Nanomechanical membrane-type surface stress sensor

Nanomechanical cantilever sensors have been emerging as a key device for real-time and label-free detection of various analytes ranging from gaseous to biological molecules. The major sensing principle is based on the analyte-induced surface stress, which makes a cantilever bend. In this letter, we present a membrane-type surface stress sensor (MSS), which is based on the piezoresistive read-out integrated in the sensor chip. The MSS is not a simple "cantilever," rather it consists of an "adsorbate membrane" suspended by four piezoresistive "sensing beams," composing a full Wheatstone bridge. The whole analyte-induced isotropic surface stress on the membrane is efficiently transduced to the piezoresistive beams as an amplified uniaxial stress. Evaluation of a prototype MSS used in the present experiments demonstrates a high sensitivity which is comparable with that of optical methods and a factor of more than 20 higher than that obtained with a standard piezoresistive cantilever. The finite element analyses indicate that changing dimensions of the membrane and beams can substantially increase the sensitivity further. Given the various conveniences and advantages of the integrated piezoresistive read-out, this platform is expected to open a new era of surface stress-based sensing.     

Modeling Piezoresistive Microcantilever Sensor Response to Surface Stress for Biochemical Sensors

This paper considers mechanical stress and strain in a piezoresistive cantilever sensor under surface stress loading, which is the loading condition that occurs in biochemical sensing applications. Finite element simulations examine the piezoresistor sensitivity due to changes in cantilever length, width, and thickness, and piezoresistor size, location, and depth. A few unexpected results are found. Unlike cantilevers designed for atomic force microscopy, cantilevers for biochemical sensing should be short and wide. While shallow piezoresistors offer good sensitivity, the piezoresistor may extend far into the thickness of the cantilever and still be quite effective. The paper concludes with comments on design guidelines for piezoresistive cantilever sensors.     

Smart MEMS Flow Sensor: Theoretical Analysis and Experimental Characterization

This paper presents analytical and experimental studies of a new microelectromechanical system (MEMS) smart flow sensor for the measurement of gas flow. The flow sensor has an array of curved-up cantilever beams that are surface-micromachined with two layers of deposition under two sets of different process parameters. The differential residual stress between the two layers of the polysilicon deposition causes the beams to curve upward from the substrate surface when the sacrificial layer is released. Each beam of the array of beams of different lengths vibrates successively as the flow rate increases, enabling more accurate sensing and identification of range of flow rates based on the vibration characteristics, thus making this a smart sensor design. Design and fabrication of these sensors are discussed. Experiments were conducted on this MEMS flow sensors to characterize the deflection of the curved cantilever beams with respect to flow rates. In addition, backflow tests were also conducted separately. Results of the analytical study are presented to investigate the cause of vibration of beams when subjected to flow. Finite-element analyses of vibration of the sensors comply with the experimental observation. Based on the analysis of fundamental natural frequencies, possible arrangement for the distribution of lengths of the beams is proposed to enhance its functionality as a sensor. Future work and plan of the on-board capacitive metrology and other practical issues are discussed     

Piezoresistive Cantilever Beam for Force Sensingin Two Dimensions

A novel two-dimensional piezoresistive nano-Newton resolution force sensing cantilever is presented. The silicon cantilever is fabricated using bulk micromachining. Two 500-nm-thick p-doped epitaxial silicon piezoresistive sensors are located on both sides of the cantilever. This structure detects both the lateral and vertical applied forces by electronic switching between two configurations of a Wheatstone bridge. A force sensitivity is measured up to 100 and 540 V/N for lateral and vertical configurations, respectively. The corresponding force resolution is estimated at 21 and 4 nN, respectively. This force-sensing cantilever can be used for measuring the contact force between manipulating tools and small objects in, e.g., living cell handling, minimally invasive surgery, and microassembly     

低量程高线性压阻式微机械加速度传感器

介绍了一种低量程高线性压阻式微机械加速度传感器的设计、工艺制作和封装 ,还较详细地介绍了静态和动态特性的测量和结果。为提高灵敏度,传感器采用悬臂梁结构。本文对结构的固有横向效应进行了计算,结果表明此种结构的三种横向效应主要是由于质量块（岛）质心偏离梁中平面,沿梁伸展方向（ x向）惯性力分量产生的弯矩所致。提出了调整上下盖板与中间质量块之间的气隙改善器件频响特性的方法。报道的压阻式微机械加速度传感器量程± 1g(g为重力加速度 ), 线性度优于 0.05％,灵敏度约 1mV/gV,阻尼比 0.7,带宽 110Hz。     

