Surface and Bulk-Silicon-Micromachined Optical Displacement Sensor Fabricated With the SwIFT-Lite™ Process

For many micromachined sensors such as microphones and accelerometers, optical displacement sensing may have advantages over capacitive sensing-offering potentially high-displacement sensitivity independent of sensor area and gap height. A particular diffraction-based optical displacement sensor design consisting of a sensing diaphragm suspended over a rigid grated electrode has demonstrated advantages in terms of sensitivity, integration and stability. When illuminated from the backside, this structure generates a zeroth and complementary higher order diffracted beams whose intensities are modulated by the diaphragm displacement with the sensitivity of a regular Michelson interferometer. In previous work, acoustic devices surface micromachined on quartz substrates using aluminum for the diaphragm and grating were presented and characterized. In this work, we present the fabrication and characterization of a surface- and bulk-silicon micromachined diffraction-based optical microphone structure fabricated with Sandia National Laboratories' SwIFT-Litetrade process, which uses silicon nitride and polysilicon structural materials that have been employed extensively in MEMS and form a more thermally matched material set robust against corrosion and fatigue. The process introduces a new design space to microscale optical displacement sensing, enabling large, soft structures with perforated back-plates ideal for microphone and inertial-sensor designs