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 e nergy distribution function (EEDF) in atomic and molecular gases in an indu ctively coupled plasma. We have discussed the precautions necessary for mak ing Langmuir probe measurements in fluorocarbon plasmas. The ionic and neut ral composition of the plasma is measured using mass spectrometry. While th e 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 (N-2, O-2, CF4). The EEDFs in argon-molecular gas mixtures change f rom Maxwellian to two-temperature distributions, as the fraction of argon i s increased in the plasma. At higher pressures, the molecular gases exhibit EEDFs reflecting the electron collision cross sections of these gases. In particular, N-2 plasmas show a "hole" in the EEDF near 3 eV due to the reso nant vibrational collisions. O-2 plasmas show a three-temperature structure , with a low-energy high-temperature electron group, a low-temperature inte rmediate-energy electron group, and a high-temperature high-energy tail. Th e fractional degree of dissociation in the N-2 and O-2 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. (C) 2000 Amer ican Institute of Physics. [S0021-8979(00)05709-1].