Gap Length Dependence of Electron Energy Distribution in Low-Pressure Ar Capacitively Coupled RF Discharges

Abstract

The electron energy distribution function (EEDF) at the midplane was measured using a Langmuir probe in symmetrically capacitively coupled RF (13.56 MHz) Ar discharges by changing the gap length from 2 cm to 10 cm over the pressure range from 0.03 Torr to 0.1 Torr, while keeping the RF current constant at 140 mArms. The EEDFs measured at pressure lower than 0.05 Torr formed a bi-Maxwellian distribution over the measured gap length. On the other hand, the EEDF structure measured at pressure higher than 0.07 Torr changed from bi-Maxwellian to Maxwellian, and to Druyvesteyn distributions with increasing gap length. The gap length dependence of the EEDF structure in the high-energy region can be explained by the transition from the stochastic to the joule heating, while that in the low-energy region can be explained by comparing the elastic collision frequency and the e-e Coulomb collision frequency.