Fabrication Optimisation of Metal-Oxide-Metal Diodes

This thesis is based on the design, research and development of devices required to successfully recover waste heat and convert it into electrical power through the use of Microsystems Technology. This takes place using optical nano-antennas, in the same way a radio antenna picks up a radio station. The main aim of this project is the rectification of this signal into a useful DC voltage. Here we have used high frequency metal-oxide-metal (MOM) diodes, which involve the use of two dissimilar metals separated by a native oxide. In order to make successful MOM diodes, the following must be considered: maximise the work function difference between the metals for asymmetry in I-V characteristics, produce a uniform oxide layer that is sufficiently thin (a few nm) for electron tunnelling to occur and reduce the diode size to sub-micron dimensions to increase the cut-off frequency. Currently the diodes consist of titanium, titanium oxide and platinum, which provides a high enough work function difference that the I-V characteristics show significant asymmetry and figure of merit values are among the best published. It has been found, using ToFSIMS and TEM analysis of the oxide, that the thickness of the oxide can be controlled between 1 nm and 7 nm using RIE etching and subsequent oxygen plasma regrowth. Different oxides have been fabricated with different stoichiometries depending on the process used. Furnace oxidation grows a complex oxide in the range 6.9 to 7.6 nm thick. By contrast a more simple oxide can be produced using a controlled reactive ion etch and subsequent plasma oxidation, with thicknesses in the range 1 to 6 nm. The final significant issue involves the cross-sectional area of the diodes, which also determines their cut-off frequency. Extrapolation of existing diode results suggests that, if made sufficiently small, they would function at high enough frequencies for rectification of radiation in the terahertz regime. Furthermore, phase shift lithography has been used to demonstrate 200 to 400nm lines in diode features, with alternative possible high scale processes discussed for future fabrication

Embroidered Rectangular Split-Ring Resonators for the Characterization of Dielectric Materials

In this paper, we report an embroidered rectangular split-ring resonator (SRR) operating at S band for material characterization based on the differences in dielectric parameters. We designed, fabricated and characterized SRR sensors on a conventional fabric that can be conformally attached over the surface of samples under investigation. The structures are made of conductive threads and can be embroidered on any dielectric fabric at low cost using conventional embroidery methods. We have demonstrated material characterization capability of the sensors using a specific design with a length of 60 mm and a width of 30 mm. We wrapped the sensors on low-density polyethylene (LDPE) bottles filled with deionized (DI) water and common solvents (ethanol, methanol, isopropanol and acetone) in our experiments. We measured the nominal resonant frequency of a specific sensor wrapped around an empty bottle as 2.07 GHz. The shifts in resonant frequencies when the bottle was filled with the solvents follow the dielectric constants of the solvents

High-Performance Atomic Layer Deposited Al2O3 Insulator Based Metal-Insulator-Metal Diode

The fabrication of metal–insulator– metal (MIM) diode using an ultrathin Al2O3 insulator layer, deposited using atomic layer deposition (ALD) is presented. The Al2O3 insulating layer was found to be highly uniform throughout the diode junction, effectively overcoming the main fabrication challenge in MIM diodes. The diodes exhibit strong non-linear current–voltage curves, have a typical zero-bias curvature coefficient of 5.4 V−1 and a zero-bias resistance of approximately 118 kΩ, a value considerably smaller than other MIM diode topologies and that allows more current to be rectified. Other results including current ratio and yield of the diode also competes favorably with the state-of-the-art MIM diodes such as the recently produced metal-octadecyltrichlorosilane (OTS)-metal structure

Integrating microfluidics and biosensing on a single flexible acoustic device using hybrid modes

Integration of microfluidics and biosensing functionalities on a single device holds promise in continuous health monitoring and disease diagnosis for point-of-care applications. However, the required functions of fluid handling and biomolecular sensing usually arise from different actuation mechanisms. In this work, we demonstrate that a single acoustofluidic device, based on a flexible thin film platform, is able to generate hybrid waves modes, which can be used for fluidic actuation (Lamb waves) and biosensing (thickness shear waves). On this integrated platform, we show multiple and sequential functions of mixing, transport and disposal of liquid volumes using Lamb waves, whilst the thickness bulk shear waves allow us to sense the chemotherapeutic Imatinib, using an aptamer-based strategy, as would be required for therapy monitoring. Upon binding, the conformation of the aptamer results in a change in coupled mass, which has been detected. This platform architecture has the potential to generate a wide range of simple sample-to-answer biosensing acoustofluidic devices

Low friction droplet transportation on a substrate with a selective Leidenfrost effect

An energy saving Leidenfrost levitation method is introduced to transport micro-droplets with virtually frictionless contact between the liquid and solid substrate. By micro-engineering the heating units, selective areas of the whole substrate can be electro-thermally activated. A droplet can be levitated as a result of the Leidenfrost effect, and further transported when the substrate is tilted slightly. The selective electro-heating produces a uniform temperature distribution on the heating units within 1 s, in response to a triggering voltage. Alongside these experimental observations, finite element simulations are conducted to understand the temperature profile of the selective heated substrate, and also generate phase diagrams to verify the Leidenfrost regime for different substrate materials. Finally, we demonstrate the possibility of controlling low friction high speed droplet transportation (~ 65 mm/s) when the substrate is tilted (~ 7 °) by structurally designing the substrate. This work establishes the basis for an entirely new approach to droplet microfluidics

Double-sided slippery liquid-infused porous materials using conformable mesh

Often wetting is considered from the perspective of a single surface of a rigid substrate and its topographical properties such as roughness or texture. However, many substrates, such as membranes and meshes, have two useful surfaces. Such flexible substrates also offer the potential to be formed into structures with either a double-sided surface (e.g. by joining the ends of a mesh as a tape) or a single-sided surface (e.g. by ends with a half-twist). When a substrate possesses holes, it is also possible to consider how the spaces in the substrate may be connected or disconnected. This combination of flexibility, holes and connectedness can therefore be used to introduce topological concepts, which are distinct from simple topography. Here, we present a method to create a Slippery Liquid-Infused Porous Surface (SLIPS) coating on flexible conformable doubled-sided meshes and for coating complex geometries. By considering the flexibility and connectedness of a mesh with the surface properties of SLIPS, we show it is possible to create double-sided SLIPS materials with high droplet mobility and droplet control on both faces. We also exemplify the importance of flexibility using a mesh-based SLIPS pipe capable of withstanding laminar and turbulent flows for 180 and 90 minutes, respectively. Finally, we discuss how ideas of topology introduced by the SLIPS mesh might be extended to create completely new types of SLIPS systems, such as Mobius strips and auxetic metamaterials

Planar selective Leidenfrost propulsion without physically structured substrates or walls

The Leidenfrost effect allows droplets to be transported on a virtually frictionless layer of vapor above a superheated substrate. The substrates are normally topographically structured using subtractive techniques to produce saw-tooth, herringbone, and other patterns and bulk heated, leading to significant challenges in energy consumption and controlled operation. Here, we propose a planar lithographic approach to levitate and propel droplets using temperature profiles, which can be spatially patterned and controlled in time. We show that micro-patterned electrodes can be heated and provide control of the pressure profile and the vapor flow. Using these almost featureless planar substrates, we achieve self-directed motion of droplets, with velocities of approximately 30 mms−1, without topographically structuring the substrate or introducing physical walls. Our approach has the potential to be integrated into applications, such as digital microfluidics, where frictionless and contactless droplet transport may be advantageous