Design and characterization of MEMS-based flow-rate and flow-direction microsensor

This study designs and characterizes a novel MEMS-based flow-rate micro-sensor consisting of a platinum resistor deposited on a silicon nitride-coated silicon cantilever beam. Due to the difference between the thermal conductivities of the silicon nitride film and the silicon beam, the tip of the cantilever structure bends slightly in the upward direction. As air travels across the upper surface of the sensor, it interferes with the curved tip and displaces the beam in either the upward or the downward direction. The resulting change in the resistor signal is then used to calculate the velocity of the air. A flow-direction micro-sensor is constructed by arranging eight cantilever structures on an octagonal platform. Each cantilever is separated from its neighbors by a tapered baffle plate connected to a central octagonal pillar designed to attenuate the aerodynamic force acting on the cantilever beams. By measuring the resistor signals of each of the cantilever beams, the micro-sensor is capable of measuring both the flow rate and the flow direction of the air passing over the sensor. A numerical investigation is performed to examine the effects of the pillar height and pillar-to-tip gap on the airflow distribution, the pressure distribution, the bending moment acting on each beam, and the sensor sensitivity. The results show that the optimum sensor performance is obtained using a pillar height of 0.75 mm and a pillar-to-tip gap of 5 mm. Moreover, the sensitivity of the octagonal sensing platform is found to be approximately 90% that of a single cantilever beam.     

Microcantilever-based weather station for temperature,humidity and flow rate measurement

The current study develops a new process for the fabrication of Pt resistor temperature detectors (RTDs), cantilevers covered with a water-absorbent polyimide layer for humidity measurement and the bending-up of cantilevers to determine the flow rate. Pt RTDs are fabricated on the silicon substrate. The temperature measurement is based on the linear resistance variations when temperature changes. The polyimide layer is spun on the cantilever to form a humidity sensing layer. A variation in humidity causes moisture-dependent bending of the micro-cantilever, which changes the measured resistance of the resistor on the micro-cantilever. The same type of micro-cantilever, without spinning on polyimide, is used to form an anemometer. It is found that the cantilever bends slightly upward as a result of the released residual stress induced in the beam during the fabrication. When wind passes over the cantilever beam, a small deformation occurs. Variations in the flow rate can therefore be determined by measuring the changes in resistance caused by the beam deflection, using a LCR meter.     

MEMS-based humidity sensor with integrated temperature compensation mechanism

This study applies conventional micro-electro-mechanical systems (MEMS) techniques to develop a novel low-cost humidity sensor comprising a silicon substrate, a freestanding cantilever and an integrated resistive thermal sensor. The cantilever has a composite structure comprising a thin layer of platinum (Pt) deposited on a silicon nitride layer and then covered with a polyimide sensing layer. The cantilever deflected in the upward direction as water molecules absorbed by the polyimide sensing layer. The humidity sensor chip caused a measurable change in the resistance of the platinum layer. By compensating the change in the measured resistance by the ambient temperature, the absolute value of the relative humidity can be directly derived. The experimental results show that the sensor has a time-response of 0.9 s when exposed to a sudden humidity change of 65%RH to 95%RH. The sensitivity of the sensors decreases as the temperature increases. Furthermore, the sensor with the longest Pt resistor has the greatest sensitivity. In additions, the temperature-calibrated resistance signal generated by the sensor varies linearly with the ambient humidity.     

The mechanics of polymer swelling on microcantilever sensors

This paper investigates the swelling mechanics of polymer capture layers integrated into piezoresistive cantilever biochemical sensors. A finite element model investigates mechanical deformations in a polymer layer affixed to a silicon microcantilever. The polymer swells during analyte absorption, inducing deformations in the silicon cantilever which are sensed by a piezoresistive sensor integrated into the cantilever. The highest sensitivity is predicted for short and wide cantilevers that are coated with stiff polymer whose thickness is twice that of the cantilever. While the polymer swelling induces the deformations, the silicon carries most of the load. When portions of the silicon beam are removed to introduce stress concentrations, the system sensitivity can increase by 18% compared to the cantilever without stress concentrations. This study of stress distributions in the cantilever system allows sensor optimization that considers the full 3D effects of polymer swelling mechanics.     

Design and optimization of parylene nanomechanical cantilevers with integrated piezoresistors for surface-stress based biochemical sensing

The sensitivity of surface-stress based cantilever sensor can be significantly increased by using polymer as the cantilever material. In our previous research, parylene nanomechanical cantilevers with integrated poly-crystal silicon piezoresistors have already been developed for biochemical applications. However, parylene cantilevers exhibit different behaviors compared with their more rigid counterparts. In this paper, a new analytical model has been developed and verified using finite element simulations. The design optimization has also been discussed based on the new analytical model, resulting in several useful design guidelines that are unique to parylene or more generally to polymer cantilevers with integrated poly-crystal or single-crystal silicon piezoresistors for surface stress sensing.     

