Mechanical and electrical characterization of resonant piezoelectric microbridges for strain sensing

Abstract

This work reports on a resonantly operated piezoelectric MEMS strain sensor based on a microbridge structure. The resonator is excited with a sputtered aluminum nitride layer and was fabricated with standard CMOS processing techniques. The mechanical frequency spectra and the mode shapes were studied with a laser Doppler vibrometer. Additionally, an electrical read out was realized by measuring the conductance spectra with an impedance analyzer. Several sensor devices varying in width have been fabricated and the differences in the resonance frequency, the mechanical displacement and the electrical conductance spectra have been evaluated, with a focus on the first two Euler-Bernoulli modes. The buckling behavior of the devices under high compressive external strain was analyzed. Furthermore, the mode veering phenomenon could be observed and is analyzed in detail. To classify this new type of strain sensor a frequency-dependent gauge factor has been defined. It shows that the sensor devices exhibit exceptionally high gauge factors, one to two orders of magnitude higher than conventional strain gauges based on the piezoresistive effect.