Formations of anode double layer and ion beam in bipolar-pulse HiPIMS (BP-HiPIMS)

As an emerging ion acceleration plasma source, the bipolar-pulse high power impulse magnetron sputtering (BP-HiPIMS) discharge provides an effective approach to improve deposited ion energy and tailor the film properties for a large range of applications. The ion acceleration mechanism in BP-HiPIMS discharge is very vital but still unclear now. In the present work, the ion acceleration mechanism is systematically investigated via the experimental measurements, particle-in-cell/Monte Carlo collision (PIC-MCC) simulation, and theoretical model together. In the experiment part, the floating potential V (f) and the ion velocity distribution function (IVDF) have been measured via the Langmuir probe and the retarding field energy analyser (RFEA) respectively. The measurements show that the V (f) at the downstream drops from +80 V to similar to+40 V after applying the positive pulse for similar to 75 mu s, suggesting the formation of the double layer. Correspondingly, the IVDF changes from the unimodal Maxwellian distribution to the bimodal distribution, suggesting the existence of the ion beam. The PIC-MCC simulation results clearly show the development process of the double layer and ion beam. A theoretical model is introduced to explore the complex plasma dynamics in the experiment and simulation. The theoretical results illustrate that (i) the sheath in front of the target surface prefers an ion sheath rather than an electron sheath, (ii) the stable position of the double layer boundary is in the magnetic null point, (iii) the potential drop across the boundary is influenced by the gas pressure p. These important theoretical results are well consistent with the measurements and simulation. In addition, the oscillation of the double layer boundary and the instabilities of the ions are briefly discussed by combining the previous works.