On density distribution of Ti atom and ion ground states near the target in HiPIMS discharge using cavity ring-down spectroscopy and laser induced fluorescence

Abstract

Cavity ring-down spectroscopy combined with laser induced fluorescence was applied during a high-power impulse magnetron sputtering of Ti target to determine the time evolutions and spatial distributions of Ti atom and ion densities of levels belonging to the ground state multiplets (GSM) near the sputtered target. From these densities, an estimate of the time evolution and spatial distribution of the ionization degree near the target was calculated. It was found that the densities (number of particles per unit volume) of levels in GSM of Ti atom and ion increases with their level energies and the highest levels in GSMs show an inversion in their populations (the density of level divided by its statistical weight) during a voltage pulse. The population inversion (a higher population of levels with a higher energy than those with lower ones) is distinct for the highest level in GSM of Ti+ ion and it persists even at 40 μs after the voltage pulse end. The sum of ground state level densities reaches values up to 8.3 × 1011 cm-3 for Ti+ ions above the target racetrack and up to 4.8 × 1011 cm-3 for Ti atoms above the target center for the regime with the peak power density of 498 W cm-2. A frequent ionization of Ti atoms above the target racetrack is manifested by distinct drops in the Ti atom spatial distribution. This leads to a very high ionization degree up to 94% at the racetrack position and the ionization degree stays also high 40 μs after the voltage pulse termination.