On-line gas-phase optical diagnostics in plasma CVD deposition of carbon films
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0. Martini and F. Polla Mattiot Eniricerche S.p.A., 00015 Monterotondo (RM), Italy (Received 19 July 1991; accepted 16 December 1991)
Space resolved, on-line spontaneous and stimulated emission, and CARS diagnostics have been employed on CH 4 /H 2 mixtures excited by rf-discharge in order to investigate the chemical processes and the gas phase kinetics in the plasma. CH4 and H2 concentration and temperature have been monitored during the process as a function of main reaction parameters (rf-power, total pressure, CH 4 /H 2 ratio). Formation of CH,H2, and H in excited states has been observed. On the basis of present spectroscopic data, a model for the gas-phase reactions accompanying the carbon film deposition is proposed.
I. INTRODUCTION Carbon in the diamond crystalline state is a material of considerable technological importance because of a combination of different properties peculiar to ceramics, e.g., hardness and stability, with good optical and electrical characteristics. The latter ones make interesting use of diamond-like carbon films in microelectronics and optoelectronics. In fact, in the diamond state carbon shows high thermal conductivity, low electrical conductivity, and a large band gap, all of which are required in highpower and high-temperature components. Furthermore, diamond-like carbon is characterized by an excellent optical transmission from FIR to UV, which, coupled with its mechanical properties, makes it useful for coating of optical elements and light detectors. Chemical processes for the synthesis of diamondlike carbon have been considered from the beginning of the century. In the sixties studies of CVD (Chemical Vapor Deposition) processes based on the pyrolysis of different hydrocarbons were started. At the moment, in the deposition of thin films, two main experimental methods are followed: hydrocarbon decomposition is obtained in a rf or microwave induced plasma or it occurs near the surface of a hot filament.1"3 CVD methods are usually applied at reduced total pressure (typically less than 10 Torr) by diluting a small hydrocarbon in hydrogen. Experimental conditions are sought under which the diamond phase (sp3) is more stable than the graphite (sp2). Addition of O2 as precursor of O atoms, which are supposed to be selective etchants of the growing sp2 phase, has also been recently proposed.4 Direct measurement of the electron current permits, to a certain extent, characterization of the plasma. Con1204 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 5, May 1992 Downloaded: 15 Mar 2015
versely, spectroscopic measurements can detect neutral and ionic fragments, also formed as reaction intermediates, and can give detailed information about their temperature from the rovibrational spectral profile.5 In spite of the large experimental effort in the low-temperature deposition of amorphous diamond-like and crystalline diamond films from hydrocarbons, the gas-phase chemical processes, involving reactant decomposition, formation of intermediates at the surface or in volume, and
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