Dust formation and charging in an Ar/SiH4 radio-frequency discharge

Abstract

The formation and charging of submicrometer dust particles in a low pressure argon/silane radio-frequency (rf) discharge was studied using laser-induced photodetachment in combination with a microwave resonance technique. This method allows a measurement of the spatially averaged electron density, the spatially resolved negative ion density, and/or the charge on small clusters in the plasma as a function of time during particle formation. The loss frequency of photodetached electrons yields information about the recharging of small clusters. During the first second after plasma ignition dust particles are formed. Simultaneously, the electron density decreases from about 2×1015 m−3 to about 4×1014 m−3. In the first 10 ms after discharge ignition, charged particles are not present in the plasma and the photodetachment experiment gives a negative ion density of 4×1015 m−3. During the first 50 ms after plasma ignition, nanocrystallites are formed, which is reflected by a strong increase of the loss frequency of photodetached electrons. After 50 ms the particles start to coalesce and acquire a negative charge, which results in a strong increase of the photodetachment signal. After 1 s of plasma operation, the charge density on particles is about 8×1016 m−3. The photodetachment signal decreases with the gas flow rate, indicating that the clusters are expelled from the plasma by the gas flow.