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CVD diamond nucleation enhanced by ultrasonic pretreatment using diamond and mixture of diamond and TaC powders
Diamond and Related Materials 11 (2002) 1683–1689
CVD diamond nucleation enhanced by ultrasonic pretreatment using diamond and mixture of diamond and TaC powders S.G. Wanga,*, Qing Zhanga, S.F. Yoona, J. Ahna, Q. Wanga, D.J. Yanga, Q.F. Huanga, Ruslia, W.Z. Tangb, F.X. Lub a
S1-B2C-20, Microelectronics Centre, School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore b School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China Received 5 October 2001; received in revised form 28 April 2002; accepted 1 May 2002
Abstract Effects of ultrasonic pretreatment on chemical vapor deposition (CVD) diamond nucleation on Si substrates were systematically studied. Pure 1.5–40 mm-diamond powder and mixtures of 1.5–5 mm-diamond as well as 5–20 mm-Tantalum Carbide (TaC) powder were used in ultrasonic pretreatment. The root-mean-square (Rms) surface roughness of the pretreated substrates, residual diamond and TaC powders left on the substrates were examined using atomic force microscopy (AFM), Raman spectroscopy and X-ray diffraction (XRD), respectively. It was observed that the Rms surface roughness increases with increasing diamond or TaC powder size, and there is some diamond or TaC powder left on the substrates after ultrasonic pretreatment. Diamond films were deposited using microwave plasma chemical vapor deposition (MPCVD) technique and characterized by field emission scanning electron microscopy (FE-SEM). It was found that CVD diamond nucleation density strongly depends on particle size of diamond or TaC powder used, the nucleation density increases with increasing diamond or TaC powder size. A mixture of diamond and TaC powders enhances CVD diamond nucleation much more significantly than that of pure diamond powder. A mixture of 1.5 mm-diamond and 20 mm-TaC powders has an equivalent nucleation enhancement efficiency, which could be caused by pure 40 mm-diamond powder. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Microwave plasma chemical vapor deposition diamond; Nucleation; Pretreated substrates
1. Introduction Chemical vapor deposition (CVD) diamond nucleation critically determines the properties, morphology and homogeneity of diamond films. High nucleation density (the number of nuclei grown on unit substrate surface) and uniform crystal size of diamond on appropriate substrates are required for the applications of CVD diamond films in optics, electronic devices and X-ray lithographic masks w1–3x. CVD diamond nucleation principally depends on surface pretreatment, substrate materials and deposition parameters. Substrate surface pretreatment prior to CVD diamond growth has a great influence on nucleation and the initial stages of growth. In most CVD processes, *Corresponding author. Tel.: q65-67904528; fax: q65-67933318. E-mail address: [email protected] (S.G. Wang).
CVD diamond nucleation density on untreated substrates is very low w4,5x. In order to improve low nucleation density, a variety of surface pretreatment methods such as scratching substrate surface with abrasives, seeding substrates with diamond grit, electrical biasing substrates and ion implantation of substrate surfaces have been developed w5–10x. Among surface pretreatment methods, the mechanical polishing and ultrasonic pretreatments with diamond particles are widely used for nucleation enhancement w5–7x. Comparing to the mechanical polishing, the ultrasonic pretreatment with diamond particle suspensions generated a more homogeneous morphology over the entire deposition surface w6,11x. Furthermore, ultrasonic pretreatment is easily applied to the substrates with complex geometry and shape, and it is less surface damage, more suitable for optical and electronic applications of diamond films. It has been demonstrated that substrate
0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 2 . 0 0 1 3 8 - 3
