Design and development of PVDF-based MEMS hydrophone and accelerometer

It is always desirable to fabricate low-cost, highly sensitive and miniaturized sensors for various applications. In this thesis, the design and processing of PVDF-based MEMS hydrophones and accelerometers have been investigated.; The basic structure of the hydrophone was fabricated on a silicon wafer using standard NMOS process technology. A MOSFET with extended gate electrode was designed as the interface circuit to a sensing material, which is a piezoelectric polymer, polyvinylidene difluoride (PVDF). Acoustic impedance possessed by this piezoelectric material provides a reasonable match to that of water, which makes it very attractive for underwater applications. The electrical signal generated by the PVDF film was directly coupled to the gate of the MOSFET. In order to minimize the parasitic capacitance underneath the PVDF film and hence improve the device sensitivity, a thick photoresist, SU-8, was first employed as the dielectric layer under the extended gate electrode. For underwater operation, the hydrophone was encapsulated by a waterproof Rho-C rubber. However, it was found that the rubber induced the degradation of the MOSFET. To improve the reliability of the hydrophone, the active device was passivated by a silicon nitride layer, which is a good barrier material to most mobile ions and solvents. The device after passivation also shows a lower noise level. A theoretical model was developed to predict the sensitivity of the hydrophone. A reasonable agreement between the theoretical and experimental results was obtained.; MEMS accelerometers based on the PVDF-MOSFET structure by attaching a seismic mass on top of the PVDF film were also fabricated. The accelerometer was calibrated using a comparison method and an average sensitivity of 0.28 mV/g was achieved. A dynamic model of the accelerometer was derived and the calculated results are in good agreement with the measured results.