MEMS based low cost piezoresistive microcantilever force sensor and sensor module

In the present work, we report fabrication and characterization of a low-cost MEMS based piezoresistive micro-force sensor with SU-8 tip using laboratory made silicon-on-insulator (SOI) substrate. To prepare SOI wafer, silicon film (0.8 µm thick) was deposited on an oxidized silicon wafer using RF magnetron sputtering technique. The films were deposited in argon (Ar) ambient without external substrate heating. The material characteristics of the sputtered deposited silicon film and silicon film annealed at different temperatures (400–1050 °C) were studied using atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. The residual stress of the films was measured as a function of annealing temperature. The stress of the as-deposited films was observed to be compressive and annealing the film above 1050 °C resulted in a tensile stress. The stress of the film decreased gradually with increase in annealing temperature. The fabricated cantilevers were 130 μm in length, 40 μm wide and 1.0 μm thick. A series of force–displacement curves were obtained using fabricated microcantilever with commercial AFM setup and the data were analyzed to get the spring constant and the sensitivity of the fabricated microcantilever. The measured spring constant and sensitivity of the sensor was 0.1488 N/m and 2.7 mV/N. The microcantilever force sensor was integrated with an electronic module that detects the change in resistance of the sensor with respect to the applied force and displays it on the computer screen.     

Mechanical behavior of mismatch strain-driven microcantilever

A model of mechanical behavior of microcantilever due to mismatch strain during deposition of MEMS structures is derived. First, a microcantilever, modeled as an Euler-Bernoulli beam, is subjected to deposition of another material and a linear ordinary differential equation which considers the through-thickness variation of the mismatch strain is derived. Second, the deposition analysis is experimentally realized by electroplating of nickel onto an atomic force microscope (AFM) cantilever beam. Young's modulus of the electroplated nickel film is determined by using Sader's method and elementary beam theory. The deflection of the AFM cantilever is in-situ measured as a function of the electroplated thin film thickness through the optical method of AFM and the mismatch strain with the through-thickness variation is determined from the experiment results.     

Interactions of single-stranded DNA on microcantilevers

The microcantilever approach has attracted considerable attention in recent years as a means of label-free detection of a variety of biomolecular and chemical reactions. The underlying physics of the intermolecular interactions that result in mechanical motions is yet to be fully explored, but it seems both rich in science and of technological importance. This paper presents an overview of experiments and theories related to interactions of single-stranded DNA immobilized on microcantilevers. Experiments and theories show that, at high grafting density, hydration forces are the dominant factor determining cantilever deflections, not electrostatics or conformational entropy.     

Detection of trace organophosphorus vapor with a self-assembled bilayer functionalized SiO2 microcantilever piezoresistive sensor

Using piezoresistive SiO₂ microcantilever technology, we present an ultra-sensitive chemical sensor for trace organophosphorus vapor detection. A self-assembled composite layer of Cu²⁺/11-mercaptoundecanoic acid is modified on the surface of the sensing cantilever as a specific coating to capture PO containing compounds. Experimental results indicate that the sensor can be quite sensitive to DMMP vapor (well known as a simulant of nerve agent). The signal-noise-limited detection resolution of the sensor is experimentally obtained as low as several parts per billion. Besides that the sensor can yield reversible and repeatable response to DMMP vapor, adsorption of DMMP on the self-assembled composite layer is well fit to the Langmuir isotherm model.     

Magnetization reversal observed by the deflection of magnetic actuator

A magnetic actuator consisting of a silicon oxide microcantilever and a silicon oxide plate deposited on ferromagnetic multilayer thin films is fabricated using electron beam lithography and electron beam evaporation, and placed in various magnetic fields to observe its flexure. The magnetic actuator is bent by magnetic torque produced by ferromagnetic multilayer thin films under an external magnetic field owing to the fabrication of a highly sensitive microcantilever and the design of elliptic ferromagnetic thin films with high magnetic shape anisotropy. The magnetic actuator is placed in four kinds of magnetic field directions to investigate the diversity of deflections; the angles between the easy axis of the ferromagnetic multilayer thin films and the direction of the external magnetic field are 90°, 70°, 45° and 20°.     

