Sheath development around a high-voltage cathode

Abstract

A secondary electron emission-capacitive probe has been used to explore the dynamic behaviour of the sheath at high voltages in a glow discharge plasma (ni approximately 1010 cm-3 N2+, T3 approximately 8 eV) generated by inductively coupled RF power ( approximately 300 W at 12-13 MHz). When a high negative potential is applied to the cathode the sheath expands rapidly but comes to a steady-state position within a few microseconds. The potential distribution then remains stable throughout the high-voltage pulse as long as the ionization rate in the plasma outside the sheath region is sufficient to replace the ions lost to the cathode. Around a sphere, this equilibrium can be described by conventional theories of the Child-Langmuir sheath. Previous experimental and theoretical investigations of the evolution of the cathodic sheath are reviewed in the light of these results which are significantly different from recently reported measurements in hot-filament and microwave multidipole discharges. It is concluded that the RF 'self-bias' of the target (which exists before application of the high voltage) and the high ionization rate in the surrounding plasma influence the time-scale on which the sheath comes to equilibrium.