plasma*

In electronegative radio-frequency plasmas, striations (STRs) can appear if the bulk plasma is dominated by positive and negative ions. Here, we investigate the behaviors of STRs and plasma parameters in dual-low-frequency (2/10 MHz) capacitively coupled CF₄ plasmas by phase-resolved optical emission spectroscopy and particle-in-cell/Monte Carlo collision simulations combined with analytical model. 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. The maximum ion density decreases slightly as a function of LF voltage, φ_LF, 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. 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.