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The fabrication of nanocrystalline diamond films using hot filament CVD
Diamond and Related Materials 13 (2004) 6–13
The fabrication of nanocrystalline diamond films using hot filament CVD T. Wang*, H.W. Xin, Z.M. Zhang, Y.B. Dai, H.S. Shen Research Institute of MicroyNano Science and Technology, Shanghai Jiao Tong University, 14, Huashan road 1954, Shanghai 200030, PR China Received 16 January 2003; received in revised form 12 August 2003; accepted 25 August 2003
Abstract By decreasing reacting gas pressure, nanocrystalline diamond films, the area of which reaches 2 inches were deposited with hot filament chemical vapor deposition (HFCVD) equipment. According to the analysis with high-resolution transmission electron microscopy (HRTEM), the grain size averages approximately 4–8 nm and the nanometer crystalline diamond grains are surrounded by amorphous ingredient. The sharp diamond and graphite peak can be shown by ultraviolet (UV) (244 nm) Raman spectroscopy. The wire drawing die coated by common diamond film and this film can last more than 10-fold move than that coated by common hard alloy. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline; Diamond films; Grain size; Raman spectroscopy
1. Introduction With the increasing development of chemical vapor deposition (CVD) technology, the deposition of diamond films with various CVD techniques has been the subject of intensive research, owing to its potential applications in microelectronics, micromechanism, optical and electronic materials, etc. However, diamond has not been utilized on a large scale by reason of the rough surface and the difficulty of manufacturing the single crystal films, and so on. While nanocrystalline diamond film has a great deal of particular properties, including smooth surfaces, the low friction coefficient, the low electron emission threshold voltage, the feasibility of being etched with dry process, these properties promise nanocrystalline diamond films a wide range of applications as abrasive resistant material, electrochemistry electrode, optical devices, film microsensor, micro-electro-mechanical systems, etc. Therefore, it is of importance to fabricate the even and smooth nanometer crystalline diamond films. Gruen et al. have shown the feasibility of depositing nanocrystalline diamond thin films from microwave Ary *Corresponding author. Tel.: q86-021-62933971; fax: q86-02762933292. E-mail address: [email protected] (T. Wang).
H2 yCH4 at different concentrations of Ar. The film structures were observed to change from microcrystalline to nanocrystalline diamond films with the increase in the Ar concentration, which illustrates that Ar is important to produce nanocrystalline diamond films since the concentration of Ar influences secondary formation of nuclei w1x. In this study, nanocrystalline diamond films were deposited with HFCVD equipment and were characterized by scanning electron microscopy (SEM), UV Raman spectroscopy, atom force microscopy (AFM), X-ray diffraction (XRD) and HRTEM, selected area electron diffraction (SAED). In order to apply this technology, the films have been coated in wire drawing die, which can last the die longevity. The technology can save tungsten usage in hard alloy, in wire drawing die and enhance productivity with decreasing cost. 2. Experiment The films were deposited in HFCVD reactor, using 1–4% acetone in hydrogen gas mixture at 0.5–6 kPa pressure. Ta filaments were operated at 1900–2300 8C and the substrate temperature was held at 850–950 8C. Details of typical deposition parameters and the HFCVD
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.08.014
T. Wang et al. / Diamond and Related Materials 13 (2004) 6–13 Table 1 Deposition parameters of nanocrystalline diamond films by HFCVD Parameters
Values
The ratio of CH3COCH3 to H2 Pressure (Torr) Deposition temperature (8C) Filament temperature (8C)
1–4 0.5–6 850–950 2100"200
equipment have been, respectively, shown in Table 1 and Fig. 1. A typical Tantalum filament was fabricated using a 12 cm length of 0.8-mm diameter wire. Four Ta filaments, stretched by fire resistant leaf springs, were parallelly and uniformpositionally fixed above the substrate with the distance of 6 mm between the filaments and substrate so that during the deposition the wires could keep straight to make sure that the substrate was held securely in an even temperature field. Before utilizing the filament for growth, they were heated in vacuum to 1800 8C for 5 min to relieve stress in the wire and then carburized for 20 min. After carburization, the Ta filament appeared light golden in color and became brittle. The substrates were silicon (100) wafers, approximately 2 inches in diameter and 0.45 mm in the thickness, which were prepared by scratching with diamond paste (0.5 um) and cleaned in an ultrasonic bath with deionized water and acetone three times. Films were grown for 5–10 h. The deposited films were imaged using SEM (Hitachi S-800) and studied using Raman scattering excited by 632.8 nm laser light and UV Raman (244 nm excitation wavelength). XRD and HRTEM were utilized to investigate the film structure and SAED was used to measure the phase composition. After pretreatment wire drawing die, the common diamond films and nanocrystalline diamond films have been orderly coated in the wire drawing die to form compound diamond films, which will been polished before usage. Wire twishing machine (RFS630) produced wires at the velocity of 28–35 mymin using the die coated with common hard alloy and the die coated with compound diamond films, respectively.
