Heat treatment of tetrahedral amorphous carbon films grown by filtered cathodic vacuum-arc technique

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1 Diamond and Related Materials 8 (1999) Heat treatment of tetrahedral amorphous carbon films grown by filtered cathodic vacuum-arc technique B.K. Tay a,*, X. Shi a, E.J. Liu a, H.S. Tan a, L.K. Cheah a, W.I. Milne b a School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore b Department of Engineering, Cambridge University, Trumpington Street, Cambridge CB2 1PZ, UK Received 31 July 1998; received in revised form 2 April 1999 Abstract Tetrahedral amorphous carbon (ta-c ) is a potential low-cost substitute for diamond in certain applications, but little is known of the temperature range over which its desirable properties are retained. The thermal stability of tetrahedral amorphous carbon (ta-c ) films has been investigated by heat treatment of the films at temperatures from room temperature to 450 C in high vacuum, low vacuum and oxygen ambient. It was found that heat treatment in oxygen ambient leads to a much more prominent variation in film thickness, stress and hardness than in both low and high vacuum. Raman studies also show an increase of the G-band frequency to higher values, an increase of the integrated intensity ratio and a narrowing of the G bands for films annealed in oxygen ambient with increasing By contrast, ta-c films exhibit a high resistance to degradation during treatment in low and high vacuum. They sustain their structure, thickness, stress and hardness for temperatures up to 400 C Elsevier Science S.A. All rights reserved. Keywords: Amorphous carbon; Cathodic arc; Heat treatment; Raman spectroscopy 1. Introduction graphitic Raman spectrum, an increased coefficient of friction, an increased index of refraction, reduced stress Recently, films of a highly sp3-bonded form of amorphous and increased wear when exposed to high carbon, denoted tetrahedral amorphous carbon The onset of these effects depends on the deposition (ta-c ), deposited by the filtered cathodic vacuum-arc method and the specific thermal treatment; it lies typi- ( FCVA) technique, have been intensively studied [1 7]. cally between 200 and 500 C. While much has been The structure of such films has been shown to consist covered for hydrogenated amorphous carbon films, the of a disordered network of sp2- and sp3-bonded atoms, thermal stability of the hydrogen-free ta-c films has not with the fraction of sp3 coordination being as high as been covered in detail. The purpose of this work is to 87%. These films have attracted many studies because examine the thermal stability of ta-c films grown by of interest in their use as a potential low-cost substitute FCVA technique. The influence of annealing environ- for diamond, owing to their unique and promising ment, such as high vacuum, low vacuum and oxygen properties. However, the usefulness of ta-c films will be gas flow, on the film stability is investigated in this work. partially determined by their thermal stability. Future applications of ta-c films will only be ensured if the properties of the material remain stable under a variety of operating conditions, including moderate elevations 2. Experimental details in The thermal stability for hydrogenated amorphous carbon films deposited by various methods 2.1. Sample preparation has been studied extensively by various researchers [8,9]. Details of the deposition system have been published In general, the films lose hydrogen and show a more elsewhere [7]. The system incorporates the off-plane double-bend (OPDB) filter [10,11] to effectively remove * Corresponding author. Tel.: ; fax: all macro particles. The substrates used were 100 address: ebktay@ntu.edu.sg (B.K. Tay) p-type silicon wafers with an average thickness of /99/$ see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S (99)

