Kyu-Yeon Park

Capacitive type surface-micromachined silicon accelerometer with stiffness tuning capability

Abstract A surface-micromachined silicon accelerometer with a novel concept, which has a stiffness tuning capability to improve the sensor resolution, is developed. Imposing an electrostatic force to the electrodes reduces the stiffness of the sensor structure. By adopting the stiffness tuning, the initially stiff structure guarantees the stability of fabrication, and the reduced stiffness, only along the sensing direction, produces the improved resolution. One of the major improvements in the developed accelerometer is the branched comb-finger type electrode which senses the relative position between the mass and the electrode. Maintaining the same capacitance variation, such electrodes allow a larger initial gap between the mass and the electrode, so that the clash problem can be easily eliminated. The accelerometer was successfully fabricated with the active size of 650×530 μm 2 , the 7-μm thick polysilicon structure, and a proof mass of about 1 μg. Experimental results show that the equivalent noise level of the accelerometer is improved by 30 dB through the stiffness tuning. The accelerometer has the bandwidth of 350 Hz, linearity of 0.3% FS, and sensing range of 50 g.

Laterally oscillated and force-balanced micro vibratory rate gyroscope supported by fish-hook-shaped springs

Abstract A new concept for a micro vibratory rate gyroscope supported by fish-hook-shaped springs, where the oscillating position sensing and force balancing take place on the wafer surface, has been developed. The gyroscope consists of a grid-type planar mass, LT-shaped position-sensing electrodes to detect the Coriolis motion, pairs of force-balancing electrodes to improve the linearity and dynamic range, prominence-shaped comb-drive electrodes to improve the resolution by increasing the oscillating displacement, and fish-hook-shaped springs to match the first and second modes with the mass oscillating and position-sensing modes, respectively. Due to the relatively high stiffness of the proposed fish-hook-shaped springs except in the desired directions, the gyroscope tends to be quite insensitive to environmental vibrations or shocks, maintaining the electromechanical stability. Also the resonance frequencies associated with lateral vibration modes are independent of the change in thickness of the polysilicon structure, which guarantees a uniform sensitivity of the products. Experimental results show that the gyroscope has an equivalent noise level of 0.1 ° s −1 at 2 Hz, a bandwidth of 100 Hz, and a dynamic range of 90 ° s −1 .

Dual-axis microgyroscope with closed-loop detection

A dual-axis microgyroscope is fabricated by a surface micromachining process. A 7.0 /spl mu/m-thick polysilicon layer deposited by LPCVD is used for the vibrating structure. We report the closed-loop rate detection of a microgyroscope based on the rotational vibration of the four plates. In particular, the structure utilizes a simple force-balancing torsional torque which does not need another top electrode layer to reduce the intrinsic nonlinearity of a capacitance-type sensor. The gyroscope is tested in a high vacuum chamber for a high Q-factor with hybrid signal-conditioning integrated circuit. The experiment resulted in a noise equivalent signal of 0.1 deg/sec.

Force-balanced dual-axis microgyroscope

The surface micromachining process realized the dual-axis microgyroscope. The 7.5 micrometers -thick polysilicon layer deposited by LPCVD is used for the vibrating structure. In this research, we present a new structure with high angular inertia momentum and compact size. In particular, this structure can utilize a simple force-balancing torsional torque which does not need another top electrode layer to reduce the intrinsic non-linearity of a capacitive-type sensor. The gyroscope is tested in a high vacuum chamber for a high Q-factor. The sensing mode is separated 2 percent from the driving mode by applying the inter-plate DC tuning bias. The experiment resulted in a nose equivalent signal of 0.1 deg/sec.

Two-input axis angular rate sensor

The surface micromachining process realized the dual-axis micro gyroscope. The 7.5 um-thick polysilicon layer deposited by LPCVD is used for the resonating structure. In this research, we report a new angularly actuated structure which detects the two-input angular rates simultaneously. One-chip is cheaper and smaller than using two gyro chips orthogonally-configured for the detection of two-input axis angular rate.

Laterally self-oscillated and force-balanced microvibratory gyroscope packaged in a vacuum package with a conditioning ASIC

A novel concept self-oscillator and dynamically tunable micro vibratory gyroscope, where oscillating, position- sensing and force-balancing take place on the wafer surface, has been developed. The gyroscope consists of: a grid-type planar mass which oscillates on the wafer surface; pairs of the differential capacitor type with LT shape position sense electrodes; a pair of force-balancing electrodes; oppositely placed comb-drive and comb-sensor for mass self-oscillation; fish hook shape springs to match the first and second modes with the mass oscillating and position sensing modes, respectively. The natural frequency of the position sensing mode is lowered and tuned by the DC bias voltage applied to the position sense electrodes and then finely tuned by DC bias on a pair of force-balancing electrodes. To reduce the mass exciting along the sensing direction, we drive the mass by the same DC and opposite AC driving voltage on the oppositely placed comb-drives. It also features that the position sensing electric interference ins reduced. The mass is self-oscillated by the condition of limit cycle, so the mass is always oscillated in the natural frequency even if the natural frequency is varied by the environment and/or it has displacement-force nonlinear behavior. The gyroscope is fabricated on the silicon wafer by surface micromachining technology and the polysilicon is used as an active structure. The gyroscope has an active size of 700 by 600 micro meters, the thickness of the structure is 7 micron meters and the proof mass of 1 micro gram. To improve the resolution of the gyro, it is packaged in the 50 mili-torr vacuum package with a conditioning ASIC. Experimental results show that the gyroscope has the equivalent noise level of 0.1 deg/sec at 2 Hz, the bandwidth of 100 Hz, linearity of 1 percent FS and the sensing range of 90 deg/sec.

A area variable capacitive microaccelerometer with force-balancing electrodes

A surface micromachined accelerometer which senses a inertial motion with an area variation is developed. The grid-type planar mass of 7 /spl mu/m thick polysilicon is supported by four thin beams and suspended above a Si substrate with a 1.5 /spl mu/m air gap. The motion sensing electrodes are formed on the substrate. The accelerometer is designed as an interdigital rib structure that has a differential capacitor arrangement. The movable electrodes are mounted on the mass and the pairs of the stationary electrodes are patterned on the substrate. In the accelerometer that has comb-type movable electrodes, the mechanical stress and the electrical pulling effects between a movable electrodes and the fixed electrodes occur. However this grid-type structure can have a large area variation in a small area relatively without stress and pulling, high sensitivity can be achieved. In order to improve the dynamic range and a linearity, a pair of comb shape force-balancing electrodes are implemented on both sides of the mass. The force-balancing electrodes are made of the same layer as the mass and anchored on a Si substrate. When acceleration is applied in the lateral direction, the difference of capacitance results from the area variation between the two capacitors and is measured using a charge amplifier. The accelerometer has a sensitivity of 95 mV/g and a bandwidth of DC/spl sim/1 kHz. A resolution of 3 mg and a non-linearity of 0.1%(F.S) is achieved for a measurement range of /spl plusmn/5 g.