Optical Emission Spectroscopy and Laser Doppler Velocimetry for an RF Thermal Plasma CVD Process
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OPTICAL EMISSION SPECTROSCOPY AND LASER DOPPLER VELOCIMETRY FOR AN RF THERMAL PLASMA CVD PROCESS T. ISHIGAKI*, Y. MORIYOSHI* AND I. IWAMOTO"* *National Institute for Research in Inorganic Materials, 1-1, Namiki, Tsukuba-shi, Ibaraki 305, Japan **Communications Research Laboratory, 4-2-1, Nukui-kitamachi, Koganei-shi, Tokyo 184, Japan ABSTRACT Optical emission spectroscopy and laser Doppler velocimetry were done for an Rf inductively coupled plasma at atmospheric pressure in order to elucidate the super-rapid coofling of the thermal plasma during the growth of diamond thin film. Attention was given to the vicinity of the watercooled substrate located 20 mm beneath the RF coil. It was found that at 1.5 mm above the substrate, the temperature of plasma was still high. At the temperature, high concentration of hydrogen atoms exist, which may take a important role in diamond growth. INTRODUCTION In the thermal plasma, which is at equilibrium or near equilibrium, not only electrons but heavy particles, like atoms and ions, have the extraordinary high energy. On leaving the energy addition region, the species experience very rapid cooling process. Application of the thermal plasma to material processing enables us to synthesize new materials. Recently, the growth of diamond thin films using thermal plasmas[an RF 2 3 inductively coupled plasma,' a DC arc plasma . ] has followed that using the thermal decomposition with a hot filament and the glow discharges. In the methods, a water-cooled substrate was inserted into Ar-Hz-CH4 plasma at atmospheric or near atmospheric pressure. The substrate cooling is thought to accelerate the cooling of plasma and enables us to utilize the thermal plasma with extremely high temperature for the growth of diamond films without co-deposition of graphite. The numerical analysis using two-temperature model for an Ar Rf induction plasma has showed that even under atmospheric condition, the wall region is not in local thermodynamic equilibrium(LTE), where electrons stay 4 at higher temperature than that of atoms/ions. The situation is expected by the insertion of a water-cooled substrate, too. For an Ar-H 2 RF plasma with a substrate insertion, a calculation of the temperature and flow patterns has reported, which includes reaction kinetics of hydrogen recombina5 tion/dissociation. Hydrogen atoms are believed to play a crucial role in diamond film formation. The calculation has shown the change of the concentration profile with substrate location, that is, the kinetic effect. In this paper, attention was paid to the plasma just above the substrate, and the observation by an optical emission spectroscopy(OES) and a laser Doppler velocimetry(LDV) was done for an Ar-H 2 RF induction plasma at atmospheric pressure. The typical concentration of methane is reported to be less than 5 % of hydrogen and we assumed that neglecting the small quantity of methane the plasma has only minor effect on the temperature and velocity distribution. Mat. Res. Soc. Symp. Proc. Vol. 190. ©1991 Materials Research Societ
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