M3P.
002
DESIGN OF T-SHAPE VECTOR HYDROPHONE BASED ON MEMS
Linggang Guan1,2, Guojun Zhang1, Chenyang Xue1,2, Wendong Zhang1,2, Jijun Xiong1,2, Jiao Xu2
1
Science and Technology on Electronic Test & Measurement Laboratory, Taiyuan, China
Key Laboratory of Instrument Science & Dynamic Measurement (North University of China), Ministry of
2
Education, Taiyuan, China
ABSTRACT At present, the difficulty of fabricating biological
This paper reports the design and fabrication of sensory mainly focused on the artificial hair. whatever the
T-shape vector hydrophone based on MEMS inspired by hair is made of SU-8, polyurethane or fiber, all the
fish lateral line. Based on the analysis of mechanical manufacture process are complex. It is hard to guarantee
properties of T-beam, the design and fabrication process the consistency of the biological sensory as well.
of microstructure are described in detail, and then the A novel MEMS vector hydrophone is proposed
characteristics of the microstructure are analyzed by the which inspired by fish lateral line, which is easily
ANSYS. The first-order resonance frequency is 1970Hz, patterned by a straightforward process assuring the
and the position of the piezoresistor is away from edges consistency. The hydrophone can realize vector detection
about 120μm. The microstructure is manufactured by of underwater acoustic signal and is particularly sensitive
means of the silicon-on-insulator (SOI) craft. The test to low frequency signal.
results show that hydrophone’s receiving sensitivity is up
to -180dB (0dB = 1V/μPa), displaying a clear directivity SENSOR DESIGN
pattern in the form of “8” shape. Principle of Operation
The peculiar sensory organ of fish is lateral line,
KEYWORDS consisting of hair shape mechanoreceptors or neuromasts,
Bionic sensor; Vector hydrophone; MEMS; SOI craft covered with a jelly like cupola, which are located on the
skin or in the canals along the body [7, 8] as shown in Fig.
INTRODUCTION 1. The fluid motion gets into pore through lateral line and
Biological sensory systems often display great delivers to mucus, causing the flowing of mucus which
performance which inspires engineers to design artificial makes the displacement of hair cell. As a result, sensory
counterparts. By imitating fish’s lateral line organ and cells are stimulated and the stimulation is transmitted to
cricket’s auditory cilia, many research institutes medulla oblongata by nerve fiber [6], which is the sensory
manufactured MEMS cilia bionic micro-sensor based on pathway of fish lateral line organs.
piezoelectric [1], piezoresistance [2] and capacitance [3].
Such as, in 2005, Nesterov and Brand [4] from Germany
developed MEMS bionic micro-probe based on
piezoresistance. In 2006, Chen [5] in Illinois State
University, Micro-Nano Technology Research Center,
manufactured MEMS flow sensor of biomimetic micro
cilia by imitating fish’s lateral line organ, and in 2006,
Dutch Krijnen [6] manufactured ciliated micro acoustic
sensor by imitating cricket’s auditory cilia.
Figure 2: (a) Schematic of the bionic microstructure of MEMS
hydrophone. (b) Schematic of a hair cell, illustrating its
functional asymmetry, as well as its efferent innervation.
According to the structure of lateral line,
microstructure of hydrophone is designed, as shown in
Fig. 2a. Sensory hair is imitated by the long cantilever
beam, sensory cells by the piezoresistors and efferent
nerve by metal lead, shown in Fig. 2b. The T-shape
hydrophone works on the same principle as fish lateral
line, the principle for the hydrophone is as follows:
Figure 1: (a) Schematic drawing of fish lateral line. （ b ） When signal acts on cantilever beam (sensory hair), it
Configuration of canal. (c) A close up of a neuromast. will lead to the deformation of cantilever beam which
978-1-4577-0156-6/11/$26.00 ©2011 IEEE 20 Transducers’11, Beijing, China, June 5-9, 2011
cause the resistance change of piezoresistors (sensory
cells). So the Wheatstone bridge, consisting of
piezoresistors R1, R2 and the reference resistors R3, R4,
will get output. According to the output, both the
direction and pressure of underwater acoustic signal can
be acquired.
In the process of application, to increase the
sensitivity, single beam is replaced by the double beams.
