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Stress relief of tetrahedral amorphous carbon films by post-deposition thermal annealing
Surface and Coatings Technology 120–121 (1999) 448–452 www.elsevier.nl/locate/surfcoat
Stress relief of tetrahedral amorphous carbon films by postdeposition thermal annealing B.K. Tay *, X. Shi, E. Liu, S.P. Lau, L.K. Cheah, Z. Sun, J. Shi Microelectronics Centre, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
Abstract The stress relief of tetrahedral amorphous carbon (ta-C ) films by post-deposition thermal annealing was investigated. The films were subjected to rapid thermal annealing (RTA) for 2 min and conventional furnace annealing (CFA) for 30 min. In both cases, the films were annealed in vacuum with argon (4×10−2 Torr) at successive higher temperatures ranging from 500 to 800°C. It was found that annealing by RTA achieved a greater stress reduction and a smaller change in the I /I ratio (obtained from the D G Raman signal of the films) than annealing by CFA. For a ta-C film subjected to 700°C RTA, the stress decreases substantially by ~90%, as compared to the ~80% achieved by CFA. The I /I ratio of the ta-C film subjected to 700°C RTA is 0.23 as compared D G to 0.27 for CFA. This suggests that a higher stress relief of ta-C films can be better achieved by a shorter annealing time without sacrificing much degradation in their diamond-like properties. Subsequent deposition and annealing steps to deposit thicker films were carried out. Films up to 0.8 mm thick with diamond-like properties have been successfully grown. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Amorphous carbon; Cathodic arc; Raman spectroscopy; Stress relief
1. Introduction Over the past few years, tetrahedral amorphous carbon (ta-C ) films grown by the filtered cathodic vacuum arc ( FCVA) technique [1–6 ] have attracted considerable interest owing to their unique combination of properties. Among the important properties of these films are high hardness, low coefficient of friction and optical transparency. The unusual combination of these properties has stimulated studies on various applications such as optical coatings, high durability and low-friction coatings. However, it is well known that typical ta-C films have very high compressive stresses (12~14 GPa) after growth [3,5,7], which are a disadvantage for practical applications because they cause adhesion failure at the film substrate interface, or prevent the growth of thick films. Thus, it is important to find ways of reducing the intrinsic stresses. Stress relief in a-C films was recently achieved by incorporating a small percentage of other elements such as boron and nitrogen to the film [8]. However, in the case of ta-C films, such an approach has so far promoted graphitization, causing a * Corresponding author. Tel.: +65-7905454; fax: +65-7912687. E-mail address: [email protected] (B.K. Tay)
reduction in hardness and bandgap, and other property changes. Another solution that has been studied to improve the stress relief of ta-C films is thermal annealing. Friedman reported that complete stress relief of ta-C films with high-quality diamond-like properties could be achieved by a short-term anneal after a 2 min anneal at 600°C [9]. Grill reported that complete stress relaxation of the films was achieved after a 4 h anneal at 440°C [10]. Because ta-C films are all based on metastable structures that remain highly influenced by their thermal history, it is of interest to compare the thermal behaviour of the films after subjecting them to different annealing times. This paper aims to study whether changes in microstructure and stress relief of ta-C films are dependent on annealing time.
