MEMS ACCELEROMETER WITH MULTIAXIAL RESPONSE BY DYNAMIC RECONFIGURATION OF PIEZORESISTIVE BRIDGES V.Ferrari, A.Ghisla*, D.Marioli, A.Taroni University of Brescia, Department of Electronics for Automation and INFM Via Branze, 38 – 25123 Brescia, Italy *Corresponding author: A.Ghisla, Phone +39 0303715896, Fax +39 030380014, e-mail alessio.ghisla@unibs.it Abstract: A MEMS accelerometer manufactured in silicon bulk micromachining with piezoresistive detection is presented. The sensor consists of a seismic mass suspended by four flexural beams that integrate two piezoresistors each. The piezoresistors can be externally connected into different Wheatstone bridges, providing acceleration sensitivity along different axes. By means of the dynamic reconfiguration of the piezoresistive bridges a triaxial response is obtained, allowing the measurement of the x, y and z components of acceleration. An electronic interface for the dynamic reconfiguration of the bridges and for the signal conditioning is proposed. Experimental results are reported demonstrating good agreement with theoretical predictions. Keywords: MEMS accelerometer, piezoresistive detection, resistive bridge, dynamic reconfiguration. 1 SENSOR DESCRIPTION Micromachined silicon accelerometers are established devices that offer good performance-to- cost ratio. In an increasing number of applications, the measurement of the acceleration along three orthogonal directions is required, demanding for multiaxial accelerometers [1]. Bulk- and surface- micromachining accelerometers present a good linearity and a low level of cross-axis sensitivity. In [2, 3] a triaxial acceleration measurement is obtained by means of multi-mass devices where the measurement is accomplished by combining the acceleration detected by each monoaxial device. In [4, 5] triaxial accelerometers are reported where piezoresistive Wheatstone bridges are implemented for the different measurement axes. In this work a piezoresistive accelerometer capable of multiaxial response was designed, manufactured in silicon bulk micromachining and tested. The device was fabricated in the SensoNor Normic Multimems process offered as a Europractice MPW service for MEMS. Figure 1 shows a schematic diagram of the sensor structure. Figure 1. Schematic diagram of the accelerometer structure. The sensor consists of a seismic mass (2.55 x 2.55 x 0.4 mm) linked to the silicon substrate by means of four suspension beams (0.55 x 0.44 x 0.023 mm) arranged symmetrically at the sides of the mass. Two piezoresistors are implanted at both ends of each beam. Figure 2 shows a top-view picture of the sensor. Figure 2. Sensor top-view picture. The applied acceleration produces the roto- translation of the mass and the deflection of the beams [4, 5]. The eight piezoresistors are only partially connected internally to the sensor, while selected nodes are made accessible through external pads. In this way, the set of piezoresistors can be either configured in two separate Wheatstone bridges for the detection of x and y components of the acceleration, or in a single bridge for the detection of the z component. Figures 3(a) and 3(b) show the displacement of the mass, the corresponding variations of the piezoresistors, and the resulting effects on the bridges for x and z accelerations. For accelerations along the x axis, the mass tilts around the y axis and only the bridge formed by piezoresistors R1 to R4 produces an unbalance. Similarly, for accelerations along the y axis, the mass tilts around the x axis and only the R5-R8 bridge undergoes an unbalance. For accelerations along the z axis, the mass translates out of plane causing the same deformation in all the beams, and the configuration of the piezoresistors produces an unbalance in the z bridge only. The symmetry of the structure and the arrangement of the piezoresistors reduce the cross-axis sensitivity. Proceedings of the Eurosensors XX Conference, Göteborg, Sweden, 17-20 September, 2006, M2B-P18. ISBN/ISSN: 97891-631-9281-4.