Diagnostics of particle genesis and growth in RF silane plasmas by ion mass spectrometry and light scattering

Abstract

Experimental identification of the precursors and processes leading to particles is essential for understanding particulate contamination in deposition plasmas. We have investigated particle formation in radiofrequency silane plasmas using light scattering (elastic and inelastic) and quadrupole ion mass spectrometry as complementary plasma diagnostics. Negative ions reach high masses (at least 500 amu) and are the only elementary species with a residence time on the scale of the powder formation time. Furthermore, a negative-ion polymerization scheme shows that the densities of high-mass anions are strongly diminished at kilohertz power modulation frequencies, at which reduced powder production is also observed. We conclude that negative ions are the particle precursors and that initial clusters grow by negative-ion polymerization in silane plasmas. In situ light scattering techniques are described to determine particle size, number density and refractive index self-consistently. Novel, visible photoluminescence measurements from particles suspended in the plasma are also reported. These diagnostics demonstrate that particle evolution proceeds by an agglomeration phase and that the particle properties are different from those of the bulk material early in particle development.