Physical Principles of Electron Microscopy: An Introduction to TEM ...

The Scanning Electron Microscope 151
5.9 Electron-Beam Lithography
Electrons can have a permanent effect on an electron-microscope specimen,
generally known as radiation damage. In inorganic materials, high-energy
electrons that are “elastically” scattered through large angles can transfer
enough energy to atomic nuclei to displace the atoms from their lattice site in
a crystal, creating displacement damage visible in a TEM image, such as the
point-defect clusters shown in Fig. 4-16. In organic materials, radiation
damage occurs predominantly as a result of inelastic scattering; the bonding
configuration of valence electrons is disturbed, often resulting in the
permanent breakage of chemical bonds and the destruction of the original
structure of the solid. This makes it difficult to perform high-resolution
microscopy (TEM or SEM) on organic materials, such as polymers (plastics)
and
impossible to observe living tissue at a subcellular level.
However, radiation damage is put to good use in polymer materials
known as resists, whose purpose is to generate structures that are
subsequently transferred to a material of interest, in a process referred to as
lithography. In a positive resist, the main radiation effect is bond breakage.
As a result, the molecular weight of the polymer decreases and the material
becomes more soluble in an organic solvent. If an SEM is modified slightly
by connecting its x and y deflection coils to a pattern generator, the electron
beam is scanned in a non-raster manner and a pattern of radiation damage is
produced in the polymer. If the polymer is a thin layer on the surface of a
substrate, such as a silicon wafer, subsequent “development” in an organic
solvent results in the scanned pattern appearing as a pattern of bare substrate.
The undissolved areas of polymer form a barrier to chemical or ion-beam
etching of the substrate (Section 4.10), hence it acts as a “resist.” After
etching and removing the remaining resist, the pattern has been transferred
to the substrate and can be used to make useful devices, often in large
number. The fabrication of silicon integrated circuits (such as computer
chips) makes use of this process, repeated many times to form complex
multilayer structures, but using ultraviolet light or x-rays as the radiation
source. Electrons are used for smaller-scale projects, requiring high
resolution but where production speed (throughput) is less important. Resist
exposure is done using either a specialized electron-beam writer or an SEM.
In a negative resist, radiation causes an increase in the extent of chemical
bonding (by “cross-linking” organic molecules), giving an increase in
molecular weight and a reduction in solubility in exposed areas. After
development, the pattern is a “negative” of the beam-writing pattern, in the
sense that resist material remains in areas where the beam was present
(similar to undissolved silver in a black-and-white photographic negative).