Rozprawy doktorskie na temat „Piezoresistor”

Silicon based micromachining technology enables the realization of high performance micro electromechanical systems (MEMS) including a range of physical and environmental sensors. Pressure sensors are used for a wide range of monitoring and control applications, e.g. environmental, industrial, aircraft, automotive. Monitoring of vehicle tyre pressures offers benefits such as improved safety, fuel economy, and tyre life. Micromachined pressure sensors are used at present, but require further research to improve their performance in terms of size, power consumption and manufacturing cost. This thesis has reviews pressure sensor technology and new developments in this area. A comparison of existing and potential future sensing mechanisms has been undertaken and identified as silicon piezoresistors. The focus of the research is motivated by the recently discovered enhanced piezoresistive effect in silicon nanowires where sensitivity can be increased by decreasing the dimension of nanowire. This thesis investigates the piezoresistive effect in p-type <110> silicon nanowires, fabricated using top down approach. It is found that the piezoresistive effect increases when the nanowire width is reduced below 400nm. Compared with micrometre sized piezoresistors, silicon nanowires have produced up to 100% enhancement. In addition, measurements indicate that the temperature coefficient of resistance (TCR) of silicon nanowire has improved with up to 40% decrease in TCR. The improvement in these two areas will be beneficial for the development of new MEMS pressure sensors. COMSOL is employed to simulate the piezoresistance effect in p-type <110> silicon for a range of doping concentrations. Simulation results demonstrate a similar trend to experimental results and publication data and show that the piezoresistance effect decreases as the doping concentration increases.

This work aims at the advancement of state-of-art accelerometer design and optimization methodology by developing an ear-plug accelerometer for race car drivers based on a novel mechanical principle. The accelerometer is used for the measurements of head acceleration when an injurious event occurs. Main requirements for such sensor are miniaturization (2×2 mm), because the device must be placed into the driver earpiece, and its measurement accuracy (i.e. high sensitivity, low crosstalk and low nonlinearity) since the device is used for safety monitoring purpose. A micro-electro-mechanical system (MEMS)-based (bulk micromachined) piezoresistive accelerometer was selected as enabling technology for the development of the sensor. The primary accelerometer elements that can be manipulated during the design stage are: the sensing element (piezoresistors), the micromechanical structure and the measurements circuit. Each of these elements has been specifically designed in order to maximize the sensor performance and to achieve the miniaturization required for the studied application. To achieve accelerometer high sensitivity and miniaturization silicon nanowires (SiNWs) as nanometer scale piezoresistors are adopted as sensing elements. Currently this technology is at an infancy stage, but very promising through the exploitation of the “Giant piezoresistance effect” of SiNWs. This work then measures the potential of the SiNWs as nanoscale piezoresistors by calculating the major performance indexes, both electrical and mechanical, of the novel accelerometer. The results clearly demonstrate that the use of nanoscale piezoresistors boosts the sensitivity by 30 times in comparison to conventional microscale piezoresistors. A feasibility study on nanowires fabrication by both top-down and bottom-up approaches is also carried out. The micromechanical structure used for the design of the accelerometer is an optimized highly symmetric geometry chosen for its self-cancelling property. This work, for the first time, presents an optimization process of the accelerometer micromechanical structure based on a novel mechanical principle, which simultaneously increases the sensitivity and reduces the cross-sensitivity progressively. In the open literature among highly symmetric geometries no other study has to date reported enhancement of the electrical sensitivity and reduction of the cross-talk at the same time. Moreover the novel mechanical principle represents advancement in the accelerometer design and optimization methodology by studying the influence of a uniform mass moment of inertia of the accelerometer proof mass on the sensor performance. Finally, an optimal accelerometer design is proposed and an optimized measurement circuit is also specifically designed to maximize the performance of the accelerometer. The new proposed accelerometer design is capable of increasing the sensor sensitivity of all axes, in particular the Z-axis increases of almost 5 times in respect to the current state-of-art-technology in piezoresistive accelerometer. This occurs thanks to the particular newly developed approach of combination of beams, proof mass geometry and measurement circuit design, together with the use of silicon nanowires as nanoscale piezoresistors. Furthermore the cross-sensitivity is simultaneously minimized for a maximal performance. The sum of the cross-sensitivity of all axes is equal to 0.4%, well below the more than 5% of the state-of-art technology counterpart reported in the literature. Future work is finally outlined and includes the electro-mechanical characterization of the silicon nanowires and the fabrication of the proposed accelerometer prototype that embeds bottom up SiNWs as nanoscale piezoresistors.

