Holger Loewe

Low temperature catalytic combustion of propane over Pt-based catalyst with inverse opal microstructure in a microchannel reactor

A novel Pt-based catalyst with highly regular, periodic inverse opal microstructure was fabricated in a microchannel reactor, and catalytic testing revealed excellent conversion and stable activity for propane combustion at low temperatures.

Decision support towards agile eco-design of microreaction processes by accompanying (simplified) life cycle assessment

Continuously running syntheses in microstructured reactors offers novel ways to intensify conventional chemical processes. An outstanding advantage of microreaction technology is the high surface-to-volume-ratio which enables intensive mixing phenomena as well as high mass and heat transfer rates. Thus, microstructured reactors may be a suitable means to improve multiphase reactions by increasing the interfacial area and the intensification of internal mixing. This improvement in reaction performances may lead to reduced environmental burdens of the process under consideration. The method of simplified life cycle assessment (SLCA) is a suitable tool to evaluate the environmental burdens caused by chemical processes. It has been applied already in research and development to identify the key parameters for a deliberate green process design of two biphasic reactions, the esterification of phenol and benzoyl chloride resulting in phenyl benzoate and the synthesis of one of the corresponding phase transfer catalysts, [BMIM]Cl. Further, SLCA is complemented by a simple cost estimation to investigate the main cost drivers relevant for possible industrial application of the syntheses investigated.

Contactless embossing of microlenses: a new technology for manufacturing refractive microlenses

Contactless embossing of microlenses (CEM) is a new fabrication technique for the production of refractive microlens arrays. The basic idea is that the surface of the microlenses has no contact with the compression molding tool during the shaping of the surface relief. A high precision matrix of holes made by LIGA microfabrication is used as a compression molding tool. This tool is pressed onto a thermoplastic sample which is heated close to the materials transformation temperature. The material bulges into the openings of the molding tool due to the applied pressure. It process conditions are properly set, the material forms lens-like spherical structures. Microlenses and arrays of microlenses with lens diameters between 30 micrometers and 500 micrometers have been fabricated in thermoplastic material. Besides highly accurate microlens arrays, CEM also provides the potential of cost-effective production and high precision mounting concepts.

Refractive microlens arrays made by contactless embossing

Contactless embossing of microlenses (CEM) with LIGA molding tools is a new fabrication techniques for the production of refractive microlens arrays which combines high accuracy in the micrometer range, cost-effective production of the devices, and cost-effective high precision mounting concepts. The name refers to the fact that the surface of the microlenses has no contact with the embossing die during the shaping of the surface relief. A high precision matrix of holes made by LIGA microfabrication is pressed onto a thermoplastic sample which is heated. The material bulges into the openings of the molding tool due to the applied pressure and forms lens-like spherical structures. The embossing die touches the lens material only outside the lens area. High-speed microlenses with f/ < f/4 and diameters of the lens aperture between 30 micrometers and 500 micrometers have been fabricated in PMMA and PC. Excellent uniformity within the microlens arrays are achieved by using LIGA microfabricated embossing dies. In addition to the excellent optical performance of the microlenses, the CEM method assists hybrid integration in micro-opto-electro- mechanical systems by providing precise auxiliary structures for easy and cost-effective mounting and adjusting.

Achieving mass fabrication of microoptical systems by combining deep-x-ray lithography, electroforming, micromolding, and embossing

The paper reviews the application of deep X-ray lithography in conjunction with electroforming, plastic molding, and stamping (LIGA) for a mass production of micro- and submicron-structured photonic devices. LIGA technology offers almost total design freedom in lateral structuring and a high aspect ratio of 100. Vertical walls with heights up to 1 mm and optical grade surfaces enable their use as functional optical surfaces. It is possible to process a variety of materials such as metals and different polymers, including those with nonlinear optical properties. Therefore, Y-branches, couplers, and structures for the positioning and fixing of fibers, detectors, and light emitters can be integrated on one chip to build up hybrid optoelectronic devices.

Highly sensitive resist material for deep x-ray lithography

The present paper describes the first chemically amplified negative-tone resist for deep x-ray lithography (DXRL). The choice of the resist material for this new resist has been oriented on the experience of the photo, electron beam and x- ray lithography (XRL) for microelectronic applications. In this work a negative tone resist containing a novolak, a crosslinker and an acid generator was developed by varying the different components. It was found that only few components, which proved to be good in thin films, were suitable for DXRL. The new resist fulfills all technological requirements and shows an increased sensitivity by a factor 15 as compared to the standard resist material, poly(methyl methacrylate). This tremendous increase in sensitivity leads to a huge cost reduction of the DXRL process. Furthermore, an excellent adhesion of this new resist to metallic substrates has been achieved which allows us to fabricate free standing columns with an aspect ratio of 80.