International Journal of Applied Engineering Research, ISSN 0973-4562 Vol.7 No.11 (2012) © Research India Publications; http://www.ripublication.com/ijaer.htm
Simulation Of Mems Based Capacitive Pressure Sensor Using Comsol Multiphysics Gitesh Mishra, Neha Paras, Arti Arora, P.J.George Kurukshetra Institute of Technology & Management, Kurukshetra
[email protected],
[email protected]
Abstract In today’s scenario, designing of the device before actual fabrication is very necessary to save economy and time both. The present work demonstrates the design of MEMS based capacitive pressure sensor using Comsol Multiphysics. It allows detailed visualization of various structures’ bend or twist, and indicates distributions of stresses and displacements and provides a wide range of simulation options. Deflection, Eigen frequency and stress of the capacitive pressure sensor are computed. The capacitance measurement is simulated for a capacitive pressure sensor for a capacitance of about 2.12e-14 farad. Keywords— MEMS, COMSOL Multiphysics, Capacitive pressure sensor, deflection, displacement.
Introduction Micromachined pressure sensors have been developed because of their small size, high performance, high reliability and low cost [1]. Different kind of pressure sensors are piezoresistive, piezoelectric, capacitive sensors which have been widely used in automotive fields [2-4]. Capacitive sensors have attracted attention because of their high sensitivity when compared with other sensors. Pressure sensors are required in applications including bio-medical systems, environmental monitoring and industrial process control. Capacitive pressure sensors provide very high pressure sensitivity, have low noise, give rapid response to change in pressure, can withstand a lot of vibration, have low temperature sensitivity and are preferred in many emerging high performance applications [5-8]. Micromachined capacitive pressure sensors have typically used an elastic diaphragm with fixed edges and a sealed cavity in between the diaphragm and the substrate below. The capacitive pressure sensor uses a pair of parallel plates which forms a capacitor. The upper plate acts as movable plate which is fixed from four sides. When pressure is applied on the upper plate it deforms which changes the distance between two plates of capacitor. This change in capacitance can then be observed to sense the pressure. The paper explores the design parameters of MEMS based capacitive pressure sensor using FEM (Finite Element Modeling). It consists of two silicon plates with air as a dielectric between the two layers. Sandwiched-type constructions with deformable intermediate layers have been used in some micro-machined sensors as well as commercial pressure mapping systems [9]. The present work demonstrates the deflection, stress, Eigen frequency analysis of capacitive pressure sensor. The behavior of square diaphragm by applying variable load on it is studied. It emphasizes on the type of substrate
used & comparisons between them using various parameters like stress, displacement etc. It also presents the best known behavior on Eigen frequency for square diaphragm. Square diaphragm is chosen because it gives maximum displacement on account of variable load. It is observed on COMSOL multiphysics that when square, hexagon and circular shape of diaphragm of same dimensions are taken, then square gives the maximum displacement, hence it is more sensitive than others. Design parameters Design of a device depends on various parameters like material, structure and shape etc. All the parameters are needed to be optimized to obtain the desired specification of the device. The design started with the selection of the substrate material and its properties. In the present work, single-crystal silicon having Young’s modulus of 169 GPa and Poisson’s ratio 0.33 has been used. Silicon is chosen because of its high melting point, low thermal expansion coefficient, less mechanical hysteresis [10] etc. Based on these properties used, the shape of the diaphragm was optimized. Square shape has been chosen as shown in Fig-1, due to the ease of anisotropic etching of the silicon in bulk. All the parameters necessary for the design of capacitive pressure sensors are optimized for this shape.
Fig-1 capacitive pressure sensor Diaphragm The optimized dimensions of silicon diaphragm are 60µm*60µm*1.5µm. Stress, Displacement and Eigen frequency is optimized using these dimensions of square diaphragm. Capacitive pressure sensor consists of two parallel plates, lower plate is fixed while the upper plate is movable. When pressure is applied on the upper plate it deforms which changes the distance between two plates of capacitor. The change in capacitance is then observed. The concept of parallel plate capacitor is expressed as follows
International Journal of Applied Engineering Research, ISSN 0973-4562 Vol.7 No.11 (2012) © Research India Publications; http://www.ripublication.com/ijaer.htm C = εoεr
(1)
Where, ‘εo’ is the permittivity of free space and ‘εr’ is the dielectric constant of the material between the plates of the capacitance. ‘A’ is the overlapping area of the plates used, and ‘d’ is the distance between the two plates. A realization function of this concept would be that the plates of the capacitor could move close to each other. The distance between two plates will decrease resulting in increase of capacitance of sensor as shown in Fig-2 simulated using FEM.
The diaphragm deflection is found to be linear with the variation in the load applied on the upper plate of the diaphragm. Fig-4 shows that the deflection or displacement in the diaphragm follows the Hooke’s law [11]. Also, Fig-5 shows the stress behavior of diaphragm with respect to variable load. It also obeys the Hooke’s law. The range for variable load is kept from 0-1000 N/m2 and the step size is 100.
Fig-4 Curve of displacement vs. variable load
Fig 2: Capacitance vs. Distance curve Resonance (Eigen) frequency of the pressure sensor has been optimized using Solid, Stress-Strain MEMS module. It is found that frequency increases with increase in thicknesses as higher frequency is needed for vibration of the silicon diaphragm.
Simulation result The deflection of the diaphragm after simulation is shown in Fig. 3
Fig-5 Stress measurement on diaphragm with variable load
Similarly, Eigen frequency and frequency response is calculated for the Diaphragm. Fig-6 shows the frequency response curve. The calculated Eigen frequency is 6.79 MHz
Fig-3 Deflection of diaphragm
International Journal of Applied Engineering Research, ISSN 0973-4562 Vol.7 No.11 (2012) © Research India Publications; http://www.ripublication.com/ijaer.htm
[9]
[10]
[11]
Fig-6 Frequency response curve
Conclusion This paper demonstrates the deflection, stress, and Eigen frequency analysis of a capacitive pressure sensor. The design parameters for the capacitive pressure sensor are optimized so as to give a capacitance of about 2.12e-14 farad.
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