Jiwei Jiao

A high-performance micromachined piezoresistive accelerometer with axially stressed tiny beams

A high-performance micromachined piezoresistive accelerometer, consisting of two axially stressed tiny beams combined with a central supporting cantilever, is developed for both much higher sensitivity and much broader bandwidth compared with conventional beam-mass piezoresistive accelerometers. With the pure axial-deformation scheme of the tiny beams, the developed accelerometer shows improvements in both sensitivity and resonant frequency. An analytic model is established for the pure axial-deformation condition of the tiny beams by adjusting the distance between the tiny beams and the central supporting cantilever. The specifications of the device, such as sensitivity and resonant frequency etc, are theoretically calculated. The analytic model is verified by using simulation of the finite element method (FEM), resulting in satisfactory agreement. Based on a figure of merit (the product of the sensitivity and the square of the resonant frequency), optimized design rules are obtained for the sensors of various measure-ranges from 0.25g to 25 000g. The accelerometers are fabricated by using silicon bulk micromachining technology. The formed 2.5g devices are characterized with a typical sensitivity of 106 mV/5 V/g and first mode resonant frequency of 1115 Hz. The testing results agree well with the design, thereby verifying the high performance of the proposed accelerometer. The developed sensors with the axially stressed tiny-beam scheme show obviously improved specifications, compared with previously published results.

Cu/Sn Isothermal Solidification Technology for Hermetic Packaging of MEMS

This paper introduces a novel wafer-level hermetic packaging technology for MEMS based on Cu/Sn isothermal solidification (IS) technology. We designed the structure of the intermediate multilayer and the pattern of sealing rings, optimized the bonding process, and analyzed some key factors affecting the hermeticity, such as the dimension of the sealing rings. Successful Cu/Sn IS bonding was realized at a low bonding temperature of 350degC under the optimal conditions. High shear strength of 27.7 MPa and excellent leak rate of around 2times10-9 atm cc/s have been achieved, which meet the requirements of MIL-STD-883E. Finally we designed and manufactured the micro resonator based on the Cu/Sn IS technology

A piezoresistive accelerometer with axially stressed tiny beams for both much increased sensitivity and much broadened frequency bandwidth

A single-wafer-based piezoresistive accelerometer, consisting of two axially stressed tiny beams and a central bending cantilever, has been proposed for both much improved sensitivity and bandwidth compared with conventional piezoresistive accelerometers. Analytical modeling has been studied for optimized design and scaling rules of the sensors. Finite element method (FEM) simulation results agree well with the analyses. The accelerometers are fabricated in silicon-on-insulator (SOI) wafer by using bulk micromachining techniques including deep-reactive-ionic-etch (DRIE). The formed devices are characterized with typical sensitivity as 106 mv/5v/g and 1/sup st/ mode resonant frequency as 1115 Hz, 10.6 and 2.23 times equivalent to the specifications of a typical conventional cantilever-mass piezoresistive accelerometer.

Nanofabrication based on MEMS technology

In this paper, a novel nanofabrication method that develops from the traditional microelectromechanical system (MEMS) technology of anisotropic etching, deep reaction ion etching, and sacrificial layer process has been reviewed based on our work. With such a technology, nano tips, nano wires, nano beams even nano devices can be fabricated in a batch process. Beams with thickness of only 12 nm, a nano tip with a heater on the beam, and a nano wire whose width and thickness is only 50 nm are demonstrated. The scale effect of the Youngs modulus of silicon has been observed and the nano-electronic-mechanical data storage has been presented

A Microgyroscope With Piezoresistance for Both High-Performance Coriolis-Effect Detection and Seesaw-Like Vibration Control

A novel piezoresistive scheme is proposed to solve the problem of piezoresistive gyros in terms of low-sensitivity and high-temperature drift. Based on the piezoresistive scheme, a micromachined vibratory gyroscope is developed. By using axially stressed piezoresistive tiny-beams, piezoresistive detection to Coriolis-acceleration can be realized with both high sensitivity and high resonant frequency, i.e., high gyro operation frequency. Thanks to the high piezoresistive sensitivity, precise frequency matching between the driving and detecting modes is unnecessary. Another four-terminal transverse piezoresistive element is used to monitor and stabilize the amplitude of the seesaw-like torsional vibration through a feedback loop. With the same feedback loop, temperature drift of the piezoresistive angular-rate sensing signal can be on-chip compensated, as the transverse piezoresistance tracks the temperature drift of the angular-rate sensing piezoresistors. The gyro is designed and fabricated by bulk micromachining. Measurement results verify the proposed piezoresistive gyro scheme and show noise-limited angular-rate resolution of 0.33deg/s for plusmn300deg/s range. Considering the mature fabrication technology and simple signal-readout for piezoresistive sensors, the piezoresistive microgyros are promising for low-cost applications