5 Torr, respectively. Obviously, it can be found that with decreasing the gas pressure, the surface images change. According to Fig. 2a, while the gas pressure is at 25 Torr, the surface is rather rough and the grain size reaches 2–3 mm. And it accords with the adequate growth of crystal and the small ratio of secondary formation of nuclei. The image shown in Fig. 2b illustrates that the diamond grains can still grow fully, and that a great deal of smaller diamond grains appear and surround the large diamond grains. However, when the gas pressure diminishes to 1.67 kPa (12.5 Torr), it is difficult to identify the grain boundary from the image in Fig. 2c since the grain size is less than 1 mm. With the further decline of the gas pressure, as illustrated in Fig. 2d–e, the grain size falls evidently into nanometer scale and the films are composed of nanocrystalline diamond. The image of nanometer crystalline diamond film deposited at 5 Torr is illustrated in Fig. 2f. The grain size is smallest. Fig. 3 is the cross-section SEM of diamond films operated with different gas pressures, which can also illustrate the change of grain size from the micron scale to nanometer scale with the decrease in the gas pressure. It can be observed from Fig. 3a that it is the columnar structure in the films, which is considered that the diamond crystals easily grow for the high proportion hydrogen can constrain secondary formation of nuclei. Nevertheless, the columnar structure can still exist, but it becomes extremely tenuous in Fig. 3b when the gas pressure declines to 10 Torr. Finally, the structure disappears and the cross-section structure is familiar with the one padded by silver sand, as shown in Fig. 3c. When the gas pressure is 5 Torr, the grain size can diminish into the nanometer scale since the ratio of secondary formation of nuclei is so low that it is difficult for crystals to grow after nucleation. The Raman spectra excited at 632.8 nm of the diamond films prepared at various gas pressures is shown in Fig. 4. There are two peaks with broad width
3. Results and discussion 3.1. Effect of reactor gas pressure The effect of the reactor gas pressure was studied by carrying out film depositions under various pressures with the same other deposition parameters showed in Table 1. Thus, the films were compared with the surface and cross-section SEM images. Fig. 2a–f show the surface SEM images of the diamond films prepared with different gas pressures which are, 25 Torr, 20 Torr, 12.5 Torr, 10 Torr, 7.5 Torr,
7
Fig. 1. Schematic diagram of the HFCVD equipment.
8
T. Wang et al. / Diamond and Related Materials 13 (2004) 6–13
Fig. 2. The surface SEM images of the diamond films prepared with different gas pressures.
at 1350 and 1580 cm y1 which were the D and G peaks relative to sp2 carbon in the Raman spectra of films grown in the low gas pressure w2,7x. But two peaks are
overemphasized than the 1332 cmy1 peak that corresponds to the sp3 structure. With the increase in gas pressure in the reaction chamber, the intensity ratio of
T. Wang et al. / Diamond and Related Materials 13 (2004) 6–13
9
Fig. 3. The cross-section SEM images of the diamond films prepared with different gas pressures.
