Low-frequency dependence of plasma characteristics in dual-frequency capacitively coupled plasma sources

It is commonly recognized that in radio frequency capacitive discharges, a higher excitation frequency can yield an enhanced electron heating rate and ion flux. Here, we reveal the low-frequency dependence of the plasma density and ion energy/angular distribution in a low-pressure (2 Pa), dual-frequency (DF) capacitively coupled argon plasma based on a combination of experiments and kinetic particle simulations. As the low frequency (LF, f(L)) is decreased from 6.8 MHz to 40 kHz, the plasma density undergoes a moderate decline initially, followed by an increase, reaching a maximum at f(L) = 400 kHz. The enhanced plasma density is attributed to a combined effect of (i) an attenuated modulation effect of the LF source on the high-frequency electron heating and (ii) enhanced emission of electron-induced secondary electrons. At a lower f(L), the ion transit time across the sheath, tau(ion), gets comparable to or shorter than the LF period, tau(LF), resulting in a higher ion energy with a narrower angular spread. The enhanced ion flux and ion energy in DF discharges operated at low frequencies in the range of hundreds of kHz are beneficial for the high-aspect-ratio plasma etching extensively used in the semiconductor industry.