Microelectromechanical components in electrical metrology

Microelectromechanical systems (MEMS) can offer a competitive alternative for conventional technology in electrical precision measurements. This article summarises recent work in development of MEMS solutions for electrical metrology. MEMS-based voltage references, RMS-to-DC converters, high frequency power sensors, and reference oscillators are discussed. The main principle of operation of the components is the balance between electrical forces and mechanical spring forces in micromachined silicon structures. In RMS sensors and RMS-to-DC converters, the quadratic voltage dependence of the force between plates of a moving-plate capacitor is utilised, and the operation of the MEMS voltage reference is based on the pull-in phenomenon of a moving-plate capacitor. Advantages of MEMS devices compared to more conventional solutions include small size, low power consumption, low price in mass production, and stability. The drift caused by electrostatic charging effects has turned out to be a major problem. This problem has not yet been solved in DC applications, but it can be circumvented by using AC actuation instead of DC and by compensating the internal DC voltages of the component. In this way, an AC voltage reference with relative drift rate below 2 ppm during a three-week test period has been constructed. Even better stability has been demonstrated with a MEMS-based reference oscillator: no changes in resonance frequency were observed at relative uncertainty level of about 0.01 ppm in a measurement which was continued for more than a month. MEMS components have also been developed for measuring RF and microwave power up to frequencies of about 40 GHz. Unlike conventional high frequency power sensors, which measure the absorbed power, the MEMS device measures the power that is transmitted through the sensor

Design, characterization and testing of a thin-film microelectrode array and signal conditioning microchip for high spatial resolution surface laplacian measurement.

Cardiac mapping has become an important area of research for understanding the mechanisms responsible for cardiac arrhythmias and the associated diseases. Current technologies for measuring electrical potentials on the surface of the heart are limited due to poor spatial resolution, localization issues, signal distortion due to noise, tissue damage, etc. Therefore, the purpose of this study is to design, develop, characterize and investigate a custom-made microfabricated, polyimide-based, flexible Thin-Film MicroElectrode Array (TFMEA) that is directly interfaced to an integrated Signal Conditioning Microchip (SCM) to record cardiac surface potentials on the cellular level to obtain high spatial resolution Surface Laplacian (SL) measurement. TFMEAs consisting of five fingers (Cover area = 4 mm2 and 16 mm2), which contained five individual microelectrodes placed in orthogonal directions (25-µm in diameter, 75-µm interelectrode spacing) to one another, were fabricated within a flexible polyimide substrate and capable of recording electrical activities of the heart on the order of individual cardiomyocytes. A custom designed SCM consisting of 25 channels of preamplification stages and second order band-pass filters was interfaced directly with the TFMEA in order to improve the signal-to-noise ratio (SNR) characteristics of the high spatial resolution recording data. Metrology characterization using surface profilometry and high resolution Scanning Electron Microscope (SEM) indicated the geometry of fabricated TFMEAs closely matched the design parameters \u3c 0.4%). The DC resistances of the 25 individual micro electrodes were consistent (1.050 ± 0.026 kO). The simulation and testing results of the SCM verified the pre-amplification and filter stages met the designed gain and frequency parameters within 2.96%. The functionality of the TFMEA-SCM system was further characterized on a TX 151 conductive gel. The characterization results revealed that the system functionality was sufficient for high spatial cardiac mapping. In vivo testing results clearly demonstrated feasibility of using the TFMEA-SCM system to obtain cellular level SL measurements with significantly improved the SNRs during normal sinus rhythm and Ventricular Fibrillation (VF). Local activation times were detected via evaluating the zero crossing of the SL electro grams, which coincided with the gold standard (dV/dt)min of unipolar electro grams within ± 1%. The in vivo transmembrane current densities calculated from the high spatial resolution SLs were found to be significantly higher than the transmembrane current densities computed using electrodes with higher interelectrode spacings. In conclusion, the custom-made TFMEASCM systems demonstrated feasibility as a tool for measuring cardiac potentials and to perform high resolution cardiac mapping experiments

Phase optimization of thermally actuated piezoresistive resonant MEMS cantilever sensors