D and G peaks decreases and the 1332 cmy1 peak become sharper. Another broad peak at 1460 cmy1 appears when the deposition gas pressure is at 12.5 and 20 Torr, which is believed to be relevant with the nanometer diamond structure w3,4x. The experiment result mentioned above illustrates that it is feasible to fabricate the nanocrystalline diamond films by decreasing gas pressure in the reaction chamber and increasing the carbon resource without adding Ar gas. 3.2. Analysis of structure of the nanocrystalline diamond films The Fig. 5a,b is the surface and cross-section SEM images of the nanocrystalline diamond deposited with 3–4% acetone in hydrogen gas mixture at 0.5–1.4 kPa pressure for several hours. It is obvious that the surface
of nanocrystalline diamond is too smooth and even to identify the crystal boundary. According to Fig. 6, the surface AFM image of the nanometer crystalline diamond, the size of grain declines into nanometer scale(100 nm). The grain size has decreased the two order of magnitude, compared with that of the common diamond films. Every grain contains several crystals. The surface roughness is approximately 30–50 nm, tested by profilometry scans. The film is familiar with the structure piled by silver sand rather than columnar shape. Fig. 7 shows the Raman spectrum of nanocrystalline diamond. The excitation wavelength is 632.8 nm in Fig. 7a where are two peaks with large width at 1350 and 1580 cmy1, namely the D and G peaks. The sharp peak at 1332 cmy1 is the characteristic line of crystalline diamond and wide enough to overlap with the D peak. Moreover, a common feature in the spectra is that a shoulder appears at approximately 1140 cmy1, which is
10
T. Wang et al. / Diamond and Related Materials 13 (2004) 6–13
Fig. 4. Raman spectra of the diamond films prepared with different gas pressures.
attributed to the nanocrystalline diamond or disordered sp3 carbon phase w5,6x. Nevertheless, the relative sensitivity of the technique to the D-band and G-band decreases with decreasing wavelength. Where the ratios of the diamond peak to graphite band change from 2:1 to ;1:50 as the excitation is moved from UV to IR. Using the UV excitation resource can increase the Raman scattering cross-section and strengthen intensity of scattering diamond w7,8x. In nanocrystalline diamond films, the grain surface area and grain boundary mainly consists of amorphous sp2 C phases. The local gap of sp3 C atoms is approximately 5.5 eV whereas that of sp2 C atoms is approximately 2 eV. Therefore, because of the p – p* resonance effect which ascribes to the comparable energy of the incident photons in visible
range of Raman spectroscopy. Hence, the Raman spectra attained with visible excitation are completely dominated by the sp2 C atoms. Raman scattering in the UV region provided significantly greater information than visible Raman spectroscopy in characterizing diamond phases by increasing the intensity from sp3 C bonding and suppressing the dominant resonance Raman scattering from sp2 C atoms w9,10x. There are two peaks, the G and diamond peak in Fig. 7b, the spectra obtained using 244 nm. The peak at 1332 cm y1 is extremely sharp, which demonstrates that the film has the good diamond structure. What’s more, the D peak disappears in the UV Raman spectra. Fig. 8 shows the XRD spectrum of the nanocrystalline diamond films, in which are the (111) and (220) diamond diffraction peaks. HRTEM is used to observe the nanometer diamond film and the image is shown in Fig. 9. It can be seen that non-uniform distribution of nano-diamond crystals in thin films, which are surrounded by amorphous structure. The area pointed by the white arrowhead is magnified and shown in the right corner of Fig. 9. The lattice distance is 0.209 nm, which is also in good agreement with the standard value of the lattice distance between the diamond crystal face {111}. Averagely, the grain size is 4–8 nm while the diameter of largest grain reaches to more than 10 nm. Some grains contain twin crystals, which are resulted from some grain secondary formation of nuclei and growth. In order to ascertain phase composition in the film, the SAED pattern of the film is shown in Fig. 10. The diffraction pattern is a series of concentric circles with different radius, which shows the diamond structure in the film. Compared with lattice distances measured from Fig. 10 and those in the ASTM standard cards, the data in Table 2 illustrate that the diffraction rings measured
Fig. 5. The surface and cross-section SEM images of the nanocrystalline diamond films.
T. Wang et al. / Diamond and Related Materials 13 (2004) 6–13
Fig. 6. The surface AFM images of the nanocrystalline diamond films.
