Piezoelectrical driven resonant force sensor:
fabrication and crosstalk
This paper presents a resonant force sensor comprising piezoelectric ZnO thin-film transducers for excitation and detection of resonant beam vibrations. A short description of the processing technique is given, i.e. deposition and passivation of the ZnO layer and separation of beam structures. The electrical behaviour of the sensor was optimized by patterning ZnO areas to minimize electrical crosstalk effects.
We introduce resonant sensors with semidigital frequency output comprising piezoelectric ZnO thin films for excitation and detection of the beam resonators. Special interest was given to the technological process integration, i.e. passivation of the ZnO layer and separation of the beam structures. An elaborate sensor design was achieved by structuring of the electrodes to optimize mode selectivity and minimizing electrical cross-talk effects.
The fabrication process of the sensor devices consists of p++-doping and backside wet ecthing of the membranes in <100>-silicon substrate. The piezoelectric ZnO transducer layer is deposited by rf-sputtering and subsequently wet etched. Top electrodes were realized by Al-metalization and following passivation. The resonator beams of the force sensor are separated by plasma etching techniques, i.e. propagation ion etching (PIE) with high selectivity and high etch rates up to 10 μm/min in silicon.
Finite Element Modeling
Extensive finite element modeling has been carried out to determine the static and dynamic behavior and to obtain an optimum sensor performance. Considering the piezoelectric excitation the effective electromechanical coupling factor and the impedance/phase-characteristics of the sensor devices were calculated leading to an optimium thickness of the ZnO layer.
The design and fabrication cycles of sensor devices can be considerably reduced and time-economized when computer simulation is used. Moreover, modeling provides a unique insight into the functioning of resonant microsensors. Numerical methods like finite element analyses (FEA) are powerful tools to model the dynamical response of resonant sensors and to predict how the sensor device will perform under changed operating conditions. The static and dynamic behaviour of bulk resonant force and pressure sensors were calculated with the general-purpose finite element code ANSYS in order to optimize the sensor performance.
Comparison of numerically calculated and optically measured flexure vibration mode shapes of a beam-like silicon force sensor. The silicon beam was excited and detected piezoelecrically by sputtered ZnO thin-film layers.
Besides mechanical calculations ANSYS allows also for coupled field analyses taking thermal-structural and piezoelectric interactions into account. Modal analyses were used to solve the eigenvalue problem by which the resonance frequencies and flexure vibration mode shapes of beams and diaphragms are determined. Combination of static and subsequent modal analysis allows to calculate the load-dependent frequency shifts of resonant sensors. Utilizing this technique the pressure-deflection and pressure-frequency characteristics of silicon diaphragm pressure sensors were investigated and optimized (PhD-Thesis).
Resonant force sensor
The resonant sensors have been experimentally characterized by means of optical and electrical measurement techniques and show a good mode selectivity, high force and pressure sensitivities combined with a low temperature cross-sensitivity. Investigations of the long term stability due to load and temperature cycles has been carried out successfully.
This work was supported by the Bundesministerium für Forschung und Technologie (today: BMBF) under contract number 13 AS 0114 / 0118. The authors would like to thank Prof. Dr.rer.nat. Stephanus Büttgenbach for assistance and project coordination.
(C) Block diagram of resonant sensor taken from “Resonant Microsensors: Fundamentals and State of the Art” by M.J. Tudor and S.P. Beeby, University of Southampton, Institute of Transducer Technology (USITT), Southampton Hampshire, England (1995)