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

A tuned, cylindrical Langmuir probe has been used to measure the electron energy distribution function (EEDF) in atomic and molecular gases in an inductively coupled plasma. We have discussed the precautions necessary for making Langmuir probe measurements in fluorocarbon plasmas. The ionic and neutral composition of the plasma is measured using mass spectrometry. While the EEDFs in argon are non-Maxwellian, the EEDFs in molecular gases are found to be approximately Maxwellian at low pressures (<20 mTorr) in the gases studied (N₂, O₂, CF₄). The EEDFs in argon-molecular gas mixtures change from Maxwellian to two-temperature distributions, as the fraction of argon is increased in the plasma. At higher pressures, the molecular gases exhibit EEDFs reflecting the electron collision cross sections of these gases. In particular, N₂ plasmas show a "hole" in the EEDF near 3 eV due to the resonant vibrational collisions. O₂ plasmas show a three-temperature structure, with a low-energy high-temperature electron group, a low-temperature intermediate-energy electron group, and a high-temperature high-energy tail. The fractional degree of dissociation in the N₂ and O₂ plasmas is below 0.1, with the parent molecules and molecular ions being the dominant species. The spatial variation of the EEDF in an oxygen plasma at low pressures (10-20 mTorr) is found to be consistent with the nonlocal theory.