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Diamond growth from a mixture of fluorocarbon and hydrogen in a microwave plasma
Diamond and Related Materials, 3 (1994) 1072-1078
1072
Diamond growth from a mixture of fluorocarbon and hydrogen in a microwave plasma Hideaki Maeda, Miki Irie, Takafumi Hino, Katsuki Kusakabe and Shigeharu Morooka* Department of Chemical Science and Technology, K yushu University, 6-10-1, Hakozaki, H igashi-ku, Fukuoka 812 (Japan) (Received November 2, 1993)
Abstract The process of diamond deposition on Si(100) substrates is examined in a microwave plasma using tetrafluoromethane (CF4) and trifluoromethane (CHF3) as well as methane (CH4) as the carbon source. Substrate pretreatment with diamond powder is effective for the enhancement of diamond nucleation for all the reactant systems tested. Well-faceted diamond of high quality is formed at a gaseous carbon concentration of about 1%. For CF4-H2 and CHF3-H2 systems the growth rate of diamond is nearly constant at temperatures above 850 °C, while in the temperature range from 640 to 850 °C it decreases not only with decreasing deposition temperature but also with increasing fluorine content of the reactant. The apparent activation energy for diamond growth is estimated as 119 kJ mol-1, 86 kJ mol-1 and 43 kJ mol-1 for the CF 4 H2, CHF 3 H z and CH4-H 2 systems respectively. The growth behaviour in the fluorocarbon gas systems differs from that in the CH4-H2 system. Fluorine compounds seem to terminate active sites of the growing diamond at temperatures lower than 850 °C and to remove non-diamond phases above 850 °C.
1. Introduction
Since the earliest reports on the successful formation of diamond from the vapour phase [1-3], diamond synthesis by CVD methods has attracted much attention because of its great potential for applications in high technology industries. Today, various CVD techniques including hot-filament [4], microwave plasma [5], r.f. plasma [6], d.c. plasma [7] and combustion flame [8] approaches have been used to synthesize diamond thin films. A variety of gases have also been tested as the carbon s o u r c e . C H 4 is one of the most popular sources, and its mixture with H 2 has been widely employed as a basic reactant system for diamond synthesis. Some recent papers [9-12] report very interesting results of the diamond formation with fluorinecontaining reactant systems. Patterson and coworkers [9-11] synthesized diamond particles under ambient pressure by thermal decomposition of fluorinecontaining gases. A simple flow tube of Monel (Ni-Cu) alloy was used, and reactant gases were continuously introduced into a hot zone kept at a temperature between 700 and 950 °C with an external electric furnace. In their experiment, molecular fluorine (F2) was added to a mixture of hydrogen and hydrocarbon, such as CH4-Fz-H 2. A mixture of hydrogen and halocarbon such as CH3F , CH2Fa, CF4, CHC1Fe, CC12F:, CC14, *Author to whom correspondence should be addressed.
0925-9635/94/$7.00 SSDI 0925-9635(93)00173-B
FaC=CH 2, C1FC=CF2, CHBr3 or CH3I was also employed. Most diamond was deposited on the substrate downstream from the hot central zone at a temperature between 750 and 250 °C. A very small amount of oxygen was found to play an important role in the diamond deposition. Wong and Wu [12] confirmed diamond deposition at 500 °C from a CHF3-CH4-Hz mixture by thermal CVD as well, but the effect of oxygen was not mentioned. Rudder et al. [13] investigated diamond deposition from the CF4-H z system by r.f. plasma CVD, and found a denser nucleation of diamond on an Si wafer without pretreatment. The concentration of the carbon source was optimum at 8%, which was much higher than in cases of traditional CH4-H 2 systems. Thus they concluded that the mechanism of diamond formation in CF4-H z mixed gas was different from that in CH4-H z. Kadono et al. [ 14], however, obtained a contradictory result. The quality of diamond deposited on an Si substrate in a mixture of C F 4 and H2 by microwave plasma was improved by decreasing the C F 4 c o n c e n t r a t i o n in the range 2.5%-40%. As mentioned above, some fluorocarbons contribute to diamond formation, but the effect of fluorine or fluorocarbons on diamond deposition has never been fully elucidated. Further research from the viewpoint of morphology and growth kinetics is needed to understand the characteristic phenomena in fluorine-containing reactant systems. In the present study, diamond depos-
© 1994 - Elsevier Sequoia. All rights reserved
H. Maeda et al. / Diamond growth from fluorocarbon-H mixture
1073
ition is performed in a microwave plasma using CF4, CHF3 and CH4 as the gaseous carbon source, and the effect of the fluorocarbon on diamond formation is discussed.
2. Experimental procedure The diamond synthesis was carried out by microwave-plasma-enhanced chemical vapour deposition (MPECVD). Figure 1 shows a schematic diagram of the apparatus employed, which is similar to that developed by Kamo et al. [5]. Microwave radiation of 2.45 G H z was introduced through a waveguide into a quartz tube reaction chamber of 43 mm inner diameter. The substrate holder, made of molybdenum metal, was completely coated with diamond film prior to the reaction. The carbon source used was CF 4, C H F 3 or CH4, and the flow rates of gaseous reactants were controlled by thermal mass-flow controllers. The substrate was a mirror-polished p-type Si(100) wafer, 5 mm by 10 mm in size, and was usually pretreated with diamond abrasive powder (8-16 pro) for 30 min by an ultrasonic method. In some runs, the substrate without pretreatment was subjected to the nucleation and growth of diamond. The substrate was placed in the microwave cavity, and the surface temperature was measured with an optical pyrometer assuming an emissivity of unity. After evacuating the quartz chamber, 0.5-8 ml rain z of the carbon source and 100 ml rain 1 of hydrogen at ambient temperature and pressure were introduced, and a mixed-gas plasma was generated. The
[~
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~ ~
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o
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o
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Fig. 1. Schematic diagram of the M P E C V D apparatus used for diamond synthesis.