Este trabalho mostra o desenvolvimento de um software, rotulado aqui como SimuPi, para simular o modelamento matemático de elementos sensores piezoresistivos. O programa desenvolvido roda modelos de primeira e segunda ordem, clássicos da literatura, os quais são indispensáveis para o desenvolvimento de elementos sensores baseados no efeito piezoresistivo do silício. Foram analisadas as principais características de programas simuladores com objetivo de observar quais seriam relevantes para serem empregadas no desenvolvimento deste trabalho. Estudou-se a forma de elaboração do programa, escolhendo sua linguagem de programação e elaborando seus diagramas detalhados de requisitos e de funcionamentos. A plataforma de testes foi desenvolvida na linguagem de programação Java. O SimuPi mostrou-se condizente com a sua proposta, apresentando gráficos e resultados equivalentes aos obtidos em testes laboratoriais a partir da análise das propriedades do silício com base em modelos matemáticos. O software pode ser ampliado para projetos de elementos sensores usando outros tipos de materiais, além de melhorias gráficas e didáticas.
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Este trabalho descreve o efeito piezoresistivo no grafite, uma forma alotrópica de carbono e sugere sua aplicação em dispositivos sensores em substituição a outros materiais cujos processos de fabricação são mais complexos quando comparados ao utilizado neste trabalho. Foram projetados e montados elementos sensores de grafite visando a sua caracterização mecânica, elétrica e térmica. Os resultados obtidos apresentam concordância com os apresentados pela literatura indicando que os modelos matemáticos utilizados para caracterização e análise do material são adequados para o processo de dispositivos sensores piezoresistivos.
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Neste trabalho é apresentado o estudo teórico-experimental a respeito das propriedades piezoresistivas de dois tipos de materiais com estrutura amorfa. O primeiro material estudado é o carbono semelhante ao diamante e o segundo é o óxido de estanho dopado com índio. O estudo compreende o levantamento bibliográfico sobre os materiais, projeto teórico e prático de estruturas individuais de testes e piezoresistores configurados em ponte completa, além da realização das caracterizações elétricas, mecânicas e térmicas de acordo com um arranjo experimental proposto. As caracterizações experimentais foram implementadas usando a técnica de flexão de uma viga engastada e a teoria das pequenas deflexões. Os diferentes materiais caracterizados e analisados apresentaram o efeito piezoresistivo e um sinal de sensibilidade mecânica condizente com as características esperadas para estes filmes. Ambos os filmes respondem as variações da temperatura de forma linear e apresentam uma direção de dependência com a temperatura. Os filmes de carbono amorfo hidrogenado livre de dopantes apresentam curvas de corrente e tensão características indicando um mecânismo de condução elétrica complexo devido a sua diversidade de microestruturas e relacionado aos parâmetros de processos de deposição. Os filmes com nitrogênio são mais estáveis termicamente com coeficientes da ordem de - 4900 ppm/ºC. Os resultados encontrados indicam a existência de dois tipos de portadores de cargas responsáveis pela mobilidade média, resistividade e efeito piezoresistivo. Os filmes de óxido de estanho dopado com índio livre e com 5 % e 10 % de oxigênio no plasma apresentam características de diminuição da resistência elétrica com o esforço mecânico e exibem efeitos de piezoresistividade na faixa de - 12 a - 23. Amostras destes filmes com oxigênio apresentaram um fator de sensibilidade mecânica muito baixa e são menos estáveis termicamente que as amostras livres de oxigênio. Os filmes estudados podem ser usados em aplicações envolvendo extensiometria ou mesmo em sensores de pressão piezoresistivos após adequação do processo de deposição e de recozimentos térmicos.
