In-plane Motion Measurements Of MEMS Resonators By Using Laser Doppler Method

With the rapid development of Micro-electro-mechanical Systems (MEMS), thedemand for characterizations of the dynamic performance of MEMS device isincreased. An investigation on MEMS motion measurements is made in thisdissertation, and the application of differential laser Doppler method in in-planemotion detection is discussed. A differential laser Doppler system is set up for measuring the in-plane vibrationof MEMS resonators, and the optical axis of the measuring system is perpendicular tothe velocity vector under test. The system consists of an optical system, a signalprocessing circuit, data analysis program and a position adjustment stage. A CCDcamera with imaging system observing the position adjustment and the resonator'svibration status is introduced to the optical arrangement. The Doppler frequency isextracted from the high-frequency output of the optical system by the circuit.Spectrum analysis and counting process are employed to calculate the resonatorvelocity, and the displacement is obtained by integration. Demonstrating the reliability and accuracy of the Doppler system, the rotation ofan electromotor and the reciprocating motion of an accelerometer calibrator aremeasured first. Then, the in-plane vibrations of four MEMS resonators are measured.The resonant frequencies, transient velocities and displacements of the movablecombs are determined. Furthermore, a computer micro-vision method is used for thecomparing experiments. A position adjustment scheme is proposed, and errors in the measurements arediscussed in detail. The vibration model of the resonators is built. The analytic equations of themovable comb's resonant frequency and amplitude are deduced. Based on silicon bulkmicromachining technology, the resonators are designed and fabricated, and theprocess flow is presented.