Silicon dioxide (SiO₂) is a critical functional material widely used as a barrier, dielectric, optical layer and protective coating across microelectronics, transportation, biomedical devices and emerging low-temperature manufacturing sectors. However, achieving high-purity, stoichiometric SiO₂ typically requires thermal processing above 1000 °C, precluding deposition on polymers, lightweight metals and temperature-sensitive components needed in modern applications. Plasma-assisted chemical vapour deposition (PACVD) offers a route to reduce this thermal budget; microwave plasmas are - characterised by high electron densities, efficient precursor dissociation and low ion bombardment and so—provide an attractive yet underexplored platform for scalable SiO₂ thin film manufacturing.
This thesis investigates the deposition of SiO₂ thin films using a microwave-assisted O₂/hexamethyldisiloxane (HMDSO) precursor system, with systematic variation of microwave power (1.2–2.0 kW) and O₂/HMDSO gas flow ratios (12:1–20:1). A multi-technique characterisation framework was constructed to establish the processing–structure–property relationships governing the as-deposited films. The framework incorporated optical emission spectroscopy (OES) plasma diagnostics, infrared and Raman spectroscopy, time-of-flight elastic recoil detection analysis, X-ray photoelectron spectroscopy, analytical scanning electron microscopy, atomic force microscopy, spectroscopic ellipsometry, nanoindentation, and mechanically-stressed dielectric breakdown testing.
The results demonstrate that increasing microwave power and oxygen fraction drive more complete precursor oxidation, leading to substantial reductions in carbon and hydrogen incorporation and the promotion of short- and medium-range structural ordering in the films. These changes are evidenced by increased Si–O–Si asymmetric stretching intensity and, reduced AS2/AS1 peak area ratios in infrared spectra, enhanced populations of 3-membered rings, and diminished large-ring signatures in Raman spectra, all collectively indicating progressive network densification within the films. This structural evolution correlates with significant improvements in mechanical performance, with hardness values reaching 8.57 GPa and reduced moduli up to ~83 GPa—approaching those of fused silica—whilst maintaining excellent optical transparency and stable refractive indices consistent with stoichiometric SiO₂.Mechanical-stress-assisted dielectric breakdown testing (842 MPa Hertzian load) reveals that densification-driven defect reduction leads to improved breakdown strengths, achieving values comparable to or exceeding commercial sputtered SiO₂ films. Importantly, the work clarifies that dielectric behaviour is governed not only by average density but also by defect topology and hydrogen-related species, explaining non-monotonic trends in specific deposition regimes.
Overall, this thesis provides the first integrated mechanistic understanding of microwave O₂/HMDSO PACVD SiO₂ growth in an industrial reactor. The findings establish clear guidelines for optimising plasma chemistry, controlling network structure, and achieving high-performance SiO₂ coatings at low substrate temperatures, enabling scalable manufacturing for advanced technological applications.