The asymmetric resonance response in thermally actuated piezoresistive cantilever sensors causes a need for optimization, taking parasitic actuation–sensing effects into account. In this work, two compensation methods based on Wheatstone bridge (WB) input voltage (VWB_in) adjustment and reference circuit involvement were developed and investigated to diminish those unwanted coupling influences. In the first approach, VWB_in was increased, resulting in a higher current flowing through the WB piezoresistors as well as a temperature gradient reduction between the thermal actuator (heating resistor: HR) and the WB, which can consequently minimize the parasitic coupling. Nevertheless, increasing VWB_in (e.g., from 1 to 3.3&thinsp;V) may also yield an unwanted increase in power consumption by more than 10 times. Therefore, a second compensation method was considered: i.e., a reference electronic circuit is integrated with the cantilever sensor. Here, an electronic reference circuit was developed, which mimics the frequency behavior of the parasitic coupling. By subtracting the output of this circuit from the output of the cantilever, the resonance response can thus be improved. Both simulated and measured data show optimized amplitude and phase characteristics around resonant frequencies of 190.17 and 202.32&thinsp;kHz, respectively. With this phase optimization in place, a phase-locked-loop (PLL) based system can be used to track the resonant frequency in real time, even under changing conditions of temperature (T) and relative humidity (RH), respectively. Finally, it is expected to enhance the sensitivity of such piezoresistive electro-thermal cantilever sensors under loading with any target analytes (e.g., particulate matter, gas, and humidity).</p

Conception et réalisation de références de tensions alternatives à base de MEMS

Les systèmes microélectromécaniques (MEMS) sont d'excellents candidats pour la métrologie électrique. En effet, grâce au couplage électromécanique dans les MEMS, il est possible de réaliser des références secondaires de tension continue (DC) ou alternative (AC) ayant des valeurs de quelques volts à quelques centaines de volts avec des stabilités relatives pouvant atteindre quelques 10-7. Celles-ci peuvent alors être une alternative aux actuelles références Zener dans le cas de la tension continue, et constitueront une première pour la tension alternative puisque aucune référence n'existe hormis celle basée sur l'effet Josephson. Ce travail de thèse a été dédié au développement et à la fabrication de plusieurs générations de structures MEMS à capacité électrique variable dans lesquelles on exploite le phénomène du pull-in pour réaliser des références de tension AC. Le design des échantillons, basé sur des architectures spécifiques correspondant à différents modes de déplacement de l'électrode mobile, est réalisé grâce à des modélisations sous ConventorWare. On distingue des structures à débattement vertical favorisant un déplacement en mode piston de la membrane mobile et des structures à peignes interdigités à déplacement dans le plan. Ces différentes structures ont été fabriquées à partir d'un procédé technologique industriel MPW (Multi-project Wafer) de la société Tronic's, basé sur un substrat SOI (Silicon On Insulator). En parallèle, un procédé technologique dédié a été mis au point pour s'adapter aux exigences particulières de nos applications. Les références de tension AC ainsi développées présentent des tensions de pull-in variant de 2 V à 100 V avec des fréquences de résonance mécanique mesurées par DLTS (Deep Level Transient Spectroscopy) de seulement quelques kilohertz. Ce dernier résultat permet donc d'envisager l'utilisation de ces références de tension AC sur une large gamme de fréquence, de quelques dizaines de kilohertz jusqu'à quelques Mégahertz. Nous avons également développé une électronique de commande spécifiquement adaptée aux caractéristiques de nos MEMS et intégrant une régulation de la température au mK près. La stabilité de la tension des MEMS a été mesurée sur plus de 150 heures avec une fluctuation inférieure au millionième à 50 kHz et 100 kHz. Les essais à plusieurs centaines de kHz sont également très prometteurs. La dépendance en température est dix fois plus petite que celle rapportée antérieurement, permettant ainsi de s'affranchir de plateformes de stabilisation thermique sophistiquées.Microelectromechanical systems (MEMS) are excellent candidates for electrical metrology. Thanks to the electromechanical coupling in MEMS, it is possible to make secondary DC and AC voltage references with values from a few volts to several hundred volts with relative stabilities of about 10-7. These standards could constitute an alternative to current Zener references in the case of DC voltage and a first in AC metrology field. This PhD work was dedicated to the development and manufacturing of several generations of MEMS structures with variable electrical capacitance in which we exploit the pull-in phenomenon to build AC voltage references. The design of the samples based on specific architectures characterized by different modes of motion of the movable electrode is achieved through ConventorWare modeling. Both MEMS structures having vertical displacement of the movable membrane and combs-drive design for in-plane motion were considered. These structures have been fabricated with an industrial MPW (Multi Project Wafer) process technology, based on an SOI (Silicon On Insulator) surface micromachining process. However, a dedicated process technology has been developed to meet the specific requirements of our applications. AC voltage references having pull-in voltages ranging from 2 V to 100 V were developed with mechanical resonant frequencies of only a few kilohertz. This makes it possible to use the AC voltage references over a wide frequency range from a few tens of kilohertz to a few megahertz. We have also developed readout electronics specifically designed to match the MEMS characteristics and where the temperature of the samples is controlled. The voltage stability of MEMS was measured over 150 hours and the relative deviation from the mean was found less than one part in 106 at 50 kHz and 100 kHz. Results at several hundred of kHz are also very promising. The temperature dependence is ten times smaller than previously reported, which allow to use less sophisticated thermal stabilization platforms

