Electrode impedance effect in dual-frequency capacitively coupled plasma

The nonlinearity of a plasma sheath generates currents having a frequency different from the drive frequencies in a capacitively coupled plasma. We demonstrated the control of these currents together with the drive currents by adjusting the impedance of an electrode using an external circuit. Linear and nonlinear effects, caused by adjusting the impedance of the bottom electrode were observed in dual-frequency capacitively coupled plasma (DFCCP) by applying 60 MHz RF power to the top electrode and 13.56 MHz RF power to the bottom electrode. The linear effect is used to control the 60 MHz current at the bottom electrode. The nonlinear effect is used to control the self-excitation and resonant growth of currents originating from the plasma nonlinearity. In particular, in addition to the harmonics of one of the drive frequencies, we also controlled the growth of inter-modulation distortions which are characteristic nonlinear currents in DFCCP. Changes in the currents at both the top and bottom electrodes were simultaneously obtained. The currents at the bottom electrode exhibit two types of peak, one of which originates from the series resonance between the plasma and the electrode impedance, and the other is caused by increases in the amplitudes of the other currents. A simplified nonlinear model including the dual-frequency input is proposed to explain the observed growth. Our results indicate that resonance is possible even below 100 MHz, and that the resonance is controllable using an external circuit.