Characterization of high power impulse magnetron sputtering discharges

Paper I: In the first paper, we present a new approach in the characterization of the high power pulsed magnetron sputtering (HiPIMS) discharge evolution—time- and species-resolved plasma imaging—employing a set of band-pass optical interference filters suitable for the isolation of the emission originating from different species populating the plasma. We demonstrate that the introduction of such filters can be used to distinguish different phases of the discharge, and to visualize numerous plasma effects including background gas excitations during the discharge ignition, gas shock waves, and expansion of metal-rich plasmas. In particular, the application of this technique is shown on the diagnostics of the 200 µs long non-reactive HiPIMS discharges using a Cr target.

Paper II: In order to gain further information about the dynamics of reactive HiPIMS discharges, both fast plasma imaging and time- and space-resolved optical emission spectroscopy (OES) are used for a systematic investigation of the 200 µs long HiPIMS pulses operated in Ar, N₂ and N ₂/Ar mixtures and at various pressures. It is observed that the dense metal plasma created next to the target propagates in the reactor at a speed ranging from 0.7 to 3.5 km s⁻¹, depending on the working gas composition and the pressure. In fact, it increases with higher N ₂ concentration and with lower pressure. The visible form of the propagating plasma wave changes from a hemispherical shape in Ar to a drop-like shape extending far from the target with increasing N₂ concentration, owing to the significant emission from molecular N₂. Interestingly, the evidence of the target self-sputtering is found for all investigated conditions, including pure N₂ atmosphere.

Paper III: Here, we report on the time- and species-resolved plasma imaging analysis of the dynamics of the 200 µs long HiPIMS discharges above a Cr target ignited in pure O₂. It is shown that the discharge emission is dominated solely by neutral and ionized oxygen, since the monitored discharge is operated above a fully poisoned (oxidized) target from which only a minimum of Cr is sputtered. No signs of self-sputtering have been detected, in contrast to the discharges in Ar, N₂ and N₂/Ar mixtures previously investigated.

Paper IV: In the fourth paper, we study different power management approaches in HiPIMS and MPPMS and their effects on the pulsed discharge evolution, plasma composition, and metal ionization estimated by OES. It is shown that HiPIMS is the only technique that enables the discharge operation in self-sputtering mode within the investigated range of applied powers, resulting in a significantly higher ionization of the sputtered metal than that reached with MPPMS. In contrast to HiPIMS, MPPMS provides a higher versatility in adjusting the pulse shape and pulse length. This feature can be particularly beneficial, for instance, in the discharge ignition. Nb coatings prepared by HiPIMS and MPPMS have very similar deposition rates that are lower than in DCMS. All films prepared at p = 1Pa possess a dense columnar structure. Coatings deposited by the two high power pulsed discharges exhibit higher compressive stress and larger out-of-plane lattice spacing than those prepared by DC sputtering under comparable conditions. At higher pressure, p = 2Pa, DCMS-grown films show a tensile stress due to a porous microstructure, while films prepared by HiPIMS and MPPMS are dense and in compression, most probably due to the substantial ion bombardment.

Paper V: In the last paper, we analyze the behavior of the HiPIMS, MPPMS and DCMS discharges in reactive O₂/Ar gas mixtures and evaluate the characteristics of the fabricated NbO_x films. We demonstrate that the surface metal oxides can be effectively sputter-eroded from the target during both HiPIMS and MPPMS pulses, and that sputtering from a partially oxide-free target is possible even at high oxygen concentrations. This results in a hysteresisfree deposition process which allows one to prepare optically transparent b₂O₅ coatings at a high growth rate without the need of feedback control commonly used in reactive DCMS. Nb₂O ₅ coatings prepared by both reactive high power pulsed discharges exhibited a high index of refraction, a low extinction coefficient, a near-zero internal stress, and high hardness and Young's modulus. The HiPIMS-deposited coatings showed the highest deposition rate and the highest index of refraction. The latter observation was related to the higher film density. In comparison, MPPMS exhibited the highest power-normalized deposition rate among the three investigated deposition techniques, possibly due to the longer period that is available for the gradual target cleaning. (Abstract shortened by UMI.)