Nanomechanical motion transducers for miniaturized mechanical systems

Reliable operation of a miniaturized mechanical system requires that nanomechanical motion be transduced into electrical signals (and vice versa) with high fidelity and in a robust manner. Progress in transducer technologies is expected to impact numerous emerging and future applications of micro- and, especially, nanoelectromechanical systems (MEMS and NEMS); furthermore, high-precision measurements of nanomechanical motion are broadly used to study fundamental phenomena in physics and biology. Therefore, development of nanomechanical motion transducers with high sensitivity and bandwidth has been a central research thrust in the fields of MEMS and NEMS. Here, we will review recent progress in this rapidly-advancing area. © 2017 by the authors

Piezoelectric and Magnetoelastic Strain in the Transduction and Frequency Control of Nanomechanical Resonators

Stress and strain play a central role in semiconductors, and are strongly manifested at the nanometer-scale regime. Piezoelectricity and magnetostriction produce internal strains that are anisotropic and addressable via a remote electric or magnetic field. These properties could greatly benefit the nascent field of nanoelectromechanical systems (NEMS), which promises to impact a variety of sensor and actuator applications. The piezoelectric semiconductor GaAs is used as a platform for probing novel implementations of resonant nanomechanical actuation and frequency control. GaAs/AlGaAs heterostructures can be grown epitaxially, are easily amenable to suspended nanostructure fabrication, have a modest piezoelectric coefficient roughly twice that of quartz, and if appropriately doped with manganese, can form dilute magnetic compounds. In ordinary piezoelectric transducers there is a clear distinction between the metal electrodes and piezoelectric insulator. But this distinction is blurred in semiconductors. An integrated piezoelectric actuation mechanism is demonstrated in a series of suspended anisotype GaAs junctions, notably pin diodes. A dc bias was found to alter the resonance amplitude and frequency in such devices. The results are in good agreement with a model of strain based actuation encompassing the diode’s voltage-dependent carrier depletion width and impedance. A bandstructure engineering approach is employed to control the actuation efficiency by appropriately designing the doping level and thickness of the GaAs structure. Actuation and frequency are also sensitively dependent on the device’s crystallographic orientation. This combined tuning behavior represents a novel type of depletion-mediated electromechanical coupling in piezoelectric semiconductor nanostructures. All devices are actuated piezoelectrically, whereas three techniques are demonstrated for sensing: optical interferometry, piezoresistance and piezoelectricity. Finally, a nanoelectromechanical GaMnAs resonator is used to obtain the first measurement of magnetostriction in a dilute magnetic semiconductor. Resonance frequency shifts induced by field-dependent magnetoelastic stress are used to simultaneously map the magnetostriction and magnetic anisotropy constants over a wide range of temperatures. Owing to the central role of carriers in controlling ferromagnetic interactions in this material, the results appear to provide insight into a unique form of magnetoelastic behavior mediated by holes