2. Experimental Tetrahedral amorphous carbon (ta-C ) films were deposited by a filtered cathodic vacuum arc (FCVA) system described elsewhere [6 ]. The system incorporates the off-plane double-bend (OPDB) filter [11,12] to effectively remove all macro-particles. During deposition, the
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 46 5 - X
B.K. Tay et al. / Surface and Coatings Technology 120–121 (1999) 448–452
C plasma leaves the self-sustaining arc spot, and then C+ ions are accelerated with a d.c. bias to the out-ofsight highly doped silicon substrate clamped onto a copper substrate holder. The arc current was kept constant at 60 A. The toroidal magnetic field for steering the carbon plasma towards the substrate was maintained at 40 mT. The chamber was evacuated to a base pressure below 2×10−6 Torr but rose to 1.5×10−5 Torr during deposition due to outgasing of the cathode. The substrate was cleaned by an Ar+ ion beam sputtering to remove any native oxide layer on its surface. Tetrahedral amorphous carbon films were deposited at 100 eV (maximum sp3 content) with the substrate held at room ˚ s−1, temperature [7]. The deposition rate was ~10 A and the thickness of a single layer film was typically 70 nm. Two sets of experiments to anneal the films in vacuum with argon (~4×10−2 Torr) were carried out. In set A, one sample piece was subjected to rapid thermal annealing (RTA) for 2 min. In set B, a separate sample deposited under the same conditions as the first piece was subjected to conventional furnace annealing (CFA) for 30 min. In both sets, samples were annealed in 100°C steps from 500 to 800°C. The annealing temperature is the temperature applied to the sample. A calibration was taken between the temperature at the sample (obtained by means of a thermocouple) and the specified oven temperature. To ensure an inert environment, both RTA and CFA chambers were vacuumed and purged with argon for three cycles. The annealing pressure (with introduction of argon) was 4×10−2 Torr. The chambers were then ramped to the set temperature to carry out annealing for the required duration of time. For set A (RTA), the heating and cooling rates at the beginning and end of the anneal were 20°C s−1. For set B (CFA), the heating rate and cooling rates at the beginning and end of the anneal were 40°C min−1. The properties of the ta-C films were characterized in the following manner. The film thickness was measured by laser spectral reflectometry to ˚ . A Tencor P-10 surface profilometer was within ±50 A used to determine the relative film stress of each film in all the sets before and after the annealing treatment. By using the radius of curvature technique, the relative change in stress was computed. The stress, s , in a thin s film of thickness t is given by Stoney’s equation [13]: c E s s= s 6(1−n ) s
t2 s t c
A
1 R
−
1 R
o
B
,
449
the 514 nm lines of Ar+ laser, focused to a spot of 50 mm in diameter. Typically, the spectra were acquired in the backscattering geometry over the range 1000–2000 cm−1 at 1.5 cm−1 interval. To avoid any laser-annealing effects, a low input power of ~5 mW was used. The optical bandgap of the films was determined by a UVISEL spectroscopic phase-modulated ellipsometer in the spectral range of 250–900 nm. The optical band gap of the films was determined by fitting the ellipsometric measurements to a Forouchi–Bloomer model [14] shown to be appropriate for amorphous diamond-like carbon films [15]. In this model, the mechanism for optical absorption is interband excitations between the bonding and antibonding p bands [16 ]. X-ray photoelectron spectroscopy ( XPS) experiments were performed on a VG Scientific ESCALAB 250 spectrometer. A monochromatic MgKa (1486.6 eV ) X-ray with spot size of ~500 mm was used as excitation source operating at 250 W. A concentric hemispherical analyser (CHA) was set at a constant analyser pass energy of 10 eV, which was connected to a six-channeltron detector for data collection.
3. Results and discussion Fig. 1 shows the relative change in stress with annealing temperature for a single-layer ta-C film. In general, the magnitude of the stress relief increases with temperature. However, the stress relief of the films annealed by RTA is comparatively higher than in CFA. At 700°C, the stress decreases by as much as ~90% in RTA, as compared to about ~80% in CFA. This result shows that for an annealing temperature range of 500–800°C, the stress of the films can be better relieved through RTA. Fig. 2 shows a typical Raman spectra with Gaussian fits for a single-layer ta-C film deposited at room temperature. In order to analyze the spectra
(1)
where E , n and t are the Young modulus, Poisson s s s ratio and thickness of the substrate. R and R are the o radii of curvature of the film–substrate composite and bare substrate, respectively. To obtain qualitative information about the changes in the structure of carbon films, the Raman spectra were measured at room temperature on a Renishaw Ramanscope System 1000 using
Fig. 1. Relative change of compressive stress ratio as a function of annealing temperature for a single-layer ta-C film annealed under RTA and CFA conditions. The stress of the as-deposited film, s , is about o 12 GPa.
