Interdigital Capacitive Sensor Research Articles

An <italic>LC</italic>-Type Passive Wireless Humidity Sensor System With Portable Telemetry Unit

This paper presents a high-sensitivity passive wireless humidity sensor system with a portable telemetry unit for applications in sealed environments. A complementary metal oxide semiconductor (CMOS) interdigital capacitive humidity sensor die was attached to an organic substrate (FR-4) on which a fixed planar spiral copper inductor was fabricated. The variable capacitor and the fixed inductor were wire bonded to form an inductor-capacitor (LC) tank circuit. The resonant frequency of the sensor tank is dependent on the sensor capacitance, which changes in response to the humidity. The sensitivity of the capacitive sensor was improved significantly using graphene oxide as a sensing material. The package-level integration was achieved by employing the embedded inductor on an organic packaging substrate. The LC-type sensor is interrogated wirelessly using our homemade portable telemetry unit, which is based on a standing wave ratio bridge to measure the real part of the readout coil impedance. Measurements show a sensitivity of -18.75 kHz/%RH over a range of 15%-95% RH. The implemented telemetry unit addresses the need for a low-cost, portable, and universal reader of the LC-type passive wireless sensors.

Bioelectric Signal Measuring System

We describe a low noise measuring system based on interdigitated electrodes for sensing bioelectrical signals. The system registers differential voltage measurements in order of microvolts. The base noise during measurements was in nanovolts and thus, the sensing signals presented a very good signal to noise ratio. An excitation voltage of 1Vrms with 10 KHz frequency was applied to an interdigitated capacitive sensor without a material under test and to a mirror device simultaneously. The output signals of both devices was then subtracted in order to obtain an initial reference value near cero volts and reduce parasitic capacitances due to the electronics, wiring and system hardware as well. The response of the measuring system was characterized by monitoring temporal bioelectrical signals in real time of biological materials such as embryo chicken heart cells and bovine suprarenal gland cells.

Rapid and molecular selective electrochemical sensing of phthalates in aqueous solution.

Reported research work presents real time non-invasive detection of phthalates in spiked aqueous samples by employing electrochemical impedance spectroscopy (EIS) technique incorporating a novel interdigital capacitive sensor with multiple sensing thin film gold micro-electrodes fabricated on native silicon dioxide layer grown on semiconducting single crystal silicon wafer. The sensing surface was functionalized by a self-assembled monolayer of 3-aminopropyltrietoxysilane (APTES) with embedded molecular imprinted polymer (MIP) to introduce selectivity for the di(2-ethylhexyl) phthalate (DEHP) molecule. Various concentrations (1-100 ppm) of DEHP in deionized MilliQ water were tested using the functionalized sensing surface to capture the analyte. Frequency response analyzer (FRA) algorithm was used to obtain impedance spectra so as to determine sample conductance and capacitance for evaluation of phthalate concentration in the sample solution. Spectrum analysis algorithm interpreted the experimentally obtained impedance spectra by applying complex nonlinear least square (CNLS) curve fitting in order to obtain electrochemical equivalent circuit and corresponding circuit parameters describing the kinetics of the electrochemical cell. Principal component analysis was applied to deduce the effects of surface immobilized molecular imprinted polymer layer on the evaluated circuit parameters and its electrical response. The results obtained by the testing system were validated using commercially available high performance liquid chromatography diode array detector system.

Hand-held unit for liquid-type recognition, based on interdigital capacitor

The implementation of a unit capable to recognize liquids based on an interdigital capacitive sensor (IDC) is presented. The sensor’s structure was fabricated on standard FR-4 PCB board. Its capacitance change relies upon permittivity change of the medium above IDC electrodes, representing dielectric properties of the liquid used. Along with the microcontroller and simple interface circuit, information about the type of the liquid is presented on a 2×16 character display and through RS232 connection on PC. Implemented unit was tested against seven liquids (benzene, phenol, acetone, ethanol, methanol, formaldehyde and distillated water) and a steady state when unit only detected air. Results imply usage of this approach for the fabrication of cost effective, portable devices for on-field sensing applications.

A self-calibration water level measurement using an interdigital capacitive sensor

A water level measurement using an interdigital capacitive sensor with low-cost, low-energy, good repeatability, high linearity, and ease of installation is proposed with a support of experimental results. This sensor comprises a printed circuit board (PCB) with configuration of two interpenetrating comb electrodes. The comb electrode is 70–80mm width, 300mm height with 1–2mm spacing between each comb. This configuration of electrode causes the capacitance between comb electrodes to vary by the water level. Microcontroller is used to calculate the capacitance between comb electrodes in terms of a discharge time correlated to the water level. A practical water level measurement technique using two comb electrodes designated as level and reference sensors is presented. This technique can directly be applied to water with different conditions without recalibration. This sensor is able to measure absolute levels of water with 0.2cm resolution over 30cm range. In addition, it is also sensitive enough to trace the variability of water level. A flood monitoring simulation is carried out in wave flume where this sensor is used to detect the rising wave.

