STUDY OF A THREE-AXIS PIEZORESISTIVE ACCELEROMETER WITH UNIFORM AXIAL SENSITIVITIES Abdelkader Benchou, PhD Canddate Nasreddne Benmoussa, PhD Kherreddne Ghaffour, PhD Unversty of Tlemcen/Unt of Materals and Renewable Energes URMER, Algera Abstract: Mcro-electro-mechancal systems (MEMS) for automotve ndustry and bomedcal applcatons (BoMEMS) have the fastest growth rate wthn the MEMS market. The Mcrosystems job market mposes to research laboratores and unverstes to respond by ncreasng the qualty of MEMS engneerng and nformatcs nterdscplnary tranng programs. In ths fact, our work conssts to study and develop a three-axs pezoresstve accelerometer havng unform senstvtes along to three axes. Ths sensor whch s made of a heavy proof mass and four long beams, allow us to obtan hgh senstvtes, by reducng the resonant frequences. Unform axal senstvtes, wth a transverse senstvty, could be obtaned usng a three-axs sensor. The stress analyss of ths sensor was performed n order to determne the postons of the pezoresstances, n the four flexure beams. Key Words: Accelerometer, MEMS, Pezoresstance, Smulaton Introducton The mcro-machned nertal sensors whch are composed of accelerometers and gyroscopes have a sgnfcant percentage of sensors contanng slcon. They can be found manly n the automotve ndustry, the bomedcal applcatons, the household electroncs, robotcs, vbraton analyss systems, navgaton systems. There are varous technques to transform the acton of acceleraton on the sensor nto electrc sgnal. These technques are based on prncples capactve, pezoresstve, pezoelectrc, and ect The concept of accelerometer s not new, but ts fabrcaton offers new market opportuntes to Mcrosystems manufacturers, so the MEMS market has motvated contnuous researches n ths knd of sensors n order to mnmze the sze and to mprove the performance. As we know, the realstc applcatons create an enormous motvaton for research on sensors MEMS, especally accelerometer. In ths modern world, the applcatons requre new sensors wth a smaller sze and hgh performances. In practce, rare are research whch can provde an effectve and complete methodology for the desgn of accelerometers. The proposed of three-axs accelerometer The three-axs accelerometer always requres small cross-axal acceleraton, hgh and lnear senstvty. We proposed a flexure confguraton that s show n fgure (1) n order to meet these crtcal characterstcs. The parameters of the structure brought to the FEM process s shown n table (1). 95
Parameters Sze (Length, Wde, Thckness) Proof mass 845x845x400 µm 3 Beams 975x80x10 µm 3 Anchors 200x200x200 µm 3 Global structure 1.5x1.5x0.5 mm 3 Fgure 1: Structure of three-axs pezoresstve accelerometer. Table 1: Parameters of structure. When an external acceleraton s appled to the sensor, the proof mass s devated. The vertcal component (Az) of acceleraton causes a vertcal dsplacement of the mass. The second type of movement s caused by transverse acceleratons (Ax and Ay). The devaton of the proof mass causes a varaton of the stress on four surfaces of the beams. Ths can be measured by twelve p-type and n- type pezoresstances dffused and assembled by three Wheatstone brdge crcuts. These pezoresstances was algned wth the crystal drecton < 110 > and < 11 0 > of slcon (100). In slcon materal, there are only three ndependent pezoresstve coeffcentsπ 11, π 12 and π 44.The longtudnal pezoresstve coeffcent 1 s defned n the case the stress s parallel to the electrc feld. Smlarly, the transverse pezoresstances coeffcent π t s defned n the case the stress s perpendcular to the electrc feld. For the orentatons < 110 > and < 11 0 > of slcon (100), these coeffcents can be expressed as follows: 1 1 πl = ( π11 + π12 + π44 ) πt = ( π11 + π12 π44 ) 2 (1) 2 (2) Desgn and smulaton usng ANSYS The fnte element method (FEM) s appled to perform analyses of the stress dstrbuton n the flexure beams. Consderng the stress dstrbuton, the pezoresstances are placed n order to elmnate the transverse senstvtes and to obtan maxmum senstvtes for the three components of acceleraton. The fnte elements model of the sensng structure was analyzed by usng software ANSYS. The boundary condton, by consderng the fxed anchor, and the free proof mass charged n the medum by acceleraton n the form of a force was appled. Fgure (2) shows the generaton of mesh for the analyss by the fnte element method. Fgure 2: The mesh generaton of the FEM model. 96