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pretreatment by diamond powder can significantly enhance diamond surface nucleation density w12,13x, besides, refractory metal carbide powder such as Tantalum carbide (TaC), WC, TiC, HfC and other hard materials such as TiN and c-BN also have a positive effect on nucleation w2,13–15x. Chakk et al. w16x showed that the diamond nucleation density increases by adding metal particle to a diamond suspension. Michau et al. w15x observed that metal carbides enhance diamond nucleation more greatly than that of metals. To date, the systematical study on the effect of adding carbide powder to a diamond suspension on diamond nucleation enhancement is scarce. In optical and microelectronic application of diamond films, besides high nucleation density, no or less substrate surface damage and contamination are of the same particular importance. In some microelectronic application of diamond films, carbide of TaC or Tantalum (Ta) is used for substrates. Besides, Ta is widely used as filament material in hot filament chemical vapor deposition (HFCVD), in these cases, the use of TaC powder in pretreatment can avoid or reduce surface contamination. We report herewith that the effects of diamond and the mixture of diamond and TaC powders on diamond nucleation enhancement. The optimum size of mixture of diamond and TaC powders, which enhances diamond nucleation greatly but causes less substrate damage than pure diamond powder does, has been obtained. 2. Experimental details Mirror-polished Si wafers were chosen as substrates for diamond deposition in this study. The substrates of 8=8 mm in size were cleaned using acetone, alcohol and de-ionized water in an ultrasonic bath for 10 min for each process in turn. Then they were ultrasonically pretreated with (1) pure 1.5, 5, 20 and 40 mm-diamond powder; (2) mixtures of 1.5, 5 mm-diamond and 5, 20 mm-TaC powders for 45 min, respectively. Diamond films were deposited using microwave plasma chemical vapor deposition (MPCVD). Substrate temperature was monitored by a thermocouple embedded in the substrate holder. The optimization deposition parameters were as follows: the substrate temperature of 800 8C, deposition pressure of 40 Torr and microwave power of 800 W. CVD diamond was grown in conventional CH4 yH2 mixture, keeping the ratio of CH4 yH2 as 1%, and total gas flow of 200 sccm. The root-mean-square (Rms) surface roughness over a 5=5 mm2 areas and surface morphology for the pretreated substrates were examined using a SPM9500J2 atomic force microscopy (AFM). The surface morphology of the diamond films is observed by a field emission scanning electron microscopy (FE-SEM) 6340F. Nucleation density was evaluated by calculating number density of diamond grains from 5 SEM images
Fig. 1. Dependence of Rms surface roughness of the substrates on diamond powder size used in ultrasonic pretreatment.
taken at the central region of each sample. The structure of residual powders left on pretreated substrates was confirmed by a Renishaw micro-Raman spectrometer and a Siemens D5005 X-ray diffractmeter. 3. Results 3.1. Effects of ultrasonic pretreatment with pure 1.5–40 mm-diamond powder on diamond nucleation Si substrates were ultrasonically pretreated with pure 1.5, 5, 20 and 40 mm-diamond powder for 45 min, respectively. The suspension used contained 6 g diamond powder in 200 ml of alcohol. Fig. 1 shows the dependence of Rms surface roughness of the substrates on diamond powder size used in ultrasonic pretreatment. It reveals that the Rms surface roughness increases with increasing diamond powder size, indicating that more substrate damage is caused using larger size of diamond powder. Fig. 2 shows AFM image for substrate ultrasonically pretreated with 1.5 mm-diamond powder. It can be clearly observed that there is some residual diamond powder (confirmed by Raman spectrometer) left and embedded on the substrate. Fig. 3 shows the diamond nucleation density, as a function of diamond powder size, the deposition time is 40 min. It reveals that nucleation density increases with increasing diamond powder size. The result obtained here is consistent with other reports w5,6x. 3.2. Effects of ultrasonic pretreatment with mixtures of diamond and TaC powders Ultrasonic pretreatments were performed on Si substrates with a mixture of 1.5 mm-diamond and 5 mm-
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Fig. 3. CVD diamond nucleation density as a function of diamond powder size used in ultrasonic pretreatment.
Fig. 2. AFM image over a 3=3 mm2 areas for substrates pretreated with 1.5 mm-diamond powder.
