A Study of Chemical and Plasma Deposition of Organosilane Thin Films

Author: Alex Sibley

Sibley, Alex, 2019 A Study of Chemical and Plasma Deposition of Organosilane Thin Films, Flinders University, College of Science and Engineering

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As industry and society in general become more away of the need for increased energy efficiency, strong, lightweight metals such as magnesium are being seriously considered for a range of applications. The high reactivity and thus corrosion rate magnesium in atmospheric conditions necessitates the development of new, environmentally benign materials and technology for corrosion prevention. Organosilanes are one such class of materials, originally pursued as adhesion promoters, but their ability to form highly cross-linked barrier coatings makes them a good candidate for corrosion prevention.

This thesis examines the fundamental interactions between the model organosilane “propyl-trimethoxysilane” (PTMS) and pure magnesium surfaces. Initially, conventional dip coating was examined with the purpose of investigating the uptake kinetics of PTMS onto magnesium. It was found that the chemical deposition of PTMS onto magnesium is oscillatory, in agreement with previously undertaken studies of a similar nature using different pure metallic surfaces. Unlike those studies, it was found here that the high reactivity of magnesium led to its rapid degradation in the acidic conditions necessary to effectively catalyse the film deposition process, making such chemical deposition unsuitable for future investigation.

A different method of film deposition was therefore investigated: plasma enhanced chemical vapour deposition. Plasma-based film deposition allows cleaning, pre-treatment and deposition to all be performed in a clean vacuum environment, without the need for organic solvents. In order to pursue this area of research, a radio frequency inductively coupled plasma (RF-ICP) chamber was constructed, including the design and fabrication of a suitable RF matching network. Through a comprehensive literature review, the deposition of PTMS in such a way has thus far not been reported, so a fundamental study was undertaken into the effects of various experimental plasma parameters on the resultant film composition and structure. Analysis of these films using a combination of X-Ray photoelectron spectroscopy and scanning Auger spectromicroscopy established that silicon containing films were deposited from PTMS, although the composition was significantly different to that of the PTMS molecule itself, with a reduction in carbon and oxygen content due to the decomposition of the molecule in the plasma environment. Additionally, it was found through elemental mapping with scanning Auger spectromicroscopy that the morphology of the magnesium itself had an effect on the uniformity of the deposited films.

To eliminate any such effects, magnesium surfaces were diamond polished to a mirror finish in order to study the effects of PTMS plasma power on the deposited films with the Fourier Transform Infra-Red spectroscopy (FTIR), in addition to the other aforementioned spectroscopies. No significant compositional change was observed with increasing plasma power however, the use of high energy resolution X-ray Photoelectron Spectroscopy (XPS) identified a change in the chemical bonding environment of silicon, with an increased amount of silicon bonded to four oxygen atoms, as opposed to three as is present in the PTMS molecule. FTIR spectroscopy gave direct evidence of crosslinking between silicon and oxygen species, as well as correlating the reduction in carbon within the film to a reduction in hydrocarbon species.

In addition to fundamental studies into film deposition from pure PTMS vapour, a proof of concept investigation was undertaken into the functionalisation of organosilane films through the mixture of PTMS and other materials within the plasma environment. The addition of carbon (in the form of methane) to the PTMS plasma led to no significant change in composition, however, an increase in silicon to carbon bonding was observed compared to films deposited from PTMS alone. By increasing the oxygen content within the plasma, either in the form of oxygen gas or water vapour, a significant reduction in carbon within the film was observed, with a greater reduction caused by oxygen. High energy resolution XPS analysis of films deposited from these precursors revealed that all silicon present within these films was attached to four oxygen atoms, implying a silicon dioxide like film. Finally, an attempt was made to incorporate nitrogen into the PTMS plasma films in order to provide a method of cost effective amine functionalisation. While initial XPS results were promising, with nitrogen detected spectrally, further efforts to locate nitrogen using Auger electron spectromicroscopy were unsuccessful. Nevertheless, the developed plasma deposition capability, and the fundamental understanding of film deposition from PTMS vapour provides a solid foundation for future studies into organosilane films.

Keywords: Organosilane, Plasma Polymerization, Surface Analysis, Scanning Auger, XPS, Thin Films, PTMS

Subject: Nanotechnology thesis

Thesis type: Doctor of Philosophy
Completed: 2019
School: College of Science and Engineering
Supervisor: Professor Jamie Quinton