Fig. 2. Single-crystal diamond used as substrate.
pressure was kept at 5.3 kPa (40 Torr) throughout the present experiment. The substrate temperature was changed between 640 and 950 °C by adjusting the microwave power. Single-crystal diamond particles were also used as the substrate, and the growth rate on the seed was determined by measurement of the particle size before and after the deposition, as previously by Kamo et al. [15]. Figure 2 shows a typical seed used in this research. Seed crystals had cubo-octahedral morphology about 0.5-1 pm in size and were prepared on an Si substrate at 800 °C in 0.5% CH4. Morphologies of deposits were observed with a field emission scanning electron microscope (Hitachi S-900) without coating. To study the early growth stage of diamond, ten points of the substrate surface were chosen as observation targets, and the change in exactly the same surface during the M P E C V D was traced as a function of reaction time by repeating the deposition and observation cycle. Products were characterized by Raman spectroscopy (Jasco NR-1100 laser Raman spectrophotometer) using the 514.5 nm line of an Ar ion laser.
3. Results and discussion 3.1. Effect of fluoromethane concentration on diamond deposition Figures 3(a)-3(c) show the morphologies and Raman spectra of products formed by the reaction at 850 °C for
H. Maeda et al. / Diamond growth from fluorocarbon-H mixture
1074
(a) 1.0 %
\
\ ,% (b) 4.8 %
soo (c) 7.4 %
_j 2000
1800
1600
! 1400
1200
,ii
1000
Wave number (cm -1 ) Fig. 3. Raman spectra and morphologies of products produced at 850 °C for 5 h in CF4-H 2 with CF4 concentrations of (a) 1%, (b) 4.8% and (c) 7.4%.
5 h with different C F 4 concentrations in H 2. Although the deposits include diamond in all cases, the amount of byproducts indicated by the Raman peak at 1400-1600 cm -~ increases with increasing CF 4 concentration. Diamond of relatively high quality is formed only when the C F 4 concentration is lower than about 1%. Higher concentrations lead to the formation of microcrystalline diamond. A similar experiment was performed for the CHFa-H 2 reactant systems. The effect of CHF 3 concentration on the quality and morphology of diamond is shown in Fig. 4, indicating that high quality diamond is obtained at 1% and 4.8% CHF3 in H2, and that the quality decreases at 7.4% CHF3. As shown above, the optimal concentration of fluorocarbon sources is nearly the same as that of methane in the C H 4 - H 2 system. This is contradictory to the result for the optimum CF4 concentration reported by Rudder et al. [13], but the reaction pressure and activation method are different in these studies. Rudder et al. performed their experiment at a relatively low pressure
of 5 Torr by r.f. plasma CVD with a frequency of 13.56 MHz. 3.2. Effect of substrate pretreatment on diamond deposition Figures 5(a) and 5(b) show the effect of substrate pretreatment with diamond powder on the morphology of diamond formed after 1 h MPECVD from CF4-H 2. The pretreatment strongly enhanced the diamond nucleation, and a continuous film comprising well-faceted crystals was formed within 1 h. In contrast, the surface without pretreatment was severely etched before diamond grew, and the population density of diamond particles was as low as 104-105 cm -2. The experiment was carried out with CHFa-H/, yielding a similar result. Nucleation enhancement by pretreatment with diamond abrasive has been reported for CH4-H2 systems by use of hot-filament CVD and microwave plasma CVD. According to Iijima et al. [16], an Si substrate surface abraded with diamond powder is coated with
H. Maeda et al. / Diamond growth from fiuorocarbon H mixture
1075
(b) 4.8 %
2000
•
8"0
1 0
'
I
1600
I
I
1400
I
I
1200
I
I
1000
Wave number (cm -1 )
Fig. 4. Raman spectra and morphologies of products produced at 850 °C for 5 h in and (c) 7.4%.
I pm
Fig. 5. Morphologies of products after 1 h MPECVD from C F a - H 2 on a substrate (a) with and (b) without pretreatment (CF4 concentration, 1%,, deposition temperature, 850 °C). small flakes of the diamond abrasive, and diamond is deposited epitaxially on the diamond residues. We [ 17] also found that pretreatment of an Si substrate with diamond abrasive powder introduced fragments of the abrasive on the substrate surface, and that diamond deposition in a microwave plasma occurred predomi-
CHF3-H 2
with CHF 3 concentrations of (a) 1%, (b) 4.8%
nantly on the surface of the implanted diamond fragments. To understand the effect of the substrate pretreatment in fluorine-containing reactant systems, the early deposition stage was observed with the field emission scanning electron microscope. Figures 6(a)-(d) show the time evolution at the same point on a pretreated substrate surface placed in C F 4 - H 2 plasma. Irregular fine particles ranging in size from a few to a few tens of nanometres were initially observed on the substrate surface as shown in Fig. 6(a), and were identified as diamond by electron diffraction analysis [17]. After a 5 m i n reaction, the substrate was severely etched by fluorination. Most diamond residues were removed, but a small number of large residues survived to act as deposition sites of diamond from the vapour phase. The growth of diamond continued in spite of the etching of the substrate. Diamond was formed where no abrasive residues were initially implanted, as is clearly indicated at the upper right-hand side in Fig. 6(c). This type of diamond formation was observed at all monitoring points. Unless the substrate was pretreated, however, the population den-
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H. Maeda et al. / Diamond growth from fluorocarbon-H mixture Temperature (°C)
(b) 5 min 10 4
1000
900
800
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'
600
700
I
'
!