Piezoresistive polysilicon film obtained by low-temperature aluminum-induced crystallization

A low-temperature deposition process employing aluminum-induced crystallization has been developed for fabrication of piezoresistive polycrystalline silicon (polysilicon) films on low cost and flexible polyimide substrates for force and pressure sensing applications. To test the piezoresistive properties of the polysilicon films, prototype pressure sensors were fabricated on surface-micromachined silicon nitride (Si₃N₄) diaphragms, in a half-Wheatstone bridge configuration. Characterization of the pressure sensor was performed using atomic force microscope in contact mode with a specially modified probe-tip. Low pressure values ranging from 5 kPa to 45 kPa were achieved by this method. The resistance change was found to be − 0.1% to 0.5% and 0.07% to 0.3% for polysilicon films obtained at 500 °C and 400 °C, respectively, for the applied pressure range.     

Epoxy based photoresist/carbon nanoparticle composites

We have fabricated composites of SU-8 polymer and three different types of carbon nanoparticles (NPs) using ultrasonic mixing. Structures of composite thin films have been patterned on a characterization chip with standard UV photolithography. Using a four-point bending probe, a well defined stress is applied to the composite thin film and we have demonstrated that the composites are piezoresistive. Stable gauge factors of 5–9 have been measured, but we have also observed piezoresistive responses with gauge factors as high as 50. As SU-8 is much softer than silicon and the gauge factor of the composite material is relatively high, carbon nanoparticle doped SU-8 is a valid candidate for the piezoresistive readout in polymer based cantilever sensors, with potentially higher sensitivity than silicon based cantilevers.     

MEMS三维微触觉力传感器标定方法

针对一种微机电系统(micro electro mechanical system,MEMS)三维微触觉力传感器,采用悬臂梁弯曲变形获得了标准微小力信号,通过测量传感器敏感梁弹性导致的传感器测杆的微小位移量,对标定过程中的误差进行了补偿,实现了三维微触觉力传感器的精确标定.建立了MEMS三维微触觉力传感器标定系统,对悬臂梁的弹性系数进行了标定,对传感器测头输出的微小电压信号设计了线性化的信号调理电路.标定过程中考虑了由于传感器敏感梁弹性变形导致的传感器测杆的微小位移量对标定精度的影响.采用高精度的纳米测量机(nano-measuring machine,NMM)对传感器测杆的位移特性进行测量,利用该参数对传感器的力特性系数进行误差补偿,最后根据传感器输出的初始电压和力特性系数建立了传感器的力特性输出方程.     

Influence of residual stress on diffusion-induced bending in bilayered microcantilever sensors

The influence of residual stress on diffusion-induced bending in bilayered microcantilever sensors has been analyzed under the framework of thermodynamic theory and Fick's second law. A self-consistent diffusion equation involving the coupling effects of residual stress and diffusion-induced stress is developed. Effects of thickness ratio, modulus ratio, diffusivity ratio and residual stress gradient of film and substrate on the curvature of bilayered cantilever are then discussed with the help of finite difference method. Results reveal that the curvature of bilayered cantilever increases with decreasing the diffusivity ratio and modulus ratio of substrate to film at a given time. Case study of the polysilicon/palladium hydrogen sensor has been finally carried out using the above developed bending theory.     

Investigation of DNA sensing using piezoresistive microcantilever probes

Piezoresistive microcantilever-based sensors maybe used in a variety of sensing applications, including chemical sensing and biological sensing. In these applications, a sensing material is functionalized so as to undergo a volumetric or dimensional change upon analyte exposure. A piezoresistive microcantilever in contact with, or embedded within, the sensing material records the dimensional change as a simple resistance change in the cantilever as it is strained by the volumetric shift in the sensing layer. Here, we describe the detection of single-strand DNA by utilizing a sensing layer material consisting of thiolated single-strand DNA attached to a gold film substrate. A piezoresistive microcantilever in direct contact with this layer in solution immediately responds to the presence of the complimentary (25 base) single strand.     