TaC powders, and a mixture of 1.5 mm-diamond and 20 mm-TaC powders. The suspension used contained 3 g diamond and 3 g TaC powders in 200 ml of alcohol. And also did with pure 1.5 mm-diamond powder for comparison, which the suspension used contained 6 g diamond powder in 200 ml of alcohol. Fig. 4 shows the dependence of Rms surface roughness of the substrates on TaC powder size, the Rms surface roughness increases with increasing TaC powder size. This is similar to the case using pure diamond powder. Fig. 5 shows the X-ray diffraction (XRD) spectra of the substrate after ultrasonic pretreatment with a mixture of 1.5 mm-diamond and 20 mm-TaC powders, the peaks for diamond and TaC are detected, indicating there are residual diamond and TaC powders left on the substrate after ultrasonic pretreatment. The results of CVD diamond nucleation density for these substrates are shown in Fig. 6, the deposition time is 40 min. It can be seen that CVD diamond nucleation density increases above 3 orders of magnitude at the mixture of 1.5 mm-diamond and 20 mm-TaC powders in comparison with pure 1.5 mm-diamond powder, indicating a much stronger enhancement of CVD diamond nucleation by the mixture of diamond and TaC powders than by pure diamond powder. The ultrasonic pretreatment was also performed on Si substrates with mixtures of 5 mm-diamond and 5, 20 mm-TaC powders for 45 min. Fig. 7 also indicates that with adding the TaC powder, much more beneficial effect of promoting diamond nucleation can be obtained. But comparing to Fig. 6, it can be seen that the mixture of 1.5 mm-diamond and 20 mm-TaC powders shows
slightly stronger enhancement effect than that of 5 mmdiamond and 20 mm-TaC powders. Based on the above experimental observations, it can be found that a mixture of diamond and TaC powders enhances CVD diamond nucleation much more greatly than that of pure diamond powder, especially when the size of diamond powder is not very large (1.5 mm). Consistent experimental results were also obtained on WC–Co substrates by the same author w17x. 3.3. Comparison of effects of ultrasonic pretreatment with pure diamond powder and mixtures of diamond and TaC powders To examine and compare the effects of ultrasonic pretreatment with pure diamond powder and mixtures
Fig. 4. Dependence of Rms surface roughness of the substrates on TaC powder size used in ultrasonic pretreatment.
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Fig. 5. XRD spectra of the substrate after ultrasonic pretreatment with a mixture of 1.5 mm-diamond and 20 mm-TaC powders.
of diamond and TaC powders on diamond nucleation and morphology, further experiments were taken. Fig. 8 shows the AFM images for the substrates after ultrasonic pretreatment with pure 40 mm-diamond powder and a mixture of 1.5 mm-diamond and 20 mm-TaC powders. It can be observed that the substrate pretreated with 40 mm-diamond powder is coarser than that with a mixture of 1.5 mm-diamond and 20 mm-TaC powders, and their Rms surface roughness of 12 and 6.6 nm, respectively. This indicates that the substrate is less damage caused by a mixture of 1.5 mm-diamond and
Fig. 6. The results of CVD diamond nucleation density on Si substrates ultrasonically pretreated using pure 1.5 mm-diamond powder and mixtures of 1.5 mm-diamond and 5, 20 mm-TaC powders for 45 min.
20 mm-TaC powders than that caused by 40 mmdiamond powder. Diamond films were deposited on the pretreated substrates under identical deposition conditions mentioned in Section 2. Fig. 9a and b show the SEM images of the diamond films on substrates ultrasonically pretreated by pure 40 mm-diamond powder, and a mixture of 1.5 mm-diamond and 20 mm-TaC powders, respectively, deposition time is 40 min. It can be observed that an approximately equivalent nucleation density of the order of 1010 cmy2, has been obtained. Fig. 9c and d show the SEM images of the diamond films after
Fig. 7. The results of CVD diamond nucleation density on Si substrates ultrasonically pretreated using pure 5 mm-diamond powder and mixtures of 5 mm-diamond and 5, 20 mm-TaC powders.
S.G. Wang et al. / Diamond and Related Materials 11 (2002) 1683–1689
Fig. 8. AFM images over a 5=5 mm2 areas for substrates after ultrasonic pretreatment with (a) pure 40 mm-diamond powder and (b) a mixture of 1.5 mm-diamond and 20 mm-TaC powders.
