Shengyi Li

A Z-axis Quartz Cross-fork Micromachined Gyroscope Based on Shear Stress Detection

Here we propose a novel quartz micromachined gyroscope. The sensor has a simple cross-fork structure in the x-y plane of quartz crystal. Shear stress rather than normal stress is utilized to sense Coriolis’ force generated by the input angular rate signal. Compared to traditional quartz gyroscopes, which have two separate sense electrodes on each sidewall, there is only one electrode on each sidewall of the sense beam. As a result, the fabrication of the electrodes is simplified and the structure can be easily miniaturized. In order to increase sensitivity, a pair of proof masses is attached to the ends of the drive beam, and the sense beam has a tapered design. The structure is etched from a z-cut quartz wafer and the electrodes are realized by direct evaporation using the aperture mask method. The drive mode frequency of the prototype is 13.38 kHz, and the quality factor is approximately 1,000 in air. Therefore, the gyroscope can work properly without a vacuum package. The measurement ability of the shear stress detection design scheme is validated by the Coriolis’ force test. The performance of the sensor is characterized on a precision rate table using a specially designed readout circuit. The experimentally obtained scale factor is 1.45 mV/°/s and the nonlinearity is 3.6% in range of ±200 °/s.

Simple, fast, and cost-effective fabrication of wafer-scale nanohole arrays on silicon for antireflection

A simple, fast, and cost-effective method was developed in this paper for the high-throughput fabrication of nanohole arrays on silicon (Si), which is utilized for antireflection. Wafer-scale polystyrene (PS) monolayer colloidal crystal was developed as templates by spin-coating method. Metallic shadow mask was prepared by lifting off the oxygen etched PS beads from the deposited chromium film. Nanohole arrays were fabricated by Si dry etching. A series of nanohole arrays were fabricated with the similar diameter but with different depth. It is found that the maximum depth of the Si-hole was determined by the diameter of the Cr-mask. The antireflection ability of these Si-hole arrays was investigated. The results show that the reflection decreases with the depth of the Si-hole. The deepest Si-hole arrays show the best antireflection ability (reflection 600 nm), which was about 28 percent of the nonpatterned silicon wafers reflection. The proposed method has the potential for high-throughput fabrication of patterned Si wafer, and the low reflectivity allows the application of these wafers in crystalline silicon solar cells.

An Axial-beam Piezoresistive Accelerometer for High-performance Crash Detection of Automotive Industry

A high-performance piezoresistive accelerometer is developed for crash assessment of automobiles. For meeting the technical requirements, a novel sensor scheme is used that features both high sensitivity and high dynamic characteristics (including high resonant frequency and precise damping ratio). A three-beam seismic mass structure is proposed with the combination of a comb damper. Considering the demand in low-cost volume production, standard silicon wafers (instead of SOI wafers) are used here for micro-fabrication of the sensors. Double-sided deep RIE is processed for the micromechanical structure that is combined with the piezoresistive integration process. Test results show that the normalized sensitivity of the accelerometer is measured as 0.11 mV/g/5V within the full-range of 2000 g. The resonance frequency is 31 KHz, with the band of plusmn5% variation in sensitivity wider than 5 kHz 15000 g shocking test has been implemented to the packaged device, resulting in safe survival.

Support Loss and Q Factor Enhancement for a Rocking Mass Microgyroscope

A rocking mass gyroscope (RMG) is a kind of vibrating mass gyroscope with high sensitivity, whose driving mode and sensing mode are completely uniform. MEMS RMG devices are a research hotspot now because they have the potential to be used in space applications. Support loss is the dominant energy loss mechanism influencing their high sensitivity. An accurate analytical model of support loss for RMGs is presented to enhance their Q factors. The anchor type and support loss mechanism of an RMG are analyzed. Firstly, the support loads, powers flowing into support structure, and vibration energy of an RMG are all developed. Then the analytical model of support loss for the RMG is developed, and its sensitivities to the main structural parameters are also analyzed. High-Q design guidelines for rocking mass microgyroscopes are deduced. Finally, the analytical model is validated by the experimental data and the data from the existing literature. The thicknesses of the prototypes are reduced from 240 μm to 60 μm, while Q factors increase from less than 150 to more than 800. The derived model is general and applicable to various beam resonators, providing significant insight to the design of high-Q MEMS devices.

