The critical role of electron induced secondary electrons in high-voltage and low-pressure capacitively coupled oxygen plasmas

Particle-in-cell/Monte Carlo collisions simulations are performed to study the effects of electron induced secondary electrons (SEs) in single-frequency (13.56 MHz) capacitively coupled oxygen discharges operated in the low-pressure regime (<1 Pa). A comparison of the plasma parameters is presented based on different surface models in the simulations. Both for electrons and ions, simple and realistic approaches are used to model their interactions with the electrode material. The simple approaches use constant coefficients (electron reflection, SE emission), irrespectively of the discharge conditions and the surface properties, while the realistic surface coefficients are functions of the energy of the particles hitting the electrode and depend on the surface properties. When electron induced secondary electron emission (SEE) is accounted for, a complex electron emission and ionization dynamics of ion induced gamma- and electron induced delta-electrons is found. When both the electron and ion induced SEE is described by energy dependent surface coefficients, an exponential increase of the plasma density and the O-2(a(1)Delta(g)) metastable concentration is observed by increasing the driving voltage amplitude, as well as a strong decrease of the electronegativity of the discharge and a transition of the discharge operation mode from the drift-ambipolar mode to the a-mode are found to take place.