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

Abstract

In electronegative radio frequency plasmas, striations 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 (DF, 2/10 MHz) capacitively coupled CF4 plasmas by PROES and PIC/MCC 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 striations appear in the plasma bulk, their number depends on the length of this region. By increasing the LF voltage, ΦLF, the sheath widths at both electrodes increase, the bulk is compressed, and the number of striations decreases. The maximum ion density decreases slightly as a function of ΦLF, too, due to the compressed plasma bulk, while the minimum of the ion density remains almost constant. The spatio-temporal distribution 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, but by the electron current at high values of ΦLF.

Finding relative negative ions concentration in dc glow discharges during undergraduate laboratory work

“…For example, So et al [54] presented the one-dimensional fluid model RF capacitive discharge at the CF 4 pressure of p = 0.2 Torr and found that α ≈ 30-40. Wang et al [57] with particle-in-cell/Monte Carlo collision simulations studied the process of striation forming in a dual-frequency RF capacitive discharge at CF 4 pressure of p = 0.75 Torr and found that in the maximum of a striation (an area with the maximum values of charged particles concentrations) α ≈ 30. The striation process was studied by authors of [22] as well.…”

The electrical asymmetry effect in electronegative CF4 capacitive RF plasmas operated in the striation mode

“…The top electrode includes a showerhead for the gas inlet as indicated by the dashed horizontal line. More detailed information on the reactor design can be found in our previous work [28].…”

“…This results in the formation of space charges, wherever density gradients are present, and ultimately in a striated pattern of regions of high and low electric field as well as plasma emission in the bulk. These self-organized striated structures have been studied as a function of the driving voltage amplitude and pressure in single frequency CCPs [25,26] as well as in dual-frequency discharges driven at substantially different frequencies [27,28]. Strong ion induced secondary electron emission from the electrodes was found to result in the disruption of the striations and to induce a mode transition into the γ-mode [29].…”

Hybrid simulation of instabilities in capacitively coupled RF CF4/Ar plasmas driven by a dual frequency source

“…Although achieving independent control of ion energy and ion flux with dual frequency sources is challenging, it still offers a relatively wide process window of good separate control of these plasma parameters [35,36]. Existing research on dual frequency sources is focused on electromagnetic effects [37][38][39][40][41], plasma discharge characteristics [42][43][44][45][46], independent regulation of ion energy and ion flux [47][48][49][50][51]. For example, Zhao et al [37] found that modulating the voltage and frequency of the LF source can suppress electromagnetic effects in a dual frequency capacitively coupled argon plasma discharge.…”