Striations in dual-low-frequency (2/10 MHz) driven capacitively coupled CF4 plasmas

Abstract

In electronegative radiofrequency plasmas, striations (STRs) can appear if the bulk plasma is dominated by positive and negative ions that can react to the driving frequency. Here, we investigate such self-organized structures in dual-frequency (2/10 MHz) capacitively coupled CF4 plasmas by phase-resolved optical emission spectroscopy and particle-in-cell/Monte Carlo collision simulations. This choice of the frequencies is made to ensure that the ions can react to both the lower (2 MHz, ‘low frequency’, LF) and the higher (10 MHz, ‘high frequency’, HF) components of the excitation waveform. A strong interplay of the two excitation components is revealed. As the STRs appear in the plasma bulk, their number depends on the length of this region. By increasing the LF voltage, ϕ LF, the sheath widths at both electrodes increase, the bulk is compressed and the number of STRs decreases. The maximum ion density decreases slightly as a function of ϕ LF, too, due to the compressed plasma bulk, while the minimum of the ion density remains almost constant. The spatio-temporal distributions of the excitation and ionization rates are modulated both by the LF and HF with maxima that occur at the first HF period that follows the complete sheath collapse at a given electrode. These maxima are caused by a high local ambipolar electric field. At a given phase within a HF period the current density is different at different phases within the LF period because of frequency coupling. The LF components of the F− ion velocity and of the electric field are much lower than the respective HF components due to the lower LF component of the displacement current in the sheaths. The LF component of the total current is dominated by the ion current at low values of ϕ LF but by the electron current at high values. The HF component of the total current is dominated by the electron current and decreases slightly as a function of ϕ LF.