ISSN 2319-8885 Vol.04,Issue.19, June-2015, Pages:3694-3698 www.ijsetr.com

Design and Simulation of MEMs Based Piezo-Resistive Pressure Sensor RAJESH KANUGANTI1, SARIKONDA RANADHEER VARMA2 1

Assoc Prof, Khammam Institute of Technology and Sciences, Khammam, TS, India, E-mail: [email protected]. 2 PG Scholar, Khammam Institute of Technology and Sciences, Khammam, TS, India, E-mail: [email protected].

Abstract: Micro electro mechanical system predicated silicon pressure sensors have undergone a consequential magnification in the last few years. The sensitivity, maximum quantifiable pressure and linear range of pressure sensors highly depend upon the diaphragm structure. In this work, single and double diaphragm predicated pressure sensors are designed and simulated and these can be utilized for high pressure quantifications. A novel method of sensitivity enhancement by optimizing the thickness of double diaphragms is presented in this work. Withal a study of the bulk micro machined silicon piezoresistive pressure sensor and surface micro machined stacked diaphragm pressure sensor are presented, simulated and compared with reverence to deflection and sensitivity. Microelectro mechanical system pressure sensors have been simulated with different diaphragm structures for obtaining wider operation range with better sensitivity. The performance of silicon and silicon on insulator pressure sensors at a given pressure are compared. The doping concentration of the piezoresistor is varied from 1015 cm-3 to 1020cm-3 and the sensitivity of pressure sensors are compared. Evaluating different structures of pressure sensors and optimizing doping concentrations as 1017cm-3, the double SOI sensor shows better pressure sensitivity. Keywords: MEMS, Piezoresistive Pressure Sensor, Surface Micromachining, Bulk Micro Machining, Sensitivity. I. INTRODUCTION MEMS or Micro-Electro-Mechanical Systems are chips that are made in semiconductor fabrications that coalesce electronic functions and mechanical actions to distribute extraordinary functionality and multifariousness. MEMS contrivances can sense and control making them valuable for numerous applications in automotive as well as in the medical field. Longer range preventive medicine and very early preemptive intervention are the essential strategies make health monitoring practical through perpetuating advances in diagnostics and wellness assessment enabled by leading-edge technology from other fields. Today, MEMS contrivances sanctions puissant and deployment of preventive and interceptive medical techniques. Sensing is the most astronomically immense category in the medical field. MEMS pressure sensors work on the principle of mechanical bending of thin silicon diaphragm by the contact medium. The diaphragm is the key sensing component of a MEMS pressure sensor and hence the realization of a high – performance diaphragm is consequential to achieve high efficiency of the sensor. The dimensions of the diaphragm like the thickness, length, radius, shape are taken into consideration and the effects of variations in pressure on the geometry of the sensing element are analyzed utilizing Intellisuite software implement [1]. In this paper the load-deflection and stress comportment of a diaphragm type pressure sensor (Si-frame), which is utilized in a spirometer, has been studied. The pressure sensing element coalesces resistors and an etched

diaphragm structure to provide an electrical signal that change with pressure. As the diaphragm, etched to a thickness estimated by the range to be quantified, moves under pressure. Stress is concentrated in categorical areas of the silicon element. Four ion implanted resistors in these areas vicissitude in value with compression. Culling the congruous location for the resistors and controlling the orientation of the resistors sanctions the MEMS designer to presage how the resistor will transmute in value for a given deflection of the diaphragm. II. RELATED WORK A. Spirometer Spirometer is a noninvasive diagnostic instrument utilized for screening and rudimentary testing of pulmonary function. Offering essential diagnostic insight into the type and extent of lung function impairment, spirometer tests can be performed expeditious at fairly low cost [2].Obstructive lung disease is a category of respiratory disease characterized by airway obstruction. It is generally characterized by inflamed and facilely collapsible airways, obstruction to airflow, quandaries exhaling and frequent office visits and hospitalizations. Types of obstructive lung disease include asthma, bronchiectasis, bronchitis and chronic obstructive pulmonary disease (COPD). Albeit COPD shares kindred characteristics with all other obstructive lung diseases, such as the denotements of coughing and wheezing, they are distinct conditions in terms of disease onset, frequency of symptoms and reversibility of airway obstruction [3]. In the light of an ever-incrementing prevalence of airway diseases,

