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Local laser-assisted chemical vapor deposition of diamond
Applied Surface Science 168 (2000) 5±8
Local laser-assisted chemical vapor deposition of diamond  . Mechlerb, P. Heszlera Z. ToÂtha,*, A a
Research Group on Laser Physics of the Hungarian Academy of Sciences, P.O. Box 406, H-6701 Szeged, Hungary b Department of Optics and Quantum Electronics, University of Szeged, P.O. Box 406, H-6701 Szeged, Hungary
Abstract Diamond spots are grown in a hot ®lament CVD reactor from CH4/H2 gas mixture on a supported thin tungsten ®lm. Local growth is achieved by con®ned heating of the substrate using the focused beam of a cw Nd-YAG laser. Thus, diamond spots with a size of 30 mm were obtained. The spots are characterized by scanning electron microscopy and micro Raman spectroscopy. The growth rate and the spot structure strongly depends on the nucleation density which could be controlled by ultrasonic treatment of the sample in a diamond powder±ethanol mixture prior to deposition. At low nucleation density the spot consists of separate crystallites with the size decreasing with the distance from the center in accordance with an inhomogeneous laser-induced temperature distribution. At high nucleation density a ¯at microcrystalline diamond disk is grown with a uniform grain size due to a ¯at top temperature pro®le. Already at the early stages of the deposition, thermal contact has been achieved between the crystallites, homogeneous temperature distribution forms due to the high thermal conductivity of the diamond ®lm. # 2000 Elsevier Science B.V. All rights reserved. Keywords: LCVD; Selective deposition; Diamond
1. Introduction The well-known extreme properties of diamond create a wide range of possibilities for it's application. In the last decades, diamond ®lms have been successfully synthesised by a number of different techniques such as hot ®lament assisted-, microwave plasma-, dc plasma- and rf plasma- chemical vapour deposition [1]. In diamond CVD, hydrogen and some hydrocarbon gas are usually employed as reagents. Laser based techniques are often applied for structuring [2±4] and polishing diamond surfaces [5,6]. Selective deposition of diamond patterns has great importance in the electronics industry. The published methods are
mainly based on deposition by large area CVD with selectively nucleated pattern [7±10]. In this case, however the thickness that can be grown is limited by the onset of spontaneous nucleation. It has been shown that a local temperature rise, induced by a focused laser beam, can be applied for local CVD of diamond spots [11]. In this work, experiments on laser assisted CVD of diamond were performed in order to investigate the dependence of the growth and the structures of spots on nucleation density and laser induced temperature distribution. It is shown that by applying an IR laser source, local growth of diamond can be achieved on a metal ®lm. 2. Experimental
*
Corresponding author. Tel.: 36-62-544-421; fax: 36-62-544-658. E-mail address: [email protected] (Z. ToÂth).
A TEM00 beam of an ADLAS DPY 321 diode pumped Nd-YAG laser (l 1064 nm) was focused
0169-4332/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 5 6 3 - 8
6
Z. ToÂth et al. / Applied Surface Science 168 (2000) 5±8
by a microscope objective to a size of 2w0 20 mm (w0 de®ned by the 1/e2 decrease in the intensity). A 100 nm thick tungsten layer supported on fused silica was used as a substrate. Prior to deposition, to enhance the nucleation, ultrasonic scratching was applied in a mixture containing ethanol and diamond powder. Poorly and heavily nucleated substrates were produced with short (3 min) and long (15 min) treatment. Finally, an ultrasonic cleaning in pure ethanol was applied. The substrate was mounted onto a watercooled copper block. The laser beam was guided through a fused silica window, reaching the tungsten layer across the fused silica substrate. The substrate holder was placed into a hot ®lament CVD reactor. The distance between the ®lament and the tungsten ®lm was 5 mm. The background temperature induced by the 22008C hot ®lament was 6008C which was measured by a thermocouple. A gas mixture of methane (CH4) and hydrogen (H2) was used as a source gas. At ¯ow rates of 100 sccm, CH4/H2 molar ratio of 1% and total pressure of 35 mbar was applied.
