Plasma Parameters in a Pre-Ionized HiPIMS Discharge Operating at Low Pressure

A high power impulse magnetron sputtering (HiPIMS) discharge assisted by a dc pre-ionizer operating at low pressure has been investigated using the Langmuir probe technique as well as energy-resolved mass spectrometry. It was found that the background plasma with densities of about 1 x 10(9) cm(-3) provided by the dc pre-ionizer can reduce the ignition delay time of the HiPIMS plasma from more than 50 mu s to less than 5 mu s at a working pressure of about 0.1 Pa. Furthermore, the technique of super-imposing HiPIMS with the dc discharge can be readily employed with a low self-sputtering yield target (Ti) at a working pressure of 0.08 Pa, with a pulse width and a repetition frequency of 100 mu s and 100 Hz, respectively. This leads to a power density of 450 Wcm(-2) at the target. At these operating conditions, the probe measurements showed a high-density plasma 6 cm in front of the target. The maximum electron density was found to be 8 x 10(11) cm(-3), with an effective electron temperature of about 3-4 eV. A study of the ion mass distribution revealed that the intensity of Ti2+ ions is larger than the peak of Ti+ in low-pressure operation. Moreover, the number of metal ions in the high-energy tail of the ion energy distribution function increased with a decreasing pressure from 0.5 to 0.08 Pa.