Langmuir-probe measurements of a pulsed and steady-state rf glow-discharge source and of an rf planar-magnetron source

Abstract

Langmuir-probe measurements were made of a conventional radio-frequency glow-discharge source (in both pulsed and constant-power modes) and of a radio-frequency planar-magnetron source. The rf-driven oscillations of the plasma were neutralized by tuning the probe impedance with an external circuit. The conventional source, with a copper cathode, exhibited temperatures of roughly 2–5 eV and electron densities of 10 9–10 10 cm −3, with the maximum electron current collected at 1 Torr. Use of a quartz sample resulted in slightly higher temperatures and an approximately 50% lower electron density. The pulsed glow discharge with a conducting cathode featured a current afterpeak (a peak immediately following the plasma termination) that can be attributed to both an expanding collection volume of the probe and to the generation of electrons from metastable-argon ionization. This afterpeak current was 3–4 times greater than the largest current collected during the dc portion of the pulsed plasma. The afterpeak current was greatest at 2–3 Torr, near the pressure at which the emission intensity is greatest for the steady-state source. The afterpeak current was much lower when an insulating sample was analyzed, particularly at higher pressures. The magnetron source exhibited a bimodal distribution of electron energies, with one population centered at 14–19 eV and the other centered at 3–6 eV. Electron densities were greater in the magnetron source, which probably accounts for the high emission and ion levels reported in previous studies.