E "o
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• CF4 101
I
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I
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1.10
10001T (K "I )
Fig. 7. Temperature dependence of the diamond growth rate between 640 and 950 °C (carbon source gas concentration, 1%).
Fig. 6. Time evolution of a substrate surface pretreated with diamond abrasive and subjected to MPECVD for (a) 0min (initial state), (b) 5 min, (c) 15 min and (d) 30 rain (CF4 concentration, 1%; deposition temperature, 850 °C).
sity of diamond particles in the fluorine-containing systems was 104-105 cm -2, equivalent to that of CH4-H2 systems using a substrate without pletreatment. Thus the sudden appearance of diamond is mostly caused by the immigration of diamond residues from another part of the substrate surface destroyed by the etching. From the above, it is shown that the pretreatment with diamond powder implants diamond fragments on the substrate and is effective in enhancing diamond deposition from fluorocarbon gas, as reported for CHa-H 2 reactant systems. 3.3. Growth kinetics of diamond in fluorine-containing systems
The growth rate of diamond during MPECVD in CF4-H2, CHF3-H2 and CH4-H2 systems was measured at temperatures between 640 and 950 °C using singlecrystal diamond particles. Figure 7 shows the temperature dependence of the growth rate in different reactant systems. The diamond growth rate at temperatures from 640 to 850 °C decreases not only with decreasing deposition temperature but also with increasing fluorine content in the gaseous carbon source. The growth rate of diamond was expressed by an Arrhenius equation in this temperature range, and the apparent activation energy estimated from the slope in Fig. 7 is 119 kJ mol-1,
86kJ mo1-1 and 43kJ mo1-1 (28kJ mo1-1, 21kcal mol -~ and 10kcal mol -~) respectively for C F 4 - H 2 , CHF3-Hz and CH4-H:. The value for CH4-H z is in good agreement with that reported by Kamo et al. [ 15] (about 45-47 kJ mol-~ at 600-800 °C) and by Kweon et al. [18] (46kJ mo1-1 below 900°C), but is lower than that in other reports [19]. As Kamo et al. [15] pointed out, the growth rate obtained in the present study does not include the nucleation stage where incubation or lead time effects are dominant. Ando et al. [20] investigated the exchange rate between hydrogen and deuterium on the diamond surface by means of Fourier transform IR spectroscopy, and obtained an activation energy of 47 kJ mo1-1 for the hydrogen exchange reaction at 600-800 °C. From the compatibility of the activation energy, our result supports their conclusion that the diffusion step of hydrogen on the diamond surface is rate determining in a series of diamond deposition reactions. The apparent activation energy of diamond growth in fluorine-containing gases is larger than that in CH4-H2, indicating that the surface reaction mechanism of fluorocarbon systems is different from that with methane. Freedman and Stinespring [21] found that three-quarters of a diamond (100) surface was covered with fluorine atoms at 300 K and that the carbon monofluoride layer was stable up to 700 K against attack by molecular oxygen or hydrogen. A further increase in temperature led to desorption of fluorine atoms, and the desorption was completed at 1100 K. Using secondaryion mass spectroscopy, Kadono et al. [ 14] determined the presence of F, Si and H impurities in diamond films prepared in CF4-H2 reactant systems by MPECVD. The fluorine content increased with increasing C F 4 concentration in the gas phase. The above results imply that fluorine can reside on the diamond surface at
H. Maeda et al. / Diamond growth from fluorocarbon H mixture
640-850 °C. The undesirable effect of fluorine-containing gases on the diamond growth in this temperature range is thus explicable by the termination of active sites of growing diamond by fluorine atoms. In the temperature range above 850 °C, the growth rate for CHF3 and C F 4 systems is nearly constant at about 1 jam h - l , while that for C H 4 systems decreases with increasing deposition temperature. As Rosner and Strakey [22] indicated, fluorine is a strong etchant of graphite, a major codeposit during diamond formation. Then the higher growth rate in fluorocarbon gases than in methane may be achieved by the rapid volatilization of non-diamond phases from the growing diamond surface by fluorination. Patterson and coworkers [ 9 - 1 1 ] and Wong and Wu [ 12] reported the low temperature synthesis of diamond by thermal decomposition of various fluorine-containing gases including CE 4 and C H F 3. However, we could not reconfirm the effectiveness of CF 4 or CHF3 on diamond deposition at low temperatures. The mechanism of diamond formation seems to be different with thermal CVD from that with M P E C V D . Figures 8(a) and 8(b) show typical morphologies of seed particles during M P E C V D growth in the C H F 3 - H 2 system at 850°C (Fig. 8(a)) and 640°C (Fig. 8(b)). Isotropic growth was commonly observed above about 700 °C for the reactant systems tested. Below 700 °C, on the contrary, the growth of a (100) surface was preferred to that of a (111) surface, and the resulting particle exhibited the octahedral morphology shown in Fig. 8(b). These variations of crystal habit are widely reported for diamond deposition with various carbon gas sources [15, 23, 24] and cannot be considered as peculiar to fluorocarbons.
