Electron density fluctuations in a dusty Ar/SiH4 rf discharge

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Abstract
The average electron density and electron density fluctuations in a dusty 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. Within the first second of plasma operation the electron density decreases with a factor of ten. This is caused by submicroscopic particles, formed in the discharge, which rapidly absorb electrons. When the particles reach a critical size they are expelled from the plasma. This process is governed by a balance between the Coulomb force, trapping the particles in the positive plasma glow and the neutral drag force, flushing them out. The periodic growth and expulsion of particles, monitored by light scattering, results in an oscillatory behavior of the electron density. From the measured oscillation period (τ), which is in the order of seconds to minutes, and its dependence on the gas flow rate (F) and on the fraction α of SiH4 in the plasma (τ[s]≊4.5×102α−1F−2 [sccm], at 10 W rf power input), the trapping force (FC) on particles can be calculated: FC[N]≊4×10−18r [nm], where r is the radius of a particle.