The application of adaptive micro-electro-mechanical systems (MEMS) device in biologically-inspired cochlear model (cochlear biomodel) has been seen as a preferable approach to mimic closely the human cochlear response. The thesis focuses on the design and fabrication of resonant gate transistor (RGT) device applied towards the development of RGT cochlear biomodel. An array of RGT devices can mimic the cochlea by filtering the sound input signals into multiple electrical outputs. The RGT device consists of two main components; a) the MEMS bridge gate structure that transduces the sound input into mechanical vibrations and b) the channel with source/drain regions underneath the bridge gate structure that transduce the mechanical vibrations into electrical signals. The created mathematical model for RGT calculates the electrical outputs that are suited for neural spike coding. The neuromorphic auditory system is proposed by integrating the RGT devices with the spike event interface circuits. The novelty of the system lies in the adaptive characteristics of the RGT devices that can self-tune the frequency and sensitivity using the feedback control signals from the neuromorphic circuits. The bridge gates have been designed to cover the audible frequency range signals of 20 Hz - 20 kHz. Aluminium and tantalum have been studied as the material for the bridge gate structure. The fabrication of a bridge gate requires a gentle etch release technique to release the structure from a sacrificial layer. The downstream etch release technique employing oxygen/nitrogen plasma has been introduced and characterised. In the first iteration, aluminium bridge gates have been fabricated. The presence of tensile stress within aluminium had caused the aluminium bridge gates of length >1mm to collapse. In order to address this issue, tantalum bridge gates have been fabricated in the second iteration. Straight tantalum bridge gates in tensile stress and buckled tantalum bridge gates in compressive stress have been characterised. The frequency range of 550 Hz - 29.4 kHz has been achieved from the fabricated tantalum bridge gates of length 0.57mm - 5.8mm. The channel and source/drain regions have been fabricated and integrated with the aluminium or tantalum bridge gate structures to create the RGTs. In this study, the n-channel and p-channel resonant gate transistor (n-RGT and p-RGT) have been considered. In n-RGT, phosphorus ions are implanted to form the source/drain regions. High subthreshold currents have been measured from the n-RGTs. Thus, p- RGTs have been employed with considerably small subthreshold current. In p-RGT, boron ions are implanted to form the source/drain regions. The threshold voltage, transconductance and subthreshold current for both n-channel and p-channel resonant gate transistor devices have been characterised. In this work, the channel conductance of the n-RGT and p-RGT devices has been modulated successfully and the sensitivity tuning within the audible frequency range has been achieved from the tantalum bridge gates of the p-RGT devices. The characterisation and optimisation of the resonant gate transistor provide the first step towards the development of the adaptive RGT cochlear biomodel for the neuromorphic auditory system application.

Cell mechanics are directly related to the biological functionality of a cell, and therefore have been extensively studied. Current understanding of the unique relationships associated with mechanical loading conditions and the biological outcomes of a cell is far from complete [1]. The main objective of this thesis work was the design of a device capable of determining mechanical properties including stiffness and Young’s modulus of a biological cell. The device was implemented using micro-electro mechanical systems technology (MEMS), and the cell testing was limited to yeast cells for the purpose of this research. The design consisted of a micro-gripper which performed controlled cell squeezing with a spring of known stiffness. Differential displacements were obtained allowing for the calculation of cell mechanical properties. The incorporation of spatially periodic structures on the moving components of the gripper enabled measurements with 10 nm precision based on discrete Fourier transformation and phase [2].

