We report a new surface micromachining technology to fabricate monocrystalline silicon membranes covering a vacuum cavity for applications like piezoresistive pressure sensors. The main process steps are: (i) local anodic etching of layered porous silicon with different porosities, (ii) tbermal rearrTgement of the porous silicon, and (iii) epitaxial growth of the silicon membrane layer. In contrast to conventional bulk micromachining the new technology has the benefit of a considerable freedom in the design of monocrystalline silicon membranes. The membrane geometry is only determined by the porous region. Further, the new fabrication method is fully CMOS compatible. In fact, except for anodic etching, all process steps are part of a standard mixed signal IC production line. Various aspects of the used key process steps are discussed, particularly with regard to the oxygen and fluorine desorption during the porous silicon annealing. A piezoresistive pressure sensor with integrated ASIC based on the new fabrication method is demonstrated.
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 analogidigital semiconductor process suited for high volume production in an IC-fah. Only two mask tayers and one electrochemical ctching 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. Fig. 2: Pressure sensor membmne with vucuum cavity under it and implantedpiezzo resistors bn top.FABRICATION Fig, 1: Mtnihmnefabricatton iechnolo,y: (a) conventional hulk niicromachinirg wifh KUH etching and glass bonding to fabricate a membrnne over a 1-efirence vacuum. (b) using porous silicon technology a monoc ysiallinr membrarre i s jix-ied bj? eppita.xiul layer growth. The porous silicon is converted in a vucuiim caviy by sintering in H2 ntmosphere.The epitaxial layer is dcposited on a porous silicon layer. The cavity is formed by subsequent thermal rearrangement of this porous silicon during the epitaxial growth process and thc following high temperature difision processes [I]. The enclosed hydrogen in the cavity during the epitaxial deposition diffuses out, but no other gases can diffuse in. This leads to a good reference vacuum in thc cavity. The square membrane is deflected by 0.7 pm / bar. It is mcchanically robust against overload because of the ''floor stopper" when it touches the bottom of the cavity. A 1 bar sensor is overload proof to more than 60 bar. The signal is sensed by a piezoresistive Wheatstone bridge, which is a measurement principle with better linearity compared to capacitive sensors. The piezoresistors are fabricated using the same diffusion layers as the electronic circuit to achieve good matching properties. The bridge signal of 55 mV / bar is ampIified by the surrounding electronics. The mixedsignal semiconductor process is suited for the automotive temperature range of -40 .. +I40 "C. The circuit is designed to be ESD and EMC proof. TRANSDUCERS'OSThe 13th lnremational ConCerencc on Solid-State Sensors, Actuators and Microsystems, Seoul, Korea, June 5-9,2005 0-7803-8952-2/0.5/$20.00 0 2 0 0 s IEEE. 35
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.
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