The effect of secondary electrons in low-pressure capacitively coupled oxygen plasmas

In this work, the role of plasma-surface interactions in capacitively coupled plasmas (CCPs) operated in oxygen is studied by Particle-in-Cell/Monte Carlo Collisions (PIC/MCC) simulations. The discharges are driven by a single-frequency (13.56 MHz) sinusoidal waveform, and they are operated in the low-pressure regime (< 1 Pa). Two different models are used to describe the interaction of electrons with the electrodes: (i) a simple one assuming only elastic reflection of the electrons with a constant probability of 0.2 and (ii) a realistic one which takes elastic reflection, inelastic reflection and secondary electron emission into account as a function of the energy and angle of incidence of the electrons. Regarding the heavy particles hitting the electrode surface, three different approximations are used: (i) the simplest one considers a constant  secondary electron emission (SEE) coefficient for the O₂⁺ ions; (ii) a more realistic one considers a  coefficient that depends on the energy of the incoming O₂⁺ ions; and (iii) the third one also takes SEE caused by O₂ neutrals into account in an energy-dependent way in addition. When the realistic model is used for electron-induced SEE, complex electron emission and ionization dynamics of ion-induced and electron-induced secondary electrons (γ- and δ-electrons, respectively) are found at low pressures, which are similar to the dynamics recently observed in argon under the same discharge conditions. In oxygen, electron-induced secondary electrons also have a remarkable effect on the O₂ (a¹Δ_g) metastable density and the electronegativity of the discharge: the metastable density increases and the plasma is found to be less electronegative as a result of the emergence of secondary electrons under the same discharge conditions. The model used for heavy particle induced SEE is also critical: at high voltage amplitudes, a transition to electropositive discharge can be observed when ion-induced secondary electron emission is treated realistically.