Micromachined accelerometers have found large attention in recent years due to their low-cost and small size. There are extensive studies with different approaches to implement accelerometers with increased performance for a number of military and industrial applications, such as guidance control of missiles, active suspension control in automobiles, and various consumer electronics devices. This thesis reports the development of various capacitive micromachined accelerometers and various integrated CMOS readout circuits that can be hybrid-connected to accelerometers to implement low-cost accelerometer systems. Various micromachined accelerometer prototypes are designed and optimized with the finite element (FEM) simulation program, COVENTORWARE, considering a simple 3-mask surface micromachining process, where electroplated nickel is used as the structural layer. There are 8 different accelerometer prototypes with a total of 65 different structures that are fabricated and tested. These accelerometer structures occupy areas ranging from 0.2 mm2 to 0.9 mm2 and provide sensitivities in the range of 1-69 fF/g. Various capacitive readout circuits for micromachined accelerometers are designed and fabricated using the AMS 0.8 &micro / m n-well CMOS process, including a single-ended and a fully-differential switched-capacitor readout circuits that can operate in both open-loop and close-loop. Using the same process, a buffer circuit with 2.26fF input capacitance is also implemented to be used with micromachined gyroscopes. A single-ended readout circuit is hybrid connected to a fabricated accelerometer to implement an open-loop accelerometer system, which occupies an area less than 1 cm2 and weighs less than 5 gr. The system operation is verified with various tests, which show that the system has a voltage sensitivity of 15.7 mV/g, a nonlinearity of 0.29 %, a noise floor of 487 Hz &micro / g , and a bias instability of 13.9 mg, while dissipating less than 20 mW power from a 5 V supply. The system presented in this research is the first accelerometer system developed in Turkey, and this research is a part of the study to implement a national inertial measurement unit composed of low-cost micromachined accelerometers and gyroscopes.

The clever designs of natural transducers are a great source of inspiration for man-made systems. At small length scales, there are many transducers in nature that we are now beginning to understand and learn from. Here, we present an example of such a transducer that is used by field crickets to produce their characteristic song. This transducer uses two distinct components—a file of discrete teeth and a plectrum that engages intermittently to produce a series of impulses forming the loading, and an approximately triangular membrane, called the harp, that acts as a resonator and vibrates in response to the impulse-train loading. The file-and-plectrum act as a frequency multiplier taking the low wing beat frequency as the input and converting it into an impulse-train of sufficiently high frequency close to the resonant frequency of the harp. The forced vibration response results in beats producing the characteristic sound of the cricket song. Based on various experimental observations reported in the literature, we model the sound production mechanism as consisting of three stages—actuator, frequency multiplier, and amplifier. We then examine how different features of the forewing govern the sound production. With careful experiments on the harp, we estimate the actual modulus of the harp cuticle and also measure the morphological features of the forewings of different field cricket species. Using this data, we construct a finite element model of the harp and carry out modal analysis to determine its natural frequency. We fine tune the model with appropriate elastic boundary conditions to match the natural frequency of the harp of a particular species—Gryllus bimaculatus. We model impulsive loading based on a loading scheme reported in the literature and predict the transient response of the harp. We show that the harp indeed produces beats and its frequency content matches closely that of the recorded song. Subsequently, we use our FEM model to show that the natural design is quite robust to structural perturbations in the file. The characteristic song frequency produced is unaffected by small variations in the spacing of file-teeth and even by larger gaps. We then attempt to predict a scaling law that crickets must use for spectrum allocation. We use our FEM model, with measurements and computations, to arrive at a predictive model that relates call frequencies of field crickets to the harp dimensions. We verify the validity of this model by using the measured dimensions of harps of nine field cricket species. We then use our model to provide possible explanations as to why the song frequency of various field crickets in our study is bounded between 3.1 kHz and 6.8 kHz. We also show that we are faced with similar challenges as crickets when designing miniature MEMS (Micro-Electro-Mechanical Systems) speakers. We present a design of MEMS speakers that is inspired by how the crickets actuate. We have been able to realize our first prototypes using simple fabrication processes. By electrostatically actuating the MEMS devices, we obtain a sound pressure of 70 dB SPL at a distance of 10 cm. We believe that with a few design and fabrication iterations, we will be able to achieve a much higher sound pressure output from the MEMS speakers.

The development and fabrication of silicon-based phononic band gap crystals has been gaining interest since phononic band gap crystals have implications in fundamental science and display the potential for application in engineering by providing a relatively new platform for the realization of sensors and signal processing elements. The seminal study of phononic band gap phenomenon for classical elastic wave localization in structures with periodicity in two- or three-physical dimensions occurred in the early 1990’s. Micro-integration of silicon devices that leverage this phenomenon followed from the mid-2000’s until the present. The reported micro-integration relies on exotic piezoelectric transduction, phononic band gap crystals that are etched into semi-infinite or finite-thickness slabs which support surface or slab waves, phononic band gap crystals of numerous lattice constants in dimension and phononic band gap crystal truncation by homogeneous mediums or piezoelectric transducers. The thesis reports, to the best of the author's knowledge, for the first time, the theory, design methodology and experiment of an electrostatically actuated silicon-plate phononic band gap quasi-crystal architecture, which may serve as a platform for the development of a new generation of silicon-integrated sensors, signal processing elements and improved mechanical systems. Electrostatic actuation mitigates the utilization of piezoelectric transducers and provides action at a distance type forces so that the phononic band gap quasi-crystal edges may be free standing for potentially reduced anchor and substrate mode loss and improved energy confinement compared with traditional surface and slab wave phononic band gap crystals. The proposed phononic band gap quasi-crystal architecture is physically scaled for fabrication as MEMS in a silicon-on-insulator process. Reasonable experimental verification of the model of the electrostatically actuated phononic band gap quasi-crystal architecture is obtained through extensive dynamic harmonic analysis and mode shape topography measurements utilizing optical non-destructive laser-Doppler velocimetry. We have utilized our devices to obtain fundamental information regarding novel transduction mechanisms and behavioral characteristics of the phononic band gap quasi-crystal architecture. Applicability of the phononic band gap quasi-crystal architecture to physical temperature sensors is demonstrated experimentally. Vibration stabilized resonators are demonstrated numerically.

