JCHPS Special Issue 9: December Page 83

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1 Design and Analysis of MEMS Based Humidity Sensor Using Comsol Multiphysics C.V. Gayathri*, V. S. SelvaKumar, L. Sujatha Department of communication systems, Rajalakshmi Engineering College, Chennai, India *Corresponding author: ABSTRACT Micro Electro Mechanical Systems (MEMS) are the devices that have the ability to sense and control environmental parameters such as temperature, pressure, humidity etc. The main motivation of this work is the necessity in the fields of automotive industry, climate control, data loggers and humidity monitoring. The capacitive humidity sensor is designed and analyzed in MEMS technology using COMSOL Multi physics. Here the sensor consists of two metal armatures between which a layer of insulating polyimide is sandwiched. This whole structure is subjected on a silicon substrate. Here simulation of humidity sensor is analyzed to detect the moisture content in the atmosphere. Humidity measurement instruments relies on the measurement criteria such as temperature, pressure, volume of air or a mechanical/electrical change in a substance when moisture is absorbed. These parameters can be optimized by using MEMS CAD tool for better performance of the sensor, and the fabrication of humidity sensor is done using Intellifab. KEY WORDS: COMSOL multi physics, capacitive humidity sensor, MEMS, polyimide. 1. INTRODUCTION A sensor is a small device that detects or measures a physical property and either records or responds to it. The most frequently used sensors are classified based on parameters such as pressure, optical, chemical, humidity, position, environment, magnetic switch, speed, ultrasonic sensors, etc. Among the available sensors, humidity sensor is most widely used and is of great importance in a variety of economic and industrial applications. The term Humidity represents the amount of water vapor in the atmosphere or in gas specifically the amount compared to the maximum amount. The higher the temperature of air, the higher is the water content present in air. It is also known as hygrometer. There are two main ways through which relative humidity can increase, when all the other variables remain equal. First, the drop in temperature causes the water content in air remains unchanged. Second, the humidity increases only if water is absorbed by air while the temperature remains the same. Principle and Design: Working Principle: There are three basic types of humidity sensors. They are capacitive, resistive and thermal humidity sensor. All these sensors monitor minute changes in the atmosphere in order to calculate humidity in the air. Due to the quick response and better sensitivity, capacitive humidity sensor is chosen. The capacitive humidity sensor consists of a dielectric material sandwiched between two electrodes which form a small capacitor. Most capacitive sensors use either polymer or plastics as the dielectric material, with a dielectric constant ranging from 2 to 15. When water vapor blows over the surface, it gets adsorbed over it. Then the adsorbed molecules diffuse in the polymer inducing a variation of its permittivity. This variation in permittivity causes variation in capacitance as expressed in terms of C = ɛ * A / d (1) Where A is the plate area, d be the distance between plates and ɛ is a dielectric material constant ɛ = relative dielectric constant x dielectric constant of vacuum = ɛ r X ɛ 0 Hence, ɛ r be a plain number, ɛ 0= 8.85 x F/m This general formula shows that, The larger the plate areas of dielectric material and smaller the distance between the two plates result in higher capacitance value. The larger the dielectric constant of the insulating (dielectric) material, higher the capacitance. Design of sensor in COMSOL multi physics: COMSOL Multi physics is a software package which is widely used for modeling geometry, meshing, defining physics and visualizing the end results. Further, it has the facility to design both 2D and 3D models and provides a 3D projection, using which a designed model can be rotated and viewed along three axes. A capacitor is a device which stores electric charge consisting of two conducting plates, which are parallel to each other and it is separated by the distance d. Hence the capacitance C depends only on the geometric factors A and d as shown in below figure.1. JCHPS Special Issue 9: December Page 83

2 Figure.1. Simulation of capacitive sensor The dimension of the capacitive sensor structure designed is tabulated as follows, Table.1. Dimensions of capacitive sensor structure S. No Element Dimension 1 Thickness of the dielectric 100µm 2 width of the conducting plate 100µm 3 Thickness of the structure 1000µm MEMS module is a part of the COMSOL Multi physics software. This module helps in designing any type of MEMS device and do further analysis. COMSOL has a number of Physics including AC-DC component, RF component and many more. Each physic is associated with its own study computation and results by default. The user has the option to select the required mesh size. The more fine the mesh size, the more accurate results are obtained. The results are computed using Finite Element Method [FEM] Algorithm. In order to get real-time accurate results, real-time boundary conditions and real-time parameters were included in the result computation. Figure 2 shows the sensor that detects the humidity based on a change of capacitance between two electrodes. Hence the static analysis of the design is performed using electrostatics Module of COMSOL Multi physics Software. Figure.2. Simulation of capacitive humidity sensor Hence the capacitive humidity sensor structure dimension is tabulated as follows, Table.2. Dimensions of the proposed capacitive humidity sensor structure S. No Element Dimension 1 Total area of the structure 1000 x 1000µm 2 Thickness of the structure 100µm 3 Parallel blocks 150 x 150 x 150µm 4 Thickness of the upper electrode 300µm 5 Thickness of the lower electrode 100µm 6 Spacing between the plates 200µm 7 Thickness of the sensitive layer 100µm (dielectric) 8 Number of grids 3 2. RESULTS AND DISCUSSION Variation of capacitance in terms of humidity: When humidity in the atmosphere increases, capacitance will also increase exponentially. Because the capacitor starts from a lower value and increases to its asymptotic value as shown in figure.3. This is based on dielectric constant change of deposited polyimide causes the capacitance change which is related to the humidity. The polymer absorbs water vapor as the relative humidity of ambient air rises and releases when humidity falls. Figure.3. Capacitance versus relative humidity of the sensor JCHPS Special Issue 9: December Page 84