450
B.K. Tay et al. / Surface and Coatings Technology 120–121 (1999) 448–452 Table 1 Influence of annealing temperature on the E bandgap for a single04 layer ta-C film annealed under RTA and CFA conditions
Fig. 2. Raman spectra of ta-C films deposited at room temperature. The individual G and D components of the fit are also shown.
quantitatively, the Raman spectra were fitted to the two Gaussian peaks using PeakFit, a least-squares computer program. The position and linewidth of the deconvoluted D and G bands are also shown. Fig. 3 shows the corresponding change in the integrated intensity ratio, I /I , with annealing temperature. It can be observed D G that both RTA and CFA annealing results show an increase in the I /I ratio with increasing temperature. D G However, the I /I ratio for films subjected to RTA is D G comparatively lower. Beyond the annealing temperature of 700°C, the gradient of the I /I plot for RTA becomes D G steeper, which indicates graphitization of the film. Therefore, 700°C is chosen as the optimum annealing temperature at which the stress can be almost fully relaxed, and the I /I ratio can still be maintained D G without any significant change. The change in the E 04 optical bandgap of the films determined from the absorption spectra is shown in Table 1. The E bandgap of 04 the as-grown (not annealed) ta-C film is about 3.4. The results showed a reduction in the optical bandgap with
Fig. 3. Variation of integrated intensity ratio, I /I , with annealing D G temperature for a single-layer ta-C film annealed under RTA and CFA conditions. For the sake of comparison, the I /I ratio of an D G as-deposited ta-C film is 0.12.
Annealing temperature (°C )
Relative change in E band 04 gap after annealing (RTA)
Relative change in E band 04 gap after annealing (CFA)
500 600 700 800
0.80 0.90 0.98 0.82
0.92 0.92 0.93 0.94
increase in annealing temperature. This is consistent with a fractional increase in the threefold carbon content. Since the annealed films are relatively stress-free, it is possible to deposit a thicker film. After annealing the first layer, another layer of 70~100 nm of coating was deposited, and the annealing procedure was repeated. In this manner, several thick films can be grown, with the thickest film being 0.8 mm. The stress and Raman results of the single (not annealed) and five-layered (0.5 mm) ta-C film are shown in Table 2. It can been seen that after several rounds of the deposition and annealing process, a thick film with a low stress and a reasonable I /I ratio can be achieved. The thick films D G adhered well to the substrate and showed no sign of delamination. Fig. 4 shows the corresponding normalized Raman spectra of the single and multi-layered thick ta-C film. As can be observed, the shape of the Raman spectrum is relatively unchanged, suggesting minor changes with annealing and indicating that the films retain their diamond-like nature. Fig. 5 shows the absorption coefficient of single layer as grown and the five-layered annealed films. A slight increase in opacity was seen for the multi-layered film. The E bandgap of 04 a five-layered ta-C film reduced by about 10% as compared to a single-layered film. To further support our findings that thick ta-C films can be grown with high diamond-like properties, the XPS C ls region of both
Fig. 4. Raman spectra of a single (~70 nm) and five (~500 nm) layered ta-C film.
451
B.K. Tay et al. / Surface and Coatings Technology 120–121 (1999) 448–452 Table 2 Stress and Raman fitting results of single (not annealed ) and five-layered ta-C films Sample
Thickness (nm)
Stress (GPa)
I /I D G
G-line width (cm−1)
G-peak position (cm−1)
Single-layer Five-layers
70 500
14.11 3.75
0.12 0.15
93.75 85.07
1570.3 1575.1
Fig. 5. Absorption coefficient spectra of a single (~70 nm) and five (~500 nm) layered ta-C film.
Similarly, when the films are rapidly cooled, the top coating layer dissipates the heat faster than the substrate. The faster ramp and cooling rates of the RTA process may have induced a greater degree of stress relief in the ta-C films. The smaller increase in the I /I ratio in D G RTA is likely to be attributed to a shorter annealing time, which gives less opportunity for reactions that result in a stable atomic structure [19]. A prerequisite for the formation of graphitic clusters is bulk and surface mobility of carbon atoms that leads to the formation of thermodynamically stable graphite phase [20]. Thus, it is likely that a shorter annealing time at a high temperature restricts the mobility of the atoms only at the surface layer, leaving the bulk properties relatively unchanged.
4. Conclusion
Fig. 6. XPS C ls region for single-layered and multi-layered ta-C films.
the single layered and multi-layered ta-C films were compared, as shown in Fig. 6. It can be seen that the shape of the spectra of both films is also relatively unchanged. It had been shown in a previous study [17] that the estimated sp3 fraction of our as-grown ta-C film based on XPS analysis is about 80%. The higher stress relief by RTA may be attributed to the higher temperature difference between the film and substrate. The thermal stress in the films originates from a difference in the expansion coefficients between the film and substrate at a particular temperature [18]. When the films are rapidly heated under a constant radiant power, the coating layer usually experiences a faster rise in temperature than the underlying substrate.
