Young-Ho Cho

Mechanical design issues in laterally-driven microstructures

Abstract The mechanical issues influencing the design and performance of laterally-driven resonant-structure micromechanical systems are identified and quantified. Two forms of resonant-structure suspensions are presented and the performance of each is calculated and compared. One represents tension-generating flexure, while the second, a ‘crab-leg’ flexure, utilizes motion-generated bending of the suspension. This second device is an unusual case of lateral motion (parallel to the substrate) with suspension elements subjected to lateral bending. The limit of linear motion (magnitude of motion subject to a constant flexural spring rate) is determined and a stress analysis is performed. An optimal design theory for each flexure is developed in which the objective is to obtain the greatest deflection per unit of allowable material stress. Preliminary results indicate that the inclusion of special design details can generate a two-to-one decrease in stress for a given level of displacement. In accordance with this optimal design theory, a prototype ‘crab-leg’ flexure is fabricated with a stress-to-displacement ratio calculated to be as low as 65 MPa/μm.

A frequency-tunable microactuator with a varied comb-width profile

A frequency-tunable microactuator with a varied comb-width profile for electrostatic, post-fabrication frequency tuning has been demonstrated. The theoretical comb-width profile was derived to obtain a constant electrostatic stiffness, independent of the position of the combs. A tunable microactuator, having 186 pairs of the varied comb-width profile, was designed and fabricated by the standard surface micromachining process. Under a biased voltage of 150 V, the resonant frequency was tuned and reduced by 55% from the initial resonant frequency of 19 kHz and the corresponding effective stiffness was decreased 80% from the initial value of 2.64 N/m. As such, this scheme can be applied to comb-shape, electrostatic-based microsensors and microactuators for active frequency tuning applications.

An electrostatic quality factor control for surface-micromachined lateral resonators

We present and apply a quality factor control method for laterally driven micromechanical resonant microstructures. The present method modifies the thickness of air-damping gap by applying an electrostatic force between the planar resonant microstructure and ground plane; thereby adjusting the quality factor of the microresonators after fabrication. Polysilicon resonators have been designed and fabricated by a 4-mask surface-micromachining process. The present method reduces the quality factor of the fabricated resonator at the rate of 120/V by applying the electrostatic voltage in the range of 1.75/spl sim/2.25 V. The maximum 50% reduction of the quality factor has been achieved at the electrostatic voltage of 2.25 V.