Instabilities and multiple steady states of radio-frequency discharges in CF4

A one-dimensional fluid model coupled with solution tracking algorithms was applied for an L = 5 mm CF4 (radio frequency) rf discharge at P = 2 Torr (low PL value of PL = I). The discharge exhibited a rich behavior of solution multiplicity as a function oi applied rf current density (or voltage). The most prominent characteristic was the development of de self-bias and asymmetry under completely symmetric discharge conditions, in agreement with experimental data by J. Butterbaugh (Ph.D. dissertation, Massachusetts Institute of Technology, 1990). The plasma can exist In three states of positive, negative, and zero self-bias, of which the first two Lire asymmetric and stable, while the third is symmetric and unstable. The two asymmetric solutions collapse again into one symmetric solution with zero self-bias at a higher voltage, a transition which depends on the value of the secondary electron emission coefficient. At lower applied currents (or voltages) the hysterisis loop of this electronegative plasma is presented between breakdown and extinction, and an unstable plasma is found between die two points. The observed phenomena are related to the transition of the discharge from diffusion-controlled to electron-oscillation-amplitude-controlled breakdowns. The sustaining mechanisms of the plasma and the electron dynamics are discussed.