Lithography
The field of lithography is vitally important to the
electronics industry. It is the only
process that can mass-produce microchips and other complex semiconductor
devices. Like many other semiconductor
processes, it's found a use in making nanostructures out of semiconductor
materials.
Lithography is actually just one step in the semiconductor
fabrication process. However, it is the
only step that can effectively guide the other steps that are required. The technique is all about patterning a
substrate (or wafer) with a desired layout.
To begin, you need to coat the substrate with photoresist. Next, the substrate has to be carefully
aligned with its 'mask'. The mask holds
the key to the patterning process. By
selectively allowing light to pass through, only designated sections of the substrate
are exposed. The light will either
break down the layer of photoresist or harden it. The portions of photoresist that are weakened as a result can be removed
with acid or other volatile substances.
Any photoresist that remains will act as a barrier for any subsequent process
like epitaxy, doping, or etching. It is
with these sequences of photoresist layering, masking, exposure, photoresist
removal, and semiconductor processing that makes up the semiconductor
fabrication process.
On a given wafer, dozens or hundreds of chips can be made at
the same time. The technique is so thoroughly
perfected that major chipmakers like Intel and AMD can churn out high-yields at
quick speeds.
In nanotechnology, lithography has proven extremely useful
in patterning a substrate for selective growth of nanostructures. For instance you can prepare a substrate so
that it will grow nanowires and quantum dots in only the selected areas. Lithography can also pave the way for a number
of other structures.
Unfortunately, there are limits to the smallest feature size
that is possible with lithography. That's
because the light that is used to pass through the mask has a fundamental
restriction known as the 'diffraction limit of light'. Many industry experts are worried about this
as the demand for faster computer chips grows.
Already, modern manufacturers are using high-power lithography to push
the limits of the technique. Using
extreme ultraviolet rays, it's possible to have feature sizes on the order of
90 nm. While it is possible to create
lithographic machines that can go smaller, prohibitive costs are preventing all
but the most powerful chipmakers to pursue the research.
Because of this, lithography will never be able to make
nanostructures directly. This is also
the reason why nanotechnology is so important for the future of
electronics. It's one of the only known
ways that future miniaturization of electronics can proceed further beyond the
fundamental limits of lithography.
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