deposition time of 70 min. It can be seen that both diamond films are pinhole-free and homogeneous. Therefore, homogeneous diamond films can be obtained through ultrasonic pretreatment of substrates with a mixture of 1.5 mm-diamond and 20 mm-TaC powders or pure 40 mm-diamond powder. 4. Discussion CVD diamond nucleation density is critically influenced by substrate surface conditions. It depends on the number of activated nucleation sites available on the substrate surface w18–20x. Figs. 3, 6 and 7 show that diamond nucleation density increases with increasing diamond or TaC powder size used in the ultrasonic pretreatment. This should be attributed to the following
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two effects: on the one hand, during ultrasonic pretreatment to the silicon substrates, surface defects can be produced on the substrates, these defects have high energy, and CVD diamond nucleates on these defect sites. The larger the diamond or TaC particles are, the more damage caused on the substrates (Figs. 1 and 4), therefore, the more the high energy and defect sites are produced, and thus the more the CVD diamond nucleation sites on the substrates w19,21x. On the other hand, it is also the seeding effect. Fig. 2 and Fig. 5 indicate there are some diamond powder residues left adherent to and embedded on the substrate surface after ultrasonic pretreatment, CVD diamond growth can occur by means of homoepitaxy on these diamond powder residues w21,22x. A mixture of diamond and TaC powders enhances diamond nucleation much more greatly than pure diamond powder, it correlates probably to the following effects of TaC: (a) TaC has a close lattice constant and a cubic crystalline structure as diamond does, which can assist diamond epitaxy w13,15,23x; (b) TaC has a relative large density w2x, it can create more high-energy and defects sites under the same pretreatment conditions, resulting in more nucleation centers; (c) TaC residues left by the pretreatment with mixed slurry (Fig. 5) influence the rate of CVD diamond growth through conversion of sp- and sp2-bonded carbon species to the sp3-bonded carbon above the growing surface. This enhancement in sp3-bonded carbon surface concentration at the initial stages of deposition prevents the small diamond residues from complete annihilation by atomic hydrogen w24x, consequently, a good nucleation enhancement efficiency has obtained with mixture of diamond and TaC powder. A mixture of 1.5 mm-diamond and 20 mm-TaC powders has a slightly stronger nucleation enhancement compared to that of 5 mm-diamond and 20 mm-TaC powders. It could be explained that 20 mm-TaC powders mainly creates defect sites (preferred nucleation sites) and 1.5 mm-diamond powder embeds more easily on defect sites of substrate surface due to its smaller size. And the submicron diamond powder residues then serve as seeds for the subsequent deposition on the substrate surface. It should be noted that: (a) Ta is widely used as filament material in HFCVD. After carburization, Ta forms carbide of TaC, therefore, using TaC in pretreatment can avoid increasing other foreign contamination on the substrate in diamond deposition by HFCVD. In addition, TaC or Ta is also used as substrates for some microelectronic applications of diamond films, in this case, it also can avoid surface contamination by using TaC powder; (b) Less substrate damage was caused by a mixture of 1.5 mm-diamond and 20 mm-TaC powders than that pure 40 mm-diamond powder, but a nearly equivalent nucleation enhancement efficiency can be
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Fig. 9. SEM images of diamond films deposited for (a) 40 min and (c) 70 min on Si substrates ultrasonically pretreated with pure 40 mmdiamond powder, and for (b) 40 min and (d) 70 min on substrates ultrasonically pretreated with a mixture of 1.5 mm-diamond and 20 mm-TaC powders, respectively.
obtained. This result should be useful for those substrates, which are required less substrate damage for optical and electronic applications.
substrates, which are required less substrate damage for optical and electronic applications. References
5. Conclusions Effects of ultrasonic pretreatment using diamond and mixtures of diamond and TaC powders on CVD diamond nucleation on Si substrates have been systematically studied. The CVD diamond nucleation density increases with increasing diamond or TaC powder size. A mixture of diamond and TaC powders enhances diamond nucleation much more greatly than that of pure diamond powder, especially when the diamond size is small (1.5 mm). A mixture of 1.5 mm-diamond and 20 mm-TaC powders has an equivalent nucleation enhancement efficiency but less substrate damage than pure 40 mmdiamond powder does. This would be useful for those
w1x T.P. Ong, R.P.H. Chang, Appl. Phys. Lett. 55 (20) (1989) 2063. w2x H.M. Liu, D.S. Dandy, Diamond Relat. Mater. 4 (1995) 1173. w3x H.E. Windischmann, G.F. PPS, J. Appl. Phys. 68 (11) (1990) 5665. w4x W. Zhu, J. Mater. Res. 10 (2) (1995) 425. w5x S. Yugo, T. Kimura, H. Kanai, Proceedings of the First International Conference of New Diamond Science and Technology, Tokyo, October 1988, KTK Scientific Pub., Tokyo, 1990, p. 119. w6x P. Ascarelli, S. Fontana, Appl. Surf. Sci. 64 (4) (1993) 307. w7x K. Kobashi, K. Nishimura, Y. Kawate, T. Horiuchi, Phys. Rev. B 38 (6) (1988) 4067. w8x W.A. Yarbrough, R. Messier, Science 247 (1990) 688. w9x S. Yugo, T. Kanai, T. Kimura, T. Muito, Appl. Phys. Lett. 58 (1991) 1036.