The development and fabrication of silicon-based phononic band gap crystals has been gaining interest since phononic band gap crystals have implications in fundamental science and display the potential for application in engineering by providing a relatively new platform for the realization of sensors and signal processing elements. The seminal study of phononic band gap phenomenon for classical elastic wave localization in structures with periodicity in two- or three-physical dimensions occurred in the early 1990’s. Micro-integration of silicon devices that leverage this phenomenon followed from the mid-2000’s until the present. The reported micro-integration relies on exotic piezoelectric transduction, phononic band gap crystals that are etched into semi-infinite or finite-thickness slabs which support surface or slab waves, phononic band gap crystals of numerous lattice constants in dimension and phononic band gap crystal truncation by homogeneous mediums or piezoelectric transducers. The thesis reports, to the best of the author's knowledge, for the first time, the theory, design methodology and experiment of an electrostatically actuated silicon-plate phononic band gap quasi-crystal architecture, which may serve as a platform for the development of a new generation of silicon-integrated sensors, signal processing elements and improved mechanical systems. Electrostatic actuation mitigates the utilization of piezoelectric transducers and provides action at a distance type forces so that the phononic band gap quasi-crystal edges may be free standing for potentially reduced anchor and substrate mode loss and improved energy confinement compared with traditional surface and slab wave phononic band gap crystals. The proposed phononic band gap quasi-crystal architecture is physically scaled for fabrication as MEMS in a silicon-on-insulator process. Reasonable experimental verification of the model of the electrostatically actuated phononic band gap quasi-crystal architecture is obtained through extensive dynamic harmonic analysis and mode shape topography measurements utilizing optical non-destructive laser-Doppler velocimetry. We have utilized our devices to obtain fundamental information regarding novel transduction mechanisms and behavioral characteristics of the phononic band gap quasi-crystal architecture. Applicability of the phononic band gap quasi-crystal architecture to physical temperature sensors is demonstrated experimentally. Vibration stabilized resonators are demonstrated numerically.

This study concerns with the testing methods for mechanical characterization at micron scale. The need for the study arises from the fact that the mechanical properties of materials at micron scale differ compared to their bulk counterparts, depending on the microfabrication method involved. Various test structures are designed according to the criteria specified in this thesis, and tested for this purpose in micron scale. Static and fatigue properties of the materials are aimed to be extracted through the tests. Static test structures are analyzed using finite elements method in order to verify the results. Test structures were fabricated by deep reactive ion etching of 100 &micro / m thick (111) silicon and electroplating 18 &micro / m nickel layer. Performance of the test structures are evaluated based on the results of tests conducted on the devices made of (111) v silicon. According to the results of the tests conducted on (111) silicon structures, elastic modulus is found to be 141 GPa on average. The elastic modulus of electroplated nickel is found to be 155 GPa on average, using the same test structures. It is observed that while the averages of the test results are acceptable, the deviations are very high. This case is related to fabrication faults in general. In addition to the tests, a novel computer script utilizing image processing is also developed and used for determination of the deflections in the test structures.

