Experimental Study On Mode Transition In Radio-frequency Capacitively Microbeam Discharge

In nanoparticles synthesis,radio-frequency capacitively microbeam discharge has a long application prospect.Different from aerosol processes,the nanoparticles synthesized from the thermal equilibrium plasma will exist three unique phenomena:particle charging,particle confinement,and selective nanoparticle heating,which makes the nanoparticle size distribution synthesized from the thermal equilibrium plasma is more narrow,more controllable and the nanoparticles are more likely to be collected.Rf capacitive microbeam technology is a good way to control particle crystallinity,particle morphology and particle size.The plasma usually form a or more rotating discharge filaments.This phenomenon is highly beneficial for the synthesis of highly particles,at the same time,the dissociation of gas will be greatly determined by the rotating discharge filaments.In previous studies,Ar and H₂are the more commonly used gases in this process.Ar is often used in gases containing H such as Si H₄,while H₂ is usually used as the reactant of halogen-containing gases such as Si Cl₄to participate in the process of nanoparticles synthesis.Therefore,this paper uses ICCD,SLR camera,high voltage probe and current probe to measure the physical characteristics of Ar,H₂ and Ar/H₂ rf capacitive microbeam discharges of different proportions,mainly to measure the spatial distribution of plasma and the conversion of discharge mode under different discharge conditions.The results show that:（1）In the Ar rf capacitive microbeam discharge,the plasma is full of the whole microbeam discharge tube at low pressure,and the discharge is glow discharge mode.With the increase of air pressure,the plasma shrinks gradually,and the luminescent region becomes shorter and thinner.As the pressure continues to increase,the plasma shrines into two discharge filaments near the high voltage electrode,and then converges into one discharge filament when the plasma transported to the ground electrode.At this time,the discharge is a mixture mode of glow discharge and filamentous discharge.With the further increase of pressure,the plasma gradually shrinks to the two electrodes,and two or three discharge filaments appear obviously.Moreover,the plasma rotates and moves in the microbeam tube,and the discharge is multi-channel filamentous discharge mode.（2）In the Ar/H2 mixed gas discharge,the low pressure is the full spatial distribution of glow discharge mode,then the medium pressure to the axial shrinkage of glow discharge mode,and finally to the high pressure of"annual"glow discharge mode;（3）In the H₂rf capacitive microbeam discharge,the low pressure is the full spatial distribution of glow discharge mode,and the high pressure is the"annual"glow discharge mode.The switch of discharge mode is caused by electron heating,skin effect and heat conduction of gas.In particular,the thermal conductivity of H₂ is much higher than that of Ar,so that the multi-channel filament-like discharge mode in high pressure Ar discharge does not appear in the mixed gas and H₂discharge.