11
are consistent with those of diamond powder. Even though it is only the diamond structure shown in Fig. 10, the amorphous graphite component exists in the nanocrystalline diamond films according to the Raman spectra. There are two keys to fabricate the nanocrystalline diamond films using CVD; the one is to increase the nucleation density to 109 –1010 during the initial stages, the other is to control the crystal grain growth into nanometer scale. It is known that the content of hydrogen and hydrocarbon, for e.g. C2, CH3, CH, etc., exert a profound effect to fabricate the diamond structure in the films. On one hand, high content of hydrogen promotes the nucleation and growth of diamond grain and accelerates growing diamond films. On the other hand, the ratio of secondary formation of nuclei increases with the decline of the content of hydrocarbon so as to restrain the grain growth, and vice versa. Therefore,
Fig. 7. Raman spectra of the nanocrystalline diamond films.
Fig. 8. XRD spectrum of the nanocrystalline diamond films.
12
T. Wang et al. / Diamond and Related Materials 13 (2004) 6–13
Fig. 9. HRTEM images of the nanocrystalline diamond films.
in this study, the two factors are utilized to fabricate the nanocrystalline diamond films by appropriate increase in carbon source and decrease in gas pressure in the reaction chamber to enhance the content of hydrogen and hydrocarbon. The decline of gas pressure cannot only result in increasing the substrate temperature and dissociation rate of hydrogen gas, but enhances the particles’ free path in the reaction chamber as well. Consequently, the particles with higher velocity and more energy bombard substrate and supply more energy to surface. The adsorbed particles on the surface become
more active and promote the ratio of secondary formation of nuclei. Moreover, it is difficult for diamond grain to grow on the occasion of the more and stronger bombardment, which also accelerates the secondary formation of nuclei. The above mentioned are the reasons to form the nanometer crystal diamond films. The experiment to twist wires has shown that the compound diamond films can increase the wire drawing die longevity approximately 10 times, since the common hard alloy die manufactured only 30 km long wire but Table 2 Measured and standard values of the interplanar crystal spacing of diamond
Fig. 10. SAED pattern of the nanocrystalline diamond films.
The serial numbers of diffraction rings (from interior to exterior)
The indices of The interplanar crystal Distinction ˚ crystallograph- spacing d (A) % ic plane (hkl) The standard The values of measured diamond values
1 2 3 4 5 6 7 8 9 10 11 12 13 14
{111} {220} {311} {400} {331} {422} {333}y{511} {440} {531} {620} {533} {444} {711} {642}
2.059 1.261 1.075 0.892 0.818 0.728 0.686 0.630 0.603 0.564 0.544 0.514 0.499 0.477
2.086 1.279 1.086 0.899 0.827 0.738 0.693 0.640 0.611 0.576 0.552 0.505 0.484 0.470
1.3 1.4 1.0 0.8 1.1 1.4 1.0 1.6 1.3 2.1 1.5 y1.8 y3.0 y1.5
T. Wang et al. / Diamond and Related Materials 13 (2004) 6–13
the compound diamond films die can produce wire more than 300 km long. Even though some sp2 structure exists along grain boundary and lower hardness a little, the structure can benefit coating polishing to obtain smooth surface in this application. 4. Conclusion The nanocrystalline diamond films were fabricated by decreasing the gas pressure and increasing the concentration of the hydrocarbon with HFCVD technique. The surface of the film is extremely smooth and the grain size is approximately 4–8 nm. According to the UV Raman spectrum with sharp diamond and graphite peaks, diamond occupies more than 90% structure in the films while a small quantity of amorphous graphite is scattered along the grain boundary. The technology can be used in the wire drawing dies. References w1x M. Gruen Dieter, Nanocrystalline diamond films, Annu. Rev. Mater. Sci. 29 (1999) 211.
