Triple-Beam Resonant Silicon Force Sensor

Based on Piezoelectric Thin Films

 

Authors

 

Journal

• Sensors and Actuators A: Physical
• Volume 42, Issues 1–3, 15 April 1994, Pages 375-380
• Also published in Proceedings of Eurosensors VIII, Budapest (1993)

Abstract

A resonant force sensor with semidigital frequency output based on a triple-beam resonator structure in silicon is presented. The piezoelectric excitation of the resonator operating in an antisymmetric vibration mode is realized by thin-film zinc oxide layers. An advantage of this triple-beam design is a high mechanical quality factor combined with high force sensitivity due to the antisymmetric vibration mode and the load-dependent stress concentration in the triple beam. Extensive finite-element modelling has been carried out to determine the static and dynamic behaviour and to obtain optimum sensor performance. Several resonator designs have been fabricated to study mechanical decoupling from the clamping region. Experimental characterization of the triple-beam sensor and of the zinc oxide thin-film properties has been performed.

Introduction

Resonant quartz and silicon sensors exhibit a wide field of appli­ca­tions and special benefits like large gauge factors, high resolu­tion and a semi-digital frequency output [Lan85, Til92a, Ste91]. The resonant force sensor described here is based on the principle that a resonator excited in a specific flexural vibra­tion mode will change its resonance frequency when mechani­cal stress is applied.

One key aspect in the design of resonant sen­sor devices is a high mechanical quality factor leading to high resolution and sensitivity. This can be achieved by placing the resonator in an evacuated cavity [Guc90, Ike90] and/or by using a special resonator design [Gre88, Ste90]. Disadvantages of vacuum encap­sulated resonator designs are the quite com­plicated fabrica­tion technolo­gy and additionally arising packaging problems.

Bulk force sensor devices fabricated in monocrystalline quartz normally use a double‑­ended‑tuning‑fork (DETF) design in order to balance the in-plane motions of the tuning fork tines and cancel out moments which are responsible for losses into the reso­nator mount [Eer88]. Though quartz is piezoelec­tric there is an easy way to excite and detect the vibration of the resonator and to operate the quartz force sensor as a mechanical one port in a feedback loop of an electrical oscilla­tor circuit. The fact that silicon is non-piezoelec­tric leads to the necessity of additional piezoelectrical thin film layers to excite the beams [Mul91].

 

In this work we used rf-sputtered zinc oxide (ZnO) thin films. Due to the induced bending moments of the silicon-ZnO-bimorph the vibrations are out of plane. To achieve a cancella­tion of moments and shear forces at the clamped ends obviously more than two beams are necessary. A resonator compri­sing a triple beam (TBTF: triple-beam tuning fork) or a quadruple beam structure, described in [Kir83, Sat89] and [Til92b], are well suited for this purpose.

In this paper we report on the realisa­tion of piezoelectrically driven resonant silicon force sensors with triple beam, where the cen­tral beam has twice the width of the outer two beams and vibrates in antiphase with them.

 

Conclusions

A triple beam resonant force sensor (TBFT: triple-beam tuning fork) with piezoelectric excitation was fabricated. Experimental characterization by means of optical and elec­tri­cal measure­ment techniques were performed. The presen­ted resonator configu­ration is capable to provide a sufficient mechanical isolation from the support by means of a flexibel de­coup­ling region and reveals a better mode selectivi­ty at the same time. Especially in combina­tion with vacuum encapsula­tion and surface micromachining, the triple beam resonator provides in­trin­sic advantages in comparison to single beam resonator designs. The benefits of the presented resonator design seems to be a promising approach for future resonant sensor applications.

 

Handbook of Force Transducers

Chapter :Resonator Force Transducers, Characteristics and Applications by Dr. Dan Mihai Ştefănescu (2020)

Source: Link.springer.com/chapter/10.1007/978-3-642-18296-9_12

 

Acknowledgements

This work was supported by the Bundesministerium für Forschung und Technologie (BMFT: today BMBF) under contract number 13 AS 0114.

• The cooperation with Dr. Gottfried Flik and Dr. Franz Lärmer (Robert Bosch GmbH, Research & development. Zentrale Forschung und Entwicklung, ZWD Gerlingen) is greatly appreciated.
• The authors would like to thank Dr. J.M. Olaf and Dipl.-Ing. W. Pfeiffer (FhG-IWM, Freiburg) for performing the indenter and rocking curve measure­ments on the zinc oxide probes.
• The authors are grateful for the technological support of Dipl.-Ing. M. Ashauer, Dipl.-Phys. W.H. Bach, Dipl.-Ing. R.-W. Gerdau and Dr. M.A.E. Wandt,

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

References

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GitHub repositories

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

Publication

• Sensors and Actuators A 41-42 (1994) 375-380
• DOI: https://doi.org/10.1016/0924-4247(94)80015-4

 

MEMS bibiography

For more MEMS bibliography of the doctoral thesis as of December 1994 have a look at MEMS literature: tfconsult.com/mems-literature

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MEMS Literature