3 Variation of capacitance in terms of thickness: Figure.4, describes the relationship between the capacitance and thickness of plate separation for an ideal parallel plate capacitor. The capacitor undergoes a discharge cycle, as the thickness of the various dielectric materials increases, capacitance decreases exponentially to zero at the end of transient state, this will happen only when the length and width of the dielectric material changes. Figure.4. Variation of capacitance versus thickness for various dielectric materials A plot of the capacitance versus thickness exponentially increases depending upon different dielectric materials, thus all the data exponentially increases due to the change in width with the constant length and thickness. As the distance between the plates increases, the capacitance would approach the ideal parallel plate capacitor value. This situation would result in a concave curve, rather than a straight line as shown in below figure. Figure.5. Change in width of various dielectric materials Variation of voltage in terms of capacitance: Capacitance voltage profiling (C V profiling) is a technique for characterizing materials and devices. For various voltage applied, the corresponding capacitance is measured and is plotted as a function of voltage as shown in Figure.6. C V profiling is used to characterize threshold voltages, device testing and other parameters to evaluate the device performance. C V measurements are also used to characterize other types of semiconductor technologies and devices, including photodiodes, bipolar junction transistors, photovoltaic cells, MEMS devices, III V compound devices, organic thin-film transistor (TFT) displays, photodiodes, and carbon nanotubes (CNTs), JFETs. Figure.6. Applied voltages versus capacitance Variation of charge in terms of capacitance: Figure.7, describes the charge that flows through the capacitor is directly proportional to the capacitance. As the capacitors ability is to store charge (Q) between its plates is proportional to the applied voltage (V), the voltage across the plates increases or decreases over the time causes the flows of current through the capacitance removes or deposits charge from its plates with the amount of charge being proportional to the applied voltage. Hence the charge of the capacitor can be calculated as Q = C x V (2) Where Q=charge of the capacitor C=capacitance V=voltage Charge is important because many devices, such as flash cameras, operate on the charge which a capacitor attains. Figure.7. Applied charges versus capacitance JCHPS Special Issue 9: December Page 85

4 Variation of resistance in terms of thickness: The resistance of a component depends on various parameters such as area, length and material of the conductor. The conductivity of the material is defined as the conductance of the block having unit dimensions made out of the material. The below figure 8 describes that when length of the gap between the plate increases, conductance also will increase linearly. The conductance is defined as the inverse of resistance. The conductance is measured in Siemens (S). The resistance (R) of an object is defined as the ratio of voltage (V) to the current (I) through it, while the conductance (G) is the inverse, which is expressed in terms of equation 3 and 4. R=V / I (3) G=I / V= 1/R (4) Figure.8. Variation of conductance versus distance 3. CONCLUSION A structure of capacitive humidity sensor is designed and simulated using COMSOL 4.2a. The result shows that the capacitive humidity sensor is having the characteristics of low hysteresis, compatibility with integrated circuit (IC) fabrication, high reliability, fast response, high sensitivity and good linearity to humidity. This humidity measurement is ideally suited for wide variety of applications such as building ventilation control, clean rooms in semiconductor, industrial drying, food/beverage, cosmetics, etc. The future concern is to fabricate the capacitive humidity sensor using photolithography techniques. REFERENCES Benmoussa N, Benichou A, Medjahdi N and Rahmoun K. Modeling and optimization of a capacitive humidity sensor response in MEMS technology, IEEE transactions on Dielectric Materials for Photovoltaic systems, 5 (15), 2014, Cheng-Long Zhao, Ming Qin, and Qing-An Huang. A fully packaged CMOS inter digital capacitive humidity sensor with polysilicon heaters, IEEE Sensors Journal, 11 (11), 2011), Cong Zhang, Li Guo, Li-Feng Wang, Jian-Qiu Huang and Qing-An Huang, Passive wireless integrated humidity sensor based on dual-layer spiral inductors, Electronics Letters, 50 (18), 2014, David Eddy S and Douglas Sparks R, Application of MEMS technology in automotive sensors and actuators, Proceedings of the IEEE, 86 (8), 1998, Fangming Deng, Yigang He, Chaolong Zhang and Wei Feng, A CMOS humidity sensor for passive RFID sensing applications, Journal of Sensors, 14, 2014, Gastone Ciuti, Leonardo Ricotti, Arianna Menciassi and Paolo Dario, MEMS sensor technologies for human centred applications in healthcare, physical activities, safety and environmental sensing, A review on research activities in Italy, Journal of Sensors, 15 (6), 2015, Hamid Farahani, Rahman Wagiran and Mohd Nizar Hamidon, Humidity sensors principle, mechanism and fabrication technologies, A comprehensive review, Journal of Sensors, 14, 2014, Hyemin Lee, Sungjun Lee, Seungwon Jung and Junghoon Lee, Nano-grass polyimide-based humidity sensors, Journal of Sensors and Actuators, 15, 2011, 2-8. Jan-Niklas Schonberg, Vitaliy Kondrashov, Anton Prokhorov and Jurgen Ruhe, Capacitive humidity and dew-point sensing, Influence of wetting of surface-attached polymer monolayers on the sensor response, Journal of Sensors and Actuators, 222, 2016, Jian-Qiu Huang, Fei Li, Min Zhao and Kai Wang. A surface micro machined CMOS MEMS humidity sensor, Journal of Micromachines, 6, 2015, Ji-Hong Kim, Byung-Moo Moon and Sung-Min Hong, Capacitive humidity sensors based on a newly designed inter digitated electrode structure, Microsystem Technology, 18, 2012, JCHPS Special Issue 9: December Page 86

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