Investigation and Integration of Piezoresistive Silicon Nanowires for MEMS applications

Ph.DDOCTOR OF PHILOSOPH

Nanoelectromechanical Sensors based on Suspended 2D Materials

The unique properties and atomic thickness of two-dimensional (2D) materials enable smaller and better nanoelectromechanical sensors with novel functionalities. During the last decade, many studies have successfully shown the feasibility of using suspended membranes of 2D materials in pressure sensors, microphones, accelerometers, and mass and gas sensors. In this review, we explain the different sensing concepts and give an overview of the relevant material properties, fabrication routes, and device operation principles. Finally, we discuss sensor readout and integration methods and provide comparisons against the state of the art to show both the challenges and promises of 2D material-based nanoelectromechanical sensing.Comment: Review pape

Mechanical Properties of Low Dimensional Materials

Recent advances in low dimensional materials (LDMs) have paved the way for unprecedented technological advancements. The drive to reduce the dimensions of electronics has compelled researchers to devise newer techniques to not only synthesize novel materials, but also tailor their properties. Although micro and nanomaterials have shown phenomenal electronic properties, their mechanical robustness and a thorough understanding of their structure-property relationship are critical for their use in practical applications. However, the challenges in probing these mechanical properties dramatically increase as their dimensions shrink, rendering the commonly used techniques inadequate. This Dissertation focuses on developing techniques for accurate determination of elastic modulus of LDMs and their mechanical responses under tensile and shear stresses. Fibers with micron-sized diameters continuously undergo tensile and shear deformations through many phases of their processing and applications. Significant attention has been given to their tensile response and their structure-tensile properties relations are well understood, but the same cannot be said about their shear responses or the structure-shear properties. This is partly due to the lack of appropriate instruments that are capable of performing direct shear measurements. In an attempt to fill this void, this Dissertation describes the design of an inexpensive tabletop instrument, referred to as the twister, which can measure the shear modulus (G) and other longitudinal shear properties of micron-sized individual fibers. An automated system applies a pre-determined twist to the fiber sample and measures the resulting torque using a sensitive optical detector. The accuracy of the instrument was verified by measuring G for high purity copper and tungsten fibers. Two industrially important fibers, IM7 carbon fiber and Kevlar® 119, were found to have G = 17 and 2.4 GPa, respectively. In addition to measuring the shear properties directly on a single strand of fiber, the technique was automated to allow hysteresis, creep and fatigue studies. Zinc oxide (ZnO) semiconducting nanostructures are well known for their piezoelectric properties and are being integrated into several nanoelectro-mechanical (NEMS) devices. In spite of numerous studies on the mechanical response of ZnO nanostructures, there is not a consensus in its measured bending modulus (E). In this Dissertation, by employing an all-electrical Harmonic Detection of Resonance (HDR) technique on ZnO nanowhisker (NW) resonators, the underlying origin for electrically-induced mechanical oscillations in a ZnO NW was elucidated. Based on visual detection and electrical measurement of mechanical resonances under a scanning electron microscope (SEM), it was shown that the use of an electron beam as a resonance detection tool alters the intrinsic electrical character of the ZnO NW, and makes it difficult to identify the source of the charge necessary for the electrostatic actuation. A systematic study of the amplitude of electrically actuated as-grown and gold-coated ZnO NWs in the presence (absence) of an electron beam using an SEM (dark-field optical microscope) suggests that the oscillations seen in our ZnO NWs are due to intrinsic static charges. In experiments involving mechanical resonances of micro and nanostructured resonators, HDR is a tool for detecting transverse resonances and E of the cantilever material. To add to this HDR capability, a novel method of measuring the G using HDR is presented. We used a helically coiled carbon nanowire (HCNW) in singly-clamped cantilever configuration, and analyzed the complex (transverse and longitudinal) resonance behavior of the nonlinear geometry. Accordingly, a synergistic protocol was developed which (i) integrated analytical, numerical (i.e., finite element using COMSOL ®) and experimental (HDR) methods to obtain an empirically validated closed form expression for the G and resonance frequency of a singly-clamped HCNW, and (ii) provided an alternative for solving 12th order differential equations. A visual detection of resonances (using in situ SEM) combined with HDR revealed intriguing non-planar resonance modes at much lower driving forces relative to those needed for linear carbon nanotube cantilevers. Interestingly, despite the presence of mechanical and geometrical nonlinearities in the HCNW resonance behavior, the ratio of the first two transverse modes f2/f1 was found to be similar to the ratio predicted by the Euler-Bernoulli theorem for linear cantilevers