Cantilever systems for the next generation of biomechanical sensors

Ieder interactief systeem gebruikt apparaten om informatie over de omgeving te verkrijgen. Ook de mens gebruikt apparaten om zijn omgeving te onderzoeken; tast- en gehoor-apparatuur voor mechanische impulsen, zicht- voor elektromagnetische en smaak- en reuk- voor chemische eigenschappen. Het gaat om thermometers, microfoons, ccd camera’s, enzovoort: allemaal sensoren die onze waarnemingsmogelijkheden vergroten, prestaties verbeteren en soms zelfs de mens vervangen in autonome systemen. In de laatste decennia is door de opkomst van nano- en biotechnologie de ontwikkeling van chemische sensoren, in het bijzonder biosensoren, in een stroomversnelling geraakt. Biosensoren worden gekenmerkt door de aanwezigheid van een biologische component (bv. een antilichaam, enzym of DNA molecuul) die een interactie aangaat met het te detecteren chemische element. Deze interactie wordt omgezet in een macroscopisch signaal welke vervolgens kan worden uitgelezen door een mens of machine. In ons dagelijks leven zijn biosensoren al terug te vinden in de vorm van zwangerschapstesten en glucosemeters, maar ook in minder opvallende toepassingen zoals voedsel- en waterveiligheid. Er zijn echter nog vele gebieden, in het bijzonder in de geneeskunde, waarin biosensoren een belangrijke rol kunnen spelen. In geval van ziekte (eenvoudige griep of allergie tot levensbedreigende kanker) produceert ons lichaam biologische markers, eiwitten, die inzicht kunnen geven in wat er zich in ons lichaam afspeelt. Daardoor kan er een betere inschatting worden gemaakt van de prognose en kan de therapie mogelijk specifiek op de patiënt worden afgestemd. Helaas is vaak niet bekend welke markers een rol spelen, en als dit wel bekend is, is de detectie veelal zeer kostbaar of zelfs niet mogelijk. In my thesis work I investigated alternative geometries of nanomechanical oscillators to be employed as biomolecular sensors. Simple mechanical oscillators, such as cantilevers and double clamped beams have been deeply investigated in the last decade and single molecule sensitivity was demonstrated. However, beside few marginal exceptions, the proof of principle demonstrations did not yet evolve into commercial devices. Alternative geometries can, in principle, improve the simple micromechanical systems studied so far, with more complex transfer functions suitable to operate also in demanding environments. The thesis work was divided in two major sections. In the first section twin cantilevers are discussed. Couples of cantilevers facing each other and separated by a nanometer gap may change their resonance response when one or more molecules are absorbed in the gap. Two different geometries have been fabricated and tested. One, with identical cantilevers, takes advantage of the shift in resonance frequency occurring upon molecular detection; the second with asymmetrical cantilevers, uses the shortest one to actuate the motion of the longer one through a molecular link. In the second part the structure of the twin cantilevers is the starting point for creating a spatially confined chemical reaction in the gap between two cantilevers facing each other. This original process is extremely precise and represents an important milestone towards the future realization of complex micro- and nanomechanical systems for biomolecular detection.

Micro-poutres résonantes à base de films minces de nitrure d’aluminium piézoélectriques, application aux capteurs de gaz gravimétriques

Resonant MEMS and NEMS are excellent candidate for the realization of low cost and high resolution gas sensing systems that have several applications in security, defense, and environment and health care domains. However, the question of the transduction technique used to couple micro or nano scale signals to the macro scale is still a key issue. Piezoelectric transduction can be advantageously exploited but has been rarely studied at the nano-scale. The objective of this PhD is thus to progress toward the realization of high-resolution gas sensor using piezoelectric micro/nano cantilevers resonators and cover the whole prototyping chain from device fabrication to proof of principle experiment. Our first contribution in this research relates the analytical modeling of the sensing performance and the system and design optimization. In particular we demonstrate that decreasing the piezoelectric active film thickness below 100 nm is particularly beneficial. The second contribution relates the fabrication, characterization and demonstration of the high sensing performances of 80 μm long cantilevers embedding a 50 nm thick piezoelectric AlN film for transduction. These devices exhibit state of the art performances in terms of resonance frequency deviation down to the 〖10〗^(-8) range. They allow thus the detection of Di-Methyl-Methyl-Phosphonate vapors, a sarin gas simulant, with concentration as low as 10 ppb. Although the level of integration of our sensing system is not sufficient for real life application, these results prove the high potential of these piezoelectric cantilever resonators for future industrial development.Les MEMS et NEMS résonants sont d'excellents candidats pour la réalisation de systèmes de détection de gaz haute résolution et faible couts ayant des applications dans les domaines de la sécurité, la défense, l'environnement et la santé. Cependant, la question du choix des techniques de transduction est toujours largement débattue. La transduction piézoélectrique pourrait être avantageusement exploitée mais elle est encore peu connue à l'échelle nanométrique. L'objectif de cette thèse est donc de progresser vers la réalisation de capteur de gaz à haute résolution à l'aide résonateurs à base de micro / nano poutres piézoélectriques en couvrant la chaîne de prototypage complète depuis les techniques de dépôt des matériaux jusqu'à l'expérience de preuve de principe de mesure de gaz. Pour cela, notre première contribution concerne la modélisation analytique des performances et l'optimisation, design et système, d'un capteur de gaz à base de poutres résonantes piézoélectriques. En particulier, nous démontrons que la diminution de l'épaisseur du film piézoélectrique actif sous la barre des 100 nm permet d'atteindre les meilleures performances. La deuxième contribution concerne la fabrication, la caractérisation et la démonstration des performances capteur de poutres résonantes de 80 μm de long exploitant un film piézoélectrique en AlN de 50 nm d'épais. Ainsi nous avons démontré expérimentalement la stabilité fréquentielle exceptionnelle de ces dispositifs atteignant des déviations standard de l'ordre de 〖10〗^(-8), au niveau de l’état de l'art. Ainsi, ils permettent la détection de vapeurs Di -Methyl -méthyl- phosphonates, un simulateur de gaz sarin, avec des concentrations aussi faibles que 10 ppb. Bien que le niveau d'intégration de notre système de détection ne soit pas suffisant, ces résultats prouvent le fort potentiel de ces résonateurs cantilever piézoélectriques pour un développement industriel futur