The stress dstrbuton on the surface of the beams, caused by the Az component of acceleraton, s shown n fgure (3). The prncple of detecton of the sensor s based on the characterstc of the p-type and n-type pezoresstances. The n-type pezoresstances decreases when the sensor s exerted by a tensle stress and contrary n the case of a p-type pezoresstances. The fgures (4.a) and (4.b) shows the stress analyss results along 1 st and the 3 rd beams when the sensor s exerted to a force caused by acceleraton along axs z. From these fgures, we can fnd the optmal locatons for the pezoresstances Wheatstone brdge of component Az. Fgure 3: The stress dstrbuton on the beams caused by the 1g acceleraton Az. Fgure 4.a Fgure 4.b Fgure 4: Longtudnal stresses on the surface of the 1 st and 3 rd beams due to 1g acceleraton Az. By the same method, the acceleraton components Ax and Ay can be detected by usng four pezoresstances on 2 nd and 4 th beams of the Wheatstone brdge of component Ax, and four pezoresstances on 1 st and 3 rd beams of the Wheatstone brdge of component Ay. The postons of the pezoresstances are also ndcated on the fgures (5.a) and (5.b). 97
Fgure 5.a Fgure 5.b Fgure 5: Longtudnal stresses on the surface of the 2 nd and 4 th beams due to 1g acceleraton Ax and Ay. From smulaton results, we would found that two normal stresses are rather smaller when comparng to the longtudnal stress σ l. The total resstance change s gven by the followng equaton: R σ = l Gl. ε = Gl. R E = x, y, z (3) Where G l s a longtudnal gauge factor: Gl = πl. E + 1 + 2. υ (4) E s a Young s modulus; υ s a Posson rato and ε s the tensle stran. In the equaton (4), we have: πl. E >> 1+ 2. υ R π l. σ l R Thus the equaton (3) becomes: (5) Vout R The electroncs senstvty can be gven by: S = =. Vn = πl. σl. Vn a R (6) Where S s the senstvty to the th acceleraton component, V n and V out are the nput and output voltage, respectvely. The followng fgure gves the Wheatstone brdge crcuts for the three acceleraton components. Fgure 6: Wheatstone brdge crcuts of three acceleraton components. 98
N- P-R P-R N- N- Table (2) gves the ncrease (+), the decrease (-), or the nvarance (0) of the pezoresstances by the applcaton of the Ax, Ay and Az components of acceleraton. P-R Z1 N-R Z2 N-R Z3 P-R Z4 P-R Y1 N- P-R X4 R Y2 R Y3 Y4 X1 R X2 R X3 Az + - - + + - - + + - - + Ay 0 0 0 0 + - - + 0 0 0 0 Ax 0 0 0 0 0 0 0 0 + - - + Table 2: Pezoresstance values changes of three acceleraton components. Concluson Ths work presents a desgn and smulaton of three-axs pezoresstve accelerometer usng MEMS technology. The sensng prncple of the sensor s the pezoresstve effect. The most mportant aspect of Fnte Element Analyss (FEA) n our desgn process s the analyss of the stress dstrbuton n the four flexure beams. The stress analyss was performed n order to determne the postons of the pezoresstances on these beams, and consequently to elmnate the transverse senstvtes for obtanng optmal three acceleraton components. Ths model of three-axs accelerometer s used n the feld of bomedcal applcatons (BoMEMS). References: [1] EDITORS-IN-CHIEF; YOGESH B. GIANCHANDANI, O. TABATA, H. ZAPPE, Comprehensve Mcrosystems volume one, Copyrght 2008 Elsever. [2] N. MALUF, K. WILLIAMS, An Introducton to Mcroelectromechancal Systems Engneerng Second Edton, Copyrght 2004 ARTECH HOUSE, INC. [3] S. BEEBY, G. ENSELL, M. KRAFT, N. WHITE, MEMS Mechancal Sensors, Copyrght 2004 ARTECH HOUSE, INC. [4] N. TIEN ANH, T. DUC TAN, A Tree-Axs Pezoresstve Accelerometer wth Unform Axal Senstvtes Cau Gay, Ha No, Vet Nam, Copyrght 2011 IEEE. [5] Y. KANDA, Pezoresstance Effect of Slcon, Sensors and Actuators, 1991, pp.83. [6] Y. KANDA, A Graphcal Representaton of the Pézorésstance Coeffcents n Slcon, IEEE Trans. On electron Devces, Vol.29, n 1, 1982. 99