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重力感应器4.pdf

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重力感应器4.pdf重力感应器4.pdf Optimized Design of a Stacked Diaphragm MEMS Pressure Sensor for Tsunami Warning System Suja K J, Santo Mathew, Rama Komaragiri Department of Electronics and Communication engineering National Institute of Technology Calicut, Kerala. rama@nitc.ac.in ...
重力感应器4.pdf
重力感应器4.pdf Optimized Design of a Stacked Diaphragm MEMS Pressure Sensor for Tsunami Warning System Suja K J, Santo Mathew, Rama Komaragiri Department of Electronics and Communication engineering National Institute of Technology Calicut, Kerala. rama@nitc.ac.in Abstract—Tsunami is a large scale, short duration vertical dis-detecting tsunamis, and a moored surface buoy for real-time placement of water column. Tsunami results in a pressure communications. An acoustic link is used to transmit data change in the sea bed. The Tsunami warning system (TWS) con-from the BPR on the seafloor to the surface buoy. The data are sist of a series of sensors which detect the change in pressure at then relayed via a GOES (Geostationary Operational Envi-the sea bed when tsunami waves are generated. Micro electro ronmental Satellites) link to ground stations. Figure 1 shows mechanical system based silicon pressure sensors have under-NOAA (National Oceanic and Atmospheric Administration) gone a significant growth in the last few years. The sensitivity, tsunami warning system. maximum measurable pressure and linear range over the deflec- tion of pressure sensor highly depend upon the diaphragm struc- ture. In this work, a stacked diaphragm pressure sensor is de- signed and simulated which can be used for under water pres- sure measurements. A novel method for sensitivity enhancement by optimizing the thickness of various layers for stacked dia- phragms is presented. Also a study of the bulk micro machined silicon piezoresistive pressure sensor and a surface micro ma- chined stacked diaphragm pressure sensor is presented, simulat- ed and compared with respect of sensitivity and deflection. Keywords— Tsunami Monitoring, MEMS, Piezoresistive Pres- sure Sensor, Surface micromachining, Bulk micro machining. 1. INTRODUCTION Tsunamis generally consist of a series of waves with periods ranging from minutes to hours, arriving in a so-called wave train. Wave heights of tens of metres can be gen-erated by large events. Although the impact of tsunamis is limited to coastal areas, their destructive power can be enor-mous has huge impact on economy, lives and environment and even can result in catastrophes. The principal generation mechanism of a tsunami is the displacement of a substantial volume of water or perturbation of the sea. This displacement of water is usually attributed to earthquakes, landslides, vol-canic eruptions and glacier calving or more rarely by meteor-ites and nuclear tests. The waves formed in this way are then sustained by gravity. Tsunamis have a small amplitude (wave height) offshore, and a very long wavelength (often hundreds of kilometres long, whereas normal ocean waves have a wave- length of only 30 or 40 metres), which is why they generally pass unnoticed at sea, forming only a slight swell usually about 300 millimetres (12 in) above the normal sea surface. Figure 1. NOAA tsunami warning system [1] They grow in height when they reach shallower water, in a wave shoaling process. A tsunami warning system (TWS) is The sensor detects the presence of a Tsunami by detecting the used to detect tsunamis in advance and issue warnings to pre-pressure variation in the sea bed [2]. Ordinary wind waves vent loss of life and damage. It is made up of two equally im-disturb only a very thin layer of water at the surface of the sea, portant components: a network of sensors to detect tsunamis and so don't affect the sea floor far, far below. Tsunami moves and a communications infrastructure to issue timely alarms to the entire water column, which in the open sea is thousands of permit evacuation of the coastal areas [1]. A TWS consists of a meters thick. So when a tsunami rolls over a sensor on the sea seafloor Bottom Pressure Recording (BPR) system capable of floor, it will cause a noticeable change in water pressure. 