A new sol-gel process for fabricating TiO2 photoresists

Abstract

A simple and cost-effective sol-gel method produces stable titanium dioxide and titanium oxynitride photoresists, and is compatible with many photolithographic techniques.Titanium dioxide (TiO 2 ) is a globally important material with a wide range of applications, including in the paint, cosmetics, and food industries. The microstructure of one of the mineral forms of TiO 2 , anatase, gives the material useful properties such as light trapping and photocatalysis, and hence anatase is of particular interest in optics and photonics research. This form of TiO 2 can be synthesized using a sol-gel method-a process in which a colloidal solution of a molecular precursor is converted to a network polymer-producing a material with excellent mechanical and chemical stability over long periods of time.Here, we present the results of our research developing a sol-gel to fabricate simple and cost-effective TiO 2 photoresists for photolithographic applications. 1 Photoresists are light-sensitive materials used to fabricate patterned thin films on the microscale or sub-microscale (i.e., photolithography). There is a large range of photolithographic techniques available, and our sol-gel is compatible with a number of these. Compared with other photoresists, the increased stability of the patterned TiO 2 thin films produced using this precursor means no post-processing (e.g., lift-off processes or etching) is required. Moreover, the density of the resulting TiO 2 thin film can easily be adjusted with heat treatment to create a dense material with a high refractive index of up to 2.2. Without heat treatment, the material is porous with a low refractive index.Our sol-gel TiO 2 photoresist fabrication method is a fivestep process. Initially, titanium tetraisopropoxide is polymerized in a mixture of methanol and butanol, producing an alcohol-insoluble (TiO) n network material. Alcohol-soluble benzoylacetone (BzAc) is added, which forms a complex with the titanium, resulting in a sol-gel. A film of the sol-gel is then deposited on a substrate. BzAc is very sensitive to UVA light, and hence when areas of the sol-gel film are exposed to UV Figure 1. Examples of four different photolithographic techniques. (a) Laser interference lithography, (b) dynamic interferometric lithography, (c) radial phase-mask lithography, and (d) microsphere lithography. TiO 2 : Titanium dioxide. BzAc: Benzoylacetone. Â: Angle between laser arms. d: Depth. : Period. -spheres: Microspheres. Continued on next page