S.G. Wang et al. / Diamond and Related Materials 11 (2002) 1683–1689 w10x H. Kawarada, C. Wild, H. Herres, R. Locher, P. Koidl, J. Appl. Phys. 81 (1997) 3490. w11x G. Popovici, M.A. Prelas, Physica Status Solid A 132 (2) (1992) 233. w12x H. Maeda, S. Masuda, K. Kusakabe, S. Morooka, J. Cryst. Growth 121 (3) (1992) 507. w13x B.V. Spitsyn, L.L. Bouilov, B.V. Derjaguin, J. Cryst. Growth 52 (1981) 219. w14x P.O. Joffreau, R. Haubner, B. Lux, Int. J. Ref. Hard Metals 7 (4) (1988) 186. w15x D. Michau, B. Tanguy, G. Demazeau, M. Couzi, R. Cavagnat, Diamond Relat. Mater. 2 (1993) 19. w16x Y. Chakk, R. Brener, A. Hoffman, Appl. Phys. Lett. 66 (1995) 2819.
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w17x S.G. Wang, W.Z. Tang, F.X. Lu, J. Univ. Sci. Tech. Beijing 20 (2) (1998) 141. w18x C.M. Niu, G. Tsagaropoulos, J. Baglio, K. Dwight, A. Wold, J. Solid State Chemistry 91 (1) (1991) 47. w19x P.A. Denning, D.A. Stevenson, Appl. Phys. Lett. 59 (13) (1991) 1562. w20x H. Maeda, S. Masuda, K. Kusakabe, S. Morooka, J. Cryst. Growth 121 (3) (1992) 507. w21x S. Iijiima, Y. Aikawa, K. Baba, J. Mater. Res. 69 (7) (1991) 1491. w22x E.J. Bienk, S.S. Eskildsen, Diamond Relat. Mater. 2 (1993) 432. w23x A.R. Badzian, T. Badzian, Surf. Coat. Technol. 36 (1988) 283. w24x Y. Chakk, R. Brener, A. Hoffman, Diamond Relat. Mater. 5 (1996) 286.
CVD diamond nucleation enhanced by ultrasonic pretreatment using diamond and mixture of diamond and TaC powders S.G. Wanga,*, Qing Zhanga, S.F. Yoona, J. Ahna, Q. Wanga, D.J. Yanga, Q.F. Huanga, Ruslia, W.Z. Tangb, F.X. Lub a
S1-B2C-20, Microelectronics Centre, School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore b School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China Received 5 October 2001; received in revised form 28 April 2002; accepted 1 May 2002
Abstract Effects of ultrasonic pretreatment on chemical vapor deposition (CVD) diamond nucleation on Si substrates were systematically studied. Pure 1.5–40 mm-diamond powder and mixtures of 1.5–5 mm-diamond as well as 5–20 mm-Tantalum Carbide (TaC) powder were used in ultrasonic pretreatment. The root-mean-square (Rms) surface roughness of the pretreated substrates, residual diamond and TaC powders left on the substrates were examined using atomic force microscopy (AFM), Raman spectroscopy and X-ray diffraction (XRD), respectively. It was observed that the Rms surface roughness increases with increasing diamond or TaC powder size, and there is some diamond or TaC powder left on the substrates after ultrasonic pretreatment. Diamond films were deposited using microwave plasma chemical vapor deposition (MPCVD) technique and characterized by field emission scanning electron microscopy (FE-SEM). It was found that CVD diamond nucleation density strongly depends on particle size of diamond or TaC powder used, the nucleation density increases with increasing diamond or TaC powder size. A mixture of diamond and TaC powders enhances CVD diamond nucleation much more significantly than that of pure diamond powder. A mixture of 1.5 mm-diamond and 20 mm-TaC powders has an equivalent nucleation enhancement efficiency, which could be caused by pure 40 mm-diamond powder. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Microwave plasma chemical vapor deposition diamond; Nucleation; Pretreated substrates
1. Introduction Chemical vapor deposition (CVD) diamond nucleation critically determines the properties, morphology and homogeneity of diamond films. High nucleation density (the number of nuclei grown on unit substrate surface) and uniform crystal size of diamond on appropriate substrates are required for the applications of CVD diamond films in optics, electronic devices and X-ray lithographic masks w1–3x. CVD diamond nucleation principally depends on surface pretreatment, substrate materials and deposition parameters. Substrate surface pretreatment prior to CVD diamond growth has a great influence on nucleation and the initial stages of growth. In most CVD processes, *Corresponding author. Tel.: q65-67904528; fax: q65-67933318. E-mail address: [email protected] (S.G. Wang).