This tesis presents the piezoresistivity theoretical and experimental study for two materials with amorphous structure. The first material is the Diamond Like Carbon and the other is the Indium Tin Oxide. The work includes the bibliographic study, theoretical and practical design of structures for testing individual and piezoresistors configured in bridge, in addition to the completion of the characterizations electrical, mechanical and thermal according to a proposed experimental arrangement. The experimental characterizations have been implemented using the technique of cantilever and the theory of small deflections. The different materials analyzed showed the piezoresistive effect with some order of magnitude and a sign of sensitivity to mechanical stress of tension consistent with the characteristics expected for these types of films. Both films respond to changes in temperature in a very linear and have a direction of dependency with the temperature according to the literature. The films of free doping have curves of current and voltage characteristics for this type of material indicating a mechanism of electric conduction very complex because of its diversity of microstructures and processes related to the parameters of the deposition and films with nitrogen are more thermally stable with coefficients of order of - 4900 ppm/ºC. The results indicate the existence of two types of charge carriers responsible for the average mobility and hence the resistivity and the piezoresistive effect. The films of indium tin oxide free and with some oxygen content in plasma presents characteristics of decreased electrical resistance to mechanical stress and exhibit effects of piezoresistive in the range of - 12 to - 23. Samples of these films with oxygen showed a factor of very low mechanical sensitivity and are less stables to thermal effect the samples free of oxygen. The films studied can be used in certain applications such strain gauges or even in piezoresistive pressure sensors, after adequate process of deposition and thermal annealing.

The chemical industry of the forthcoming years will be shaped by a number of emerging global megatrends strictly related to the growth and aging of the world population (nine billion people in 2050). This will result in demand of innovative materials able to solve new needs in different fields: health, communication, energy, environmental sustainability, etc. In this diversified context, conducting organic polymers (COPs) are expected to play an important role thanks to their polyhedric properties. Among them, polyaniline is one of the more investigated COPs owing to its peculiar properties which make it a potential substitute of conventional materials in different fields (electronics, fenestration, textile industry, sensors and many others). However, to date many aspects related to its synthesis and application are still open. Scope of the present work is to provide alternative eco-friendly methods to the traditional synthetic routes towards PANI-based materials and enlarge their present applications in view of the novel requirements. This study has been organized in three main sections. In the first section a new green protocol will be present to prepare PANI/metal oxides nanocomposites, innovative materials in the field of EMI shielding. For the first time the double role of magnetic nanoparticles, as catalysts of the reaction and magnetic fillers of the final products, will be illustrated. Conducting/magnetic materials are particularly tempting for their ability to reduce the electromagnetic interferences (EMI) originated by the increasing use of electronic devices and telecommunication equipment. Preliminary results in terms of their microwave absorbing properties will be shown. The possibility to improve the health and quality of life for millions of people worldwide is, in fact, the overall goal of tissue engineering. Nanostructured PANI in form of fibers or wires could find application as novel conductive scaffolds in neuronal or cardiac stimulations. In the second section, the possibility to produce highly pure polyaniline nanofibers by electrospinning technique will be showed. These materials, characterized by high values of conductivity and cytocompatibility, could represent an alternative to traditional solutions for cardiac and neuronal stimulation. Regarding the third section of the work, the amazing piezoresistive properties of PANI, especially in form of film, will be for the first time herein presented. Herein, the extraordinary high GF values of PANI-based films (more than 10 times higher than those of commercial piezoresistors) will be reported. The mechanical monitoring in large and small scale (buildings/touch-technology) needs of highly sensitive stress/strains sensors and PANI-based materials are particularly promising in this sector. All these characteristics contribute to make PANI and its composites innovative materials which could offer new solutions for many challenges of the future.