Micromachined bar-structure gyroscope with high Q-factors for both driving and sensing mode at atmospheric pressure

In this paper, we report the design and fabrication of a novel micromachined electro-magnetically driven fork tuning type gyroscope with bar-structure working at atmospheric pressure. Instead of common squeeze film damping, slide film damping effect plays an important role in this sensor, which enables it to achieve high Q-factors for both driving and sensing mode at atmospheric pressure. The angular rate is sensed by detecting the differential change of capacitance between the bar structures electrodes and the fixed interdigitated electrodes on the glass substrate. The measured Q-factors at atmospheric pressure are 1005 for driving mode and 365 for sensing mode, respectively. The sensitivity of the sensor is about 10mV//spl deg/ /S, and the nonlinearity is less than 0.5%.

A TIA-based readout circuit with temperature compensation for MEMS capacitive gyroscope

This paper presents an integrated readout circuit based on trans-impedance amplifier (TIA) for MEMS capacitive gyroscope. The feedback resistors in TIA are realized in T-network pattern, which provides on-chip trans-impedance gains up to 22MΩ. A CMOS temperature-variable gain circuit is proposed to compensate the temperature induced sensitivity variance in MEMS and TIA. The demodulator, instrumental amplifier and low-pass filter for signal processing are also integrated on chip. In order to simulate the response of the circuit to the vibrating gyroscope, a gyroscope simulation model implemented in Verilog-A HDL is established, which can take the MEMS parameters influence into account. The circuits measure 0.8×1.7mm2 in a standard 0.35µm CMOS process. The simulation results show that the TIA achieves a capacitive resolution of 0.42aF/−Hz at 2.5kHz with 75ppm/°C temperature coefficient from a single 5V supply.

A novel tuning fork vibratory microgyroscope with improved spring beams

To increase the sensitivity and bandwidth for a tuning fork vibratory microgyroscope, a novel structure with improved spring beams is presented. The shape of the suspension beams are optimized by a cellular automata approach. Electrostatic excitation and capacitive detection mechanism are employed. The gyroscope works at atmospheric pressure, and the dominant air damping of the gyroscope is the slide-film damping, which ensures high Q-factors for both driving and sensing modes even at atmospheric pressure. The improved gyroscope is fabricated by the bulk silicon micromachining technology and packaged at atmospheric pressure. Experimental results show that the sensitivity and bandwidth of the microgyroscope are both promoted.

A high-sensitivity piezoresistive gyroscope with torsional actuation and axially-stressed detection

A piezoresistive gyroscope is presented, which is excited by electrostatic force and detected piezoresistively. With axially deformed tiny beams serving as piezoresistors to detect coriolis acceleration, the sensitivity can be much improved. With a four-terminal piezoresistive sensing element monitoring the driving-mode output versus temperature, the gyroscope temperature drift can be eliminated effectively. So the gyroscope enables a combination of self-compensation temperature drift and high sensitivity, and the gyroscope sensitivity will be so high as 3.07mV/7s/5V. Besides, the gyroscope can operate at atmosphere, as its high Q value is about 1000 in detection-mode. To verify the axially stressed detection scheme and develop fabrication technology, the detection part of the gyroscope has been fabricated in silicon-on-insulator (SOI) wafer by bulk micromachining techniques including deep-reactive-ionic-etch (DRIE). Testing results show both high resonant frequency of 1115Hz for operation and high sensitivity of 106mV/g/5V for detection. The detection scheme is proved to effectively improve the gyroscope sensitivity.

A TIA-based interface for MEMS capacitive gyroscope

This paper presents a CMOS readout and driving interface for micro-electro-mechanical systems (MEMS) capacitive gyroscope. A low-noise fully differential trans-impedance amplifier (TIA) scheme is used both in readout channel and driving loop for capacitive signal detection. A linear model of amplitude control loop and the design of an AGC-based analog drive loop for gyroscope are presented. The model allows the small signal analysis and behavior prediction of the driving loop. The interface circuits are designed and implemented in a 0.35µm 2P4M CMOS process. The simulation results show that the TIA readout circuit achieves a capacitive resolution of 0.73aF/√Hz and dynamic range of 93dB from a single 5V supply.