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Time and temperature-dependent changes in the structural properties of tetrahedral amorphous carbon films
Surface and Coatings Technology 130 Ž2000. 248᎐251
Time and temperature-dependent changes in the structural properties of tetrahedral amorphous carbon films B.K. Tay U , D. Sheeja, S.P. Lau, X. Shi, B.C. Seet, Y.C. Yeo Microelectronics Centre, School of Electrical and Electronic Engineering, Nanyang Technological Uni¨ ersity, Nanyang 639798, Singapore Received 13 October 1999; accepted in revised form 5 April 2000
Abstract The time and temperature-dependent changes in the structural properties of tetrahedral amorphous carbon Žta-C. films were accessed continuously by Raman spectroscopy. It has been found that a film of 70-nm thickness remains structurally stable after annealing in air at up to 300⬚C for 4 h. Although some degree of graphitization was observed on a film annealed at 400⬚C, the film begins to oxidize and lose thickness only after annealing at 500⬚C for more than 2 h. This reflects the high thermal stability of the films. In general, annealing results in a narrowing and an upshifting of the G-band together with an increase in the I DrIG ratio. Most of these changes were observed during the first 2 h of annealing, after which the structure of the film appears to stabilize, with the exception at 500⬚C in which the film deteriorates further as oxidation occurs. It was also observed that thin film has better thermal stability against graphitization than thick film. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Amorphous carbon film; Annealing; Graphitization; Raman spectra; Thermal stability
1. Introduction Hard amorphous diamond-like carbon coatings are now being used in several applications Ži.e. computer hard discs, magnetic storage tape heads and drums, sunglasses, barcode readers and orthopedic implants. because of their good wear resistance and low friction in dry sliding. The thermal behavior of these films annealed in vacuum has been extensively studied w1,2x. However, the changes in film properties and structural relaxation due to annealing in air have been much less studied. It is therefore, important to investigate the properties of the films annealed in air as it represents the condition of most practical importance for coating applications. In this paper, the time᎐temperature annealing behavior of tetrahedral amorphous carbon ŽtaC. films deposited by the filtered cathodic vacuum arc ŽFCVA. technique was examined by Raman spectroU
Corresponding author.
scopy. This is made possible by a programable heater stage, which enables the film to be heat-treated continuously and allows us to map the evolution of the Raman spectra with increasing time and temperature.
2. Experimental details 2.1. Film preparation Tetrahedral amorphous carbon Žta-C. films were deposited by a filtered cathodic vacuum arc ŽFCVA. system described elsewhere w3x. The system incorporates the off-plane double-bend ŽOPDB. filter w4,5x to effectively remove all macro-particles. During deposition the C plasma leaves the self-sustaining arc spot, and then Cq ions are accelerated with a DC bias to the out-of-sight 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 main-
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 6 9 9 - X
B.K. Tay et al. r Surface and Coatings Technology 130 (2000) 248᎐251
249
tained at 40 mT. The chamber was evacuated to a base pressure below 2 = 10y6 t but rose to 1.5= 10y5 t during deposition due to outgasing of the cathode. The substrate was cleaned by an Arq ion beam, which is generated by an ion source supplied by Ion Tech Inc., to remove any native oxide layer on its surface. Tetrahedral amorphous carbon films were deposited at 100 eV Žmaximum sp 3 content of approx. 88%. with the substrate held at room temperature w6x, and the deposi˚ tion rate was ; 10 Ars. 2.2. Annealing process A Linkam THMS 600 stage heater was used to anneal the films in air. Two sets of experiments were carried out to investigate the time᎐temperature annealing behavior of the films. In set A, all the films were 70-nm thick and each was annealed at different temperatures ranging from 150 to 550⬚C. At each temperature, Raman spectra were acquired at every 15-min interval during the first hour followed by every 30-min interval during the second hour and subsequently at every 1-h interval up to 4 h. In set B, films of different thicknesses Ž30 nm, 70 nm and 100 nm. were heated continuously in air, from ambient temperature up to 500⬚C at every 100⬚C interval. Before ramping to the next higher temperature, the heater stage was programed to hold at each temperature for approximately 5 min for the system to reach a steady temperature as well as for the Raman spectra to be acquired. The temperature stability during the measurements was "2⬚C, and the heating and cooling rates at the beginning and end of annealing were 40⬚Crmin. The temperature difference between the top of the film and the bottom of the silicon substrate is found to be approximately 10⬚C for a 508-m silicon wafer, and hence the temperature gradient is approximately 20⬚Crmm.
