M. New Wang Diamond et al. and Frontier Carbon Technology 29 Vol. 15, No. 1 2005 MYU Tokyo NDFCT 472 Deposition of C-F Thin Films by Sputtering and Their Micromechanical Properties Mei Wang, Shuichi Watanabe and Shojiro Miyake Nippon Institute of Technology, 4-1 Gakuendai, Miyashiro-machi, Minamisaitama-gun, Saitama 345-8501, Japan (Received 13 September 2004; accepted 20 December 2004) Key words: C-F bonding, atomic force microscopy, viscoelasticity, polytetrafluoroethylene Three different carbon and fluorine-containing (C-F) thin films were deposited on silicon (100) wafers by radio frequency (RF) magnetron sputtering in an atmosphere of argon plasma using graphite and polytetrafluoroethylene (PTFE) as target materials. From Fourier transform infrared spectroscopy (FT-IR) analysis, the films prepared by using a PTFE target were found to exhibit C-F bonding and an amorphous carbon structure. The effects of C-F bonding and the amorphous carbon structure on the surface free energy and viscoelasticity were investigated using the liquid drop method and atomic force microscopy (AFM). It was found that the surface free energy of the silicon wafers was decreased after surface modification and the water-repellence and viscoelasticity properties of the silicon wafers were significantly improved due to C-F bonding. 1. Introduction Since lubrication under conditions such as high relative humidity, extremely high or low temperature and high vacuum has become more and more important, many protective friction-reducing coatings have been developed to act as solid lubrication, such as amorphous carbon (a-c) films, which are widely used in the hard-disk industry and space devices, and amorphous carbon-fluorine films (a-c: F: H). The latter films are very interesting due to the fact that they can combine the mechanical properties of hard a-c films and the tribological properties of fluorine-based lubricants. (1,2) On the other hand, it is well known that polytetrafluoroethylene (PTFE) has been used widely an engineering plastic because of its outstanding thermal stability, good solvent resistance, and low friction coefficient. However, it is difficult to form high-quality solid lubrication films. Surface modification by physical vapor deposition (PVD) has been greatly investigated and applied in recent years. The sputtering method of depositing solid lubricants is one way * Corresponding author: e-mail: wm64@yahoo.com 29
30 New Diamond and Frontier Carbon Technology, Vol. 15, No. 1 (2005) of solving the above-mentioned problems. It has been reported that deposition of PTFE on silicon wafers was successfully performed to improve its self-lubrication and waterrepellence due to the weak interaction of C-F combination between their superficial molecules. (3) Fluoropolymers have many outstanding properties including low surface tension, extreme chemical inertness and high oxidative and thermal resistance. Solid materials at ambient temperature, such as PTFE, have been adopted as sputtering targets. (4 6) Contrary to expectations, the tribological and dielectric properties of RF sputtered PTFE films are inferior to those of the parent material because of structural changes. (7) The possibility of improving these film properties by modifying the sputtering process has been investigated. (8) In this report, we describe the fabrication of carbon- and carbon-fluorine-containing films on silicon wafers to improve friction, water-repellence, and mechanical and viscoelastic properties. We defined the expression graphite film to mean a carbon film formed by using graphite as a target, the expression graphite-ptfe film to mean a carbon-fluorinecontaining film formed by using graphite and PTFE as targets and the expression PTFE film to mean a fluorine-containing film formed by using PTFE as a target. The surface conditions of the modified silicon wafers were characterized, and the films deposited using different targets and deposition times were compared. The viscoelastic properties of the films were also evaluated using atomic force microscopy (AFM) in forced modulation mode. 2. Experimental Method Figure 1 shows a schematic illustration of RF magnetron sputtering equipment. Silicon (100) wafers were used as the substrate. First, graphite-ptfe films were synthesized on the silicon wafer in the sputter deposition chamber in an atmosphere of argon plasma. PTFE and graphite were used as targets, and the size of the targets and the deposition time were set as shown in Table. 1. Second, the deposition of graphite films was carried out using only a Fig. 1. Schematic illustration of RF sputtering apparatus.