1077
deposition behaviour of diamond was compared with that observed with C H 4. Well-faceted diamonds of high quality were formed at a carbon gas concentration of about 1% in all the reactant systems employed. Substrate pretreatment with diamond abrasive was effective in enhancing diamond nucleation. The growth rate of diamond above 850 °C was nearly constant at about 1 jam h 1 for the CF4-H2 and C H F 3 - H 2 systems, while that for CH4 H2 systems decreased with increasing deposition temperature. In the temperature range from 640 to 850 c'C, however, the growth rate decreased with decreasing deposition temperature and with increasing fluorine content in the gaseous reactants. The temperature dependence of the growth morphology was actually the same for all the reactants tested. The difference in growth behaviour for fluorine-containing reactants was apparently caused by the termination of active sites of the growing diamond below 850 C and by the rapid removal of non-diamond phases from the growing surface by fluorination above 850 C .
Acknowledgments We would like to express our gratitude to Professor Hiroshi K o m i y a m a of the University of Tokyo for his useful advice, and to Mr. Masayuki Notoya for R a m a n spectroscopy measurements. Silicon wafers were supplied by Sumitomo Sitix Corporation. This work was supported by the Research Project for Fundamental Engineering of CVD of the Society of Chemical Engineers, Japan.
References 4. Conclusion Diamond was synthesized on an Si(100) substrate using CF 4 and CHF3 in a microwave plasma, and the
Fig. 8. Crystal habits of diamond after MPECVD (a) at 850 °C for 30 min and (b) at 640 °C for 2 h in CHF3 (1%).
1 W. G. Eversole, US Patents 3,030,187, 3,030,188, 1962. 2 B. V. Derjaguin, D. V. Fedssev, V. M. Lukyanovich, B. V. Spitsyn, V. A. Ryabov and A. V. Lavrentyev, J. Cryst. Growth, 2 (1968) 386. 3 J. C. Angus, H. A. Will and W. S. Stanko, J. Appl. Phys., 39 (1968) 2915. 4 S. Matsumoto, Y. Sato, M. Tsutsumi and S. Setaka, J. Mater. Sei., I7 t 1982) 3106. 5 M. Kamo, Y. Sato, S. Matsumoto and N. Setaka, J. Cryst. Growth, 62 (1983) 642. 6 S. Matsumoto, J. Mater. Sci. Lett., 4 (1985) 600. 7 K. Suzuki, A. Sawabe, H. Yasuda and T. lnuzuka, Appl. Phys. Lett., 50 (1987) 728. 8 Y. Hirose and M. Mitsuizumi, New Diamond. (Japan New Diamond Forum), 4 (3) (1987) 34. 9 D. E. Patterson, B. J. Bai, C. J. Chu, R. H. Hauge and J. L. Margrave, Proc. 2nd Int. Conf. on New Diamond Science and Technology, Materials Research Society, Pittsburgh, PA, 1991, p. 433. 10 D. E. Patterson, C. J. Chu, B. J. Bai, Z. L. Xiao, N. J. Komplin, R. H. Hauge and J. L. Margrave, paper presented at Int. Conf. on Metallurgical Coatings and Thin Films. San Diego, CA, April 1991.
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H. Maeda et al. / Diamond growth from fluorocarbon-H mixture
11 D. E. Patterson, C. J. Chu, B. J. Bai, N. J. Komplin, R. H. Hauge and J. L. Margrave, in Y. Tzeng, M. Yashikawa, M. Murakawa and A. Feldman (eds.), Applications of Diamond Films and Related Materials, Proc. 1st Int. Conf. on the Application of Diamond Films and Related Materials, Materials Science Monographs, Vol. 73, Elsevier, New York, 1991, p. 569. 12 M. S. Wong and C. H. Wu, Diamond Relat. Mater., I (1992) 369. 13 R. A. Rudder, G. C. Hudson, J. B. PosthiU, R. E. Thomas and R. J. Markunas, Appl. Phys. Lett., 59 (1991) 791. 14 M. Kadono, T. Inoue, A. Miyadera and S. Yamazaki, Appl. Phys. Lett., 61 (1992) 772. 15 M. Kamo, T. Ando, Y. Sato, K. Bando and J. Ishikawa, Diamond Relat. Mater., 1 (1992) 104. 16 S. Iijima, Y. Arakawa and K. Baba, Appl. Phys. Lett., 57 (1990) 2646.
17 H. Maeda, S. Ikari, K. Kusakabe and S. Morooka, J. Mater. Sci., 28 (1993) 129. 18 D. W. Kweon, J. Y. Lee and D. Kim, J. Appl. Phys., 69 (1991) 8329. 19 E. Kondoh, T. Ohta, T. Mitomo and K. Ohtsuka, Appl. Phys. Lett., 59 (1991) 488. 20 T. Ando, T. Aizawa, M. Ishii, M. Kamo and Y. Sato, Abstr. 6th Diamond Symp., Japan New Diamond Forum, Saitama, 1992, p. 58. 21 A. Freedman and C. D. Stinespring, Appl. Phys. Lett., 57 (1990) 1194.
22 D. E. Rosner and J. P. Strakey, J. Phys. Chem., 77 (1973) 690. 23 B. V. Spitsyn, L. L. Bouilov and B. V. Derjaguin, J. Cryst. Growth, 52 (1981) 219. 24 K. Hirabayashi, New Diamond, 7 (1) (1991) 14.