High Frequency Thermally Actuated Single Crystalline Silicon Micromechanical Resonators with Piezoresistive Readout

Over the past decades there has been a great deal of research on developing high frequency micromechanical resonators. As the two most common and conventional MEMS resonators, piezoelectric and electrostatic resonators have been at the center of attention despite having some drawbacks. Piezoelectric resonators provide low impedances that make them compatible with other low impedance electronic components, however they have low quality factors and complicated fabrication processes. In case of electrostatic resonators, they have higher quality factors but the need for smaller transductions gaps complicates their fabrication process and causes squeezed film damping in Air. In addition, the operation of both these resonators deteriorates at higher frequencies. In this presented research, thermally actuated resonators with piezoresistive readout have been developed. It has been shown that not only do such resonators require a simple fabrication process, but also their performance improves at higher frequencies by scaling down all the dimensions of the structure. In addition, due to the internal thermo-electro-mechanical interactions, these active resonators can turn some of the consumed electronic power back into the mechanical structure and compensate for the mechanical losses. Therefore, such resonators can provide self-Q-enhancement and self-sustained-oscillation without the need for any electronic circuitry. In this research these facts have been shown both experimentally and theoretically. In addition, in order to further simplify the fabrication process of such structures, a new controlled batch fabrication method for fabricating silicon nanowires has been developed. This unique fabrication process has been utilized to fabricate high frequency, low power thermal-piezoresistive resonators. Finally, a new thermal-piezoresistive resonant structure has been developed that can operate inside liquid. This resonant structure can be utilized as an ultra sensitive biomedical mass sensor

Nanomechanical Resonators: Toward Atomic Scale

The quest for realizing and manipulating ever smaller man-made movable structures and dynamical machines has spurred tremendous endeavors, led to important discoveries, and inspired researchers to venture to new grounds. Scientific feats and technological milestones of miniaturization of mechanical structures have been widely accomplished by advances in machining and sculpturing ever shrinking features out of bulk materials such as silicon. With the flourishing multidisciplinary field of low-dimensional nanomaterials, including one-dimensional (1D) nanowires/nanotubes, and two-dimensional (2D) atomic layers such as graphene/phosphorene, growing interests and sustained efforts have been devoted to creating mechanical devices toward the ultimate limit of miniaturization— genuinely down to the molecular or even atomic scale. These ultrasmall movable structures, particularly nanomechanical resonators that exploit the vibratory motion in these 1D and 2D nano-to-atomic-scale structures, offer exceptional device-level attributes, such as ultralow mass, ultrawide frequency tuning range, broad dynamic range, and ultralow power consumption, thus holding strong promises for both fundamental studies and engineering applications. In this Review, we offer a comprehensive overview and summary of this vibrant field, present the state-of-the-art devices and evaluate their specifications and performance, outline important achievements, and postulate future directions for studying these miniscule yet intriguing molecular-scale machines

Human and Biological Skin-Inspired Electronic Skins for Advanced Sensory Functions and Multifunctionality

