Contrary to paradigm, magnetron discharge runaway cannot always be related to self-sputtering. We report here that the high density discharge can be observed with all conducting targets, including low sputter yield materials such as carbon. Runaway to a high density discharge is therefore generally based on self-sputtering in conjunction with the recycling of gas atoms in the magnetic field-affected pre-sheath. A generalized runaway condition can be formulated, offering a pathway to a time-dependent model for high-power impulse magnetron sputtering that includes rarefaction and an explanation for the termination of runaway.
A bipolar HiPIMS discharge with a rectangular positive voltage pulse (with controllable amplitude, delay after the main negative pulse and positive pulse length) was systematically investigated by mass spectroscopy. The time-averaged spectra of ions measured at the substrate position exhibit a prominent high-energy peak. It is shown that the position of the peak can be varied by the positive pulse amplitude, its magnitude scales with the pulse length and its width can be slightly influenced by the length of the delay interval. Measurements of the plasma potential at the mass spectrometer position and time-resolved mass spectroscopy clearly show that the high-energy peak is formed by ions accelerated by the elevated plasma potential during the positive pulse. In addition, fine details of the ion energy distribution functions related to the plasma potential transients at the start of the positive pulse are identified. The presented results are beneficial for the optimisation of the parameters of the positive pulse in experiments implementing the bipolar HiPIMS technology.
We systematically investigate the reactive behaviour of two types of high-power pulsed magnetron discharges above a Nb target using either square voltage pulses (denoted as HiPIMS) or custom-shaped pulses (denoted as MPPMS), and compare it with that of a dc magnetron sputtering (DCMS) discharge. We demonstrate that the surface metal oxides can be effectively sputter-eroded from the target during both HiPIMS and MPPMS pulses operated in reactive O2/Ar gas mixtures, and that sputtering from a partially oxide-free target is possible even at high oxygen concentrations. This results in a hysteresis-free deposition process which allows one to prepare optically transparent high refractive index Nb2O5 coatings exhibiting an elevated deposition rate without the need for feedback control commonly used in reactive DCMS. The cathode voltage was identified as the principal parameter that affects the reactive discharge behaviour.
We have determined the local plasma parameters using the Langmuir probe measurements with a sub-microsecond time resolution during positive voltage pulses of a bipolar high-power impulse magnetron sputtering discharge using an unbalanced magnetron with a titanium target. The effects of the positive voltage pulse amplitude and the delay between the negative voltage pulse end and the positive voltage pulse initiation are investigated as well as the spatial dependence of the plasma parameters at three distances from the target. From the results, the values of the average energy flux of ions during the positive voltage pulse to the substrate are estimated. We have found that the time evolution of the plasma parameters has similar developments which are independent of the positive voltage pulse parameters and the distance from the target, although the values of the plasma parameters are different. During the initial part of the positive voltage pulse, a large difference (up to 200 V) between the plasma and the floating potential accompanied by a high electron temperature (up to 150 eV) and a significant decrease of electron density (up to one order of magnitude) is registered. After this part, the difference of the potentials and the electron temperature are low (<2 V and ≲1 eV, respectively). The short delays between the negative voltage pulse end and the positive voltage pulse initiation as well as the higher positive voltage amplitudes have a beneficial effect on the average energy flux of ions during the positive voltage pulse to the grounded and insulated substrates.
We systematically investigate and quantify different physical phenomena influencing the deposition rate, a D , 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 a D 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 a D 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.
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