Etch-induced stress failures of SiO2 cantilever beams

Acute angles formed in the underlying silicon during anisotropic etching to free silicon dioxide cantilever beams are found to be the point of maximum stress during etch. Consequently, failures of the cantilever structure originate at these locations. The origin of this stress concentration is due to mechanical loading of the cantilever during etch, and this loading effect is smaller for shorter or narrower cantilevers. Finite element modelling of a partially etched cantilever microstructure, as used to predict the location of maximum stress, as well as the probable vector direction failures, will follow. Cantilevers free of etch-induced failure are fabricated by isotropic etching and by the modification of etch geometry through the addition of ribbed segments to the cantilever.     

Surface patterned polymer micro-cantilever arrays for sensing

Microinjection molding was employed to fabricate low-cost polymer cantilever arrays for sensor applications. Cantilevers with micrometer dimensions and aspect ratios as large as 10 were successfully manufactured from polymers, including polypropylene and polyvinylidenfluoride. The cantilevers perform similar to the established silicon cantilevers, with Q-factors in the range of 10-20. Static deflection of gold coated polymer cantilevers was characterized with heat cycling and self-assembled monolayer formation of mercaptohexanols. A hybrid mold concept allows easy modification of the surface topography, enabling customized mechanical properties of individual cantilevers. Combined with functionalization and surface patterning, the cantilever arrays are qualified for biomedical applications.     

变宽度悬臂梁的FBG流速传感器

通过对光纤传感器进行设计,提出了一种基于变宽度悬臂梁的光纤(Bragg)光栅(FBG)流速传感器.传感部分由不锈钢材质的悬臂梁和粘贴在其特定位置上的FBG构成,悬臂梁采用等腰梯形和矩形相结合的外形结构设计,传感头两部分之间的衔接不需要用销子固定,整个传感头浑然一体,无额外附加重量,制作方法简易,且实验设置参考光栅,实验结果不受温度变化的影响.实验表明:传感器的Bragg波长漂移量与流速变化有很好的线性关系,传感器的灵敏度为0.025 m/s.可测流速范围为0～2 m/s,传感器不仅实现了对温度的补偿,而且提高了测量精度、灵敏度.     

Design and Characterization of Artificial Haircell Sensor for Flow Sensing With Ultrahigh Velocity and Angular Sensitivity

We report the development of an artificial hair cell (AHC) sensor with design inspired by biological hair cells. The sensor consists of a silicon cantilever beam with a high-aspect-ratio cilium attached at the distal end. Sensing is based on silicon piezoresistive strain gauge at the base of the cantilever. The cilium is made of photodefinable SU-8 epoxy and can be up to 700-mum tall. In this paper, we focus on flow-sensing applications. We have characterized the performance of the AHC sensor both in water and in air. For underwater applications, we have characterized the sensor under two flow conditions: steady-state laminar flow (dc flow) and oscillatory flow (ac flow). The detection limit of the sensor under ac flow in water is experimentally established to be below 1 mm/s. A best case angular resolution of 2.16deg is also achieved for the sensor's yaw response in air.     

Elastic–plastic modeling of heat-treated bimorph micro-cantilevers

This paper models the residual stress distributions within micro-fabricated bimorph cantilevers of varying thickness. A contact model is introduced to calculate the influence of contact on the residual stress following a heat treatment process. An analytical modeling approach is adopted to characterize bimorph cantilevers composed of thin Au films deposited on thick poly-silicon or silicon-dioxide beams. A thermal elastic–plastic finite element model (FEM) is utilized to calculate the residual stress distribution across the cantilever cross-section and to determine the beam tip deflection following heat treatment. The influences of the beam material and thickness on the residual stress distribution and tip deflections are thoroughly investigated. The numerical results indicate that a larger beam thickness leads to a greater residual stress difference at the interface between the beam and the film. The residual stress established in the poly-silicon cantilever is greater than that induced in the silicon-dioxide cantilever. The results confirm the ability of the developed thermal elastic–plastic finite element contact model to predict the residual stress distributions within micro-fabricated cantilever structures with high accuracy. As such, the proposed model makes a valuable contribution to the development of micro-cantilevers for sensor and actuator applications.     