The deposited diamond spots were characterised by optical and scanning electron microscopy (SEM), atomic force microscopy (AFM) and micro-Raman spectroscopy. 3. Laser induced local deposition of diamond Through a suitable choice of substrate background temperature and laser power, isolated islands of several tenths of a mm in diameter of polycrystalline diamond could be deposited on the tungsten surface. The patterns obtained on poorly and heavily nucleated areas as shown in the SEM images in Fig. 1a and b, respectively, differ in both spot diameter and crystallite size. On the poorly nucleated surface, well separated single crystals and crystal clusters were grown with gradually increasing size towards the beam centre due to the inhomogeneous temperature pro®le. The diameter of the spot was 30 mm. Wider spots of smaller crystallite size are observed for higher nuclea-
Fig. 1. SEM images of diamond spots grown on tungsten surface. Deposition time: 1h, background temperature: 6008C, laser power: 70 mW. (a) Deposition onto a poorly and (b) heavily nucleated area and the corresponding Raman spectra of the middle of the spots.
Z. ToÂth et al. / Applied Surface Science 168 (2000) 5±8
tion densities. In the central part of these spots the crystallites are overlapping each other (dark region in Fig. 1b). The corresponding Raman spectra of the centre of the spots are also shown in Fig. 1. The deposits are of a good quality displaying a pronounced sharp peak at 1332 cmÿ1 which is characteristic to diamond. The broad features at 1350 and 1580 cmÿ1, however indicate some carbon content also. In both cases the outer parts of the spots show better quality diamond crystal formation, as can be seen in the higher magni®cation SEM images in Fig. 1. The laser induced temperature increase was calculated numerically by solving the two dimensional heat ¯ow equation using the method of ®nite differences. The laser beam which is focused to a 20 mm diameter spot, induced a local temperature rise on a larger area due to heat ¯ow. Fig. 2 shows the calculated temperature distributions on a clean tungsten surface (curve `a') and the distribution after deposition of a continuous 35 mm diameter 1 mm thick diamond spot (curve `b'). There is a signi®cant difference as one can see by comparing the two curves. The explanation for this is that, due to the high thermal conductivity of the diamond phase, as soon as the ®rst intact layer of diamond has formed the heat conduction along the substrate plain increases by a factor of at least 3±5 times. This, according to the calculations, causes the drop of maximum temperature and a rather homogeneous temperature distribution along the laser heated spot (curve `b' in Fig. 2). Therefore, the growth
Fig. 2. Temperature distributions induced by a 70 mW cw NdYAG laser beam focused onto 20 mm diameter spot (a) in a 100 nm thick W ®lm supported onto fused silica (b) in a 35 mm diameter diamond spot deposited onto the W ®lm.
7
Fig. 3. Arrhenius plot calculated for the crystallites in the low nucleation density spot assuming local temperatures as shown by the curve a in Fig. 2, d represents the lateral size of the crystallites grown for 1 h.
rate becomes lower in the centre as compared to the uncompacted diamond layer, resulting in smaller crystallites of homogeneous distribution. With the enhanced heat transport to the edges of the spot, the laser induced temperature distribution widens leading to the corresponding widening of the diameter of the diamond spot. These effects were experimentally observed, see Fig. 1a and b. The effect of local substrate temperature on onedimensional growth rate is shown in Fig. 3. At temperatures below 1000 K the Arrhenius plot indicates the existence of a thermally activated process. The activation energy was determined and found to be 1:45 105 J/mol (34 kcal/mol) which is comparable with the activation energy of diamond growth by conventional hot ®lament CVD [1,12]. Above 1000 K there is a breakpoint in the Arrhenius plot, the growth rate shows saturation with increasing temperature. This effect can be explained either by the onset of other processes, or by the onset of transport limitation of the precursor species to the growing surface. It is known from the research of diamond CVD that higher substrate temperatures result in deposition of graphitic phase in higher amounts [1,12]. The transport limitation is also a possible explanation. It is well-known that high ¯ow rates may signi®cantly increase the growth rate during diamond CVD [1]. It is clear from the comparison of SEM images and Raman spectra in Fig. 1, that in the continuous diamond spot (Fig. 1b) which corresponds to lower temperatures, better crystalline properties
8
Z. ToÂth et al. / Applied Surface Science 168 (2000) 5±8
Fig. 4. (a) Graphitic structure grown on tungsten surface. Deposition time: 1h, background temperature: 6008C, laser power: 90 mW and (b) the corresponding Raman spectrum.
