they impinge on the target. As a result, it contributes to lower deposition rate. In addition, because of their low speed (and large mass), W neutrals sputtered from the target are ionized over small distances from the target and a high fraction of sputtered atoms are redeposited on the target [11]. This statement is sustained by the fact that the target sputtered at high power density (40 W cm 2 ), after a few minutes, becomes incandescent (red hot) in all investigated pressure range. Moreover, at high gas pressure, the sputtered metallic atoms are also submitted to a greater number of collisions and can be scattered off, which accounts for a lowering deposition rate. The XRD analysis (Fig. 9) shows the influence of the target power density in W film structure. At low target power density (8 W cm 2 ), the films seem to be mostly amorphous and present a diffraction peak corresponding to WO 3. We suggest that the oxygen impurities in the deposited film came from

TIRON et al.: CARBON AND TUNGSTEN SPUTTERING IN A HELIUM MAGNETRON DISCHARGE 1583 TABLE 1 OPERATION PARAMETERS AND RESULTS OF W AND C FILMS DEPOSITED IN PURE He ON OXIDIZED SILICON WAFERS Fig. 3. Three-dimensional AFM images of the C thin film deposited in He atmosphere at 10 W cm 2. Fig. 5. (a) SEM surface morphology and (b) cross-sectional images of W thin film deposited in He atmosphere at 8 W cm 2. Fig. 4. Three-dimensional AFM images of the C thin film deposited in He atmosphere at 34 W cm 2. the oxidized silicon substrate due to bombardment with W reflected neutrals and from residual gas (the ultimate pressure in the sputtering device was 10 4 Pa). Increasing the target power density (40 W cm 2 ), the WO 3 peak increases, and α-w and β-w phases are also obtained. The α-w phase is a stable body-centered cubic (a =0.316 nm) structure with the greater density, 19.3 g cm 3. The β-w structure is a metastable phase with a primitive cubic lattice (a =0.505 nm) and a lower density of 14.6 g cm 3 [12]. It was suggested that oxygen incorporation might play an important role in formation of the metastable β-w phase. In the case of C film deposition, the XRD analysis reveals that the thin layer is amorphous. IV. CONCLUSION For each target-gas combination, both roughness and thickness of the deposited film increased with the target power Fig. 6. (a) SEM surface morphology and (b) cross-sectional images of W thin film deposited in He atmosphere at 40 W cm 2. density. For a constant operating pressure the deposition rate of both C and W increased with the power density. For the same power density and target-gas combination, the

