Electron and metastable densities in parallel-plate radio-frequency discharges

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Abstract
Electron densities were measured in continuous and pulse‐modulated, 13.56‐MHz, helium and argon discharges. These measurements were made in a symmetrically driven Gaseous Electronics Conference Reference Cell. Pulse modulation of the applied potential allowed observation of the time required for the electron density to achieve steady state. In general, helium discharges reached steady state in approximately 1.5 ms, taking three to ten times longer than argon discharges under similar operating conditions. As much as a threefold increase of the electron density was observed in the afterglow of a pulse‐modulated helium discharge, indicative of large metastable densities. Absorption spectroscopy was used to measure the helium singlet and triplet metastable densities throughout the volume of the discharge. For a 1.0‐Torr discharge, helium triplet metastable densities at the center of the discharge were as large as 2.5×1011 cm−3 while the peak singlet density was 3.0×1010 cm−3. The steady‐state electron density varied from 3.8×1010 cm−3 at 50 V to 2.3×1011 cm−3 at 200 V for the 1.0‐Torr helium discharge.