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RAJESH KANUGANTI, SARIKONDA RANADHEER VARMA pulmonary function instruments have become indispensable volume loop is engendered by having the patient inhale diagnostic implements, in clinical and office settings, in deeply to total lung capacity (TLC), forcefully exhale until industrial and preventive medicine, as well as in the lungs have been vacated to residual volume (RV), and epidemiology. Represents spirometer instrumentation. rapidly inhale to reach TLC. Flow is plotted on the Y axis Screening of individuals in peril, rudimental testing of sick and the volume on the X axis; a typical loop is shown in patients and treatment follow-up are key applications of Fig.2. The upper portion of the curve reflects the expiratory portion of the coerced vital capacity (FVC) maneuver and is spirometer. withal referred to as the maximal expiratory flow-volume Spirometer has shown considerable magnification in past (MEFV) curve. Depending on their location (intra thoracic or 30 years for several reasons (a) published standards and extra thoracic), they incline to deport differently during testing guidelines, (b) ameliorated spirometer and software, inhalation and exhalation. (c) evidence that both patients and medicos have erroneous perceptions of the astringency of the airflow obstruction, (d) evidence that history taking and physical examination by themselves are not auxiliary in identifying patterns of lung disease, (e) recommendations that spirometer be included in the assessment of patients suspected of having asthma and recommendations of objective quantifications to reduce the impact of chronic pulmonary disease. The incipient spirometry technique is superior to subsisting spirometry techniques (pneumotach, ultrasound, sultry wire anemometer) for the following reasons. (a)Low cost, robust design; (b) Ease ofsterilization and maintenance (c) High precision; (d) More parameters can be quantified than similarly priced contrivances (e) Data transfer and storage capabilities.

Fig.2. Flow Volume Spirograms.

Fig.1. Spirometer Instrumentation. Spirometer measures the flow and volume of gas (air) moving in and out of the lungs during a breathing manoeuvre. The quantified flow and volume values are plotted as graphs called the spirograms (Fig.1) that are utilized for diagnosis of the patient. The spirometer is predicated on the quantification of either the flow rate or the volume of gas inhaled and exhaled during respiration. The pressure quantification system is additionally a flow rate quantification-predicated method where the flow rate is indirectly determined by quantifying the pressure (e.g., orifice or venturi tube meters). The flow volume curve depicts the cognation between the lung volume and the maximum rate of airflow as lung volume changes during a coerced expiration. The Fig.2 corresponds to flow volume spirograms to different calibers of disabilities. Mundane or salubrious person; (b) fine-tuned airway obstruction; (c) extra thoracic obstruction and (d) airflow obstruction [4]. A flow-

Breathing maneuvers need to be injuctively authorized limpidly; especially the coerced exhalation should be fortified by incentive commands. For evaluation, maximum coerced Flow rates (FEFs) are picked from the recording. Evaluation of spirometer testing is carried out by comparing quantified data with soothsaid normsor reference data, derived from body weight, height, age, and gender. Prognostications are calculated from equations published and recommended by scientific societies. When comparing quantified with soothsaid values, the standard deviation, a bespeaker of the variation of the tested parameter in a salubrious population, needs to be taken into consideration[5]. Lung volumes and lung capacities refer to the volume of air associated with different phases of the respiratory cycle. Lung volumes are directly quantified; Lung capacities are inferred from lung volumes. The average total lung capacity of an adult human male is about 6 litres of air,but only an iota of this capacity is utilized during mundane breathing. Tidal breathing is mundane, reposing breathing; the tidal volume is the volume of air that is inhaled or exhaled in only a single such breath. The average human respiratory rate is 30-60 breaths per minute at birth, decrementing to 12-20 breaths per minute in adults [6]. III. IMPLEMENTATION A. Modeling and Design of a Micro Differential Pressure Sensor A. Analytical Modeling The piezoresistive pressure sensor (Fig.3) measures the applied pressure on one side of the diaphragm. The stress