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Diamond growth from a mixture of fluorocarbon and hydrogen in a microwave plasma Hideaki Maeda, Miki Irie, Takafumi Hino, Katsuki Kusakabe and Shigeharu Morooka* Department of Chemical Science and Technology, K yushu University, 6-10-1, Hakozaki, H igashi-ku, Fukuoka 812 (Japan) (Received November 2, 1993)
Abstract The process of diamond deposition on Si(100) substrates is examined in a microwave plasma using tetrafluoromethane (CF4) and trifluoromethane (CHF3) as well as methane (CH4) as the carbon source. Substrate pretreatment with diamond powder is effective for the enhancement of diamond nucleation for all the reactant systems tested. Well-faceted diamond of high quality is formed at a gaseous carbon concentration of about 1%. For CF4-H2 and CHF3-H2 systems the growth rate of diamond is nearly constant at temperatures above 850 °C, while in the temperature range from 640 to 850 °C it decreases not only with decreasing deposition temperature but also with increasing fluorine content of the reactant. The apparent activation energy for diamond growth is estimated as 119 kJ mol-1, 86 kJ mol-1 and 43 kJ mol-1 for the CF 4 H2, CHF 3 H z and CH4-H 2 systems respectively. The growth behaviour in the fluorocarbon gas systems differs from that in the CH4-H2 system. Fluorine compounds seem to terminate active sites of the growing diamond at temperatures lower than 850 °C and to remove non-diamond phases above 850 °C.
1. Introduction
Since the earliest reports on the successful formation of diamond from the vapour phase [1-3], diamond synthesis by CVD methods has attracted much attention because of its great potential for applications in high technology industries. Today, various CVD techniques including hot-filament [4], microwave plasma [5], r.f. plasma [6], d.c. plasma [7] and combustion flame [8] approaches have been used to synthesize diamond thin films. A variety of gases have also been tested as the carbon s o u r c e . C H 4 is one of the most popular sources, and its mixture with H 2 has been widely employed as a basic reactant system for diamond synthesis. Some recent papers [9-12] report very interesting results of the diamond formation with fluorinecontaining reactant systems. Patterson and coworkers [9-11] synthesized diamond particles under ambient pressure by thermal decomposition of fluorinecontaining gases. A simple flow tube of Monel (Ni-Cu) alloy was used, and reactant gases were continuously introduced into a hot zone kept at a temperature between 700 and 950 °C with an external electric furnace. In their experiment, molecular fluorine (F2) was added to a mixture of hydrogen and hydrocarbon, such as CH4-Fz-H 2. A mixture of hydrogen and halocarbon such as CH3F , CH2Fa, CF4, CHC1Fe, CC12F:, CC14, *Author to whom correspondence should be addressed.
0925-9635/94/$7.00 SSDI 0925-9635(93)00173-B
FaC=CH 2, C1FC=CF2, CHBr3 or CH3I was also employed. Most diamond was deposited on the substrate downstream from the hot central zone at a temperature between 750 and 250 °C. A very small amount of oxygen was found to play an important role in the diamond deposition. Wong and Wu [12] confirmed diamond deposition at 500 °C from a CHF3-CH4-Hz mixture by thermal CVD as well, but the effect of oxygen was not mentioned. Rudder et al. [13] investigated diamond deposition from the CF4-H z system by r.f. plasma CVD, and found a denser nucleation of diamond on an Si wafer without pretreatment. The concentration of the carbon source was optimum at 8%, which was much higher than in cases of traditional CH4-H 2 systems. Thus they concluded that the mechanism of diamond formation in CF4-H z mixed gas was different from that in CH4-H z. Kadono et al. [ 14], however, obtained a contradictory result. The quality of diamond deposited on an Si substrate in a mixture of C F 4 and H2 by microwave plasma was improved by decreasing the C F 4 c o n c e n t r a t i o n in the range 2.5%-40%. As mentioned above, some fluorocarbons contribute to diamond formation, but the effect of fluorine or fluorocarbons on diamond deposition has never been fully elucidated. Further research from the viewpoint of morphology and growth kinetics is needed to understand the characteristic phenomena in fluorine-containing reactant systems. In the present study, diamond depos-
© 1994 - Elsevier Sequoia. All rights reserved
H. Maeda et al. / Diamond growth from fluorocarbon-H mixture
1073
ition is performed in a microwave plasma using CF4, CHF3 and CH4 as the gaseous carbon source, and the effect of the fluorocarbon on diamond formation is discussed.
2. Experimental procedure The diamond synthesis was carried out by microwave-plasma-enhanced chemical vapour deposition (MPECVD). Figure 1 shows a schematic diagram of the apparatus employed, which is similar to that developed by Kamo et al. [5]. Microwave radiation of 2.45 G H z was introduced through a waveguide into a quartz tube reaction chamber of 43 mm inner diameter. The substrate holder, made of molybdenum metal, was completely coated with diamond film prior to the reaction. The carbon source used was CF 4, C H F 3 or CH4, and the flow rates of gaseous reactants were controlled by thermal mass-flow controllers. The substrate was a mirror-polished p-type Si(100) wafer, 5 mm by 10 mm in size, and was usually pretreated with diamond abrasive powder (8-16 pro) for 30 min by an ultrasonic method. In some runs, the substrate without pretreatment was subjected to the nucleation and growth of diamond. The substrate was placed in the microwave cavity, and the surface temperature was measured with an optical pyrometer assuming an emissivity of unity. After evacuating the quartz chamber, 0.5-8 ml rain z of the carbon source and 100 ml rain 1 of hydrogen at ambient temperature and pressure were introduced, and a mixed-gas plasma was generated. The
[~
Optical pyrometer
Quartz viewport ~ Quartz chamber
~ ~
Cooling water
I~IX~--,P- CF4 I
o
I
o
Thermal ] - ~ n -,~- CHF3 mass-flow controllers i~::~::l._.~._ CH4
--7-
~¢k:p--~--
i Three-stub tuner Pla ;ma Power monitor [~ J. J, Isolator
H2
,,u
B
I
J I I micr°wave
B~ ~Waveguide z~
I generat°r
[1~" Substrate [ ~Mo holder ~
Vacuum gauge
[5--
-" Quartz rod
Flow control valves
Rotary pump
Fig. 1. Schematic diagram of the M P E C V D apparatus used for diamond synthesis.
