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Enabling high efficiency sensing with optomechanical devices
No full textInternational audienceAvancées du LICA dans le domaine des capteurs optomécanique
Characterization of nanomechanical resonators based on silicon nanowires
Get PDFEls sensors de massa nanomecànics han atret un gran interès darrerament per la seva alta sensibilitat, que ve donada per les petites dimensions del ressonador que actua com a element sensor. Aquesta tesi tracta sobre la fabricació i caracterització de ressonadors nanomecànics per a aplicacions de sensat de massa. Aquest objectiu inclou diferents aspectes: 1) el desenvolupament d’una tecnologia de fabricació per a ressonadors nanomecànics basats en nanofils de silici, 2) la caracterització de la seva resposta freqüencial utilitzant mètodes elèctrics i 3) l’avaluació del seu rendiment com a sensors de massa.
Durant aquest treball hem fabricat ressonadors nanomecànics basats en nanofils de silici doblement fixats, utilitzant dues estratègies de fabricació diferents: els nanofils crescuts amb mètodes bottom-up (“de baix a dalt”), i els definits amb mètodes de litografia top-down (“de dalt a baix”). Aprofitant les característiques d’ambdues tècniques, hem fabricat nanofils amb dimensions laterals de fins a 50 nanòmetres, i amb un alt nombre de dispositius per xip, aconseguint un alt grau de rendiment per a estructures d’aquestes dimensions.
Hem aplicat esquemes avançats de detecció elèctrica basats en la mescla de senyals cap a freqüències baixes per tal de caracteritzar la resposta freqüencial dels ressonadors. Hem demostrat que el mètode de freqüència modulada (FM) proporciona la millor eficiència en la transducció de l’oscil·lació mecànica en una senyal elèctrica. Aquesta tècnica ha permès detectar múltiples modes de ressonància del ressonador, a freqüències de fins a 590 MHz. La detecció de modes de ressonància superiors és important per tal de solucionar una de les principals problemàtiques en el camp dels sensors de massa nanomecànics: desacoblar els efectes de la posició i la massa de la partícula dipositada. També hem combinat la informació obtinguda de la caracterització elèctrica amb simulacions d’elements finits per tal de quantificar l’estrès acumulat als nanofils durant la seva fabricació.
Hem estudiat els sistemes de transducció electromecànica en ressonadors basats en nanofils de silici comparant l’eficiència de tres mètodes de detecció: el mètode FM ja esmentat i els mètodes de dos generadors, 1ω i de dos generadors, 2ω. D’aquesta manera hem demostrat que dos mecanismes de transducció diferents coexisteixen en els nanofils de silici bottom-up: el mecanisme lineal (en què la senyal transduïda és proporcional al moviment del ressonador) i el quadràtic (en què la senyal transduïda és proporcional al quadrat del moviment del ressonador). Per altra banda, en els ressonadors top-down només és present el mecanisme de transducció lineal. Aquest mecanisme lineal és el que permet la gran eficiència del mètode FM per a la caracterització de la resposta freqüencial de ressonadors basats en nanofils de silici.
Per tal d’utilitzar els ressonadors nanomecànics com a sensors de massa, el seguiment de la freqüència de ressonància en temps real és indispensable. Hem dissenyat i implementat una configuració en llaç tancat basada en la caracterització FM i un algorisme de detecció de pendent. Aquest sistema permet el seguiment de canvis en la magnitud i freqüència de la resposta del ressonador, possibilitant la detecció de massa en temps real i la caracterització de l’estabilitat temporal del sistema. D’aquesta manera s’ha pogut avaluar l’eficiència del sistema per a aplicacions de sensat de massa. La sensibilitat en massa dels sensors de dimensions més reduïdes és de l’ordre de 6 Hz/zg (1 zg = 6·10-21 g), i les mesures d’estabilitat en freqüència en llaç tancat mostren una resolució en massa de 6 zg a temperatura ambient.Nanomechanical mass sensors have attracted interest during the last years thanks to their unprecedented sensitivities, which arise from the small dimensions of the resonator which comprises the sensing element. This thesis deals with the fabrication and characterization of nanomechanical resonators for mass sensing applications. This objective comprises three different aspects: 1) the development of a fabrication technology of nanomechanical resonators based on silicon nanowires (SiNW), 2) the characterization of their frequency response by electrical methods and 3) the evaluation of their performance as mass sensors.
During this work, we have fabricated nanomechanical resonators based on SiNW clamped-clamped beams, using two different approaches: bottom-up growth of SiNW and top-down definition by lithography methods. By exploiting the advantages of each technique, we have succeeded in fabricating nanowires of small lateral dimensions, in the order of 50 nanometers, and with high number of devices per chip, achieving a high throughput taking into account the dimensions of these structures.
We have applied advanced electrical detection schemes based on frequency down-mixing techniques for the characterization of the frequency response of the devices. We have found that the frequency modulation (FM) detection method provides the best efficiency in transducing the mechanical oscillation into an electrical signal. This technique has enabled the detection of multiple resonance modes of the resonator at frequencies up to 590 MHz. The detection of high modes of resonance is important to address one of the issues in nanomechanical mass sensing, decoupling the effects of the position and mass of the deposited species. Moreover, by combining the information obtained from the experimental characterization of the frequency response with FEM simulations, we have quantified the stress accumulated in the SiNWs during the fabrication.
