Sensitivity analysis of mems capacitive pressure sensor using carbon diaphragm

Nội dung text: Sensitivity analysis of mems capacitive pressure sensor using carbon diaphragm

1. SENSITIVITY ANALYSIS OF MEMS CAPACITIVE PRESSURE SENSOR USING CARBON DIAPHRAGM Toan P.M., Thu N.T.K.* School of Engineering and Technology, Vinh University ABSTRACT In this paper, we present simulation and evaluation of sensitivity of electrical and mechanical effects of MEMS based capacitive pressure sensor with rectangle diaphragm using COMSOL Multiphysics. This includes diaphragm deflection, sensitivity and linearity analysis, capacitance and thermal considerations. Capacitance values are plotted under uniform external pressure 10kPa. Selected materials include silicon and carbon, acknowledging that carbon has shown the best and steady result. Simulation results also show how the capacitance and diaphragm deformation varies under increasing pressure. Keywords: Carbon diaphragm, COMSOL Multiphysics, diaphragm displacement, MEMS, sensitivity, Pressure sensor. 1. INTRODUCTION The most attractive features of MEMS capacitive pressure sensors are low power consumption, high sensitivity, and immutability temperature effects [1]. This model performs an analysis of a hypothetical sensor design using the electromechanics interface. The effect of a rather poor choice of packaging solution with the performance of the sensor is also considered. The results demonstrate the importance of considering packaging in the MEMS design process. Vacuum comparment Dividing diaphragm with electrode Base with counterelectrode Pressurized compart ment Fig. 1. One quarter of the MEMS capacitive pressure sensor in 3D. 1476
2. 2. STRUCTURE The model geometry is illustrated in Figure 1. The pressure sensor is part of a carbon die that has been bonded to a silica glass plate at 400 °C. Since the geometry is symmetric, only a single quadrant of the geometry needs to be included in the model, and it is possible to use symmetry boundary condition. The pressure sensor consists of a carbon layer structure that includes a micrometer-thick diaphragm situated between two silica glass layers. Figure 2 shows the geometry and the dimensions are given in Table I. In addition, two 1mm2 rectangular plates at the pressurized compartment‘s top and bottom form the electrodes [1,2,5,6]. Vacuum comparment Dividing diaphragm Base with with electrode counterelectrode Pressurized compart ment Fig. 2. 2D view of a pressure sensor [2]. Table 1. Device component dimensions and materials [1] Dimensions and Materials Property Top and bottom Vacuum Pressurized Middle layer layers compartment compartment Rectangle with Symmetric Shape Rectangular Rectangular Engraved cavities trapezoid Width/ 2.5 mm top: 1.9 mm 2.5 mm 1.5 mm 1.5 mm Length diaphr.: 1.5 mm bottom: 1.5 mm 0.5 mm Height 0.5 mm 0.475 mm 5 µm diaphr.: 20 µm Material Silica glass Carbon Vacuum Air 1477
3. 3. MATERIAL PROPERTIES 3.1 Silica glass Silica glass is a kind of glass which is composed of almost only SiO2, while other glasses are composed of various kinds of elements. The amount of metallic impurity contained in silica glass is extremely little, and even silica glass having a large amount of metallic impurity has only several 10 ppm and silica glass having the small amount of metallic impurity has less than 10 ppb. In this way, the fact that the purity is extremely high brings to silica glass it's the excellent characteristics which cannot be seen in other glasses [3]. 3.2 Carbon Carbons are unique tubular structures of nanometer diameter and large length/diameter ratio. The amazing mechanical and electronic properties of the nanotubes stem in their quasi-one-dimensional structure and the graphite-like arrangement of the carbon atoms in the shells. Thus, the nanotubes have high Young‘s modulus and tensile strength, which makes them preferable for composite materials with improved mechanical properties. The nanotubes can be metallic or semiconducting depending on their structural parameters. This paves the ways for application of the nanotubes as central elements in electronic devices, including field-effect transistors and MEMS sensor [4]. The material properties of Silica glass and Carbon have been presented in Table 2. Table 2. Material properties of Slica glass and Carbon [1] Material Property Silica glass Carbon Modulus Young 73.1e9 [Pa] 105e9 [Pa] Poisson‘s ratio 0.55 0.1 Coefficient of thermal expansion 1.45e-6 [1/K] 0.8e-6 [1/K] Density 2203 [kg/m3] 3515 [kg/m3] 4. RESULTS AND DISCUSSION 4.1 Sensor Deformation, Stresses and Electric Field Figure 3 shows the results from the 3D model when the sensor is in operation: it is exposed to a pressure of one atmosphere at 15 °C. The largest stress on the diaphragm appears near the position where the diaphragm connects the surrounding material. Fig. 3. Sensor deformation with carbon membrane when being exposed to ambient pressure for A temperature conditions 1478
