Biomedical Engineering Reference
In-Depth Information
Fig. 2
Photolithography steps of surface patterning
The technical limits of photolithography are related to the wavelength of light
used, which also determines the smallest feature sizes achievable. A limitation is
dictated by the lens characteristics: if the used wavelength is made too small, the
lens simply absorbs the light in its entirety and do not allow curing of the photore-
sist. This implies that photolithography cannot reach the superfi ne sizes of some
other technologies, such as electron beam lithography, as the wavelength of typical
light, even UV lasers, is somewhat limited. Small-wavelength UV light (deep UV
techniques) can now be employed to obtain fi ner nanostructures.
A technology that allows for fabrication of much smaller nanostructures than
photolithography is that of electron beam (e-beam) lithography. The use of an elec-
tron beam to draw a pattern nanometer by nanometer and expose an e-beam resist
layer (usually made of a polymer) allows achieving incredibly small sizes (on the
order of 20 nm). This technique, though, is much more expensive and time consum-
ing than photolithography, as the e-beam exposure is carried out line-by-line or
dot-by-dot, unlike the photolithography by which the whole area of, e.g., 12-in.
diameter Si wafer can be exposed all at once. It takes orders of magnitude longer to
complete the patterning with e-beam, so it is therefore diffi cult to sell the e-beam
lithography for large-scale industry applications.
New nanolithography technologies are continued to being researched, leading to
smaller possible sizes and access to more complex surface geometries and proper-
ties. Extreme ultraviolet (EUV) lithography, for example, is capable of using light
at wavelengths of 13.5 nm and fabricating ~30-nm regime features. While hurdles
still exist in this new fi eld, it promises the possibility of sizes far below those pro-
duced by current industry standards. Another lithography technique proposed for
the realization of nanoscale features is the direct-write, “dip-pen” nanolithography
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