The behaviour of negative oxygen ions in the afterglow of a reactive HiPIMS discharge

Using a single Langmuir probe, the temporal evolution of the oxygen negative ion, n-, and electron, ne, densities in the afterglow of a reactive HiPIMS discharge operating in argon-oxygen gas mixtures have been determined. The magnetron was equipped with a titanium target and operated in 'poisoned' mode at a frequency of 100 Hz with a pulse width of 100 mu s for a range of oxygen partial pressures, pO(2) /p(total) = 0.0-0.5. In the initial afterglow, the density of the principle negative ion in the discharge (O-) was of the order of 10(16)m(-3) for all conditions. The O- concentration was found to decay slowly with characteristic decay times between 585 mu s and 1.2 ms over the oxygen partial pressure range. Electron densities were observed to fall more rapidly, resulting in long-lived highly electronegative afterglow plasmas where the ratio, alpha = n(-)/ne, was found to reach values up to 672(+/- 100) for the highest O-2 partial pressure. By comparing results to a simple plasma-chemical model, we speculate that with increased pO2 /ptotal ratio, more O(-)ions are formed in the afterglow via dissociative electron attachment to highly excited metastable oxygen molecules, with the latter being formed during the active phase of the discharge. After approximately 2.5 ms into the off-time, the afterglow degenerates into an ion-ion plasma and negative ions are free to impinge upon the chamber walls and grounded substrates with flux densities of the order of 10(18)m(-2) s(-1), which is around 10% of the positive ion flux measured during the on-time. This illustrates the potential importance of the long afterglow in reactive HiPIMS, which can act as a steady source of low energy O-ions to a growing thin film at the substrate during periods of reduced positive ion bombardment.