FABRICATION OF GLASS MICROSTRUCTURES BY MICRO-MOLDING OF SOL-GEL PRECURSORS

This paper demonstrates that the molding of a sol-gel precursor against an elastomeric replica of the desired features (a form of soft lithography) is a convenient method to generate submicrometer patterns of glasses on a substrate. We were able to fabricate glass (silicon dioxide doped with boron oxide, titanium oxide, or aluminum oxide) structures with micrometer-scale features supported on a flat Si/SiO2 substrate, as well as free-standing membranes. In molding, an initially fluid material is allowed to acquire its final geometry by solidifying in a mold. This technique allows the reproduction of the fine details of the mold: replica molding of structures in polymers has generated structures with 10 nm sized features. The molding of sol-gel precursor solutions has produced monolithic silica pieces as well as Fresnel lenses or gratings with sub-micrometer periods. Replica molding is a method that has several potentially useful features. It does not require photolithography and can be used without access to a clean room. Once the mold is fabricated, many replicas can be produced. It has a theoretical limit to resolution that is much below that that can be achieved by photolithography. It can be used to make patterns on curved substrates. The throughput of a process based on molding can be high and its cost low. Unlike electron-beam or scanning tunneling microscopy (STM) writing, molding allows parallel fabrication. We believe that micro-molding is a type of process that will be widely useful in microfabrication. The sol-gel process is a versatile method for synthesizing many inorganic oxides. This method generates materials with controlled chemical composition and low levels of impurities. Most common glasses, with the exception of some halide glasses, have been successfully synthesized; these compositions range from high purity silica to an eight-component glass ceramic. Other materials that have been prepared by sol-gel process include PZT ceramics (i.e., Pb(Zr,Ti)O3), [7,8] electro-optic films, high efficiency phosphors, and electrochromic glasses. The use of sol-gel chemistry to prepare materials has one unattractive characteristic: the shrinkage induced in the drying stage gives rise to high stresses in the structures. These stresses can cause the deformation and breaking of the structures. In order to reduce these stresses, drying additives can be used. The incorporation of non-hydrolyzing organic groups in the material (methyl or phenyl) gives structures with higher compliance and allows structural relaxation during the drying stage. Controlled slow drying can also decrease the risk of cracking of the glass; it will, however, result in long process times. Good adhesion of the glass to the substrate is necessary to prevent the delamination of the structure. We wished to fabricate glass structures with dimensions in the range of 0.1 mm to several micrometers by non-lithographic methods, and have examined the molding of solgels in an elastomeric mold. This paper first summarizes this method and then shows and characterizes different geometries of structures that we were able to fabricate. General Procedure: The reaction involved in the gelation of the precursor solution (sol) is a two-step hydrolysis and polycondensation of tetramethylorthosilicate (TMOS), the main constituent of all glasses studied. Other alkoxides react similarly. In the case of molding between a substrate and a patterned elastomer, a drop of precursor was put on the freshly cleaned substrate and the mold was then placed face down on the surface (Fig. 1). A pressure of roughly 10 psi (ca. 69 kN m, i.e., ca. 0.7 atm) was applied and the area of the patterned surface was typically 1±5 cm with feature sizes in the micrometer range. It is important that the liquid dewets the surface and allows contact of the elastomer and the substrate in regions where no sol-gel±derived material is desired. The dewetting is driven by both the applied pressure and the difference of the interfacial tensions of the precursor and the stamp. More precisely, the dewetting speed is proportional to S, where S = gLS + gLE ± gSE; gLS is the liquid±substrate interfacial tension, gLE is the liquid±elastomer interfacial tension, and gSE is the substrate±elastomer interfacial tension. Since gSE is fixed, we had to increase the interfacial tensions of the precursor solution in order to accelerate the dewetting. Although pressure improves the definition of the features, it cannot be increased too much because of the deformations induced in the mold. Diluting the precursor in acetonitrile, a polar solvent with low viscosity and high surface tension (g = 29.3 mJ m) that does not swell the mold, allowed us to obtain satisfactory dewetting for all the patterns that we