Effect of pressure on discharge mode transition in radio-frequency capacitively coupled micro-beam plasmas

Abstract

Micro-beam radio-frequency (RF) capacitive discharges have been widely used in the plasma enhanced chemical vapor deposition of nanocrystalline particles such as nano silicon crystal. However, the plasma column shrinks radially at a sufficiently high gas pressures as manifested by their glow not entirely filling the radial cross-section of the discharge tube. This greatly limits the dissociation rate of gas in plasma. In order to obtain the information about the plasma column varying with gas pressure, the formation of different gas discharge mode under different pressure is discussed. In this paper the spatial characteristics of micro-beam RF capacitive discharges are investigated by using an intensified charged-coupled device (ICCD) and a single lens reflex camera (SLR camera). Furthermore, high voltage probe and current probe are used to record the electrical characteristics of the high voltage electrode. The results indicate that in a pure argon discharge, the discharge mode evolves from a glow discharge into a filament discharge with the increase of pressure. As the pressure continues to increase, the filament is split: a single channel of plasma is split into two or more filaments at a certain gas pressure. However, the glow discharge in a mixture of 99% argon and 1% hydrogen at a low pressure is observed: the plasma spreads throughout the tube. As the pressure increases, the filament disappears, and the plasma column still can be observed in the center of quartz tube. The glow shrinks in the radial center at a moderate pressure. At a high pressure, the "annulus" glow discharge is achieved as manifested by a glow ring on the surface of the discharge tube. In addition, in pure hydrogen discharges, the discharge mode evolves from the full-space glow discharge into an "annulus" glow discharge with pressure increasing. Finally, through the interaction between the electron heating by the radio frequency electric field and heat conduction of gas, the filament discharge in a low thermal conduction gas is explained. In addition, special attention is paid to the pure argon filamentation, which is the splitting of a single channel of plasma into two or more smaller filaments as a result of the skin effect.