CVD diamond nucleation density on untreated substrates is very low w4,5x. In order to improve low nucleation density, a variety of surface pretreatment methods such as scratching substrate surface with abrasives, seeding substrates with diamond grit, electrical biasing substrates and ion implantation of substrate surfaces have been developed w5–10x. Among surface pretreatment methods, the mechanical polishing and ultrasonic pretreatments with diamond particles are widely used for nucleation enhancement w5–7x. Comparing to the mechanical polishing, the ultrasonic pretreatment with diamond particle suspensions generated a more homogeneous morphology over the entire deposition surface w6,11x. Furthermore, ultrasonic pretreatment is easily applied to the substrates with complex geometry and shape, and it is less surface damage, more suitable for optical and electronic applications of diamond films. It has been demonstrated that substrate
0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 2 . 0 0 1 3 8 - 3
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pretreatment by diamond powder can significantly enhance diamond surface nucleation density w12,13x, besides, refractory metal carbide powder such as Tantalum carbide (TaC), WC, TiC, HfC and other hard materials such as TiN and c-BN also have a positive effect on nucleation w2,13–15x. Chakk et al. w16x showed that the diamond nucleation density increases by adding metal particle to a diamond suspension. Michau et al. w15x observed that metal carbides enhance diamond nucleation more greatly than that of metals. To date, the systematical study on the effect of adding carbide powder to a diamond suspension on diamond nucleation enhancement is scarce. In optical and microelectronic application of diamond films, besides high nucleation density, no or less substrate surface damage and contamination are of the same particular importance. In some microelectronic application of diamond films, carbide of TaC or Tantalum (Ta) is used for substrates. Besides, Ta is widely used as filament material in hot filament chemical vapor deposition (HFCVD), in these cases, the use of TaC powder in pretreatment can avoid or reduce surface contamination. We report herewith that the effects of diamond and the mixture of diamond and TaC powders on diamond nucleation enhancement. The optimum size of mixture of diamond and TaC powders, which enhances diamond nucleation greatly but causes less substrate damage than pure diamond powder does, has been obtained. 2. Experimental details Mirror-polished Si wafers were chosen as substrates for diamond deposition in this study. The substrates of 8=8 mm in size were cleaned using acetone, alcohol and de-ionized water in an ultrasonic bath for 10 min for each process in turn. Then they were ultrasonically pretreated with (1) pure 1.5, 5, 20 and 40 mm-diamond powder; (2) mixtures of 1.5, 5 mm-diamond and 5, 20 mm-TaC powders for 45 min, respectively. Diamond films were deposited using microwave plasma chemical vapor deposition (MPCVD). Substrate temperature was monitored by a thermocouple embedded in the substrate holder. The optimization deposition parameters were as follows: the substrate temperature of 800 8C, deposition pressure of 40 Torr and microwave power of 800 W. CVD diamond was grown in conventional CH4 yH2 mixture, keeping the ratio of CH4 yH2 as 1%, and total gas flow of 200 sccm. The root-mean-square (Rms) surface roughness over a 5=5 mm2 areas and surface morphology for the pretreated substrates were examined using a SPM9500J2 atomic force microscopy (AFM). The surface morphology of the diamond films is observed by a field emission scanning electron microscopy (FE-SEM) 6340F. Nucleation density was evaluated by calculating number density of diamond grains from 5 SEM images
Fig. 1. Dependence of Rms surface roughness of the substrates on diamond powder size used in ultrasonic pretreatment.
taken at the central region of each sample. The structure of residual powders left on pretreated substrates was confirmed by a Renishaw micro-Raman spectrometer and a Siemens D5005 X-ray diffractmeter. 3. Results 3.1. Effects of ultrasonic pretreatment with pure 1.5–40 mm-diamond powder on diamond nucleation Si substrates were ultrasonically pretreated with pure 1.5, 5, 20 and 40 mm-diamond powder for 45 min, respectively. The suspension used contained 6 g diamond powder in 200 ml of alcohol. Fig. 1 shows the dependence of Rms surface roughness of the substrates on diamond powder size used in ultrasonic pretreatment. It reveals that the Rms surface roughness increases with increasing diamond powder size, indicating that more substrate damage is caused using larger size of diamond powder. Fig. 2 shows AFM image for substrate ultrasonically pretreated with 1.5 mm-diamond powder. It can be clearly observed that there is some residual diamond powder (confirmed by Raman spectrometer) left and embedded on the substrate. Fig. 3 shows the diamond nucleation density, as a function of diamond powder size, the deposition time is 40 min. It reveals that nucleation density increases with increasing diamond powder size. The result obtained here is consistent with other reports w5,6x. 3.2. Effects of ultrasonic pretreatment with mixtures of diamond and TaC powders Ultrasonic pretreatments were performed on Si substrates with a mixture of 1.5 mm-diamond and 5 mm-
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Fig. 3. CVD diamond nucleation density as a function of diamond powder size used in ultrasonic pretreatment.
