ELECTRON-DENSITY FLUCTUATIONS IN A DUSTY AR SIH4 RF DISCHARGE/

The average electron density and electron density fluctuations in a du sty Ar/SiH4 rf discharge have been studied using a microwave resonance technique. The average electron density increases with rf input power and it has a maximum as a function of pressure at about 30 mTorr. Wit hin the first second of plasma operation the electron density decrease s with a factor of ten. This is caused by submicroscopic particles, fo rmed in the discharge, which rapidly absorb electrons. When the partic les reach a critical size they are expelled from the plasma. This proc ess is governed by a balance between the Coulomb force, trapping the p articles in the positive plasma glow and the neutral drag force, flush ing them out. The periodic growth and expulsion of particles, monitore d by light scattering, results in an oscillatory behavior of the elect ron density. From the measured oscillation period (tau), which is in t he order of seconds to minutes, and its dependence on the gas how rate (F) and on the fraction alpha of SiH4 in the plasma (tau[s]approximat e to 4.5X10(2) alpha(-1)F(-2) [seem], at 10 W rf power input), the tra pping force (F-C) on particles can be calculated: F-C[N]approximate to 4X10(-18)r [nm], where r is the radius of a particle. (C) 1995 Americ an Institute of Physics.