Environmental sensors based on micromachined cantilevers with integrated read-out (original) (raw)
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Simulation and design of piezoelectric microcantilever chemical sensors
2005
This paper presents an analytical modeling of a piezoelectric multi-layer cantilever used as a micro-electro-mechanical-system (MEMS) chemical sensor. Selectively coated microcantilevers have been developed for highly sensitive chemical sensor applications. The proposed piezoelectric chemical sensor consists of an array of multi-layer piezoelectric cantilevers with voltage output in the millivolt range that replaces the conventional laser-based position-sensitive detection systems. The sensing principle is based upon changes in the deflection induced by environmental factors in the medium where a microcantilever is immersed. Bending of the cantilever induces the potential difference on opposite sides of the piezoelectric layer providing an information signal about the detected chemicals. To obtain an application specific optimum design parameters and predict the cantilever performance ahead of actual fabrication, finite element analysis (FEM) simulations using CoventorWare (a MEMS design and simulation program) were performed. Analytical models of multi-layer cantilevers as well as simulation concept are described. Both mechanical and piezoelectric simulation results are carried out. The cantilever structures are analyzed and fabrication process steps are proposed.
Performance Analysis of Cantilever Based MEMS Sensor for Environmental Applications
In this paper we presents a MEMS (Micro-electromechanical System) cantilever based humidity sensor for various applications such as environmental monitoring, electronics, agriculture and biomedical fields. The main focus of this paper is to design, simulate and analyze the performance of MEMS based T shaped microcantilevers using different sensing materials such as Al2O3, Porous Silicon and Poly Silicon. The simulation is done through finite element tool and parameters like the maximum induced stress; deflection and sensitivity of the diaphragms have been analyzed using the software INTELLISUITE version 8.7. The change in humidity element is bending of the microcantilever that modifies the measured displacement between the substrate and the microcantilever. This change in displacement gives the measure of amount of water vapor present in that environment. The outcome of these studies can be used to enhance the sensitivity of these devices. Here we observe that the best sensitivity output responses are obtained in the range of 10%RH to 100% RH and also the maximum sensitivity of 21.85 (m/%RH).
AFM Microcantilevers used as sensors for monitoring humidity
A microcantilever sensor is presented. Functionalization of the cantilever with a polyaniline (PANI) sensitive layer and its use as a humidity sensor were investigated. Polyaniline was produced by interfacial synthesis and the sensitive layer was deposited on the microcantilever surface by the spin-coating method. The microcantilever deflection at various levels of relative humidity (RH) was read by means of the optical lever of an atomic force microscope (AFM Veeco Dimension V). A range of RH from 20% to 70% was introduced into the AFM chamber by mixing streams of dry and wet nitrogen. The sensitivity and reversibility of the sensors were assessed at various RH and temperatures (10, 20 and 30 C). A large deflection was observed in the coated microcantilever sensors, with faster response time at 10 C and better sensitivity and reversibility at 30 C. These results demonstrate that the spin-coated microcantilever can be used as a sensor to detect relative humidity at various different temperatures
Unconventional uses of cantilevers for chemical sensing in gas and liquid environments
2010
Microcantilevers used as (bio)chemical sensors are usually coated with a chemically sensitive layer. The coated devices operate either in a static bending regime or in a dynamic flexural mode. While the coated devices operate generally well in both the static and dynamic mode, they do suffer from certain shortcomings depending on the medium of operation and the application, including lack of selectivity and of reversibility of the sensitive coating and a reduced quality factor due to the surrounding medium. In particular, the performance of microcantilevers excited in their standard out-of-plane dynamic mode drastically decreases in viscous liquid media. Moreover, the responses of coated cantilevers operating in the static bending mode are often difficult to interpret. To resolve those performance issues, unconventional uses of microcantilever are reviewed in this paper, which consist of the use of the dynamic mode without sensitive coating, the use of in-plane (flexural and longitudinal) vibration modes in liquid media, and fully accounting for the viscoelastic effects of the coatings in the static mode of operation. The advantages and drawbacks of these unconventional uses of microcantilevers for chemical sensing in gas and liquid environments are discussed.