Thesis (Ph. D.)--Mechanical Engineering, Georgia Institute of Technology, 2007.
Committee Chair: King, William; Committee Member: Allen, Mark; Committee Member: Brand, Oliver; Committee Member: Glezer, Ari; Committee Member: Joshi, Yogendra.

Im Mittelpunkt dieser Arbeit stand die Darstellung sensitiver Blockcopolymere und deren Gele, die als Ausgangsmaterialien in Sensor- und Aktorsystemen einsetzbar sind. Die Vereinigung verschiedener Ansprechparameter stellt erhöhte Anforderung an die Synthese. Geringe Ansprechzeiten lassen sich mit einer Gelgröße im µm-Bereich erreichen. Hydrogele dieser Größenordnungen können durch nachträgliche Vernetzung funktioneller linearer Polymere ermöglicht werden. Die Makroinitiatormethode ermöglichte den Aufbau verschiedener linearer photovernetzbarer Blockcopolymere. Zum Einen wurde das temperatursensitive P(n-BuAc)-block-P(PNIPAAm-co-DMIAAm) erhalten, des Weiteren gelang die Darstellung der multi-sensitiven Blockcopolymere P2VP-block-P(NIPAAm-co-DMIAAm) und P4VP-block-P(NIPAAm-co-DMIAAm). Die Blockcopolymere wurden mit variierenden Blocklängen und Verhältnissen sowie mit unterschiedlichem Vernetzergehalt dargestellt. Die Charakterisierung der Blockcopolymere erfolgte mittels 1H-NMR-Spektroskopie, GPC-Messungen (Zusammensetzung) und DSC-Messungen (thermische Eigenschaften). Das Löslichkeitsverhalten in wässrigen Medien wurde durch Dynamische Lichtstreuung bestimmt. Die Beschreibung des Quellverhaltens der vernetzten Schichten erfolgte durch vornehmlich durch optische Methoden (SPR/OWS, WAMS, Ellipsometrie). Die Veränderung des E-Moduls in Abhängigkeit äußerer Parameter konnte mittels AFM untersucht werden. Die Reaktion der Schichten wurde gegenüber Temperatur, pH-Wert und Salzkonzentrationen getestet. Die charakterisierten Filme konnten im Anschluss als sensitive Schichten in piezoresistiven Sensorsystemen verwendetet werden.

Cette étude portait sur l’évaluation des performances mécaniques et des mécanismes de défaillance des composites cousus dans différentes conditions de chargement. Des plaques stratifiées et des raidisseurs renforcés par tufting ont été fabriqués avec différents paramètres de couture afin d'évaluer leur effet sur les propriétés des composites. L'investigation a été assistée par une caractérisation multi-instrumentée pendant les tests. Les plaques cousues soumises à des tests de cisaillement à poutre courte sont utilisées dans l'analyse du comportement de la densité et de l'angle de couture dans des conditions de chargement en mode II, tandis que des tests d'impact et de compression après impact (CAI) sur la tolérance aux dommages. Des tests de fatigue en éprouvettes trouées ont également été réalisés afin d’évaluer la réponse des coutures, en particulier leur position par rapport au trou central, à la concentration de déformation générée par le trou. La suite de ce travail a consisté en des tests mécaniques sur panneau raidi oméga renforcé par tufting. La procédure a optimisé les paramètres de touffetage utilisés pour renforcer les structures du lot précédent d'échantillons jusqu'à atteindre un point optimal où les propriétés principales, principalement trouvées dans les tests d'arrachement, sont égales ou supérieures à celles des échantillons témoins. Cette amélioration tenait également compte des modifications de la forme des raidisseurs. En outre, une nouvelle approche basée sur l’effet piézorésistif des coutures en fibres de carbone lors du chargement des éprouvettes composites est réalisée. Cela peut faciliter la surveillance de l’état de santé des fils cousus et donc du composite en raison de la nature structurelle des coutures. Les résultats ont montré que les renforts par tufting sont capables d'augmenter considérablement la ténacité entre les composites et la tolérance aux endommagements des composites, principalement en raison de leurs phénomènes de pontage des fissures. Les paramètres de tufting sont des facteurs décisifs pour obtenir les meilleures propriétés mécaniques. Cependant, ces travaux ont montré que les fils de coutures sont également responsables de la création de fissures dues à la concentration de contrainte et aux défauts causés par leur insertion et, par conséquent, à la diminution de la résistance des composites. L'enquête conclut que l'insertion aléatoire des touffes n'est pas idéale pour la performance du matériau et doit donc être évitée. Le développement de