Nonisochronous Oscillations in Piezoelectric Nanomechanical Resonators

Nanoelectromechanical systems (NEMS) have proven an excellent test bed for exploring nonlinear dynamics due to short decay times, weak nonlinearities, and large quality factors. In contrast to previous research in nonlinear dynamics involving driven or phase fixed NEMS, where time is referenced by an external source, we describe phenomena classified by phase free phenomena. Here we describe NEMS embedded into feedback oscillators with weak nonlinearities. We make measurements of this mechanical nonlinearity by developing a transduction scheme, the piezoelectric/piezoresistive (PZE/PZR) transduction, which emphasizes the detector dynamic range over absolute sensitivity. Using these measurements, projections on quantum nondemolition schemes involving the mechanical nonlinearity as a detector are made. These measurements also are important for understanding the detection limits of NEMS sensor technology, which uses a mechanical resonator as a frequency reference in a phase locked loop (PLL). This work identifies ways to reduce noise within ‘nonlinear’ feedback oscillators, and these results have implications for sensing systems using nonlinear mechanical resonators embedded in PLLs. Since the mechanical nonlinearity of PZE/PZR resonators can be accurately calibrated, we make predictions for the behavior of these dynamical systems based on the given mechanical and electrical parameters. We show, theoretically, that local isochronicity above critical nonlinear amplitudes can create special operating points in feedback oscillators at which parametric fluctuations may cause less phase noise in the oscillator than in feedback oscillators driven below critical amplitudes. For these predictions, we present data that show quantitative agreement for the amplitude and frequency, and qualitative agreement for the phase noise. Finally, we show synchronization, assisted by nonisochronicity, between two feedback NEMS oscillators. We develop a general theoretical framework for two saturated feedback oscillators which use resonators with nonlinear stiffness. In the limit of small coupling, we show that the system obeys the Adler equation with analytical predictions for the oscillators’ individual amplitudes and net frequency difference. We develop an experiment in which the three important parameters of the system (detuning, nonisochronicity, and coupling) can be tuned, and show data that agrees with the predictions for a large range of coupling. We include data on phase slipping between two oscillators in which the aperiodic frequency difference is clearly observed. Finally, we present data on phase noise in synchronized oscillators.</p

Advanced Applications of Nanoelectromechanical Systems

Nanoelectromechanical systems (NEMS) have advanced the technologies in a wide spectrum of fields, including nonlinear dynamics, sensors for force detection, mass spectrometry, inertial imaging, calorimetry, and charge sensing. Due to their low power consumption, fast response time, large dynamic range, high quality factor, and low mass, NEMS have achieved unprecedented measurement sensitivity. For optimized system functionalization and design, precise characterization of material properties at the nanoscale is essential. In this thesis, we will discuss three applications of NEMS: mechanical switches, using anharmonic nonlinearity to measure device and material properties, and mass spectrometry and inertial imaging. The first application of NEMS we discuss is NEMS switches, switches with physical moving parts. Conventional electronics, based largely on silicon transistors, is reaching a physical limit in both size and power consumption. Mechanical switches provide a promising solution to surpass this limit by forcing a jump between the on and off states. Graphene, which is a single sheet of carbon atoms arranged in a hexagonal structure, has high mechanical strength and strong planar bonding, making it an ideal candidate for nanoelectromechanical switches. In addition, graphene is conductive, which decreases resistive heating at the contact area, therefore reducing bonding issues and subsequently reducing degradation. We demonstrate using exfoliated graphene to fabricate suspended graphene NEMS switches with successful switching. The second application of NEMS we discuss in this thesis is the use of mechanical nonlinearity to measure device and material properties. While the nonlinear dynamics of NEMS have been used previously to investigate the longitudinal speed of sound of materials at nano- and micro-scales, we correct a previously attempted method that employs the anharmonicity of NEMS arising from deflection-dependent stress to interrogate the transport of RF acoustic phonons at nanometer scales. In contrast to existing approaches, this decouples intrinsic material properties, such as longitudinal speed of sound, from properties associated with linear dynamics, such as tension, of the structure. We demonstrate this approach through measurements of the longitudinal speed of sound in several NEMS devices composed of single crystal silicon along different crystal orientations. Good agreement with literature values is reported. The third application of NEMS we discuss is mass spectrometry and inertial imaging. Currently, only doubly clamped beams and cantilevers have been experimentally demonstrated for mass spectrometry. We extend the one-dimension model for mass spectrometry to a novel method for inertial imaging. We further extend the theory of mass spectrometry and inertial imaging to two dimensions by using a plate geometry. We show that the mode shape is critical in performing NEMS mass spectrometry and inertial imaging, and that the mode shapes in plates deviate from the ideal scenario with isotropic stress. We experiment with various non-ideal conditions to match non-ideal mode shape observed.</p