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w2x X.S. Sun, N. Wang, W.J. Zhang, H.K. Woo, X.D. Han, I. Bello, C.S. Lee, S.T. Lee, Synthesis of nanocrystalline diamond by the direct ion beam deposition method, J. Mater. Res. 14 (8) (1999) 3204. w3x H. Yoshikawa, C. Moral, Y. Koga, Synthesis of nanocrystalline diamond films using microwave plasma CVD, Diamond Relat. Mater. (10) (2001) 1588. w4x R. Kapil, B.R. Mehta, V.D. Vankar, C. Roy, A. Pradhan, Growth of diamond thin films in a cyclic growth-etch oxyacetylene flame process, Thin Solid Films 322 (1–2) (1998) 74. w5x R.J. Nemanich, J.T. Glass, G. Lucovsky, R.E. Shroder, J. Vac. Sci. Technol. A 6 (1988) 1783. w6x B. Marcus, L. Fayette, M. Mermoux, L. Abello, G. Lucazeau, J. Appl. Phys. 76 (1994) 3463. w7x Z. Sun, J.R. Shi, B.K. Tay, S.P. Lau, UV Raman characteristics of nanocrystalline diamond films with different grain size, Diamond Relat. Mater. (9) (2000) 1979. w8x S.M. Leeds, P.W. May, M.N.R. Ashford, T.J. Davis, Use of different excitation wavelength for the analysis of CVD diamond by laser Raman spectroscopy, Diamond Relat. Mater. (2-5) (1998) 233. w9x V.I. Merkulov, J.S. Lannin, C.H. Munro, S.A. Asher, V.S. Veerasamy, W.I. Milne, Phys. Rev. Lett. 78 (1997) 4869. w10x S.M. Leeds, T.J. Davis, P.W. May, C.D.O. Pickard, M.N.R. Ashfold, Diamond Relat. Mater. 7(2–5) 233.
The fabrication of nanocrystalline diamond films using hot filament CVD T. Wang*, H.W. Xin, Z.M. Zhang, Y.B. Dai, H.S. Shen Research Institute of MicroyNano Science and Technology, Shanghai Jiao Tong University, 14, Huashan road 1954, Shanghai 200030, PR China Received 16 January 2003; received in revised form 12 August 2003; accepted 25 August 2003
Abstract By decreasing reacting gas pressure, nanocrystalline diamond films, the area of which reaches 2 inches were deposited with hot filament chemical vapor deposition (HFCVD) equipment. According to the analysis with high-resolution transmission electron microscopy (HRTEM), the grain size averages approximately 4–8 nm and the nanometer crystalline diamond grains are surrounded by amorphous ingredient. The sharp diamond and graphite peak can be shown by ultraviolet (UV) (244 nm) Raman spectroscopy. The wire drawing die coated by common diamond film and this film can last more than 10-fold move than that coated by common hard alloy. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline; Diamond films; Grain size; Raman spectroscopy
1. Introduction With the increasing development of chemical vapor deposition (CVD) technology, the deposition of diamond films with various CVD techniques has been the subject of intensive research, owing to its potential applications in microelectronics, micromechanism, optical and electronic materials, etc. However, diamond has not been utilized on a large scale by reason of the rough surface and the difficulty of manufacturing the single crystal films, and so on. While nanocrystalline diamond film has a great deal of particular properties, including smooth surfaces, the low friction coefficient, the low electron emission threshold voltage, the feasibility of being etched with dry process, these properties promise nanocrystalline diamond films a wide range of applications as abrasive resistant material, electrochemistry electrode, optical devices, film microsensor, micro-electro-mechanical systems, etc. Therefore, it is of importance to fabricate the even and smooth nanometer crystalline diamond films. Gruen et al. have shown the feasibility of depositing nanocrystalline diamond thin films from microwave Ary *Corresponding author. Tel.: q86-021-62933971; fax: q86-02762933292. E-mail address: [email protected] (T. Wang).