l'insertion des coutures dans les raidisseurs oméga a été soutenu par la caractérisation multi-instrumentée qui a permis d'optimiser le renforcement de la structure. Bien que l’étude ait permis d’obtenir des propriétés mécaniques nettement supérieures à celles des panneaux oméga renforcés par touffetage, il est évident que la procédure employée n’est pas optimale. Le présent travail propose également un modèle préliminaire d'éléments finis pour surmonter le coût et la perte de temps des tests expérimentaux. Il vise principalement à optimiser les paramètres de tufting dans la structure. Le modèle développé était capable de prédire les mêmes endommagements que ceux observés expérimentalement, mais encore éloignés des prévisions quantitatives des résultats. Le contrôle de l’état structurel des stratifiés composites cousus par les fils de carbone semble prometteur et pourrait aider à l’avenir à fournir des informations sur l’état de santé des coutures sous chargement qui ne sont pas atteintes par les méthodes de caractérisation classiques utilisées dans ce travail
This study focused mainly on the assessment of the mechanical performance and the failure mechanisms of tufted composites under divers loading conditions. Laminated plates and stiffened panels reinforced by tufting was manufactured with different tufting parameters to evaluate their effect in the properties of the composites. Multi-instrumented characterization carried out during the tests assisted the investigation. The tufted plates subjected to short-beam shear tests aided especially in the behavior analysis of tufting density and angle in mode Il loading condition, while impact and compression after impact (CAI) tests on the damage tolerance. Open-hole fatigue tests were also performed to evaluate the tufts response, especially regarding their position to the center hole, to the strain concentration factor generated by the hole. The following part of this work consisted of the mechanical tests on omega stiffened panel reinforced by tufting. The procedure optimized the tufting parameters employed for reinforcing the structures from the previous batch of specimens until reaching an optimal point that the main properties, primarily found in pull-off tests, are equal or superior to those of the control specimens. This improvement also considered the modifications in the shape of the stiffeners. Furthermore, a novel approach based on the piezoresistive effect of carbon tufts under loading of the composite specimens is performed. This may support the monitoring of the health status on the tufted threads and therefore of the composite because of the structural nature of the tufts. The results showed that tufting reinforcements are capable of increasing the interlaminar fracture toughness and damage tolerance of the composites considerably owing mainly to their crack bridging phenomena. The tufting parameters are decisive factors for achieving the best mechanical properties. However, this work reported that tuft threads are also responsible for generating cracks due to the strain concentration and defects caused by their insertion and consequently, can decrease the strength of the composites. The investigation concludes that the random insertion of the tufts is not ideal for the performance of the material and thus must be avoided. The development of the tufting insertion in the omega stiffeners was supported by the multi-instrumented characterization that led to optimizing reinforcement in the structure. Although the study achieved the goal of obtaining mechanical properties significantly superior to the omega panels reinforced by tufting, it is noticeable that the procedure employed is not optimal. The present work also proposes a preliminary finite element model to overcome the costly and time consuming of the experimental tests. It intends primarily optimizing the tufting parameters in the structure. The model developed was capable of predicting the same damage events as observed experimentally, but it still distant from the quantitative predictions of the results. The structural health monitoring of the tufted composite laminates by the carbon threads seems promising and could help in the future for supplying data about the tufts health status under loading that are not achieved by the conventional characterization methods employed in this work

Thesis (M.Eng.)--University of Louisville, 2008.
