The mechanism of frequency coupling in low pressure dual-frequency capacitively coupled plasmas revisited based on the Boltzmann term analysis*

The electron power absorption dynamics is investigated for radio-frequency (RF) argon capacitively coupled plasmas (CCPs) at low pressure excited by a dual frequency waveform with frequencies of 27.12 MHz and 1.937 MHz. Based on the spatio-temporal dynamics of the ambipolar electric field outside the sheath, a novel interpretation of the mechanism of frequency coupling is given, which is not based on electron reflection from a Hard Wall sheath edge model, as in previous explanations. 

In the framework of the Boltzmann term analysis, frequency coupling arises due to the decreased spatial extent of the ambipolar region outside the sheath when the low frequency sheath is close to its full expansion, which leads to a decreased ionization in this region. The spatio-temporally averaged ambipolar power absorption shows a nonmonotonic behaviour as a function of pressure, which is shown to ultimately be connected to the mechanism of frequency coupling.