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HiPIMS plasma diagnostic to study the repeatability of low temperature deposition of crystalline titania

Delfour-Peyrethon, Brice (2018) HiPIMS plasma diagnostic to study the repeatability of low temperature deposition of crystalline titania. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

In the last few decades, Titanium dioxide (also called Titania or TiO2) has experienced significant scientific interest due to its range of properties and possibility for further developments in a range of applications, including photocatalysis. Initially, the material was widely used in this application in powder form for water decontamination applications. It was later developed as a thin film coating onto glasses and metals (industrial, construction scale) for its self-cleaning properties. Indeed, the scientific community realised that TiO2 coatings could present both antimicrobial properties (via degradation of cell walls) and a strong hydrophilic behaviour. Recently, thin films technologies, and more particularly magnetron sputtering processes, have undergone a new expansion: the HiPIMS (High Power Impulse Magnetron Sputtering) process. The HiPIMS process is 15 years old and its principal aim is to provide a more ionized deposition flux (by an order of magnitude), easing the control of the deposited film properties (density, structure, composition, targeted areas on the substrate, etc…). It has also been shown to facilitate low substrate temperatures during operation. The combination of both characteristics should lead to the possible deposition of crystalline films at low temperature. The first aim in this work is to provide the reader with a deep understanding of the HiPIMS discharge and how it can influence the deposition process. To do so, plasma diagnostics have been carried out on different coating systems (rigs) and using different power supplies to show how both of these aspects can drastically change the processes occurring within the discharge. For the rig used for deposition of the films, thermal and deposition rates measurements have also been carried out. This leads to the establishment of a process envelope, suitable for the deposition of crystalline titanium dioxide films. Subsequently, TiO2 coatings have been deposited by HiPIMS onto various substrates and the influences of the deposition parameters studied to better understand the plasma behaviour and its consequences on the thin film properties, in order to optimize the latter’s crystallography. The final aim of this project is to deposit a crystalline photo-active (i.e. photocatalytic, hydrophilic) film onto a polymeric substrate at low temperature. This project implies the use of a large range of characterisation techniques. The structural, mechanical, chemical and crystallographic properties have been studied by using a large array of techniques including X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Raman spectroscopy, etc… The photocatalytic activity was assessed using dye degradation tests under UV light, and the hydrophilicity by contact angle measurements. In this project, it has been shown that a modification of the deposition rig can bring drastic changes to the plasma. These changes are even more important when it comes to the reactive plasma. Each rig has its own process envelope and transferring from a laboratory scaled rig to an industrial rig is not a simple task. Parameters influences on the plasma can also vary from one configuration to another and optimized deposition conditions are not necessarily repeatable from one rig to another. Mainly, it seems that the oxidation process is greatly modified by the rig configuration. This thesis presents plasma diagnostic conducted using only an oscilloscope, to show that this is a simple and cheap way for industrials to deposit in reactive atmosphere. Here, the author has optimized deposition conditions on a given apparatus, where crystalline Anatase (TiO2) has been deposited at low temperature, typically, below 60 °C. These films have also been deposited onto polymeric substrates with no destruction of the latter. The oxygen content during deposition appears to be the more important point to control during deposition. For the rig used here, films have been deposited in the poisoned mode, at low pressure, high frequency and short pulse width as it seemed to provide the more ionized discharge, facilitating the films crystallization.

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