Fig. 1. Fitting of a typical Raman spectrum with two Gaussian curves.
Fig. 2. I D rI G ratio of ta-C films annealed at different temperatures and time.
2.3. Film characterization The Raman spectra were measured on a Renishaw Rama scope System 1000 using the 514 nm lines of the Arq laser, focused to a spot of 50 m in diameter. Typically the spectra were acquired in the backscattering geometry over the range from 100 to 4000 cmy1 at 1.5 cmy1 intervals. To avoid any laser annealing effects, a low input power of ; 5 mW was used. The data were then fitted to two Gaussian curves, ‘G’ and ‘D’ curves as shown in Fig. 1 using PeakFit, a least squares computer program.
3. Results and discussion Fig. 2 shows the intensity ratio ID rIG as a function of annealing time at different temperatures ranging from 150 to 550⬚C. The ID rIG ratio increased with annealing time and temperature. In general, for an-
Fig. 3. G-line width of ta-C films annealed at different temperatures and time.
250
B.K. Tay et al. r Surface and Coatings Technology 130 (2000) 248᎐251
Fig. 4. G-peak position of ta-C films annealed at different temperatures and time.
nealing temperatures below 500⬚C, the ID rIG ratio increased gradually during the first 2 h of annealing, beyond which it saturated at a certain level. However, for annealing at 500⬚C, a sharp increase in the ID rIG ratio was observed at the third hour of annealing. This could be attributed to the surface oxidation process, which resulted in a decrease of film thickness and served as a catalyst for sp 3-to-sp 2 bond relaxation w7x. The sp 3-to-sp 2 bond relaxation affects the ID rIG ratio. The rate of oxidation increased more dramatically when annealed at 550⬚C, in which a complete loss of film thickness was observed after 30 min of annealing, and hence further data could not be obtained. Fig. 3 shows the G-line width ŽGaussian width. as a function of annealing time at different temperatures. The G-line width is observed to be decreasing with increasing annealing temperature. Schwan et al. w8x have reported an increasing linear relation between G-line width and stress. Hence, a narrower G-line
Fig. 5. Raman spectra of ta-C films annealed at different temperatures after 2 hrs.
Fig. 6. Raman spectra of a ta-C film annealed at 500⬚C up to 4 hrs.
width observed was associated with lower film stress. It is thus interesting to observe a significant reduction in film stress at 150⬚C, considering only a minor increase in the sp 2 fractions as evidenced by its relatively low ID rIG ratio. The decrease in stress could be attributed to the migration of carbon interstitials to the surface upon annealing w9x. It has been known that the impingement of energetic carbon ions during film growth often results in an amorphous network that is full of defects Ži.e. interstitials, vacancies, dangling bonds, etc... However, when the interstitials have gained sufficient mobility to migrate and recombine with the vacancies near the surface, it results in a reduced film density and stress. It is possible that at moderate temperatures Ži.e. 150⬚C., the thermal energy is sufficient for the unbonded interstitials to migrate, but may be insufficient to trigger a significant sp 3-to-sp 2 bond re-
Fig. 7. Raman spectra of ta-C films annealed in different environments.
B.K. Tay et al. r Surface and Coatings Technology 130 (2000) 248᎐251
laxation, which explains why the ID rIG ratio remains relatively at a low value. Fig. 4 shows the G-peak position as a function of annealing time at different temperatures. A noticeable change in the G-peak position is observed between anneals performed at 300 and 400⬚C. For annealing up to 300⬚C, the film is structurally stable but at temperatures of 400⬚C and above, the films began to graphitize as evidenced by the shift of the G-peak position towards 1585 cm, which is the Raman peak for nanocrystalline graphite w10x. Fig. 5 shows the Raman spectra of the films annealed for 2 h at different temperatures. It can be observed that the intensity of the spectra remains relatively unchanged, which reflects a high thermal stability of the films. This is in good agreement with that reported by Anders et al. w11x. The spectra do not show a well separated D and G peak even at 550⬚C, this might probably be due to the distinctive nature of hydrogenfree ta-C films prepared by the FCVA technique. Fig. 6 shows that the films annealed at 500⬚C experienced a reduction of intensity only after 3 h of annealing, as a result of loss of film thickness due to oxidation. Fig. 7 shows the Raman spectra of films, which are annealed at 400⬚C in ambient air, low and high vacuum as well as in the oxygen environment. It is clear from the figure that, there is not much difference in spectra, which were obtained in ambient air, low or high vacuum. This shows that the dominant mode of sp 3-to-sp 2 conversion is caused by temperature-induced graphitization when the annealing is done in ambient air.
