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

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.