An experimental study of a parallel-plate radio-frequency discharge: Measurements of the radiation temperature and electron density

Microwave diagnostic techniques have been used to measure the radiation temperature and electron density in parallel-plate capacitively coupled radio-frequency (2.5 MHz) discharges. Evidence is presented for the existence and importance of an energetic electron beam, produced by the large cathode sheath voltage, which sustains and excites the plasma. The measured radiation temperature of the bulk electrons is 500 K for helium and 800 K for argon. An upper limit on the radiation temperature is obtained for N2 (1200 K) and CF4 (3000 K) discharges. Measurements of the electron density in helium as a function of electrode spacing show a peak in the electron density that is consistent with the theory of ionization by beam electrons. The electric fields required to carry the rf current by drift in the bulk of the glow are low and consistent with the measured radiation temperature. The implications of these measurements on the nature of the rf plasma-sustaining mechanism. The voltage distribution, and the electron energy distribution are discussed.