In summary, a higher stress relief of ta-C films can be better achieved by a shorter annealing time. The greater stress reduction achieved in RTA may be due to a greater temperature difference between the film and substrate, as a result of a faster temperature ramp rate. The smaller change in the I /I ratio in RTA is likely D G to be attributed to a shorter annealing time, which restricts the graphitization of the film to only occur on the surface layer, thus leaving the bulk properties relatively unchanged. Thick ta-C films can be successfully deposited. The Raman and XPS spectra of annealed films show only subtle changes, indicating that the films retain their diamond-like nature.
References [1] P.J. Martin, S.W. Filipczuk, R.P. Netterfield, J.S. Field, D.F. Whitnall, D.R. McKenzie, J. Mater. Sci. Lett. 7 (1988) 410. [2] P.J. Martin, R.P. Netterfield, T.J. Kinder, L. Descotes, Surf. Coat. Technol. 49 (1991) 239. [3] D.R. McKenzie, D. Muller, B.A. Pailthorpe, Phys. Rev. Lett. 67 (1991) 773. [4] S. Falabella, D.B. Boercker, D.M. Sanders, Thin Film Solids 236 (1993) 82. [5] P.J. Fallon, V.S. Veerasamy, C.A. Davis, J. Robertson, G.A.J. Amaratunga, W.L. Milne, J. Koskinen, Phys. Rev. B 48 (1993) 4777. [6 ] X. Shi, B.K. Tay, H.S. Tan, Y.Q. Tu, S.R.P. Silva, W.L. Milne, J. Appl. Phys. 79 (9) (1996) 7239.
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[7] B.K. Tay, X. Shi, H.S. Tan, H.S. Yang, Z. Sun, Surf. Coat. Technol. 105 (1998) 155. [8] M. Chhowalla, Y. Yin, G.A.I. Amaratunga, D.R. McKenzie, T. Faueheim, Appl. Phys. Lett. 69 (1996) 2344. [9] T.A. Friedmann, J.P. Sullivan, J.A. Knapp, D.R. Tallant, D.M. Follstaedt, Appl. Phys. Lett. 71 (1997) 3820. [10] A. Grill, V. Patel, B.S. Meyerson, J. Mater. Res. 5 (11) (1990) 2531. [11] X. Shi, D. Flynn, B.K. Tay, H.S. Tan, in: Filtered Cathodic Arc Source PCT/GB96/00389, 20 February (1995). [12] X. Shi, M. Fulton, D. Flynn, H.S. Tan, in: Deposition Apparatus, PCT/GB96/00390, 20 February (1995).
[13] G.G. Stoney, Proc. R. Soc. Lond. 82 (1909) 172. [14] A.R. Forouhi, I. Bloomer, Handbook of Optical Constants of Solids II, Academic Press, New York, 1991. Chapter 151. [15] S.A. Alterovitz, N. Savvides, F.W. Smith, J.A. Woollam, Handbook of Optical Constants of Solids II, Academic Press, New York, 1991. [16 ] J. Roberston, E.P. O’Reilly, Phys. Rev. B 35 (1987) 2946. [17] B.K. Tay, D. Chua, Surf. Interf. Anal. J. 27 (1999) in press. [18] M. Ohring, The Material Science of Thin Films, Academic Press, New York, 1992. [19] J.P. Hirvonen, J. Koskinen, J. Appl. Phys. 81 (1997) 7248. [20] P. Reinke, P. Oelhafen, J. Appl. Phys. 81 (1997) 2396.