Enhancing the sensitivity and measuring the deflection of MEMS microcantilevers

This thesis investigates how the deflection sensitivity of microcantilever sensors can be increased, and how the deflections of an array of these cantilevers can be measured up to a few micrometers, with sub-nanometric resolution, using a simple optical system. The deflection of microcantilevers for a given stimulus can be increased by increasing the length or by decreasing the thickness of the beam, but these result in lower resonant frequencies. A lower resonant frequency makes the cantilever susceptible to thermal noise and low-frequency vibrations. The sensitivity of cantilevers can be increased by simultaneously increasing the deflection and the resonant frequency. However, a method of achieving this has not been demonstrated in the literature. This thesis shows that the deflection and resonant frequency of cantilevers can be simultaneously increased by creating perforations in a manner that reduces its mass by a larger fraction than the reduction of its spring constant. Analytical models are developed to describe the deflection and the resonant frequency of perforated microcantilevers. Results obtained from these models show good agreement with finite element method simulation results obtained using ANSYS, which validates the models. The variations of the deflections and resonant frequencies of cantilevers with perforation parameters are characterised using these models. Using these results, the cantilever profile that optimises sensitivity is determined. Using the developed analytical models, models from the literature and simulations using ANSYS, it is established that cantilevers with triangular profiles have larger deflection—resonant frequency combinations compared with standard rectangular cantilevers, and are proposed as being more suitable for sensor applications than standard rectangular beams. The reasons for the measurement range of the standard interdigital interferometric method being limited to a quarter of the wavelength ë of the optical source are also investigated. Using a novel mathematical approach, the far-field optical intensity pattern is decomposed into the sum of spatial harmonic functions. The spatial frequencies of these harmonic functions are shown to be determined by the distances between each cantilever of the array, and the phase terms are shown to be dependent on the amount of deflection. It is shown that by making each moving cantilever in the array to have distinct deflections for a given stimulus, and extracting the phase terms of the spatial harmonic functions from the Fourier transformation of the far-field diffraction pattern, the measurement range can be increased independent of the quarter-wavelength limitation. A principle to correct the errors induced by the misalignment of the image sensor is established, as well as principles to determine the width and the locations of individual cantilevers. The requirements of the photo sensor, namely, the maximum allowable pixel spacing, the size of the image sensor and the resolution of the Analog to Digital Converter for a given measurement resolution are also determined. By simulating an example cantilever array, deflections up to 3250 nm are demonstrated to be measurable with a 0.2 nm measurement resolution, compared with the 162.5 nm deflection range of standard interferometric measurement techniques