Development and Evaluation of Portable Low Cost Testing System for Phthalates

Abstract A portable low-cost microcontroller based testing system was designed and constructed to detect phthalates in aqueous media. Phthalates, especially di (2-ethylhexyl) phthalate is the most ubiquitous endocrine disrupting compound (EDC) posing highest reproductive toxicity threat to all living species on earth. Frequency response analyser (FRA) approach was used to develop the rapid response, non-invasive electrochemical impedimetric system for detection. A smart thin film gold interdigital electrodes capacitive sensor with enhanced penetration depth was used with the proposed system for detection of the said EDC. The performance of the developed system was evaluated by comparing the results to the commercially available electrochemical Impedimetric frequency response analyzer equipment. Complex nonlinear least square (CNLS) curve fitting algorithm was used to deduce the equivalent circuit for the developed system. The DEHP detection results by the developed system were validated using high performance liquid chromatography (HPLC) diode array detection confirming that the proposed system was able to detect the concentration of phthalates in aqueous medium.

Electrochemical impedance spectroscopy based MEMS sensors for phthalates detection in water and juices

Phthalate esters are ubiquitous environmental and food pollutants well known as endocrine disrupting compounds (EDCs). These developmental and reproductive toxicants pose a grave risk to the human health due to their unlimited use in consumer plastic industry. Detection of phthalates is strictly laboratory based time consuming and expensive process and requires expertise of highly qualified and skilled professionals. We present a real time, non-invasive, label free rapid detection technique to quantify phthalates' presence in deionized water and fruit juices. Electrochemical impedance spectroscopy (EIS) technique applied to a novel planar inter-digital (ID) capacitive sensor plays a vital role to explore the presence of phthalate esters in bulk fluid media. The ID sensor with multiple sensing gold electrodes was fabricated on silicon substrate using micro-electromechanical system (MEMS) device fabrication technology. A thin film of parylene C polymer was coated as a passivation layer to enhance the capacitive sensing capabilities of the sensor and to reduce the magnitude of Faradic current flowing through the sensor. Various concentrations, 0.002ppm through to 2ppm of di (2-ethylhexyl) phthalate (DEHP) in deionized water, were exposed to the sensing system by dip testing method. Impedance spectra obtained was analysed to determine sample conductance which led to consequent evaluation of its dielectric properties. Electro-chemical impedance spectrum analyser algorithm was employed to model the experimentally obtained impedance spectra. Curve fitting technique was applied to deduce constant phase element (CPE) equivalent circuit based on Randle's equivalent circuit model. The sensing system was tested to detect different concentrations of DEHP in orange juice as a real world application. The result analysis indicated that our rapid testing technique is able to detect the presence of DEHP in all test samples distinctively.

Foil-to-foil lamination and electrical interconnection of printed components on flexible substrates

• We described and compared two foil-to-foil lamination and interconnection methods. • Anisotropic conductive adhesive (ACA) method requires less processing steps. • Through foil vias (TFV) method proved to be more robust to cyclic bending forces. • ACA and TFV methods shown to have high reliability during environmental ageing. • The methods are demonstrated for two types of foil-based humidity sensors. This paper describes and compares two integration methods to structurally laminate and interconnect foil-based components with flexible polymeric substrates. The first method uses isotropic conductive adhesives (ICA) confined in laser-ablated through foil vias (TFV), while the second one uses an anisotropic conductive adhesive (ACA). Both procedures were successfully demonstrated by interconnecting silver-based inkjet printed and gold-sputtered interdigitated capacitive humidity sensors onto flexible PEN carrier substrates, showing functionality, high process yield and low-complexity. The robustness of the assemblies was tested and compared for adhesion, bending, high humidity and temperature cycling. Confined ICA vias show higher mechanical robustness to bending, while both methods remained functional after more than 900 h of environmental ageing. Finally, the interconnections were fully validated at different levels of relative humidity (RH) by comparing the sensor response to that of a commercial sensor.

High‐performance bulk silicon interdigital capacitive temperature sensor based on graphene oxide

A high-performance bulk silicon interdigital capacitive temperature sensor based on graphene oxide (GO) is presented. Compared with typical CMOS integrated temperature sensors, the sensitivity of this bulk silicon interdigital capacitive temperature sensor was improved significantly by using GO as sensing material, and this temperature sensor can operate in the range of − 70 to 40°C.

