Procedia Chemistry Procedia Chemistry 1 (2009) 540–543
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Proceedings of the Eurosensors XXIII conference
Piezoelectric Tunable Resonant Microsensors for High Resolution and Ghost-Suppressive Ultrasonic Measurement Kaoru Yamashita*, Kenji Tomiyama, Keita Yoshikawa, Minoru Noda Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585, Japan
Abstract
A phased array measurement technique is proposed for high resolution and ghost-suppressive measurement based on a synthesized directivity pattern using multiple frequencies. Tunable resonant-type piezoelectric ultrasonic microsensors have been developed for the measurement technique. The ultrasonic sensor consists of a silicon-micromachined diaphragm structure and a piezoelectric PZT (Pb(Zr,Ti)O3) capacitor on it. The capacitor has inner and outer top electrodes; the inner electrode for ultrasonic sensing, and the outer electrode for resonant frequency tuning through converse-piezoelectrically induced stress by applying external dc voltage. Two kinds of fabrication process have been developed to make flat and deflected diaphragms. Resonant frequency has increased nearly twice on the flat diaphragm and decreased to a half on the deflected diaphragm under a bias voltage application in the range of ±8 V. Keywards: Ultrasonic sensor, Piezoelectric stress, Frequency tuning, High resolution, Ghost suppression
1. Introduction In phased array measurement, a higher angular resolution is achieved with a larger array size but a too wide interelement spacing causes a ghost [1]. That is, a large number of array elements is inevitably required for a high resolution and ghost-suppressive measurement. This restriction is a severe issue to microsensors because of their size limitation for applications and complicated wiring to many sensor elements especially in a two dimensional array. The authors propose a measurement technique which realizes both high resolution and ghost suppression under a limited array size and a limited number of sensor elements, and have developed piezoelectric ultrasonic microsensors for the measurement technique. This measurement technique is based on a synthesized directivity pattern using multiple frequencies, and the developed ultrasonic sensors can be tuned in their resonant frequency by using converse piezoelectric effect, that is, the individual sensor can detect ultrasound in multiple frequencies. In this paper, the principle and a simple example of the multiple frequency measurement are introduced in Section 2, and design, fabrication process and evaluation of the developed ultrasonic microsensors are described in Section 3.
* Corresponding author. Tel.: +81-75-724-7446; fax: +81-75-724-7400. E-mail address:
[email protected].
1876-6196/09 © 2009 Published by Elsevier B.V. Open access under CC BY-NC-ND license. doi:10.1016/j.proche.2009.07.135
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2. Principle of the Multiple Frequency Measurement Figure 1 (a) shows a schematic illustration of a linear array configuration for phased array ultrasonic measurement. It is assumed for simplicity that the ultrasound reflection point is far enough compared to the array size and the incident ultrasound can be treated as a parallel wave, that is, the incident angle θ to the sensor elements are all the same. Waveforms received by the sensor elements are delayed individually and are summed up to steer the directivity of the array toward the direction φ. The synthesized waveform wφ(t) by the summation of the delayed waveforms of wi(t) is expressed as N −1
wφ (t ) =
∑ wi (t − δ i (φ ))
(1)
i =0
where N is the number of sensor elements and δi(φ) is the delay time for i-th element, which is expressed as
δ i (φ ) =
ib sin φ , C
(2)
and C is the sound speed. The incident angle θ is estimated to be equal to φ such that maximizes wφ(t). The directivity d(θ,φ), which represents relative sensitivity to direction θ of the array steered to direction φ, is expressed as
d (θ , φ ) = cosθ
sin(πα sin θ ) sin( Nπβ (sin θ − sin φ )) πα sin θ N sin(πβ (sin θ − sin φ ))
(3)
where
α = a/λ , β = b/λ ,
(4)
and a is the size of the sensor element, b is the period of the adjacent elements and λ is the wavelength of the ultrasound. The second part of the right hand of Eq. (3) represents the effect of the sensor size and the third part represents that of the inter-element spacing. In most airborne ultrasonic microsensors, the sensor size is small enough to the wavelength and the second part can be approximately regarded as constant 1. For example in literatures [2, 3], a = 0.5~0.7 mm and λ = 2.7~3.4 mm, and the second term is accordingly no less than 0.92.
Fig. 1. (a) a schematic illustration of a linear array configuration and directivity steering by delay & summation; (b) directivity patterns calculated from Eq. (3) for 50 kHz and 100 kHz airborne ultrasound received by a five-element and 3.4 mm-spacing linear array.
