This document discusses the design and simulation of a MEMS-based surface acoustic wave (SAW) gas sensor. SAW gas sensors use piezoelectric materials and operate by detecting changes in oscillation frequency when gas molecules are absorbed by a sensing film on the surface. The document describes modeling the sensor geometry and materials in COMSOL Multiphysics to simulate sensor performance. Simulations analyzed deformation and electrical potential at resonance and anti-resonance frequencies for different piezoelectric materials. Results showed lithium tantalate produced better responses than lithium niobate. SAW gas sensors are widely used to detect gases for applications like vehicle emissions monitoring.

1. 
Abstract— Characteristics of Piezo electrical material are
given the highest significance when gas sensitive layer is
etched on it for application like detection of dangerous gases.
Choice of material dominates the behavior of the system and
results in malfunctioning if proper material is not chosen.
Surface Acoustic Wave (SAW) gas sensor is one, which is
widely used in numerous engineering applications which is
widely used now a days. Detection of gases as its one of the
major applications SAW gas sensor extended its services into
the field of medical and even in power plants. This paper
reviews the significance of piezoelectric material and focuses
on MEMS based SAW gas sensor model.
Keywords:-MEMS, Inter Digital Transducer (IDT), Surface
Acoustic Wave (SAW) as Sensor, Piezo Electric Material,
COMSOL Multiphysics.
I. INTRODUCTION
Sophisticated technology made sensors as a part of our
daily lives. Demand for high precision sensor technology
continues to drive the production a higher volume of
smaller, cheaper, and more sensitive sensors at tailor made
design. Acoustic wave (AW) devices have received
increasing interest in recent years in a wide range of
applications where they are currently used as resonators,
filters, sensors and actuators.
Fig. 1: SAW gas sensor, showing the IDT electrodes
(in black), the thin PIB film (light gray), and the
LiNbO3 substrate (dark gray).
MEMS—micro-scale devices are now available as mass-
production is possible and due to compatibility with
standard micro fabrication processes they are particularly
well suited to fit the required demand. One class of MEMS
sensors, the surface acoustic wave (SAW) sensor are known
for ruggedness, reliability, low cost, and simplistic design
[1], is of particular interest due to its adaptability to many
different applications in telecommunication, textile,
chemical, cement, steel factories, automobile, aeronautical
and in power plants. It is estimated that approximately
around four billion SAW gas sensors are being produced
every year.
Fig. 2: Model geometry of SAW gas sensor.
II. THEORY OF OPERATION
SAW gas sensor consists of Inter Digital Transducers
(IDT’s) that are placed on Piezo electric material with the
gas sensing film in between these two transducers. The
frequency range of operation of SAW transducers is in the
range of 50 MHz and several GHz [2]. Typical SAW-based
sensors are coated with thin polymer films. This device
exhibits a decrease in frequency when the gas molecules are
adsorbed directly on the surface of the sensitive film that is
on piezoelectric substrate. The variation of oscillating
frequency is proportional to the mass of foreign molecules
deposited on the crystal surface and the center frequency of
the piezoelectric crystal.
III. DESIGN PROCESS
The design of the MEMS based structure includes
defining the variables for the required geometry and
selection of the parameters. The 2D geometry has been
constructed in the drawing mode of COMSOL
Multiphysics. This model consists of SAW gas sensor slice
Review on Significance of Piezoelectric Material for Manufacturing
of SAW Gas Sensor
Sai Pavan Rajesh. V, Department of Control Systems, St. Mary’s Group of Institutions, Affiliated to
Jawaharlal Nehru Technological University- Hyderabad, Kukatpally, Hyderabad-50085, India.
pavanlbce@gmail.com.
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2. which is removed to reveal the modeled unit cell (in white)
from the base model that is shown in figure 1. This sensor
is equipped with rectangular shaped two electrodes made of
aluminum, etched on different piezoelectric substrates and
is covered with polyisobutylene (PIB) film. The width of
substrate is 4 μm with a height of 22.5 μm. The length of
PIB material is 4 μm with a height of 0.5 μm.
Fig. 3 Model geometry of SAW with meshing.
In the model, boundary is fixed to a zero displacement.
The Poisson’s ratio of PIB material is taken to be 0.48,
which corresponds to a rather rubbery material. The density
of the PIB film is taken from the experimental results of K.
Ho [3]. The PIB film covers two 1 μm-wide electrodes on
top of the Piezo substrate, which is 500 nm. The model
geometry of SAW gas sensor with base as Piezo electric
crystal and two electrodes along with sensitive film are
shown in figure 2.
Fig. 4: Deformed shaped plot of SAW model at
resonance for Lead Zirconate Titanate (PZT-8)
material.
Fig. 5: Deformed shaped plot of SAW model at Anti-
resonance for Lithium Niobate material.
Fig. 6: Electric potential distribution and deformation
at resonance, symmetric with respect to center of each
electrode for Quartz material.
