FABRICATION METHODOLOGIES FOR INTEGRATED PHOTONIC DEVICES IN LITHIUM NIOBATE

Ph.DDOCTOR OF PHILOSOPH

Study of organic molecules and nano-particle/polymer composites for flash memory and switch applications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 205-218).Organic materials exhibit fascinating optical and electronic properties which motivate their hybridization with traditional silicon-based electronics in order to achieve novel functionalities and address scaling challenges of these devices. The application of organic molecules and nano-particle/polymer composites for flash memory and switch applications is studied in this dissertation. Facilitating data storage on individual small molecules as the approach the limits in miniaturization for ultra-high density and low power consumption media may enable orders of magnitude increase in data storage capabilities. A floating gate consisting of a thin film of molecules would provide the advantage of a uniform set of identical nano-structured charge storage elements with high molecular area densities which can result in a several-fold higher density of charge-storage sites as compared to quantum dot (QD) memory and even SONOS devices. Additionally, the discrete charge storage in such nano-segmented floating gate designs limits the impact of any tunnel oxide defects to the charge stored in the proximity of the defect site. The charge retention properties of molecular films was investigated in this dissertation by injecting charges via a biased conductive atomic force microscopy (AFM) tip into molecules comprising the thin films. The Kelvin force microscopy (KFM) results revealed minimal changes in the spatial extent of the charge trapping over time after initial injection. Fabricated memory capacitors show a device durability over 105 program/erase cycles and hysteresis window of up to 12.8 V, corresponding to stored charge densities as high as 5.4x 1013 cm-2, suggesting the potential use of organic molecules in high storage capacity memory cells. Also, these results demonstrate that charge storage properties of the molecular trapping layer can be engineered by rearranging molecules and their a-orbital overlaps via addition of dopant molecules. Finally, the design, fabrication, testing and evaluation of a MEMS switch that employs viscoelastic organic polymers doped with nano-particles as the active material is presented in this dissertation. The conductivity of the nano-composite changes 10,000-fold as it is mechanically compressed. In this demonstration the compressive squeeze is applied with electric actuation. Since squeezing initiates the switching behavior, the device is referred to as a "squitch". The squitch is essentially a new type of FET that is compatible with large area processing with printing or photolithography, on rigid or flexible substrates and can exhibit large on-to-off conduction ratio.by Sarah Paydavosi.Ph.D

Silver Filament Formation/Dissolution Dynamics Through a Polymer/Ionic liquid Composite by Direct-write

A direct-write, electrochemical approach to the formation and dissolution of silver nanofilaments is demonstrated through a novel polymer electrolyte consisting of a UV-crosslinkable polymer, polyethylene glycol diacrylate (PEGDA) and an ionic liquid (IL), 1-butyl-3-methylimadozolium hexafluorophosphate ([BMIM]PF6). Nanofilaments are formed and dissolved at pre-programmed locations with a conductive atomic force microscope (c-AFM) using a custom script. Although the formation time generally decreases with increasing bias from 0.7 to 3.0 V, an unexpected non-monotonic maximum is observed ~2.0 V. At voltages approaching this region of inverted kinetics, IL electric double layers (EDLs) become detectable; thus, the increased nanofilament formation time can be attributed to electric field screening, which hinders silver electromigration and deposition. Scanning electron microscopy confirms that nanofilaments formed in this inverted region have significantly more lateral and diffuse features. Time dependent formation currents reveal two types of nanofilament growth dynamics: abrupt, where the resistance decreases sharply over as little as a few ms, and gradual where it decreases more slowly over hundreds of ms. Whether the resistance change is abrupt or gradual depends on the extent to which the EDL screens the electric field. Silver nanofilaments with gradual growth dynamics have potential application in neuromorphic computing. In this study, a linear (R2 > 0.9) dependence of conductance on the number of bias pulses is demonstrated—a signature feature that is required for neuromorphic application. Hundreds of distinguishable conductance states ranging from 235 to 260 microsimens can be accessed using a low read bias. These results show that novel PEGDA/IL composite electrolyte enables the gradual formation and dissolution of silver nanofilament with tunable conductance states, making it a promising candidate to advance neuromorphic applications