Figure 1 (b) shows an example case of airborne ultrasonic measurement by using 50 kHz and 100 kHz ultrasound. A five-element array with the element period b = 3.4 mm (λ/2 on 50 kHz and λ on 100 kHz) is assumed. The main lobe for 100 kHz is sharper than that for 50 kHz. Steering the directivity to 30º as shown in Fig. 2, however, the element period over a half of the wavelength causes a grating lobe [1] and results in a ghost in the 100 kHz measurement as shown in Fig. 2 (a). Although these problems are inevitable in measurement using one frequency, the sharp main lobe without a large grating lobe is achieved by combining these two measurements as shown in Fig. 2 (b), where a new directivity is synthesized by simply multiplying the two directivities for 50 kHz and for
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100 kHz. This kind of measurement can be realized using an array composed by sensors each of which has sensitivity to both 50 kHz and 100 kHz. A tunable resonant sensor is a promising candidate for this measurement.
Fig. 2. (a) directivity patterns steered to 30° of the arrays composed by 50 kHz and by 100 kHz sensors; (b) the directivity pattern synthesized by multiplying the two directivities for 50 kHz and for 100 kHz shown in Fig. 2 (a).
3. Piezoelectric Tunable Ultrasonic Microsensors Resonant frequency shift of piezoelectric diaphragms by dc bias voltage were reported [3, 4], yet the tunabilities were not enough for the measurement technique described in the previous section. Figure 3 (a) illustrates a newly developed ultrasonic sensor structure having the double top electrodes. The inner electrode is used for ultrasonic sensing through piezoelectric effect and is optimized in size for high sensitivity [5, 6]. The outer electrode is used for resonant frequency tuning by lateral stress induced through converse piezoelectric effect by applying external dc voltage. Frequency shifts reported in Refs. [3, 4] show reversed shapes each other and the authors assume that the reversal is caused by buckling of the structure [7]. Two kinds of fabrication process have been developed as shown in Fig. 3 (b). The diaphragm structure in the step (II-2) includes a thermally oxidized silicon layer and causes spontaneous buckling because of its compressive stress. The PZT film derived through sol-gel method causes a tensile stress which prevents buckling of the structure during the steps (I-2) to (I-3), while the stress does not make the structure completely flat [8] in the step (II-3).
Fig. 3. (a) schematic illustration of a sensor structure having the double top electrode; (b) fabrication processes of the sensors with (I) a flat diaphragm and (II) a deflected diaphragm.
Oxidized silicon wafers have been used for the flat diaphragms while oxidized SOI (silicon on insulator) wafers have been used for the deflected diaphragms and buried oxide layer has been removed at the step (II-3) after PZT formation. The static deflection profiles and photographs of the fabricated diaphragms are shown in Figs. 4 (a) and (b), respectively. The resonant frequency tunabilities have been evaluated by using response waveforms to a pulse ultrasound under dc voltage application, and the results are shown in Fig. 4 (c). Each tunability shows a butterfly curve due to the ferroelectric polarization hysteresis, and the butterflies reverse to each other on the flat and deflected structures, as designed. Tunabilities over −50% (154 kHz to 75 kHz) on the deflected diaphragm and up to +90% (117 kHz to 223 kHz) on the flat diaphragm have been achieved under dc 8 V application.
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Fig. 4. (a) a static deflection profiles; (b) photographs; (c) measured tunabilities of the fabricated sensors in the process shown in Fig. 3 (b) for (I) the flat and (II) the deflected diaphragms.
4. Conclusions A phased array measurement technique is proposed to realize both high angular resolution and ghost suppression in limited number of sensor elements. This measurement technique is based on synthesized directivity by multiple frequency measurements. New piezoelectric ultrasonic microsensors having an inner sensing electrode and an outer tuning electrode have been developed, which sensors’ resonant frequency can be widely changed for the proposed measurement technique in multiple frequency detection by individual sensors. The resonant frequency tunability is precisely controlled through fabrication process to realize flat and deflected diaphragms. Fabricated sensors have changed their resonant frequency nearly twice on the flat diaphragm and a half on the deflected diaphragm under ±8 V bias application. The realized tunabilities are large enough for the proposed technique and an array composed by the fabricated sensors promises the high resolution and ghost-suppressive ultrasonic measurement.
Acknowledgements This work was partially supported by Grant-in-Aid for Scientific Research (C) 20560299 from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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