A thermal-calorimetric gas flow meter with improved isolating feature

This paper presents design and characterization of a novel thermal-calorimetric flow-meter using suspended-cantilever-structure. There is an air gap between the heater and each individual thermistor providing a good thermal isolation. Due to the suspended-structure which consists of three cantilevers, the thermal convection effect is present on both sides of the active area. Also the velocity boundary layer thickness of the cantilever is much less than closed-membrane one. This characteristic enhances the sensitivity of sensor. The simulation results indicate that the average temperature difference between upstream and downstream thermistors are 36.5 and 1.04 K for flow rate of 1 m/s and the worst case of 0.1 m/s respectively. This solution significantly improves the sensitivity compared to the closed-membrane-structures. The maximum temperature difference causes 94 mV at the output of Wheatstone bridge with 3 V of voltage supply. The calculated and simulated results show that the maximum power consumption of sensor is 4.7 mW at the maximum flow velocity of 1 m/s. The operational range of the designed flow meter is from 0 to 1 m/s. The features of the device are analytically evaluated and simulated under various conditions.

     

深海高流速传感器研制

阐述了深海高流速传感器的工作原理、结构设计、工艺研究及信号处理技术,该传感器中流速测量采用毕托管原理,利用差压传感器测量流体总压与静压差,流向测量采用水向标原理,角度变化由磁阻芯片检测,并采用单片机进行数据采集和后续处理,使得传感器具有测量范围宽、准确度高、耐高静压、分辨力高、抗振动冲击、耐腐蚀、适于深海中流场测量等特点。     

Elastic–plastic modeling of heat-treated bimorph micro-cantilevers

This paper models the residual stress distributions within micro-fabricated bimorph cantilevers of varying thickness. A contact model is introduced to calculate the influence of contact on the residual stress following a heat treatment process. An analytical modeling approach is adopted to characterize bimorph cantilevers composed of thin Au films deposited on thick poly-silicon or silicon-dioxide beams. A thermal elastic–plastic finite element model (FEM) is utilized to calculate the residual stress distribution across the cantilever cross-section and to determine the beam tip deflection following heat treatment. The influences of the beam material and thickness on the residual stress distribution and tip deflections are thoroughly investigated. The numerical results indicate that a larger beam thickness leads to a greater residual stress difference at the interface between the beam and the film. The residual stress established in the poly-silicon cantilever is greater than that induced in the silicon-dioxide cantilever. The results confirm the ability of the developed thermal elastic–plastic finite element contact model to predict the residual stress distributions within micro-fabricated cantilever structures with high accuracy. As such, the proposed model makes a valuable contribution to the development of micro-cantilevers for sensor and actuator applications.

     

MEMS cantilever sensor for non-destructive metrology within high-aspect-ratio micro holes

Tactile metrology with deep and narrow micro holes was addressed using extremely slender piezoresistive micro cantilever sensors. Linear strain–displacement characteristics were observed with this sensor operated under transversal and axial loading. From noise, non-linearity and repeatability measurements the resolution and uncertainty of the cantilever sensors were determined to few nm and few tens of nm, respectively, within a micron displacement span. Under axial loading buckling of the cantilevers was observed after exceeding the critical limit of an Euler beam under the boundary conditions of a clamped-pinned beam. The cantilevers typically survived displacements well above the buckling limit, i.e., fracture of 3-mm long cantilevers was only observed at displacements of more than 200 μm. The feasibility of the cantilever as an active 1D touch probe for high-aspect-ratio blind holes was demonstrated at a dry-etched silicon high-aspect-ratio microstructure. As an application example with a high-volume product we investigated the form and roughness of diesel injector nozzle spray holes.     

A 2-D microcantilever array for multiplexed biomolecular analysis

An accurate, rapid, and quantitative method for analyzing variety of biomolecules, such as DNA and proteins, is necessary in many biomedical applications and could help address several scientific issues in molecular biology. Recent experiments have shown that when specific biological reactions occur on one surface of a microcantilever beam, the resulting changes in surface stress deflect the cantilever beam. To exploit this phenomenon for high-throughput label-free biomolecular analysis, we have developed a chip containing a two-dimensional (2-D) array of silicon nitride cantilevers with a thin gold coating on one surface. Integration of microfluid cells on the chip allows for individual functionalization of each cantilever of the array, which is designed to respond specifically to a target analyte. An optical system to readout deflections of multiple cantilevers was also developed. The cantilevers exhibited thermomechanical sensitivity with a standard deviation of seven percent, and were found to fall into two categories-those whose deflections tracked each other in response to external stimuli, and those whose did not due to drift. The best performance of two "tracking" cantilevers showed a maximum difference of 4 nm in their deflections. Although "nontracking" cantilevers exhibited large differences in their drift behavior, an upper bound of their time-dependent drift was determined, which could allow for rapid bioassays. Using the differential deflection signal between tracking cantilevers, immobilization of 25mer thiolated single-stranded DNA (ssDNA) on gold surfaces produced repeatable deflections of 80 nm or so on 0.5-/spl mu/m-thick and 200-/spl mu/m-long cantilevers.     

