On double-layer and reverse discharge creation during long positive voltage pulses in a bipolar HiPIMS discharge

Abstract

Time-resolved Langmuir probe diagnostics at the discharge centerline and at three distances from the target (35mm, 60mm, and 100mm) was carried out during long positive voltage pulses (a duration of 500μs and a preset positive voltage of 100V) in bipolar High-Power Impulse Magnetron Sputtering of a Ti target (a diameter of 100mm) using an unbalanced magnetron. Fast-camera spectroscopy imaging recorded light emission from Ar and Ti atoms and singly charged ions during positive voltage pulses. It was found that during the long positive voltage pulse, the floating and the plasma potentials suddenly decrease, which is accompanied by the presence of anode light located on the discharge centerline between the target center and the magnetic null of the magnetron's magnetic field. These light patterns are related to the ignition of a reverse discharge, which leads to the subsequent rise in the plasma and the floating potentials. The reversed discharge is burning up to the end of the positive voltage pulse, but the plasma and floating potentials have lower values than the values from the initial part of the positive voltage pulse. Secondary electron emission induced by the impinging Ar+ ions to the grounded surfaces in the vicinity of the discharge plasma together with the mirror configuration of the magnetron magnetic field are identified as the probable causes of the charge double-layer structure formation in front of the target and the ignition of the reverse discharge.

Effect of magnetic field configuration on double layer formation and reverse discharge ignition in bipolar HiPIMS

“…The onset of the RD ignition is detected by time-resolved OES imaging and by measuring the floating potential. The study confirms the connection between DL formation and RD ignition, as proposed by Pajdarová et al [17], for all studied magnetic field configurations. The timing of RD ignition is found to be strongly correlated to the magnetic field configuration, and the positioning of external electrodes near the discharge plasma is identified as influencing the RD.…”

“…As the accumulation of the electrons behind the magnetic trap continues, a drop in plasma and floating potential occurs in this region, resulting in the formation of the DL structure [3,[12][13][14][15]. Subsequently, electrons are accelerated across the DL's potential rise, leading to the ignition of the discharge as evidenced by the observation of 'jet-like' or 'bulb-like' anode light patterns (ALP) by optical emission spectroscopy (OES) imaging of the plasma [10,16,17]. These accelerated electrons can also ionize atoms located inside the ALP core.…”

“…In our recent paper [17], we proposed that the geometry and strength of the magnetron's magnetic field play important roles in the formation of the DL structure and the RD ignition. This can explain the inconsistent results of plasma potential evolution obtained for balanced and unbalanced magnetic fields reported in the literature.…”

“…In this work, a systematic investigation of the effect of the magnetic field on DL formation and RD ignition is performed using a magnetron with an adjustable magnetic field. The magnetic field topology varies between more unbalanced and more balanced configurations with respect to the standard unbalanced configuration used in [9,17,18]. The onset of the RD ignition is detected by time-resolved OES imaging and by measuring the floating potential.…”

“…This way, the magnetic field at the target center is weakened, and thus, the configuration becomes even more unbalanced. The standard unbalanced configuration used in [9,17,18] is with the magnets fully inserted (both at the position 0 mm). The geometric factor G = z null /R c [20], where z null is the vertical distance of the magnetic null point from the target, and R c is the target radius, characterizes the degree of magnetron unbalance and varies between 0.66 and 1.16 in our experiments.…”