978-1-4799-1095-3/13/$31.00 ?2013 IEEE 346 development direction, prevention medical patient disputes, bad event, hospital of technology of, digital, network, intelligent of, information became currently and future measure hospital management level and operation efficiency of a key factors. currently domestic of medium hospital are this aspects constantly improve hospital information level, Technology management level, medical level in order to occupy a strong position in the market competition. The weak electricity system construction project is going digital, centralized management of the network. perform advanced, stable, reasonable structure, strong expansion, economic and practical standards of the integrated management system. Engineering range this times weak system construction project of engineering covers following system: integrated wiring system (including network system and voice communications system), wired TV system, information guide and the released system, monitoring system, access management system, parking management system, surgery monitoring teaching system, ICU visits system, call on told management system, queued station system, buildings control system, weak intelligent of management system. In the construction of weak current system, we believe that the needs of each independent and stable operation of the subsystem, and mutual interaction between and mutual support is a very important step. Because Pressure sensors are one of the most common Micro Electro ty are reported [2]. The load deflection method that describes Mechanical System (MEMS) devices. The MEMS sensor fab-the relation between displacement and applied pressure for a rication technology enables miniaturization of complex sys-tems flat square diaphragm is given by [3] eqn. (1). by integrating the sensing, controlling and actuating functions on a single chip. Diaphragm dimensions are deter-mined by the 4 3 Pay ,4.2 , y , 1.58 , pressure range and maximum pressure that needs to be measured. , , (1) , , , , MEMS based pressure sensors are clas-sified into piezoresistive 4 2Eh ,1,, , , h , ,1,, , , h , and capacitive based depending upon the on pressure sensing Where E is Young’s modulus, ν is Poisson’s ratio of the dia-mechanism. The piezoresistive pres-sure sensor utilises the phragm material and h is the diaphragm thickness. According piezoresistive property of silicon to measure applied pressure. The to the load-deflection method, the deflection range is divided piezoresistive device works on the principle of change in into two regions: a small deflection region where deflection resistance with the deflection of the diaphragm due to applied less than 25% of the diaphragm thickness described by the pressure. Generally Wheatstone bridge configuration is formed by linear term in eqn. (1) and a large deflection region described using four different resistors. by the non-linear, cubic term in eqn. (1).The square dia- phragm has the highest induced stress with the application of a given pressure [4] and thus a square diaphragm is preferred for the design of pressure sensor in this work. For a square plate clamped at the edges, the maximum stress σ at the max middle of each edge is given by eqn. (2). 2 0.308Pa , , (2) 2 h max Figure 2. Structure of a piezoresistive silicon pressure sensor The minimum thickness of silicon which gives the maximum Figure 2 shows the cross section of a bulk micro machined sensitivity without damaging the diaphragm is given by eqn. square diaphragm with diffused piezoresistors. The substrate (3). is silicon and the cavity is produced by anisotropic etching on 1 , 0.39P , one side. Piezoresistors are implanted on other side of the sub- 2 h,strate to sense the induced stress due to applied pressure. The min , B , (3) , performance of the piezoresistive pressure sensor depends on , max , the dimensions of the diaphragm and the position of the resis- “P” is the applied pressure and “a” is side length of the square tors on the diaphragm. In order to obtain high deflection dia- phragm should be thin. A very thin diaphragm not only inca- diaphragm under consideration. pable of measuring high pressure but also results in non-linear The maximum deflection for a square diaphragm with a given effects. P-type diffused resistors with large piezoresistive co- side length and thickness is given by eqn. (4). efficients on an N type layer are used in pressure sensors. 4 W , 0.00126Pa (4) max D Where D is the bending rigidity of the diaphragm material and is given by eqn. (5). 3 Eh2 Figure 3. Structure of a SOI MEMS pressure sensor D , 121 ,, (5) Figure 3 shows the schematic cross section of a SOI pressure , , sensor implemented by surface micromachining process. The The maximum deflection is inversely proportional to bending insulator material can be SiO, SiN or any other dielectric. 234rigidity. As thickness of diaphragm increases, rigidity increas-The total thickness h of the diaphragm is the sum of insulator es which in turn decreases the deflection. The output voltage thickness h and top silicon layer thickness h. 12and sensitivity of the diaphragm depends on the stress induced which in turn is a function of deflection. The dimensions of 2. 2. THEORY AND DESIGN the diaphragm can be calculated such that the stress generated The diaphragm can be modelled as a well-known thin-plate on application of pressure is less than fracture stress of silicon, problem [1].Analytical model for burst pressure and sensitivi- which is 7 GPa, throughout the operating range of the device. 