Capacitive Fringing Field Sensor Design for Moisture Measurement

The fringing field capacitive sensor is described for moisture measurement like soil, food application. Interdigitated electrode capacitive sensor has been utilized in numerous applications at low frequency. A sensor performance was evaluated by monitoring the output capacitance variation with and without moisture content. The variation in capacitance is directly measured by using interface circuit. Switched capacitor dual slope capacitor to digital converter has been utilized as an interface circuit. The interface circuit has been designed and simulated with variable capacitance by sensor performance. Printed circuit board technology is particularly advantageous for realizing this type of sensor by fabricating the interdigitated electrode structures in the patterned Cu foil instead of realizing these type of sensors in various technologies.

A Capacitive Fringing Field Sensor Design for Moisture Measurement Based on Printed Circuit Board Technology

Interdigitated electrode capacitive fringing field sensors have been utilized in numerous applications. Although various technologies are used to realize these types of sensors, printed circuit board technology is particularly advantageous for realizing this type of sensor through fabricating the interdigitated electrode structures in the patterned Cu foil. Additionally, the solder mask coating can insulate the electrodes to prevent shorting in the presence of water. Using this approach, prototype sensors were designed, simulated, fabricated, and successfully evaluated. Applications include water detection and quantity measurement and soil moisture content measurement.

Inkjet Printing of Interdigitated Capacitive Chemical Sensors with Reduced Size by the Introduction of a Dielectric Interlayer

A new fabrication strategy is proposed to reduce the size of sensors based on inkjet-printed interdigitated electrodes (IDE). The process is compatible with fabrication at low temperature and can be applied to flexible organic foils. The new fabrication concept consists of the deposition of a thin dielectric layer of parylene-C onto the first electrodes comb prior to the printing of the second one. In this way, the combs are not longer on the same plane, preventing undesired short-circuits and solving the typical problems of definition and yield associated to inkjet printing of IDE devices with small pitch. The proposed strategy results especially relevant for capacitive-based devices, where short-circuits render the device non-functional. Moreover, it permits enhancing their capacitance per surface area. The validation of the exposed process for sensors application has been carried out by functionalizing the IDE structure with a humidity sensing layer, and characterizing the resulting sensor against changes in relative humidity (R.H.). (C) 2012 Elsevier Ltd .... Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o.

A Fully Packaged CMOS Interdigital Capacitive Humidity Sensor With Polysilicon Heaters

This paper presents a fully packaged CMOS interdigital capacitive humidity sensor with polysilicon heaters. The sensor was fabricated with the industrial standard CMOS process and the MEMS postprocessing step. Polyimide was used as the sensing material for its good linearity. To enlarge the depth of the recesses between the interdigital electrodes, the polysilicon heater was placed under the Al electrode and also the passivation between the interdigital electrodes was partly etched. As a result, thicker polyimide can be deposited between the Al electrodes and thus the sensitivity of the sensor can be improved. The etching resistant passivation between Al electrodes and polyimide was used to ensure the reliability of the sensor. To put it into practical use, the sensor was fully packaged, and by using the polysilicon heaters, both the hysteresis and the recovery time of the sensor could be greatly improved. The packaged humidity sensor was measured to show a good linearity and repeatability over the whole testing range.

Polymer Coated Microfabricated Interdigitated Electrodes Arrays for Gas Sensing Applications

InterDigitated Capacitive (IDC) sensor arrays are fabricated with conventional microelectronics-micromachining technologies on quartz substrates. After fabrication, a polymeric well is patterned around each IDC to precisely define the sensing area and thus deposit coatings of various polymers, by drop casting, in a reproducible and controlled manner. The gas sensing performance of the IDC array is presented for humidity and p-xylene.

Capacitive sensor arrays with controllable deposition of the sensing polymer area for VOCs applications: Design and measurement considerations

InterDigitated Capacitive (IDC) sensor arrays are fabricated with conventional microelectronics-micromachining technologies on quartz substrates. After the IDC fabrication, a polymeric well is patterned around each IDC to precisely define the sensing area and thus deposit coatings of various polymers, by drop casting, in a reproducible and controlled manner. The performance of the coated IDC array is evaluated in terms of IDC critical dimension, measurement frequency and for two analytes and guidelines for improved sensing performance are proposed. Through careful selection of the polymeric coatings in conjunction with suitable signal processing, discrimination of VOCs is possible.