ENABLING HARDWARE TECHNOLOGIES FOR AUTONOMY IN TINY ROBOTS: CONTROL, INTEGRATION, ACTUATION

The last two decades have seen many exciting examples of tiny robots from a few cm3 to less than one cm3. Although individually limited, a large group of these robots has the potential to work cooperatively and accomplish complex tasks. Two examples from nature that exhibit this type of cooperation are ant and bee colonies. They have the potential to assist in applications like search and rescue, military scouting, infrastructure and equipment monitoring, nano-manufacture, and possibly medicine. Most of these applications require the high level of autonomy that has been demonstrated by large robotic platforms, such as the iRobot and Honda ASIMO. However, when robot size shrinks down, current approaches to achieve the necessary functions are no longer valid. This work focused on challenges associated with the electronics and fabrication. We addressed three major technical hurdles inherent to current approaches: 1) difficulty of compact integration; 2) need for real-time and power-efficient computations; 3) unavailability of commercial tiny actuators and motion mechanisms. The aim of this work was to provide enabling hardware technologies to achieve autonomy in tiny robots. We proposed a decentralized application-specific integrated circuit (ASIC) where each component is responsible for its own operation and autonomy to the greatest extent possible. The ASIC consists of electronics modules for the fundamental functions required to fulfill the desired autonomy: actuation, control, power supply, and sensing. The actuators and mechanisms could potentially be post-fabricated on the ASIC directly. This design makes for a modular architecture. The following components were shown to work in physical implementations or simulations: 1) a tunable motion controller for ultralow frequency actuation; 2) a nonvolatile memory and programming circuit to achieve automatic and one-time programming; 3) a high-voltage circuit with the highest reported breakdown voltage in standard 0.5 μm CMOS; 4) thermal actuators fabricated using CMOS compatible process; 5) a low-power mixed-signal computational architecture for robotic dynamics simulator; 6) a frequency-boost technique to achieve low jitter in ring oscillators. These contributions will be generally enabling for other systems with strict size and power constraints such as wireless sensor nodes

Exploring the physics of 3D magnetic nanowires and 3D artificial spin-ice lattices