微悬臂梁生化传感器在液体环境中的应用

微悬臂梁由于其体积小、响应速度快、灵敏度高和易于集成等特点,已迅速发展成为一门新兴的传感器技术.本文简要介绍了微悬臂梁传感器的工作原理和信号读出方法,概括了近年来它在液体环境中的研究现状,并对其性能特点和局限性进行了讨论,展望了该类型传感器今后的研究趋势.     

Mechanical and electronic approaches to improve the sensitivity of microcantilever sensors

Advances in the field of micro electro mechanical systems and their uses now offer unique opportunities in the design of ultrasensitive analytical tools. The analytical community continues to search for cost-effective, reliable, and even portable analytical techniques that can give reliable and fast response results for a variety of chemicals and biomolecules. Microcantilevers （MCLs） have emerged as a unique platform for label-free chem-sensor or bioassay. Several electronic designs, including piezoresistive, piezoelectric, and capacitive approaches, have been applied to measure the bending or frequency change of the MCLs upon exposure to chemicals. This review summarizes mechanical, fabrication, and electronics approaches to increase the sensitivity of MCL sensors.     

Linear and non-linear vibration and frequency response analyses of microcantilevers subjected to tip-sample interaction

Despite their simple structure and design, microcantilevers are receiving increased attention due to their unique sensing and actuation features in many MEMS and NEMS. Along this line, a non-linear distributed-parameters modeling of a microcantilever beam under the influence of a nanoparticle sample is studied in this paper. A long-range Van der Waals force model is utilized to describe the microcantilever-particle interaction along with an inextensibility condition for the microcantilever in order to derive the equations of motion in terms of only one generalized coordinate. Both of these considerations impose strong nonlinearities on the resultant integro-partial equations of motion. In order to provide an understanding of non-linear characteristics of combined microcantilever-particle system, a geometrical function is wisely chosen in such a way that natural frequency of the linear model exactly equates with that of non-linear model. It is shown that both approaches are reasonably comparable for the system considered here. Linear and non-linear equations of motion are then investigated extensively in both frequency and time domains. The simulation results demonstrate that the particle attraction region can be obtained through studying natural frequency of the system consisting of microcantilever and particle. The frequency analysis also proves that the influence of nonlinearities is amplified inside the particle attraction region through bending or shifting the frequency response curves. This is accompanied by sudden changes in the vibration amplitude estimated very closely by the non-linear model, while it cannot be predicted by the best linear model at all.     

A grating-based optical readout method for microcantilever biosensor

For measuring the deflections of the microcantilever biosensor, a reflective grating microcantilevers based on SOI were designed and fabricated, a high precision optical readout approach based on diffraction spectrum balancing feedback control was presented. The diffraction spectrum image was collected by a 12-bit digital area array monochrome CCD. According to the sum gray value of the image which subtracted each other at the balancing position and bending position to control a high precision motorized rotation stage revolve make the sum gray value remained in the balancing position always, then the motorized rotation stage revolving angle is just the cantilever bend angle. The resolution of motorized rotation stage is 35 × 10⁻⁶ deg, the system practical measurement resolution is 1 × 10⁻⁴ deg, that is to say, for a length of 250 μm microcantilever, the tip measure resolution is up to 0.043 nm. Measurement results clearly demonstrate that this reflective grating microcantilever biosensor and this read out method have a great potential for biological and chemical applications.     

Design and fabrication of in-plane resonant microcantilevers

An in-plane resonant bimetal microcantilever based on thermal actuation mechanism was developed utilizing the photolithography technique. The microcantilever structure consists of a platform, a long narrow anchor, and a U-shape actuation loop. Niobium and gold are used as materials for fabrication of the microcantilever. In the cantilever design the shortcoming of low actuation frequency was overcome by separating the thermal actuator part and the microcantilever part. According to the dynamic property tests, the in-plane resonant frequency of our microcantilevers is one order of magnitude higher than the out-of-plane one. With further optimizing the design, our microcantilevers may have applications as actuators and biosensors.