Short- and long-term plasma phenomena in a HiPIMS discharge

Using a time-resolved Langmuir probe the temporal evolution of the bulk plasma parameters in a high-power impulse magnetron sputtering (HiPIMS) discharge was investigated for a number of different discharge conditions. The magnetron was operated in argon between 0.5 and 1.6 Pa with a titanium target and with peak target power densities up to 1000 W cm−2. The pulse width and repetition rate were held constant at 100 µs and 100 Hz, respectively. Using an OML analysis as well as a Druyvesteyn formulation, the electron densities, effective temperatures and energy distribution functions were obtained throughout the pulse period (0–9 ms), including a detailed study of the first 10 µs, which was achieved with a temporal resolution better than 0.5 µs. In the initial phase of the voltage pulse (t ~ 1–4 µs), three distinct groups of electrons (indistinguishable from Maxwellian electrons) were observed, namely 'super-thermal', 'hot' and 'cold' populations with effective temperatures of 70–100 eV, 5–7 eV and 0.8–1 eV, respectively. After 4 µs these groups become energetically indistinguishable from each other to form a single distribution with an electron temperature that decays from about 5 to 3 eV during the rest of the pulse on-time. The presence of the 'super-thermal' electron group pushes the probe floating potential to a very negative value (significantly deeper than −95 V) during the initial period of the pulse. In the off-time, the electron density decays with two-fold characteristic times, revealing initially short-term (30–40 µs) and ultimately long-term (3–4 ms) decay rates. These long decay times lead to a relative high density remnant plasma (2 × 109 cm−3) at the end of the off-time, which serves to seed the next voltage pulse. The electron temperature and plasma potential also exhibit two-fold decay in the off-time, but with typically somewhat faster decays, particularly for the long-term decay (100–500 µs) up to the end of the off-time. The time evolution of the plasma potential shows that for a considerable fraction of the on-time the plasma potential remains negative (down to −12 V) only becoming positive after t ~ 60 µs which corresponds to a time of maximum plasma density (typical values of 2 × 1012 cm−3). The generation of super-thermal electrons in the initial phase of the discharge is argued through the development of a simple magnetized-electron bounce model of the expanding sheath.