| Capacitive MEMS accelerometer has the advantage of high sensitivity, large dynamic range, good noise performance, and low temperature sensitivity. They are widely used in the areas like inertia navigation, micro gravity test and oil prospecting.A capacitive sandwich accelerometer with double-sided beam-mass structure fabricated by self-stop etching is developed. In the beam-mass structure, eight straight beams symmetrically connect to the corners of the proof mass on both sides, and result in low off-axis sensitivity. The beam-fabrication process which combines ICP dry etching and KOH anisotropic self-stop etching realizes the double-sided beam-mass structure with a single (100) wafer. Through this simple beam-fabrication approach, beam parameters can be well controlled, the uniformity will not be affected by the wafer thickness and intrinsic stress in the beams is minimized. Accelerometers with different sensitivities can be easily fabricated by varying the depth of ICP etching without making any change to the masks.Firstly, a model of the beam-mass structure is established. The relationship between the performance of the device and the structure parameters are studied carefully. The optimized parameters are chosen: mass 2500um×2500um×415um, beam 770um×18um×17um and capacitance gap 4um.Then, key points of the fabrication of the accelerometer are studied. Evolvement of the cross-section of the beam when etched in KOH is theoretically analyzed. With wire-bonding cavities in the top layer, the pads of the top electrode and movable electrode are got simultaneously by just one metallization process step.And then, the fabrication flow of the device is designed. The devices are fabricated.Finally, preliminary characterization of the accelerometers is performed in terms of resonance frequency, quality factor and sensitivity. For a device with 17μm-thick beams, the quality factor and the sensitivity with an interface circuit are 45 and 0.35 V/g. |
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