The dynamics of dust particles in silane glow discharges between parallel plates

Abstract

In order to avoid particle contamination of substrates during the plasma-enhanced chemical vapour deposition of amorphous silicon, we have investigated the dependence on temperature and gas flow conditions of the dynamics of submicrometre particles in discharges in silane. We report on light scattering experiments in an RF-powered plasma reactor with heatable parallel electrodes. The motion and trapping of particles was found to be strongly influenced by the gas flow conditions and the temperature gradient in the discharge. To understand this behaviour the equation of motion of a particle was solved in one dimension including gravitation, electric force on a 'dressed' particle, friction force and thermophoresis. The validity of the friction force is verified by observing experimentally the sinking of particles without electric force after switching off the discharge. Results for calculated particle positions and velocities as a function of time are presented for the cases of particles with radii of 0.1 and 0.15 mu m. When the discharge is on, the light particle is trapped at the sheath boundary. In the case of a temperature gradient (heating the lower electrode to 80 degrees C) the lighter particles are driven upward by thermophoresis and may be trapped at the sheath edge. Finally, a strong downward-directed gas flow (feeding gas through the upper electrode) of 30 cm s-1 prohibits the trapping of particles at the lower sheath boundary in agreement with the experimental observation. Electrode heating, together with proper gas flow, is found in our experiments to completely avoid the trapping of particles in the interelectrode space.