In-situ Plasma Diagnosis of Chemical Species in Microwave Plasma-assisted Chemical Vapor Deposition for the Growth of Ca
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In-situ Plasma Diagnosis of Chemical Species in Microwave Plasma-assisted Chemical Vapor Deposition for the Growth of Carbon Nanotubes Using CH4/H2/NH3 Gases Y.S.Woo1, I.T.Han, N.S.Lee, J.E. Jung, D.Y. Jeon1, and J.M.Kim 1 Dept. of Materials Sci. and Eng., Korea Advanced Institute of Science and Technology, Taejon, Korea. Display Lab., Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, Korea, *[email protected] ABSTRACT Synthesis of multi-wall carbon nanotubes (MWNTs) was attempted by microwave plasma enhanced chemical vapor deposition using CH4/H2/NH3 gases on Ni/Cr-coated glass at low temperature. The synthesis was investigated by optical emission spectroscopy and quadrupole mass spectroscopy. It was observed that MWNTs could be grown within a very restrictive range of gas compositions. An addition of a small amount of NH3 resulted in a decrease of C2H2, which can be used to estimate the amount of carbon sources in plasma for the growth of MWNTs, and an increase of CN and Hα radicals acting as etching species of carbon phases. These results show that carbon nanotubes can be grown only under an appropriate condition that the growing process surpasses the etching process. The optimum C2H2 /Hα ratio in a gas mixture was found to be between 1 and 3 for the MWNT growth at low temperature. INTRODUCTION Since the first observation of multi-wall carbon nanotubes (MWNTs) by Ijima, many methods to synthesize MWNTs have been reported such as arc discharge [1,2], laser vaporization [3,4], pyrolysis [5], and plasma-enhanced chemical vapor deposition (CVD) [6-9]. For practical applications, low temperature growth of MWNTs on glass is required because glass is widely used for commercial purposes and its deformation temperature is rather low. However, low temperature deposition results in such an involvement of amorphous carbon impurities. Thus an etching process to remove such carbon impurities during the MWNT growth is critical to the synthesis of high quality MWNTs. An in-situ diagnosis of the plasma during the growth may suggest how to enhance the etching reaction. Optical emission spectroscopy (OES) has been widely used as a diagnostic tool to identify active radical species in a plasma [10-12]. It has also been well known that CH3 radicals [13-15] and C2H2 molecules [15] are regarded as important precursors for the deposition of diamond films. Only a few results, however, has been reported about active chemical species for the growth of carbon nanotubes in arc discharge [16] and laser vaporization [4], which have suggested C and C2 dimers as precursors for the carbon nanotube growth. In this study, an NH3 gas was added to a CH4/H2 gas mixture to enhance the etching reaction. And the plasma was diagnosed during the deposition process using OES and quadrupole mass spectroscopy (MS) to identify active species for the carbon nanotube growth. EXPERIMENTAL MWNTs were grown on Ni/Cr-coated glass substrates using microwave plasma-enhanced CVD (MPECVD). A Cr film with the thickness of 1500Å was coated on
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