microchip revolution has seen a steady shrinking of features on silicon chips,
packing in more transistors and wires to boost chips’ speed and data capacity.
But in recent years, the technologies behind these chips have begun to bump up
against fundamental limits, such as the wavelengths of light used for critical
steps in chip manufacturing.
a new technique developed by researchers at the Massachusetts Institute of
Technology (MIT) and the University of Utah offers a way to break through one
of these limits, possibly enabling further leaps in the computational power
packed into a tiny sliver of silicon. A paper describing the process was
published in Physical Review Letters.
Trisha Andrew PhD ’10 of MIT’s Research Laboratory of Electronics, a co-author
of this paper as well as a 2009 paper that described a way of creating finer
lines on chips, says this work builds on that earlier method. But unlike the
earlier technique, called absorbance modulation, this one allows the production
of complex shapes rather than just lines, and can be carried out using less
expensive light sources and conventional chip-manufacturing equipment. “The
whole optical setup is on a par with what’s out there” in chip-making plants,
she says. “We’ve demonstrated a way to make everything cheaper.”
in the earlier work, this new system relies on a combination of approaches:
namely, interference patterns between two light sources and a photochromic
material that changes color when illuminated by a beam of light. But, Andrew
says, a new step is the addition of a material called a photoresist, used to
produce a pattern on a chip via a chemical change following exposure to light.
The pattern transferred to the chip can then be etched away with a chemical
called a developer, leaving a mask that can in turn control where light passes
through that layer.
traditional photolithography is limited to producing chip features larger than
the wavelength of the light used, the method devised by Andrew and her
colleagues has now been shown to produce features one-eighth that size. Others
have achieved similar sizes before, Andrew says, but only with equipment whose
complexity is incompatible with quick, inexpensive manufacturing processes.
new system uses “a materials approach, combined with sophisticated optics, to
get large-scale patterning,” she says. And the technique should make it
possible to reduce the size of the lines even further, she says.
key to beating the limits usually imposed by the wavelength of light and the
size of the optical system is an effect called stimulated emission depletion
imaging, or STED, which uses fluorescent materials that emit light when
illuminated by a laser beam. If the power of the laser falls below a certain
level, the fluorescence stops, leaving a dark patch. It turns out that by
carefully controlling the laser’s power, it’s possible to leave a dark patch
much smaller than the wavelength of the laser light itself. By using the dark
areas as a mask, and sweeping the beam across the chip surface to create a
pattern, these smaller sizes can be “locked in” to the surface.
process has previously been used to improve the resolution of optical
microscopes, but researchers had thought it inapplicable to photolithographic
chip making. The innovation by this MIT and Utah team was to combine STED with the
earlier absorbance-modulation technique, replacing the fluorescent materials
with a special polymer whose molecules change shape in response to specific
wavelengths of light.
addition to enabling the manufacture of chips with finer features, the
technique could also be used in other advanced technologies, such as the
production of photonic devices, which use patterns to control the flow of light
rather than the flow of electricity. “It can be used for any process that uses
optical lithography,” Andrew says.
Stefan Hell, head of the Department of NanoBiophotonics at the Max Planck
Institute for Biophysical Chemistry in Göttingen,
this work “strikingly simple and elegant” and “a most impressive demonstration
of the idea of using photochromic molecules to create features that are both
finer and closer together than half the wavelength of the light.”
work shows a concrete pathway to creating tiny and dense features at the
nanoscale.” he adds. “Because of its future potential it needs to be actively
pursued. … These methods have the potential of shifting the paradigm of what
we think that focused light can do for making nanosized features and hence
mastering the nanoworld.”