are reached. At the same time, in the central part of the poorly nucleated spot where temperature falls between 1000 and 1200 K, due to increased renucleation, ball like diamond structures are grown. Higher power illumination results in clearly nondiamond deposition in the central region as shown in Fig. 4a. The Raman spectrum of the central spot is also plotted (see Fig. 4b). The two wide peaks that can be seen in the Raman spectrum reveal the turbostratical character of the studied material. The reason for graphitic carbon deposition is the high central temperature [1,12]. 4. Conclusions In conclusion, it is shown that by local Nd-YAG laser heating, the diamond growth can be con®ned to predetermined areas on thin metal ®lms. For deposition of high quality compact diamond disks, application of substrates of high nucleation density are favourable, as shown by our experiments and calculations. Laser-assisted diamond CVD onto poorly nucleated areas provided a series of data from one experiment which could be used to study the kinetics of the growth process. It is shown that diamond deposition at a temperature lower than 1000 K is an Arrhenius type process, while above 1000 K the growth is limited either by precursor transport to the reaction zone or the formation of the non-diamond phases.
Acknowledgements Nato's Scienti®c Affairs Division in the framework of the Science for Peace Programme (Sf P-971934), Hungarian FKFP 0422/1999 and OTKA T 02298 funds supported this work. Z. ToÂth is indebted for Bolyai Scolarship. References [1] H. Liu, D.S. Dandy, Diamond Chemical Vapour Deposition: Nucleation and Early Growth Stages, Noyes Publications, Park Ridge, New Jersey, 1995. [2] V.G. Ralchenko, S.M. Pimenov, Diamond-Films Technol. 7 (1) (1997) 15±40. [3] G.A. Shafeev, S.M. Pimenov, E.N. Loubin, Appl. Surf. Sci. 86 (1995) 392±397. [4] G.A. Shafeev, E.D. Obraztsova, S.M. Pimenov, Appl. Phys. A. 65 (1997) 29±32. [5] R.K. Singh, Dong-Gu-Lee, J. Electr. Mat. 25 (1996) 137±142. [6] A.M. Ozkan, A.P. Malshe, W.D. Brown, Diamond Related Mater. 6 (1997) 1789±1798. [7] S.M. Pimenov, G.A. Shafeev, A.A. Smolin, V.I. Konov, B.K. Vodolaga, Appl. Surf. Sci. 86 (1995) 208±212. [8] M.L. Terranova, M. Rossi, V. Sessa, A. Alippi, Chem. Vapour Depos. 3 (1997) 301±306. [9] H. Sugimura, K. Ushiyama, Y. Sato, O. Takai, Y. Sakamoto, M. Takaya, N. Nakagiri, J. Vac. Sci. Technol. B. 17 (1999) 1919±1922. [10] Y. Sakamoto, M. Takaya, H. Sugimura, O. Takai, N. Nakagiri, Diamond Related Mater. 8 (1999) 1423±1426. [11] M. Lindstam, M. Boman, J.O. Carlsson, Appl. Surf. Sci. 109110 (1997) 462±466. [12] E. Kondoh, T. Ohta, T. Mitomo, K Ohtsuka, J. Appl. Phys. 72 (2) (1992) 705±711.