abstract: Harsh environments have conditions that make collecting scientific data difficult with existing commercial-off-the-shelf technology. Micro Electro Mechanical Systems (MEMS) technology is ideally suited for harsh environment characterization and operation due to the wide range of materials available and an incredible array of different sensing techniques while providing small device size, low power consumption, and robustness. There were two main objectives of the research conducted. The first objective was to design, fabricate, and test novel sensors that measure the amount of exposure to ionizing radiation for a wide range of applications including characterization of harsh environments. Two types of MEMS ionizing radiation dosimeters were developed. The first sensor was a passive radiation-sensitive capacitor-antenna design. The antenna's emitted frequency of peak-intensity changed as exposure time to radiation increased. The second sensor was a film bulk acoustic-wave resonator, whose resonant frequency decreased with increasing ionizing radiation exposure time. The second objective was to develop MEMS sensor systems that could be deployed to gather scientific data and to use that data to address the following research question: do temperature and/or conductivity predict the appearance of photosynthetic organisms in hot springs. To this end, temperature and electrical conductivity sensor arrays were designed and fabricated based on mature MEMS technology. Electronic circuits and the software interface to the electronics were developed for field data collection. The sensor arrays utilized in the hot springs yielded results that support the hypothesis that temperature plays a key role in determining where the photosynthetic organisms occur. Additionally, a cold-film fluidic flow sensor was developed, which is suitable for near-boiling temperature measurement. Future research should focus on (1) developing a MEMS pH sensor array with integrated temperature, conductivity, and flow sensors to provide multi-dimensional data for scientific study and (2) finding solutions to biofouling and self-calibration, which affects sensor performance over long-term deployment. / Dissertation/Thesis / Ph.D. Engineering 2013

The field of microfluidics has the potential to provide a number of products to better everyday life, but is still not well understood. In previous research performed in the field, microfluidics has been shown to exhibit behavior different from what would be expected through normal pipe flow theory. While some research has shown that fluid flow through microchannels does conform to the theoretical flow mechanics, and thus can be predicted and understood through use of well-known relations; other research performed has indicated that fluid flow through microchannels experiences higher or lower pressure losses than would be expected with macro scale theory. This work strives to further explore and explain this anomaly by focusing on simple straight rectangular channels of varying hydraulic diameters from 24 µm to 88 µm, in order to form a more basic understanding for fluid flow in microchannels. Water was pumped through each of these channels at a number of different flow rates, and the static pressure was measured in two locations, a set length apart. The measured pressure loss over this length for each flow rate was then recorded and analyzed to provide relations between pressure loss and hydraulic diameter. Through the data obtained in this study, microfluidic flow of Reynolds numbers greater than 40 and in channels as small as 48 µm in diameter experienced pressure losses predicted from macroscale theory. Below these values, the data was more random, but still showed some conformance to theory. A clear relationship between measured pressure loss and hydraulic diameters over the entire range of channels was also found for two different flow rates. It is hoped that the data obtained will provide a better understanding of microfluidics and pave the way for potential applications to be realized.

In this work, we studied the electronic behavior of polymer-derived ceramics (PDCs) and applied them for the synthesis of carbon nanotube reinforced ceramic nanocomposites and ceramic MEMS (Micro-Electro-Mechanical Systems) structures. Polymer-derived SiCN ceramics were synthesized by pyrolysis of a liquid polyureasilazane with dicumyl peroxide as thermal initiator. The structural evolution during pyrolysis and post-annealing was studied using FTIR, solid state NMR and Raman. The results revealed that the resultant ceramics consisted of SiCxNx-4 as major building units. These units were connected with each other through C-C/C=C bonds or by shearing N/C. The amount of sp2 free carbon strongly depends on composition and processing condition. Electron paramagnetic resonance (EPR) was used to investigate electronic structure of PDCs; the results revealed that the materials contain unpaired electron centers associated with carbons. Electronic behavior of the SiCN ceramics was studied by measuring their I-V curves, temperature dependence of d.c.-conductivities and impendence. The results revealed that the SiCN ceramics exhibited typical amorphous semiconductor behavior, and their conductivity varied in a large range. The results also revealed that the materials contain more than one phase, which have the different electronic behavior. We explored possibility of using polymer-derived ceramics to make ceramic MEMS for harsh environmental applications with a lithography technique. The cure depth of the polymer precursor was measured as a function of UV intensity and exposure time. The experimental data was compared with the available theoretical model. A few typical SiCN parts were fabricated by lithography technique. We also prepared carbon nanotube reinforced ceramic nanocomposites by using PDC processing. The microstructures of the composites were characterized using SEM and TEM; the mechanical properties were studied characterized using nanoindentation. The significant improvement in mechanical properties was observed for the nanocomposites.