International Journal of Scientific Engineering and Technology Research Volume.04, IssueNo.19, June-2015, Pages: 3694-3698

Design and Simulation of MEMs Based Piezo-Resistive Pressure Sensor vicissitude in the diaphragm causes the resistance change of σl can be related with σ t via where ν is the Poisson ratio. The the piezoresistor. differential output voltage V of an ideally balanced bridge with assume 0 d identical (but opposite in sign) resistance change, ∆R, in response to an applied pressure P, on the sensor is given by (6)

Fig.3. A Typical structure of piezoresistive pressure sensor. The deflection of a uniformly loaded square diaphragm with clamped edge is given by (1) Where Y is the deflection at cen 0 ter of the diaphragm, p is the applied pressure, a is the dimension of side of the square diaphragm, E is the modulus of elasticity of diaphragm material, H is the thickness of the diaphragm, and ν is the Poisson‟s ratio. The maximum longitudinal stress σl is at the edge and it is given by (2) The maximum tangential stress σt is at the center and it is given by

(3) Fig.3 shows the Wheatstone bridge arrangement of piezoresistors. The longitudinal stress σl and transverse stress σt is experienced by the piezoresistors R1 and R3 respectively. Then, the piezoresistors R2 and R4 experience longitudinal stress σt and transverse stress σl which are rotated 90° compared with the stresses experienced by R 1 and R3. The piezoresistive coefficients π l and πt are given by (4) In a cubic semiconductor, the matrix of piezoresistive coefficients contains only three independent values, conventionally labeled as 11 12 π ,π , and π 44 .For a diffused resistor subjected to longitudinal and transverse stress components σl and σt respectively, the resistance change is given by (5)

Fig.4. Schematic in Architect. The differential pressure can the distinction between pressures between the two diaphragms and the differential voltage can be the distinction between the voltage experienced by the two signal conditioning circuits i.e. the Wheatstone bridge circuits. The analytical simulation is carried out in MATLAB. Table 1 shows the analytical simulation results of differential pressure sensor. b. Numerical Modeling The numerical design is done utilizing the FEM design implement Coventorware. Silicon of thickness 400 µm is deposited and anisotropically etched backside to get the diaphragm1 of thickness 10 µm. The piezoresistors are ion implanted on the diaphragm for transduction. The silicon of 2 mm thickness is deposited and etched to get high range over bulwark layer. Again Silicon of thickness 400 µm is deposited and anisotropically etched front side to get the diaphragm2 of thickness 10 µm. The square diaphragm is of size (500 × 500 × 10) µm3. The wafer thickness of 400 µm and sidewall angle of -35.3° is considered. The piezoresistors are of size (70 X 8 X 1) µm3. The diaphragm structure for differential pressure is engendered by designating process step in the process editor with the mask layout. The model is meshed with mapped mesh of linear element size 50. Architect is a module in MEMS design and simulation software Coventor Ware. It is utilized to find the optimized location of Piezoresistors on the diaphragms. The sensor has two diaphragms which work complementarily to each other by the applied differential pressure [3]. The output of Piezoresistors in Wheatstone bridge configuration is conditioned utilizing operational amplifiers and the same is being simulated in Architect, which is a module in Coventorware. The stress results got from MemMech analysis from designer is carried to architect module for signal conditioning. Fig.3 shows the 3D model of the sensor. Fig.4 shows the Schematic in Architect. Fig.5 shows the diaphragm deflection after numerical simulation.