Local laser-assisted chemical vapor deposition of diamond  . Mechlerb, P. Heszlera Z. ToÂtha,*, A a
Research Group on Laser Physics of the Hungarian Academy of Sciences, P.O. Box 406, H-6701 Szeged, Hungary b Department of Optics and Quantum Electronics, University of Szeged, P.O. Box 406, H-6701 Szeged, Hungary
Abstract Diamond spots are grown in a hot ®lament CVD reactor from CH4/H2 gas mixture on a supported thin tungsten ®lm. Local growth is achieved by con®ned heating of the substrate using the focused beam of a cw Nd-YAG laser. Thus, diamond spots with a size of 30 mm were obtained. The spots are characterized by scanning electron microscopy and micro Raman spectroscopy. The growth rate and the spot structure strongly depends on the nucleation density which could be controlled by ultrasonic treatment of the sample in a diamond powder±ethanol mixture prior to deposition. At low nucleation density the spot consists of separate crystallites with the size decreasing with the distance from the center in accordance with an inhomogeneous laser-induced temperature distribution. At high nucleation density a ¯at microcrystalline diamond disk is grown with a uniform grain size due to a ¯at top temperature pro®le. Already at the early stages of the deposition, thermal contact has been achieved between the crystallites, homogeneous temperature distribution forms due to the high thermal conductivity of the diamond ®lm. # 2000 Elsevier Science B.V. All rights reserved. Keywords: LCVD; Selective deposition; Diamond
1. Introduction The well-known extreme properties of diamond create a wide range of possibilities for it's application. In the last decades, diamond ®lms have been successfully synthesised by a number of different techniques such as hot ®lament assisted-, microwave plasma-, dc plasma- and rf plasma- chemical vapour deposition [1]. In diamond CVD, hydrogen and some hydrocarbon gas are usually employed as reagents. Laser based techniques are often applied for structuring [2±4] and polishing diamond surfaces [5,6]. Selective deposition of diamond patterns has great importance in the electronics industry. The published methods are
mainly based on deposition by large area CVD with selectively nucleated pattern [7±10]. In this case, however the thickness that can be grown is limited by the onset of spontaneous nucleation. It has been shown that a local temperature rise, induced by a focused laser beam, can be applied for local CVD of diamond spots [11]. In this work, experiments on laser assisted CVD of diamond were performed in order to investigate the dependence of the growth and the structures of spots on nucleation density and laser induced temperature distribution. It is shown that by applying an IR laser source, local growth of diamond can be achieved on a metal ®lm. 2. Experimental
*
Corresponding author. Tel.: 36-62-544-421; fax: 36-62-544-658. E-mail address: [email protected] (Z. ToÂth).
A TEM00 beam of an ADLAS DPY 321 diode pumped Nd-YAG laser (l 1064 nm) was focused
0169-4332/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 5 6 3 - 8
6
Z. ToÂth et al. / Applied Surface Science 168 (2000) 5±8
by a microscope objective to a size of 2w0 20 mm (w0 de®ned by the 1/e2 decrease in the intensity). A 100 nm thick tungsten layer supported on fused silica was used as a substrate. Prior to deposition, to enhance the nucleation, ultrasonic scratching was applied in a mixture containing ethanol and diamond powder. Poorly and heavily nucleated substrates were produced with short (3 min) and long (15 min) treatment. Finally, an ultrasonic cleaning in pure ethanol was applied. The substrate was mounted onto a watercooled copper block. The laser beam was guided through a fused silica window, reaching the tungsten layer across the fused silica substrate. The substrate holder was placed into a hot ®lament CVD reactor. The distance between the ®lament and the tungsten ®lm was 5 mm. The background temperature induced by the 22008C hot ®lament was 6008C which was measured by a thermocouple. A gas mixture of methane (CH4) and hydrogen (H2) was used as a source gas. At ¯ow rates of 100 sccm, CH4/H2 molar ratio of 1% and total pressure of 35 mbar was applied.