1584 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37, NO. 8, AUGUST 2009 Fig. 9. XRD data of tungsten films deposited at two different values of target power density (thin line 8 W cm 2 and thick line 40 W cm 2 ). Fig. 7. (a) SEM surface morphology and (b) cross-sectional images of C thin film deposited in He atmosphere at 10 W cm 2. Fig. 8. (a) SEM surface morphology and (b) cross-sectional images of C thin film deposited in He atmosphere at 34 W cm 2. amount of deposited film decreased when increasing the gas pressure. By injecting more density power into the target, the mean grain size was increasing for the both sputtered materials. Formation of the nanocrystalline structure can be related to a greater amount of oxygen incorporation into the film from substrate during film formation. ACKNOWLEDGMENT The authors would like to thank Dr. L. Leonte for his help with the XRD analysis. REFERENCES [1] W. Eckstein, Physical sputtering and reflection processes in plasma wall interaction, J. Nucl. Mater., vol. 248, pp. 1 8, Sep. 1997. [2] A. Kallenbach, P. T. Lang, R. Dux, C. Fuchs, A. Herrmann, H. Meister, V. Mertens, R. Neu, T. Pütterich, and T. Zehetbauer, Tungsten as first wall material in fusion devices, J. Nucl. Mater., vol. 337 339, pp. 732 736, Mar. 2005. [3] C. Paturaud, G. Farges, M. C. Sainte Catherine, and J. Machet, Influence of particle energies on the properties of magnetron sputtered tungsten films, Surf. Coat. Technol., vol. 98, no. 1 3, pp. 1257 1261, Jan. 1998. [4] E. Harry, Y. Pauleau, M. Adamik, P. B. Barna, A. Sulyok, and M. Menyhard, Growth characteristics of tungsten-carbon films deposited by magnetron sputtering, Surf. Coat. Technol., vol. 100/101, pp. 291 294, Mar. 1998. [5] C. Wang, P. Brault, C. Zaepffel, J. Thiault, A. Pineau, and T. Sauvage, Deposition and structure of W Cu multilayer coatings by magnetron sputtering, J. Phys. D, Appl. Phys., vol. 36, no. 21, pp. 2709 2713, Nov. 2003. [6] C. Paturaud, G. Farges, M. C. Sainte Catherine, and J. Machet, Influence of sputtering gases on the properties of magnetron sputtered tungsten films, Surf. Coat. Technol., vol. 86/87, pp. 388 393, Dec. 1996. [7] C. F. Lo, P. McDonald, D. Draper, and P. Gilman, Influence of tungsten sputtering target density on physical vapor deposition thin film properties, J. Electron. Mater., vol. 34, no. 12, pp. 1468 1473, Dec. 2005. [8] T. Karabacak, A. Mallikarjunan, J. Singh, D. Ye, G. C. Wang, and T. M. Lu, β-phase tungsten nanorod formation by oblique-angle sputter deposition, Appl. Phys. Lett., vol. 83, no. 15, pp. 3096 3098, Oct. 2003. [9] R. A. Roy, J. J. Cuomo, and D. S. Yee, Control of microstructure and properties of copper films using ion-assisted deposition, J. Vac. Sci. Technol. A, Vac. Surf. Films, vol. 6, no. 3, pp. 1621 1626, May 1988. [10] N. Maréchal, E. Quesnel, and Y. Pauleau, Deposition process and characterization of chromium-carbon coatings produced by direct sputtering of a magnetron chromium carbide target, J. Electrochem. Soc., vol. 141, pp. 1691 1698, 1994. [11] R. Neu, K. Asmussen, M. Bessenrodt-Weberpals, S. Deschka, R. Dux, W. Engelhardt, A. Thoma, J. C. Fuchs, J. Gaffert, C. García-Rosales, A. Herrmann, K. Krieger, F. Mast, J. Roth, V. Rohde, M. Weinlich, and U. Wenzel, The tungsten experiment in ASDEX upgrade, J. Nucl. Mater., vol. 241 243, pp. 678 683, Feb. 1997. [12] J. Ligot, S. Benayoun, J. J. Hantzpergue, and J. C. Remy, Sputtered tungsten film on polyimide, an application for X-ray masks, Solid State Electron., vol. 43, no. 6, pp. 1075 1078, Jun. 1999. Vasile Tiron, photograph and biography not available at the time of Codrin Andrei, photograph and biography not available at the time of

TIRON et al.: CARBON AND TUNGSTEN SPUTTERING IN A HELIUM MAGNETRON DISCHARGE 1585 Andrei V. Nastuta, photograph and biography not available at the time of George B. Rusu, photograph and biography not available at the time of Catalin Vitelaru, photograph and biography not available at the time of Gheorghe Popa was born in Lucacesti, Romania, on October 25, 1943. He received the Diploma (M.Sc.) and Ph.D. degrees in physics from Alexandru Ioan Cuza University, Iasi, Romania, in 1966 and 1974, respectively. Since 1966, he has been with the Plasma Physics Department, Faculty of Physics, Alexandru Ioan Cuza University, where he was promoted in all academic steps until 1990 when he was nominated as Full Professor. He was invited as Guest Researcher and Professor in plasma physics with the University of Innsbruck, Innsbruck, Austria, Shizuoka University, Ohya, Japan, Nagoya Institute of Technology, Nagoya, Japan, University of Nantes, Nantes, France, and Paris-Sud University, Orsay, France. He worked on the research in plasma discharges and their applications as ion nitridation, plasma polymerization, surface treatment of polymers, and thin-layer deposition. He also made fundamental research in Q-machine and D. P.-machine plasmas on the following: ionization waves, ion acoustic waves and soliton, ion spacecharge instabilities, and ion cyclotron waves and instabilities. He developed methods for plasma diagnostics. Prof. Popa is a member of the European Physical Society and a Fellow of IOP.