Experimental study and analytical modelling of the effect of the driving frequencies on dual frequency capacitively coupled plasmas

Abstract

A capacitively coupled radio frequency discharge driven by two harmonics substantially different from each other allows some degrees of independent control of the ion energy and ion flux. The low frequency (ωLF) source controls the ion energy, while the ion flux is controlled by the high frequency (ωHF) source. The choices of the driving frequencies can influence the properties of the dual frequency capacitively coupled plasmas (2f CCP). Here, we study the effect of the driving frequencies on the properties of the 2f CCPs, i.e., on the generation of a DC self-bias, the excitation of the non-linear Plasma Series Resonance (PSR) effect as well as the independent control over the mean ion energy and the flux in three different 2f CCP experiments: namely (2.26 + 13.56) MHz, (2.26 + 27.12) MHz, and (13.56 + 27.12) MHz. We also use a non-liner global model that consists of a description of the plasma bulk based on a fluid dynamic approach coupled to a separate model of the sheath. We use argon and cover a wide range of operating conditions. We find the choices of different ωLF and ωHF's result in substantial changes of the decoupling of the mean ion energy from the ion flux. We also observe that increasing ωHF decreases the DC self-bias at a fixed value of ωLF. The PSR effect and, therefore, the electron power deposition strongly depend on the high frequency source. The experimental results are consistent with the predictions of the non-linear global model which proves the usefulness of such a rather simplistic model to study of 2f CCP discharges.