Self-compensating fiber optic flow sensor system and its field applications

A self-compensating fiber optic flow sensor system based on the principle of broadband white-light interferometers and cantilever beam bending is described. The fiber optic sensor system uses two fiber ferrule sensors that are bonded on either side of a cantilever beam to measure the flow rate by monitoring the air-gap changes caused by the bending of the cantilever beam. Cross sensitivity of the temperature and pressure dependence of the sensor can be compensated for automatically. The prototype sensor system was constructed, laboratory characterized, and field tested. The results from the field testing have demonstrated high resolution, repeatability, and stability for on-line detection of the flow rates of fluids.     

多晶硅梁的加速疲劳实验

MEMS器件在循环振动载荷作用下,器件可能会发生断裂、软化等疲劳失效现象.本文中选取了以表面工艺加工的多晶硅结构—固支梁与悬臂梁作为实验研究对象,并在微结构梁上利用光刻的方法对两个被测结构分别引入了凹槽和切口两种缺陷形式,并在其上加载循环静电载荷,进行加速疲劳实验.实验利用激光多普勒测振仪测量谐振频率的变化,来表征微梁结构等效弹性模量的改变.实验结果表明,无论固支梁或者悬臂梁,其谐振频率都发生了明显的偏移:固支梁结构初始频率为170.749 kHz,实验后谐振频率增大,偏移量达到15.618 kHz,其相对变化量为9.15%,而悬臂梁结构初始频率为112.357 kHz,实验后谐振频率变小,减小量达到1.342 kHz,相对偏移为1.34%,器件性能发生明显退化.     

多晶硅微构件拉伸破坏特性的测量

硅表面工艺是微型机械制造的重要工艺。而多晶硅是表面工艺中的主要结构层,它的机械特性直接关系到微型机电系统的可靠性和使用寿命,因此对其破坏特性的研究是非常必要的。本文在描述了国外对体硅工艺和表面工艺形成的材料破坏特性研究情况的基础上,提出了利用旋转台对表面工艺形成的多晶硅微粱进行拉伸破坏实验的方法。     

Characterization of the Temperature Dependence of the Pressure Coefficients of n- and p-Type Silicon Using Hydrostatic Testing

Piezoresistive stress sensors on the (111) surface of silicon offer the unique ability to measure the complete stress state at a point in the (111) material. However, four-point bending or wafer-level calibration methods can measure only four of the six piezoresistive coefficients for p- and n-type resistors required for application of these sensors. In this work, a hydrostatic test method has been developed in which a high-capacity pressure vessel is used to apply a triaxial load to a single die over the -25degC to+100 degC temperature range. The slopes of the adjusted resistance change versus pressure plots yield pressure coefficients for p- and n-type silicon that provide the additional information necessary to fully determine the complete set of piezoresistive coefficients.     

炭纤维水泥石压敏传感器的温湿度补偿方法

介绍了适合混凝土结构局部压应力、 压应变状态监测的嵌入式炭纤维水泥石压敏传感器的概念及其传感原理, 并研究了环境温度和湿度两个因素对炭纤维水泥石压敏传感器零点输出的影响; 同时从传感器电路设计角度探讨了温度、 湿度的补偿方法及其在结构中的应用方式。研究表明: 炭纤维水泥石压敏传感器的零点输出对环境温度、 湿度的变化比较敏感, 温度、 湿度的最大影响分别约为6%和200%; 对炭纤维水泥石压敏传感器采用"测试-补偿"对耦设置构成的补偿电路可以消除因环境温度、 湿度以及极化作用产生的噪声信号对传感器零点输出的影响。      

Electrostatically actuated resonant microcantilever beam in CMOS technology for the detection of chemical weapons

The design, fabrication, and testing of a resonant cantilever beam in complementary metal-oxide semiconductor (CMOS) technology is presented in this paper. The resonant cantilever beam is a gas-sensing device capable of monitoring hazardous vapors and gases at trace concentrations. The new design of the cantilever beam described here includes interdigitated fingers for electrostatic actuation and a piezoresistive Wheatstone bridge design to read out the deflection signal. The reference resistors of the Wheatstone bridge are fabricated on auxiliary beams that are immediately adjacent to the actuated device. The whole device is fabricated using a 0.6-/spl mu/m, three-metal, double-poly CMOS process, combined with subsequent micromachining steps. A custom polymer layer is applied to the surface of the microcantilever beam to enhance its sorptivity to a chemical nerve agent. Exposing the sensor with the nerve agent simulant dimethylmethylphosphonate (DMMP), provided a demonstrated detection at a concentration of 20 ppb or 0.1 mg/m/sup 3/. These initial promising results were attained with a relatively simple design, fabricated in standard CMOS, which could offer an inexpensive option for mass production of a miniature chemical detector, which contains on chip electronics integrated to the cantilever beam.     

