Origin of the Delayed Current Onset in High-Power Impulse Magnetron Sputtering

Repetitive pulses of voltage and current are applied in high-power impulse magnetron sputtering. The current pulse usually lags the applied voltage by a significant time, which, in some cases, can reach several tens of microseconds. The current time lag is generally highly reproducible and jitters less than 1% of the delay time. This work investigates the time lag experimentally and theoretically. The experiments include several different target and gas combinations, voltage and current amplitudes, gas pressures, pulse repetition rates, and pulse durations. It is shown that, in all cases, the inverse delay is approximately proportional to the applied voltage, where the proportionality factor depends on the combination of materials and the conditions selected. The proportionality factor contains the parameters of ionization and secondary-electron emission. The statistical time lag is negligible, while the formative time lag is large and usually dominated by ion motion (inertia), although, at a low pressure, the long free path of magnetized electrons causing ionization contributes to the delay.