Correlation between excitation temperature and electron temperature with two groups of electron energy distributions

The relationship between the electron excitation temperature $(Texc)$ determined by optical emission spectroscopy and the electron temperature $(Te)$ using a rf-compensated Langmuir probe was investigated in argon capacitively coupled plasmas. In the experiment performed at the gas pressure range of 30 mTorr to 1 Torr and the rf power range of 5–37 W, the electron energy probability function (EEPF) obtained from the probe current versus voltage characteristic curve showed two energy groups of electrons. The measured EEPF demonstrated that the electron energy distribution changed from Druyvesteyn to single Maxwellian as the discharge current was increased and from bi-Maxwellian to Druyvesteyn as the pressure was increased. As a result, $Texc$ showed a tendency identical to that of $Te$ of the high energy part of electrons as pressure and rf power were varied. This suggests that electron temperature can be determined from the measured $Texc$ through a calibration experiment by which the ratio between electron and excitation temperatures is measured.