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3. IV. SIMULATION
In this study, the simulations are performed using the
Sensor module under the MEMS model in COMSOL
Multiphysics, which is designed specifically to support the
numerical modulation of resonant frequency of SAW gas
sensor. The lowest SAW Eigen mode has its wavelength
equal to the width of the geometry, 4 μm. The Eigen
frequency of this mode multiplied by 4 μm hence gives the
velocity of the wave. Simulation comprises of calculation of
electrical potential and analysis of deformation at resonance
and anti resonance. The presence of the aluminum IDT
electrodes and the PIB film cause the lowest SAW mode to
split up in two Eigen solutions, the lowest one representing
a series resonance, where propagating waves interfere
constructively and the other one a parallel (“anti-”)
resonance, where they interfere destructively. These two
frequencies constitute the edges of the stop band, within
which no waves can propagate through the IDT. The
resonance and anti-resonance frequencies evaluate to
approximately 839 MHz and 849 MHz, respectively.
Exposing the sensor to a 100 ppm concentration of DCM in
air leads to a resonance frequency shift of approximately
200 Hz downwards. This is computed by evaluating the
resonance frequency before and after increasing the density
of adsorbed DCM to that of the PIB domain. The
parameters for the model geometry were listed in table 1.
Table .1 Parameters for SAW gas sensor model geometry.
Description Expression Value
Air pressure p 101.325[kPa]
Air temperature T 25[degC]
Gas constant R 8.3145[Pa*m^3/(K*mol)]
DCM
concentration in
air
c_DCM_
air
100e-6*p/(R*T)
Molar mass of
DCM
M_DCM 84.93[g/mol]
PIB/air
partition
constant for
DCM
K 30.346
Mass
concentration of
DCM in PIB
rho_DCM_
PIB
0.010534kg/m3
Density of PIB rho_PIB 918.00kg/m3
Young's
modulus of PIB
E_PIB 10[Gpa]
Poissons ratio
of PIB
nu_PIB 0.48
Relative
permittivity of
PIB
eps_PIB 2.2
V. RESULTS AND DISCUSSION
Using software, two kinds of studies has been explored
i.e., study 1 is calculation of deformation for resonance and
anti –resonance of SAW model. Whereas study two
comprises of analysis of electrical potential distribution with
respect to the center of each electrode for different
eigenmodes. In this model a parametric sweep is set up with
respect to the amount of adsorbed species on the sensor, and
eigenfrequencies are searched near 850MHz. So to calculate
the deformation and potential the eigenvalues are selected
for 8.387092e8, 8.489836e8, from the eigenfrequency list.
Simulation results were applied for tellurium dioxide as
shown in fig 7.
Fig.7. Total displacement for Tellurium Dioxide.
To see all computed eigenfrequencies as a table for full
precision, the first 6 digits of the eigenfrequency are the
same. Subtracting the new value from the previous value
shows that the eigenfrequency with gas exposure is lower
approximately 200 Hz.
Table. 2: Total displacement at Resonance & Anti
Resonance
S. No Material Surface
Displacement
in μm at
Resonance
Surface
Displacement
in μm at anti
Resonance
1 Lithium
Niobate
6.2241x10-3
2.0002x10-3
2 Lithium
Tanatalate
1.555 x10-3
0.1553
Table. 3: Electrical potential at Resonance & Anti
Resonance
S. No Material Electrical
Potential at
Resonance
Electrical
Potential at
Anti-
Resonance
1 Lithium
Niobate
5.4428 5.8003
2 Lithium
Tanatalate
5.1009 5.6111
Proceedings International Conference On Advances In Engineering And Technology
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4. CONCLUSION
MEMS based SAW gas sensor model is designed for
different piezoelectric materials. Surface displacement and
electrical potential when analyzed for resonant frequency
and anti resonance, results that lithium tantalite materials
showed good response when compared with the lithium
niobate, in the same environmental conditions. It was
observed that when quartz material is considered the
resonance and anti resonance frequencies were the same
and produced very less values when compared with the
based model parameters. These results help in better choice
of material for SAW gas sensor manufacturing. This sensor
is widely used for detection of alcohol, propane, hydrogen,
methane, carbon dioxide gases and now being used in car
navigation systems, the telematics, and even in cellular
phones.
ACKNOWLEDGMENT
The author would like to thank the faculty of
Instrumentation and College of Lakireddy Balireddy
College of Engineering, for providing an opportunity to
work in MEMS Laboratory.
REFERENCES
[1] John Vetelino and Aravind Reghu, Introduction to Sensors, CRC Press,
Florida, 2011.
[2] E. Benes, M. Gr, W. Burger, and M. Schmid, “Sensors based on
piezoelectric resonators,” Sens. Actuators A, Phys., vol. 48, pp. 1–21,
1995.
[3] K. Ho and others, “Development of a Surface Acoustic Wave Sensor for
In-Situ Monitoring of Volatile Organic Compounds”, Sensors vol. 3, pp.
236–247, 2003.
Proceedings International Conference On Advances In Engineering And Technology
ISBN NO: 978 - 1503304048
www.iaetsd.in
International Association of Engineering & Technology for Skill Development
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