Ionization by Drift and Ambipolar Electric Fields in Electronegative Capacitive Radio Frequency Plasmas

Unlike $\alpha$- and $\gamma$-mode operation, electrons accelerated by strong drift and ambipolar electric fields in the plasma bulk and at the sheath edges are found to dominate the ionization in strongly electronegative discharges. These fields are caused by a low bulk conductivity and local maxima of the electron density at the sheath edges, respectively. This drift-ambipolar mode is investigated by kinetic particle simulations, experimental phase-resolved optical emission spectroscopy, and an analytical model in ${\mathrm{CF}}_4$. Mode transitions induced by voltage and pressure variations are studied.

Figure 1

PIC simulation results: spatiotemporal plots of the electron heating rate (first column), ionization rate (second column), electric field (third column), and electron density (fourth column) in Ar and ${\mathrm{CF}}_4$ discharges driven at 13.56 MHz and 80 Pa with an electrode gap of 1.5 cm. First row: Ar, 100 V, $\gamma =0$. Second row: Ar, 200 V, $\gamma =0.2$. Third row: ${\mathrm{CF}}_4$, 400 V, $\gamma =0.1$. The color scales are given in units of ${10}^5\mathrm{W}{\mathrm{m}}^{-3}$ (heating rate), ${10}^{21}{\mathrm{m}}^{-3}{\mathrm{s}}^{-1}$ (ionization rate), ${10}^3\mathrm{V}{\mathrm{m}}^{-1}$ (electric field), and ${10}^{15}{\mathrm{m}}^{-3}$ (electron density).

Figure 2

Profiles of (a) the electric field obtained from the simulation (black solid line) and (1) (dashed red line); (b) the second (drift field) and fourth (ambipolar field) terms of (1) in the bulk at $t\approx 26\mathrm{ns}$ [vertical dashed lines in Figs. 1i, 1j, 1k, 1l]. Discharge conditions: ${\mathrm{CF}}_4$, 13.56 MHz, 400 V, 80 Pa, $\gamma =0.1$.

Figure 3

Spatiotemporal ionization rate obtained from the simulation in units of ${10}^{21}{\mathrm{m}}^{-3}{\mathrm{s}}^{-1}$ (top row) and measured plasma emission at 250 nm in a.u. (bottom row) in a ${\mathrm{CF}}_4$ discharge at different voltage amplitudes (13.56 MHz, 40 Pa).

Figure 4

Spatiotemporal ionization rate obtained from the simulation in units of ${10}^{21}{\mathrm{m}}^{-3}{\mathrm{s}}^{-1}$ (top row) and measured plasma emission at 250 nm in a.u. (bottom row) in a ${\mathrm{CF}}_4$ discharge at different pressures (13.56 MHz, 200 V).