Realistic treatment for secondary electron emission in hybrid DC/DF capacitively coupled discharge

Abstract

Particle-in-cell/Monte Carlo collision simulations are performed to investigate the effects of using realistic models for secondary electron emission induced by different plasma species on the discharge characteristics in direct current (DC) superposed radio-frequency (RF) capacitively coupled plasmas (CCPs). A dual-frequency (DF, 60/2 MHz) source is applied on one of the electrodes to sustain the discharge, and an auxiliary DC source is fixed on the opposite electrode to generate energetic secondary electrons (SEs). Realistic models are employed to calculate the secondary electron yields (SEYs) induced by electrons and heavy particles (i.e. ions and fast neutrals) impacting the ‘dirty’ surfaces (e.g. oxidized metal), respectively, in argon discharge at a fixed pressure of 1.5 Pa. The results are compared to those obtained by assuming a constant ion-induced SEYs of γ = 0.1, and a constant elastic reflected electron yield of ηel. = 0.2. As the effective SEY γ* (arising from heavy particle impact) calculated from the realistic model is enhanced at the DC electrode, numerous γ-electrons can be induced under the effect of the DC-bias. These γ-electrons then bombard the opposite RF electrode with mean energies corresponding to the value of DC-bias, which further induce significant emission of δ-electrons (i.e. electron-induced SEs). Moreover, the δ-electrons originating from the RF electrode can be trapped and bounced back by the opposite DC sheath toward the RF electrode, inducing δ-electron emission once more. Consequently, this positive-feedback source of δ-electrons, leads to an enhanced ionization in the first half-cycle of the low frequency period, and a further growth in plasma density in hybrid DC/RF discharge.