Abstract
Electron energy distribution functions (EEDFs) were measured with increasing discharge voltages in hydrogen capacitively coupled plasmas by means of radio-frequency compensated Langmuir probe. The results are compared with EEDF in argon plasmas. It was found that, in the hydrogen capacitive discharge, abnormally low-energy electrons became highly populated and the EEDF evolved to a non-Maxwellian distribution as the discharge voltage was increased. This voltage dependence of the EEDF in the hydrogen is contrary to argon capacitively coupled plasma, where at high discharge voltage, low-energy electrons are significantly thermalized due to gamma heating and the EEDF evolves to the Maxwellian distribution. The highly populated low-energy electrons at high gas pressure, which was not observed in capacitively coupled argon plasma, show that the gamma heating mechanism is somehow inefficient in terms of the molecular gas in capacitive discharges. It appears that this inefficient gamma heating seems to be attributed to an efficient vibrational excitation in hydrogen capacitive plasma.