Analyses of electron and negative atomic oxygen ion density by means of 160 GHz microwave interferometry and laser photodetachment in oxygen cc-rf plasma

Microwave interferometry at 160.28 GHz with Gaussian beam propagation and laser photodetachment were simultaneously applied for analysis of negative atomic oxygen ions in the bulk plasma of an asymmetric capacitively coupled 13.56 MHz discharge. The line integrated negative oxygen ion density amounts between 2*10$^{14}$ m$^{-2}$ and 10$^{15}$ m$^{-2}$ depending on total oxygen pressure and rf power. Furthermore, the decay of the detachment signal reveals two modes of rf oxygen plasma characterized by different electronegativity $\alpha $=n$_{-}$/n$_{e}$. In the case of $\alpha >$2 the decay time constant amount to few $\mu $s, only, whereas in oxygen plasmas with low electronegativity, $\alpha <$1, the relaxation of electron density needs much longer with typical decay time constants up to about 100 $\mu $s. The transition between the two modes shows a step-like characteristic and was observed at a specific rf power depending on the oxygen pressure. For high electronegativity the electron density relaxation can be described by a simple 0D-attachment-detachment model taking into consideration a constant density for positive ions and neutral oxygen species. This model indicates that the metastable O$_{2}$(a$^{1}\Delta _{g})$ plays an important role, both in formation and loss of negative atomic oxygen ions.