Deposition rate enhancement in HiPIMS without compromising the ionized fraction of the deposition flux

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We systematically investigate and quantify different physical phenomena influencing the deposition rate, aD, of Nb coatings prepared by high power impulse magnetron sputtering (HiPIMS), and propose a straightforward approach for deposition rate enhancement through the control of the magnetron's magnetic field. The magnetic field strength at the target surface, B, of a 50 mm diameter magnetron was controlled by the application of paramagnetic spacers with different thicknesses in between the magnetron surface and the target. We found that lowering B achieved by the application of a 2.8 mm thick spacer led to an increase in aD by a factor of ∼4.5 (from 10.6 to 45.2 nm min−1) when the discharge was operated at a fixed average pulse target power density (2.5 kW cm−2). However, the ionized fraction of the deposition flux onto the substrate was found to be comparable, despite a large difference in B-dependent discharge characteristics (magnetron voltage and discharge current). We show that the decrease in aD commonly observed in HiPIMS (ranging from 33% to 84% in comparison with dc magnetron sputtering in the presented experiments) is governed by different physical processes, depending on the value of B: for high B, the back-attraction of the target ions towards the target is the dominant effect, while for low B the ion back-attraction, the sub-linear dependence of the sputtering yield on the ion energy, and the variation in material transport effects are all important. Finally, we offer a theoretical background for the observed results, demonstrating that the here-presented conclusions may be applicable to HiPIMS discharges using different metal targets and different inert gases.