347 development direction, prevention medical patient disputes, bad event, hospital of technology of, digital, network, intelligent of, information became currently and future measure hospital management level and operation efficiency of a key factors. currently domestic of medium hospital are this aspects constantly improve hospital information level, Technology management level, medical level in order to occupy a strong position in the market competition. The weak electricity system construction project is going digital, centralized management of the network. perform advanced, stable, reasonable structure, strong expansion, economic and practical standards of the integrated management system. Engineering range this times weak system construction project of engineering covers following system: integrated wiring system (including network system and voice communications system), wired TV system, information guide and the released system, monitoring system, access management system, parking management system, surgery monitoring teaching system, ICU visits system, call on told management system, queued station system, buildings control system, weak intelligent of management system. In the construction of weak current system, we believe that the needs of each independent and stable operation of the subsystem, and mutual interaction between and mutual support is a very important step. Because 3. RESULTS AND ANALYSIS length increases, deflection increases. For a given side length and thickness SOI diaphragm gives better deflection than sili- con diaphragm. The silicon diaphragm is realized by conven-3.1 Comparative study of silicon diaphragm tional bulk micromachining and the horizontal and vertical and stacked diaphragm. edges of the diaphragm are essentially integral part of the sub-The structure was created and simulated using Coventor-strate. In contrast to this, the stacked diaphragm is realised by Ware? tool. Diaphragms with a side length of 500µm, 600µm surface micromachining where the edges of the diaphragm are and 700µm with various thicknesses are simulated. Deflection, not integral part of the substrate. Hence deflection is more and stress and output voltage are noted. The maximum pressure induced stress is less for stacked diaphragm than silicon dia-applied is 0.9MPa which is less than one tenth of the burst phragm with same dimensions. pressure used for calculating the minimum diaphragm thick- ness. The maximum stress induced is one tenth of the fracture stress as expected. Figure 4 shows the deflection and stress for a silicon diaphragm in which it is satisfying the two conditions necessary for the design. Figure 6. Comparison of diaphragm deflection for silicon and SOI diaphragm Figure 4. Deflection and stress analyses of silicon pressure sensors Figure 5. Simulation structure of a square diaphragm Figure 7. Comparison of diaphragm deflection for silicon and SOI diaphragm Figure 4 shows the Deflection and stress for a silicon pressure sensor with a side length of 500µm. Figure 5 shows the de- flection and stress distribution of a square diaphragm when a uniform load is applied. The deflection is maximal at the cen- tre of the diaphragm and stress induced is maximal at the middle of the edges. An applied pressure results in a bridge unbalance which results in an output. Figure 6 shows a com- parison of stress induced for silicon and SOI diaphragms for a given pressure for different diaphragm thickness. The stress induced is less for SOI diaphragm than a silicon diaphragm for a given pressure and diaphragm dimensions. Figure 7 and 8 respectively show a comparison of deflection and stress respectively for silicon and SOI diaphragms for different diaphragm dimensions and an applied pressure of 0.9 Figure 8. Comparison of diaphragm deflection for silicon and SOI diaphragm MPa. As the thickness of the diaphragm increases, the bend- ing rigidity which is proportional to the thickness increases Figure 8 shows a comparison of sensitivity of silicon dia- and deflection decreases. For a particular thickness, as side 348 development direction, prevention medical patient disputes, bad event, hospital of technology of, digital, network, intelligent of, information became currently and future measure hospital management level and operation efficiency of a key factors. currently domestic of medium hospital are this aspects constantly improve hospital information level, Technology management level, medical level in order to occupy a strong position in the market competition. The weak electricity system construction project is going digital, centralized management of the network. perform advanced, stable, reasonable structure, strong expansion, economic and practical standards of the integrated management system. Engineering