Fully integrated transducer platform with cavity optomechanical readout

Research and development of transducers based on cavity optomechanics is a topic of high interest particularly because these transducers enable measurement of mechanical motion down to the fundamental limit of precision imposed by quantum mechanics. The development of an on-chip cavity optomechanical transducer platform that combines high bandwidth and sensitivity near the standard quantum limit with compactness, robustness, small size, and potential for low cost batch fabrication inherent in MEMS is demonstrated as a proof of concept study. Design, fabrication and characterization of fully integrated and fiber pigtailed transducers is presented. The devices combine high sensitivity (0.14 - 40 fm·Hz^(-1/2), high bandwidth optomechanical readout and built-in thermal and electrostatic actuation. It is implemented by a double-side wafer-scale microfabrication process combining one e-beam, six stepper, and three contact mask aligner lithography steps. The SiN probes can be actuated using an electrical signal supplied to an integrated thermal or electrostatic actuator. The probe is evanescently coupled to a high-Q (10^5 - 2 x 10^6) optical whispering gallery mode of the optical microdisk cavity and the motion is detected by measuring the resonance frequency shift of the optical cavity mode. The actuator can be used to dynamically move the probe as well as to tune the distance between the cantilever and the optical cavity, to change the sensitivity and range of measurement of the cantilever. One side of the probe overhangs the edge of the chip, where it can be easily coupled to a variety of off-chip samples and physical systems of interest. The modular design of the transducer allows for parallelization, which enables the possibility of sensor arrays for simultaneous detection of multiple forces or other physical properties. Parallelization is shown on a 2x1 array, which can be easily extended to larger array architectures. The application of the probe arrays and single probes in a commercial scanning probe microscope is shown. In addition the flexibility of this transducer approach is demonstrated with membrane transducers and acceleration sensors. The performance of all presented transducers is studied, focusing on displacement sensitivity, frequency stability and readout gain tuning.Forschung und Entwicklung von Wandlern basierend auf kavität- optomechanischen Elementen ist ein Forschungsgebiet von hohem Interesse. Sie kombiniert hohe Bandbreiten und Empfindlichkeit nahe dem Standardquantumlimit mit Kompaktheit, Robustheit, kleinen Abmessungen und dem Potential für eine wirtschaftliche Massenproduktion systemimmanent bei mikroelektromechanischen Systemen. Vollintegrierte Wandler erlauben demnach Bewegungsmessungen bis hin zum fundamentalen Quantenlimit. In dieser Arbeit werden Design, Herstellung und Charakterisierung eines vollintegrierten und glasfasergekoppelten Wandlers in einer Machbarkeitsstudie dargestellt. Das System kombiniert hohe Verschiebungsauflösungen 0.14 - 40 fm· Hz^(-1/2), optomechanische Detektion mit hoher Bandbreite und eine eingebaute thermische und elektrostatische Anregung. Die Herstellung erfolgt in einem doppelseitigen mikro- und nanotechnischen Fertigungsverfahren auf Waferbasis, in einer Kombination aus einem Elektronenstrahllithographieschritt, sechs Projektionslithographieschritten und drei Kontaktlithographie Schritten. Die Siliziumnitrid-Sonden können mittels eines elektrischen Signals, angelegt an den integrierten thermischen oder elektrostatischen Aktuator, angeregt werden. Sie sind optisch über das evanecente Feld mit einer optischen Kavität hoher Güte (10^5 - 2 x 10^6) gekoppelt. Die Bewegung der Sonde wird detektiert über eine Veränderung der Resonanzfrequenz der Kavität. Die eingebauten Aktuatoren ermöglichen die Einstellung des Abstandes zwischen Sonde und optischer Kavität, welche die Einstellung der Sensitivität ermöglicht. Eine Seite der Sonde steht über die Kante des Siliziumchips, um die Kopplung mit einer Vielzahl von Proben und physikalischen Systemen zu erlauben. Die modulare Bauweise des Wandlers schafft die Grundlage zur Parallelisierung der Sonden für die gleichzeitige Messung mehrerer Kräfte oder physikalischer Eigenschaften. Die Parallelisierung wird in dieser Arbeit am Beispiel eines 2x1 Array gezeigt, welche mit geringem Aufwand auf größere Arrayarchitekturen angepasst werden kann. Zur Demonstration der Funktion von Einzelsonden und Sondenarrays, wird die Sondenanwendung in der Rasterkraftmikroskopie präsentiert. Des Weiteren wird die Flexibilität der Wandlerbauweise an der Herstellung von Membrane- und Beschleunigungswandlern belegt. Das Verhalten aller hergestellten Wandler wird hinsichtlich der Bewegungsempfindlichkeit, Frequenzstabilität, und Einstellbarkeit der Auslesung analysiert