Application of Sculptured Thin Film Technology to Metal Oxide Gas Sensors

Abstract

The main goal of this research has been the application of glancing angle deposition (GLAD) techniques to create highly porous sculptured thin films (STF) structures for use as gas sensors. The glancing angle technique utilizes physical vapor depositions at highly oblique (>70°) angles of incidence to form porous film structures. Process conditions including the deposition rate, rotation speed, and flux angle were investigated to produce pillar and chevron formations.

Control of the anatase/rutile percentages in TiO2 gas sensors is critical for design and development. Thin films were deposited using reactive e-beam evaporation with a titanium source melt at various partial pressures of oxygen and argon. Post deposition anneal studies were conducted in air from 150-900 °C. The combination of reactive evaporation at high oxygen partial pressures and low temperature (150 °C) anneals in air formed films with the highest percentage of anatase. Depositions at lower oxygen partial pressures yielded films with a higher percentage of rutile, which supports the role of oxygen vacancies as nucleation centers for the anatase to rutile transformation. Higher temperature annealing produced rutile films as expected from the thermodynamics of the TiO2 system. After the formation of anatase STF structures, the films displayed a high degree of temperature stability with subsequent annealing. Anatase films structures were still evident even after annealing at 1200 °C in air.

The GLAD technique produces films with preferred crystal orientations which differ based on the deposition angle. A series of experiments looked at the effect of the angle of incidence, the deposition conditions, and the substrate material on the crystal structure of the STF structures. The titanium films did show a dependence of the crystalline structure on the angle of incidence for both as-deposited films and after oxidation at 900 °C in air.

A conductive MOS sensor was designed based on an anatase chevron formation. Following the fabrication of a sensor test chamber, the sensor response was characterized versus temperature for in argon, air, and hydrogen in argon.