Three-dimensional magnetic nanostructures have become subject to recent intense interest due to the availability of new fabrication techniques. 3D nanostructured materials provide access to a host of new phenomena such as novel spin textures established by exotic 3D geometries and curvature, ultrafast domain walls beating the Walker limit, controlled spin-wave emission, and a plethora of technological applications. Two-photon lithography (TPL) is a powerful tool facilitating the fabrication of 3D magnetic nanostructures, as has been demonstrated in recent work realizing a 3D lattice of nanowires arranged in a diamond bond structure. Initial experimental work has shown that this technique, when combined with thermal evaporation, can be used to produce 3D artificial spin-ice (3DASI) systems. After providing a background to the relevant physics and experimental techniques, chapter 4 outlines a detailed micromagnetic study of key geometries that make up the experimental 3DASI lattice. This study provides a detailed understanding of switching in individual wires, coordination-two bipod structures present on the surface and coordination-four tetrapod structures present within the bulk. These studies provide a deeper understanding of measurements performed upon the system. TPL with line-of-sight (LOS) deposition results in magnetic nanowires with a crescent-shaped cross-section where non-uniform thickness and curvature leads to novel switching mechanisms and perturbs domain wall structure. Simulations show that the individual wires in the lattice are Ising-like, single domain with sharp reversals between two well-defined states. The crescent-shaped cross-section perturbs domain wall structure and introduces novel edge states impacting switching. The Ising-like condition continues to hold in more complex geometries comprising these wires. Simulations exploring the switching in single wires, bipod systems, and tetrapod systems are explored and compared to experimental optical magnetometry. Every permutation of magnetization within coordination-two and coordination-four vertices are simulated to obtain spin textures, energies, and magnetic surface charge density of conventional artificial spin-ice vertex types. The energies of ice-rule states are found to be almost degenerate, and high energy singly charged monopole states are shown to be stable. Doubly charged monopole states are not stable within the simulation geometries. Computed magnetic surface charge density aids the identification of vertex types measured using magnetic force microscopy, enabling the identification of magnetic charges propagating through the lattice. The energy associated with a monopole excitation upon the surface coordination-two vertices of the 3DASI is shown to be a factor of ~3 higher than an excitation in the coordination-four vertices of the bulk. The utilisation of the calculated energies within Monte Carlo simulations performed by collaborators allowed a reasonable agreement to be obtained with experimental results. Despite the success of TPL and LOS deposition as a tool for magnetic nanostructure fabrication, a limitation in the methodology comes from a thin film of the functional material being deposited on the substrate. In the case of magnetic materials, the substrate film may interact with the functional components and unwanted signals in measurements using MOKE or other techniques where relatively large spot sizes capture background film. The presence of substrate film is then a limiting factor in the types of structures that may be fabricated using TPL and studied. Chapter 5 explores a modification to the TPL fabrication procedure to include a poly(acrylic acid) sacrificial layer compatible with TPL and laser ablation to create a process that removes the substrate film. The novel sacrificial layer process is used to produce isolated magnetic nanowires with no detectable material upon the substrate. MOKE measurements upon a simple nanowire show hysteresis loops with a sharp transition at 9.9 mT, the introduction of a large nucleation pad reduces the wire switching field to 1.63 mT, demonstrating controlled domain wall injection into the nanowire. We present a proof-of-principle of using 3D nanostructuring to introduce out-of-plane perturbations to control domain wall motion in the wires. Finite difference simulations elucidate the pinning mechanism at the proposed perturbation, and MOKE magnetometry suggests a 3mT pinning field. The validity of the pinning measurements is discussed. MOKE measurements performed on 3DASI lattices fabricated using the sacrificial layer approach show that previously detected low-field features due to background film were eliminated; this shows potential for MOKE as a technique to obtain depth-dependent switching information from our lattice. In particular, it is shown that the experimental parameters associated with MOKE, such as polarization and analyzer angle, can be used to help elucidate switching taking place upon the different sublattices. The successful implementation of a sacrificial layer enables the use of TPL with line-of-sight deposition to produce a wide variety of interesting 3D geometries that have been explored within the literature. Examples include gaussian surfaces, which stabilize skyrmions and other topological spin textures. Experimental feasibility of domain wall injection into a 3D nanowire system opens the possibility of realizing more complex domain wall circuits, approaching racetrack like devices. Finally, the work upon sacrificial layers and depth-dependent switching also has important implications for the study of 3DASI systems. Soon the group intends to study thermal systems. Here the removal of the sheet film will eliminate possible spurious signals from the substrate, whilst depth-dependent switching will allow the dynamic route to ground state to be studied

Resistance switching devices based on amorphous insulator-metal thin films

Nanometallic devices based on amorphous insulator-metal thin films are developed to provide a novel non-volatile resistance-switching random-access memory (RRAM). In these devices, data recording is controlled by a bipolar voltage, which tunes electron localization length, thus resistivity, through electron trapping/detrapping. The low-resistance state is a metallic state while the high-resistance state is an insulating state, as established by conductivity studies from 2K to 300K. The material is exemplified by a Si3N4 thin film with randomly dispersed Pt or Cr. It has been extended to other materials, spanning a large library of oxide and nitride insulator films, dispersed with transition and main-group metal atoms. Nanometallic RRAMs have superior properties that set them apart from other RRAMs. The critical switching voltage is independent of the film thickness/device area/temperature/switching speed. Trapped electrons are relaxed by electron-phonon interaction, adding stability which enables long-term memory retention. As electron-phonon interaction is mechanically altered, trapped electron can be destabilized, and sub-picosecond switching has been demonstrated using an electromagnetically generated stress pulse. AC impedance spectroscopy confirms the resistance state is spatially uniform, providing a capacitance that linearly scales with area and inversely scales with thickness. The spatial uniformity is also manifested in outstanding uniformity of switching properties. Device degradation, due to moisture, electrode oxidation and dielectrophoresis, is minimal when dense thin films are used or when a hermetic seal is provided. The potential for low power operation, multi-bit storage and complementary stacking have been demonstrated in various RRAM configurations.Comment: 523 pages, 215 figures, 10 chapter

