Thermal Plasma Physical Vapor Deposition of Nanostructured SiC Coatings
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Thermal Plasma Physical Vapor Deposition of Nanostructured SiC Coatings Xinhua Wang, Keisuke Eguchi, Atsushi Yamamoto1 and Toyonobu Yoshida Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-Ku, Tokyo 113-8656, Japan 1 National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan ABSTRACT Nanostructured and thick SiC coatings have been successfully deposited on Si and graphite substrates by thermal plasma physical vapor deposition (TPPVD) using ultrafine SiC powder as a starting material. The control of processing parameters such as substrate temperature, composition of plasma gases, permits to the deposition of SiC coatings with a variety of microstructures and with various morphologies from dense to columnar. The maximum deposition rate reached 200 nm/s. Seebeck coefficient up to –480 µV/K was obtained for the non-doped coatings with stoichiometric composition. Nitrogen doping to the coatings made it possible to decrease the electrical resistivity from 10-2~10-3 to 10-4~10-5 Ωm and showing the maximum power factor of 1.0×10-3 Wm-1K-2 at 973 K. INTRODUCTION Silicon carbide films and coatings attracted growing attention in the past decades due to the unique physical properties, thermal and chemical stability and excellent mechanical properties [1]. A wide range of applications has been reported such as for electronic and optoelectronic devices, protective coatings against corrosion, X-ray mask, thermonuclear reactor walls’ protection coating, and so on. Moreover, recent development and investigations showed that SiC is a promising candidate for a high temperature thermoelectric material because of their high thermoelectric power, high thermal stability and high resistance to oxidation and corrosion, and their nontoxicity [2,3]. Various techniques have been used to deposit SiC coatings. The most common ones are thermal CVD, plasma enhanced CVD, sputtering, laser-assisted methods. The common disadvantage of these processes is the low deposition rate (generally 50 sccm), the deposition rate for stationary substrate was larger than that for rotating substrate. Figure 3 shows the crystallite size and the composition as a function of H2 flow rate for the rotating case. With increasing hydrogen concentration, the crystallite size is also increased, and the film composition changed from C-rich to Si-rich, which suggests that the increase of hydrogen content favors the escape of
(a)
(b)
Figure 1. SEM images of coatings deposited at rf power of 70 kW, CH4 flow rate: (a) 0; (b) 200 sccm.
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Deposition Rate / nm/s
200 150 100 Stationary substr. Rotating substr.
50 0
200
400
600
800 1000
CH4 flow rate / sccm
1.4
20
1.2
15
1.0
10
0.8
5
0.6
0
2
4
6
8
10
0 12
Crystallite size / nm
C/Si atomic ratio
Figure 2. Influence of CH4 flow rate on deposition rate
Hydrogen flow rate/ slm
Figure 3. Influence of hydrogen content on crystallite size and composition the C source. The influence of methane flow rat
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