251
4. Conclusion The time᎐temperature behavior of tetrahedral amorphous carbon Žta-C. films annealed in air was studied. It was found that the ta-C film remained structurally stable up to 300⬚C for 4 h. Graphitization of the film began to occur at 400⬚C and at 550⬚C, a complete loss of film thickness within 30 min of annealing. References w1x T.A. Friedmann, K.F. McCarty, J.C. Barbour, M.P. Siegal, D.C. Dibble, Appl. Phys. Lett. 68 Ž1996. 1643. w2x S. Anders, J. Diaz, J.W. Ager, Appl. Phys. Lett. 71 Ž1997. 3367. w3x S. Xu, B.K. Tay, H.S. Tan, Z. Li, Y.Q. Tu, J. Appl. Phys. 79 Ž1996. 7234. w4x X. Shi, D. Flynn, B.K. Tay, H.S. Tan, Filtered Cathodic Arc Source, PCTrGB96r00389, 20th February 1995. w5x X. Shi, M. Fulton, D. Flynn, H.S. Tan, Deposition Apparatus, PCTrGB96r00390, 20th February 1995. w6x B.K. Tay, X. Shi, H.S. Tan, C.S.Tay, Analysis of hydrogen free diamond-like carbon films by Raman spectroscopy, submitted to Diamond Related Materials. w7x A.A. Voevodin, J.S. Zabinski, Proceedings of the 1st International Specialist Meeting on Amorphous Carbon, SMAC 1997, pp. 237. w8x J. Schwan, S. Ulrich, V. Batori, H. Ehrhardt, J. Appl. Phys. 80 Ž1996. 440. w9x J. Schwan, S. Ulrich, T. Theel, H. Roth, H. Ehrhardt, J. Appl. Phys. 82 Ž1997. 6024. w10x B.K. Tay, X. Shi, H.S. Tan, H.S. Yang, Z. Sun, Surf. Coat. Technol. 105 Ž1998. 155. w11x S. Anders, J.W. Ager, G.M. Pharr, T.Y. Tsui, I.G. Brown, Thin Solid Films 308r309 Ž1997. 186.
Time and temperature-dependent changes in the structural properties of tetrahedral amorphous carbon films B.K. Tay U , D. Sheeja, S.P. Lau, X. Shi, B.C. Seet, Y.C. Yeo Microelectronics Centre, School of Electrical and Electronic Engineering, Nanyang Technological Uni¨ ersity, Nanyang 639798, Singapore Received 13 October 1999; accepted in revised form 5 April 2000
Abstract The time and temperature-dependent changes in the structural properties of tetrahedral amorphous carbon Žta-C. films were accessed continuously by Raman spectroscopy. It has been found that a film of 70-nm thickness remains structurally stable after annealing in air at up to 300⬚C for 4 h. Although some degree of graphitization was observed on a film annealed at 400⬚C, the film begins to oxidize and lose thickness only after annealing at 500⬚C for more than 2 h. This reflects the high thermal stability of the films. In general, annealing results in a narrowing and an upshifting of the G-band together with an increase in the I DrIG ratio. Most of these changes were observed during the first 2 h of annealing, after which the structure of the film appears to stabilize, with the exception at 500⬚C in which the film deteriorates further as oxidation occurs. It was also observed that thin film has better thermal stability against graphitization than thick film. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Amorphous carbon film; Annealing; Graphitization; Raman spectra; Thermal stability
1. Introduction Hard amorphous diamond-like carbon coatings are now being used in several applications Ži.e. computer hard discs, magnetic storage tape heads and drums, sunglasses, barcode readers and orthopedic implants. because of their good wear resistance and low friction in dry sliding. The thermal behavior of these films annealed in vacuum has been extensively studied w1,2x. However, the changes in film properties and structural relaxation due to annealing in air have been much less studied. It is therefore, important to investigate the properties of the films annealed in air as it represents the condition of most practical importance for coating applications. In this paper, the time᎐temperature annealing behavior of tetrahedral amorphous carbon ŽtaC. films deposited by the filtered cathodic vacuum arc ŽFCVA. technique was examined by Raman spectroU
Corresponding author.
scopy. This is made possible by a programable heater stage, which enables the film to be heat-treated continuously and allows us to map the evolution of the Raman spectra with increasing time and temperature.
