Electrical Admittance Spectroscopy for Piezoelectric MEMS

A simple measurement architecture based on a transimpedance amplifier is demonstrated for the electrical admittance spectroscopy of small-scale transducers. The simplicity and low cost of the measurement system as compared to dedicated network analyzers may make spectroscopy measurements more widely accessible. The approach is adaptable to cover a broad frequency range and can be used for transducers with very small capacitance and high impedance. Measurements are demonstrated on piezoelectric micromachined transducers fabricated on silicon-on-insulator wafers and composed of 20-μm -thick epitaxial beams with 800-nm-thick lead zirconate titanate films along the surface. A complete system identification (ID) procedure based purely on measured phase spectra is also summarized. A unique perspective to the system ID procedure is provided based on poles and zeros of the admittance transfer function and geometry in the complex plane. A rigorous procedure for simultaneously extracting the transducer coupling ratio, undamped natural frequency, and damping ratio is summarized which is particularly useful for lightly coupled sensors, in which case fitting parameters by inspection is challenging and can lead to errors. This system identification approach is summarized on beams with coupling coefficients as small as 0.35%. The system ID procedure further consists of extracting effective e₃₁ coefficients, which, for the sensors in this work, are in the range 8.7-9.3 C/m².