Solder-bonded micromachined capacitive pressure sensors

We report the first solder-bonding of low-cost silicon absolute pressure sensors. The goal of the work is to solder a pressure sensor wafer and a CMOS wafer containing the signal conditioning circuitry together face to face under vacuum. The result is a very flat capacitive absolute pressure sensor which can be used in harsh environments for automotive, medical, barometric, and other applications. We successfully demonstrated this approach using sensor wafers with micromachined silicon membranes and CMOS wafers without signal conditioning circuitry. Solder frames surrounding the membrane are electroplated on the sensor wafer. Different solder materials such as Au/Sn and SnPb were examined. Characterization of the hermetic prototypes in a pressure chamber showed a sensitivity of 0.8 fF/mbar, in good agreement with finite (FE) element simulations. With special regard to the sensitivity of the sensor a quadratic membrane, a rectangular membrane and a square membrane with a boss, all with an area of 2.25 mm(2), were compared using FE analysis. The rectangular membrane has the largest sensitivity.