A microelectromechanical systems (MEMS)-based environmental monitoring platform was presented in this dissertation. All devices were realized using thin-film piezoelectric-on-substrate (TPoS) technology, which provides a path to integrate various functionalities on a single substrate with MEMS components. TPoS resonators exhibit high quality factors (Qs) in air and are capable of low-power oscillator implementation, which further qualifies such a platform for mobile and portable systems. To validate the TPoS platform, gravimetric humidity sensing was demonstrated with thermally-corrected output by an uncoated "reference" temperature sensor. Also presented were TPoS sensors for toluene and xylene, which are pollutants of great importance for indoor and outdoor air quality as well as health screenings. Silicon dual-mode resonators and oscillators for self-temperature sensing were also explored. Dual-mode thermometry exploits the inherent frequency-temperature dependence of silicon to accurately and locally measure device temperature, forming an essential building block of highly stable oscillators and sensors. Multi-axis piezo-on-Si kinetic energy harvesting (KEH) devices with integrated frequency-upconverting transducers were also introduced. Devices were micromachined on the same substrate as TPoS resonant sensors and have an individual volume in mm3, enabling applications in wireless autonomous sensor nodes. In remote locations where continuous operation may be required, TPoS energy harvesters can provide battery replacement or recharging alternatives that do not increase overall system size.

Thesis (M.S.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 44-46). Also available on the World Wide Web.

Thesis (Ph. D.)--University of Oregon, 2006. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 204-213). Also available for download via the World Wide Web; free to University of Oregon users.

This dissertation presents theoretical and experimental investigations into the dynamical behavior of Micro electromechanical systems (MEMS) resonators and their exploitation for filtering, sensing, and logic applications. The dissertation is divided into two parts: MEMS coupled structures and MEMS dynamic logic devices. First, a theoretical and experimental investigation is presented on both electrostatically and mechanically coupled resonator. Static and dynamic analysis is presented for weakly electrostatically coupled silicon microbeams and also for strongly mechanically coupled polyimide microbeams. The static analysis focuses on revealing pull-in characteristics, while the dynamic analysis focuses on the frequency response of the system and its exploitation for potential applications in filtering and amplification. Next, the phenomenon of mode localization is explored theoretically and experimentally on both electrostatically and mechanically weakly coupled resonators. Eigenvalue analysis is conducted and the dynamic response of the coupled system under different external perturbations is investigated. It is observed, that the exploitation of mode localization depends on the choice of the resonator to be under direct excitation, its stiffness to be perturbed, and which resonator is used to record the output results. These understandings will potentially help improve the performance of MEMS mode-localized sensors. Finally, three techniques to realize cascadable MEMS logic devices are presented. MEMS logic device vibrates at two steady states; a high on-resonance state (1) and a low off-resonance state (0). First, a MEMS logic device is presented capable of performing the AND/NAND logic gate and a tri-state logic gate using mixed-frequency excitation. This work is based on the concept of activation (1) and deactivation (0) of combination resonances due to the mixing of two or more input signals. Second, exploitation of subharmonic resonance under an AC only excitation to perform AND logic operation is presented. Finally, another MEMS logic device is presented working on the principal of activation (1) and deactivation (0) of second resonant mode of a clamped-clamped microbeam. This device is capable of performing OR, XOR and NOT gate. Experimental demonstration of the cascadability is shown for this case cascading OR and NOT gate to perform a logically complete NOR logic gate.

