Comparison of three humidity sensors for a pulmonary functiondiagnosis microsystem

Three capacitive humidity sensors developed for a portable clinical application are presented and compared. The first structures consist of interdigitated electrodes covered by a polyimide sensitive layer. The second structures have the same geometry but include a benzocyclobutene sensitive layer and a heating. resistor. The third structure has been developed with a new geometry, with the electrodes being stacked. Humidity measurement results are presented, in particular sensor response time in absorption that must be very small (less than 500 ms). The influence of the heating on the response time is described. In conclusion, the three sensors are compared and the most suitable structure for our application is indicated. Although polyimide has been widely used for realization of capacitive humidity sensors, we demonstrate here that it is not the best sensitive material for our application     

气象用湿敏电容传感器的稳定性测试与分析

为研究湿敏电容传感器的稳定性特征,利用14支湿敏电容传感器静态测试数据,用误差年漂移量定量表征湿敏电容传感器的稳定性,并对误差年漂移量的变化规律及影响因素进行分析。结果表明,湿敏电容传感器的稳定性受温度、温度和湿度的交互作用以及厂家制造水平的影响,低温时稳定性较差;室温时稳定性随湿度升高而降低。经过一年的使用,78.6%的湿敏电容传感器无法满足技术指标要求。     

The influence of electric-field bending on the nonlinearity ofcapacitive sensors

Three-layered electrode structures are often employed in multiple-electrode capacitive position sensors. Even when advanced algorithms and well-designed guarding electrodes are used, the electric-field-bending effect is still one of the major contributors to the nonlinearity of capacitive position sensors. In this paper, the effects of electric-field bending on linearities of five capacitive linear-position sensors have been studied based on a physical model of the capacitive sensor. It is shown that the effect of electric-field bending on linearities strongly depends on the sensor structures, and that it is significantly reduced when advanced sensor structures and algorithms are used. The results are very useful for optimizing the sensor structure according to its application     

Durable microfabricated high-speed humidity sensors

We describe a durable microfabricated humidity sensor made of interdigitated rhodium electrodes on a silicon substrate covered with a sensing film of Nafion perfluorosulfonate ionomer. Rhodium electrodes are much less prone to oxidative degradation compared to previously described gold electrode-based sensors. Even with dc excitation, Rh electrode sensors exhibit excellent long-term response stability. It has been found that low-amplitude (+/-1 V) square wave excitation can prolong the usability of gold electrode-based sensors to at least several months; however, this mode of interrogation cannot provide subsecond response times. Rhodium deposition on the microsensors is much more difficult than that of gold. We were able to attain crack-free Rh deposits by adaptation of pulsed electroplating techniques. At excitation voltages of >2 V dc, the Rh sensors respond to moisture with 10 <--> 90% rise and fall times of 30-50 ms. These are the fastest microfabricated water vapor sensors reported to date. We demonstrate applications as a breath monitor. Such sensors should also be of utility in atmospheric eddy measurements. Short-term repeatability is better than 0.6% RSD (n = 7).     

Equilibrium and nonequilibrium electrode processes on porous silicon

The electrode potential of porous silicon (por-Si) in aqueous electrolytes of variable acidity has been measured for the first time. It is shown that por-Si electrode can be a promising pH sensor. The results of the electrode potential measurements show that water condensed in capillaries of capacitive humidity sensors with a thick layer of mesoporous silicon is probably subject to electrolysis.     

Modeling and Optimization of a Microscale Capacitive Humidity Sensor for HVAC Applications

This paper presents a comprehensive numerical study of the performance of a capacitive humidity sensor for heating, ventilation, and air conditioning (HVAC) applications. The proposed sensor comprises a sensing layer sandwiched between an array of top and bottom electrodes. A combination of both parallel plate and interdigitated electrode arrangements is considered to achieve their distinctive advantages. Polyimide is used as the humidity sensing material due to good sensing characteristics and aluminum is used as the electrode material because of the ease of fabrication. A layer of polyimide covers the top electrodes to provide protection from atmospheric contamination thus improving durability. The influence of relative humidity on the dielectric constant of the sensing layer is determined theoretically using the models of Looyenga and Shibata The model is validated by comparing model predictions with experimentally measured data for a previously reported capacitive humidity sensor. The model is then used to simulate and predict the performance of the proposed humidity sensor. The effects of design configuration, sensing layer thickness, electrode polarity, electrode width and thickness, and electrode gap are studied. The influence of operating conditions including relative humidity, temperature and voltage is investigated. Based on the simulation results, the optimum design configuration is identified.     