2. Experimental details 2.1. Film preparation Tetrahedral amorphous carbon Žta-C. films were deposited by a filtered cathodic vacuum arc ŽFCVA. system described elsewhere w3x. The system incorporates the off-plane double-bend ŽOPDB. filter w4,5x to effectively remove all macro-particles. During deposition the C plasma leaves the self-sustaining arc spot, and then Cq ions are accelerated with a DC bias to the out-of-sight 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 main-
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 6 9 9 - X
B.K. Tay et al. r Surface and Coatings Technology 130 (2000) 248᎐251
249
tained at 40 mT. The chamber was evacuated to a base pressure below 2 = 10y6 t but rose to 1.5= 10y5 t during deposition due to outgasing of the cathode. The substrate was cleaned by an Arq ion beam, which is generated by an ion source supplied by Ion Tech Inc., to remove any native oxide layer on its surface. Tetrahedral amorphous carbon films were deposited at 100 eV Žmaximum sp 3 content of approx. 88%. with the substrate held at room temperature w6x, and the deposi˚ tion rate was ; 10 Ars. 2.2. Annealing process A Linkam THMS 600 stage heater was used to anneal the films in air. Two sets of experiments were carried out to investigate the time᎐temperature annealing behavior of the films. In set A, all the films were 70-nm thick and each was annealed at different temperatures ranging from 150 to 550⬚C. At each temperature, Raman spectra were acquired at every 15-min interval during the first hour followed by every 30-min interval during the second hour and subsequently at every 1-h interval up to 4 h. In set B, films of different thicknesses Ž30 nm, 70 nm and 100 nm. were heated continuously in air, from ambient temperature up to 500⬚C at every 100⬚C interval. Before ramping to the next higher temperature, the heater stage was programed to hold at each temperature for approximately 5 min for the system to reach a steady temperature as well as for the Raman spectra to be acquired. The temperature stability during the measurements was "2⬚C, and the heating and cooling rates at the beginning and end of annealing were 40⬚Crmin. The temperature difference between the top of the film and the bottom of the silicon substrate is found to be approximately 10⬚C for a 508-m silicon wafer, and hence the temperature gradient is approximately 20⬚Crmm.
Fig. 1. Fitting of a typical Raman spectrum with two Gaussian curves.
Fig. 2. I D rI G ratio of ta-C films annealed at different temperatures and time.
2.3. Film characterization The Raman spectra were measured on a Renishaw Rama scope System 1000 using the 514 nm lines of the Arq laser, focused to a spot of 50 m in diameter. Typically the spectra were acquired in the backscattering geometry over the range from 100 to 4000 cmy1 at 1.5 cmy1 intervals. To avoid any laser annealing effects, a low input power of ; 5 mW was used. The data were then fitted to two Gaussian curves, ‘G’ and ‘D’ curves as shown in Fig. 1 using PeakFit, a least squares computer program.
3. Results and discussion Fig. 2 shows the intensity ratio ID rIG as a function of annealing time at different temperatures ranging from 150 to 550⬚C. The ID rIG ratio increased with annealing time and temperature. In general, for an-
Fig. 3. G-line width of ta-C films annealed at different temperatures and time.
250
B.K. Tay et al. r Surface and Coatings Technology 130 (2000) 248᎐251
Fig. 4. G-peak position of ta-C films annealed at different temperatures and time.
nealing temperatures below 500⬚C, the ID rIG ratio increased gradually during the first 2 h of annealing, beyond which it saturated at a certain level. However, for annealing at 500⬚C, a sharp increase in the ID rIG ratio was observed at the third hour of annealing. This could be attributed to the surface oxidation process, which resulted in a decrease of film thickness and served as a catalyst for sp 3-to-sp 2 bond relaxation w7x. The sp 3-to-sp 2 bond relaxation affects the ID rIG ratio. The rate of oxidation increased more dramatically when annealed at 550⬚C, in which a complete loss of film thickness was observed after 30 min of annealing, and hence further data could not be obtained. Fig. 3 shows the G-line width ŽGaussian width. as a function of annealing time at different temperatures. The G-line width is observed to be decreasing with increasing annealing temperature. Schwan et al. w8x have reported an increasing linear relation between G-line width and stress. Hence, a narrower G-line
Fig. 5. Raman spectra of ta-C films annealed at different temperatures after 2 hrs.