This dissertation investigates dissipation in microelectromechanical (MEMS) resonators via detailed analysis and modeling of the energy loss mechanisms and provides a framework toward creating resonant devices with ultra-low dissipation. Fundamental mechanisms underlying acoustic energy loss are explored, the results of which are applied to understanding the losses in resonant MEMS devices. Losses in the materials, which set the ultimate limits of the achievable quality factor of the devices, are examined. Other sources of loss, which are determined by the design of the resonator, are investigated and applied to example resonant MEMS structures. The most critical of these designable loss mechanisms are thermoelastic dissipation (TED) and support (or anchor) loss of acoustic energy through the attachment of the MEMS device to its external environment. The dissipation estimation framework enables prediction of the quality factor of a MEMS resonator, which were accurate within a factor of close to 2 for high-frequency bulk acoustic wave MEMS resonators, and represents a signficant step forward by closing one of the largest outstanding problems in MEMS devices: how to predict the quality factor for a given device. Dissipation mitigation approaches developed herein address the most critical dominant loss mechanisms identified using the framework outlined above. These approaches include design of 1D phononic crystals (PCs) and novel 3D MEMS structures to trap and isolate vibration energy away from the resonator anchors, optimization of resonator geometry to suppress thermoelastic dissipation, and analysis of required levels of surface polish to reduce surface dissipation. Phononic crystals can be used to manipulate the properties of materials. In the case of the 1D PC linear acoustic bandgap (LAB) structures developed here, this manipulation arises from the formation of frequency stop bands, or bandgapwhich convert silicon from a material capable of supporting acoustic waves to a material which rejects acoustic propagation at frequencies in the bandgap. The careful design of these LAB structures is demonstrated to be able to enhance the quality factor and insertion loss of MEMS resonators without significant detrimental effects on the overall device performance.

In this thesis, the design, fabrication and characterisation of graphene electromechanical resonators have been presented. Graphene features ultrahigh Young’s modulus and large surface to volume ratio that make it ideal for radio frequency (RF) components, sensors and other micro/nano-electromechanical systems (MEMS/NEMS). A novel batch fabrication process for graphene electromechanical resonators has been developed by using poly-Si film as sacrificial layer. Previously reported fabrication processes of graphene resonators use SiO2 as sacrificial layer only because graphene is visible on 300nm SiO2/Si substrate. However, the wet etching of SiO2 involves HF, which is not compatible with metal connections or SiO2 serving as dielectric or passivation layer in graphene NEMS devices. Moreover, the liquid surface tension during drying after wet etching could damage graphene bridges even critical point drying is used. Therefore, in this work, poly-Si is adopted as the sacrificial material. To facilitate the fabrication of graphene resonators, a poly-Si/SiO2/Si substrate has been designed and optimised to make graphene visible under optical microscope for the first time to the author’s knowledge. Chemical vapour deposition (CVD)-grown monolayer graphene sheet has been transferred onto the optimised poly-Si/SiO2/Si substrate and patterned into strips. Metal electrodes have been deposited by lift-off process to make electrical connections, which is prerequisite for integrating graphene resonator into practical devices. The graphene bridges have been released by etching the poly-Si layer with XeF2 in vapour phase, which completely avoids the capillary force induced damage to the graphene bridges. De-fluorination process has been performed by hydrazine reduction to recover graphene’s conductivity. This fabrication process is scalable for massive production of graphene electromechanical resonators, thus furthering their practical application. One-source current mixing characterisation setup has been constructed to test the graphene resonators. Besides the fundamental peak, the activation and enhancement of the second mode of doubly clamped resonator by electrostatic actuation have been observed for the first time. The second mode amplitude reaches 95% of the fundamental mode, whereas only odd higher modes of small intensity have been reported before in other MEMS/NEMS resonators actuated by electrostatic force or magnetomotive force. The findings in this thesis could lead to substantial increase of the sensitivity of sensors based on the graphene resonators. Modal analysis based on Euler-Bernoulli equation has been performed to understand the mechanism behind the activation and enhancement of the second mode. Finite element analysis agrees very well with experimental results and complies with the theoretical model. Finally, a set of novel alignment marks has been designed, which can be incorporated to process mechanically exfoliated 2D material flakes of micron size and irregular shape with conventional photolithography tools, as have been demonstrated by the successful fabrication of a graphene transistor. This optical alignment technique provides an alternative for prototype device development besides electron beam lithography to prevent electron-induced damage to fragile 2D materials.

