Proceedings International Conference On Advances In Engineering And Technology
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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.
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.
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.
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 massproduction is possible and due to compatibility with standard micro fabrication processes they are particularly
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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
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Proceedings International Conference On Advances In Engineering And Technology
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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.
ISBN NO: 978 - 1503304048
Fig. 5: Deformed shaped plot of SAW model at Antiresonance 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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Proceedings International Conference On Advances In Engineering And Technology
IV. SIMULATION
V. RESULTS AND DISCUSSION
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 c_DCM_ 100e-6*p/(R*T) concentration in air air Molar mass of M_DCM 84.93[g/mol] DCM PIB/air K 30.346 partition constant for DCM Mass rho_DCM_ 0.010534kg/m3 concentration of PIB DCM in PIB Density of PIB rho_PIB 918.00kg/m3 Young's E_PIB 10[Gpa] modulus of PIB Poissons ratio nu_PIB 0.48 of PIB Relative eps_PIB 2.2 permittivity of PIB
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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
1
Lithium Niobate Lithium Tanatalate
2
Surface Displacement in μm at Resonance 6.2241x10-3
Surface Displacement in μm at anti Resonance 2.0002x10-3
1.555 x10-3
0.1553
Table. 3: Electrical potential at Resonance & Anti Resonance S. No
Material
Electrical Potential at Resonance
1
Lithium Niobate Lithium Tanatalate
5.4428
Electrical Potential at AntiResonance 5.8003
5.1009
5.6111
2
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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.
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