Fig. 2. AFM image over a 3=3 mm2 areas for substrates pretreated with 1.5 mm-diamond powder.
TaC powders, and a mixture of 1.5 mm-diamond and 20 mm-TaC powders. The suspension used contained 3 g diamond and 3 g TaC powders in 200 ml of alcohol. And also did with pure 1.5 mm-diamond powder for comparison, which the suspension used contained 6 g diamond powder in 200 ml of alcohol. Fig. 4 shows the dependence of Rms surface roughness of the substrates on TaC powder size, the Rms surface roughness increases with increasing TaC powder size. This is similar to the case using pure diamond powder. Fig. 5 shows the X-ray diffraction (XRD) spectra of the substrate after ultrasonic pretreatment with a mixture of 1.5 mm-diamond and 20 mm-TaC powders, the peaks for diamond and TaC are detected, indicating there are residual diamond and TaC powders left on the substrate after ultrasonic pretreatment. The results of CVD diamond nucleation density for these substrates are shown in Fig. 6, the deposition time is 40 min. It can be seen that CVD diamond nucleation density increases above 3 orders of magnitude at the mixture of 1.5 mm-diamond and 20 mm-TaC powders in comparison with pure 1.5 mm-diamond powder, indicating a much stronger enhancement of CVD diamond nucleation by the mixture of diamond and TaC powders than by pure diamond powder. The ultrasonic pretreatment was also performed on Si substrates with mixtures of 5 mm-diamond and 5, 20 mm-TaC powders for 45 min. Fig. 7 also indicates that with adding the TaC powder, much more beneficial effect of promoting diamond nucleation can be obtained. But comparing to Fig. 6, it can be seen that the mixture of 1.5 mm-diamond and 20 mm-TaC powders shows
slightly stronger enhancement effect than that of 5 mmdiamond and 20 mm-TaC powders. Based on the above experimental observations, it can be found that a mixture of diamond and TaC powders enhances CVD diamond nucleation much more greatly than that of pure diamond powder, especially when the size of diamond powder is not very large (1.5 mm). Consistent experimental results were also obtained on WC–Co substrates by the same author w17x. 3.3. Comparison of effects of ultrasonic pretreatment with pure diamond powder and mixtures of diamond and TaC powders To examine and compare the effects of ultrasonic pretreatment with pure diamond powder and mixtures
Fig. 4. Dependence of Rms surface roughness of the substrates on TaC powder size used in ultrasonic pretreatment.
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Fig. 5. XRD spectra of the substrate after ultrasonic pretreatment with a mixture of 1.5 mm-diamond and 20 mm-TaC powders.
of diamond and TaC powders on diamond nucleation and morphology, further experiments were taken. Fig. 8 shows the AFM images for the substrates after ultrasonic pretreatment with pure 40 mm-diamond powder and a mixture of 1.5 mm-diamond and 20 mm-TaC powders. It can be observed that the substrate pretreated with 40 mm-diamond powder is coarser than that with a mixture of 1.5 mm-diamond and 20 mm-TaC powders, and their Rms surface roughness of 12 and 6.6 nm, respectively. This indicates that the substrate is less damage caused by a mixture of 1.5 mm-diamond and
Fig. 6. The results of CVD diamond nucleation density on Si substrates ultrasonically pretreated using pure 1.5 mm-diamond powder and mixtures of 1.5 mm-diamond and 5, 20 mm-TaC powders for 45 min.
20 mm-TaC powders than that caused by 40 mmdiamond powder. Diamond films were deposited on the pretreated substrates under identical deposition conditions mentioned in Section 2. Fig. 9a and b show the SEM images of the diamond films on substrates ultrasonically pretreated by pure 40 mm-diamond powder, and a mixture of 1.5 mm-diamond and 20 mm-TaC powders, respectively, deposition time is 40 min. It can be observed that an approximately equivalent nucleation density of the order of 1010 cmy2, has been obtained. Fig. 9c and d show the SEM images of the diamond films after
Fig. 7. The results of CVD diamond nucleation density on Si substrates ultrasonically pretreated using pure 5 mm-diamond powder and mixtures of 5 mm-diamond and 5, 20 mm-TaC powders.