Humidity sensor structures with thin film porous alumina for on-chip integration

Our aim was to produce a cheap, reliable, low-power and CMOS-MEMS process compatible relative humidity (RH) capacitive sensor that can be incorporated into a state-of-the art wireless sensor network. Porous alumina, produced by electrochemical oxidation of aluminum thin film under anodic bias (AAO, Anodic Aluminum Oxide) was used for the purpose of the sensing layer. We prepared two different capacitive sensor structures on silicon substrate using semiconductor processing steps and anodic oxidation in addition. The first sensor has an ultra-thin, vapor-permeable palladium upper electrode, while in the second case, an electroplated gold-grid is used for the same purpose. The highest achieved average sensitivity is approx. 15 pF/RH%, which is much higher than the values found in product catalogues of discrete, off-the-shelf capacitive humidity sensors (0.2-0.5 pF/RH%).     

Mass transfer and gas-phase calibration of implanted oxygen sensors

A protocol is described for validation of implanted oxygen sensors, in which sensors are calibrated in the gas phase where concentration boundary layers are absent. Calibration prior to sensor implantation and confirmation after sensor explantation allows separation of tissue mass transfer effects from sensor variance and drift. A model is given here that describes the oxygen-dependent signal current in terms of oxygen mass transfer to the sensor, permeability of the sensor membrane, and electrode area. The parameter used in the model to describe mass transfer to implanted sensors is consistent with experimental observations and allows comparisons with nonimplanted sensors. This method provides a bridge between the complementary approaches of empirical calibration and model-based calculation for determining oxygen concentration from the sensor response.     

Capacitive Transduction for Liquid Crystal Based Sensors, Part II: Partially Disordered System

This paper presents the capacitive transduction technique involved with liquid crystal (LC) based sensors in partially disordered systems. These sensors have the potential applications in chemical and biological systems. The theory for tracking the average molecular deformation (state of alignment) and degree of ordering of anisotropic and partially disordered LC film via capacitive sensing is investigated. This system is modeled using the Q-tensor approach in modeling uniaxial LC material. The proposed sensor design is an interdigitated electrodes structure. Transverse and fringing capacitances as function of the molecular deformation are calculated. It is verified that three capacitance measurements are required to track the average molecular orientation and the degree of disorder in the LC film. The sensitivity for the sensor at different alignments and ordering degree is also studied. Toward practical sensor, neuro-fuzzy system is modeled to simulate the capacitive transduction and to monitor the LC profile. Sensors are fabricated and tested. Both the experimental and calculated capacitances are presented and compared.      

A simple interface circuit to measure very small capacitancechanges in capacitive sensors

This paper presents an easy-to-design interface circuit to measure very small-percentage capacitance variations in capacitive sensors, especially suitable for industrial measurements. A computer-controlled 24-bit A/D converter is employed to obtain a higher resolution. This interface circuit can be used with various types of capacitive sensors. The most interesting thing is, that the measurement results through this interface circuit are independent of the initial capacitance of the sensor. In addition, the double differential operating principle used here minimizes the error caused by coupling and stray capacitance of sensor probes. The operating principle of the designed interface circuit, the major assumptions made, test data, and possible future developments are discussed     

Method for measuring thickness of dielectric films using microdielectric fringe-effect sensors

A method for noninvasive thickness measurements of dielectric films using fringe-effect (FE) sensors is developed and experimentally validated. The fringing electrical field, created by electrodes microfabricated at the film substrate, depends on the film thickness and dielectric permittivity of the film under test (FUT). The unknown film thickness is estimated by matching the theoretical prediction of thickness-dependent sensor admittance with the measured value. In the case of FE sensors with spatially periodic, interdigitated electrode (IDE) configuration, the admittance prediction is simplified, which allows for the real-time measurements of changing thickness. The developed method can be used to continuously measure the changing dielectric permittivity of the FUT material, which makes it possible to determine the thickness of films of changing dielectric properties, caused by chemical or other transformations. The application of the developed method is demonstrated experimentally by measuring the thickness of silicon nitride film deposited in several increments on the quartz substrate of the IDE sensor. In the expected range of sensor sensitivity, the results show an excellent agreement with the independent thickness measurements.     