A method to etch undoped silicon cantilever beams

A simple method has been designed to etch cantilever beams oriented in the <100> direction on (100) silicon wafers without back-etching, heavily doped boron etch stop, or anodic oxidation etch stop. The scheme requires only two levels of masking. Silicon dioxide and evaporated gold film are used as passivation materials. Anisotropic etching is performed in a sodium hydroxide bath. Silicon cantilevers with background doping concentration levels and having vertical edges are produced     

Novel fabrication method for surface micromachined thin single-crystal silicon cantilever beams

This paper describes a novel technique for the fabrication of surface micromachined thin silicon cantilever beams using merged epitaxial lateral overgrowth (MELO) of silicon and chemical-mechanical polishing (CMP). The objective is to demonstrate the feasibility of using this novel technique for the fabrication of arrays of ultrathin, low-stress, single-crystal silicon cantilever beams for use in ultrahigh sensitivity surface-stress or resonant-frequency-based chemical or biological detection schemes. The process flow used in this work is described in detail and the issues that were faced during the fabrication are discussed. Cantilever beams with thickness of 0.3-0.5 /spl mu/m that were 10-25-/spl mu/m wide and 75-130-/spl mu/m long were fabricated. Mechanical characterization of the cantilever beams were performed by measuring their spring constant using the "added mass" method, which also demonstrated the use of these initial structures to detect masses as low as 10-100 pg. Further work is underway to scale the thickness of these beams down to the sub-100-nm regime.     

Silicon nitride cantilevers with oxidation-sharpened silicon tips for atomic force microscopy

High-resolution atomic force microscopy (AFM) of soft or fragile samples requires a cantilever with a low spring constant and a sharp tip. We have developed a novel process for making such cantilevers from silicon nitride with oxidation-sharpened silicon tips. First, we made and sharpened silicon tips on a silicon wafer. Next, we deposited a thin film of silicon nitride over the tips and etched it to define nitride cantilevers and to remove it from the tips so that they protruded through the cantilevers. Finally, we etched from the back side to release the cantilevers by removing the silicon substrate. We characterized the resulting cantilevers by imaging them with a scanning electron microscope, by measuring their thermal noise spectra, and by using them to image a test sample in contact mode. A representative cantilever had a spring constant of /spl sim/0.06 N/m, and the tip had a radius of 9.2 nm and a cone angle of 36/spl deg/ over 3 /spl mu/m of tip length. These cantilevers are capable of higher resolution imaging than commercially available nitride cantilevers with oxidation-sharpened nitride tips, and they are especially useful for imaging large vertical features.     

A static turbine flow meter with a micromachined silicon torque sensor

A new class of flow sensors is introduced where a static turbine converts the volume flow into a torque. The structure consists of a turbine fixed to a torque sensor, which in turn is connected to the pipe wall giving a perfectly symmetric, bidirectional flowmeter. In contrast to conventional turbine flowmeters, the wheel does not rotate and consequently it is insensitive to bearing friction and wear that conventional, rotating turbines experience. Furthermore, the flow passing the wheel is distributed over the circumference of the wheel and levels out nonuniform flow profiles leading to profile independent volumetric flow measurement. The torque-sensing element is a 300-/spl mu/m-thick silicon cantilever, 2 mm wide and 16 mm long. The stiffness of the torque sensor, and thereby the sensitivity, is mainly determined by lateral dimensions of specially designed stiffness reduction parts defined by photolithography, thus giving good control of the sensitivity. Two polysilicon strain gauges are placed on each side of the cantilever, measuring the bending moment from the turbine wheel. The torque sensor has been evaluated for different geometries and together with the turbine to evaluate the flow sensing performance. A turbine with a blade length of 2.7 mm and a blade angle of 30/spl deg/ has a sensitivity of 4.0 /spl mu/V/V/(1/min)/sup 2/ when measured using the silicon torque sensor. The output signal shows good symmetry between different flow directions.     

表面部分电极含金属芯压电纤维气流传感特性研究

含金属芯压电纤维可以模仿昆虫的毛发感受器,感知周围环境的气流变化。在含金属芯压电纤维的部分纵向表面涂镀电极,制备了表面部分电极含金属芯压电纤维传感器PMPF（Partial electrode of Metal core Piezoelectric Fiber）。基于平均电荷法,建立了悬臂梁结构PMPF准静态气流传感模型,分析了PMPF产生的电荷值与表面电极分布角度、纤维长度、气流流速和气流方向的关系。搭建了实验系统,测试了PMPF对冲击气流的响应,验证了理论模型。结果表明,PMPF的传感信号和气流流速成指数关系,和气流方向成“8”字形关系。PMPF具有气流速度和方向的传感性能。