H2 yCH4 at different concentrations of Ar. The film structures were observed to change from microcrystalline to nanocrystalline diamond films with the increase in the Ar concentration, which illustrates that Ar is important to produce nanocrystalline diamond films since the concentration of Ar influences secondary formation of nuclei w1x. In this study, nanocrystalline diamond films were deposited with HFCVD equipment and were characterized by scanning electron microscopy (SEM), UV Raman spectroscopy, atom force microscopy (AFM), X-ray diffraction (XRD) and HRTEM, selected area electron diffraction (SAED). In order to apply this technology, the films have been coated in wire drawing die, which can last the die longevity. The technology can save tungsten usage in hard alloy, in wire drawing die and enhance productivity with decreasing cost. 2. Experiment The films were deposited in HFCVD reactor, using 1–4% acetone in hydrogen gas mixture at 0.5–6 kPa pressure. Ta filaments were operated at 1900–2300 8C and the substrate temperature was held at 850–950 8C. Details of typical deposition parameters and the HFCVD
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.08.014
T. Wang et al. / Diamond and Related Materials 13 (2004) 6–13 Table 1 Deposition parameters of nanocrystalline diamond films by HFCVD Parameters
Values
The ratio of CH3COCH3 to H2 Pressure (Torr) Deposition temperature (8C) Filament temperature (8C)
1–4 0.5–6 850–950 2100"200
equipment have been, respectively, shown in Table 1 and Fig. 1. A typical Tantalum filament was fabricated using a 12 cm length of 0.8-mm diameter wire. Four Ta filaments, stretched by fire resistant leaf springs, were parallelly and uniformpositionally fixed above the substrate with the distance of 6 mm between the filaments and substrate so that during the deposition the wires could keep straight to make sure that the substrate was held securely in an even temperature field. Before utilizing the filament for growth, they were heated in vacuum to 1800 8C for 5 min to relieve stress in the wire and then carburized for 20 min. After carburization, the Ta filament appeared light golden in color and became brittle. The substrates were silicon (100) wafers, approximately 2 inches in diameter and 0.45 mm in the thickness, which were prepared by scratching with diamond paste (0.5 um) and cleaned in an ultrasonic bath with deionized water and acetone three times. Films were grown for 5–10 h. The deposited films were imaged using SEM (Hitachi S-800) and studied using Raman scattering excited by 632.8 nm laser light and UV Raman (244 nm excitation wavelength). XRD and HRTEM were utilized to investigate the film structure and SAED was used to measure the phase composition. After pretreatment wire drawing die, the common diamond films and nanocrystalline diamond films have been orderly coated in the wire drawing die to form compound diamond films, which will been polished before usage. Wire twishing machine (RFS630) produced wires at the velocity of 28–35 mymin using the die coated with common hard alloy and the die coated with compound diamond films, respectively.
5 Torr, respectively. Obviously, it can be found that with decreasing the gas pressure, the surface images change. According to Fig. 2a, while the gas pressure is at 25 Torr, the surface is rather rough and the grain size reaches 2–3 mm. And it accords with the adequate growth of crystal and the small ratio of secondary formation of nuclei. The image shown in Fig. 2b illustrates that the diamond grains can still grow fully, and that a great deal of smaller diamond grains appear and surround the large diamond grains. However, when the gas pressure diminishes to 1.67 kPa (12.5 Torr), it is difficult to identify the grain boundary from the image in Fig. 2c since the grain size is less than 1 mm. With the further decline of the gas pressure, as illustrated in Fig. 2d–e, the grain size falls evidently into nanometer scale and the films are composed of nanocrystalline diamond. The image of nanometer crystalline diamond film deposited at 5 Torr is illustrated in Fig. 2f. The grain size is smallest. Fig. 3 is the cross-section SEM of diamond films operated with different gas pressures, which can also illustrate the change of grain size from the micron scale to nanometer scale with the decrease in the gas pressure. It can be observed from Fig. 3a that it is the columnar structure in the films, which is considered that the diamond crystals easily grow for the high proportion hydrogen can constrain secondary formation of nuclei. Nevertheless, the columnar structure can still exist, but it becomes extremely tenuous in Fig. 3b when the gas pressure declines to 10 Torr. Finally, the structure disappears and the cross-section structure is familiar with the one padded by silver sand, as shown in Fig. 3c. When the gas pressure is 5 Torr, the grain size can diminish into the nanometer scale since the ratio of secondary formation of nuclei is so low that it is difficult for crystals to grow after nucleation. The Raman spectra excited at 632.8 nm of the diamond films prepared at various gas pressures is shown in Fig. 4. There are two peaks with broad width
3. Results and discussion 3.1. Effect of reactor gas pressure The effect of the reactor gas pressure was studied by carrying out film depositions under various pressures with the same other deposition parameters showed in Table 1. Thus, the films were compared with the surface and cross-section SEM images. Fig. 2a–f show the surface SEM images of the diamond films prepared with different gas pressures which are, 25 Torr, 20 Torr, 12.5 Torr, 10 Torr, 7.5 Torr,
7
Fig. 1. Schematic diagram of the HFCVD equipment.
