Effect of parallel resonance on the electron energy distribution function in a 60 MHz capacitively coupled plasma

In general, as the radio frequency (RF) power increases in a capacitively coupled plasma (CCP), the power transfer efficiency decreases because the resistance of the CCP decreases. In this work, a parallel resonance circuit is applied to improve the power transfer efficiency at high RF power, and the effect of the parallel resonance on the electron energy distribution function (EEDF) is investigated in a 60 MHz CCP. The CCP consists of a power feed line, the electrodes, and plasma. The reactance of the CCP is positive at 60 MHz and acts like an inductive load. A vacuum variable capacitor (VVC) is connected in parallel with the inductive load, and then the parallel resonance between the VVC and the inductive load can be achieved. As the capacitance of the VVC approaches the parallel resonance condition, the equivalent resistance of the parallel circuit is considerably larger than that without the VVC, and the current flowing through the matching network is greatly reduced. Therefore, the power transfer efficiency of the discharge is improved from 76%, 70%, and 68% to 81%, 77%, and 76% at RF powers of 100 W, 150 W, and 200 W, respectively. At parallel resonance conditions, the electron heating in bulk plasma is enhanced, which cannot be achieved without the VVC even at the higher RF powers. This enhancement of electron heating results in the evolution of the shape of the EEDF from a bi-Maxwellian distribution to a distribution with the smaller temperature difference between high-energy electrons and low-energy electrons. Due to the parallel resonance effect, the electron density increases by approximately 4%, 18%, and 21% at RF powers of 100 W, 150 W, and 200 W, respectively.