2 B.K. Tay et al. / Diamond and Related Materials 8 (1999) mm. The silicon surfaces were precleaned with the ellipsometric measurements to a Forouchi Bloomer deionized water in an ultrasonic bath. Buffered hydrofluoric model [14] shown to be appropriate for amorphous acid was also used during the cleaning process diamond-like carbon films [15]. In this model the mecha- to remove the native oxide layer on the surface of the nism for optical absorption is interband excitations silicon wafer. Tetrahedral amorphous carbon films were between the bonding and antibonding p bands [16]. deposited at 100 ev ion energy (maximum sp3 content) with the substrate held at room The film thickness was typically 70 nm. Three sets of experiments 3. Experimental results and discussion to anneal the films at different temperatures ranging from 100 to 450 C were carried out. The first set (A) Fig. 1 shows the relative change in film thickness as was annealed in high vacuum (~10 6 torr), the second a function of heat-treatment In general, set ( B) in low vacuum (~10 2 torr) and the third set the film thickness decreases when exposed to higher (C) in an oxygen flow (~0.5 torr). In the last set, the Heat treatment in oxygen ambient (set C) chamber was vacuum-pumped to 10 2 torr after which at 450 C led to considerable loss of film thickness ( 26%) oxygen gas was introduced via mass flow controller to due to oxidation, while heat treatment in high vacuum increase the pressure to 0.5 torr. The samples were kept (set A) and low vacuum (set B) resulted in only a small at each annealing temperature for 35 min and allowed change (5%). Owing to complete loss of the film when to cool down to room temperature for other measure- subjected to heat treatment in oxygen ambient at above ments. The heating rate at the beginning of the annealing 500 C, no thermal behaviour at higher temperature was cycle was about 40 C min 1. observed. Fig. 2 shows the relative change in compressive stress as a function of heat treatment. The stress 2.2. Film characterization also decreases with higher heat-treatment At around 400 C, it can be seen that the stress decreases The thermal stability of the ta-c films was characterized by 10% and 20% for set A and set B, respectively. For in the following manner. The film thicknesses in set C, the stress decreases much more prominently by all the three sets were measured by laser spectral reflectometry as much as 60%. A linear decreasing trend with increas- to within ±50 Å. A Tencor P-10 surface profi- ing heat treatment was also observed for set C, at which lometer was used to determine the stress of each film in the film is completely etched off the substrate at around all the sets. By using the radius of curvature technique 500 C. Fig. 3 shows the relative change in hardness of [12], the relative change in stress before and after heat the films before and after heat treatment. Heat treatment treatment was computed for each film. The hardness of of the films in low and high vacuum does not change the films was examined by nanoindentation [13]. The measurements were performed with a sharp Berkovich diamond indenter. Nanoindentation measures the hardness as a function of contact depth and is influenced by the substrate. Hence the relative change in hardness before and after heat treatment, rather the absolute hardness, was determined. Atomic force microscopy (AFM) studies were performed with a Digital Instruments Nanoscope IIIa system. The AFM data were used to derive a surface roughness R, defined as q R = Z Z /N, where Z is the average of the Z q i ave ave height values within a given area, Z is the current Z i value, and N is the number of points within a given area. To obtain qualitative information about changes in the structure of carbon films with heat treatment, Raman spectra were measured at room temperature on a Renishaw system using the nm line of an Ar+-ion laser as excitation source. The Raman spectra were acquired over the range from 1100 cm 1 to 2000 cm 1 at 1 cm 1 resolution. Low input power was used in order to minimize any possible heating effects of the beam. The optical bandgap of the films were determined by a UVISEL spectroscopic phase-modulated ellipsometer in the nm spectral range. The optical bandgap of the films was determined by fitting the hardness values significantly, at least up to 300 C, while for heat treatment in oxygen ambient the hardness starts to change significantly at about 100 C. Fig. 4 Fig. 1. Change of film thickness as a function of heat-treatment

3 1330 B.K. Tay et al. / Diamond and Related Materials 8 (1999) Fig. 2. Change of compressive stress as a function of heat-treatment Fig. 4. Surface roughness as a function of heat-treatment Fig. 3. Change of hardness as a function of heat-treatment Fig. 5. Raman spectra before and after heat treatment. the thickness, stress, hardness and roughness values. shows that that surface roughness also increases with This can probably to be attributed to the surface oxida- heat-treatment Films in set A and B do tion process, which resulted in a decrease of film thickness not exhibit a significant change in surface roughness and served as a catalyst for sp3-to-sp2 bond with the high- and low-vacuum heat treatments. In both relaxation. sets, the surface topography retained the initial smoothness Fig. 5 shows the Raman spectra between 1100 cm 1 of the silicon substrate ( nm) up to and 2000 cm 1 for ta-c films subjected to annealing at ~400 C. However, a significant increase of the surface a temperature of 400 C in low vacuum, high vacuum roughness at higher heat-treatment temperature was and oxygen ambient. The Raman spectra of amorphous observed for films in set C. hydrogen-free ta-c differ essentially from those of the Figs. 1 to 4 demonstrate that the films deposited at hydrogenated materials, despite the generally amorphous 100 ev are relatively stable in vacuum before and after structure of the two types of film [ 17,18]. In treatment up to 400 C. This is evidence of the high particular, the spectra exhibit a single, broad, asymmetric thermal stability of the film. On the other hand, films peak centred around 1550 cm 1. This main peak, annealed in oxygen ambient show distinct changes in conventionally attributed to highly localized sp2-