Nanostructured Metal Oxide Thin Films for Humidity Sensors

Capacitive humidity sensors were fabricated using countersunk interdigitated electrodes coated with amorphous nanostructured TiO₂, SiO₂, and Al₂O₃ thin films grown by glancing angle deposition. The capacitive response and response times for each sensor were measured. The sensor utilizing TiO₂ exhibited the largest change in capacitance, increasing exponentially from ~ 1 nF to ~ 1muF for an increase in relative humidity from 2% to 92%. Adsorption and desorption response times were measured using flow rates of 2.5 l/min and were between 90 ms and 300 ms for the sensors studied here. A simple model of the capacitive response of the devices has been developed and used to calculate the dielectric constant of the combined system of our films and adsorbed water. The obtained dielectric constants are found to be much higher than bulk or literature values for similar systems.     

电极结构对高分子电阻型湿度传感器性能的影响

本文以聚苯乙烯磺酸钠为湿敏材料,制备了以金叉指电极为基底的高分子电阻型湿度传感器。研究了电极基片材料和叉指电极构型对传感器湿敏响应特性的影响。研究表明,采用多孔结构的基片材料可降低传感器电阻,增强湿敏膜与基片的结合能力从而提高传感器的稳定性;叉指电极构型对传感器的电阻大小有一定影响,增加电极中心线间距离使传感器的稳定性提高。     

Intelligent sensors using computationally efficient Chebyshev neural networks

Intelligent signal processing techniques are required for auto-calibration of sensors, and to take care of nonlinearity compensation and mitigation of the undesirable effects of environmental parameters on sensor output. This is required for accurate and reliable readout of the measurand, especially when the sensor is operating in harsh operating conditions. A novel computationally efficient Chebyshev neural network (CNN) model that effectively compensates for such non-idealities, linearises and calibrates automatically is proposed. By taking an example of a capacitive pressure sensor, through extensive simulation studies it is shown that performance of the CNN-based sensor model is similar to that of a multilayer perceptron-based model, but the former has much lower computational requirement. The CNN model is capable of producing pressure readout with a full-scale error of only plusmn1.0% over a wide operating range of -50 to 200degC.     

Tuning sensitivity and selectivity of complementary metal oxide semiconductor-based capacitive chemical microsensors

New details on selectivity and sensitivity of fully integrated CMOS-based capacitive chemical microsensor systems are revealed. These microsystems have been developed to detect volatile organics in ambient air and rely on polymeric sensitive layers. The sensitivity and selectivity changes induced by thickness variation of the sensitive polymer layer allow for tuning of the layer parameters to achieve desired sensor features. Cross-sensitivity to interfering agents can be drastically reduced, as is shown for two important cases: (a). rendering the capacitive sensor insensitive to a low-dielectric-constant analyte (lower than that of the polymer) and (b). reducing the influence of a high-dielectric-constant analyte, such as water, on the sensor response. The second case is of vital importance for capacitive sensors, since water is omnipresent and evokes large capacitive sensor signals. The thickness-induced selectivity is explained as a combination of dielectric constant change and swelling and has been confirmed by measurements. Experimentally determined sensitivities qualitatively and quantitatively coincide with the calculated values implying understanding of the sensing mechanism.     

Tracking Control of a Nanopositioner Using Complementary Sensors

Piezoelectric tube actuators are widely used in atomic force and scanning tunneling microscopy (STM) for nanoscale positioning. There has been a consistent effort to increase the scan speed of these actuators using feedback control techniques. A feedback controller requires a measurement of the scanner's deflection, which is often provided by a capacitive sensor. Such measurements are corrupted by sensor noise, typically in the order of 20 pm/ radicHz rms. Over a bandwidth of 10 kHz, this translates into an rms noise of 2 nm, clearly inadequate for applications that require subnanometer positioning accuracy, e.g., STM. In this paper, we illustrate how the strain voltage induced in a free electrode of the scanner can be used as an additional displacement signal. The noise level corresponding to the strain signal is about three orders of magnitude less than that of a capacitive sensor, making it an ideal choice for nanopositioning applications. However, it cannot be used for dc and low-frequency measurements. A two-sensor-based controller is designed to use the capacitive sensor signal at low frequencies, and the strain displacement signal at high frequencies. By limiting the capacitive sensor feedback loop bandwidth to less than 100 Hz, the rms value of the noise is reduced to well below 1 nm. For almost the same noise level, the two-sensor-based control structure achieves a closed-loop bandwidth of more than three times that of the single-sensor-based controller.     

