대기압 플라즈마 제트를 이용한 다이아몬드의 고속증착

윤종선, 박동화

인하대학교 화학공학과

High growth rate diamond synthesis in a atmospheric plasma jet

Jong-Sun Yun, Dong-Wha Park

Department of Chemical Engineering, Inha University

253 Younghyun-dong, Nam-gu, Inchon 402-751, KOREA

INTRODUCTION

Studies of synthesized diamond thin films have attracted much attention for the last two decades because of diamond,s remarkable properties such as its high hardness, thermal conductivity, electrical resistivity, chemical inertness, radiation-resistance, etc [Ko and park, 1998]. Now, diamond films can be synthesized by various chemical vapor deposition (CVD) techniques such as D.C, R.F, hot filament CVD, and microwave-assisted CVD, etc. Among them, Although most reports refer to deposition at subatmospheric conditions, there have been some reports of diamond deposition at atmospheric pressure both with radio frequency (RF) induction thermal plasmas and with dc thermal plasma arc-jet. Low-pressure synthesis techniques usually yield low growth rates. The growth rates for the low-pressure plasma-enhance CVD are typically 1-3 □h-1. The thermal plasma jet was used in this experiment, and grow rates of the order of 180 □h-1 could be realized.

EXPERIMENTAL

Diamond film was synthesized on molybdenum substrate by chemical reaction of methane and hydrogen gas using the DC plasma jet at atmospheric pressure. The experimental apparatus is schematically shown in Fig. 1. The apparatus consist of a dc arc discharge plasma torch and its associated power generator, a reaction chamber, a cool down substrate holder, and vacuum pump. In this experiment, the plasma gas was used argon, and the source gas was used methane. Methane was mixed hydrogen (CH4/H2 ratio of 2∼7%), which was poured into the reaction chamber from two gas inlet nozzles. The flow rates of the methane and hydrogen were controlled with mass flow controllers. Cathode and anode cooling was provided by circulating high pressure water through the torch. Further, the chamber was jacketed reactor to allow water cooling.

Molybdenum substrates of 25.4 mm in diameter were firstly polished with sandpaper, followed by diamond paste. The polished molybdenum substrates were then cleaned in acetone and rinsed with deionized water, before being mounted on a water-cooled holder made of copper. The substrate temperature was carefully adjusted by changing the distance between the substrate and torch exit and by controlling cooling water flow rate. The optical pyrometer was used to monitor the substrate temperature. The experimental conditions are listed in the following Table 1.

The deposited diamond films were analyzed with scanning electron microscopy (Hitach S-4200), X-ray diffractometry (Philips PW 3124) and Raman spectroscopy (SPEX 1403).

Table 1. Experimental conditions

Power	6kW(200A, 30V)	Substrate Temperature	800∼1200℃
Ar flow rate	15□/min	Pressure	760 torr
CH4 flow rate	0.02∼0.28□/min	Distance from torch to substrate	20 mm∼50 mm
H2 flow rate	2~5□/min	Deposition time	60 min
CH4/H2 ratio	2∼7%	Substrate	Molybdenum

Fig. 1. Schematic diagram of experimental apparatus

RESULTS AND DISCUSSION

In order to determine optimum conditions for high growth rate and good crystal, the experiments were conducted by changing of CH4/H2 ratio (2∼7%) and substrate temperature (850∼1100℃). The film had a good crystal at CH4/H2 ratio of 4% and substrate temperature of 950℃. Increasing CH4 concentration, the film structure changed to rectangular (100) faces with secondary nucleation.

Fig. 2 displays the Raman spectra of diamond films grown at different CH4/H2 ratio. The narrow peak at 1332 cm-1 is the characteristic for crystalline diamond, while the two peaks at 1320∼1360 cm-1 and 1500∼1550 cm-1 belong to amorphous sp2-bonded carbon [Ralchenko et al., 1997]. The full width at half maximum of the diamond peak at 1332 cm-1 ranged from 7 cm-1 to 11 cm-1, the minimum value corresponding to the 4% methane film. At 7% CH4, non-diamond hump is showed. These non-diamond forms of carbon are mainly located at grain boundaries, evinced by the fact that the broad Raman peak in 7% CH4 is enhanced by small grains. It is plausible that this broad peak indicates the existence of various local carbon clusters with relaxed sp2 bindings modified by surrounding vacancies [Zhu et al., 1995].

Fig. 3 shows the scanning electron microscopic (SEM) photographs of the cross-sectional views of the films, which were deposited, at different substrate temperatures. The CH4 concentration is 4%. When the substrate temperature is 850℃, the thickness of the film is about 80 □  and the film is smoother than that of other substrate temperature. When the substrate temperature is 950℃, the thickness of film increased about 120 □. At the temperature of 1000℃, the film which has great surface roughness is deposited. The thickness of the film is about 180 □ and it shows that the growth rate of this film is 180 □h-1. Above the 1000℃, the thickness of the film is almost the same as that of 1000℃.

Fig. 4 shows the growth rate and Raman intensity ratio as a function of substrate temperature for deposited films at 4% CH4. The growth rate is observed to reach a maximum at about 1000℃ and the best quality film was achieved at 950℃. It is known that the temperature ranges of the best quality film and high growth rate are at 950∼1000℃.

Fig. 2.  Raman spectra of diamond films deposited at five different CH4/H2 ratio.

Fig. 3. SEM photographs of cross sections of diamond films deposited at various            deposition temperature. deposition time is 1h.

Fig. 4. Effect of substrate temperature on growth rate and diamond to graphite               Raman intensity ratio (Id/Ig). CH4/H2 ratio is 4%.

REFERENCES

Ko, C.K. and Park, D.-W., "Deposition of Diamond Film by Hydrogen-oxyacetylene Combustion Flame", The Korean Journal of Ceramics, 4, 1 (1998).

Ralchenko, V.G., Smolin, A.A., Korotushenko, K.G. and Obraztsova, E.D., "Structural studies of diamond thin films grown from dc arc plasma", J. Mater. Res., 12, 2540 (1997).

Zhu, W., Tan, B.H., Ahn, J. and Tan, H.S., "Studies of diamond films/crystals synthesized by oxyacetylene combustion flame technique", J. Mater. Sci., 26, 2135 (1995).