Heng Yang

Energy transfer model for squeeze-film air damping in low vacuum

High quality factors are essential for vibratory microsensors. Therefore, the vibrating structure of the sensors is often encapsulated in a housing where the air is evacuated for reduced air damping. However, the vacuum is usually low and the quality factor is still mainly determined by the energy losses to the surrounding air molecules. Air damping in low vacuum is usually estimated using the free molecular model proposed by Christian (Christian R 1966 Vacuum 16 175–8). The major drawback of the model is that the effect of the nearby objects (e.g. the electrodes for electrostatic driving) and the dimensions of the plate cannot be considered. Therefore, the damping effect is often significantly underestimated for real structures. This paper proposes a new model for air damping of microstructures in low vacuum. In this model, the damping effect is calculated by using an energy transfer mechanism instead of the momentum transfer mechanism in Christians model. For an isolated oscillating plate, the calculated quality factor by the model is the same as that by Christians model. However, for an oscillating plate with a neighboring object, the damping effect by the new model is related to the dimensions of the vibrating plate and the gap between the plate and the nearby object. The quality factors calculated agree with experimental data better than with Christians model by about an order of magnitude.

Fabrication of a CMOS compatible pressure sensor for harsh environments

The fabrication and characteristics of CMOS compatible absolute pressure sensors for harsh environments are presented in this paper. The sensor which was fabricated using post-processing surface micromachining consists of 100 circular membranes with a total capacity of 14 pF. PECVD SiC was used due to its good mechanical properties, but since SiC has high resistivity, aluminium layers were used for electrodes. The stiction problems were avoided by using polyimide PI2610 as a sacrificial layer. The pressure sensors were fabricated and the change of capacitance over full pressure range, 5 bar, was 3.4 pF.

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.

Squeeze-film air damping of thick hole-plate

Abstract The damping effect of air flow in holes is considered based on Poiseuille equation and the damping effect of lateral flow is considered by conventional Reynolds’ equation for infinite thick hole-plate. The expression for damping ratio is obtained and the conditions for minimum damping ratio can be found. A modified Reynolds’ equation is established for thick hole-plate with finite dimensions. The equation is a good approximation for hole-plates in typical MEMS devices. As it is also effective for non-hole-plate as well, it is more general than the conventional Reynolds’ equation. The distribution of damping pressure under a hole-plate can be found by solving the modified Reynolds’ equation with appropriate boundary conditions. As an example, the damping pressure of squeeze-film air damping of a long rectangular hole-plate is considered. Analytical expressions for the damping pressure, damping force and damping ratio are found.

Effects of electrostatic forces generated by the driving signal on capacitive sensing devices

Abstract In measuring the capacitance of a variable mechanical capacitor used in a capacitive mechanical sensor, an electrical driving signal is usually needed. The electrostatic forces caused by the driving signal on the mechanical capacitor may interfere with the measurement and the normal operation of the devices significantly. In this paper, quantitative analyses on the effects of driving signal are made for single-sided driving, double-sided driving and double-sided driving with voltage feedback (i.e., force-balanced measurement schemes). The effects caused by the driving signal are found to be: (1) the zero offset of the sensors for single-sided driving signal, (2) the change of the measurement sensitivity, and (3) the reduction of the critical measurand signal level causing the pull-in effect that hampers the normal operation of the device. The levels of critical measurand signal for specific driving signal levels are found quantitatively. Based on the analyses, the conclusions are: (1) the level of driving signal can be selected by the compromise among the requirements on the sensitivity, the accuracy and the reliability of the sensors devices for a specific configuration, (2) the side effects of the driving signal can be minimized by using the testing scheme of double driving with voltage feedback.

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

Two-dimensional excitation operation mode and phase detection scheme for vibratory gyroscopes

We propose a novel operation mode of two-dimensional excitation and phase detection for vibratory gyroscopes. It has been proven that the angular rate output is both amplitude-modulated and phase-modulated when the structure is driven into vibration in both the x- and y-directions. The output can be converted into a narrow-band phase-modulated signal by an amplitude limiter. Therefore, the angular rate signal of a vibratory gyroscope can be measured by detecting the phase change of the output signal. This operation scheme features high accuracy, high immunity against interference and a low-temperature coefficient. A bulk micromachined gyroscope with piezoresistive sensing elements is tested in the two-dimensional excitation mode with the phase detection scheme and the experimental results are 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

A CMOS compatible SiC accelerometer

In this paper we describe the fabrication process and preliminary results of a CMOS compatible surface micromachined vertical accelerometer. The vertical accelerometer together with lateral one fabricated in the same process creates 3D surface micromachined accelerometer. PECVD silicon carbide and aluminium layers were used as mechanical material and electrodes, respectively. As the maximum processing temperature is 400/spl deg/C, the sensor can be made on top of the CMOS readout circuit as post-processing module. The sensor is designed to operate in range -5/+5g with sensitivity 1.8fF/g and 2.3f.F/g in vertical and lateral direction, respectively. The sensor has been fabricated and is under measurement. The initial measurement shows that the initial capacitance of vertical accelerometer is 0.42pF.

A novel technique for measuring etch rate distribution of Si

Three-dimensional etch rate distribution of Si is necessary in analyzing the anisotropic etching of Si. As three-dimensional etch rate distribution can be composed by a series of two-dimensional distributions, a novel method is suggested in this paper for measuring the two-dimensional distributions with simple tools and high efficiency. For example, vertical sidewalls of a series of U-shaped trenches in (0mn) silicon wafers are first created by deep reactive ion etching (DRIE) technique. By measuring the etch rates in normal directions of the vertical sidewalls, two-dimensional distributions of etch rate in (0mn) planes can be found. As the height of vertical sidewalls formed by DRIE can be very large, the planes of sidewall can withstand a relatively long etch time before they are replaced by emerging slow etching planes. No special measuring facilities but a conventional microscope is needed for the measurement. Presented in this paper are etch rate distributions in (001) and (011) crystal planes in 40% KOH and 25% TMAH.