High sensitivity interdigited capacitive sensors using branched treelike carbon nanotubes on silicon membranes

Branched treelike carbon nanotubes on silicon substrate have been exploited for the realization of high sensitivity interdigital capacitive pressure sensors. The interdigital structure has been realized using a micromachining technique on silicon membranes, whereas the growth of nanotubes has been achieved using a direct-current plasma enhanced chemical vapor deposition method. A sequential growth and hydrogenation has led to the formation of multiple branched structures of nanotubes. The growth in an interdigital manner results in a high overlap between neighboring fingers and consequently a magnified response to mechanical variations in the membrane as a result of applying an external pressure is observed. An oscillatory behavior has been observed which may be attributed to the vibration of nanotubes on thinned membranes.

Interdigital Capacitive Array of tree-like Carbon Nanotubes on Silicon-based Membranes for Sensor Applications

AbstractWe have grown tree-like vertically-aligned carbon nanotubes (CNTs) on silicon substrate, suitable for highly sensitive interdigital capacitive sensors. As an application, we present a sensitive pressure sensor with branched CNTs as its capacitance plates.After realization of the interdigital structure, the growth of CNTs has been achieved through direct-current plasma enhanced chemical vapor deposition (DC-PECVD) method. A sequential growth and hydrogenation has led to the formation of multiple branched structures of nanotubes. The growth of tree-like CNTs on the interdigitally patterned substrate results in a high overlap between adjacent fingers and consequently a significant response to mechanical variations of the membrane as a result of the applied pressure.

Single chip interdigitated electrode capacitive chemical sensor arrays

In this paper the design considerations, fabrication and first evaluation results of a capacitive type single chip chemical sensor array based on interdigitated electrode structure is presented. The electrode arrays have been fabricated with conventional semiconductor processes. The final structure consists of four interdigital capacitive sensors each one coated with a different polymer sensitive film by using only successive lithographic steps. The array sensors have good response to controlled concentrations of volatile organic compounds (∼35fF, ∼7fF, ∼12fF for the PHEMA covered sensor when exposed to 100 ppm of water, methanol and ethanol vapors and ∼2.5fF, in the case of 100 ppm of toluene vapors for the PDMS covered sensor).

Detection and Discrimination Capabilities of a Multitransducer Single-Chip Gas Sensor System

The performance of a single-chip, three-transducer, complementary metal oxide semiconductor gas sensor microsystem has been thoroughly evaluated. The monolithic gas sensor system includes three polymer-coated transducers, a mass-sensitive cantilever, a thermoelectric calorimetric sensor, and an interdigitated capacitive sensor that are integrated along with all electronic circuits needed to operate these sensors. The system additionally includes a temperature sensor and a serial interface unit so that it can be directly connected to, for example, a microcontroller. Several multitransducer chips have been coated with various partially selective polymers and then have been exposed to different volatile organic compounds. The sensitivities of the three different polymer-coated transducers to defined sets of gaseous analytes have been determined. The obtained sensitivity values have then been normalized with regard to the partition coefficients of the respective analyte/polymer combination to reveal the transducer-specific effects. The results of this investigation show that the three different transducers respond to fundamentally different molecular properties, such as the analyte molecular mass (mass-sensitive), its dielectric coefficient (capacitive), and its sorption heat (calorimetric) so that correlations between the determined sensitivity values and the different molecular properties of the absorbed analytes could be established. The information as provided by the system, hence, represents a body of orthogonal data that can serve as input to appropriate signal processing and pattern recognition techniques to address issues such as the quantification of analytes in mixtures.

Dielectric response of interdigital chemocapacitors: The role of the sensitive layer thickness

The most important characteristics of a chemical sensor are its sensitivity and selectivity. In choosing a sorbent material for a chemical sensor all possible interactions with the vapour of interest should be understood if our goal is to obtain a desirable sensitivity or/and selectivity. In this work, an analytical model for the capacitance of interdigital capacitive sensors was used to explain the relationship between the sensor response and its geometrical parameters. This study reveal the strong link between the sensor response (sensitivity and selectivity) and the sensor geometric parameters, in particular what concerns the sensitive layer (sorbent material) thickness for a given electrode geometry. The knowledge of these relationships enables the right choice of a sensor in order to obtain the desired sensitivity and/or selectivity to a set of analytes. This can be done not only by the correct choice of the sorbent material but as well by the right choice of its thickness and sensor geometry. On the other hand, the understanding of the sensor response opens the possibility of making sensors or a combination of sensors with a reduced sensitivity to unwanted analytes. The dynamic of the response was studied as well, to identify the physical process involved which can be used to understand the sensing mechanisms and as well to increase the number of features to be extracted from the sensors.