Stress relief of tetrahedral amorphous carbon films by postdeposition thermal annealing B.K. Tay *, X. Shi, E. Liu, S.P. Lau, L.K. Cheah, Z. Sun, J. Shi Microelectronics Centre, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
Abstract The stress relief of tetrahedral amorphous carbon (ta-C ) films by post-deposition thermal annealing was investigated. The films were subjected to rapid thermal annealing (RTA) for 2 min and conventional furnace annealing (CFA) for 30 min. In both cases, the films were annealed in vacuum with argon (4×10−2 Torr) at successive higher temperatures ranging from 500 to 800°C. It was found that annealing by RTA achieved a greater stress reduction and a smaller change in the I /I ratio (obtained from the D G Raman signal of the films) than annealing by CFA. For a ta-C film subjected to 700°C RTA, the stress decreases substantially by ~90%, as compared to the ~80% achieved by CFA. The I /I ratio of the ta-C film subjected to 700°C RTA is 0.23 as compared D G to 0.27 for CFA. This suggests that a higher stress relief of ta-C films can be better achieved by a shorter annealing time without sacrificing much degradation in their diamond-like properties. Subsequent deposition and annealing steps to deposit thicker films were carried out. Films up to 0.8 mm thick with diamond-like properties have been successfully grown. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Amorphous carbon; Cathodic arc; Raman spectroscopy; Stress relief
1. Introduction Over the past few years, tetrahedral amorphous carbon (ta-C ) films grown by the filtered cathodic vacuum arc ( FCVA) technique [1–6 ] have attracted considerable interest owing to their unique combination of properties. Among the important properties of these films are high hardness, low coefficient of friction and optical transparency. The unusual combination of these properties has stimulated studies on various applications such as optical coatings, high durability and low-friction coatings. However, it is well known that typical ta-C films have very high compressive stresses (12~14 GPa) after growth [3,5,7], which are a disadvantage for practical applications because they cause adhesion failure at the film substrate interface, or prevent the growth of thick films. Thus, it is important to find ways of reducing the intrinsic stresses. Stress relief in a-C films was recently achieved by incorporating a small percentage of other elements such as boron and nitrogen to the film [8]. However, in the case of ta-C films, such an approach has so far promoted graphitization, causing a * Corresponding author. Tel.: +65-7905454; fax: +65-7912687. E-mail address: [email protected] (B.K. Tay)
reduction in hardness and bandgap, and other property changes. Another solution that has been studied to improve the stress relief of ta-C films is thermal annealing. Friedman reported that complete stress relief of ta-C films with high-quality diamond-like properties could be achieved by a short-term anneal after a 2 min anneal at 600°C [9]. Grill reported that complete stress relaxation of the films was achieved after a 4 h anneal at 440°C [10]. Because ta-C films are all based on metastable structures that remain highly influenced by their thermal history, it is of interest to compare the thermal behaviour of the films after subjecting them to different annealing times. This paper aims to study whether changes in microstructure and stress relief of ta-C films are dependent on annealing time.
2. Experimental Tetrahedral amorphous carbon (ta-C ) films were deposited by a filtered cathodic vacuum arc (FCVA) system described elsewhere [6 ]. The system incorporates the off-plane double-bend (OPDB) filter [11,12] to effectively remove all macro-particles. During deposition, the
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 46 5 - X
B.K. Tay et al. / Surface and Coatings Technology 120–121 (1999) 448–452
C plasma leaves the self-sustaining arc spot, and then C+ ions are accelerated with a d.c. bias to the out-ofsight highly doped silicon substrate clamped onto a copper substrate holder. The arc current was kept constant at 60 A. The toroidal magnetic field for steering the carbon plasma towards the substrate was maintained at 40 mT. The chamber was evacuated to a base pressure below 2×10−6 Torr but rose to 1.5×10−5 Torr during deposition due to outgasing of the cathode. The substrate was cleaned by an Ar+ ion beam sputtering to remove any native oxide layer on its surface. Tetrahedral amorphous carbon films were deposited at 100 eV (maximum sp3 content) with the substrate held at room ˚ s−1, temperature [7]. The deposition rate was ~10 A and the thickness of a single layer film was typically 70 nm. Two sets of experiments to anneal the films in vacuum with argon (~4×10−2 Torr) were carried out. In set A, one sample piece was subjected to rapid thermal annealing (RTA) for 2 min. In set B, a separate sample deposited under the same conditions as the first piece was subjected to conventional furnace annealing (CFA) for 30 min. In both sets, samples were annealed in 100°C steps from 500 to 800°C. The annealing temperature is the temperature applied to the sample. A calibration