Christoph Schelling

Next generation pressure sensors in surface micromachining technology

One of the first MEMS products - the pressure sensor - has still room for innovation. We report a completely new pressure sensor generation based on a novel surface micromachining technology. Using porous silicon the membrane fabrication can be monolithically integrated with high synergy in an analog/digital semiconductor process suited for high volume production in an IC-fabrication facility. Only two mask layers and one electrochemical etching step are inserted at the beginning of a standard IC-process to transform the epitaxial silicon layer from the electronic process into a monocrystalline membrane with a vacuum cavity under it.

Monocrystalline Si membranes for pressure sensors fabricated by a novel surface micromachining process using porous silicon

We developed a novel surface micromachining process to fabricate monocrystalline silicon membranes covering a vacuum cavity without any additional sealing steps. Heart of the process is anodic etching of porous silicon, annealing and epitaxial growth. The porous silicon layer consists of two parts, a starting mesoporous silicon layer with low surface porosity and a nanoporous silicon layer with a high porosity. The following annealing step removes native oxide within the later cavity, and the surface is sealed for the subsequent epitaxial layer deposition. The observed stacking fault density in the epitaxial layer about 1E5 cm-2. The temperature budget of the following ASIC-process leads to a complete transformation of the nanoporous silicon layer into a large cavity. The whole structure can be used as a pressure sensor. The estimated pressure in the cavity is smaller than 1 mbar. First integrated pressure sensors have been fabricated using this process. The sensors show a good linearity over the whole pressure range of 200 mbar to 1000 mbar. This novel process has several advantages compared to already published processes. It is a “MEMS first” process, which means that after the epitaxial growth the surface of the wafer is close to a standard wafer surface. Due to full IC compatibility, standard ASIC processes are possible after the fabrication of the membrane. The use of porous silicon enables a high degree of geometrical freedom in the design of membranes compared to standard bulk micromachining (KOH, TMAH). The monocrystalline membranes can be fabricated with surface micromachining without any additional sealing or backside processing steps.

Application of hydrogel-coated microcantilevers as sensing elements for pH

This note reports on cantilever-based sensor elements coated with a hydrogel. The hydrogel responds with a volume change on varying the pH value of surrounding liquids. The change in volume leads to a static deflection of the cantilevers, which is detected using integrated piezoresistors. To increase deflection sensitivity of the sensor elements, sub-micron, multilayered cantilevers consisting of polycrystalline silicon and silicon oxide are used. A new cantilever design is developed, which decreases the cantilever sensitivity to in situ stresses and thermal bimorph effects. A theoretical model for the sensor elements is introduced providing the output signal of multiple cantilevers connected in a full Wheatstone bridge. Measurements of deflection sensitivity prove the theoretical model. Finally, the cantilevers are coated with a 2-hydroxyethyl methacrylate and 2-(dimethylamino) ethyl methacrylate copolymer-based hydrogel, and changes in the pH value from pH 4 to pH 10 are measured.

Low-cost microbolometer with nano-scaled plasmonic absorbers for far infrared thermal imaging applications

We present a scalable low-cost microbolometer technology platform which is based on separate fabrication of MEMS and read-out ASIC CMOS wafers. Mechanical, electrical and hermetical connection is achieved by Cu-based thermocompression bonding. The performance loss due to the resulting backside illumination of the sensor is compensated by an optimized microbolometer design including nano-scaled plasmonic absorbers, a dedicated pixel geometry and the use of highly temperature sensitive devices. The low-cost approach features CMOS compatible MEMS processes, wafer level packaging and uncooled operation of the sensor.