Understanding the ion acceleration mechanism in bipolar HiPIMS: the role of the double layer structure developed in the after-glow plasma

The physical vapor deposition techniques are nowadays widely used at industrial scale to produce thin films and surface coatings. Despite their proven utilities and benefits, these techniques still need to be improved in order to build other exigent and innovative coating systems, able to satisfy the market demand and to meet modern society's needs. In this context, the bipolar high power impulse magnetron sputtering (BP-HiPIMS) technology is gaining ground and popularity due to its extraordinary ability to control the energy of the incoming ion flux to the growing film, enabling an energy-enhanced deposition process thanks to the existence of a dynamic double layer (DL) structure which develops in the after-glow plasma. In this work, the HiPIMS discharge was operated in bipolar mode, in which the negative pulses are followed by positive pulses whose amplitude, duration and delay can be independently controlled. The temporal and spatial evolutions of the plasma potential in BP-HiPIMS discharge were investigated using an emissive probe. Energy-resolved mass spectrometry was used to study the influence of the pulsing configuration (positive/negative pulse duration) on the energy distribution and flux of the ionic species bombarding the substrate. It was found that the ion energy distributions and ion flux were mainly determined by the specific time-and-space evolution of the plasma potential throughout the cycle of the voltage pulses applied to the target and the spatial distribution of the ions at the onset of the positive pulse. The ion propagation dynamics is strongly related to the potential drop across the DL structure whose characteristic features are mainly influenced by the amplitude and duration of the positive pulse. Short negative HiPIMS pulses followed by short and highly positive reverse pulses are favorable for an efficient acceleration mechanism of the metal ions in the potential drop of the DL.