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Title: The study of spokes in HiPIMS discharges with applications to the production of superconducting thin films
Author: Lockwood Estrin, Francis
ISNI:       0000 0004 6422 867X
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2017
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HiPIMS is a method of physical vapour deposition (PVD) used to make thin films. Regions of high optical emission, known as spokes, have recently been discovered in HiPIMS. Spokes have been linked to the production of high energy ions, which can be beneficial for the production of high density thin films. The aim of this PhD has been to develop new methods and diagnostic devices capable of studying spokes in HiPIMS. These are used to gain a greater insight into the make-up of spokes and ultimately to use this knowledge in order to produce higher quality niobium films for potential use in SRF cavities. The strip probe is one such diagnostic device. It consists of a radial section of the target, electrically isolated from the remainder, but kept at the same potential so that the local current drawn by the plasma above the strip can be measured. The strip probes reveal that in an argon plasma with an aluminium target, the spokes cause the local current to increase by up to 52 % above its base value. Moreover, the amplitude and nature of spokes change in reaction to changing current and power. There were found to be three regimes of spokes: chaotic short lived spokes at lower power and pressure; a spokeless regime at higher power and pressure, where no spokes are visible; and the coherent regime, where spokes are regular and periodic, in-between. It was discovered that there exists an optimum power for each pressure at which spoke amplitude is at a maximum; this effect was found for both aluminium and niobium targets. The velocity of the spokes in the ExB direction approximately matches the CIV of argon and aluminium, for an argon plasma with an aluminium target. However, at lower pressures of argon, with a niobium target, the spoke velocities are far higher then the CIV. The strips were also used simultaneously with optical imaging. Different ways of representing the strip probe current are investigated with both the strip probe current plotted as an image and the camera images linearised to a 1D time trace, like the strip probe output. These representations showed that the strip probe current matches the optical emission intensity closely. The effect of the different spoke conditions on film deposition was tested by making niobium films at different angles to the target in the chaotic, coherent and spokeless regimes and comparing them to films created by DC sputtering. The results indicated that pressure was the dominant effect, rather then the spoke regime. Another diagnostic technique developed in this thesis was the triple probe. This was designed to take readings of ne, Te, Vf and Vp with a time resolution sufficient to investigate spokes. The triple probe was used to analyse the full HiPIMS pulse, the different spoke regimes and the spokes themselves, just above the target racetrack. The results match those previously taken by the Langmuir probe. It was found that electron temperature was an important factor in the formation of spokes, with spokes mostly absent when Te 2.5 eV. The triple probe was also used with the strip probe to investigate spokes and Te was found to be in anti-phase with ne. The peaks in the strip probe current also seem to have an influence on Te, indicating a heating effect from the spokes. This led to a model of rarefaction and heating above the target as the spokes passed beneath. Suggested future work includes the continued use of the strip probe to detect spokes in systems where optical imaging is impractical and a more in-depth investigation into the effect of spoke regimes and amplitudes on deposited films.
Supervisor: Bradley, J. W. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral