Building a novel nanofabrication system using MEMS

Micro-electromechanical systems (MEMS) are electrically controlled micro-machines which have been widely used in both industrial applications and scientific research. This technology allows us to use macro-machines to build micro-machines (MEMS) and then use micro-machines to fabricate even smaller structures, namely nano-structures. In this thesis, the concept of Fab on a Chip will be discussed where we construct a palette of MEMS-based micron scale tools including lithography tools, novel atomic deposition sources, atomic mass sensors, thermometers, heaters, shutters and interconnect technologies that allow us to precisely fabricate nanoscale structures and conduct in-situ measurements using these micron scale devices. Such MEMS devices form a novel microscopic nanofabrication system that can be integrated into a single silicon chip. Due to the small dimension of MEMS, fabrication specifications including heat generation, patterning resolution and film deposition precision outperform traditional fabrication in many ways. It will be shown that one gains many advantages by doing nano-lithography and physical vapor deposition at the micron scale. As an application, I will showcase the power of the technique by discussing how we use Fab on a Chip to conduct quench condensation of superconducting Pb thin films where we are able to gently place atoms upon a surface, creating a uniform, disordered amorphous film and precisely tune the superconducting properties. This shows how the new set of techniques for nanofabrication will open up an unexplored regime for the study of the physics of devices and structures with small numbers of atoms

Energy dissipation in hollow beam resonators in function of microfluidic channel off-axis placement

Suspended Microchannel Resonators (SMRs) are hollow resonant structures containing an embedded u-shaped micro fluidic channel. Having the sample fluid confined inside the resonator allows for real time detection of liquid compounds while the device quality factor remains almost unaffected. Theoretical studies and experimental results have shown that these devices present a nonmonotonic energy dissipation as the fluid viscosity is increased or decreased. This is in contrast with conventional cantilevers immersed in fluid, where the quality factor monotonically decreases as the fluid viscosity increases. In the simplest configuration, the channel contained inside the cantilever is perfectly centered about the neutral axis of the resonant structure. Analytical models have shown variation of the device quality factor by several orders of magnitude when the microfluidic channel is placed out of the beam neutral axis. This project aims to experimentally demonstrate the relation between SMRs’ energy dissipation and off-axis placement of the microfluidic channel, in function of fluidic properties as compressibility and dynamic viscosity. The student’s tasks are the following: • Review and understand the state of the art about theoretical studies concerning energy dissipation of hollow beam resonators; - Propose an experimental plan presenting interesting geometries and fluidic properties to be tested; - Design and fabricate samples to be tested; - Optimize an experimental setup; - Collect experimental results and compare with theoretical studies found in literature

Development of predictable palladium based optomechanical hydrogen sensors

In addition to its use as an energy storage medium or fuel, hydrogen gas has a variety of commercial applications such as methanol and ammonia production. Given the volatility and flammability of hydrogen, as well as its small molecular size, fast and accurate sensors capable of operating in a variety of environments are necessary. A large subset of hydrogen gas sensors rely on palladium metal, which is known to reversibly react with hydrogen to form palladium hydride. This results in a change in the optical, electrical and mechanical properties of the film. These changes are a result of a change the Fermi level and band structure of the metal, as well as an increase in lattice constant in the presence of hydrogen. The change in complex refractive index plays a role in both reflection/transmission, and for determining resonances or guided modes in waveguides and other sub-wavelength features. However, the increase in the lattice constant of the metal, a process called hydrogen induced lattice expansion, was found to be equally important in modeling the response of the sensors, both from an optical and a mechanical perspective. This dissertation is concerned with the simulation, fabrication, and testing of palladium based optomechanical sensors, particularly to elucidate the role of hydrogen induced lattice expansion in their design and functionality. Two specific sensor designs: a nano-aperture based sensor and a cantilever based sensor were designed, fabricated, characterized, and modeled. The first sensor developed was based on a single nano-aperture etched into a palladium coated fiber facet. Designed to operate based on the principle of extraordinary transmission and the change in optical constants of the palladium, this sensor showed experimental sensitivity down to 150ppm in transmission and 50 ppm reflection. However, without inclusion of the mechanical effects, the device behavior was unpredictable. Separate work was thus carried out to characterize lattice expansion in thin palladium films using quantitative phase imaging techniques, so that a new sensor could be designed and accurately modelled. This second fabricated sensor consisted of a Pd coated cantilever which operated based on optical probing of mechanical deflections. For more thorough characterization, the cantilever was measured using the same phase imaging techniques. The results of this analysis further improved the understanding of thin film expansion and the capabilities of diffraction phase microscopy for material analysis. Furthermore, this culminated in the fabrication of a sensitive and reliable optomechanical hydrogen sensor whose response matched theory

Optical and Mechanical Studies of Semiconductor Resonators

This thesis concerns the investigation of micromechanical resonators formed either from group III-V semiconductors, or two-dimensional (2D) transition metal dichalcogenides (TMDs). The work is motivated by furthering the understanding of nonlinear resonator dynamics at the micron-scale, and, separately, the possibility of coupling the mechanics to an embedded quantum emitter. The latter is of particular interest for sensing applications using micromechanical resonators. First, gallium arsenide (GaAs) nanowires (NWs) are grown with cross-sectional dimensions of varied elongation, and the effects of the elongation on the resonator dynamics are studied. The single-mode dynamics of the NWs are found to agree with predictions made using Euler-Bernoulli (EB) beam theory. The NWs are then driven into the large amplitude regime of motion, and the nonlinear response is used to estimate the cubic Duffing nonlinearity. The nonlinear response of NWs gives rise to coupled mode dynamics. In the coupled mode regime, a quadratic dependence between the change in the fundamental (and second order) mode frequencies on the drive amplitude of the coupled mode is observed. Depending on the NW elongation, and which flexural modes are driven, a reversal in the direction of the frequency change is observed. This response is explained using the coupled, nonlinear Duffing equations of motion. Strain coupling between the mechanical motion of a GaAs cantilever and the emission properties of an embedded indium arsenide (InAs) quantum dot (QD) is then investigated. The cantilever is driven at the fundamental resonance frequency, and the effect of the cantilever motion on the QD emission energy is evaluated. The QD emission energy is modulated at the cantilever’s resonance frequency via the deformation potential, and is used to estimate the QD-cantilever optomechanical coupling rate. Computational modelling is used to predict the strain fields within the cantilever, and therefore estimate the optomechanical coupling rate. This is found to be in good agreement with predictions made from the experimental observations. This research is working towards the realisation of strain-induced sensing applications using micromechanical resonators formed from III-V semiconductors. Next, GaAs cantilevers, similar to those studied for the previous strain tuning application, are integrated with a one-dimensional (1D) photonic crystal cavity (PhCC), and a 1D perturbing PhC structure. The PhCC acts as an on-chip spectral filter or cavity for enhancement of the QD emission. In this system, displacement of the cantilever results in an out-of-plane separation between the PhCC and the perturbing PhC structure, which can be used to tune the PhCC mode resonance indirectly. Here, indirect tuning of the PhCC resonance is attempted through electrostatic actuation of the cantilever. Computational modelling is carried out to predict the optical response of the PhCC in response to the out-of-plane separation of the perturbing PhC structure, and the technological challenges involved with fabricating the structures are outlined. This research has applications in on-chip integrated quantum optical circuits. Finally, monolayer tungsten diselenide (WSe2) integrated within an optically and electrically active van der Waals heterostructure is studied, with specific focus given to the emission properties of embedded single defect emitters (SDEs). Electrical tuning of the SDEs is demonstrated, which has promising applications for quantum information processing (QIP). Observations of SDEs in monolayer TMDs motivated the study of the mechanical properties of suspended molybdenum diselenide (MoSe2) monolayer resonators, which could be used as mechanical strain sensors. The resonators are electrostatically driven by applying a bias to the suspended structures with time varying (AC) and constant voltage (DC) components. The initial tension within the monolayer is tuned by controlling DC bias, which in turn allows for tuning of the resonator’s resonance frequency. Then, the monolayers are driven into the large amplitude regime of motion (similar to previous demonstrations using GaAs NWs and cantilevers) and nonlinear motion is observed. These observations contribute to the fundamental understanding of the dynamical properties of TMD monolayer resonators

Nano-electro-mechanical systems fabricated by tip-based nanofabrication

This dissertation explores the use of a heated AFM tip for fabrication of NEMS devices. Two critical challenges hindering TBN from NEMS fabrication are addressed in this thesis. First, we experimentally found out that polystyrene nanopatterns deposited by a heated AFM tip can serve directly as etch mask and transfer the nanopatterns to solid-state materials such as silicon and silicon oxide through one step of etching, solving the first challenge for NEMS device fabrication using TBN; second, we developed a process that makes this TBN method seamlessly compatible with conventional nanofabrication processes. Polystyrene nanopatterns deposited can serve together with optical lithography patterned mask and transfer both micropatterns defined by optical lithography and nanopatterns defined by the heated AFM tip to silicon. After solving the two critical challenges, we demonstrated various types of silicon NEMS mechanical resonators such as single-clamped, double-clamped, wavy-shaped, spider-like and spiral-shaped using this TBN method with a heated AFM tip. Laser interferometer measurement on two NEMS resonators showed resonance frequencies of 1.2MHz and 2.2 MHz, close to the simulated resonance frequencies. Moreover, we demonstrated PDMS nanofluidic channels with arbitrary shapes using this TBN method with a heated AFM tip. Both ion conductance measurement and fluorescence measurement confirmed the functionality of the TBN-fabrication nanofluidic channels. Finally, we demonstrated a MESFET transistor using this TBN method with a heated AFM tip. MESFET devices with one, two, four and eight fins were fabricated, demonstrating the capability of this TBN method. I-V measurements proved the functionality of the transistor. This thesis work demonstrated that TBN with a heated AFM tip held great potential in nanodevice fabrication due to its simplicity, robustness, flexibility and compatibility with existing device nanofabrication process. For example, the whole TBN process takes place in ambient conditions and is very simple. And this TBN method is additive so that the heated AFM tip only deposits polymer where needed, thus only resulting in minimal contamination. Future work should improve the throughput and scalability to make this TBN method commercially available for NEMS fabrication

Strain engineering of graphene

The focus of this thesis is on using mechanical strain to tailor the electronic properties of graphene. The first half covers the electro-mechanical coupling for graphene in different configurations, namely a hexagonal Y-junction, various shaped bubbles on different substrates, and with kirigami cuts. For all of these cases, a novel combination of tight-binding electronic structure calculations and molecular dynamics is utilized to demonstrate how mechanical loading and deformation impacts the resulting electronic structure and transport. For the Y-junction, a quasi-uniform pseudo magnetic field induced by strain restricts transport to Landau-level and edge-state-assisted resonant tunneling. For the bubbles, the shape and the nature of the substrate emerge as decisive factors determining the effectiveness of the nanoscale pseudo magnetic field tailoring in graphene. Finally, for the kirigami, it is shown that the yield and fracture strains of graphene, a well-known brittle material, can be enhanced by a factor of more than three using the kirigami structure, while also leading to significant enhancements in the localized pseudo magnetic fields. The second part of the thesis focuses on dissipation mechanisms in graphene nanomechanical resonators. Thermalization in nonlinear systems is a central concept in statistical mechanics and has been extensively studied theoretically since the seminal work of Fermi, Pasta, and Ulam (FPU). Using molecular dynamics and continuum modeling of a ring-down setup, it is shown that thermalization due to nonlinear mode coupling intrinsically limits the quality factor of nanomechanical graphene drums and turns them into potential test beds for FPU physics. The relationship between thermalization rate, radius, temperature and prestrain is explored and investigated

Parametric Optimization of Visible Wavelength Gold Lattice Geometries for Improved Plasmon-Enhanced Fluorescence Spectroscopy

The exploitation of spectro-plasmonics will allow for innovations in optical instrumentation development and the realization of more efficient optical biodetection components. Biosensors have been shown to improve the overall quality of life through real-time detection of various antibody-antigen reactions, biomarkers, infectious diseases, pathogens, toxins, viruses, etc. has led to increased interest in the research and development of these devices. Further advancements in modern biosensor development will be realized through novel electrochemical, electromechanical, bioelectrical, and/or optical transduction methods aimed at reducing the size, cost, and limit of detection (LOD) of these sensor systems. One such method of optical transduction involves the exploitation of the plasmonic resonance of noble metal nanostructures. This thesis presents the optimization of the electric (E) field enhancement granted from localized surface plasmon resonance (LSPR) via parametric variation of periodic gold lattice geometries using finite difference time domain (FDTD) software. Comprehensive analyses of cylindrical, square, star, and triangular lattice feature geometries were performed to determine the largest surface E-field enhancement resulting from LSPR for reducing the LOD of plasmon-enhanced fluorescence (PEF). The design of an optical transducer engineered to yield peak E-field enhancement and, therefore, peak excitation enhancement of fluorescent labels would enable for improved emission enhancement of these labels. The methodology presented in this thesis details the optimization of plasmonic lattice geometries for improving current visible wavelength fluorescence spectroscopy

ADVANCED MEMS RESONATOR FOR MASS DETECTION AND MICROMECHANICAL TRANSISTOR

2010/2011Cantilever sensors have been the subject of growing attention in the last decades and their use as mass detectors proved with attogram sensitivity. The rush towards the detection of mass of few molecules pushed the development of more sensitive devices, which have been pursued mainly through downscaling of the cantilever-based devices. In the field of mass sensing, the performance of microcantilever sensors could be increased by using an array of mechanically coupled micro cantilevers of identical size. In this thesis, we propose three mechanically coupled identical cantilevers, having three localized frequency modes with well-defined symmetry. We measure the oscillation amplitudes of all three cantilevers. We use finite element analysis to investigate the coupling effect on the performance of the system, in particular its mass response. We fabricated prototype micron-sized devices, showing that the mass sensitivity of a triple coupled cantilever (TCC) system is comparable to that of a single resonator. Coupled cantilevers offer several advantages over single cantilevers, including less stringent vacuum requirements for operation, mass localization, insensitivity to surface stress and to distributed a-specific adsorption. We measure the known masses of silica beads of 1µm and 4µm in diameter using TCC. As it is difficult to obtain one single bead at the free end of the cantilevers, we choose to use the Focused Ion Beam. By sequential removing mass from one cantilever in precise sequence, we proved that TCC is also unaffected from a-specific adsorption as is, on the contrary, the case of single resonator. Finally, we proposed shown the use of TCC can be as micromechanical transistor device. We implemented an actuation strategy based on dielectric gradient force which enabled a separate actuation and control of oscillation amplitude, thus realizing a gating effect suitable to be applied for logic operation.XXIV Ciclo198