Negative corona discharge: Application to nanoparticle detection in rf reactors

Negative corona discharges have been studied for their possible use to detect nanoparticles in capacitively coupled plasma (CCP) reactors. Silicon nanoparticles (below 20 nm in diameter) were produced in a pulsed CCP discharge in a silane-argon-hydrogen mixture. An emissive probe with a tungsten filament biased to a sufficient negative potential (in the range 350-450 V) was used to create a negative corona in the postplasma regime when the rf power was switched off. Due to surface contamination, the probe was operated in the regime of weak electron emission to allow stable operation. Nevertheless, the filament temperature was high enough for nanoparticle deceleration/acceleration by induced thermophoretic force. It appears that positively charged nanoparticles decelerated near the filament can switch the negative corona from a pulsed (subnormal) discharge mode into a normal glow discharge mode. Hence the negative corona may have potential for detecting nanoparticles in CCP reactors operating in silane-containing mixtures.