P. Schwarz

Quality-factor amplification in piezoelectric MEMS resonators applying an all-electrical feedback loop

An all-electrical velocity feedback control to enhance the quality factor of piezoelectric aluminium nitride (AlN)-based microcantilevers and microbridges was implemented. Two alternatives to obtain a velocity-proportional signal were demonstrated depending on the top electrode configuration. For a straightforward electrode design in one-port configuration (i.e. self-actuation and self-sensing), a velocity signal, proportional to the piezoelectric current, was used in the feedback loop by cancelling out the dielectric current electronically. For top electrodes allowing a two-port configuration (i.e. one for actuation and one for sensing), the piezoelectric current is directly extracted and its relationship with velocity is analysed taking the symmetry of the modal shape into account. Standard operational amplifier-based configurations for the feedback circuits were implemented on a printed circuit board. Quality factors were determined from the transient electrical response of the devices. Comparable results were obtained from the displacement spectrum applying a laser Doppler vibrometer. Quality factors as high as 2 × 105, corresponding to an enhancement factor of about 200, were achieved in air for the lowest gain margin achievable before the circuit becomes unstable, making this kind of device more competitive for mass sensor applications due to enhanced spectral resolution.

Electrical crosstalk in two-port piezoelectric resonators and compensation solutions

Crosstalk is an impediment to electrically interfaced two-port resonators. The overall output function of two-port piezoelectric resonator is a superposition of the mechanical resonance behavior and electrical crosstalk, the latter coming mainly from the coupling feedthrough capacitance. In this paper, two crosstalk compensation solutions have been developed for an aluminum nitride-based doubly clamped beam resonator. The first solution demonstrates an on-chip self-cancellation technique of the feedthrough capacitance by using a compensation electrode and applying a complementary voltage to it, while the second solution applies an adjustable compensation voltage to the common bottom electrode. A specifically designed compensation-readout circuit is presented. Experimental investigations of the output signal have proved the efficiency of both crosstalk compensation solutions.

Study on the lower resolution limit and the temperature-dependent performance of a surface micromachined gyroscope

This paper reports on the characterization of a novel micromachined angular rate sensor with a single point mechanical suspension targeted for automotive applications. In detail, the lower resolution limit of the sensor element was determined at application-relevant ambient pressure levels ranging from 1 to 10 mbar. Furthermore, the temperature-dependent performance of the gyroscope was measured at an ambient pressure of 3 mbar which was found to be an optimized value for achieving high performance at acceptable excitation voltage levels. The temperature coefficients of sensitivity (TCS) and frequency (TCF) were determined experimentally in a custom-built measurement set-up.

Decoupled Surface Micromachined Gyroscope With Single-Point Suspension

This paper focuses on the performance of a new micromechanical gyroscope for automotive and consumer applications. Its most characteristic properties are a single-point mechanical suspension, seismic masses vibrating in antiphase tuning fork motion, as well as the spatial separation of the drive oscillator and the sense oscillator for minimizing electromechanical crosstalk between drive mode and sense mode. In detail, nonlinearities in the damping behavior of the drive unit were measured and analyzed theoretically. They can be attributed to fringe field effects in the comb drive unit. New design rules were defined to overcome this effect. Furthermore, an ambient pressure level of 1 mbar was found to yield optimum sensitivity of the sensor element for a given excitation amplitude of the drive unit. Further reduction of pressure does not improve the results. The temperature-dependent performance of the gyroscope was measured at an ambient pressure of 3 mbar. The temperature coefficient of frequency was measured to be -45.3 ppm/K for the drive mode (bending) and -35.5 ppm/K for the sense mode (torsional). The temperature coefficient of sensitivity was determined to be -858 ppm/K in the case of operating with constant drive amplitude. This value could be reduced to 17.5 ppm/K over the full observed temperature range by adapting the drive amplitude as a linear function of temperature in an optimal way. The resolution limit of the sensor element was found to be about 0.08°/ s/√{Hz} at application-relevant ambient pressure levels ranging from 1 to 10 mbar.

Air damping of micro bridge resonator vibrating close to a surface with a moderate distance

The vibration of micro resonators is strongly influenced by the hydrodynamics of the surrounding fluid in the vicinity of a rigid wall. While most prior efforts to model this hydrodynamic loading have focused on squeeze film damping with very narrow gaps, in many practical applications, the resonators vibrate close to a surface with a moderate distance. Two recently developed models which deal with this problem are reviewed. Experiments by using a micro bridge resonator with a big range of gaps are performed at controlled gas pressures, and are compared with predictions from these theoretical models. The unsteady Navier–Stokes model yields the best agreement with experiments.

AlN micromechanical radial-contour disc resonator

Micromechanical resonators that have high performance and are small in size are competitive candidates for radio frequency (RF) device minimization, paving the way for high performance monolithic transceivers. A piezoelectric aluminium nitride radial-contour mode disc resonator with a CMOS-compatible fabrication process is presented. The piezoelectric properties of the resonator disc, concerning the driving voltage and resonance mode deformation, are analysed, revealing the advantages introduced when using a sputter deposited AlN thin film. The radial-contour resonance mode of the disc resonator was investigated by finite element method simulation and theoretical evaluation. The resonance frequency was found to be thickness-independent, which is beneficial for fabrication and integration. In view of CMOS compatibility, a fabrication process with a low thermal budget and a tungsten titanium sacrificial layer was designed; devices were fabricated on 4 inch silicon wafers. The RF performance of the resonators with a diameter of 150 µm was measured using a HP4395A network analyzer, yielding a resonant frequency of 14.11 MHz with a Q value of 3125 and a return loss of 31.46 dB. These results indicate that this device is attractive for use in RF frequency-selecting and generation devices in high performance wireless communication systems.