EEE5425 Introduction to Nanotechnology 1

NANOFABRICATION -3NOVEL PROCESSES

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Focused Ion Beam

A focused ion beam system (FIB) is a relatively new tool that has a high degree of analogy with a focused electron beam system such as a scanning electron microscope or a transmission electron microscope. In these systems the electron beam is directed towards the sample, and upon interaction it generates signals that are used to create high magnification images of the sample. The major difference with a focused ion beam system is the use of a different particle to create the primary beam that interacts with the sample. As the name FIB indicates, ions are used instead of electrons.

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Ions are positive, large, heavy and slow whereas electrons are negative, small, light and fast.The most important consequence of the properties listed above is that ion beams will remove atoms from the substrate and because the beam position, dwell time and size are so well controlled it can be applied to remove material locally in a highly controlled manner, down to the nanometer scale.

The choice of Ga+ ionsAs a source, Ga+ ions are used in a FIB for various reasons:•Low melting temperature and hence it is a very convenient material to construct a compact gun with limited heating.•A high brightness is obtained due to the surface potential, the flow properties of the Ga, the sharpness of the tip•The element Ga is nicely positioned in the center of the periodic table (element number 31) and its momentum transfer capability is optimal

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SE image made with ion beam ofcatalyst covered ceramic balls. The use of FIB allows any individual particle to be selected and analyzed for industrial quality control. In this case the FIB has machined a TEM sample of the top surface. Time to result: 60 minutes.

Ion beam deposited tungsten nano-wires for direct electrical measurements (4 point probe) of nano structures, in this case a carbon nanotube.

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Vapor-Liquid-Solid Method -1The vapor-liquid-solid method (VLS) is a mechanism for the growth of one-dimensional structures, such as nanowires, from chemical vapor deposition. Growth of a crystal through direct adsorption of a gas phase on to a solid surface is generally very slow. The VLS mechanism circumvents this by introducing a catalytic liquid alloy phase which can rapidly adsorb a vapor to supersaturation levels, and from which crystal growth can subsequently occur from nucleated seeds at the liquid-solid interface. The physical characteristics of nanowires grown in this manner depend, in a controllable way, upon the size and physical properties of the liquid alloy.

The VLS mechanism was proposed in 1964 as an explanation for silicon whisker growth from the gas phase in the presence of a liquid gold droplet placed upon a silicon substrate. The explanation was motivated by the absence of axial screw dislocations in the whiskers (which in themselves are a growth mechanism), the requirement of the gold droplet for growth, and the presence of the droplet at the tip of the whisker during the entire growth process.

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The VLS mechanism is typically described in three stages:

1. Preparation of a liquid alloy droplet upon the substrate from which a wire is to be grown

2. Introduction of the substance to be grown as a vapor, which adsorbs on to the liquid surface, and diffuses in to the droplet

3. Supersaturation and nucleation at the liquid/solid interface leading to axial crystal growth

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3. Lithography techniques can also be used to controllably manipulate the diameter and position of the droplets (and as you will see below, the resultant nanowires).

The VLS process takes place as follows:1. A thin (~1-10 nm) Au film is deposited onto a silicon (Si) wafer substrate by sputter deposition or thermal evaporation.

2. The wafer is annealed at temperatures higher than the Au-Si eutectic point, creating Au-Si alloy droplets on the wafer surface (the thicker the Au film, the larger the droplets). Mixing Au with Si greatly reduces the melting temperature of the alloy as compared to the alloy constituents. The melting temperature of the Au:Si alloy reaches a minimum (~363 °C) when the ratio of its constituents is 4:1 Au:Si, also known as the Au:Si eutectic point.

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Vapor-Liquid-Solid Method -44. One-dimensional crystalline nanowires are then grown by a liquid metal-alloy droplet-catalyzed chemical or physical vapor deposition process, which takes place in a vacuum deposition system. Au-Si droplets on the surface of the substrate act to lower the activation energy of normal vapor-solid growth. For example, Si can be deposited by means of a SiCl4:H2 gaseous mixture reaction (chemical vapor deposition), only at temperatures above 800 °C, in normal vapor-solid growth. Moreover, below this temperature almost no Si is deposited on the growth surface. However, Au particles can form Au-Si eutectic droplets at temperatures above 363 °C and adsorb Si from the vapor state (due to the fact that Au can form a solid-solution with all Si concentrations up to 100%) until reaching a supersaturated state of Si in Au. Furthermore, nanosized Au-Si droplets have much lower melting points (ref) due to the fact that the surface area-to-volume ratio is increasing, becoming energetically unfavorable, and nanometer-sized particles act to minimize their surface energy by forming droplets (spheres or half-spheres).

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5. Si has a much higher melting point (~1414 °C) than that of the eutectic alloy, therefore Si atoms precipitate out of the supersaturated liquid-alloy droplet at the liquid-alloy/solid-Si interface, and the droplet rises from the surface. This process is illustrated in the figure.

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Oblique Angle Deposition -1

Oblique angle deposition technique (also know as glancing angle deposition) has attracted the interest of many researchers due to its ability to generate nanostructures relatively easily. Oblique angle growth, as illustrated in the figure, basically combines a typical deposition system with a tilted and rotating substrate. Due to the shadowing effect, the incident flux of material that comes to the surface with an oblique angle is preferentially deposited on to the top of surface features with larger values in height. This referential growth dynamic gives rise to the formation of isolated columnar structures.

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From T. Karabacak, U Arkansas, Little Rock

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•Simple, cheap, & effective: 3D nanostructures through physical self-assembly• Structures that are not possible to produce by lithographical techniques (e.g. springs, slanted rods, balls)• Almost no materials limit• Can be grown on almost any substrate material• Control of nanostructure size and separation (tens– hundreds of nm)• Novel material properties

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From T. Karabacak, U Arkansas, Little Rock