Atomic force microscopy of laser-conditioned and laser-damaged amorphous TiO 2 sol-gel thin films

Abstract

Amorphous TiO<SUB>2</SUB> films are spin-cast from highly acidic ethanol precursor solutions of titanium ethoxides onto fused silica and single crystal silicon substrates at 2000 to 3000 rpm. Immediately after spinning, the resulting homogeneous film is approximately 350 nm thick and has a refractive index of 1.6 as determined from UV-VIS-NIR transmission measurements. After 20 hours at room temperature, the refractive index increases to 1.8 and is accompanied by a decrease in film thickness. Mild heat-treatment of the film results in a further increase of the refractive index to 2.0. Thicker, homogenous films can be prepared by depositing subsequent layers. Raman spectra of these films indicate the presence of the amorphous phase of titania. Prior to mild heat-treatment, the amorphous titania films are observed to crystallize to the anatase phase upon exposure to cw radiation from an Argon-ion laser at a fluence of 2.5 Mw cm<SUP>-2</SUP>. Crystallization is initiated approximately 1.2 seconds after exposure and the illuminated spot becomes fully crystalline after 3.6 seconds. However, the laser fluence can be increased to 6.0 Mw cm<SUP>-2</SUP> without resulting in crystallization if the amorphous film is conditioned by exposure to progressively greater laser fluence beginning below the damage threshold. Similarly, films that have experienced a mild heat-treatment prior to irradiation have a damage threshold greater than 6.0 Mw cm<SUP>-2</SUP>. In this study, atomic force microscopy is used to evaluate the microstructure of the laser-conditioned and laser-damaged titania films at the micron to submicron scale.