International Journal of Scientific Engineering and Technology Research Volume.04, IssueNo.19, June-2015, Pages: 3694-3698

RAJESH KANUGANTI, SARIKONDA RANADHEER VARMA Table 2 shows the numerical results of micro differential ±5% of the nominal value of the thickness and the same procedure is reiterated for the diaphragm side and Modulus pressure sensor. of elasticity, considering each parameter individually at a A. Design Under Uncertainty time. For carrying out skepticality analysis numerically, the The deflection of a uniformly loaded square diaphragm vary analysis is performed in Coventorware, with all the with clamped edge under uncertainty is given by dimensional parameters and material properties 5% less than the nominal value and 5% more than the nominal values. The differential output voltage is calculated analytically by inputting all inputs in interval utilizing INTLAB. INTLAB is (7) an implement for calculating interval arithmetic‟s utilizing The maximum longitudinal stress ∆σl is at the edge and MATLAB. under uncertainty it is given by IV. CONCLUSION A conventional silicon MEMS pressure sensor and a SOI pressure sensor diaphragm were modeled. Optimizing the (8) doping concentration of piezoresistor as 1017cm-3 , the The maximum tangential stress ∆σt is at the center and double SOI shows better pressure sensitivity. The results under uncertainty it is given by shows that the silicon pressure sensor exhibited lower deflection and more stresses at its edges, than the SOI pressure sensors. When connected in Wheatstone bridge (9) configuration, the MEMS pressure sensor with SOI where ∆Y0 is the deflection at center of the diaphragm under diaphragm provides higher voltage output and sensitivity skepticality, P is the applied pressure under dubiousness, ∆a when compared to its silicon obverse. SOI pressure sensor is the dimension of side of the square diaphragm under withal provides wider range and better sensitivity. Hence SOI dubiousness, E∆ is the modulus of elasticity of diaphragm predicated double diaphragm predicated MEMS pressure material under skepticality, h∆ is the thickness of the sensors can be utilized for high pressure applications with diaphragm under skepticality, andν is the Poisson‟s ratio. better sensitivity when compared to its silicon counterparts. Interval Analysis is a technique used to estimate the bounds on sundry model outputs predicated on the bounds of the V. REFERENCES model inputs and parameters. [1] M. X. Zhouet. Al, “A Novel Capacitive Pressure Sensor In the interval method approach, skeptical parameters are postulated to be unknown but bounded and each of them has upper and lower limits without a probabilistic structure. Every dubious parameter is described by an interval [6]. An interval is a close set in R, which included the possible range of a number. In this paper, an interval will be represented by the authoritatively mandated pair [a, b] = {x: a =x = b} where „a‟ is the lower limit of the interval and „b‟ is the upper limit of the interval and „a‟ and „b‟ are authentic numbers. The number is kenned to lie between values but the exact value is unknown. Interval arithmetic is an elegant implement for practical work with inequalities, approximate numbers, error bounds, and more generally with certain convex and bounded sets. Let x = [a, b] and y = [c, d] be two interval numbers, a and c are lower limits, b and d are upper limits and a, b, c, d are authentic. 1. Addition: x + y = [a, b] + [c, d] = [a + c, b + d] 2. Subtraction: x - y = [a, b] - [c, d] = [a -d, b-c] 3. Multiplication: xy = [min(ac,ad,bc,bd), max(ac,ad,bc,bd)] 4. Division: 1 / x = [1/b, 1/a] When the pressure of 10 kPa is applied to diaphragm1 (which is at top) and pressure of 20 kPa is applied (which is at bottom), the analytical differential output voltage with nominal values for all the parameters is 6.8986v. When the same analysis is carried out for uncertainties in the range of 5% to the parameters like diaphragm thickness, diaphragm side and Modulus of elasticity, the differential output voltage is [1.6455, 12.4292] v. On the kindred line, considering a single dimensional parameter for example thickness, the sensor is analyzed with

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International Journal of Scientific Engineering and Technology Research Volume.04, IssueNo.19, June-2015, Pages: 3694-3698

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International Journal of Scientific Engineering and Technology Research Volume.04, IssueNo.19, June-2015, Pages: 3694-3698