Electroosmotic Soft Actuators

This dissertation details the research involved in creating the first paper-based soft actuator driven by electroosmosis. To accomplish this, research breakthroughs were made in the fields of electrokinetic pumping and device manufacturing using soft materials. Electroosmosis is an electrically induced microfluidic flow phenomenon. When an electric field is applied to the fluid, across the microchannels, electroosmotic flow occurs in the direction of the applied electric field. In this work, liquid was electroosmotically displaced within a flexible microfluidic device to actuate an elastomeric membrane. The goal of this work was to create a fully sealed fluidic actuator. It was therefore necessary to encapsulate the pumping fluid within the device, and to maximize pressure it was necessary to eliminate compliance caused by trapped gases. Electrolytic gas formation is well known to disrupt pumping in DC electroosmotic systems that use water as the pumping liquid. In this work, electrolysis was eliminated by replacing water with propylene carbonate (PC): PC was determined to be electrochemically stable up to at least 10 kV, in the absence of moisture or salt contaminants. Bubble-free electroosmotic pumping with PC was achieved within sealed miniature actuators, which could be continuously operated for at least one hour. Benchtop fabrication techniques were developed to build encapsulated fluidic actuators composed entirely of soft, flexible materials. Stretchable electrochemically stable electrodes were made using a conductive paint made by mixing carbon nanoparticles into a silicone base. High-density microchannel networks were incorporated by using paper and other flexible porous materials, instead of conventional planar replica-molded microchannels. The device was filled with pumping fluid without the use of external tubing, and then encapsulated by casting a film of elastomer over the filled reservoir to form the actuating membrane. The resulting actuators were flexible and stretchable, demonstrating significant membrane deformations (hundreds of micrometers) within seconds of applying the electric field and ability to lift large loads (tens of grams). These polymeric electroosmotic actuators are unique among electroactive polymer actuators because they are able to simultaneously generate high force as well as large stroke. It is envisioned that this research will pave the way for the creation of artificial muscles and smart shape-changing materials that can be actuated by electroosmosis

Nanocellulose and Nanocarbons Based Hybrid Materials

This highly informative and carefully presented book discusses the preparation, processing, characterization and applications of different types of hybrid nanomaterials based on nanocellulose and/or nanocarbons. It gives an overview of recent advances of outstanding classes of hybrid materials applied in the fields of physics, chemistry, biology, medicine, and materials science, among others. The content of this book is relevant to researchers in academia and industry professionals working on the development of advanced hybrid nanomaterials and their applications

A low cost direct writing process for flexible circuit and interconnect fabrication

This thesis investigates the development of a low cost fabrication process for flexible electronics and interconnects. By using a ‘direct writing’ process, the use of vacuum-­‐based metal evaporation and photoresist steps is not necessary and so less complex equipment is needed. The process forms silver embedded on top of a polyimide substrate and was first tested using a UV laser to perform writing before switching to a blue laser due to excessive substrate degradation observed from UV exposures. The blue light was combined with a biologically friendly photo reducing agent, which was found to be much more efficient at the creation of silver. The methods of silver formation by various means are the main focus of investigation in this thesis but process expansion and improvement were the main goals. To this end, a chemical, rather than light-­‐based, process for silver creation was found to produce more consistent silver coatings, however the patterning by this method was found to be more challenging. The process was also extended to a different substrate in polyetherimide