Growth and nucleation of diamond-like carbon (DLC) film on aluminum

Transcription

1 Nuclear Instruments and Methods in Physics Research B 206 (2003) Growth and nucleation of diamond-like carbon (DLC) film on aluminum L.H. Li a,b, X.B. Tian a, P.K. Chu a, *, Y.H. Zhang b, X.M. Cui b, H.Q. Zhang b a Department of Physics and Materials Science, City University of Hong Kong, 88 Tat Chee Avenue, Kowloon, Hong Kong, China b 702 Department, Mechanical Engineering School of Beijing, University of Aeronautic and Astronautic, Beijing, China Abstract Diamond-like carbon (DLC) films were deposited on aluminum substrate using vacuum arc deposition (VAD) and plasma immersion ion implantation deposition (PIII-D). Even though Raman and XPS results show that the carbon films deposited by VAD and PIII-D both possess DLC structures, atomic force microscopy (AFM) reveals that the two sets of films have very different surface morphologies. The growth mechanisms in these two techniques thus differ and the different nucleation and growth mechanisms of the DLC films deposited by VAD and PIII-D can be explained using the statistical formation theory. Growth models for VAD and PIII-D DLC films are proposed. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: A; Sz; ; Dy Keywords: DLC films; Nucleation; Growth 1. Introduction Successful diamond-like carbon (DLC) film deposition on Al substrate is very valuable in space applications since DLC films have low friction coefficients especially in high vacuum. Some structural models have been proposed and the relationship between the DLC properties/structures and deposition methods/parameters have been proposed and investigated [1 7]. In contrast, the nucleation and growth mechanisms on aluminum are not as well understood, and to our knowledge, there have been few reports comparing the growth phenomena using different deposition techniques. Because the ion energies used in different fabrication methods and sometimes in the same deposition technique with different tilt angles are different, the nucleation and growth mechanisms are expected to vary [8,9]. In this work, we investigate the nucleation and growth mechanisms of two common techniques: vacuum arc deposition (VAD) and plasma immersion ion implantation deposition (PIII-D). 2. Experimental * Corresponding author. Tel.: ; fax: / address: paul.chu@cityu.edu.hk (P.K. Chu). Two sets of samples were prepared by VAD and PIII-D, respectively. Detailed descriptions of the X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi: /s x(03)

2 692 L.H. Li et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) Fig. 1. Changes reflecting the growth patterns of the vacuum arc deposited DLC films (intervals of about 4 s). equipment and experimental methods can be found elsewhere [10 12]. Both of the substrates were LY12 aluminum alloy with a diameter of 40 mm and thickness of 5 mm. Before deposition, the substrates were cleaned by 1 kv argon in a glow discharge plasma for 30 min. In the vacuum arc deposition experiment, a 150 ma bias current was applied to the substrates for 3 min. Argon was used as the ambient gas, and a carbon cathodic arc plasma source was used. The samples were placed 250 mm from the exit of the arc source. The deposition current was 66 A at a voltage of 30 V. The pressure was about 0.1 Pa. In PIII-D, the C 2 H 2 plasma was sustained by 1 kw hot filament discharge. )20 kv pulses with a pulse width of 27 ls were applied to the substrates at a frequency of 128 Hz. The implantation deposition time was 2h. Raman spectra were acquired by a SPEX 1403 Raman spectrometer employing a laser wavelength of 488 nm. The resolution was 2 cm 1 and XPS spectra were taken on a PHI-5700 equipped with a non-monochromatic MgK a radiation source (hm ¼ 1253:6 ev). The energy resolution was ev. The analytical area was within a 0.4 mm diameter circle. 3. Film growth Deposition processes were visually observed through a window on the deposition chamber and very interesting growth phenomena were observed. In VAD, growth always started from one spot that is referred to as the starting spot in this paper. The location of this starting spot was random but at the edge of the substrate. Before this spot appeared, even if the arc had been ignited for a long time, no carbon film could be observed. As soon as this starting point emerged, the film began to expand on the surface rapidly in a ripple fashion spreading to the other side of the film. Visually, the growth appeared as propagation of colorful wave fronts and the first DLC layer was often transparent. The subsequent ripples then emerged from the same starting spot. As the DLC film got thicker, it became yellow, red and then black. The visual changes on the samples in the VAD process with time (at intervals of about 4 s) are depicted in Fig. 1. This growth characteristic is unique to vacuum arc deposition, as no starting spot or ripple effect can be observed in PIII-D. 4. Results To characterize the structure of the films, Raman shifts and X-ray photoelectron spectroscopy (XPS) spectra were measured from both the red and black areas of the VAD sample shown in Fig. 1(f) and the PIII-D sample. The Raman spectra acquired from both the VAD and PIII-D samples exhibit asymmetrical peaks and are in good agreement with results reported in the literature