8
T. Wang et al. / Diamond and Related Materials 13 (2004) 6–13
Fig. 2. The surface SEM images of the diamond films prepared with different gas pressures.
at 1350 and 1580 cm y1 which were the D and G peaks relative to sp2 carbon in the Raman spectra of films grown in the low gas pressure w2,7x. But two peaks are
overemphasized than the 1332 cmy1 peak that corresponds to the sp3 structure. With the increase in gas pressure in the reaction chamber, the intensity ratio of
T. Wang et al. / Diamond and Related Materials 13 (2004) 6–13
9
Fig. 3. The cross-section SEM images of the diamond films prepared with different gas pressures.
D and G peaks decreases and the 1332 cmy1 peak become sharper. Another broad peak at 1460 cmy1 appears when the deposition gas pressure is at 12.5 and 20 Torr, which is believed to be relevant with the nanometer diamond structure w3,4x. The experiment result mentioned above illustrates that it is feasible to fabricate the nanocrystalline diamond films by decreasing gas pressure in the reaction chamber and increasing the carbon resource without adding Ar gas. 3.2. Analysis of structure of the nanocrystalline diamond films The Fig. 5a,b is the surface and cross-section SEM images of the nanocrystalline diamond deposited with 3–4% acetone in hydrogen gas mixture at 0.5–1.4 kPa pressure for several hours. It is obvious that the surface
of nanocrystalline diamond is too smooth and even to identify the crystal boundary. According to Fig. 6, the surface AFM image of the nanometer crystalline diamond, the size of grain declines into nanometer scale(100 nm). The grain size has decreased the two order of magnitude, compared with that of the common diamond films. Every grain contains several crystals. The surface roughness is approximately 30–50 nm, tested by profilometry scans. The film is familiar with the structure piled by silver sand rather than columnar shape. Fig. 7 shows the Raman spectrum of nanocrystalline diamond. The excitation wavelength is 632.8 nm in Fig. 7a where are two peaks with large width at 1350 and 1580 cmy1, namely the D and G peaks. The sharp peak at 1332 cmy1 is the characteristic line of crystalline diamond and wide enough to overlap with the D peak. Moreover, a common feature in the spectra is that a shoulder appears at approximately 1140 cmy1, which is
10
T. Wang et al. / Diamond and Related Materials 13 (2004) 6–13
Fig. 4. Raman spectra of the diamond films prepared with different gas pressures.
attributed to the nanocrystalline diamond or disordered sp3 carbon phase w5,6x. Nevertheless, the relative sensitivity of the technique to the D-band and G-band decreases with decreasing wavelength. Where the ratios of the diamond peak to graphite band change from 2:1 to ;1:50 as the excitation is moved from UV to IR. Using the UV excitation resource can increase the Raman scattering cross-section and strengthen intensity of scattering diamond w7,8x. In nanocrystalline diamond films, the grain surface area and grain boundary mainly consists of amorphous sp2 C phases. The local gap of sp3 C atoms is approximately 5.5 eV whereas that of sp2 C atoms is approximately 2 eV. Therefore, because of the p – p* resonance effect which ascribes to the comparable energy of the incident photons in visible
range of Raman spectroscopy. Hence, the Raman spectra attained with visible excitation are completely dominated by the sp2 C atoms. Raman scattering in the UV region provided significantly greater information than visible Raman spectroscopy in characterizing diamond phases by increasing the intensity from sp3 C bonding and suppressing the dominant resonance Raman scattering from sp2 C atoms w9,10x. There are two peaks, the G and diamond peak in Fig. 7b, the spectra obtained using 244 nm. The peak at 1332 cm y1 is extremely sharp, which demonstrates that the film has the good diamond structure. What’s more, the D peak disappears in the UV Raman spectra. Fig. 8 shows the XRD spectrum of the nanocrystalline diamond films, in which are the (111) and (220) diamond diffraction peaks. HRTEM is used to observe the nanometer diamond film and the image is shown in Fig. 9. It can be seen that non-uniform distribution of nano-diamond crystals in thin films, which are surrounded by amorphous structure. The area pointed by the white arrowhead is magnified and shown in the right corner of Fig. 9. The lattice distance is 0.209 nm, which is also in good agreement with the standard value of the lattice distance between the diamond crystal face {111}. Averagely, the grain size is 4–8 nm while the diameter of largest grain reaches to more than 10 nm. Some grains contain twin crystals, which are resulted from some grain secondary formation of nuclei and growth. In order to ascertain phase composition in the film, the SAED pattern of the film is shown in Fig. 10. The diffraction pattern is a series of concentric circles with different radius, which shows the diamond structure in the film. Compared with lattice distances measured from Fig. 10 and those in the ASTM standard cards, the data in Table 2 illustrate that the diffraction rings measured
Fig. 5. The surface and cross-section SEM images of the nanocrystalline diamond films.