Title and description from thesis home page (viewed September 12, 2008). Department of Mechanical Engineering. Vita. "June 2008." Includes bibliographical references (p. 95-102).

High performance and low cost sensors based on microelectromechanical systems (MEMS) have become commonplace in today's world. MEMS sensors, such as accelerometers, gy- roscopes, pressure sensors, and microphones, are routinely used in consumer electronics, automobiles, industrial and aerospace applications. Basically, all these devices mea- sure tiny displacements of micromachined mechanical structures in response to external stimuli. One of the widely used techniques to detect these displacements is piezoresistive sensing. Piezoresistive sensors are popular in MEMS due to their simplicity and robustness. Traditionally, silicon has been the material of choice for piezoresistors due to its high strain sensitivity or gauge factor. Whereas metal lm piezoresistors typically have low gauge factor that puts them out of favour when compared to silicon. But metal lm piezoresistors have several advantages compared to their semiconductor counterparts, including simple and low-cost fabrication, low resistivity and generally low noise. Low resistance sensors become desirable particularly when the devices are scaled down to nanoelectromechanical systems (NEMS), where signal-to-noise ratio (SNR) performance becomes crucial. Enhancing the gauge factor of metal lms while keeping their low resistance advantage can dramatically improve their SNR performance for NEMS. This thesis reports a simple method we have developed to enhance the gauge factor of metal lm piezoresistors. We demonstrate this method on specially designed micro- cantilever devices. Using controlled electromigration, we are able to engineer the microstructure of gold lm and transform it into a locally inhomogeneous conductor which resembles a percolation network. This results in more than 100 times higher gauge factor at low to moderate sensor resistance. The SNR possible with our piezoresistor at high frequencies exceeds that of most available systems by at least an order of magnitude. Our locally inhomogeneous metal lm piezoresistor is a promising candidate for high-performance NEMS-based sensors of the future.

High performance and low cost sensors based on microelectromechanical systems (MEMS) have become commonplace in today's world. MEMS sensors, such as accelerometers, gy- roscopes, pressure sensors, and microphones, are routinely used in consumer electronics, automobiles, industrial and aerospace applications. Basically, all these devices mea- sure tiny displacements of micromachined mechanical structures in response to external stimuli. One of the widely used techniques to detect these displacements is piezoresistive sensing. Piezoresistive sensors are popular in MEMS due to their simplicity and robustness. Traditionally, silicon has been the material of choice for piezoresistors due to its high strain sensitivity or gauge factor. Whereas metal lm piezoresistors typically have low gauge factor that puts them out of favour when compared to silicon. But metal lm piezoresistors have several advantages compared to their semiconductor counterparts, including simple and low-cost fabrication, low resistivity and generally low noise. Low resistance sensors become desirable particularly when the devices are scaled down to nanoelectromechanical systems (NEMS), where signal-to-noise ratio (SNR) performance becomes crucial. Enhancing the gauge factor of metal lms while keeping their low resistance advantage can dramatically improve their SNR performance for NEMS. This thesis reports a simple method we have developed to enhance the gauge factor of metal lm piezoresistors. We demonstrate this method on specially designed micro- cantilever devices. Using controlled electromigration, we are able to engineer the microstructure of gold lm and transform it into a locally inhomogeneous conductor which resembles a percolation network. This results in more than 100 times higher gauge factor at low to moderate sensor resistance. The SNR possible with our piezoresistor at high frequencies exceeds that of most available systems by at least an order of magnitude. Our locally inhomogeneous metal lm piezoresistor is a promising candidate for high-performance NEMS-based sensors of the future.