The vibration model of the microstructure is built, and the
mechanical parameter is analyzed. The first resonant
frequency is
0.55966 EK 2 (1)
f1 = Figure 3: Deformation and stress distribution of the structure.
l2 ρ
When the force F was applied to the beam, the stress
on the surface of beam is
F (l − x)
σ= a (2)
I
1 2
S ∫s
Where E is the Young’s modulus, K 2 = r dS is
the radius of gyration, if the section of beam is rectangle
a×b ( a and b are the thickness and width of beam), we get
the K 2 = a 2 12 ; I = b × a 3 12 is the second moment of
area, ρ is the density of the silicon, l is the length of beam.
Figure4: Curve of stress distribution on the beam.
Simulation of Structure
The model of hydrophone is established by ANSYS, SENSOR FABRICATION
both the structure’s size and piezoresistors’ location can Fabrication of Microstructure
be confirmed by simulation. In order to confirm the The fabrication of microstructure by means of SOI
structure’s size, the resonant frequency of the technology, and it mainly consists of
microstructure, the feasibility and stability of the process oxidation, photoetching, etching, ion implantation and
should be taken into considered. For piezoresistors’ vapor deposit. Figure 5 shows the fabrication procedures.
location, Piezoresistors should not be only arranged in the Clean and oxidize to form SiO2 layer in the wafer (step a).
maximum stress area but also avoid be arranged in the Use SiO2 as a mask, the implantation windows have been
nonlinear area. By simulation, the final size is photolithographically patterned and boron has been
20×130×3500μm3 (thickness×width×length). 1Pa loads implanted to form piezoresistors (step b). The next step
are given along the Z direction to the cantilever beam, and (step c) is re-oxidation, form the ohmic contact holes
the stress distribution is shown in Fig. 3, from which it using the same process, implantation dense boron to form
can be seen that the maximum stress area locates at the ohmic contact. Subsequently, sputtering TiPtAu and
root of beam. Curve of stress distribution on beam is making the metal wire (step d). Lastly step (step e), the
obtained by path definition in ANSYS, as shown in Fig. 4, cantilevers have been etched in ICP (Inductively Coupled
from which it can be figured out that the piezoresistors Plasma) reactor, and then make the back cavity by EPW
can be located at the region away from root 120μm. (Ethylene Pyrocatechol Water) aeolotropic corrosion.
The working frequency of vector hydrophone is often Figure 6 shows the SEM images of the microstructure.
below the microstructure’s first-order resonance
frequency, which can make the structure’s frequency
response consistent and stable. So in the process of
design, the microstructure’s first-order frequency should
be increased as much as possible. Through the analysis in
equation (1), it can be concluded that the resonant
frequency is related to beam’s thickness and length, but
not related to beam’s width. The first-order resonance (a)Prepare wafer and oxidation.
frequency is 1969.3Hz by ANSYS simulation. Substitute
the structure’s size to formula (1), we can obtain
f1=1970.2Hz which is the almost the same as ANSYS
simulation results.
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Packaging of Sensor
The microstructure could not detect the underwater
signals directly without a kind of sound-transparent,
insulated and waterproof package [9]. According to the
neuromast structure shown in Fig. 1(c), a new package
method is proposed. That is, polyurethane instead of
(b)Implant boron to form piezoresistors. cupula, and castor oil instead of mucus. A MEMS vector
hydrophone is eventually produced which is shown in Fig.
7 (The size of hydrophone is aboutΦ17×100mm).
(c)Re-oxidation and implantation dense boron.
Figure 7: Photo of packaged T-shape vector hydrophone
(d)Sputter metal to form lead. SENSOR CHARACTERIZATION
The characterization of vector hydrophone mainly
refers to the sensitivity and directivity. The measurement
were carried out in the standing wave tube calibration
setup, Figure 8 shows the schematic diagram. The
measurement setup is mainly composed of function
generator, power amplifier and calibration tube. The sine
wave generated by the function generator was send to the
emission transducer after amplification. The
(e)Corrode back cavity and release the structure. measurement adopts comparison calibration [10] to
acquire the sensitivity of tested hydrophone. That is,
comparing the voltage output of tested MEMS
hydrophone with the reference hydrophone. The
expression of sensitivity is:
Figure 5: The process of manufacturing technology.