Fig. 2. Single-crystal diamond used as substrate.
pressure was kept at 5.3 kPa (40 Torr) throughout the present experiment. The substrate temperature was changed between 640 and 950 °C by adjusting the microwave power. Single-crystal diamond particles were also used as the substrate, and the growth rate on the seed was determined by measurement of the particle size before and after the deposition, as previously by Kamo et al. [15]. Figure 2 shows a typical seed used in this research. Seed crystals had cubo-octahedral morphology about 0.5-1 pm in size and were prepared on an Si substrate at 800 °C in 0.5% CH4. Morphologies of deposits were observed with a field emission scanning electron microscope (Hitachi S-900) without coating. To study the early growth stage of diamond, ten points of the substrate surface were chosen as observation targets, and the change in exactly the same surface during the M P E C V D was traced as a function of reaction time by repeating the deposition and observation cycle. Products were characterized by Raman spectroscopy (Jasco NR-1100 laser Raman spectrophotometer) using the 514.5 nm line of an Ar ion laser.
3. Results and discussion 3.1. Effect of fluoromethane concentration on diamond deposition Figures 3(a)-3(c) show the morphologies and Raman spectra of products formed by the reaction at 850 °C for
H. Maeda et al. / Diamond growth from fluorocarbon-H mixture
1074
(a) 1.0 %
\
\ ,% (b) 4.8 %
soo (c) 7.4 %
_j 2000
1800
1600
! 1400
1200
,ii
1000
Wave number (cm -1 ) Fig. 3. Raman spectra and morphologies of products produced at 850 °C for 5 h in CF4-H 2 with CF4 concentrations of (a) 1%, (b) 4.8% and (c) 7.4%.
5 h with different C F 4 concentrations in H 2. Although the deposits include diamond in all cases, the amount of byproducts indicated by the Raman peak at 1400-1600 cm -~ increases with increasing CF 4 concentration. Diamond of relatively high quality is formed only when the C F 4 concentration is lower than about 1%. Higher concentrations lead to the formation of microcrystalline diamond. A similar experiment was performed for the CHFa-H 2 reactant systems. The effect of CHF 3 concentration on the quality and morphology of diamond is shown in Fig. 4, indicating that high quality diamond is obtained at 1% and 4.8% CHF3 in H2, and that the quality decreases at 7.4% CHF3. As shown above, the optimal concentration of fluorocarbon sources is nearly the same as that of methane in the C H 4 - H 2 system. This is contradictory to the result for the optimum CF4 concentration reported by Rudder et al. [13], but the reaction pressure and activation method are different in these studies. Rudder et al. performed their experiment at a relatively low pressure
of 5 Torr by r.f. plasma CVD with a frequency of 13.56 MHz. 3.2. Effect of substrate pretreatment on diamond deposition Figures 5(a) and 5(b) show the effect of substrate pretreatment with diamond powder on the morphology of diamond formed after 1 h MPECVD from CF4-H 2. The pretreatment strongly enhanced the diamond nucleation, and a continuous film comprising well-faceted crystals was formed within 1 h. In contrast, the surface without pretreatment was severely etched before diamond grew, and the population density of diamond particles was as low as 104-105 cm -2. The experiment was carried out with CHFa-H/, yielding a similar result. Nucleation enhancement by pretreatment with diamond abrasive has been reported for CH4-H2 systems by use of hot-filament CVD and microwave plasma CVD. According to Iijima et al. [16], an Si substrate surface abraded with diamond powder is coated with
H. Maeda et al. / Diamond growth from fiuorocarbon H mixture
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(b) 4.8 %
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•
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1 0
'
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Fig. 4. Raman spectra and morphologies of products produced at 850 °C for 5 h in and (c) 7.4%.
I pm
Fig. 5. Morphologies of products after 1 h MPECVD from C F a - H 2 on a substrate (a) with and (b) without pretreatment (CF4 concentration, 1%,, deposition temperature, 850 °C). small flakes of the diamond abrasive, and diamond is deposited epitaxially on the diamond residues. We [ 17] also found that pretreatment of an Si substrate with diamond abrasive powder introduced fragments of the abrasive on the substrate surface, and that diamond deposition in a microwave plasma occurred predomi-
CHF3-H 2
with CHF 3 concentrations of (a) 1%, (b) 4.8%
nantly on the surface of the implanted diamond fragments. To understand the effect of the substrate pretreatment in fluorine-containing reactant systems, the early deposition stage was observed with the field emission scanning electron microscope. Figures 6(a)-(d) show the time evolution at the same point on a pretreated substrate surface placed in C F 4 - H 2 plasma. Irregular fine particles ranging in size from a few to a few tens of nanometres were initially observed on the substrate surface as shown in Fig. 6(a), and were identified as diamond by electron diffraction analysis [17]. After a 5 m i n reaction, the substrate was severely etched by fluorination. Most diamond residues were removed, but a small number of large residues survived to act as deposition sites of diamond from the vapour phase. The growth of diamond continued in spite of the etching of the substrate. Diamond was formed where no abrasive residues were initially implanted, as is clearly indicated at the upper right-hand side in Fig. 6(c). This type of diamond formation was observed at all monitoring points. Unless the substrate was pretreated, however, the population den-
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H. Maeda et al. / Diamond growth from fluorocarbon-H mixture Temperature (°C)
(b) 5 min 10 4
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Fig. 7. Temperature dependence of the diamond growth rate between 640 and 950 °C (carbon source gas concentration, 1%).
