Thomas Schaller

Fabrication of microlenses by combining silicon technology, mechanical micromachining and plastic molding

Silicon can be subjected to plasmaless isotropic etching in mixtures of elemental bromine and fluorine. BrF3 is generated in the etching process. This ensures a high etching rate on smooth surfaces. The addition of noble gases, e.g. xenon, allows extremely smooth surfaces to be etched. Thermally oxidized SiO2 layers are applied as the etching mask. Among other applications, this technique can be used to manufacture microlenses. As a consequence of the complete isotropy of the etching process, spherical depressions of 100 to 500 micrometers in diameter are produced in the silicon when small circular holes of 5 to 50 micrometers are underetched in the SiO2 mask. After removal of the SiO2 mask the silicon sample can be used as a mold insert for plastic molding. The molded microlenses have been checked dimensionally and verified optically. The microlenses are planned for technical use in a miniaturized endoscope. This requires further processing of the silicon sample. As no hemispherical recesses but calotte shells are needed, the silicon surface must be machine prior to molding. This is done by microgrinding with variable-grain diamond tools on CNC high- precision machines. To generate adjusting devices, stoppers, and holding structures, the ground silicon sample and a mechanically microstructured perforated plate are combined in a modular multi-level mold insert. The microlenses molded by hot embossing or injection molding are separated mechanically. They can then be integrated in the endoscope with a holding unit manufactured independently.

Large area polymer replication for microfluidic devices

A huge market development is expected for modern drug discovery and genomic analysis when rapid parallel analysis of a large number of samples gets available at affordable costs. The state of the art shows that low cost devices can be fabricated in mass production by micromolding of polymers. In close collaboration, Greiner Bio-One and Forschungszentrum Karlsruhe have developed a single-use plastic microfluidic capillary electrophoresis (CE) array in the standardized microplate footprint. This paper presents the results of experiences which show that hot embossing with a mechanically micromachined molding tool is the appropriate technology for low cost mass fabrication. A subsequent sealing of the microchannels allows sub-microliter sample volumes in 96- channel multiplexed microstructures.