was taken between the temperature at the sample (obtained by means of a thermocouple) and the specified oven temperature. To ensure an inert environment, both RTA and CFA chambers were vacuumed and purged with argon for three cycles. The annealing pressure (with introduction of argon) was 4×10−2 Torr. The chambers were then ramped to the set temperature to carry out annealing for the required duration of time. For set A (RTA), the heating and cooling rates at the beginning and end of the anneal were 20°C s−1. For set B (CFA), the heating rate and cooling rates at the beginning and end of the anneal were 40°C min−1. The properties of the ta-C films were characterized in the following manner. The film thickness was measured by laser spectral reflectometry to ˚ . A Tencor P-10 surface profilometer was within ±50 A used to determine the relative film stress of each film in all the sets before and after the annealing treatment. By using the radius of curvature technique, the relative change in stress was computed. The stress, s , in a thin s film of thickness t is given by Stoney’s equation [13]: c E s s= s 6(1−n ) s
t2 s t c
A
1 R
−
1 R
o
B
,
449
the 514 nm lines of Ar+ laser, focused to a spot of 50 mm in diameter. Typically, the spectra were acquired in the backscattering geometry over the range 1000–2000 cm−1 at 1.5 cm−1 interval. To avoid any laser-annealing effects, a low input power of ~5 mW was used. The optical bandgap of the films was determined by a UVISEL spectroscopic phase-modulated ellipsometer in the spectral range of 250–900 nm. The optical band gap of the films was determined by fitting the ellipsometric measurements to a Forouchi–Bloomer model [14] shown to be appropriate for amorphous diamond-like carbon films [15]. In this model, the mechanism for optical absorption is interband excitations between the bonding and antibonding p bands [16 ]. X-ray photoelectron spectroscopy ( XPS) experiments were performed on a VG Scientific ESCALAB 250 spectrometer. A monochromatic MgKa (1486.6 eV ) X-ray with spot size of ~500 mm was used as excitation source operating at 250 W. A concentric hemispherical analyser (CHA) was set at a constant analyser pass energy of 10 eV, which was connected to a six-channeltron detector for data collection.
3. Results and discussion Fig. 1 shows the relative change in stress with annealing temperature for a single-layer ta-C film. In general, the magnitude of the stress relief increases with temperature. However, the stress relief of the films annealed by RTA is comparatively higher than in CFA. At 700°C, the stress decreases by as much as ~90% in RTA, as compared to about ~80% in CFA. This result shows that for an annealing temperature range of 500–800°C, the stress of the films can be better relieved through RTA. Fig. 2 shows a typical Raman spectra with Gaussian fits for a single-layer ta-C film deposited at room temperature. In order to analyze the spectra
(1)
where E , n and t are the Young modulus, Poisson s s s ratio and thickness of the substrate. R and R are the o radii of curvature of the film–substrate composite and bare substrate, respectively. To obtain qualitative information about the changes in the structure of carbon films, the Raman spectra were measured at room temperature on a Renishaw Ramanscope System 1000 using
Fig. 1. Relative change of compressive stress ratio as a function of annealing temperature for a single-layer ta-C film annealed under RTA and CFA conditions. The stress of the as-deposited film, s , is about o 12 GPa.
450
B.K. Tay et al. / Surface and Coatings Technology 120–121 (1999) 448–452 Table 1 Influence of annealing temperature on the E bandgap for a single04 layer ta-C film annealed under RTA and CFA conditions
Fig. 2. Raman spectra of ta-C films deposited at room temperature. The individual G and D components of the fit are also shown.
quantitatively, the Raman spectra were fitted to the two Gaussian peaks using PeakFit, a least-squares computer program. The position and linewidth of the deconvoluted D and G bands are also shown. Fig. 3 shows the corresponding change in the integrated intensity ratio, I /I , with annealing temperature. It can be observed D G that both RTA and CFA annealing results show an increase in the I /I ratio with increasing temperature. D G However, the I /I ratio for films subjected to RTA is D G comparatively lower. Beyond the annealing temperature of 700°C, the gradient of the I /I plot for RTA becomes D G steeper, which indicates graphitization of the film. Therefore, 700°C is chosen as the optimum annealing temperature at which the stress can be almost fully relaxed, and the I /I ratio can still be maintained D G without any significant change. The change in the E 04 optical bandgap of the films determined from the absorption spectra is shown in Table 1. The E bandgap of 04 the as-grown (not annealed) ta-C film is about 3.4. The results showed a reduction in the optical bandgap with
Fig. 3. Variation of integrated intensity ratio, I /I , with annealing D G temperature for a single-layer ta-C film annealed under RTA and CFA conditions. For the sake of comparison, the I /I ratio of an D G as-deposited ta-C film is 0.12.
