Argon metastables in HiPIMS: time-resolved tunable diode-laser diagnostics

Abstract

Time-resolved tunable diode-laser absorption spectroscopy measurements were performed on the argon metastable (Ar^m) level 3s²3p⁵ 4s excited at 801.478 nm, in the dense plasma region in front of the magnetron target in a high power impulse magnetron sputtering (HiPIMS) discharge. From the Doppler profile the evolution of the temperature and density was derived during the pulse as well as during the plasma decay, i.e. in the afterglow. It is shown that the Ar^m density sharply increases at the beginning of the discharge pulse, followed by a severe Ar^m depletion along with increasing gas temperature around the peak of the HiPIMS discharge current. The strong dynamics of these parameters involve many elementary processes such as electron-impact excitation, electron-impact de-excitation and ionization of Ar^m, gas rarefaction, electron temperature increase at the end of the pulse and gas diffusion. These phenomena are discussed with respect to several parameters: distance from the target, peak discharge current during the pulse, pulse length, and gas pressure.