Measurements of the electron energy distribution function in molecular gases in a shielded inductively coupled plasma

A cylindrical Langmuir probe has been used to measure the electron energy d istribution function (EEDF) in atomic and molecular gases in a shielded ind uctively coupled plasma. We report the EEDFs in these gases as a function o f pressure. While the electron properties in a discharge depend on the prod uct of the neutral number density (N-0) and the effective discharge dimensi on (d(eff)) for a given gas, this dependence is different for different gas es. We find that pressure is a convenient parameter for comparison of the E EDFs in these gases. The EEDFs in inert (Ar, Kr, Xe) and molecular gases (H -2,N-2,O-2,H2O,CO2,CF4) in the low pressure limit (below 1 mTorr) show a "t hree-temperature" structure. Since this wide range of gases display similar EEDF shape, we propose this structure to be common to all gas discharges i n this limit. The EEDF in all of the gases shows a two-temperature structur e with apparent tail depletion at 3 mTorr. The similarity of the EEDFs in a ll of the above gases is probably due to nonlocality of the electrons at th ese low pressures. The molecular gases exhibit a nearly Maxwellian EEDF bet ween about 10 and 30 mTorr, while the EEDF in argon is non-Maxwellian in th is range. At pressures above 30 mTorr, the EEDFs in molecular gases show de viations from a Maxwellian distribution, reflecting the electron-neutral co llision cross sections of each gas. The EEDFs in molecular gases at 100 mTo rr show significant deviations from a Maxwellian distribution. We find that the EEDF in molecular gases can be approximated by a Maxwellian distributi on over a fairly large pressure range of 3-50 mTorr for the purposes of mod eling these discharges. (C) 2000 American Institute of Physics. [S0021-8979 (00)00320-0].