Department of Energy Engineering (Energy Engineering)The electronic skin (e-skin) technology is an exciting frontier to drive next generation of wearable electronics owing to its high level of wearability to curved human body, enabling high accuracy to harvest information of users and their surroundings. Altough various types of e-skins, based on several signal-transduction modes, including piezoresistive, capacitive, piezoelectric, triboelectric modes, have been developed, their performances (i.e. sensitivity, working range, linearity, multifunctionality, etc.) should be improved for the wearable applications. Recently, biomimicry of the human and biological skins has become a great inspiration for realizing novel wearable e-skin systems with exceptional multifunctionality as well as advanced sensory functions. As an ideal sensory organ, tactile sensing capabilities of human skin was emulated for the development of e-skins with enhanced sensor performances. In particular, the unique geometry and systematic sensory system of human skin have driven new opportunities in multifunctional and highly sensitive e-skin applications. In addition, extraordinary architectures for protection, locomotion, risk indication, and camouflage in biological systems provide great possibilities for second skin applications on user-interactive, skin-attachable, and ultrasensitive e-skins, as well as soft robots. Benefitting from their superior perceptive functions and multifunctionality, human and biological skins-inspired e-skins can be considered to be promising candidates for wearable device applications, such as body motion tracking, healthcare devices, acoustic sensor, and human machine interfaces (HMI). This thesis covers our recent studies about human and biological skin-inspired e-skins for advanced sensory functions and multifunctionality. First, chapter 1 highlights various types of e-skins and recent research trends in bioinspired e-skins mimicking perceptive features of human and biological skins. In chapter 2, we demonstrate highly sensitive and tactile-direction-sensitive e-skin based on human skin-inspired interlocked microdome structures. Owing to the stress concentration effect, the interlocked e-skin experiences significant change of contact area between the interlocked microdomes, resulting in high pressure sensitivity. In addition, because of the different deformation trends between microstructures in mutual contact, the interlocked e-skin can differentiate and decouple sensor signals under different directional forces, such as pressure, tensile strain, shear, and bending. In chapter 3, interlocked e-skins were designed with multilayered geometry. Although interlocked e-skin shows highly sensitive pressure sensing performances, their pressure sensing range is narrow and pressure sensitivity continuously decreases with increasing pressure level. The multilayer interlocked microdome geometry can enhance the pressure-sensing performances of e-skins, such as sensitivity, working range, and linearity. As another approach of e-skin with multilayered geometry, we demonstrate multilayered e-skin based on conductivity-gradient conductive materials in chapter 4. The conducive polymer composites with different conductivity were coated on the microdome pattern and designed as interlocked e-skin with coplanar electrode design, resulting in exceptionally high pressure-sensing performances compared with previous literatures. In chapter 5, inspired by responsive color change in biological skins, we developed mechanochromic e-skin with a hierarchical nanoparticle-in-micropore architecture. The novel design of hierarchical structure enables effective stress concentration at the interface between nanoparticle and porous structure, resulting in impressive color change under mechanical stimuli. In chapter 6, we emulate ultrahigh temperature sensitivity of human and snake skin for temperature-sensitive e-skin. The thermoresponsive composite based on semi-crystalline polymer, temperature sensor shows ultrahigh temperature sensitivity near the melting point of semi-crystalline polymer. In addition, integration of thermochromic composite, mimicking biological skins, enables dual-mode temperature sensors by electrical and colorimetric sensing capabilities. Finally, in chapter 7, we summarize this thesis along with future perspective that should be considered for next-generation e-skin electronics. Our e-skins, inspired by human and biological skin, can provide a new paradigm for realizing novel wearable electronic systems with exceptional multifunctionality as well as advanced sensory functions.clos