This paper reports on the development of a piezoresistive microcantilever sensor read-out circuitry to detect acceleration, biological or chemical activities. Laser micromachining technique is used in fabricating the piezoresistive microcantilever sensor as well as assisting in the cantilever beam and piezoresistor shape formation. In order to test the sensor performance, a Wheatstone bridge which acts as resistive sensor is integrated with three other resistors and the fabricated sensor. A set of amplifier circuit consisting of INA128 is developed to amplify and extract the electrical signal component of the bridge circuit. The resistance and output voltage characteristic of the Wheatstone bridge is investigated, where the percentages difference between the calculated and measured output voltage is very low and similar to each other. The sensor response to vibration is also studied using an electro-dynamic vibration system. The system is designed specifically to enable the accessibility of a small resistivity change due to outside reaction.
Modeling of Piezoresistive Cantilevers for Chemical Sensing
International Journal of Engineering Applied Sciences and Technology
This paper presents and analytical model to provide good results for cantilevers with point load at free end and to describe the micro cantilevers with surface stress. It has been shown that the width of the cantilever has to be increased to optimize the sensitivity. The placement of the piezoresistor was found to be in the centre of the cantilevers clamped end and the area of the piezoresitor should be minimized.
The spring constant calibration of the piezoresistive cantilever based biosensor
2010
Piezoresistive microcantilevers are widely applied to measurements of low forces, masses and viscosity. After surface functionalization they might be used as a biochemical sensors being capable of the intermolecular force investigation. The problem is that such sensors change its mechanical properties in the environment they operate. Therefore there is a need for a high accuracy technique being capable of measuring of mechanical properties of functionalized cantilevers operating in the target environment. We suppose that such conditions meet the analysis of thermomechanical oscillation noise.
Longitudinal vibration mode of piezoelectric thick-film cantilever-based sensors in liquid media
Applied Physics Letters, 2010
We report on the fabrication of a self-actuated resonant-microsensor, based on a thick-film piezoelectric cantilever, dedicated to either ͑bio͒chemical detection in gaseous or liquid media or fluid characterization. The aim of this paper is to demonstrate that longitudinal modes can be used in highly viscous environments. Lower levels of fluid-solid interaction in comparison with classical flexural modes are expected from the results of our analytical model of a cantilever oscillating in a fluid. For example, in various fluid ranging from air to a Newtonian fluid of 300 cP viscosity, measured quality factors for the first longitudinal mode range from 300 to 20.
Sensing of liquid level at micron resolution using self-excited millimeter-sized PZT-cantilever
Sensors and Actuators A-physical, 2005
Composite resonant millimeter-sized piezoelectric cantilevers were fabricated and their mass change sensitivity was evaluated using liquid evaporation experiments. The cantilever sensor was immersed to 1 mm depth in two volatile fluids, and one non-volatile fluid. Deionized water and 50% ethanol–water solution showed a resonant frequency change of 19 ± 1 and 23 ± 1 Hz/min, respectively. The cantilever liquid level change sensitivity (ϕ) was determined as 0.26 ± 0.01 μm/Hz. Experimental measurement of liquid level change of deionized water at room temperature and humidity was found to be 5.0 ± 0.1 μm/min, and is in good agreement with first principles model prediction of 5.5 μm/min. We have modeled the added mass effects as proportional to displaced liquid mass. Such a model agrees with experimental results for immersion depths of less than 1 mm. The model provides a method for determining cantilever's spring constant and the added mass coefficient. The significance of the results we report is that millimeter-sized piezoelectric-actuated cantilevers have the sensitivity for measuring micron-level liquid level changes.
Chemical sensing: millimeter size resonant microcantilever performance
Journal of Micromechanics and Microengineering, 2004
Based on the use of resonant cantilever, a mass sensitive gas sensor for the detection of Volatile Organic Compounds (VOC) has been developed. Analyte gases are absorbed by a sensitive layer deposited on cantilever: the resulting mass change of the system implies the cantilever resonant frequency decrease. In this paper, the process technology, based on the use of SOI wafer, is described. To integrate the measurement, piezoelectric and electromagnetic excitations are investigated and for the detection of microcantilever vibrations, piezoresistive measurement is performed. Then, the polymer choice and the spray coating system are detailed. Using various geometrical microcantilevers, the frequency dependence to mass change is measured and allows to estimate the mass sensitivity (0.06Hz/ng). In gas detection the first experiments exhibit the sensor response, then by calculating the partition coefficient (K=977), the minimum detectable concentration of ethanol is deduced and permits to estimate the gas sensor resolution (14 ppm).Finally a comparison between millimeter size and micrometer size cantilever shows the importance of noise in the design of an integrated sensor.