T. Wang et al. / Diamond and Related Materials 13 (2004) 6–13
Fig. 6. The surface AFM images of the nanocrystalline diamond films.
11
are consistent with those of diamond powder. Even though it is only the diamond structure shown in Fig. 10, the amorphous graphite component exists in the nanocrystalline diamond films according to the Raman spectra. There are two keys to fabricate the nanocrystalline diamond films using CVD; the one is to increase the nucleation density to 109 –1010 during the initial stages, the other is to control the crystal grain growth into nanometer scale. It is known that the content of hydrogen and hydrocarbon, for e.g. C2, CH3, CH, etc., exert a profound effect to fabricate the diamond structure in the films. On one hand, high content of hydrogen promotes the nucleation and growth of diamond grain and accelerates growing diamond films. On the other hand, the ratio of secondary formation of nuclei increases with the decline of the content of hydrocarbon so as to restrain the grain growth, and vice versa. Therefore,
Fig. 7. Raman spectra of the nanocrystalline diamond films.
Fig. 8. XRD spectrum of the nanocrystalline diamond films.
12
T. Wang et al. / Diamond and Related Materials 13 (2004) 6–13
Fig. 9. HRTEM images of the nanocrystalline diamond films.
in this study, the two factors are utilized to fabricate the nanocrystalline diamond films by appropriate increase in carbon source and decrease in gas pressure in the reaction chamber to enhance the content of hydrogen and hydrocarbon. The decline of gas pressure cannot only result in increasing the substrate temperature and dissociation rate of hydrogen gas, but enhances the particles’ free path in the reaction chamber as well. Consequently, the particles with higher velocity and more energy bombard substrate and supply more energy to surface. The adsorbed particles on the surface become
more active and promote the ratio of secondary formation of nuclei. Moreover, it is difficult for diamond grain to grow on the occasion of the more and stronger bombardment, which also accelerates the secondary formation of nuclei. The above mentioned are the reasons to form the nanometer crystal diamond films. The experiment to twist wires has shown that the compound diamond films can increase the wire drawing die longevity approximately 10 times, since the common hard alloy die manufactured only 30 km long wire but Table 2 Measured and standard values of the interplanar crystal spacing of diamond
Fig. 10. SAED pattern of the nanocrystalline diamond films.
The serial numbers of diffraction rings (from interior to exterior)
The indices of The interplanar crystal Distinction ˚ crystallograph- spacing d (A) % ic plane (hkl) The standard The values of measured diamond values
1 2 3 4 5 6 7 8 9 10 11 12 13 14
{111} {220} {311} {400} {331} {422} {333}y{511} {440} {531} {620} {533} {444} {711} {642}
2.059 1.261 1.075 0.892 0.818 0.728 0.686 0.630 0.603 0.564 0.544 0.514 0.499 0.477
2.086 1.279 1.086 0.899 0.827 0.738 0.693 0.640 0.611 0.576 0.552 0.505 0.484 0.470
1.3 1.4 1.0 0.8 1.1 1.4 1.0 1.6 1.3 2.1 1.5 y1.8 y3.0 y1.5
T. Wang et al. / Diamond and Related Materials 13 (2004) 6–13
the compound diamond films die can produce wire more than 300 km long. Even though some sp2 structure exists along grain boundary and lower hardness a little, the structure can benefit coating polishing to obtain smooth surface in this application. 4. Conclusion The nanocrystalline diamond films were fabricated by decreasing the gas pressure and increasing the concentration of the hydrocarbon with HFCVD technique. The surface of the film is extremely smooth and the grain size is approximately 4–8 nm. According to the UV Raman spectrum with sharp diamond and graphite peaks, diamond occupies more than 90% structure in the films while a small quantity of amorphous graphite is scattered along the grain boundary. The technology can be used in the wire drawing dies. References w1x M. Gruen Dieter, Nanocrystalline diamond films, Annu. Rev. Mater. Sci. 29 (1999) 211.
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