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.
Highlights
Among the techniques currently implemented for the deposition of thin films, magnetron sputtering is one of the most commonly used [1]
This indicates that the measured dynamics are not specific to high power impulse magnetron sputtering (HiPIMS), but rather a feature of bipolar magnetron sputtering
In a previous work [9], the Nb+ ion energy distribution functions (IEDFs) were measured with the CERN-balanced setup in HiPIMS at main pulse duration of 30 μs, positive pulse (PP) delay of 4 μs, PP duration of 100 and 250 μs, PP amplitude of +50 V, and 1 kHz repetition rate, reaching 1 A cm−2 peak current density averaged over the whole target surface
Summary
Among the techniques currently implemented for the deposition of thin films, magnetron sputtering is one of the most commonly used [1]. This leads to higher peak power densities (∼1 kW cm−2) and plasma densities up to three orders of magnitude above those of DCMS, with the consequent ionization of the sputtered target atoms [4] These ions can be accelerated onto the growing film either by biasing the substrate surface with a negative voltage [5,6,7,8], or by applying a positive pulse (PP) at the target during the afterglow between consecutive discharge pulses [9,10,11]. Two different setups have been used in this work to investigate HiPIMS with Ar as the process gas at 8 × 10−3 mbar Both systems include planar magnetrons featuring a 50 mm diameter Nb target, and an MEA-Pfeiffer plasma process monitor 422 that provides time-integrated ion energy distribution functions (IEDFs). Complementary floating potential Vfl measurements of the LP are performed to confirm the afterglow dynamics by directly connecting the LP tip to the digital oscilloscope, bypassing the LP amplification module used for the I –V curve measurements
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