Electron Beam Lithography Due to small wavelength of the 30-100 keV electrons, the resolution of electron beam nanolithography is much higher than that of optical lithography. Unlike optical lithography, the resolution of electron lithography systems is not limited by diffraction, but by electron scattering in the resists and by various aberrations of the electron optics. The commercial electron lithography systems offer small spot sizes of the order of 4-5 nm along with dynamic focus corrections when writing large field sizes. This enables large areas to be written without worrying about changes in the size of features that occurs due to improper focus during off optic axis writing. Electron exposure of resists occurs through bond breaking (positive resist) or the formation of bonds or crosslinks between polymer chains (negative resist). Since the incident electron energies are many times greater than bond energies in resist molecules, both bond breaking and bond formation can occur simultaneously. Which predominates determines whether the resist is positive or negative. In a positive resist bond scission predominates and hence the exposure leads to lower molecular weights thereby resulting in an improved solubility in a developer. On the other hand, in a negative resist, exposure to an electron beam results in the development of cross-links between molecules and thus making the polymer less soluble in a developer. The most common type of resist used in electron lithography is poly(methylmethacrylate), PMMA. It can be used as positive or negative resist. Other commonly used resists are calixerene and ZEP-520. The smallest feature that can be written in a PMMA resist is about 4 nm. Spin-coatable oxide resists are the latest members in the club and they are known to give a resolution of about 8 nm. The resist resolution is determined by two major factors: electron scattering and swelling of the resist in the developer. When electrons are incident on a resist, they enter the resist and lose energy by scattering producing secondary electrons and X-rays. This process limits the resolution of the resist to an extent that depends on resist thickness, beam energy and the type of substrate used. A thinner resist, a higher accelerating voltage of electrons and a substrate composing of lighter elements would increase resolution. The swelling of the resists has two principal effects, especially in a negative resist. Two adjacent lines of resist may swell enough that they end up touching each other. In the rinsing cycle, these lines may or may not separate, thus forming “bridges”. This also gives rise to increased line edge roughness of the resist and thereby reducing the resolution. Secondly, the expansion and contraction of the resist during process can have a bad effect on the adhesion of small features and can potentially weaken them. The steps to produce a structure using electron beam nanolithography are schematically shown in figure . The sample, typically a cleaned silicon wafer, is coated with a thin layer of positive PMMA. The desired structure is exposed with certain electron dose. The exposed area shows a change in solubility when developed using a developer. If a metallic structure is desired then a metallic film is evaporated onto the sample. A treatment in acetone would result in dissolution of unexposed PMMA leaving the metallic structure in the substrate. Focused Ion Beam Nanolithography The focused ion beam (FIB) employs rastering of a Ga+ ion beam for imaging with either secondary electrons or secondary ions. High energy (30 keV) Ga+ ions are focused into spots as small as 10 nm to form pixel-by-pixel images. Imaging using secondary electrons provides surface information with similar resolution to that obtainable from an SEM; as the image is created, atoms from the surface are sputtered by the incident Ga+ ions, meaning that we can acquire images from different depths ("slices") within the sample. The main applications arise from the use of ions as the scanned species. These include compositional imaging via secondary ions, direct etching of material in selected regions for in-situ sectioning and imaging, microfabrication, transmission electron microscopy specimen preparation, and localised deposition and implantation of metal and insulator structures. Main application is in nanofabrication of devices. The unique combination of 10 nm resolution imaging with the ability both to remove and to deposit material in selected areas provides a means of performing materials studies or device fabrication processes which would otherwise be impossible or unreasonably time-consuming. Introduction As the diagram on the below shows, the gallium (Ga+) primary ion beam hits the sample surface and sputters a small amount of material, which leaves the surface as either secondary ions (i+ or i-) or neutral atoms (n0). The primary beam also produces secondary electrons (e-). As the primary beam rasters on the sample surface, the signal from the sputtered ions or secondary electrons is collected to form an image. At low primary beam currents, very little material is sputtered; modern FIB systems can achieve 10 nm imaging resolution. At higher primary currents, a great deal of material can be removed by sputtering, allowing precision milling of the specimen down to a sub micron scale. In addition to primary ion beam sputtering, the system permits local "flooding" of the specimen with a variety of gases such as iodine. These gases can either interact with the primary gallium beam to provide selective gas assisted chemical etching, or selective deposition of either conductive or insulating material by decomposition of the deposition gas by the primary ion beam.