Optimizing the ion diffusion in bipolar-pulse HiPIMS discharge (BP-HiPIMS) via an auxiliary anode

The investigation of plasma dynamics and optimization of target particle diffusion are particularly important and urgent in the recently emerging bipolar-pulse high-power impulse magnetron sputtering (BP-HiPIMS) discharge. In this paper, a novel approach, in which an external auxiliary anode was installed in front of the sputtering target, was proposed to optimize electric field distribution and guide particle diffusion, named as AABP-HiPIMS. A positive pulse voltage was applied to the auxiliary anode in synchronization with the target positive pulse voltage, which was achieved by connecting the target and auxiliary anode through a diode in series. Advantageously, no additional power supply needs to be provided in AABP-HiPIMS. The discharge characteristics of this new plasma source were investigated in detail. A theoretical model was successfully built to reveal the transformation process of the target current from the net ion current to the net electron current by systematically analyzing the collapsing process of the target sheath and charged-particle movement in E x B fields. The optimization of ion diffusion was investigated by diagnosing plasma parameters using a Langmuir probe and emissive probe together. The measurements illustrate that, as expected, the electron density in AABP-HiPIMS during the positive pulse phase has been effectively increased similar to 2.5 times on average compared to that in the conventional BP-HiPIMS. Correspondingly, the plasma potential distribution illustrates that the potential gradient pointing to the auxiliary anode centre is more beneficial for control of ion diffusion and to enhance ion flux towards the downstream. Additionally, for better selection of the magnetron process parameters, the characteristic time of plasma decay during the positive pulse was estimated using the Bohm diffusion theory. The rough calculation of flight time for electrons from z = 35 to 95 mm was similar to 70 mu s in BP-HiPIMS and similar to 50 mu s in AABP-HiPIMS, respectively. The effects of negative pulse duration and positive pulse amplitude on plasma parameters were investigated. Measurements of plasma potentials suggest that adopting a relatively short duration negative pulse and a slightly higher amplitude positive pulse, as well as employing the externally applied auxiliary anode, are the optimum choices for improving both the flux and energy of deposited particles when preparing films on the floating substrate.