4. Figure 4 shows the results from the 2D mode at the same conditions. The figure is arbitrarily scaled and is focused on the left half of the lower cavity. The diaphragm deforms toward the vacuum with maximum deformation in the middle. Maximum stresses appear at the upper corners of the lower cavity where the membrane attaches to the silicon boundaries. The streamlines show the electric field in the lower cavity. The lines are vertically between the two electrodes. Some field lines appear outside of the electrode region, but the field strength is very small there [1]. Capacitance values computed from the electric field for two conditions: Conditions A has bonding taken place at 400 °C and the sensor is then cooled down to 22 °C. For Condition D, thermal deformation does result from the ambient temperature. Fig. 4. Deformation, stresses and electric field of carbon membrane when being exposed to ambient pressure at A temperature conditions Figure 5 shows the results of sensor deformation, stresses, electric field with silicon diaphragm. The graph illustrates that the displacements of the silicon membrane are less than the carbon diaphragm. Fig. 5. Deformation, stresses and electric field for silicon membrane when exposed to ambient pressure at A temperature conditions Simulation results show the mean and maximum displacements of the membrane as a function of applied pressure. These results indicate the mean diaphragm and the maximum diaphragm results are very close which increases the stability of the sensor. 1479
5. 4.2 Sensitivity Comparison of Silicon and Carbon Materials This result shows the response of device in presence of packaging stress. The operating temperature of the device is 22oC and bonding temperature is 400oC. Thermal stresses are introduced due to miss-match in thermal coefficient of expansion of two different materials. Thermal stress makes device output temperature dependent due to its dependence on temperature. The graph shows that due to thermal stresses, displacement is more dependent on pressure. The deformation of the membrane due to applied pressure causes a change in capacitance. The capacitance of the device decreases non-linearly with applied pressure as shown in the graph. Table 3. Sensitivity comparison of Silicon and Carbon materials Silicon Carbon Pressure [kPa] Sensitivity [nF/Pa] Sensitivity [nF/Pa] Sensitivity [nF/Pa] Sensitivity [nF/Pa] (At condition A) (At condition D) (At condition A) (At condition D) 2 0.088285 0.098588 0.108055 0.109068 4 0.079442 0.087537 0.095177 0.092777 6 0.072228 0.078719 0.085070 0.082213 8 0.066218 0.071503 0.076904 0.074123 10 0.061127 0.065482 0.070160 0.067578 For silicon diaphragm, results shows that as temperature increases, the sensitivity value decreases at uniform external applied pressure. For carbon diaphragm, results show that as temperature increases, sensitivity value increases at uniform external applied pressure. Due to thermal stress, the sensor response has become temperature dependent. 5. CONCULSION This research focus on the sensitivity evaluation of different materials that are used in designing diaphragm of the capacitive pressure sensor. The results demonstrate that the sensitivity in MEMS capacitive pressure sensors with a carbon diaphragm is higher than that in silicon. The reason is that the amount of thermal effect change in deflection is bigger. The simulation results are very promising which shows high sensitivity, small size of the effects of temperature, and accordingly may be used for various MEMS applications. REFERENCES [1] COMSOL Multiphysics, ―Capacitive Pressure Sensor,‖ MEMS Module Model Library. Retrieved 4/21, 2017, from [2] Amith.V, Sushil, Vyasaraj.T, Gururaj Hatti, Vikram Kumar, Suraj Kumar, Vandana Kumari, Divya S Kamble, ―Modelling& Simulation of Capacitive Pressure Sensor Using COMSOL Multiphysics 5.0,‖ IJIRSET, vol. 5. Issue 5, 2016, pp. 8407-8415. J. Clerk Maxwell, A Treatise on Electricity and Magnetism, 3rd ed., vol. 2. Oxford: Clarendon, 1892, pp.68–73. [3] TOSOH EUROPE BV, ―Silica-glass-characteristics,‖. Retrieved 4/21, 2017, from www.tosoheurope.com/our-products/silica-glass/silica-glass-characteristics. 1480
6. [4] Valentin N. Popov, ―Carbon nanotubes: properties and application,‖ Materials Science and Engineering, R 43, 2004, pp. 61–102. [5] Li, E. Kubba, Ahmed Hasson, Ammar, I. Kubba, and Gregory Hall1, ―A microcapacitive pressure sensor design and modelling,‖ J. Sens. Sens. Syst, vol. 5, 2016, pp. 95– 112. [6] Nallathambi, T. Shanmuganantham, ―Design of Diaphragm Based MEMS Pressure Sensor with Sensitivity Analysis for Environmental Applications,‖ Sensors & Transducers, vol. 188, Issue 5, 2015, pp. 48-54. 1481