We have studied the electromechanical transduction mechanisms in SiNW resonators by the comparative performance of three electrical detection methods: the aforementioned FM and two more detection techniques (namely the two-source, 1ω and the two-source, 2ω). We have proved that two different transduction mechanisms co-exist in bottom-up grown SiNWs: linear (in which the transduced signal is proportional to the motion of the resonator) and quadratic (in which the transduced signal is proportional to the square of the motion of the resonator). On the other hand, in the top-down nanowires only the linear transduction mechanism is present. It is this newly found linear transduction which enables the outstanding performance of the FM detection method when characterizing the frequency response of SiNW resonators.
For the use of nanomechanical resonators in mass sensing applications, the real-time tracking of their resonance frequency is needed. We have designed and implemented a novel closed-loop configuration, based on the FM detection technique and a slope detection algorithm. It allows the monitoring of changes in the magnitude and the frequency of the response of the resonator, enabling not only the real time detection of mass, but also the characterization of the temporal stability of the system. In this way, its overall performance for mass sensing applications has been characterized. The mass sensitivity of the system for the smallest resonators stands in the range of 6 Hz/zg (1 zg = 6·10-21 g) and the frequency stability measurements in the closed loop configuration reveal a mass resolution of 6 zg at room temperature
Caracterització d'oscil·ladors basats en CMOS-M/NEMS i la seva aplicació com a sensors de massa
No full textEl present projecte tracta sobre la caracterització d'oscil·ladors basats en ressonadors micro/nanoelectromecànics (M/NEMS) i la seva aplicació com a sensors de massa. Els oscil·ladors utilitzats es basen en un pont i una palanca ressoants M/NEMS, integrats en tecnologia CMOS. En primer lloc s'ha fet una introducció teòrica als conceptes utilitzats per a entendre el funcionament i la caracterització dels dispositius. A continuació s'han realitzat proves per tal de caracteritzar els paràmetres importants per a l'ús dels oscil·ladors com a sensors de massa, com la seva estabilitat en freqüència. Per acabar s'han aplicat aquests sensors en la caracterització i modelització del flux de massa a través d'obertures de dimensions micromètriques.El presente proyecto trata sobre la caracterización de osciladores basados en resonadores micro/nanoelectromecánicos (M/NEMS) y su aplicación como sensores de masa. Los osciladores utilizados se basan en un puente y una palanca resonantes M/NEMS, integrados en tecnología CMOS. En primer lugar se ha realizado una troducción teórica a los conceptos utilizados para entender el funcionamiento y caracterización de los dispositivos. A continuación se han realizado pruebas para caracterizar los parámetros importantes para el uso de los osciladores como sensores de masa, como su estabilidad en frecuencia. Para terminar, se han aplicados estos sensores en la caracterización y modelado del flujo de masa a través de aperturas de dimensiones micrométricas.The aim of the present project is the characterization of micro/nanoelectromechanical (M/NEMS)-based oscillators and their application as mass sensors. The studied oscillators are based on a bridge and a cantilever resoant M/NEMS, integrated in CMOS technology. Firstly, an introduction to the concepts used to understand the operation and characterization of the devices has been written. Then important parameters for the oscillator working as a mass sensor, like his frequency stability, have been characterized. Finally, the sensors have been used in the characterization of flux passing through micrometer-size apertures.Nota: Aquest document conté originàriament altre material i/o programari només consultable a la Biblioteca de Ciència i Tecnologia
Top-down silicon microcantilever with coupled bottom-up silicon nanowire for enhanced mass resolution
No full textVidal-Álvarez, Gabriel et al.© 2015 IOP Publishing Ltd. A stepped cantilever composed of a bottom-up silicon nanowire coupled to a top-down silicon microcantilever electrostatically actuated and with capacitive or optical readout is fabricated and analyzed, both theoretically and experimentally, for mass sensing applications. The mass sensitivity at the nanowire free end and the frequency resolution considering thermomechanical noise are computed for different nanowire dimensions. The results obtained show that the coupled structure presents a very good mass sensitivity thanks to the nanowire, where the mass depositions take place, while also presenting a very good frequency resolution due to the microcantilever, where the transduction is carried out. A two-fold improvement in mass sensitivity with respect to that of the microcantilever standalone is experimentally demonstrated, and at least an order-of-magnitude improvement is theoretically predicted, only changing the nanowire length. Very close frequency resolutions are experimentally measured and theoretically predicted for a standalone microcantilever and for a microcantilever-nanowire coupled system. Thus, an improvement in mass sensing resolution of the microcantilever-nanowire stepped cantilever is demonstrated with respect to that of the microcantilever standalone.This work has been partially funded by the Spanish government and the European Union FEDER program under project TEC2012-32677 (NEMS-in-CMOS), the Gicserv-6 NGG-193 program, the Israel Science Foundation (1475/09), and the Technion Russell Berrie Nanotechnology Institute.Peer Reviewe
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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