dip pen nano lithography

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    DIP PEN NANOLITHOGRAPHY

    Dip pen nanolithography is a direct writing technique which is used to create nanostructure on

    the substrate of interest by delivering collection of molecules via a capillary from an AFM tip.

    In DPN molecules are deposited on the surface from an atomic force microscope tip The tip is

    inked with a solution of the molecule of interest and brought into contact with the sample

    surface. Under normal ambient conditions, a capillary forms between an AFM tip and the

    surface that it contacts. In DPN this capillary functions as a liquid bridge to facilitate transfer of

    fluid from the tip to the surface. Control of the ambient humidity therefore has an influence on

    the sizes of the features formed.

    Thus DPN uses an atomic force microscope (AFM) tip as a nib, a solid-state substrate (in this

    case, Au) as paper, and molecules with a chemical affinity for the solid-state substrate as

    ink..The first demonstrations of DPN were based on the deposition of alkanethiols onto goldsurfaces.

    It allows binding structures to be placed at will on the surface so that other units can be bound

    on the surface,thusdpn could be used as glue pen that puts glue at point where we wants to fix

    things

    ATTRIBUTES:

    The one of the most important attribute of dpn is that the same device is used to image and

    write the pattern

    Creating nanostructures using DPN is a single step process which does not require the use of

    resists

    By writing simultaneously with an array of cantilevers, DPN also offers the possibility of

    generating multiple structures in parallel.

    Using a conventional atomic force microscope (AFM) it is possible to achieve ultra-high

    resolution features with linewidths as small as 10-15 nm with ~ 5 nm spatial resolution.

    Material Flexibility: Wide range of inks can be deposited onto a variety of different surfaces

    Multiplexing: Multiple ink deposition onto the same substrate

    Operate in Ambient Conditions: No UHV necessaryno clean room needed

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    DRAWBACK

    DPN is a serial method features are created one after the other. Serial writing

    is time-consuming,

    EPITAXY

    Epitaxy refers to the method of depositing a monocrystalline film on a monocrystalline substrate. The

    deposited film is denoted as epitaxial film or epitaxial layer. The term epitaxycomes from the Greek

    roots epi, meaning "above", and taxis, meaning "in ordered manner". It can be translated "to arrange

    upon".

    Homoepitaxy is a kind of epitaxy performed with only one material. In homoepitaxy, a crystalline film is

    grown on a substrate or film of the same material. This technology is used to grow a film which is more

    pure than the substrate and to fabricate layers having different doping levels.

    Heteroepitaxy is a kind of epitaxy performed with materials that are different from each other. In

    heteroepitaxy, a crystalline film grows on a crystalline substrate or film of a different material. This

    technology is often used to grow crystalline films of materials for which single crystals cannot otherwise

    be obtained and to fabricate integrated crystalline layers of different materials

    Another distinction is made by phase from which film has been deposited

    y Vapor phase epitaxy

    y Liquid phase epitaxi

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    y Molecular beam epitaxy

    VAPOR PHASE EPITAXY:

    Epitaxial silicon is usually grown using vapor-phase epitaxy (VPE), a modification ofchemical vapor

    deposition.

    In this technique the required elements are transported as components of gaseous compounds such as

    metal alkyls and non-metal hydrides to a suitable chamber where they flow over the surface of a heated

    substrate. These compounds break down and react so as to deposit the relevant semiconductor, with

    the remaining waste gases being removed from the chamber.

    Silicon is most commonly deposited from silicon tetrachloride in hydrogen at approximately 1200 C:

    SiCl4(g) + 2H2(g) Si(s) + 4HCl(g)

    This reaction is reversible, and the growth rate depends strongly upon the proportion of the two source

    gases. Growth rates above 2 micrometres per minute produce polycrystalline silicon, and negative

    growth rates (etching) may occur if too much hydrogen chloride byproduct is present. (In fact, hydrogen

    chloride may be added intentionally to etch the wafer.) An additional etching reaction competes with

    the deposition reaction:

    SiCl4(g) + Si(s) 2SiCl2(g)

    Silicon VPE may also use silane, dichlorosilane, and trichlorosilane source gases. For instance, the silane

    reaction occurs at 650 C in this way:

    SiH4 Si + 2H2

    LIQUID PHASE EPITAXY:

    Liquid phase epitaxy (LPE) is a method to grow semiconductor crystal layers from the melt on solid

    substrates. This happens at temperatures well below the melting point of the deposited semiconductor.The semiconductor is dissolved in the melt of another material. At conditions that are close to the

    equilibrium between dissolution and deposition the deposition of the semiconductor crystal on the

    substrate is slow and uniform.

    Typical deposition rates for monocrystalline films range from 0.1 to 1 m/minute. The equilibrium

    conditions depend very much on the temperature and on the concentration of the dissolved

    semiconductor in the melt. The growth of the layer from the liquid phase can be controlled by a forced

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    cooling of the melt.Impurity introduction can be strongly reduced. Doping can be achieved by the

    addition of dopants.

    The method is mainly used for the growth of compound semiconductors. Very thin, uniform and high

    quality layers can be produced. A typical example for the liquid phase epitaxy method is the growth of

    ternary and quarternery III-V compounds on gallium arsenide (GaAs) substrates. As a solvent quite often

    gallium is used in this case. Another frequently used substrate is indium phosphide (InP)

    MOLECULAR BEAM EPITAXY:

    MBE is a growth technique in which epitaxial, single atomic layers(approx. 0.2 -0.3nm) are grown on a

    heated substrate under UHV conditions, using either atomic or molecular beams evaporated from

    effusion sources with openings directed towards a heated substrate usually consisting of a thin

    (aapprox.0.5mm) wafer cut from a bulk single crystal. The sources can be either solid or gaseous and an

    MBE machine will typically have an array of multiple sources, which can be shuttered to allow layered,

    alternating hetero-structures to be produced. Semiconductor quantum wells, superlattices and

    quantumwires and metallic or magnetic multilayers for spin valve structures are deposited using this

    technique.

    Standard MBE uses elements in a very pure form as solid sources contained within a number of Knudsen

    cells. In operation the cells are heated to the temperature at which the elements evaporate, producing

    beams of atoms which leave the cells. The beams intersect at the substrate and deposit the appropriate

    semiconductor, atomic layer by atomic layer.The substrate is rotated to ensure even growth over its

    surface. By operating mechanicalshutters in front of the cells, it is possible to control which

    semiconductor or metal is deposited. For example, opening the Ga and As cell shutters results in the

    growth of GaAs. Shutting the Ga cell and opening the Al cell switches the growth to AlAs.

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