Design of a fuzzy PID controller for a MEMS tunable capacitor for noise reduction in a voltage reference source

This study presents a conventional Ziegler-Nichols (ZN) Proportional Integral Derivative (PID) controller, having reviewed the mathematical modeling of the Micro Electro Mechanical Systems (MEMS) Tunable Capacitors (TCs), and also proposes a fuzzy PID controller which demonstrates a better tracking performance in the presence of measurement noise, in comparison with conventional ZN-based PID controllers. Referring to importance and impact of this research, the proposed controller takes advantage of fuzzy control properties such as robustness against noise. TCs are responsible for regulating the reference voltage when integrated into Alternating Current (AC) Voltage Reference Sources (VRS). Capacitance regulation for tunable capacitors in VRS is carried out by modulating the distance of a movable plate. A successful modulation depends on maintaining the stability around the pull-in point. This distance regulation can be achieved by the proposed controller which guarantees the tracking performance of the movable plate in moving towards the pull-in point, and remaining in this critical position. The simulation results of the tracking performance and capacitance tuning are very promising, subjected to measurement noise. Article Highlights This article deals with MEMS tunable capacitor dynamics and modeling, considering measurement noise. It designs and applies fuzzy PID control system for regulating MEMS voltage reference output. This paper contributes to robustness increase in pull-in performance of the tunable capacitor

Design of a fuzzy PID controller for a MEMS tunable capacitor for noise reduction in a voltage reference source

This study presents a conventional Ziegler-Nichols (ZN) Proportional Integral Derivative (PID) controller, having reviewed the mathematical modeling of the Micro Electro Mechanical Systems (MEMS) Tunable Capacitors (TCs), and also proposes a fuzzy PID controller which demonstrates a better tracking performance in the presence of measurement noise, in comparison with conventional ZN-based PID controllers. Referring to importance and impact of this research, the proposed controller takes advantage of fuzzy control properties such as robustness against noise. TCs are responsible for regulating the reference voltage when integrated into Alternating Current (AC) Voltage Reference Sources (VRS). Capacitance regulation for tunable capacitors in VRS is carried out by modulating the distance of a movable plate. A successful modulation depends on maintaining the stability around the pull-in point. This distance regulation can be achieved by the proposed controller which guarantees the tracking performance of the movable plate in moving towards the pull-in point, and remaining in this critical position. The simulation results of the tracking performance and capacitance tuning are very promising, subjected to measurement nois

Portable low-power electronic interface for explosive detection using microcantilevers

Microcantilevers have been recently revealed as a highly effective technique for gas detection at trace level when acting as chemical sensors. However, an important milestone still remains to achieve a full-scale development in commercial applications: the cumbersome systems traditionally used to read-out its responses. To accomplish this, a portable low-power electronic interface, based on an analog lock-in amplifier processing square signals, which is fully capable of creating the excitation signal as well as obtaining the response values from resonating microcantilevers functionalized with zeolite based coatings has been herein attempted. The so obtained read-out results are in good agreement with the commercial lock-in amplifier's measurements, demonstrating the accuracy and reliability of the electronic interface. Finally, its performance has been validated for 2-nitrotoluene (o-MNT) detection at ppm V level, as an example of an explosive-related molecule, with BEA zeolite coated microcantilevers. Theoretical limit of detection (LOD) values below 100 ppb have been obtained for Co exchanged BEA modified sensors