Fig. 6. Time evolution of a substrate surface pretreated with diamond abrasive and subjected to MPECVD for (a) 0min (initial state), (b) 5 min, (c) 15 min and (d) 30 rain (CF4 concentration, 1%; deposition temperature, 850 °C).
sity of diamond particles in the fluorine-containing systems was 104-105 cm -2, equivalent to that of CH4-H2 systems using a substrate without pletreatment. Thus the sudden appearance of diamond is mostly caused by the immigration of diamond residues from another part of the substrate surface destroyed by the etching. From the above, it is shown that the pretreatment with diamond powder implants diamond fragments on the substrate and is effective in enhancing diamond deposition from fluorocarbon gas, as reported for CHa-H 2 reactant systems. 3.3. Growth kinetics of diamond in fluorine-containing systems
The growth rate of diamond during MPECVD in CF4-H2, CHF3-H2 and CH4-H2 systems was measured at temperatures between 640 and 950 °C using singlecrystal diamond particles. Figure 7 shows the temperature dependence of the growth rate in different reactant systems. The diamond growth rate at temperatures from 640 to 850 °C decreases not only with decreasing deposition temperature but also with increasing fluorine content in the gaseous carbon source. The growth rate of diamond was expressed by an Arrhenius equation in this temperature range, and the apparent activation energy estimated from the slope in Fig. 7 is 119 kJ mol-1,
86kJ mo1-1 and 43kJ mo1-1 (28kJ mo1-1, 21kcal mol -~ and 10kcal mol -~) respectively for C F 4 - H 2 , CHF3-Hz and CH4-H:. The value for CH4-H z is in good agreement with that reported by Kamo et al. [ 15] (about 45-47 kJ mol-~ at 600-800 °C) and by Kweon et al. [18] (46kJ mo1-1 below 900°C), but is lower than that in other reports [19]. As Kamo et al. [15] pointed out, the growth rate obtained in the present study does not include the nucleation stage where incubation or lead time effects are dominant. Ando et al. [20] investigated the exchange rate between hydrogen and deuterium on the diamond surface by means of Fourier transform IR spectroscopy, and obtained an activation energy of 47 kJ mo1-1 for the hydrogen exchange reaction at 600-800 °C. From the compatibility of the activation energy, our result supports their conclusion that the diffusion step of hydrogen on the diamond surface is rate determining in a series of diamond deposition reactions. The apparent activation energy of diamond growth in fluorine-containing gases is larger than that in CH4-H2, indicating that the surface reaction mechanism of fluorocarbon systems is different from that with methane. Freedman and Stinespring [21] found that three-quarters of a diamond (100) surface was covered with fluorine atoms at 300 K and that the carbon monofluoride layer was stable up to 700 K against attack by molecular oxygen or hydrogen. A further increase in temperature led to desorption of fluorine atoms, and the desorption was completed at 1100 K. Using secondaryion mass spectroscopy, Kadono et al. [ 14] determined the presence of F, Si and H impurities in diamond films prepared in CF4-H2 reactant systems by MPECVD. The fluorine content increased with increasing C F 4 concentration in the gas phase. The above results imply that fluorine can reside on the diamond surface at
H. Maeda et al. / Diamond growth from fluorocarbon H mixture
640-850 °C. The undesirable effect of fluorine-containing gases on the diamond growth in this temperature range is thus explicable by the termination of active sites of growing diamond by fluorine atoms. In the temperature range above 850 °C, the growth rate for CHF3 and C F 4 systems is nearly constant at about 1 jam h - l , while that for C H 4 systems decreases with increasing deposition temperature. As Rosner and Strakey [22] indicated, fluorine is a strong etchant of graphite, a major codeposit during diamond formation. Then the higher growth rate in fluorocarbon gases than in methane may be achieved by the rapid volatilization of non-diamond phases from the growing diamond surface by fluorination. Patterson and coworkers [ 9 - 1 1 ] and Wong and Wu [ 12] reported the low temperature synthesis of diamond by thermal decomposition of various fluorine-containing gases including CE 4 and C H F 3. However, we could not reconfirm the effectiveness of CF 4 or CHF3 on diamond deposition at low temperatures. The mechanism of diamond formation seems to be different with thermal CVD from that with M P E C V D . Figures 8(a) and 8(b) show typical morphologies of seed particles during M P E C V D growth in the C H F 3 - H 2 system at 850°C (Fig. 8(a)) and 640°C (Fig. 8(b)). Isotropic growth was commonly observed above about 700 °C for the reactant systems tested. Below 700 °C, on the contrary, the growth of a (100) surface was preferred to that of a (111) surface, and the resulting particle exhibited the octahedral morphology shown in Fig. 8(b). These variations of crystal habit are widely reported for diamond deposition with various carbon gas sources [15, 23, 24] and cannot be considered as peculiar to fluorocarbons.