3 L.H. Li et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) [13,14]. By using Gaussian multi-peak fitting, the Raman spectra acquired from the VAD sample can be deconvoluted into two sub-peaks at cm 1 and at cm 1, respectively. The spectrum of PIII-D sample is a composite consisting of a peak at cm 1 and another one at cm 1. Because carbon films with various structures can yield similar Raman spectra, XPS or other characterization techniques should be employed to corroborate the results [15]. XPS spectra were taken form the black area of VAD sample, red area of the VAD sample, and PIII-D sample, respectively. All the peaks are asymmetrical and exhibit a shift towards a higher energy (285.5 ev, diamond C1s binding energy) from ev (typical graphite C1s binding energy) indicating sp 3 bond characteristics in both samples and the shifts towards higher binding energies are 0.35, 0.1 and ev, respectively. The results are also confirmed by nano-indentation hardness tests [16]. In order to study the detailed surface morphology of the DLC films, atomic force microscopy (AFM) was performed and the results are shown in Fig. 2. As shown in Fig. 2(a) (VAD specimen), the film surface mimics the substrate morphology, whereas that observed on the PIII-D specimen (Fig. 2(b)) is very different resembling that of a wasp nest. 5. Discussion When approaching the substrate, electrically neutral particles enter a field of attractive London forces (the attractive forces for neutral atoms, generally called van der Waals forces or dispersion forces) originating from the instantaneous dipole and quadrupole moments of the surface atoms or molecules. The atoms from the gas phase collide with the substrate and predominantly inelastically transfer their kinetic energy to the substrate lattice [17]. Condensation of the atoms onto the substrate is determined by the absorption energy E a and the sublimation energy E su. Above a critical condensation temperature, one can assume that E a E su. Under this condition, the lifetime of the atoms is Fig. 2. AFM images: (a) VAD sample and (b) PIII-D sample. too small to permit nucleation and condensation may not take place unless the incident atoms are supersaturated on the surface. In this case, the nucleation mechanism can be explained using the statistical formation theory. If E a E su, nucleation does not require saturation. If E a ¼ E su, nucleation needs moderate saturation and can be explained by thermodynamic theories. In vacuum arc deposition, the incident particles are mainly singly or multiply charged ions with a relatively small percentage of atoms. The substrate bias also contributes to the surface mobility, but as indicated by the difficulty to deposit DLC films on Al substrates using VAD unless the conditions are proper, it can be concluded that carbon ions have low wettability on Al. In addition, the energy of the carbon ions is not as high as that of the carbon

4 694 L.H. Li et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) ions in PIII-D. The nucleation efficiency is thus low because a critical vapor stream density is not achieved. Under certain conditions, though the necessary nucleation centers cannot be supplied by the clean substrate, they may be provided by the deep potential minima as well as impurities especially at the edge of the Al substrate. This is believed to be the reason that the film nucleation and growth in VAD always start from the edge of the substrate plane. When the nucleation center forms and grows to a critical size, it can proliferate very quickly because the absorbed particles on the substrate surface have high mobility. The carbon atoms diffuse to the first nucleation center, and stair-like nucleation centers having the critical size are formed at the first nucleation center. The formation of centers elsewhere is masked by the rapid proliferation of the first nucleation center [see Fig. 3(a)], and as a result, the edge of nucleation center propagates like ripples and the surface morphology resembles that of the underlying aluminum substrate. In comparison, the carbon ion density in PIII-D is much less than that in VAD, and so the formation of critical carbon nucleation centers is more difficult. However, owing to the higher incident ion energy ()20 kv bias), nucleation centers can form at multiple locations where carbon atoms aggregate. The nucleation centers distribute evenly on the substrate surface. The growth model of PIII-D is illustrated in Fig. 3(b). Film growth proceeds simultaneously at multiple centers without a specific starting point, resulting in surface morphologies such as that displayed in Fig. 2(b). 6. Conclusion Even though Raman and XPS spectra show that the carbon films by VAD and PIII-D are both DLC, the growth mechanism and surface morphology of these two types of DLC films are quite different. In VAD of DLC on Al, nucleation sites are not abundant because the carbon ion stream does not possess a sufficient critical vapor density. In contrast, in PIII-D, the higher ion incident energy obviates the need for supersaturation, and film growth can initiate on multiple areas of the surface. In this case, the energetic pinned carbon ions can aid the formation of critical nuclei, film grows uniformly, and the surface morphology resembles the shape of a wasp nest. Acknowledgements The work described in this work was supported by City University of Hong Kong SRG # References Fig. 3. Postulated DLC film growth model: (a) stair-like layer VAD film growth; (b) PIII-D film growth. [1] J.C. Angus, Thin Solid Films 216 (1992) 126. [2] J. Robertson, Surf. Coat. Technol. 50 (1992) 185. [3] B.F. Coll, P. Sathrum, R. Aharonov, M.A. Tamor, Thin Solid Films 209 (1992) 165. [4] D. Roth, B. Rau, S. Roth, J. Mai, K.-H. Dittrich, Surf. Coating Technol (1995) 637. [5] S.R.P. Silva, K.J. Clay, S.P. Speakman, G.A.J. Amaratunga, Diamond Relat. Mater. 4 (1995) 977. [6] M.J. Paterson, K.G. Orrman-Rossiter, D.K. Sood, Diamond Relat. Mater. 2 (1993) [7] H. Hofs ass, H. Binder, T. Klumpp, E. Reeknagel, Diamond Relat. Mater. 3 (1993) 137. [8] D. Drescher, J. Koskinnen, H.-J. Scheibe, A. Mensch, Diamond Relat. Mater. 7 (1998) 1375.

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