Sensitivity analysis and structure design for tri-mass structure micromachined gyroscope

The design method for increasing gyroscope sensitivity is studied. By designing lumped parameter models of uni-mass, bi-mass, and tri-mass gyroscope structures, the influence between sensitivity and model parameters is analyzed. These three gyroscope structures are optimized in the same condition and the result shows that the sensitivity ratio of the three structures is 1:7:800 in the condition of frequency matching. Tri-mass structures high sensitivity shows its advantage in high performance design.

A High-Performance Monolithic Triaxial High-G Accelerometer

A high-performance monolithic triaxial piezoresistive accelerometer is developed for 50000-100000g measure range. An axial-stressed tiny-beam scheme is developed for the X and Y axial elements and a three-beam twin-mass structure is developed for Z-axis element. Both of these structures feature high sensitivity and high resonant frequency. The three sensing elements are monolithically integrated by micromachining techniques of double-sided deep etching combined with piezoresistive processes. A dropping-hammer calibration system is used to characterize the accelerometer. The sensitivity of the three elements is measured as 2.17, 2.25 and 2.64muV/g/5V respectively and the resonant frequency is 308, 303 and 164 kHz for each axial element. A novel time-domain separation method using a long and thin aluminum alloy bar is developed for the dropping test of cross-axis sensitivity. With this measurement technique, satisfactory cross-axis sensitivity is experimentally obtained for all the three axial elements, which contributes to the high-performance monolithic triaxial shock accelerometer.

Inexpensive and fast fabrication of ordered gold nanocone arrays

Metallic nanocones are particularly interesting recently for the electromagnetic field concentration occur near sharp tips, but the fabrication is still a problem for their application. In this paper, a simple self-assembly based templating approach was presented for the fabrication of ordered gold nanocone arrays. The double-layered silica colloidal crystal was utilized as etching masks to develop inverted tapered silicon nanopits onto silicon substrates. Gold nanocone arrays can then be replicated from silicon templates simply by sacrificial layer removing and adhesive peeling process. The resulting gold nanocone array was characterized by field emission scanning electron microscopy (SEM).

Sidewall arris flatting process in quartz gyroscope fabrication

The fabrication of quartz gyroscope is studied through the quartz wet etching experiments in this paper. The quartz gyroscope is made of quartz wafer covered by Cr/Au mask in etchant and need to form electrodes on the sidewall of quartz beam. The key problem in fabrication is two steps arris in on the sidewall because of anisotropy in quartz etching. 100Å Cr and 2000Å Au films are deposited at double sides of 500μm Z-cut quartz wafer as the etch mask. The quartz etchant was a mixture of HF and NH4F in proportions HF:NH4F = 1:1. Wet etching experiments are carried out every 5°C from 50°C to 80°C. Etch rate nonlinearly increases with temperature, and high temperature makes the roughness of sidewall surface increase. After 27h etching, the two steps arris are flatted because the etch rates of main surface on the quartz sidewall are different. This flatting process has been used in the fabrication of the gyroscope.

Analyzing the frequency response of micromachined capacitive inertial devices by the third harmonic detection

Its a challenge to figure out the frequency response of a capacitive MEMS device due to the large coupling noises introduced by the parasitic capacitors. We present a new method to measure the frequency response based on the third harmonic detection. Different from previous methods, the frequency of the measured signal in our method is triple as that of the exciting voltage, so there is no coupling noise from the exciting voltage through the parasitic capacitors, and a higher SNR is achieved. In this paper, the principle of this method is introduced, and the configuration of the measurement system is presented. The simulation results as well as the experimental results show that this new method works very well.

High performance structurally decoupled micromachined gyroscope

In order to suppress the quadrature error in micromachined gyroscopes, various decoupled structures have been designed employing independent suspension beams for the drive and sense modes. The new decoupled structure presented in this paper employs a vibration isolation frame and independent inertial masses for the drive and sense modes. This frame can not only prevent the oscillation of the drive masses coupling to the sense masses, but also pass the Coriolis forces on the drive masses to the sense masses. A gyroscope sample and its read-out circuit are fabricated. The quality factors of the drive and sense modes in atmosphere are measured as 800 and 34, and the resonance frequencies of the drive and sense modes are 2.981 kHz and 2.813 kHz. The scale factor of this gyroscope is 38 mV/(deg/sec) with a nonlinearity better than 0.8% in a measurement range of plusmn100deg/sec, and the short-term bias stability in half an hour is 0.28deg/sec.