Design, fabrication and development of polymer microcantilever for flow rate measurement and thermal actuation Research Supervisors: Prof. K. Rajanna and Dr. G. R. Jayanth Microcantilevers are sensitive micromechanical platforms used to detect small forces and surface stresses arising due to changes in physical environment. They are popularly used as mechanical probes in scanning probe microscopy to obtain 3D surface topography of samples upto atomic scale resolution. These microcantilevers find applications in biosensing, environment monitoring, air flow measurement, microbolometry, Atomic Force Microscopy (AFM), etc,. Furthermore, microcantilevers form versatile, compliant platforms for producing mechanical actuation. Microcantilever based actuators are used as RF switches, biomanipulators, microrelays and microfluidic valves. Conventionally, microcantilevers are fabricated using silicon, silicon nitride or silicon dioxide. However, in recent times polymers are being used as alternate materials for fabricating microcantilevers. These polymer microcantilevers offer several advantages and versatilities. The aim of the present thesis work is to the design, fabricate, characterize and evaluate the performance of piezoresistive SU8 microcantilevers for low flow rate measurement and thermal actuation. Finite element (FE) simulation was used to determine the stress distribution across a stressed microcantilever structure. The results of FE simulations enable suitable piezoresistor design for integration with the cantilever. Various surface micromachining techniques were attempted to fabricate freely suspended SU8 microcantilevers with gold thin film piezoresistors. Electrical interconnection was established using ball bump aided epoxy bonding technique. The fabricated SU8 microcantilever sensor was mechanically characterized and its strain sensitivity was evaluated. These sensors were employed for low gas flow rate measurement in the range 0 to 100 mL/min. The sensor response was found to be linear, repeatable and consistent with different flow rates. The fabricated SU8 microcantilever device also exhibited thermomechanical actuation. Hence, the performance of the device due to Joule heating of the piezoresistor was studied in detail. A nonlinear thermomechanical model was proposed to accurately estimate the thermal behaviour of the polymer microcantilever. This study underscores the need to consider nonlinear thermo-elastic properties of polymers while modeling their thermomechanical response. Both finite element simulation and experimental result indicate nonlinear thermomechanical response of the SU8 based thermal actuator. The developed microsystem presents simultaneous sensing and actuation mechanisms. Hence, they are suitable for integration with Lab-on-chip-devices. This thesis in divided into 8 chapters and the brief summary is as follows: Chapter 1 This chapter gives a brief introduction to the state-of-art scenario of MEMS technology and its relevance in the field of sensors and actuators. Later, an overview of micromachining techniques used for the fabrication of MEMS devices is discussed. Microcantilever based devices and their applications are discussed. In particular, their use as non-thermal flow sensors is presented. Also, the need for polymeric microcantilever sensors for low gas flow rate measurement is discussed. At the end, the objective, scope of present work and the organization of the thesis are discussed. Chapter 2 The aspect of SU8 microcantilever design for flow measurement is presented. Relevant piezoresistivity theory required for the design of thin film piezoresistor is explained. Finite element simulation was used to identify regions of maximum stress in the microcantilever due to fluid flow interactions. The geometry and shape of thin film piezoresistor was chosen based on the simulation results. Finally, the optimal design parameters of piezoresistive SU8 microcantilever sensor are summarized. Chapter 3 This chapter describes the processes involved in the fabrication of piezoresistive SU8 microcantilevers. Surface micromachining techniques such as wet oxidation, lift-off, thin film deposition, sacrificial layer etching etc were used during the fabrication. Wet oxidation was used to grow uniform, dense oxide for sacrificial layer. Gold thin films were deposited using RF sputtering technique and patterned using UV photolithography. SU8 microcantilevers were patterned using photolithography and freely suspended SU8 microcantilevers were obtained by selectively etching the sacrificial layer. The issues of residual stress in suspended SU8 microcantilever are discussed. Finally, an optimal fabrication process was obtained to build SU8 