In this thesis, the design and fabrication of a bulk micromachined and wafer bonded pressure sensor for high temperature applications is described. The device design is based on the magnetic coupling principle as described by the Biot-Savart law. By combining the mechanical properties of single crystal silicon with magnetic coupling, the designed sensor can be operated up to 600°C. The key components within the sensor are two inductive coils, a silicon diaphragm and a hermetic vacuum cavity.
The modeling based on a nine-turn single level coil device and a 300 μm x 300 diaphragm indicates an output rms voltage range of 70 mV with an input current of 100 mA and frequency of 200 MHz at pressures ranging from 0 kPa to 300 kPa for a sensitivity of 11 μV/mA.MHzkPa at 300°C. The output voltage doubles to 150 mV at 600 °C for the same pressure range. Experiments on 6-turn single-level aluminum foil coils showed a linear decrease in output with the reduction in coil dimensions as the Young\u27s modulus decreases. Experiments indicate that double-level or multi-level coils give substantially larger output.
The sensor fabrication plan combines standard IC processing, anistropic etch of silicon and silicon wafer bonding. A KOH solution is used to etch the silicon and define the diaphragm. The diaphragm is formed by a boron diffusion technique. The diaphragm thickness is controlled by the diffusion depth and etch-stop technology. The silicon wafer bonding uses sputtered Pyrex as an intermediate adhesive layer. Pyrex has good thermal expansion of coefficient with that of silicon. This would ensure a good thermal match between the silicon and glass together with a good thermal stability at high temperatures
