Article from Journal of Micromechanics and Microengineering, Vol. 5, No. 2 (1995) 143-146, pesented at The Fifth European Workshop on Micromechanics (MME ’94), 5-6 September 1994, Pisa, Italy:

 

Piezoelectrical driven resonant force sensor:
fabrication and crosstalk

by K. Funk, T. Fabula*, G. Flik, F. Lärmer

 

Robert Bosch GmbH, Postfach 10 60 50, D-70049 Stuttgart, Germany

*HSG-IMIT, Wilhelm-Schickard-Str. 10, D-78052 Villingen-Schwenningen, Germany

 

Abstract

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.

 

Introduction

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.

Technology

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.

(C) Electronics: 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)

 

DOI

Link: https://doi.org/10.1088/0960-1317/5/2/022

 

GitHub repositories

• github.com/ThomasFabula/Modeling-of-Resonant-Silicon-Microsensors
• github.com/ThomasFabula/piezoelectric_simulation
• github.com/ThomasFabula/ANSYS_MEMS

 

References

1. G. Stemme, Resonant silicon sensors, J. Micromech. Microeng. 1(1991), p. 113-125
2. H.A.C. Tilmans, M. Elwenspoek and J.H.J. Fluitman, Micro resonant force gauges, Sensors and Actuators A, 30, no. 1-2 (1992) 35-53
3. C.J. van Mullem, H.A.C.Tilmans, A.J. Mouthaan and J.H.J. Fluitman, Electrical cross-talk in two-port resonators – the fresonant silicon beam force sensor, Sensors and Actuators A, 31 (1992) 168-173
4. F.R. Blom, D.J. Yntema, F.C.M. van den Pol, M. Elwenspoek and Th.J.A. Popma, Thin-film ZnO as micromechanical actuator at low frequencies, Sensors and Actuators A, 21-23 (1990) p. 226-228
5. F.R. Blom, F.C.M. van den Pol, G. Bauhuis and Th.J.A. Popma, R.F.-planar magnetron sputtered ZnO-films – part II: electrical properties, Thin Solid Film 204 (1991), p. 356-376
6. C.J. van Mullem, F.R. Blom, J.H.J. Fluitman and M. Elwenspoek, Piezoelectrically driven silicon beam force sensor, Sensors and Actuators A, 25-27 (1991) p. 379-383

 

Acknowledgements

This work was supported by the Bundesministerium für Forschung und Technolo­gie (former BMFT, today BMBF) under contract number 13 AS 0114 / 0118. The authors would like to thank Prof. Dr.rer.nat. Stephanus Büttgenbach (TU Braunschweig) for assistance and project coordination.

 

Testimonial

“Dr. Fabula gave us valuable support in the application-specific optimization of MEMS sensors. I value Dr. Fabula as a competent partner whose initiative, drive and wealth of ideas always excelled in the course of our many years of collaboration.” ~ Dr. Franz Lärmer, Project manager MEMS, Robert Bosch GmbH

Dr.rer.nat. Franz Lärmer | Robert Bosch GmbH, Gerlingen