The role of hydrogen in the radical polymerization mechanism of hydrocarbons and chlorosilanes in a low pressure microwave plasma

Abstract

The introduction of propylene (C3H6) or tetrachlorosilane (SiCl4) in concentrations of 3–10 vol.% into a microwave (2.45 GHz) plasma of Ar or mixtures of Ar+H2, at low pressures (1–10 Torr) and flow velocity in the range of 103 cm⋅s−1 results with the formation of solid carbon (pyrocarbon) or silicon, respectively. In the plasma state, both hydrocarbons and chlorosilanes, dissociate and polymerize to CxHy and SixCly forms with x>1 and x>y. It was found that polymerization process in the Ar (without H2) microwave plasma was mainly propagated by an ion–molecule mechanism, while additions of H2 (6–20 vol.%) to the Ar plasma propagates the polymerization mainly by a radical mechanism. The two mechanisms were determined by sampling the microwave plasmas with a quadrupole mass spectrometer (QMS), by indirect measurement of the plasmas free radicals concentration in an electron paramagnetic resonance (EPR) spectrometer and by measuring the deposition rate of pyrocarbon and silicon on substrates floated, grounded or biased to minus 100 V.