Single-Wafer-Processed Self-Testable High-$g$ Accelerometers With Both Sensing and Actuating Elements Integrated on Trench-Sidewall

A single-wafer-processed high-$g$ piezoresistive accelerometer is reported. The microsensor has an in-plane self-caging cantilever configuration, in which an electrostatic self-testing function is integrated on-chip. Both the sensing piezoresistors and the self-test actuating electrodes are integrated on vertical sidewalls of the laterally deflecting cantilever. For single-wafer-based fabrication of the self-testable piezoresistive accelerometer, a trench-sidewall micromachining technology is developed, which is capable of integration of both boron-diffused piezoresistive sensors and electrostatic actuators on deep trench sidewalls. In addition, the technology can realize electrical continuity from the vertical trench-sidewall to the wafer surface. After design and fabrication of the accelerometers for a 200 000 g measure-range, characterization was performed to evaluate the developed trench-sidewall integration technology and to test the self-testable high-$g$ accelerometers. A linear I-V relationship for the sidewall-diffused piezoresistor is measured with satisfactory sidewall-to-surface electric-transfer properties. The electrical isolation between adjacent elements on the sidewall shows a breakthrough voltage of about 55 V. Moreover, with the single-chip integrated lateral-actuating structure, both static and dynamic self-testing functions are realized. The measurement of the accelerometer results in a sensitivity of about 1 $~mu$V/g/3.3 V, noise-limited vibration resolution of about 1 g and zero-point temperature drift of lower than 100 ppm/$^{circ}$ C.      

浮动元件壁面剪切应力传感器研究进展

介绍了浮动元件的类型和浮动元件壁面剪切应力测量的方法,阐述了基于浮动元件的电容式、压阻式和光学式壁面剪切应力传感器的基本原理和研究现状,分析了上述3种类型的浮动元件壁面剪切应力传感器的优缺点,指出可通过优化浮动元件与传感器封装件之间的移动间隙以及浮动元件与被测面间的平齐度来提升浮动元件壁面剪切应力传感器的性能。展望了浮动元件壁面剪切应力传感器在湍流测量、判断边界层转捩、维护飞行器安全和优化飞行器结构等领域的发展方向,提出未来可通过发展MEMS技术、优化传感器后端处理电路和温度补偿方式、采用一体化设计加工方式等,进一步提升浮动元件壁面剪切应力传感器的小型化程度、检测壁面剪切应力极低值时的灵敏度和精度、测量的可靠性和准确性。     

Multimode Air-Coupled Excitation of Micromechanical Structures

The development of an acoustic measurement system for multimode air-coupled excitation and detection of micrometer-scale cantilever structures, which are, for example, used in micro-electromechanical systems (MEMSs), is detailed and reported for the first time. The source of noncontact vibrational excitation is a pulsed acoustic field generated by an air-coupled transducer. In the experimental system, the transient response of the cantilever beam is obtained at various points along the beam axis to extract its resonance frequencies and corresponding mode shapes. We demonstrate that measurable amplitudes of vibrations can be obtained at various excitation levels in the megahertz range, and higher harmonics of vibration of a microbeam can be excited by the air-coupled mechanism from a distance on the order of 10 mm. In the specific utilizations of the reported system, resonance frequencies and mode shapes can be related to the mechanical properties and geometric attributes (dimensions and defects), as well as the residual stress state in a microstructural element using various established computational and experimental inverse techniques. Another potential application area of the reported system is in the sensors for detecting the bending stiffness of deposited films on cantilever oscillators (in addition to its film mass loading) to increase the detection sensitivity and selectivity in a single sensing element.     

Nonlocal nonlinear bending and free vibration analysis of a rotating laminated nano cantilever beam

In this article, we present the nonlocal, nonlinear finite element formulations for the case of nonuniform rotating laminated nano cantilever beams using the Timoshenko beam theory. The surface stress effects are also taken into consideration. Nonlocal stress resultants are obtained by employing Eringen’s nonlocal differential model. Geometric nonlinearity is taken into account by using the Green Lagrange strain tensor. Numerical solutions of nonlinear bending and free vibration are presented. Parametric studies have been carried out to understand the effect of nonlocal parameter and surface stresses on bending and vibration behavior of cantilever beams. Also, the effects of angular velocity and hub radius on the vibration behavior of the cantilever beam are studied.