Preparation of micropatterns with profile heights up to 30 microns from silica sols

Abstract

A new synthesis and processing route for SiO2 glass like micropatterns with heights up to 30 µm by gel embossing and thermal densification has been developed. For this reason an organically modified nanoparticulate sol prepared by acid catalysis of methyl- and phenyl-triethoxysilane and tetraethyl orthosilicate in combination with colloidal silica sol was used. Sol coatings with thicknesses up to 15 µm are obtained by dipping of float glass substrates. After a predrying step of about 60 s micropatterns are obtained using a pressure of only 2.5 mN/mm2. Due to this low pressure, flexible and low cost silicon rubber stampers can be used. The gelation time of the sol can be extended from 5d to 16 days and the working time for embossing can be extended from 60 s up to 100 s by a partial replacement of methyl silane by phenyl silane from 0 to 20 mole %. After embossing and drying at 50°C the patterned layer was densified at temperatures up to 500°C to 95 % density as indicated by refractive index measurements. It is assumed that the densification process is strongly promoted by the used colloidal silica nano particles. The linear shrinkage of the micropatterns is limited to about 25 % due to the high solid content of the sol and the high green density of the layers. Since the structures are densified at temperatures far below Tg sharp edged patterns can be obtained as shown by high resolution secondary electron microscopy. The capability of this technique is demonstrated by the fabrication of light trapping structures with pyramides of 7 µm in height and 10 µm in width on an area of 20 X 20 mm2 and micro lens arrays of lenses with 30 µm in height and 600 µm in diameter on an area of 20 X 30 mm2.