Fig. 6. Raman spectra of a ta-C film annealed at 500⬚C up to 4 hrs.
width observed was associated with lower film stress. It is thus interesting to observe a significant reduction in film stress at 150⬚C, considering only a minor increase in the sp 2 fractions as evidenced by its relatively low ID rIG ratio. The decrease in stress could be attributed to the migration of carbon interstitials to the surface upon annealing w9x. It has been known that the impingement of energetic carbon ions during film growth often results in an amorphous network that is full of defects Ži.e. interstitials, vacancies, dangling bonds, etc... However, when the interstitials have gained sufficient mobility to migrate and recombine with the vacancies near the surface, it results in a reduced film density and stress. It is possible that at moderate temperatures Ži.e. 150⬚C., the thermal energy is sufficient for the unbonded interstitials to migrate, but may be insufficient to trigger a significant sp 3-to-sp 2 bond re-
Fig. 7. Raman spectra of ta-C films annealed in different environments.
B.K. Tay et al. r Surface and Coatings Technology 130 (2000) 248᎐251
laxation, which explains why the ID rIG ratio remains relatively at a low value. Fig. 4 shows the G-peak position as a function of annealing time at different temperatures. A noticeable change in the G-peak position is observed between anneals performed at 300 and 400⬚C. For annealing up to 300⬚C, the film is structurally stable but at temperatures of 400⬚C and above, the films began to graphitize as evidenced by the shift of the G-peak position towards 1585 cm, which is the Raman peak for nanocrystalline graphite w10x. Fig. 5 shows the Raman spectra of the films annealed for 2 h at different temperatures. It can be observed that the intensity of the spectra remains relatively unchanged, which reflects a high thermal stability of the films. This is in good agreement with that reported by Anders et al. w11x. The spectra do not show a well separated D and G peak even at 550⬚C, this might probably be due to the distinctive nature of hydrogenfree ta-C films prepared by the FCVA technique. Fig. 6 shows that the films annealed at 500⬚C experienced a reduction of intensity only after 3 h of annealing, as a result of loss of film thickness due to oxidation. Fig. 7 shows the Raman spectra of films, which are annealed at 400⬚C in ambient air, low and high vacuum as well as in the oxygen environment. It is clear from the figure that, there is not much difference in spectra, which were obtained in ambient air, low or high vacuum. This shows that the dominant mode of sp 3-to-sp 2 conversion is caused by temperature-induced graphitization when the annealing is done in ambient air.
251
4. Conclusion The time᎐temperature behavior of tetrahedral amorphous carbon Žta-C. films annealed in air was studied. It was found that the ta-C film remained structurally stable up to 300⬚C for 4 h. Graphitization of the film began to occur at 400⬚C and at 550⬚C, a complete loss of film thickness within 30 min of annealing. References w1x T.A. Friedmann, K.F. McCarty, J.C. Barbour, M.P. Siegal, D.C. Dibble, Appl. Phys. Lett. 68 Ž1996. 1643. w2x S. Anders, J. Diaz, J.W. Ager, Appl. Phys. Lett. 71 Ž1997. 3367. w3x S. Xu, B.K. Tay, H.S. Tan, Z. Li, Y.Q. Tu, J. Appl. Phys. 79 Ž1996. 7234. w4x X. Shi, D. Flynn, B.K. Tay, H.S. Tan, Filtered Cathodic Arc Source, PCTrGB96r00389, 20th February 1995. w5x X. Shi, M. Fulton, D. Flynn, H.S. Tan, Deposition Apparatus, PCTrGB96r00390, 20th February 1995. w6x B.K. Tay, X. Shi, H.S. Tan, C.S.Tay, Analysis of hydrogen free diamond-like carbon films by Raman spectroscopy, submitted to Diamond Related Materials. w7x A.A. Voevodin, J.S. Zabinski, Proceedings of the 1st International Specialist Meeting on Amorphous Carbon, SMAC 1997, pp. 237. w8x J. Schwan, S. Ulrich, V. Batori, H. Ehrhardt, J. Appl. Phys. 80 Ž1996. 440. w9x J. Schwan, S. Ulrich, T. Theel, H. Roth, H. Ehrhardt, J. Appl. Phys. 82 Ž1997. 6024. w10x B.K. Tay, X. Shi, H.S. Tan, H.S. Yang, Z. Sun, Surf. Coat. Technol. 105 Ž1998. 155. w11x S. Anders, J.W. Ager, G.M. Pharr, T.Y. Tsui, I.G. Brown, Thin Solid Films 308r309 Ž1997. 186.