Triple-Beam Resonant Silicon Force Sensor Based on Piezoelectric Thin Films
- 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)
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.
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 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.
A triple beam resonant force sensor 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.
This work was supported by the Bundesministerium für Forschung und Technologie (BMFT: today BMBF) under contract number 13 AS 0114.
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,
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- DOI: https://doi.org/10.1016/0924-4247(94)80015-4