Measurements of rf bias effect in a dual electron cyclotron resonance-rf methane plasma using the Langmuir probe method

Low-pressure (0.35 Pa) methane $(CH4)$ plasma was generated by a dual microwave electron cyclotron resonance (ECR)-radio frequency (rf) discharge. The plasma parameters and electron energy distribution functions (EEDFs) were measured using an automated cylindrical Langmuir probe. Optical emission spectroscopy was achieved as a complementary measurement to the probe results. The measured electron and ion densities increase from $109$ to $1010 cm−3$ as a function of the applied rf power. Plasma parameters obtained from both orbital motion limited theory and EEDF integrals show a consistent trend on the applied rf biasing. The EEDFs measured are well represented by Maxwellian distribution functions and show a significant increase in the electron temperature when rf biasing is applied. This contribution of hot electrons observed above 10 eV in the presence of rf biasing may lead to enhancement of the ionization and dissociation processes of $CH4$ in our dual ECR-rf discharge.