Effect of the low-frequency voltage on nonlinear standing wave excitation in dual-frequency asymmetric capacitive discharges

It is known that in very-high-frequency capacitively coupled plasmas (CCPs), the higher harmonics generated by nonlinear sheath motion can enhance the standing wave effect, leading to a center-peaked plasma density. It is also known that the dual-frequency (DF) CCP is widely used for the independent control of the ion flux and ion bombarding energy. In this work, a nonlinear transmission line model is coupled to a bulk plasma model and a numerical sheath model to study the effect of the low-frequency (LF) voltage on nonlinear standing wave excitation and plasma uniformity in DF asymmetric capacitive argon discharges at 3 Pa. The simulations are conducted with a 2 MHz LF voltage varied from 0 to 700 V and a 60 MHz high-frequency (HF) voltage fixed at 50 V. Simulation results indicate that when the LF voltage is 0 V, the standing wave effect enhanced by nonlinear harmonics leads to a significant center peak of the plasma density. As the LF voltage increases, the surface wavelength increases due to a thicker sheath width, and the amplitudes of the nonlinear harmonics decay dramatically. As a result, the plasma density profile becomes more uniform with increasing the LF voltage.