M. Wang et al. 31 Table 1 Deposition conditions for graphite-ptfe films. Apparatus Radio frequency (RF) sputtering equipment Gas presure (Torr) Argon, 3.4 10 2 Target (mm 2 ) Graphite (circle with 10 mm radius), PTFE (7 mm 7 mm) RF power (kw) 0.2 0.5 Deposition time (min) 20 Table 2 Deposition conditions for graphite film. Apparatus Radio frequency (RF) sputtering equipment Gas presure (Torr) Argon, 3.4 10 2 Target (mm 2 ) Graphite (circle with 10 mm radius) RF power (kw) 0.2 0.5 Deposition time (min) 15 graphite target under the deposition conditions shown in Table. 2. In order to evaluate the tribological properties of the C-F bonding films deposited on the silicon substrates, the C- F films deposited under different conditions were examined by analytical techniques to determine C-F bonding, thickness, self-lubrication, and water-repellence. Contact angle measurements were performed using refined water and methyl iodide to measure surface free energy. Hydrophobicity was determined by measuring the contact angle of a droplet of distilled water on the specimen. The surface free energy and diffusion coefficients of three liquids on the samples were also evaluated. By Fourier transform infrared spectroscopy (FT-IR), the chemical bonding features of C-F films were investigated. IR absorption spectra were obtained using K Br pellets. When argon was used for the sputtering gas, these spectra had peaks at 1220 cm 1 (high intensity) and 1640 cm 1 (low intensity) and no other appreciable peaks were observed. The high intensity peak is assigned to the C-F stretching of perfluoroalkyl groups and the low intensity peak is assigned to the C=C stretching of alkenyl groups. (2) Viscoelastic properties were evaluated using the force modulation method of scanning probe microscopy (SPM) as shown in Fig. 2. This apparatus was added to a nanoindentation system along with a lock-in amplifier. The Berkovich diamond indenter was vibrated vertically in the test. Phase lag and displacement were evaluated according to the response of the tip indenter, which was controlled using a transducer. Viscoelastic properties such as storage modulus, loss modulus and tanδ were analyzed with a computer. The tests were performed by scanning a diamond tip with an applied a load of 20 µn for vibration of frequencies ranging from 0 to 400 Hz. After the tests, the sample was scanned using the same tip under a load of 2 nn with a vibration of a frequency of 50 Hz. Viscoelastic properties such as storage modulus, loss modulus and tanδ were evaluated by means of atomic force microscopy (AFM). (9) These properties of C-F films were compared with those of untreated silicon and PTFE films reported in our previous paper (10) to clarify the effects of modification under different deposition conditions.
32 New Diamond and Frontier Carbon Technology, Vol. 15, No. 1 (2005) Fig. 2. Measurement system of AFM. 3. Result and Discussion 3.1 Measurement of surface free energy The surface properties of the graphite-ptfe, graphite and PTFE films deposited on the silicon substrates were analyzed by measuring surface contact angles and surface free energies. The surface free energy of these films was evaluated using a drop of liquid. Figure 3 shows the drop profile images of the films. Compared with a drop contact angle 36.5 for distilled water on an untreated specimen, the contact angles of the graphite, graphite-ptfe and PTFE deposited films respectively increase to 79.7, 85.5 and 108, which indicates that the surface tension of deposited silicon wafers is decreased by surface modification due to the weak interactions of C-F bonds, (11) which indicates an increase in the contact angle as the PTFE target was used, leading to greater incorporation of fluorine in the films. (12,13) Figure 4 shows the contact angles measured with three liquids such as refined water, and the surface free energies of the films are shown in Fig. 5. According to the extended Fowkes s theory of 3, surface free energy (γs) is the sum of the dispersion force (γsd), the dipole force (γsp) and the hydrogen bond (γsh) strength as in the following equation: γs = γsd + γsp + γsh. (1) The surface free energies are 71.5 mn/m for untreated silicon, 40.2 mn/m for the graphite-pfpe-coated silicon wafer, and 17 mn/m for the PTFE-coated silicon wafer, which suggests that C-F bonds might be formed on the silicon by graphite and PTFE sputtering treatments using a PTFE target. 3.2 FT-IR measurement of C-F bonding IR is a common characterization tool for C-F compounds, as the C-F bond shows a distinct feature. A distinct absorption band assigned to C-F stretching is noted around 1200 cm 1. (2,14) In IR absorption spectra obtained using KBr pellets, as shown in Fig. 6, there is
M. Wang et al. 33 Fig. 3. wafers. Distilled water drop profiles of untreated, graphite and graphite-ptfe deposited on silicon Fig. 4 (left). Contact angles of untreated, PTFE, graphite, graphite-ptfe films deposited on silicon wafers. Fig. 5 (right). Surface free energies of untreated, PTFE, graphite, graphite-ptfe films deposited on silicon wafers. Fig. 6. FT-IR spectra of untreated, PTFE, graphite, graphite-ptfe films deposited on silicon wafers.
