The Sol-Gel Process For Realisation Of Optical Micro-Structures in Glass

We have applied the sol-gel process to generate optical microstructures in glass using in-house developed sol-gel materials. The synthesis of our sol-gel glass materials is based on the formation of particles of controlled size from organo-metallic compounds. The overall shrinkage during annealing of the gel material is negligible and does not imply crack formation. Silica, titania and binar y oxides of silica titania oxide sol-gel materials have been used. A two-step pattern transfer is employed to replicate the structure in glass: first a polydimeth ylsiloxane (PDMS) replica is obtained from a Si, polymethylmethacr ylate (PMMA) or polyimide (P I) master structure, and secondly, a la yer of sol-gel material is applied on the PDMS `soft-replica' to get diffractive micro-/nano-structures in glass after drying and annealing. These structures are characterised b y a scanning electron microscope (SEM) and b y an optical diffraction setup. Introduction Sub-micrometer structures, particularl y optical elements, are necessar y in advanced microopto-electro-mechanical systems (MOEMS) and, therefore represent a research area of great importance. Such elements can today be fabricated through the use of expensive processes, like ion etching, e-beam/laser writing, which are limited in performance [1,2]. For example, it is difficult to obtain a smooth concave, convex or prismatic profile using these techniques; on the other hand, pol ymer/plastic materials are easily microfabricated using molding techniques and are currently in use for micro-optical applications [3]. However, for certain applications, plastic structures present problems, for example in those where optical components are exposed to strong irradiation, giving rise to degradation of the plastic polymer chains, or in applications where temperature of operation is strongl y var ying, so that incompatibilities between dilatation of plastic structures and glass parts lead to a deterioration of the optical device functioning. Therefore, an attractive option is to develop a molding technology in glass. However, the mechano-chemical inertness of glass makes it hard for microstructuring. The only possible wa y to replicate glass is the direct casting from a liquid solution. Melt molding of glass is not a feasible solution, but chemical processing of a glass using liquid precursors provides a better opportunity to microstructure glass components with desired physico-optical properties. This process, known as the sol-gel process, can be easily modified to prepare a desired glass network. We have realized arra ys of glass micro-optical elements like microlenses, channel gratings, holographic gratings, fresnel lenses using in-house developed sol-gel materials [4,5]. Experimental Two classes of sol-gel materials are prepared: (i) polymeric and (ii) nano-colloidal materials. The principal oxides used are silicon oxide, titanium oxide, and their binar y oxides. Polymeric sol-gel materials consist of sub-micron size linear chains of an oxide network with reactive h ydrox yl groups. Nano-colloidal sol gel materials are composed of nano-sized metal Key Engineering Materials Vols. 264-268 (2004) pp. 371-374 online at http://www.scientific.net © 2004 Trans Tech Public