Ki Bang Lee

A triangular electrostatic comb array for micromechanical resonant frequency tuning

Abstract A new electrostatic comb structure has been presented for the post-fabrication frequency tuning of laterally driven micromechanical resonators. The frequency tuning comb structure, composed of a triangular comb array of linearly varied finger lengths, generates a linear electrostatic tuning force from the translational motion of the DC-biased comb array. Simple analytic formulae for the electrostatic tuning force, the modified stiffness, and the resonant frequency have been derived in terms of the tuning bias voltage. A set of frequency tunable microresonators has been designed and fabricated by the 4-mask surface-micromachining process. The resonant frequency of the microfabricated microresonator has been measured as 2,42 kHz at the reduced pressure of 1 Torr. The resonant frequency has been reduced by 3.3% for a tuning voltage increase of 20 V. The theoretical frequency shift, including the electrostatic levitation effect, is compared with the experimental results. The higher tuning voltage results in the lower effective stiffness, thereby decreasing resonant frequency of the microresonator.

Vibratory structure, method for controlling natural frequency thereof and sensor and actuator adopting the vibratory structure

A vibratory structure and a method for controlling the natural frequency thereof are provided. The vibratory structure has an elastic member one end portion of which is fixed to a first support end, an inertial object for vibrating by the elastic force of the elastic member, a moving electrode attached integrally to the inertial object, an effective stiffness controlling electrode fixed to a second support end, and a power supplier for generating electric force between the moving electrode and the effective stiffness controlling electrode. The moving electrode and the effective stiffness controlling electrode are formed into a predetermined shape, so that the electric force varies linearly according to the displacement of the inertial object. Thus, the natural frequency of the vibratory structure is controlled by changing the voltage applied across the moving electrode and the effective stiffness controlling electrode.

Laterally driven electrostatic repulsive-force microactuators using asymmetric field distribution

We present a new electrostatic actuation method using a lateral repulsive-force induced by an asymmetric distribution of planar electrostatic field. The lateral repulsive-force has been characterized by a simple analytical equation, derived from a finite element simulation. Quality-factors are estimated from the computer simulation based on creep flow model. A set of repulsive-force polycrystalline silicon microactuators has been designed and fabricated by a four-mask surface-micromachining process. Static and dynamic response of the fabricated microactuators has been measured at the atmospheric pressure for the driving voltage range of 0-140 V. The static displacement of 1.27 /spl mu/m is obtained at the dc voltage of 140 V. The resonant frequency of the repulsive-force microactuator increases from 11.7 kHz to 12.7 kHz when the dc induction voltage increases from 60 V to 140 V. The measured quality-factors are increased from 12 to 13 in the voltage range of 60-140 V. Fundamental characteristics of the force, frequency and quality-factor of the electrostatic repulsive-force microactuator have been discussed and compared with those of the conventional electrostatic attractive-force microactuator.

Frequency tuning of a laterally driven microresonator using an electrostatic comb array of linearly varied length

We present a new post-fabrication frequency-tuning method for laterally driven electrostatic microresonators using a DC-biased electrostatic comb array of linearly varied finger-length. The electrostatic tuning force, and the modified stiffness, due to the DC-biased tuning-comb array, have been derived in terms of the tuning voltage. A set of frequency tunable microresonators has been designed and fabricated by 4-mask surface-micromachining process. The resonant frequency of the microfabricated microresonator has been measured for varying tuning voltage at the reduced pressure of 1 torr. The maximum 3.3% reduction of the resonant frequency is measured for a tuning voltage increase of 20 V.

Electrostatic control of mechanical quality factors for surface-micromachined lateral resonators

We investigate a laterally driven, surface-micromachined microstructure, whose mechanical quality factor can be adjusted electrostatically after fabrication. An electrical method for the mechanical quality factor control is presented and applied for laterally driven micromechanical resonators. The present method is based on the damping-gap change caused by an electrostatic force between parallel planar microstructures. Micromechanical resonators for quality factor control tests have been designed and fabricated by a 4-mask surface-micromachining process, including the dry etching of a -thick LPCVD polycrystalline silicon layer. In the experimental test performed at atmospheric pressure, the quality factors of the microfabricated resonators have been reduced rapidly at the rate of in the control voltage range of 1.75 - 2.25 V; thereby demonstrating that 50% reduction of the mechanical quality factor of a microfabricated resonator can be achieved in the control voltage range of 1.75 - 2.25 V with the maximum 0.8% modification of resonant frequency.

A surface-micromachined tunable microgyroscope

We present a surface-micromachined microgyroscope, whose resonant frequency is electrostatically-tunable after fabrication. The microgyroscope has two oscillation modes: a sensing mode and a actuating mode. In a theoretical study, the microgyroscope has been designed so that the resonant frequency in the sensing mode is higher than that in the actuating mode. A 4-mask surface-micromachining process for the tunable microgyroscope has been described, including the deep RIE of a 6 /spl mu/m-thick LPCVD polycrystalline silicon structure layer. In a experimental study, the resonant frequency in the sensing mode has been matched to that in actuating mode by applying an inter-plate bias voltage. Frequency matching for the fabricated microgyroscope has been achieved at 5.8 kHz under the bias voltage of 2 V in a reduced pressure of 0.1 torr. For an input angular rate of 50/spl deg//sec, an output signal of 20 mV has been measured from the tuned microgyroscope for an AC drive voltage of 2 V with a DC bias voltage of 3 V.

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.