microcantilever with integrated piezoresistor. Chapter 4 The fabricated flow sensor needs to be connected with the external circuitry via electrical interconnects. This chapter discusses the process of packaging and electrical interconnection with the fabricated SU8 microcantilever sensor. The issues of making wire bonding onto SU8 chip using conventional wire bonding techniques are described. Alternate wire bonding techniques such as epoxy bonding was attempted. Finally, ball bump aided epoxy bonding technique was developed and used for making electrical interconnection with the sensor. Chapter 5 In this chapter the fabricated and packaged microcantilever sensor was characterized to evaluate its electro-mechanical performance. The sensor response was evaluated experimentally by providing known mechanical displacement via precisely controlled piezostage. At the end, the sensor characteristics such as gauge factor of the piezoresistor, deflection sensitivity of the microcantilever sensor, its hysteresis, linearity and repeatability were also obtained. Chapter 6 This chapter describes the performance study of piezoresistive SU8 cantilever sensor for low gas flow rate measurement in the range 10 to 500 mL/min. The measured flow sensitivity was about 1.103×10-5 mL/min. Finite element simulations were used to estimate the cantilever deflection due to gas flow. The simulation results show quadratic dependence of cantilever deflection on gas flow rate. For a flow rate between 0 to100 mL/min, the experimental results agree well with the simulation results showing a linear trend in this range. Chapter 7 This chapter presents the nonlinear thermomechanical analysis and thermal actuation of fabricated SU8 microcantilevers. The thermomechanical analysis of the actuator incorporates nonlinear temperature-dependent properties of SU8 polymer to accurately model its thermal response during actuation. The issues of residual stress developed within the SU8 microstructure during fabrication are discussed and a novel strategy was proposed to release the residual stress in the fabricated actuators. The thermomechanical response of the actuator was obtained experimentally. The measured average actuation range of about 8.5 μm was produced for an actuation current of 5 mA. It was found that the results of nonlinear thermomechanical analysis agree well with the experimental result. Chapter 8 The chapter summarizes the results and conclusions drawn from the present work. Also, the scope of future work is discussed.

博士
國立臺灣大學
應用力學研究所
103
This research developed a microaccelerometer via Computer-Aid-Design(CAE) and Micro Electro Mechanical Systems(MEMS) with high performance in linearity and cross-axis sensitivity. Unlike the conventional sensing elements which are always embedded at the position of maximum displacement, the present study situated the sensors at the locations where the maximum displacements of the structure are generated in order to raise up the maximal output than the former. The core elements of accelerometer includes a vertical, double-ended flexural beam, a proof mass integrated at the middle section of the beam, and four suspended piezoresistors fixed at the mass block and across the trenches to the anchor pads. The mass block had maximum displacements of the dynamic structure which would activate the sensors to deliver maximal output. It was simulated by numerical method to analyze how much and where the maximal stress would be. The sensing chip was fabricated on a silicon-on-insulator(SOI) wafer through MEMS processes and installed by Dual-In-Package. The accelerometer was placed on a rate table that provided stable centrifugal acceleration up to approximately 3000 G for quasi-static testing. The output voltage of the accelerometer was digitized and radiofrequency transmitted for remote data acquisition. The natural frequency was about 232.4 kHz from mode analysis. After numerous experiments, the correlations for the individual runs showed that the accelerometer had a sensitivity of 3.0015 μV/Vexc/G with extraordinary performance. The best linearity of the sensing output was only 0.11% of full scale output (FS, or 59 dB), as deduced from the average standard deviation of all test runs. The average of the maximum reading deviations from the corresponding correlated curves was approximately 0.26% FS. Moreover, the cross-axis sensitivity for the two orthogonal directions nearly vanished in the test range. With the high rigidity of the microstructure, the accelerometer exhibited an ultra high performance factor of 25.8 x 10^6 MHz. The accelerometer possessed exceptional sensitivity, linearity, and repeatability, and extremely low cross-axis interference and noise.