Nanostructured thin films for humidity sensing

Humidity sensors are found in a variety of applications with differing requirements. The sensitivity, range of humidity operation, and response time of a humidity sensor are important in determining suitable applications. The performance of a humidity sensor is largely determined by the properties of the sensing medium including porosity, surface area, and morphology. Glancing angle deposition (GLAD) is a physical vapour deposition (PVD) technique capable of fabricating highly porous thin films with controlled morphology on the 10 nm length scale. The high porosity, large surface area, controlled morphology, and ability to use virtually any source material that can be deposited via PVD make GLAD films ideal for humidity sensing studies.;An optical humidity sensor was fabricated from a TiO2 film with periodic nanoscale porosity variations designed to yield a narrow-bandpass optical interference filter with a humidity sensitive transmittance spectrum. Comparisons to capacitive sensors were made and advantages identified.;The research presented in this thesis investigates the use of GLAD films for capacitive and optical based humidity sensors. Capacitive sensors were fabricated by coating interdigitated capacitor (IDC) substrates with metal oxide GLAD films. Optimized sensors exhibited a three orders of magnitude change in the capacitance over the entire relative humidity range, and had response times in the 400 ms range. The capacitive response was found to diminish with time and attempts to regenerate the response via heat treatments had some success. The geometry of the IDC substrates was observed to have a significant impact on sensitivity. TiO2 GLAD films were found to be the most sensitive in comparison to similarly fabricated sensors utilizing SiO 2 and Al2O3 films. The overall capacitance change and high humidity sensitivity was discovered to be a result of the first few hundred nanometres of GLAD film. The response time was found to be determined predominantly by film thickness and ranged from a couple hundred milliseconds to over one second for TiO2 films 0.28--8.5 mum thick. The capacitive response and response time were also affected by the incident vapour flux angle (alpha) used during film deposition.