Micromachined Polysilicon Resonating Xylophone Bar Magnetometer: Resonance Characteristics
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well as integration of microelectronics. In our laboratory, we have been investigating the miniaturization of a wide dynamic range, high sensitivity xylophone bar magnetometer [1-3]. The active element of the xylophone magnetometer is a conductive bar supported at the nodes of its fundamental mode of flexural vibration [4]. A sinusoidal current is supplied to the bar at this frequency through the support arms, and in the presence of a magnetic field component parallel to the surface of the xylophone bar and normal to the drive current direction gives rise to a harmonically varying Lorentz force that drives the xylophone bar at its resonance. The response amplitude is linearly proportional to the drive current (I), magnetic field (B), and mechanical quality-factor (Q) of the xylophone resonator [1-3]. EXPERIMENT The polysilicon xylophone bars, support arms and mounting were fabricated at the MCNC MEMS foundry [5]. SEM images of a typical device are shown in Figure 1. The lower electrode consists of a 0.5 gim thick polysilicon layer patterned on the silicon nitride-coated silicon substrate. The xylophone bar, support electrodes, and
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Mat. Res. Soc. Symp. Proc. Vol. 605 © 2000 Materials Research Society
Figure 1. Scanning electron microscopy images of a microfabricated polysilicon xylophone bar (500 m x 50 m), support arms and mounting pads. mounting pads are fabricated from 2 gim thick polysilicon suspended 2 pim above the silicon nitride layer (following release of the sacrificial silicon dioxide layer). The mounting pads are attached to the lower electrode by the indicated patterns to provide a rigid anchor, which minimizes vibrational coupling to the xylophone bar [6]. Contact between the xylophone bar and the silicon nitride layer is minimized by having 0.75 jim deep dimples in the center and toward the ends of the xylophone bar. Evaluation of these polysilicon xylophone bar resonators has been carried out in an evacuated chamber, using a bench-top beam deflection microscope [1-3]. Magnitude and phase data were obtained in a mT static magnetic field by scanning and detecting at the frequency of the sinusoidal drive current through a 500 m x 50 m xylophone bar with 4 m x 50 m support arms at a chamber pressure of 15 mTorr. A resonance frequency of 78.2 kHz and a Q of almost 22,000 were obtained from these data. For reference, the predicted frequency for a polysilicon free-free resonating bar (assuming a Young s modulus of 160 GPa) is 69.2 kHz; the higher measured values have been attributed to the torsional resistance offered by the support beams [3]. To further explore this difference in measured and predicted frequency, experimental data were also obtained from xylophone bars with 6 and 10 gim wide support arms. The measured resonance frequencies of these xylophone bars were 84.9 and 95.6 kHz, respectively. MODELING A mathematical model of the xylophone magnetometer incorporating an approximation for the mass and stiffness contributions of the support arm was developed to test the validity of the above no
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