Development of piezoresistive sensors for biomedical applications

Tese de doutoramento em Engenharia Electrónica Industrial e de ComputadoresIn the last decades there has been an increase in sensing systems applied in a variety of situations with a large variety of sensor ranges. This represents a growing area with high potential. One of the areas of sensor development that require a great deal of attention is the area of sensor for biomedical applications and biosensors. These sensors have to overcome a number of challenges and limitations inherent to the environment where they are introduced. These difficulties lead to the necessity of using new materials and new techniques for their construction together with the more traditional materials, e.g. silicon based, which have already proven their potential in this area. Among the various materials, polymers have proven to be a good choice, due to a set of advantages such as simple processing, flexibility and facility of being obtained in different shapes. Therefore it is interesting to fabricate polymer based piezoresistive sensors for functional coatings of implantable hip prosthesis. These sensors will allow coating the prosthesis and provide new functionalities to the implants such as the possibility to measure forces and deformations between the prosthesis and the bone and therefore improving the postoperative diagnostic. In this works, a model of hip prosthesis with coated sensors was developed. For this purpose, flexible piezoresistive sensors have been developed that allow being implanted. Strain sensors were fabricated based on thin films of n+-nc-si.H by the technique of hot-wire chemical vapor deposition at a temperature of 150 ºC on a polymeric substrate, using the lithographic technique to construct the various layers of the sensors. The sensor has a gauge factor of -28 for low frequency deformation cycles. In the last decades there has been an increase in sensing systems applied in a variety of situations with a large variety of sensor ranges. This represents a growing area with high potential. One of the areas of sensor development that require a great deal of attention is the area of sensor for biomedical applications and biosensors. These sensors have to overcome a number of challenges and limitations inherent to the environment where they are introduced. These difficulties lead to the necessity of using new materials and new techniques for their construction together with the more traditional materials, e.g. silicon based, which have already proven their potential in this area. Among the various materials, polymers have proven to be a good choice, due to a set of advantages such as simple processing, flexibility and facility of being obtained in different shapes. Therefore it is interesting to fabricate polymer based piezoresistive sensors for functional coatings of implantable hip prosthesis. These sensors will allow coating the prosthesis and provide new functionalities to the implants such as the possibility to measure forces and deformations between the prosthesis and the bone and therefore improving the postoperative diagnostic. In this works, a model of hip prosthesis with coated sensors was developed. For this purpose, flexible piezoresistive sensors have been developed that allow being implanted. Strain sensors were fabricated based on thin films of n+-nc-si.H by the technique of hot-wire chemical vapor deposition at a temperature of 150 ºC on a polymeric substrate, using the lithographic technique to construct the various layers of the sensors. The sensor has a gauge factor of -28 for low frequency deformation cycles.In the last decades there has been an increase in sensing systems applied in a variety of situations with a large variety of sensor ranges. This represents a growing area with high potential. One of the areas of sensor development that require a great deal of attention is the area of sensor for biomedical applications and biosensors. These sensors have to overcome a number of challenges and limitations inherent to the environment where they are introduced. These difficulties lead to the necessity of using new materials and new techniques for their construction together with the more traditional materials, e.g. silicon based, which have already proven their potential in this area. Among the various materials, polymers have proven to be a good choice, due to a set of advantages such as simple processing, flexibility and facility of being obtained in different shapes. Therefore it is interesting to fabricate polymer based piezoresistive sensors for functional coatings of implantable hip prosthesis. These sensors will allow coating the prosthesis and provide new functionalities to the implants such as the possibility to measure forces and deformations between the prosthesis and the bone and therefore improving the postoperative diagnostic. In this works, a model of hip prosthesis with coated sensors was developed. For this purpose, flexible piezoresistive sensors have been developed that allow being implanted. Strain sensors were fabricated based on thin films of n+-nc-si.H by the technique of hot-wire chemical vapor deposition at a temperature of 150 ºC on a polymeric substrate, using the lithographic technique to construct the various layers of the sensors. The sensor has a gauge factor of -28 for low frequency deformation cycles. Sensors with larger flexibility were also developed though inkjet printing technique. Various configurations and materials were used to evaluate which materials are most appropriate for these types of sensors. Sensors with a gauge factor of approximately 2.5 for an active layer of PeDOT were obtained. A sensor matrix of 4 x 5 sensors was fabricated with an active area of 1.8 x 1.5 mm2 per sensor. These sensors were subjected to a set of electromechanical tests to evaluate its performance in situations close to end use. So the prosthesis was coated with the various sensors, cemented and subjected to deformation cycles for three levels of force according to standard ISO7206. An adaptive system read-out electronic circuit was developed and built that allows reading piezoresistive sensors with different characteristics. This system allows measuring a matrix of 8x8 sensors, but can be scaled to a large number of sensors. The readable range of the system is between 50 Ω and 100 kΩ according to the needs of the sensors being implanted. The total area of the circuit is 135 mm2, according to the requirements of a circuit to be used in in-vivo applications. An energy management system was also implemented that allows to activate and deactivate parts of the circuit when they are not needed, reducing the energy consumption. The system was validated by measuring a matrix of sensors with different characteristics. Finally, simulations were performed in order to evaluate the best options for the development of a wireless communications system. Three possible operation frequency ranges were used for three types of standard antennas. The communication system was introduced into a model simulating the characteristics of the various layers that constitute the human body. These simulations allow evaluate the frequency range most appropriate for implantable devices, the most appropriate antenna and the best location within the body. So the frequency chosen for the implementation was 868 Mhz for a Inverted- F antenna (IFA). In conclusion, the key elements for the implementations of an instrumented hip prosthesis were development and validated. The developed and/or simulated elements, including sensors, circuits for reading and communication system can also be used in other applications due to characteristics.These simulations allow evaluate the frequency range most appropriate for implantable devices, the most appropriate antenna and the best location within the body. So the frequency chosen for the implementation was 868 Mhz for a Inverted- F antenna (IFA). In conclusion, the key elements for the implementations of an instrumented hip prosthesis were development and validated. The developed and/or simulated elements, including sensors, circuits for reading and communication system can also be used in other applications due to characteristics. Neste trabalho foi desenvolvido um modelo de prótese de anca com implementação de sensores. Para atingir esse objectivo, foram desenvolvidos sensores piezoresitivos flexíveis que permitam ser implantados. Assim foram fabricados sensores de deformação baseados em filmes finos de n+-nc-si.H pela técnica de hot-wire chemical vapor deposition a uma temperatura de 150ºC sobre um substrato polimérico. Recorreu-se a técnica de litografia para construir as várias camadas do sensor. Os sensores apresentam um gauge factor de -28, para ciclos de baixa frequência em testes de four-point-bending. Foram ainda desenvolvidos sensores com uma maior flexibilidade através da técnica de inkjet printing. Para esse desenvolvimento foram usadas várias configurações e materiais, para avaliar quais os materiais mais adequados para este tipo de sensores. Na caracterização destes sensores obteve-se um gauge factor de aproximadamente 2.5 para uma camada ativa de PeDOT. Com os melhores sensores obtidos foram construídas matrizes de 4 x 5 sensores que apresentam uma área ativa de 1.8 x 1.5mm2 por sensor. Estes sensores foram sujeitos a um conjunto de ensaios electromecânicos, para avaliar o seu desempenho em situações próximas da utilização final. Desta forma foi revestida uma prótese com os diferentes sensores, cimentada e sujeita a ciclos de deformação para três níveis de força, segundo a norma ISO7206. Foi desenvolvido e construído um sistema de leitura adaptável que permite medir sensores piezoresistivos com diferentes características entre eles. Este sistema permite medir uma matriz de 8x8 sensores, mas pode ser escalada para um número maior de sensores. A gama de leitura do sistema varia entre 50 Ω e 100 kΩ, de acordo com as necessidades dos sensores a serem implementados. A área total deste circuito é de 135 mm2, de acordo com as necessidades de um circuito a ser utilizado em aplicações in-vivo. Foi também implementado um sistema de gestão de energia que permite ativar e desativar partes do circuito quando estas não são necessárias, permitindo, desta forma, reduzir os consumos de energia. O sistema foi validado através da medição de uma matriz de sensores com diferentes características. foram realizadas simulações de forma a avaliar as melhores opções para o desenvolvimento do sistema de comunicação sem fios. Foram usadas três possíveis gamas de frequência de operação para três tipos de antenas standard. O sistema de comunicação foi introduzido num modelo simulando as características das várias camadas que constituem o corpo humano. Estas simulações permitem aferir a gama de frequências mais adequadas para os dispositivos implantáveis, a antena mais adequada e a sua melhor localização, pois foi verificado como as várias camadas que constituem o corpo humano influenciam a comunicação. Assim, a frequência escolhida para a implementação foi de 868 MHz e a antena foi a IFA. Em conclusão, os elementos principais para a implementação de uma prótese de anca instrumentada, foram desenvolvidos e validados. Os elementos desenvolvidos e/ou simulados, incluindo os sensores, circuitos de leitura e sistema de comunicação, poderão igualmente ser utilizados em outras aplicações devido às suas boas características