Annealing temperature (°C )
Relative change in E band 04 gap after annealing (RTA)
Relative change in E band 04 gap after annealing (CFA)
500 600 700 800
0.80 0.90 0.98 0.82
0.92 0.92 0.93 0.94
increase in annealing temperature. This is consistent with a fractional increase in the threefold carbon content. Since the annealed films are relatively stress-free, it is possible to deposit a thicker film. After annealing the first layer, another layer of 70~100 nm of coating was deposited, and the annealing procedure was repeated. In this manner, several thick films can be grown, with the thickest film being 0.8 mm. The stress and Raman results of the single (not annealed) and five-layered (0.5 mm) ta-C film are shown in Table 2. It can been seen that after several rounds of the deposition and annealing process, a thick film with a low stress and a reasonable I /I ratio can be achieved. The thick films D G adhered well to the substrate and showed no sign of delamination. Fig. 4 shows the corresponding normalized Raman spectra of the single and multi-layered thick ta-C film. As can be observed, the shape of the Raman spectrum is relatively unchanged, suggesting minor changes with annealing and indicating that the films retain their diamond-like nature. Fig. 5 shows the absorption coefficient of single layer as grown and the five-layered annealed films. A slight increase in opacity was seen for the multi-layered film. The E bandgap of 04 a five-layered ta-C film reduced by about 10% as compared to a single-layered film. To further support our findings that thick ta-C films can be grown with high diamond-like properties, the XPS C ls region of both
Fig. 4. Raman spectra of a single (~70 nm) and five (~500 nm) layered ta-C film.
451
B.K. Tay et al. / Surface and Coatings Technology 120–121 (1999) 448–452 Table 2 Stress and Raman fitting results of single (not annealed ) and five-layered ta-C films Sample
Thickness (nm)
Stress (GPa)
I /I D G
G-line width (cm−1)
G-peak position (cm−1)
Single-layer Five-layers
70 500
14.11 3.75
0.12 0.15
93.75 85.07
1570.3 1575.1
Fig. 5. Absorption coefficient spectra of a single (~70 nm) and five (~500 nm) layered ta-C film.
Similarly, when the films are rapidly cooled, the top coating layer dissipates the heat faster than the substrate. The faster ramp and cooling rates of the RTA process may have induced a greater degree of stress relief in the ta-C films. The smaller increase in the I /I ratio in D G RTA is likely to be attributed to a shorter annealing time, which gives less opportunity for reactions that result in a stable atomic structure [19]. A prerequisite for the formation of graphitic clusters is bulk and surface mobility of carbon atoms that leads to the formation of thermodynamically stable graphite phase [20]. Thus, it is likely that a shorter annealing time at a high temperature restricts the mobility of the atoms only at the surface layer, leaving the bulk properties relatively unchanged.
4. Conclusion
Fig. 6. XPS C ls region for single-layered and multi-layered ta-C films.
the single layered and multi-layered ta-C films were compared, as shown in Fig. 6. It can be seen that the shape of the spectra of both films is also relatively unchanged. It had been shown in a previous study [17] that the estimated sp3 fraction of our as-grown ta-C film based on XPS analysis is about 80%. The higher stress relief by RTA may be attributed to the higher temperature difference between the film and substrate. The thermal stress in the films originates from a difference in the expansion coefficients between the film and substrate at a particular temperature [18]. When the films are rapidly heated under a constant radiant power, the coating layer usually experiences a faster rise in temperature than the underlying substrate.
In summary, a higher stress relief of ta-C films can be better achieved by a shorter annealing time. The greater stress reduction achieved in RTA may be due to a greater temperature difference between the film and substrate, as a result of a faster temperature ramp rate. The smaller change in the I /I ratio in RTA is likely D G to be attributed to a shorter annealing time, which restricts the graphitization of the film to only occur on the surface layer, thus leaving the bulk properties relatively unchanged. Thick ta-C films can be successfully deposited. The Raman and XPS spectra of annealed films show only subtle changes, indicating that the films retain their diamond-like nature.
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