Plasma potential and ion energy characteristics in BP-HiPIMS discharge with double layer

Abstract

As an emerging ion acceleration plasma source, the bipolar-pulse high power impulse magnetron sputtering (BP-HiPIMS) discharge has been widely concerned by academia and industry due to its ability to adjust the ion kinetic energy. Formation of the double layer (DL) potential structure during the BP-HiPIMS positive pulse is very vital for accelerating ions, but its structure characteristics are still unclear now. In this work, to understand the DL characteristics affected by various discharge parameters, evolutions of plasma potential Vp and ion energy in BP-HiPIMS discharge with copper target has been investigated systematically via the emissive probe and mass spectrometer together. Spatial plasma potential measurements show that the DL is established in front of the target during the positive pulse, whose boundary potential drop UDL to accelerate ions can be increased to ~60 V at a lower operating gas pressure (p＝0.6 Pa) and a higher applied positive pulse voltage (U+＝200 V). The ignition onset time of DL after applying the positive pulse can be shortened to ~25 μs by decreasing the gas pressure and increasing the positive pulse voltage or negative pulse duration. After the DL ignition, a group of high-energy copper ions with energy higher than the surrounding plasma potential can be recognized in the ion energy distribution function (IEDF) curves at the downstream plasma. This result illustrates that the copper ions can be ionized at the high-potential plasma region and be accelerated by the DL boundary potential drop. In addition, a global current balance model of the DL in BP-HiPIMS is developed, which suggests that the UDL can be well adjusted by increasing the positive pulse voltage U+ especially for the U+＞200 V as verified by the experimental potential measurements.