Magnetron Sputtering Discharge Plasmas And Non-evaporative Getters

Numerous techniques have been developed to deposit high-quality thin films, such as chemical vapor deposition (CVD) and physical vapor deposition (PVD). Physical sputtering deposition is one form of PVD that has a great technological and industrial impact. The work presented in this thesis represents a comprehensive study of the plasma dynamics in E x B magnetron discharges, the effect of these dynamics on the deposition process, and the deposition of non-evaporative getter (NEG) coatings.As the most common type of E×B devices, magnetron sputtering includes direct current magnetron sputtering (DCMS) operated with low power density, pulsed magnetron sputtering (PMS) operated in the intermediate regime, and high power impulse magnetron sputtering (HiPIMS) which operates in high power density, all of which have been systematically investigated in this thesis. The effective regimes of operation were then applied to the deposition of non-evaporative getter (NEG) thin films in very narrow, long tubes with inner diameters of less than 10 mm for the specific needs of soft x-ray facilities for diffraction-limited storage rings (DLSR) in the frame of Advanced Light Source Upgrade (ALS-U) project. After activation, the NEG coatings help to achieve Ultra High Vacuum (UHV) in the range from 10-9 Torr to 10-12 Torr at room temperature.In the first part of this thesis, the study of plasma dynamics focuses on two major low frequency plasma oscillations:(i) rotating spoke oscillations, which propagate along the "racetrack" erosion profile, also called the azimuthal direction, and (ii) breathing oscillations in the axial direction.The plasma dynamics in E x B discharges such as magnetron sputtering plasmas and how they affect the deposition of thin films are among the leading subjects in magnetron research. In magnetron sputtering, HiPIMS discharges have been extensively studied, revealing plasmas that are rich in structure, featuring rotating plasma spokes, plasma flares, and azimuthally asymmetric charged particle jets, etc. However, instabilities in the DCMS and PMS regimes are significantly less studied. Fast camera imaging (frame and streak), Langmuir probes, and ion energy and mass spectroscopy have been employed to analyze the oscillations.Rotating spoke oscillations-Spokes in HiPIMS propagate in the E x B direction, while in DCMS they propagate in the opposite, -E x B direction. In PMS operated in the intermediate regime, a reversal in the propagation direction of spokes has been discovered. The hypothesis is that the propagation direction is associated with localized electron heating and a balance of ionization rate versus supply of neutrals.Breathing oscillations-Axial plasma oscillations, with characteristic frequencies between 10-100 kHz have been discovered. They are more significant at low pressures and low currents, i.e. in DCMS and low current PMS. Transitions from periodic to chaotic behavior and to a spoke-free mode were found as the current increased. Axial variations of plasma parameters can be associated with an ionization rate that is strongly determined by the supply of neutrals including background gas and fluxes from the target. A mode diagram has been developed to plot the spoke mode and breathing mode in relation to current and pressure.Spoke-free oscillations-In HiPIMS operated at very high currents, the disappearance of spokes has been discovered for targets with high sputtering yields. Ion energy gains associated with localized ionization spokes have been measured. The interpretation is that the disappearance of spokes is related to an abundant supply of neutrals from the sputtered target, causing an increased rate of electron cooling, dampening plasma instabilities.In the second part of this thesis, based on the universal features and physical theories discovered in magnetron sputtering plasmas, the focus is on the advancement of NEG coatings deposited by pulsed sputtering in very narrow chambers with 6 mm inner diameter, although the deposition conditions were somewhat different.A wire cathode suspended vertically with reasonable tension was used to deposit NEG coatings on the inside of 6 mm inner diameter copper tubes. Pulsed sputtering has been employed to power the discharges due to its versatility. Observing Paschen's scaling law, the deposition was done at unusually high pressure to compensate for the unusually small distance between the cathode and anode. Two types of wire cathodes have been compared:Ti, V, and Zr wires twisted together, and single Ti-V-Zr alloy wire. The characterization of the getter thin films has been carried out by Scanning Electron Microscopy (SEM) for the surface morphology, Energy Dispersive X-ray Spectroscopy (EDX) for the bulk chemical composition, and Rutherford Backscattering Spectrometry (RBS) for composition depth profile. The pumping performance of the NEG coatings have been investigated.SEM images clearly reveal that the morphology of NEG films is relatively uniform at low magnification. At higher magnification, the films have cauliflower-like surface structures and the nanostructures have average size of approximately 10-100 nm. Such large surface area is favorable for gas absorption.The eventual unraveling of the twisted wires during sputtering introduces two significant problems. Firstly, the elemental composition of the deposited getter films greatly varies depending on which wire the particular surface is mostly facing. Secondly, experimentally, it is difficult to keep the unraveling wires from protruding towards the copper tube, causing a short circuit. The elemental composition becomes inhomogeneous as the twisted wires gradually unravel during the sputtering process. The deposition using alloy wire cathode shows higher stability both in film composition and in the deposition process. The elemental concentrations of the deposited NEG coatings are approximately consistent with those of the single alloy wire cathode. Different discharge voltage with different pulse frequency has been used to deposit NEG films, showing that higher voltage results in denser films, more uniform morphology and higher deposition rate. A getter coating of atomic percentages 46% Ti,15%V ,29%Zr, and 10% O has been successfully prepared for pumping performance tests, providing a solution for the UHV required by the soft x-ray facilities for diffraction-limited storage rings (DLSR) in the frame of Advanced Light Source Upgrade (ALS-U) project.