The deposited diamond spots were characterised by optical and scanning electron microscopy (SEM), atomic force microscopy (AFM) and micro-Raman spectroscopy. 3. Laser induced local deposition of diamond Through a suitable choice of substrate background temperature and laser power, isolated islands of several tenths of a mm in diameter of polycrystalline diamond could be deposited on the tungsten surface. The patterns obtained on poorly and heavily nucleated areas as shown in the SEM images in Fig. 1a and b, respectively, differ in both spot diameter and crystallite size. On the poorly nucleated surface, well separated single crystals and crystal clusters were grown with gradually increasing size towards the beam centre due to the inhomogeneous temperature pro®le. The diameter of the spot was 30 mm. Wider spots of smaller crystallite size are observed for higher nuclea-
Fig. 1. SEM images of diamond spots grown on tungsten surface. Deposition time: 1h, background temperature: 6008C, laser power: 70 mW. (a) Deposition onto a poorly and (b) heavily nucleated area and the corresponding Raman spectra of the middle of the spots.
Z. ToÂth et al. / Applied Surface Science 168 (2000) 5±8
tion densities. In the central part of these spots the crystallites are overlapping each other (dark region in Fig. 1b). The corresponding Raman spectra of the centre of the spots are also shown in Fig. 1. The deposits are of a good quality displaying a pronounced sharp peak at 1332 cmÿ1 which is characteristic to diamond. The broad features at 1350 and 1580 cmÿ1, however indicate some carbon content also. In both cases the outer parts of the spots show better quality diamond crystal formation, as can be seen in the higher magni®cation SEM images in Fig. 1. The laser induced temperature increase was calculated numerically by solving the two dimensional heat ¯ow equation using the method of ®nite differences. The laser beam which is focused to a 20 mm diameter spot, induced a local temperature rise on a larger area due to heat ¯ow. Fig. 2 shows the calculated temperature distributions on a clean tungsten surface (curve `a') and the distribution after deposition of a continuous 35 mm diameter 1 mm thick diamond spot (curve `b'). There is a signi®cant difference as one can see by comparing the two curves. The explanation for this is that, due to the high thermal conductivity of the diamond phase, as soon as the ®rst intact layer of diamond has formed the heat conduction along the substrate plain increases by a factor of at least 3±5 times. This, according to the calculations, causes the drop of maximum temperature and a rather homogeneous temperature distribution along the laser heated spot (curve `b' in Fig. 2). Therefore, the growth
Fig. 2. Temperature distributions induced by a 70 mW cw NdYAG laser beam focused onto 20 mm diameter spot (a) in a 100 nm thick W ®lm supported onto fused silica (b) in a 35 mm diameter diamond spot deposited onto the W ®lm.
7
Fig. 3. Arrhenius plot calculated for the crystallites in the low nucleation density spot assuming local temperatures as shown by the curve a in Fig. 2, d represents the lateral size of the crystallites grown for 1 h.
rate becomes lower in the centre as compared to the uncompacted diamond layer, resulting in smaller crystallites of homogeneous distribution. With the enhanced heat transport to the edges of the spot, the laser induced temperature distribution widens leading to the corresponding widening of the diameter of the diamond spot. These effects were experimentally observed, see Fig. 1a and b. The effect of local substrate temperature on onedimensional growth rate is shown in Fig. 3. At temperatures below 1000 K the Arrhenius plot indicates the existence of a thermally activated process. The activation energy was determined and found to be 1:45 105 J/mol (34 kcal/mol) which is comparable with the activation energy of diamond growth by conventional hot ®lament CVD [1,12]. Above 1000 K there is a breakpoint in the Arrhenius plot, the growth rate shows saturation with increasing temperature. This effect can be explained either by the onset of other processes, or by the onset of transport limitation of the precursor species to the growing surface. It is known from the research of diamond CVD that higher substrate temperatures result in deposition of graphitic phase in higher amounts [1,12]. The transport limitation is also a possible explanation. It is well-known that high ¯ow rates may signi®cantly increase the growth rate during diamond CVD [1]. It is clear from the comparison of SEM images and Raman spectra in Fig. 1, that in the continuous diamond spot (Fig. 1b) which corresponds to lower temperatures, better crystalline properties
8
Z. ToÂth et al. / Applied Surface Science 168 (2000) 5±8
Fig. 4. (a) Graphitic structure grown on tungsten surface. Deposition time: 1h, background temperature: 6008C, laser power: 90 mW and (b) the corresponding Raman spectrum.
