Electron Beam Lithography

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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.
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