4 B.K. Tay et al. / Diamond and Related Materials 8 (1999) Table 1 E 04 bandgap values after heat-treatment process 25 C 3.49 Heat treatment in low vacuum (400 C ) 3.53 Heat treatment in high vacuum (400 C ) 3.68 Heat treatment in oxygen ambient (400 C ) 4.13 E 04 bandgap (ev) films from sets A and B, the intensity ratio I /I is D G relatively constant up to 400 C, increasing slightly from 0.10 to The change in the ratio is more pronounced for films from set C, increasing from 0.10 to 0.40 with increasing heat-treatment temperature up to 450 C. The intensity ratio result correlates very well with the peak position and linewidth of the G band. The dependence of the films G-peak position and linewidth on the heattreatment temperature is shown in Fig. 7. It shows an upward frequency shift of the G band and a narrowing Fig. 6. The variation of integrated intensity ratio, I /I, with heat- D G of the G band with increasing heat-treatment tempertreatment ature. Similar observations have also been reported by other researchers for hydrogenated amorphous carbon bonded clusters residing in the network, can be approxi- films. It has been reported [19] that the position of the mated by two Gaussians at about 1350 cm 1 and G band and its linewidth in the Raman spectrum for 1550 cm 1 (the so-called D and G lines). It can be seen amorphous carbon films are correlated with at least two from Fig. 5 that for the set annealed in oxygen ambient, film properties the sp3/sp2 ratio and the film stress, the film properties are much more temperature-sensitive. where a lower sp3 content and/or lower stress leads to The oxygen ambient spectra show considerable changes a lower G linewidth. In these all cases, as well as in the in comparison to the unheated film sample, indicating current study, these changes in the Raman spectrum some change in the structure of the ta-c film, possibly suggest graphitization of the films, as evidenced by the related to an increase in the sp2/sp3 bonding ratio. By shift in position of the G peak towards 1585 cm 1, contrast, the Raman spectra are only slightly changed which is the Raman peak for nanocrytalline graphite. after heat treatment in low and high vacuum. Fig. 6 The relatively small shift in the G-peak position and the shows the variation in the integrated intensity ratio of change in G linewidth observed for ta-c films suggest the D peak to the G peak (I /I ) determined from the D G that the ta-c films are structurally stable when subjected fits as a function of heat-treatment For to heat treatment in vacuum. The E optical bandgaps of the films determined 04 from the absorption spectra are shown in Table 1. Of particular interest is that all films showed a slight but definite increase in optical bandgap. We do not have a good explanation for the observed increase; possibly some relaxation of structural defects might be responsible. Further work on rapid thermal (<2 min) and long (>60 min) annealing times will be carried out to investigate this observed effect, as ta-c films are all basically metastructure films. 4. Conclusion Fig. 7. The variation of peak position and linewidth of the G band with heat-treatment In conclusion, we have explored the temperature stability of vacuum-annealed ta-c films. It was found that the thermal stability of the films is greatly affected by the addition of a background gas such as oxygen. Raman studies show an increase of the G-band frequency to higher values, an increase of the integrated

5 1332 B.K. Tay et al. / Diamond and Related Materials 8 (1999) intensity ratio and a narrowing of the G band for films [6] P.J. Fallon, V.S. Veerasamy, C.A. Davis, J. Robertson, annealed at increasing Films exposed to G.A.J. Amaratunga, W.I. Milne, J. Koskinen, Phys. Rev. B 48 (1993) 7. heat treatment in oxygen ambient exhibit the highest [7] S. Xu, B.K. Tay, H.S. Tan, L. Zhong, Y.Q. Tu, S.R.P. Silva, W.I. oxidization rate. They began to graphitize at a temper- Milne, J. Appl. Phys. 79 (9) (1996) ature of 300 C and above, as evidenced by the shift of [8] T.A. Friedman, K.F. McCarty, J.C. Barbour, M.P. Siegal, D.C. the G peak towards 1585 cm 1, which is the Raman Dibble, Appl. Phys. Lett. 68 (12) (1996) peak for nanocrytalline graphite. By contrast, ta-c films [9] D.R. Tallant, J.E. Parmeter, M.P. Siegal, R.L. Simpson, Diamond Relat. Mater. 4 (1995) 191. exhibit a high resistance to degradation during treatment [10] X. Shi, D. Flynn, B.K. Tay, H.S. Tan, Filtered cathodic arc source, in high and low vacuum. They sustain their structure, PCT/GB96/00389, 20 February thickness, stress and hardness up to 400 C. [11] X. Shi, M. Fulton, D. Flynn, H.S. Tan, Deposition apparatus, PCT/GB96/00390, 20 February [12] A. Argon, V. Gupta, H. Landis, J. Cornie, Mater. Sci. Eng. A References 107 (1989) 41. [13] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) [14] A.R. Forouhi, I. Bloomer, Handbook of Optical Constants of [1] I.I. Aksenov, V.A. Belous, V.G. Padalka, V.M. Khoroshikh, Fiz Solids vol. II, Academic Press, New York, 1991, p Plazmy, Sov. J. Plasma Phys. 4 (1978) 425. [15] S.A. Alterovitz, N. Savvides, F.W. Smith, J.A. Woollam, Handbook [2] I.I. Aksenov, S.I. Vakula, V.G. Padalka, V.E. Strelnitskii, V.M. of Optical Constants of Solids vol. II, Academic Press, New Khoroshikh, Sov. Phys. Tech. Phys. 25 (1980) York, 1991, p [3] P.J. Martin, S.W. Filipczuk, R.P. Netterfield, J.S. Field, D.F. [16] J. Roberston, E.P. O Reilly, Phys. Rev. B 35 (1987) Whitnall, D.R. McKenzie, J. Mater. Sci. Lett. 7 (1988) 410. [17] M.A. Tamor, W.C. Vassell, J. Appl. Phys. 76 (6) (1994) [4] P.J. Martin, R.P. Netterfield, T.J. Kinder, L. Descotes, Surf. Coat. [18] S. Anders, J.W. Ager III,, G.M. Pharr, T.Y. Tsui, I.G. Brown, Technol. 49 (1991) 239. Thin Solid Films (1997) 186. [5] D.R. McKenzie, D. Muller, B.A. Pailthorpe, Phys. Rev. Lett. 67 [19] J. Schwan, S. Ulrich, V. Batori, H. Ehrhardt, S.R.P. Silva, J. Appl. (1991) 773. Phys. 80 (1) (1996) 440.