Fluid simulation of the superimposed dual-frequency source effect in inductively coupled discharges

Superimposition of dual frequencies (DFs) is one of the methods used for controlling plasma distribution in an inductively coupled plasma (ICP) source. The effects of a superimposed DF on the argon plasma characteristics have been investigated using a two-dimensional self-consistent fluid model. When both currents are fixed at 6 A, the plasma density drops with decrease in one of the source frequencies due to less efficient heating and the plasma uniformity improves significantly. Moreover, for ICP operated with superimposed DFs (i.e., 4.52 MHz/13.56 MHz and 2.26 MHz/13.56 MHz), the current source exhibits the same period as the low frequency (LF) component, and the plasma density is higher than that obtained at a single frequency (i.e., 4.52 and 2.26 MHz) with the same total current of 12 A. However, at superimposed current frequencies of 6.78 MHz/13.56 MHz, the plasma density is lower than that obtained at a single frequency of 6.78 MHz due to the weaker negative azimuthal electric field between two positive maxima during one period of 6.78 MHz. When the superimposed DF ICP operates at 2.26 and 13.56 MHz, the rapid oscillations of the induced electric field become weaker during one period of 2.26 MHz as the current ratio of 2.26 MHz/13.56 MHz rises from 24 A/7 A to 30 A/1 A, and the plasma density drops with the current ratio due to weakened electron heating. The uniformity of plasma increases due to sufficient diffusion under the low-density condition.