range this times weak system construction project of engineering covers following system: integrated wiring system (including network system and voice communications system), wired TV system, information guide and the released system, monitoring system, access management system, parking management system, surgery monitoring teaching system, ICU visits system, call on told management system, queued station system, buildings control system, weak intelligent of management system. In the construction of weak current system, we believe that the needs of each independent and stable operation of the subsystem, and mutual interaction between and mutual support is a very important step. Because layer where the piezoresistors are integrated which is applied phragm and SOI diaphragm for a given dimension and applied pressure. The sensitivity of SOI diaphragm is greater than that to the piezoresistor decreases. So sensitivity decreases as the of silicon diaphragm since the deflection obtained is greater insulator layer thickness increases. Sensitivity shows the same for SOI diaphragm. Also the range of operation for stacked trend for any applied pressure. diaphragm pressure sensor is more because of less stress in- duced for a given pressure. Hence a stacked diaphragm is al- ways preferred over a silicon diaphragm. 3.2 Sensitivity enhancement of stacked dia- phragm pressure sensors Figure 9 and 10 respectively show the deflection of SOI dia- phragm when the insulation layer thickness is varied from 0% to 60% of the total thickness for two different applied pres- sures. The deflection shows an increasing trend with an insu- lation layer thickness up to 30% of total diaphragm thickness Figure 11.Sensitivity of SOI diaphragm for various insulation layer thickness-and beyond that, it is found to be almost saturating due to rap-es. (Pressure applied = 0.9MPa) id increase in flexural rigidity. The device is simulated for two diffrent pressure ranges. The range of pressure applied are 0.1 MPa to 0.9 MPa and 1 to 9 kPa. In both pressure ranges the change in deflection shows the same trend. It shows an in- creasing trend with insulation layer thickness up to 30% of total diaphragm thickness and beyond that it is found to be almost saturating. Figure 12. Sensitivity of SOI diaphragm for various insulation layer thick-nesses (pressure applied = 9kPa) 3.3 Sensitivity analysis of single diaphragm P-type piezoresistors are oriented in the <110> direction on a (100) diaphragm plane, which is connected in Wheatstone bridge configuration and is used as a sensing element. The Figure 9. Deflection of SOI diaphragm for various insulation layer thickness-piezoresistor can be treated as a point at the middle of the dia-es. (Pressure applied = 0.9MPa) phragm sides, so the bending moment per unit length acting on the edges parallel to the y axis and x axis are M and M and are given by eqn. (6) and eqn. (7). , w , 2 2 w2 2 M , ,D , ,, , , x, , ,x ,y , (6) , 2 2 , w M , ,D , 2, , , w, y2 , , ,y ,x , 22 M , ,0.0513, PL , M , ,0.0513, PL (7) xy Figure 10. Deflection of SOI diaphragm for various insulation layer thick-The stress can be calculated by eqn. (8) and eqn. (9). nesses. (Pressure applied = 9kPa) M x Figure 11 and 12 respectively show the variation in sensor 2 , , , , ,6 h(8) xlsensitivity when thickness of the insulation layer changes. Sensitivity increases initially, reaches a maximum when insu- M y lator layer thickness is 30% of total diaphragm thickness and , , , , ,6 2 (9) yththen decreases. As the oxide layer thickness increases, the σ and σ are the transverse and longitudinal stresses respec- ltstress relaxation becomes more and stress at the top silicon 349 development direction, prevention medical patient disputes, bad event, hospital of technology of, digital, network, intelligent of, information became currently and future measure hospital management level and operation efficiency of a key factors. currently domestic of medium hospital are this aspects constantly improve hospital information level, Technology management level, medical level in order to occupy a strong position in the market competition. The weak electricity system construction project is going digital, centralized management of the network. perform advanced, stable, reasonable structure, strong expansion, economic and practical standards of the integrated management system. Engineering range this times weak system construction project of engineering covers following system: integrated wiring system (including network system and voice communications system), wired TV system, information guide and the released system, monitoring system, access management system, parking management system, surgery monitoring teaching system, ICU visits system, call on told management system, queued station system, buildings control system, weak intelligent