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deposition behaviour of diamond was compared with that observed with C H 4. Well-faceted diamonds of high quality were formed at a carbon gas concentration of about 1% in all the reactant systems employed. Substrate pretreatment with diamond abrasive was effective in enhancing diamond nucleation. The growth rate of diamond above 850 °C was nearly constant at about 1 jam h 1 for the CF4-H2 and C H F 3 - H 2 systems, while that for CH4 H2 systems decreased with increasing deposition temperature. In the temperature range from 640 to 850 c'C, however, the growth rate decreased with decreasing deposition temperature and with increasing fluorine content in the gaseous reactants. The temperature dependence of the growth morphology was actually the same for all the reactants tested. The difference in growth behaviour for fluorine-containing reactants was apparently caused by the termination of active sites of the growing diamond below 850 C and by the rapid removal of non-diamond phases from the growing surface by fluorination above 850 C .
Acknowledgments We would like to express our gratitude to Professor Hiroshi K o m i y a m a of the University of Tokyo for his useful advice, and to Mr. Masayuki Notoya for R a m a n spectroscopy measurements. Silicon wafers were supplied by Sumitomo Sitix Corporation. This work was supported by the Research Project for Fundamental Engineering of CVD of the Society of Chemical Engineers, Japan.
References 4. Conclusion Diamond was synthesized on an Si(100) substrate using CF 4 and CHF3 in a microwave plasma, and the
Fig. 8. Crystal habits of diamond after MPECVD (a) at 850 °C for 30 min and (b) at 640 °C for 2 h in CHF3 (1%).
1 W. G. Eversole, US Patents 3,030,187, 3,030,188, 1962. 2 B. V. Derjaguin, D. V. Fedssev, V. M. Lukyanovich, B. V. Spitsyn, V. A. Ryabov and A. V. Lavrentyev, J. Cryst. Growth, 2 (1968) 386. 3 J. C. Angus, H. A. Will and W. S. Stanko, J. Appl. Phys., 39 (1968) 2915. 4 S. Matsumoto, Y. Sato, M. Tsutsumi and S. Setaka, J. Mater. Sei., I7 t 1982) 3106. 5 M. Kamo, Y. Sato, S. Matsumoto and N. Setaka, J. Cryst. Growth, 62 (1983) 642. 6 S. Matsumoto, J. Mater. Sci. Lett., 4 (1985) 600. 7 K. Suzuki, A. Sawabe, H. Yasuda and T. lnuzuka, Appl. Phys. Lett., 50 (1987) 728. 8 Y. Hirose and M. Mitsuizumi, New Diamond. (Japan New Diamond Forum), 4 (3) (1987) 34. 9 D. E. Patterson, B. J. Bai, C. J. Chu, R. H. Hauge and J. L. Margrave, Proc. 2nd Int. Conf. on New Diamond Science and Technology, Materials Research Society, Pittsburgh, PA, 1991, p. 433. 10 D. E. Patterson, C. J. Chu, B. J. Bai, Z. L. Xiao, N. J. Komplin, R. H. Hauge and J. L. Margrave, paper presented at Int. Conf. on Metallurgical Coatings and Thin Films. San Diego, CA, April 1991.
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11 D. E. Patterson, C. J. Chu, B. J. Bai, N. J. Komplin, R. H. Hauge and J. L. Margrave, in Y. Tzeng, M. Yashikawa, M. Murakawa and A. Feldman (eds.), Applications of Diamond Films and Related Materials, Proc. 1st Int. Conf. on the Application of Diamond Films and Related Materials, Materials Science Monographs, Vol. 73, Elsevier, New York, 1991, p. 569. 12 M. S. Wong and C. H. Wu, Diamond Relat. Mater., I (1992) 369. 13 R. A. Rudder, G. C. Hudson, J. B. PosthiU, R. E. Thomas and R. J. Markunas, Appl. Phys. Lett., 59 (1991) 791. 14 M. Kadono, T. Inoue, A. Miyadera and S. Yamazaki, Appl. Phys. Lett., 61 (1992) 772. 15 M. Kamo, T. Ando, Y. Sato, K. Bando and J. Ishikawa, Diamond Relat. Mater., 1 (1992) 104. 16 S. Iijima, Y. Arakawa and K. Baba, Appl. Phys. Lett., 57 (1990) 2646.
17 H. Maeda, S. Ikari, K. Kusakabe and S. Morooka, J. Mater. Sci., 28 (1993) 129. 18 D. W. Kweon, J. Y. Lee and D. Kim, J. Appl. Phys., 69 (1991) 8329. 19 E. Kondoh, T. Ohta, T. Mitomo and K. Ohtsuka, Appl. Phys. Lett., 59 (1991) 488. 20 T. Ando, T. Aizawa, M. Ishii, M. Kamo and Y. Sato, Abstr. 6th Diamond Symp., Japan New Diamond Forum, Saitama, 1992, p. 58. 21 A. Freedman and C. D. Stinespring, Appl. Phys. Lett., 57 (1990) 1194.
22 D. E. Rosner and J. P. Strakey, J. Phys. Chem., 77 (1973) 690. 23 B. V. Spitsyn, L. L. Bouilov and B. V. Derjaguin, J. Cryst. Growth, 52 (1981) 219. 24 K. Hirabayashi, New Diamond, 7 (1) (1991) 14.