A switched-capacitor interface for capacitive sensors based onrelaxation oscillators

A switched-capacitor (SC) interface for capacitive sensors based on a modified Martin's relaxation oscillator is proposed. The output signal is the duty-cycle of a pulse-width modulated square-wave voltage or a binary-coded digital signal which is directly related to the capacitance ratio of an unknown capacitance and reference capacitance. The circuit can be implemented in a monolithic IC form using CMOS technology. It requires a relatively small device count integrable onto a small chip area and its suited particularly for the on-chip interface circuitry for microprocessors     

Transparent and Flexible Cellulose Nanocrystal/Reduced Graphene Oxide Film for Proximity Sensing

The rapid development of touch screens as well as photoelectric sensors has stimulated the fabrication of reliable, convenient, and human‐friendly devices. Other than sensors that detect physical touch or are based on pressure sensing, proximity sensors offer controlled sensibility without physical contact. In this work we present a transparent and eco‐friendly sensor made through layer‐by‐layer spraying of modified graphene oxide filled cellulose nanocrystals on lithographic patterns of interdigitated electrodes on polymer substrates, which help to realize the precise location of approaching objects. Stable and reproducible signals generated by keeping the finger in close proximity to the sensor can be controlled by humidity, temperature, and the distance and number of sprayed layers. The chemical modification and reduction of the graphene oxide/cellulose crystal composite and its excellent nanostructure enable the development of proximity sensors with faster response and higher sensitivity, the integration of which resolves nearly all of the technological issues imposed on optoelectronic sensing devices.     

Quantitative measurements of dielectric spectra with microdielectric fringe-effect sensors

The parallel-plate method is a gold standard for measuring dielectric properties of materials. However, it requires sampling of the material under testing (MUT), which makes it less suitable for real time, dynamic, and in situ measurements. The alternative to the parallel-plate method is to use the microdielectric fringe-effect (FE) sensors, which can be placed inside the process or laboratory equipment to provide rapid, on-line, and noninvasive characterization of the dielectric properties. An additional potential advantage of the FE measurements is the ability to obtain spatially localized and interfacial measurements, which may be important in some applications. Unfortunately, interpretation of the FE sensor measurements is difficult because of the spatial nonuniformity of the electrical excitation field created by the FE sensor and the extraneous contributions from the sensor substrate and unknown stray elements. The objective of this study is to summarize the theoretical basis of the dielectric measurements using planar interdigitated sensors and to use it in the development of a new method for obtaining quantitative measurements with FE sensors. As the first step, the basic correlation between the impedance measurements obtained with the FE sensor and the dielectric properties of the MUT is elucidated. The theoretical results are then used to analyze the contribution of the sensor substrate and unknown stray components to the overall measurements. A novel calibration method to eliminate extraneous contributions is then proposed. The application example demonstrates the application of the developed method to the measurement of the dielectric permittivities of a polydispersed cis-polyisoprene samples. The results are compared with those obtained using the parallel-plate measurements and show excellent agreement. Experimental comparison with the alternative calibration methods is also performed, indicating significant improvement in accuracy of dielectric measurements over a broad range of frequencies.     

Bulk acoustic wave sensors for sensing measurand-induced electrical property changes in solutions

A variety of quartz thickness shear mode (TSM) resonant sensors with different electrode configurations have been designed, fabricated, and tested in liquids for probing liquid electrical property changes. The resonant frequency of the sensors was found to have more than an order of magnitude increase in sensitivity over the standard AT-cut quartz resonant sensor. The increase in sensitivity is due to the ability of the sensors to detect changes in the electrical properties in the liquid