of management system. In the construction of weak current system, we believe that the needs of each independent and stable operation of the subsystem, and mutual interaction between and mutual support is a very important step. Because tively. The fractional change in resistance of a resistor is sub- becomes too large for sufficient isolation from the bulk for jected to longitudinal and transverse stress is given by eqn. reasonable layer thickness. However, for surface concentra- 18-3(10). tions below 10 cm π will reach its limiting values. Figure 4413 shows the change in resistance due to stress in piezo re- ,R , , , , , , (10) sistance for different surface concentration. lltt R The values of πl and πt in <110> are given by eqn. (11) and A better sensor design requires two important performance eqn. (12). factors, namely high output voltage at a given burst pressure ,1 and high sensitivity. It is difficult to design a sensor with both l ,110, ,,, , , , , ,2 (11) the parameters maximized simultaneously, which translates 11 4412 into performance trade-offs. If a sensor exhibits higher sensi- ,,, t ,110, 12 ,,, , , , 1 (12) tivity, it typically offers low burst pressure, as the pressure 11 44 2 sensitivity is inversely proportional to burst pressure. Output Compared with π and π, π value dominates in P type ma- voltage of a sensor is expressed as in eqn. (20). 111244 terials. Then eqn. (11) and eqn.(12) can be reduced as, , ,R , V,V 1 out , ,in (20) , , , R (13) , , 2 l,110 , 44 Sensitivity can be expressed in eqn. (21). ,,t,110 , 44 , 1 V mV (14) V out 2 P The stress on the longitudinal resistor and transverse resistor S , P (21) V in are equal but act in 90? to each other. The total change in lon- gitudinal and transverse resistance is expressed by eqn. (15) and eqn. (16) as , ,R , ,, ,,,,,, (15) lt l , , R , ,l , ,R , , ,, , , , (16) , t t , , , l R , , t Thus, ,R , , ,R , , (17) , , , , , Figure13. Resistance change due to induced stress for different surface , R , , R , l t con-centration (in set change in resistance on log scale) π, π are negligible when compared to π for p - type dif-111244fused resistors. The general expression for fraction change in p type piezo resistance oriented in <110> direction on (100) diaphragm is given by eqn. (18). , ,, , , , , ,R (18) 44tl R The resistance change due to the pressure induced stress is expressed as in eqn. (19) [5]. 2 Figure 14. Structure of a double diaphragm pressure sensor ,R , ,0.1539, PL(19) R 44 2 hDouble diaphragm based piezoresistive pressure sensor can In the above equations, R and R are the change in resistance and handle a large range of pressure without compromising the actual resistance respectively. The p-type doping concen-tration 15-320-3linearity and sensitivity of the device compared with single of the piezoresistor is varied from 10 cm to 10 cm and diaphragm [10]. Figure 14 shows the structure of a double doped with boron to make it p-type. The fractional change of diaphragm. For a given pressure the deflection is more for resistance due to stress is less and almost constant for high-er double diaphragm than a single diaphragm as shown in fig-surface concentration values. For surface concentrations below ure15. Two set of piezoresistors were diffused, one set on the 18-310cm, the average resistivity of the diffused layer edges of inner diaphragm and other at the edges of outer dia- 350 development direction, prevention medical patient disputes, bad event, hospital of technology of, digital, network, intelligent of, information became currently and future measure hospital management level and operation efficiency of a key factors. currently domestic of medium hospital are this aspects constantly improve hospital information level, Technology management level, medical level in order to occupy a strong position in the market competition. The weak electricity system construction project is going digital, centralized management of the network. perform advanced, stable, reasonable structure, strong expansion, economic and practical standards of the integrated management system. Engineering range this times weak system construction project of engineering covers following system: integrated wiring system (including network system and voice communications system), wired TV system, information guide and the released system, monitoring system, access management system, parking management system, surgery monitoring teaching system, ICU visits system, call on told management system, queued station system, buildings control system, weak intelligent of management system. In the construction of weak current system, we believe that the needs of each independent and stable operation of the subsystem, and mutual interaction between and mutual support is a very important step. Because
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