Open loop and servo-controlled micromachined capacitive pressure sensors.

摘要

A sealed capacitive pressure sensor with a novel structural design is proposed. In this design, a skirt extends outward from the periphery of the vacuum-sealed cavity, acting as the electrode that deflects under pressure. The stationary electrode is metal patterned on the substrate below this skirt. This approach permits the device to be fabricated in just three masking steps, because there is no need for lead transfer from the inside of the cavity to the outside. Also, this structure can be operated in a servo-controlled mode by using electrostatic force to oppose the pressure-induced deflection of the skirt electrode. Compared to past efforts on servo-controlled pressure sensors, the proposed device has an important advantage, because it does not need an extra element on the top of the diaphragm to actuate the deflected diaphragm.; To evaluate the device concept, the structure of the pressure sensor is analyzed with FEA. The analysis for open loop operation includes displacement of diaphragm and electrode, temperature coefficients, sensitivity of the device. For closed loop operation, restoring bias voltage, and the relation between the capacitance and bias voltage are analyzed with FEA.; The proposed pressure sensor is fabricated by using the dissolved wafer process. Particular means have been developed to permit lead transfer, and enhance fabrication yield. The dissolved wafer process provides well-controlled material properties and residual stress, which are important to mechanical sensors, such as pressure sensors, and is compatible with batch-fabrication. The fabrication results show that the dissolved wafer process is suitable to fabricate the pressure sensor.; The fabricated pressure sensors are sealed within the metal package which is filled with an inert dielectric liquid. The inert liquid improves the pressure sensor performance in open and closed servo-controlled operation. The high dielectric constant of the liquid improves sensitivity in open loop operation, and reduces the operating voltage in servo-controlled operation, which is helpful for compatibility with electronic interfaces or integration. Also, the measurement noise and mechanical noise noises are theoretically reduced due to the high dielectric constant and the high viscosity of the inert liquid.