Fundamentals on Bionanotechnologies - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 22

where NA is the numerical aperture of the imaging lens and ˇ Š 0:8. The ratio
ˇ=NA is nearly 1 in the optical projection exposure method, the resolution being
directly proportional with the excitation wavelength.
In a typical optical lithographic system, which uses mercury arc lamps with
pronounced emission peaks at 365 and 435 nm, only geometrical line widths
exceeding 0:25m can be obtained. If thinner line widths are needed, excimer lasers
such as ArF ( D 193 nm), F 2 ( D 157 nm), and KrF ( D 248 nm) must be used
to lower the geometrical line widths up to 0:13 m.
Higher resolutions, of 30 nm, and interlevel alignments as small as 10 nm can
be achieved with extreme ultraviolet lithography (EUV), which uses wavelengths
of 10-14 nm. However, there are many difficulties in mask fabrications at these
wavelengths, the reflective elements of EUV masks being multilayered Bragg
mirrors deposited on Si. A synchrotron or plasma source illuminates the mask,
the tolerances of the imaging system in the projection exposure method being of
only few angstroms. Despite these difficulties, the EUV displays very high yields,
including a high speed of feature patterning, of about 10 11 features/s. At the extreme
end of the electromagnetic spectrum, lithography with X-rays uses high-energy
(few keV). X-ray sources such as Cu target systems emitting X-rays or electron
synchrotrons. X-ray lithography works in the proximity-printing mode and has a
resolution of 50 nm. The main drawback of this method is the 1:1 scale mask
fabrication.
The particle nanolithography is based on electrons or ions, the lithographical
process displaying very good performances, with resolutions of 10 nm for ions and
50 nm for electron lithography. Even smaller features, of 5 nm, can be obtained using
this method.
The electron beam lithography (EBL) technique relies on two configurations. In
the first, direct writing EBL, a focalized electron source, is directed to a substrate
or a substrate covered with a resist such as PMMA. The electron beam is then
scanned with the help of magnetic or electrical deflection systems and writes the
desired pattern in the resist. The disadvantage of this method is the very slow
writing process, which needs hours to write a high-resolution pattern. The second
EBL method is the electron beam projection lithography, similar to the optical
lithographical technique with the same name described above. In this case, the
electron beam passing through the mask, which is a membrane with holes, focuses
an image of the pattern on a resist with the help of an imaging system. The interested
reader can consult ( Tseng et al. 2003 ) for an excellent review on this subject.
The focused ion beam (FIB) lithography consists of scanning directly a substrate
with a focused high-energy ion beam. This technique does not require masks
or resists, and the point-by-point lithographical process relies on two different
principles: (1) subtraction of surface atoms or (2) decomposition over the substrate
of an organic vapor. In the first case, the desired pattern is imprinted directly on
the substrate by scanned sputtering of atoms from the surface, while in the second
case, the desired pattern is formed by the material deposited on the substrate.
The FIB lithographical process can be monitored/imaged in real time by the ions
and electrons emitted as a result of the interaction between the substrate and the