Applications of Reflectometry Towards the Development of MEMS Gas Sensors

Reflectometry or reflectance spectroscopy is a relatively simple characterization technique based on the analysis of reflected light from a surface. Herein, reflectometry is used to attain significant insights towards the development of micro-electro-mechanical systems (MEMS) based gas sensors. Two main reflectometry applications are demonstrated which led to formation of unique MEMS gas sensor devices. The first application was the use of in-situ reflectance spectroscopy to characterize the growth behavior of metal oxide films grown by atmospheric pressure-spatial atomic layer deposition. The technique revealed an initial film nucleation period, where the length of the nucleation time was sensitive to the deposition process parameters. The in-situ reflectometry technique was then used to study and monitor the growth behavior of metal oxide films on various non-conventional surfaces. Doing so allowed for the accurate deposition of zinc oxide films on a variety of surfaces with desired thickness. This was instrumental, as it enabled the integration of the zinc oxide films into a novel MEMS resonant cantilever architecture for gas sensing. In this device, the zinc oxide layer serves as both the cantilever structural layer as well as the gas sensitive receptor layer. A key advantage of the approach was the reduction in overall mass of the cantilever which can lead to an enhancement in sensitivity to low quantities of analyte gases. The sensor had an outstanding sensitivity to low levels of relatively humidity(RH) when compared to other frequency shift-based humidity sensors. The zinc oxide cantilever demonstrated a sensitivity of 23649 ppm⁄%RH at 5.9 % RH and an average sensitivity of 1556 ppm⁄%RH in the range of 30-60% RH. The second application of reflectometry was its use as a screening technique to find suitable gas sensitive receptor materials for static deflection type MEMS gas sensors. The reflectance intensity of various materials exposed to gases, was monitored, where changes in the intensity indicated that the material was physically changing in the presence of the gas. This expansion behavior is ideal for static deflection type MEMS gas sensors such as the nanomechanical membrane type surface stress sensor (MSS) architecture. The reflectance screening technique identified that laser treated two dimension materials such as graphene oxide, molybdenum disulfide and tungsten disulfide were suitable candidates to be integrated as the receptor layer in the MSS platform. The sensing response of the coated devices was obtained for a select group of volatile organic compounds. The results showed that the laser treatment technique was advantageous to enhancing the sensor response and sensitivity, as it introduces defects, dopants and functional groups to the receptor materials for improved gas adsorption