S.G. Wang et al. / Diamond and Related Materials 11 (2002) 1683–1689
Fig. 8. AFM images over a 5=5 mm2 areas for substrates after ultrasonic pretreatment with (a) pure 40 mm-diamond powder and (b) a mixture of 1.5 mm-diamond and 20 mm-TaC powders.
deposition time of 70 min. It can be seen that both diamond films are pinhole-free and homogeneous. Therefore, homogeneous diamond films can be obtained through ultrasonic pretreatment of substrates with a mixture of 1.5 mm-diamond and 20 mm-TaC powders or pure 40 mm-diamond powder. 4. Discussion CVD diamond nucleation density is critically influenced by substrate surface conditions. It depends on the number of activated nucleation sites available on the substrate surface w18–20x. Figs. 3, 6 and 7 show that diamond nucleation density increases with increasing diamond or TaC powder size used in the ultrasonic pretreatment. This should be attributed to the following
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two effects: on the one hand, during ultrasonic pretreatment to the silicon substrates, surface defects can be produced on the substrates, these defects have high energy, and CVD diamond nucleates on these defect sites. The larger the diamond or TaC particles are, the more damage caused on the substrates (Figs. 1 and 4), therefore, the more the high energy and defect sites are produced, and thus the more the CVD diamond nucleation sites on the substrates w19,21x. On the other hand, it is also the seeding effect. Fig. 2 and Fig. 5 indicate there are some diamond powder residues left adherent to and embedded on the substrate surface after ultrasonic pretreatment, CVD diamond growth can occur by means of homoepitaxy on these diamond powder residues w21,22x. A mixture of diamond and TaC powders enhances diamond nucleation much more greatly than pure diamond powder, it correlates probably to the following effects of TaC: (a) TaC has a close lattice constant and a cubic crystalline structure as diamond does, which can assist diamond epitaxy w13,15,23x; (b) TaC has a relative large density w2x, it can create more high-energy and defects sites under the same pretreatment conditions, resulting in more nucleation centers; (c) TaC residues left by the pretreatment with mixed slurry (Fig. 5) influence the rate of CVD diamond growth through conversion of sp- and sp2-bonded carbon species to the sp3-bonded carbon above the growing surface. This enhancement in sp3-bonded carbon surface concentration at the initial stages of deposition prevents the small diamond residues from complete annihilation by atomic hydrogen w24x, consequently, a good nucleation enhancement efficiency has obtained with mixture of diamond and TaC powder. A mixture of 1.5 mm-diamond and 20 mm-TaC powders has a slightly stronger nucleation enhancement compared to that of 5 mm-diamond and 20 mm-TaC powders. It could be explained that 20 mm-TaC powders mainly creates defect sites (preferred nucleation sites) and 1.5 mm-diamond powder embeds more easily on defect sites of substrate surface due to its smaller size. And the submicron diamond powder residues then serve as seeds for the subsequent deposition on the substrate surface. It should be noted that: (a) Ta is widely used as filament material in HFCVD. After carburization, Ta forms carbide of TaC, therefore, using TaC in pretreatment can avoid increasing other foreign contamination on the substrate in diamond deposition by HFCVD. In addition, TaC or Ta is also used as substrates for some microelectronic applications of diamond films, in this case, it also can avoid surface contamination by using TaC powder; (b) Less substrate damage was caused by a mixture of 1.5 mm-diamond and 20 mm-TaC powders than that pure 40 mm-diamond powder, but a nearly equivalent nucleation enhancement efficiency can be
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Fig. 9. SEM images of diamond films deposited for (a) 40 min and (c) 70 min on Si substrates ultrasonically pretreated with pure 40 mmdiamond powder, and for (b) 40 min and (d) 70 min on substrates ultrasonically pretreated with a mixture of 1.5 mm-diamond and 20 mm-TaC powders, respectively.
obtained. This result should be useful for those substrates, which are required less substrate damage for optical and electronic applications.
substrates, which are required less substrate damage for optical and electronic applications. References
5. Conclusions Effects of ultrasonic pretreatment using diamond and mixtures of diamond and TaC powders on CVD diamond nucleation on Si substrates have been systematically studied. The CVD diamond nucleation density increases with increasing diamond or TaC powder size. A mixture of diamond and TaC powders enhances diamond nucleation much more greatly than that of pure diamond powder, especially when the diamond size is small (1.5 mm). A mixture of 1.5 mm-diamond and 20 mm-TaC powders has an equivalent nucleation enhancement efficiency but less substrate damage than pure 40 mmdiamond powder does. This would be useful for those
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