are reached. At the same time, in the central part of the poorly nucleated spot where temperature falls between 1000 and 1200 K, due to increased renucleation, ball like diamond structures are grown. Higher power illumination results in clearly nondiamond deposition in the central region as shown in Fig. 4a. The Raman spectrum of the central spot is also plotted (see Fig. 4b). The two wide peaks that can be seen in the Raman spectrum reveal the turbostratical character of the studied material. The reason for graphitic carbon deposition is the high central temperature [1,12]. 4. Conclusions In conclusion, it is shown that by local Nd-YAG laser heating, the diamond growth can be con®ned to predetermined areas on thin metal ®lms. For deposition of high quality compact diamond disks, application of substrates of high nucleation density are favourable, as shown by our experiments and calculations. Laser-assisted diamond CVD onto poorly nucleated areas provided a series of data from one experiment which could be used to study the kinetics of the growth process. It is shown that diamond deposition at a temperature lower than 1000 K is an Arrhenius type process, while above 1000 K the growth is limited either by precursor transport to the reaction zone or the formation of the non-diamond phases.
Acknowledgements Nato's Scienti®c Affairs Division in the framework of the Science for Peace Programme (Sf P-971934), Hungarian FKFP 0422/1999 and OTKA T 02298 funds supported this work. Z. ToÂth is indebted for Bolyai Scolarship. References [1] H. Liu, D.S. Dandy, Diamond Chemical Vapour Deposition: Nucleation and Early Growth Stages, Noyes Publications, Park Ridge, New Jersey, 1995. [2] V.G. Ralchenko, S.M. Pimenov, Diamond-Films Technol. 7 (1) (1997) 15±40. [3] G.A. Shafeev, S.M. Pimenov, E.N. Loubin, Appl. Surf. Sci. 86 (1995) 392±397. [4] G.A. Shafeev, E.D. Obraztsova, S.M. Pimenov, Appl. Phys. A. 65 (1997) 29±32. [5] R.K. Singh, Dong-Gu-Lee, J. Electr. Mat. 25 (1996) 137±142. [6] A.M. Ozkan, A.P. Malshe, W.D. Brown, Diamond Related Mater. 6 (1997) 1789±1798. [7] S.M. Pimenov, G.A. Shafeev, A.A. Smolin, V.I. Konov, B.K. Vodolaga, Appl. Surf. Sci. 86 (1995) 208±212. [8] M.L. Terranova, M. Rossi, V. Sessa, A. Alippi, Chem. Vapour Depos. 3 (1997) 301±306. [9] H. Sugimura, K. Ushiyama, Y. Sato, O. Takai, Y. Sakamoto, M. Takaya, N. Nakagiri, J. Vac. Sci. Technol. B. 17 (1999) 1919±1922. [10] Y. Sakamoto, M. Takaya, H. Sugimura, O. Takai, N. Nakagiri, Diamond Related Mater. 8 (1999) 1423±1426. [11] M. Lindstam, M. Boman, J.O. Carlsson, Appl. Surf. Sci. 109110 (1997) 462±466. [12] E. Kondoh, T. Ohta, T. Mitomo, K Ohtsuka, J. Appl. Phys. 72 (2) (1992) 705±711.