Development and characterisation of traceable force measurement for nanotechnology

Traceable low force metrology should be an essential tool for nanotechnology. Traceable measurement of micro- and nanonewton forces would allow independent measurement and comparison on material properties, MEMS behaviour and nanodimensional measurement uncertainties. Yet the current traceability infrastructure in the UK is incomplete. This thesis describes the incremental development of the low force facility at the National Physical Laboratory (NPL). The novel contribution of this thesis has three components. First, specific modifications to the NPL Low Force Balance were undertaken. This involved developing novel or highly modified solutions to address key issues, as well as undertaking detailed comparions with external ans internal traceability references. Second, a triskelion force sensor flexure was proposed and mathematically modelled using both analytical and finite element techniques, and compared to experimentally measured spring constant estimates. The models compared satisfactorily, though fabrication defects in developed prototype artefacts limited the experimental confirmation of the models. Third, a piezoelectric sensor approach for quasistatic force measurement was proposed, experimentally evaluated and rejected. Finally, an improved design for a low force transfer artefact system is presented, harnessing the findings of the reported investigations. The proposed design combines proven strain-sensing technology with the advantageous triskelion flexure, incorporating an external stage and packaging aspects to achieve the requirements for a traceable low force transfer artefact

Review of low-cost sensors for the ambient air monitoring of benzene and other volatile organic compounds

This report presents a literature review of the state of the art of sensor based monitoring of air quality of benzene and other volatile organic compounds. Combined with information provided by stakeholders, manufacturers and literature, the review considered commercially available sensors, including, PID based sensors, semiconductor (resistive gas sensor) and portable on-line measuring devices (sensor arrays). The bibliographic collection includes the following topics: sensor description, field of application in fixed, mobile, indoor and ambient air monitoring, range of concentration levels and limit of detection in air, model descriptions of the phenomena involved in the sensor detection process, gaseous interference selectivity of sensors in complex VOC matrix, validation data in lab experiments and under field conditions.JRC.C.5-Air and Climat

Development and evaluation of a calibration free exhaustive coulometric detection system for remote sensing.

Most quantitative analytical measurement techniques require calibration to correlate signal with the quantity of analyte. The purpose of this study was to employ exhaustive coulometry, an implementation of coulometric analysis in a stopped-flow, fixed-volume, thin-layer cell, to attain calibration-free measurements that would directly benefit intervention-free analysis systems designed for remote deployment. This technique capitalizes on the short diffusion lengths (\u3c 100 µm) to dramatically reduce the time for analysis (\u3c 30 sec). For this work, a thin-layer fluidic cell was designed in software, fabricated via CNC machining, and evaluated using Ferri/Ferrocyanide {Fe(CN)63-/4-} as a model analyte. The 2 µL fixed volume incorporated an oval, 8mm by 4 mm, thin-film gold electrode sensor with an integrated Ag|AgCl pseudo-reference electrode. The flow cell area matched the shape of the sensor, with a volume set by the thickness of a laser-cut silicone rubber gasket (~80 µm). A semi-permeable membrane isolated the working electrode and counter electrode chambers to prevent analyte diffusion. A miniaturized custom potentiostat was designed and built to measure reaction currents ranging from 10 mA to 0.1 nA. Software was developed to perform step voltammetry as well as cyclic voltammetry analysis for verifying electrode condition and optimal redox potential levels. Experimentally determined oxidation/reduction potentials of -100 mV and 400 mV, respectively, were applied to the working electrode for sample concentrations ranging from 50 µM to 10,000 µM. The current generated during the reactions was recorded and the total charge captured at each concentration was obtained by integrating the amperograms. When compared to the expected charge for a 2 µL sample, the total charge vs. concentration plots displayed a near perfect linearity over the full concentration range, and the expected charge (100 % converted) was reached within 20 seconds. The reaction currents ideally should have decayed to background levels, but exhibited constant offset values slightly larger than the background signal, a phenomenon assumed to be lingering residual flow from sample injection. After adding rigid tubing and external valves, the thin-layer cell was shown to remain within 6% of the theoretical charge after 200 seconds. Continued development of this system will offer the possibility of remote, calibration-free determinations of real-world analytes such mercury and lead