Sharon Marilyn Melpolder

Phase transformations in TiO2/SiO2 sol-gel films as a function of composition and heat-treatment

The refractive indices of titania/silica sol-gel films are known to vary over a wide range with composition. However, little work has been done to investigate the possible variations in refractive index that can be produced for one specific sol-gel composition by varying the heat-treatment time and temperature. In this study, three different titania/silica sol-gel compositions containing 40, 60 and 80 mol% titania were studied after thermal processing at temperatures from 175–1050°C for variable lengths of time. The refractive indices and thicknesses of the sol-gel films spin-coated on to silicon wafers were determined using a combined technique of ellipsometry and reflectance spectroscopy. The microstructural differences in the films were investigated using X-ray diffraction and transmission electron microscopy techniques.

Film/substrate/vacuum-chuck interactions during spin-coating

Thickness variations that are associated with the vacuum chuck were observed in wet-chemical-derived dielectric films applied by the spin-coating technique. These thickness variations are controlled by factors such as the thermal properties of the substrate material, the evaporation behavior of the coating solution, and the physical design of the vacuum chuck. Atechnique is described for evaluating the magnitude ofthis effect.

Laser processing of chemically derived dichroic filters

Optical interference filters were fabricated using multilayer stacks derived from sol-gel SiO 2 and SiO 2 -TiO 2 coatings. Laser processing was then used to modify the spectral properties (color) of local regions of these stacks. Changes in color were analyzed with respect to changes in the individual film thicknesses and refractive indices. Design considerations and some basic limitations of laser firing for tuning interference filter colors are also discussed.

Optimization of Sol-Gel Film Properties

The preparation and characterization of organic-modified and organic/ iorganic-modified silica sol-gel spin-coated thin films is presented as a function of thermal processing. The ability to alter the films final refractive index, thermal expansion coefficient and the polar contribution to the films surface energetic term is reported. The stability of the coating solution at elevated temperatures was studied by analyzing changes in molecular weight, viscosity and particle size during the sol-to-gel transition. Control of the sol-gel thin film properties allows the design of a film/substrate interface where both thermal mismatches and surface energetic differences are minimized. Decreased stresses should be induced during processing, resulting in increased adhesion between the coating and the substrate.

Laser processing of sol-gel-derived multilayer interference filters

Optical interference filters were fabricated using multilayers derived from sol-gel SiO2 and SiO2-TiO2 thin films. Laser processing was then used to modify the spectral properties of local regions of these stacks. The feasibility of using laser processing for selectively changing the optical properties of thin film devices was thus demonstrated. Design considerations and some basic limitations of this technique for tuning interference filter colors are then discussed.

Laser Processing of Multilayer Interference Filters

The use of lasers to modify the optical properties of multilayer interference filters is examined. The effects of laser firing on the optical properties of sol-gel deposited and electron-beam evaporated filters are compared experimentally. Filters made from sol-gel films are found to have tunability of at least 15%, while electron-beam evaporated filters are tunable by laser firing to less than 1%. This difference is explained by applying the Lorentz-Lorentz equation, which describes the effects of residual organics, water, and porosity in the sol-gel films on their optical properties. Possible routes to tailoring film microstructure and composition to optimize filter tunability are also discussed.