Time-resolved tunable diode laser absorption spectroscopy of excited argon and ground-state titanium atoms in pulsed magnetron discharges

Abstract

Time-resolved investigations of excited argon atom density and temperature and ground-state titanium atom density during high-power impulse magnetron sputtering (HiPIMS, repetition frequency 100 Hz) and direct current pulsed magnetron (repetition frequency 2.5 kHz) discharges (PMDs) in argon employing a titanium target were performed. Atom density and temperature were measured with the help of tunable diode laser absorption spectroscopy. Excited argon atoms form during the discharge pulse and again by three-body electron ion recombination in the afterglow. Similarly, the temperature of excited (metastable) argon atoms rises during the plasma on phase and again during the afterglow. The observed temporal evolution of the temperature is faster than expected from thermal conductivity considerations, which is taken as an indication that metastable and ground-state argon atoms are not in thermal equilibrium. The time dependence of titanium atoms can be explained by recombination and diffusion. The results provide new insights into the physics of PMDs.