U x sin kd 0
Mx = (3)
P0 cos kd
Piezoresister
Where Ux is the open-circuit voltage of the MEMS
hydrophone, and Mx is the receiving sensitivity of the
hydrophone. k is the wave number, d and d0 are the
distance from water surface to the MEMS hydrophone
and reference hydrophone, respectively. P0 can be
acquired by measuring the open-circuit voltage of the
reference hydrophone. Figure 9 shows the frequency
Pad
response ranging from 20 Hz to 2.5 KHz, which indicates
the resonance peak appearing at 2 KHz.
Figure 6: SEM images (top view) of the microstructure.
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According to the test results, the receiving sensitivity can
be up to -180dB (0 dB = 1V / μPa), it exhibits a directivity
pattern in the form of “8” shape. This hydrophone will be
expected to detect and localize the underwater
low-frequency acoustic signal in future.
REFERENCES
[1] Z. Fan, J. Chen, J. Zou, et al, “Design and fabrication
of artificial lateral-line flow sensors”, J. Micromech.
Microeng, Vol.12, pp.655-661, 2002.
[2] Y. Ozaki, T. Ohyama, T. Yasuda, et al. “Air flow
sensor modeled on wind receptor hairs of insects”
Proceedings of the IEEE International Conference
Figure 8: Schematic diagram of the experimental setup for on Micro Eletro Mechanical Systems (MEMS2000),
evaluating the performance of the hydrophones. Miyazaki, Japan, Jan 23-27, 2000, pp.531-536.
[3] J. V. Baar, M. Dijkstra, R. Wiegerink, et al. “Arrays
Using a similar measurement setup and by placing the of cricket-inspired sensory hairs with capacitive
vector hydrophone on a rotary stage, the signal response motion detection”, Proceedings of the IEEE
was recorded for every 15º rotation angles with respect to International Conference on Micro Eletro
the source, then the directivity pattern of T-shape vector Mechanical Systems (MEMS2005), Miami Beach,
hydrophone can be acquired. Directivity pattern at the USA, Jan 30-Feb 3, 2005, pp. 646-649.
frequency 257Hz and 966Hz are shown in Fig. 10. [4] V. Nesterov, U. Brand, “Modelling and investigation
of the silicon twin design 3D micro probe”, J.
Micromech Microeng, Vol. 15, pp.514-520, 2005.
-150
Measurement
[5] J. Chen, J. M. Engel, C. Liu, “Development of
-160 Curve fitting Polymer-Based Artificial Haircell Using Surface
Micromachining and 3D Assembly”, Digest Tech.
Sensitivity[dB]
-170
Papers, Transducers 2003 Conference, Boston, MA,
-180 USA,June 8-12, 2003, pp.1035-1038.
[6] G. J. M. Krijnen, M. Dijkstra, J. J.van Baar et al.,
-190
“MEMS based hair flow-sensors as model systems
-200 for acoustic perception studies”, Nanotechnology,
Vol.17, pp.S84-S89,2006.
-210 1
10 10
2
10
3 [7] S. Coombs and J. C. Montgomery, “The enigmatic
Frequency[Hz] lateral line system” , in Comparative Hearing:
Fishes and Amphibians, A. N. Popper and R. R. Fay,
Figure 9: Receiving sensitivity of T-shape vector hydrophone. Eds. Springer Handbook of Auditory Research,
Vol.11, pp.319-362, Springer-Verlag, N.Y., 1999.
[8] N. Izadi, M. J. de Boer, J. W. Berenschot, et al.,
“Fabrication of Dense Flow Sensor Arrays on
Flexible Membranes”, Digest Tech. Papers,
Transducers 2009 Conference, Denver, CO, USA,
June 21-25, 2009, pp.1075-1078.
[9] G. J. Zhang, P. P.Wang, L G Guan, et al.,
“Improvement of the MEMS bionic vector
hydrophone”, Microelectr. J, in press.
[10] V. A. Gordienko, “Absolute pressure calibration of
acoustic receivers in a vibrating column of liquid”,
Acoust Phys, Vol.40, pp.243-246, 1994.
CONTACT
* Wendong Zhang, tel: 86-351-3920399;
wdzhang@nuc.edu.cn
* Jijun Xiong, tel: 86-351-3924575;
xiongjijun@nuc.edu.cn
Figure 10: Directivity pattern of T-shape vector hydrophone.
CONCLUSION
A T-shape vector hydrophone based on MEMS has
been fabricated, and its performance evaluated.
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