On singlet metastable states, ion flux and ion energy in single and dual frequency capacitively coupled oxygen discharges

We apply particle-in-cell simulations with Monte Carlo collisions to study the influence of the singlet metastable states on the ion energy distribution in single and dual frequency capacitively coupled oxygen discharges. For this purpose, the one-dimensional object-oriented particle-in-cell Monte Carlo collision code oopd1 is used, in which the discharge model includes the following nine species: electrons, the neutrals O(P-3) and O-2(X-3 Sigma(-)(g)), the negative ions O , the positive ions O+ and O-2(+), and the metastables O(D-1), O-2(a(1)Delta(g)) and O-2(b(1)Sigma(+)(g)). Earlier, we have explored the effects of adding the species O-2(a(1)Delta(g)) and O-2(b(1)Sigma(+)(g)), and an energy-dependent secondary electron emission yield for oxygen ions and neutrals, to the discharge model. We found that including the two molecular singlet metastable states decreases the ohmic heating and the effective electron temperature in the bulk region (the electronegative core). Here we explore how these metastable states influence dual frequency discharges consisting of a fundamental frequency and the lowest even harmonics. Including or excluding the detachment reactions of the metastables O-2(a(1)Delta(g)) and O-2(b(1)Sigma(+)(g)) can shift the peak electron temperature from the grounded to the powered electrode or vice versa, depending on the phase difference of the two applied frequencies. These metastable states can furthermore significantly influence the peak of the ion energy distribution for O-2(+)-ions bombarding the powered electrode, and hence the average ion energy upon bombardment of the electrode, and lower the ion flux.