34 New Diamond and Frontier Carbon Technology, Vol. 15, No. 1 (2005) a peak near 1200 cm 1 corresponding to the PTFE target because PTFE possesses a typical C-F combination. Graphite-PTFE and PTFE films display a clear peak around 1200 cm 1. However, the absorption does not appear in the graphite film. Typical bonds of C-F appear indicating that the C-F combination is formed when depositing PTFE and graphite-ptfe films using PTFE as a target. 3.3 Evaluation of viscoelastic properties Comparing the viscoelastic properties of silicon wafers coated with carbon fluorinate and carbon films with those of untreated silicon wafers, it is found that their viscoelastic properties are improved due the formation of C-F bonds on the silicon surface. As shown in Fig. 7(a), PTFE used as a target displays a typical storage modulus which is smaller than 10 GPa. Correspondingly, the storage moduli of the PTFE-deposited and graphite-ptfecoated silicon wafers are low, under 20 GPa when the loading vibration frequency is less than 100 Hz. It is inferred from this that the storage modulus decreases due to C-F bonding. This result corresponds to that obtained from FT-IR measurements, whereas the storage moduli of the untreated substrate and graphite-deposited silicon wafer show a high value over 20 (a) (b) (c) Fig. 7. Viscoelastic properties of the untreated, PTFE, graphite, graphite-ptfe films deposited on silicon wafers; (a) storage modulus, (b) loss modulus and (c) tanδ of the films.
M. Wang et al. 35 GPa. Increasing the loading vibration frequency to 150 300 Hz causes all storage moduli to attain a value of about 20 GPa. Figure 7(b) shows that the loss modulus of untreated silicon is similar to that of the graphite-deposited silicon. As the film was deposited using PTFE and graphite targets, its loss modulus decreased to 15 GPa. On other hand, it is very obvious that the loss modulus is significantly changed to a small value which is the same as the loss modulus of the PTFE target. It is confirmed that the loss modulus could be reduced markedly due to the weak interaction of C-F bonding formed in films. Figure 7(c) indicates that tanδ of a deposited PTFE film is under 0.1 over a range of loading vibration frequencies from 0 to 220Hz. As noted from Fig. 6, the viscoelastic properties of the graphite-ptfe-deposited silicon lie between those of the graphite- and PTFE-deposited silicon samples. 4. Conclusion The PTFE, graphite-ptfe, graphite films were deposited on silicon wafers by means of magnetic sputtering using PTFE and two PTFE and graphite targets, respectively. Surface structure conditions, water-repellence and tribological properties were investigated. (1) By using PTFE as a target, carbon and fluorine-containing films were obtained. The results indicate that the self-lubricating and water-resistance properties can be improved due to the weak interactions of C-F bonds. (2) The tribological properties of the films were investigated using AFM in force modulation mode. The results show that the effects of the deposited film on tribological properties depend on the use of PTFE or graphite targets and the tribological properties of the modified silicon wafers are improved remarkably due to C-F bonding. References 1) L. G. Jacobsohn, D. F. Franceschini, M. E. H. Maia da Costa and F. L. Freire Jr: Vac. Sci. Technol. A 18 (2000) 2449. 2) I. Sugimoto and S. Miyake: Thin Solid Films 128 (1988) 51. 3) Y. Enomoto and S. Miyake: Tribology of Thin Films (University of Tokyo Press, Tokyo, 1994) (in Japanese). 4) R. Harrop and P. J Harrop: Thin Solid Films 3 (1972) 109. 5) I. H. Pratt and P. C. Lausman: Thin Solid Films 10 (1972) 151. 6) H. Biellerman: Vacuum 31 (1981) 285. 7) T. Robertson and T. Morrison: Thin Solid Films 27 (1975) 19. 8) P. Niederhauser, M. Maillet and H. E. Hinterman: Proc. Ist Eur. Symp. On Space Mechanisms and Tribology, ESA SP-196 (European Space Agency, 1983) pp. 119 123. 9) S. A. Syed Asif, K. J. Wahl and R. J. Colton: Rev. Sci. Instrum. 70 (1999) 2408. 10) N. Shimizu, S. Watanabe, and S. Miyake: Proceeding of Mechanical Congress, Japan, vol. II, 8 (2000) p. 395 (in Japanese). 11) N. Shimizu, S. Watanabe, M. Wang and S. Miyake: Proceeding of 105th Meeting of the Surf. Fin. Soc. of Jap. 179 (2002) p. 4 (in Japanese). 12) R. Prioli, L. G. Jacobsohn, M. E. H. Maia da Costa and F. L. Freire, Jr: Tribology Letters 15 (2003) 177. 13) P. B. Leezenberg, W. H. Johnston and G. W. Tyndall: J. Appl. Phys. 89 (2001) 3498. 14) Infrared Spectra Handbook of Inorganic Compounds (Sadtler Research Laboratories, 1984).