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Title: Electron diagnostics of magnetron discharges
Author: Ryan, Peter
ISNI:       0000 0004 9347 3316
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2019
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The magnetron is a weakly magnetised plasma source used for physical vapour deposition to produce high-quality thin films and coatings for technological applications. In this research, electron plasma property measurements were performed in conventional DC magnetron discharges and in high power impulse magnetron sputtering (HiPIMS) discharges. The two main topics of this thesis are: (i) a comparison of electron plasma property measurements made by Langmuir probe and incoherent laser Thomson scattering, and (ii) an investigation of electron dynamics in HiPIMS discharges using a combination of incoherent laser Thomson scattering and optical emission spectroscopy. Electrons are responsible for driving many important processes in low temperature discharge plasmas, such as, plasma heating, ionisation and plasma chemistry. The Langmuir probe is commonly employed to measure electron plasma properties, but its data can be difficult to interpret, especially from magnetised plasma. The aim of the first part of the research was to assess the accuracy of electron plasma property measurements made by Langmuir probes in weakly magnetised plasma, by comparing the results from the probe measurements with reliable results obtained via laser Thomson scattering. Standard unmagnetised theories were used to interpret the probe data. The range of magnetic field strength, electron temperature and electron density in the study were 1 < B[mT] < 33, 0.1 < T_e[eV] < 5.9 and 4x10¹⁶ < n_e[m⁻³] < 7x10¹⁹, respectively. The results showed that there was good agreement between the diagnostics during the pulse-on time of HiPIMS at all of the measurement positions. This is a significant outcome because previous Langmuir probe studies were, in general, restricted to regions where electron magnetisation was insignificant due to the difficulty of interpreting probe data and concerns over plasma perturbation. In contrast, large discrepancies were observed for the lower-density DC magnetron mode, even when the magnetic field strength was insignificant for electron magnetisation. For some discharge conditions, the electron density determined by laser Thomson scattering was over an order of magnitude greater than the plasma density obtained by the Langmuir probe, using both ion and electron collection theories. In addition, the low energy part of the electron energy distribution function determined by the probe was depleted at all of the measurement positions for the DC mode. The possible reasons for the discrepancies are discussed, with the conclusion being that the plasma was significantly perturbed by the probe stem. HiPIMS discharges have a high ionisation fraction of sputtered atoms in the deposition flux, which leads to improved film properties. Previous investigations have reported reliable electron plasma properties for only isolated spatial regions of the discharge. The aim of the second part of the research was to perform a comprehensive survey of electron dynamics in HiPIMS discharges. Electron plasma properties were measured using laser Thomson scattering, and optical emission spectroscopy provided information about excited atomic and ionic states. The combination of these two diagnostics gives an insight into the electron-heavy species interactions occurring in the plasma. The results were consistent with a dense (n_e > 10¹⁹ m⁻³), metal-rich plasma propagating along the axial direction from the magnetic trap region to a typical substrate position. The main outcomes from this research were experimental confirmation of the expected features of HiPIMS discharges using the reliable laser Thomson scattering technique, and validation of trends identified in previous Langmuir probe studies at a typical substrate position. Therefore, the results provide a solid foundation for the understanding of electron dynamics in HiPIMS.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral