Stress free quad beam optical silicon accelerometer. 1
J.A. Plaza1, A. Llobera1, J.Berganzo2, J.Garcia2, C.Dominguez1, J.Esteve1 Centro Nacional de Microelectrónica, campus UAB, Cerdanyola del Vallés, Barcelona, Spain,
[email protected], Tel: +34 935947700, Fax: +34 935801496 2 Ikerlan, Electronics and Components Department
Topics: 3. Mechanical and thermal sensors, 4. Optoelectronic/photonic sensors
Abstract The paper is focus on the design, simulation, fabrication and preliminary characterisation of a new optical accelerometer. The working principles of the commercial micro-accelerometers are mainly piezoresistive and capacitive. However, these working principles have inherent drawbacks as temperature dependence, low sensitivity, and high sensitive to electromagnetic interference. Optical accelerometers are insensitive to these factors and have a high sensitivity [1-3]. . The accelerometer presented is a bulk Quad beam design. It consists of one central mass connected to the silicon frame by four beams. The beams have a L-shape to increase the mechanical sensitivity of the device. There is defined an ARROW structure on the mass that its aligned to two identical wavesguide structures on the frame of the chip, figure 1. Therefore, the applied acceleration force a displacement of the mass that cause a misalignment between the waveguide of the mass and the waveguides on the frame. The misalignment reduces the output intensity as a function of the acceleration. The accelerometer has been optimised by the Finite Element Method (FEM) using ANSYS V5.7. The device was designed to have a mechanical sensitivity (displacement of the mass versus the acceleration) of 1µm/g. The cross sensitivities, resonance frequency and modal modes were also simulated. The accelerometers were fabricated on 450µm thick BESOI (Bond and Etch back Silicon On Insulator) wafers. Firstly the ARROW structures were defined by growing, depositing and patterning silicon oxide and silicon nitride layers. Special efforts were done to reduce the intrinsic stress of these layers. Secondly the mechanical structure that defined the accelerometer was defined buy anisotropic etching and dry etch process. The finished silicon wafer were anodically bonded to glass wafers The glass wafer were previous etched to have cavities to permit the movement of the masses and to control the damping of the devices. The chip size is 6.2 x 6.2 mm2. The preliminary static and dynamic characterisation of the accelerometers will be also presented on the conference. a)
b) Optic Fibers
Wave guides
Accelerometer Acceleration
Fig.1: a) Schematic view of the working principle of the optical accelerometer and b) photographic picture of the accelerometer with the optics fibers aligned References: [1] E.Abbaspour-Sani, R.Huang, and C.Y.Kwok, "A novel optical accelerometer", IEEE Electron Device Letters, vol. 16, no.5, May, pp. 166-168, 1995. [2] M.Ohkawa, M.Izutsu, and T.Sueta, "Integrated Optic accelerometer Employing a Cantilever on Silicon Substrate", Japanese Journal of Applied Physics, vol. 28, no 2, pp. 287-288, 1989. [3] J.Kalenik and R.Pajak, "A cantilever optical-fiber accelerometer", Sensors and Actuators A, 68, pp. 350-355, 1998.
