Afterglow dynamics of plasma potential in bipolar HiPIMS discharges

Abstract

In bipolar magnetron sputtering, the plasma afterglow is initiated by switching the target bias from a negative to positive voltage. In the following, the plasma potential evolution in this configuration is characterized, being responsible for the ion acceleration at the substrate sheath potential fall, in particular in high power impulse magnetron sputtering (HiPIMS). A mass-energy analyzer and a Langmuir probe respectively measure the ion energies and the plasma/floating potential at different positions within HiPIMS discharges. A plasma potential drop and rise in the first 45 μs of the afterglow is observed, settling in the plasma bulk towards values below the applied positive bias. The measured ion energies agree with the plasma potential values before and after the drop-rise. To gain more comprehensive insights into the mechanisms responsible for such a potential evolution, particle-in-cell Monte Carlo 3D simulations of bipolar direct current magnetron sputtering discharges are explored in equivalent geometries. Despite their average power being orders of magnitude lower compared to the HiPIMS configuration, a similar afterglow behavior is observed. This indicates that the measured dynamics are not specific to HiPIMS, but rather a feature of bipolar magnetron sputtering. The responsible mechanisms are studied further: the effects of various system parameters are decoupled, with the magnetic field configuration emerging as crucial for the plasma potential drop-rise dynamics and the associated re-ionization close to the target.

Ion energy distribution and non-linear ion dynamics in BP-HiPIMS and ACBP-HiPIMS discharge

“…As reported by Avino et al [50], recently, a local net negative charge and consequent plasma potential drop appear in front of the target centre after applying the positive pulse for ∼30 μs, obtained by PIC-MCC simulation. When this larger potential drop appears close to the target, the ionization events may occur, resulting in the formation of the anode glow and plasma double layer [50,51]. Therefore, in the present work for ACBP-HiPIMS discharge with an auxiliary anode, we believe that the potential distribution along z-direction is in favour of plasma double layer shown in figure 8(b) (blue line), instead of the electron sheath and anode glow, as deduced via the following evidences:…”

“…This implies that increasing the strength of the coil magnetic field speeds up the re-distribution of plasma potential structure. As reported in [27,31,33,50] for the regular BP-HiPIMS discharge, a double layer potential structure was found or inferred, which was considered as the critical structure to accelerate ions. However, in [27], this double layer potential only appears at the beginning of the positive pulse with a short-life time.…”

“…Figure 8(b) shows three possible plasma potential V p distributions in z-direction, including the so-called electron sheath, anode glow and plasma double layer, according to [31]. As reported by Avino et al [50], recently, a local net negative charge and consequent plasma potential drop appear in front of the target centre after applying the positive pulse for ∼30 μs, obtained by PIC-MCC simulation. When this larger potential drop appears close to the target, the ionization events may occur, resulting in the formation of the anode glow and plasma double layer [50,51].…”

“…For a magnetron discharge, the A E /A W is typical in the order of 10 −2 -10 −1 , and μ/α e is in the order of 10 −3 ; thence, the electron sheath structure may be not established in the case of a larger target area, which often occurs in the small probe tip. However, as simulated by Avino et al [50], a net negative charge density appears in front of the target centre after applying the positive pulse for ∼30 μs, leading to a potential drop correspondingly. In their work, this result can be attributed to that the electron population is repelled from the guarding line at target edge regions and drained to the target centre following the magnetic field lines.…”

Reverse coating technique for the production of Nb thin films on copper for superconducting radio-frequency applications

“…Thanks to this system, the niobium and copper films can be coated consecutively without venting the system in between, hence avoiding the risk of contamination due to contact with air. Before the coatings, the bakeout of the system was performed at 120 • C for 24 h. During the coatings, the equilibrium temperature reached by the system is 150 • C. All the coatings in this study were performed via the physical vapour deposition (PVD) method known as Bipolar High Power Impulse Magnetron Sputtering (Bipolar-HiPIMS) [ [11][12][13]. This technique has been shown to provide denser niobium films on copper substrate than the consolidated Direct Current Magnetron Sputtering technique, and is particularly suitable for coating surfaces at grazing incidence angles with respect to the direction of motion of the sputtered impinging ions [11,14,15].…”