This thesis investigated single nanoparticle/molecule detections using a whispering gallery mode (WGM) microcavity, with focuses on sensing with the cavity optomechanical oscillation (OMO). The high quality (Q) factor and small mode volume properties of a WGM microcavity make it possible to establish a strong intracavity power density with a small amount of input optical power. Such a high optical power density exerts a radiation pressure that is sufficient to push the cavity wall moving outward. The dynamic interaction between the optical field and the mechanical motion eventually results in a regenerative mechanical oscillation of the WGM cavity, which is termed as the optomechanical oscillation. With a high Q spherical microcavity, the observation of OMO in heavy water is reported. To the best knowledge of the author, this is the first demonstration of the cavity OMO in an aqueous environment. Furthermore, by utilizing the properties of reactive sensing, cavity OMO, and optical spring effect, we demonstrated a new sensing mechanism that improves the WGM microcavity sensing resolution by several orders of magnitude. Finally, we conducted the demonstration of in-vitro molecule sensing by detecting single bindings of the 66 kDa Bovine Serum Albumin (BSA) protein molecules at a signal-to-noise ratio of 16.8. / Graduate

This thesis presents a detailed study of chip based silicon photonic waveguide technologies for chemical sensing applications. The project specifically focuses on the use of strip and slot waveguide based micro-ring resonators (MRRs) integrated with graphene and graphene oxide (GO) as potential functional sensor coatings. The primary objective is to understand the effect of graphene/GO on the optical properties of such a device, to assess performance in bio-/chemical sensing applications and to identify ways in which such a device may be optimised. A detailed analysis of how the MRR cavity optical extinction ratio (ER) varies with the interaction length of surface integrated graphene reveals, for the first time using this technique, the in-plane graphene linear absorption coefficient, αgTE = 0.11 ± 0.01dBμm⁻¹. A model of the MRR cavity optical losses for different graphene lengths and heights (above the waveguide surface) provides a predictive capability for the design rules of optimised performance in sensing and photo-detector based applications. The graphene integrated MRRs were also characterized by a Raman mapping technique from which careful analysis of the graphene G and 2D scattering peak frequencies and relative intensities revealed that the graphene is electrically intrinsic where it is suspended over the MRR yet moderately hole-doped where it sits on top of the waveguide structure. This 'pinning' of the graphene Fermi level at the graphene-silicon/SiO2 interface is the result of 'trapped' ad-charges, the concentration of which may be increased at dangling bond sites after relatively aggressive (O2 plasma) cleaning of the silicon/SiO2 surface prior to graphene transfer. Quantifying this substrate doping effect is critically important when attempting to determine graphene's optical properties and should be taken into account when designing graphene-silicon hetero-structures for opto-electronic devices. The large absorption coefficient determined for the graphene integrated MRR devices means that cavity losses are far too high for practical realisation of refractive index based sensing. However, an alternative approach using GO as the functional layer for improved MRR based refractive index sensors remains a possibility on account of the much lower transmission loss. GO also has distinct advantages over graphene; ease of integration, a high density of surface functional groups and micro-porosity. Transmission spectral analyses of both bare (uncoated) MRRs and those coated with different GO concentrations revealed the in-plane linear absorption coefficient for the GO film to be αGOTE = 0.027±0.02dBμm⁻¹, which is much lower than that for graphene. Construction of a gas cell and integrated 'bubbler' arrangement for delivering variable vapour concentrations to the graphene/GO integrated MRR devices under test is presented. Both bare and GO coated MRRs were exposed to vapours from a series of typical organic solvents; ethanol, pentene and acetone delivered by a carrier gas (N2). Dynamic optical tracking of the MRR cavity resonance wavelength during vapour exposure, at different flow rates (vapour concentrations) reveals the sensitivity of the device(s) to small changes in refractive index. The dynamic response of the GO coated MRRs to the vapours were up to three times faster than the uncoated MRR with similar improvements in sensitivity and limit of detection, largely attributable to the porous nature and molecular binding affinity of the GO. Critically, these experiments reveal that the detection sensitivity and response of the GO is solvent dependent, which may mean that it is capable of providing a degree of selectivity, which is normally difficult to achieve in refractive index based gas sensing.

Thesis (Ph. D.)--West Virginia University, 2008. / Title from document title page. Document formatted into pages; contains xi, 105 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 48-51).

published_or_final_version / Electrical Engineering / Master / Master of Philosophy