On electron power absorption in low pressure electronegative capacitive discharges

One-dimensional particle-in-cell/Monte Carlo collisional (PIC/MCC) simulations have been performed on capacitive discharges in argon, oxygen, and chlorine, driven by a sinusoidal rf voltage. The properties of the discharges, the reaction rates for creation and loss of a few key species, the electron energy probability function, and the primary electron power absorption processes are explored and compared as the gas pressure and the inclusion of secondary electron emission processes in the discharge model is varied. Neglecting secondary electron emission we compare the electron power absorption dynamics for low pressure rf driven capacitively coupled discharges in oxygen and chlorine. The oxygen discharge is weakly electronegative at 13 Pa, and electron power absorption is mainly due to pressure heating, while it is highly electronegative at 1.3 Pa, and the electron power absorption occurs mainly within the electronegative core. The chlorine discharge is highly electronegative, in the pressure range 1 - 50 Pa, and becomes more resistive as the pressure is increased. In the oxygen discharge the singlet delta metastable oxygen molecule plays a significant role in both the creation and loss of the negative ion O^- and it is detachment dominated, while in the chlorine discharge the negative ion Cl^- is almost entirely created by dissociative attachment and lost through ion-ion recombination, and therefore the capacitive chlorine discharge is recombination dominated. We discuss how the addition of argon to the chlorine discharge influences the electronegativity, the electron power absorption and the plasma chemistry.