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The
science-fiction fantasy of nanotechnology — building
novel structures, devices, and materials at the atomic
or molecular scale — is becoming a reality. For the
great potential of nanoscience and nanotechnology to be
fully realized, however, research efforts must cross many
disciplines, from electrical engineering, mechanical engineering,
materials science, and computer science to bioengineering,
chemistry, and physics.
Nowhere
is this cross-disciplinary approach fostered more than
at UC Berkeley. Each month, Lab Notes is proud
to present the work of nanotechnology researchers from
the College of Engineering and our collaborators across
the campus.
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The Lighter Side of Next-Generation Lithography
by David Pescovitz
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version
In
this illustration, the height — about 13.4 nanometers
— is a measure of an EUVL optic's deviation from perfect
shape.
Courtesy
Lawrence Berkeley National Laboratory
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At the dawn of
the Digital Age, Silicon Valley pioneer and Intel co-founder Gordon
Moore predicted that the number of transistors on an integrated
circuit, and hence, its speed, would double every year or at least
every 18 months. He was right, more or less. But soon, Moore's law
will collide with a much less flexible set of laws — the laws
of physics. Within the next decade, the technology used to manufacture
integrated circuits, known as optical lithography, may reach its
practical limits. Quite simply, today's best commercial fabrication
methods — which produce transistors 130 nanometers in size
using light — probably won't be dexterous enough to shrink
transistors smaller than 50 nanometers.
"When the industry is ready to make nanotransistors, how will they
mass produce them?" asks Jeffrey Bokor, a professor of Electrical
Engineering and Computer Sciences at UC Berkeley.
Bokor is a pioneer in extreme ultraviolet lithography (EUVL), widely
accepted as the heir to the optical lithography throne. For nearly
15 years, Bokor has been helping EUVL get ready for prime time so
that computers will continue to grow in power even as they shrink
in size and price. Most recently, he and a team of collaborators
from UC Berkeley's Center for X-Ray Optics, the Virtual National
Laboratory, and Lawrence Berkeley National Laboratory's (LBNL) Advanced
Light Source built one of the most accurate optical system testing
machines in the world to put EUVL technology through its paces.
Jeff
Bokor (center) with collaborating LBL scientists Ken Goldberg
(left) and Patrick Naulleau (right) beside Beamline 12.0
at LBL's Advanced Light Source where many of the EUVL experiments
are conducted.
Courtesy
Lawrence Berkeley National Laboratory
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In today's optical
lithography technology, light shining through a glass mask (essentially
a stencil of the chip's features) projects the chip pattern onto
a silicon wafer coated with photoresist, an organic film that hardens
when exposed to light. In the areas not exposed, the film can be
washed off, leaving bare areas where a previously deposited film
of insulator or conductor can then be etched away. By repeating
this process on many different layers, the transistors and wires
that make an IC chip are formed. The shorter the wavelength of light
projecting through the mask, the smaller the features on the chip.
Extreme Ultraviolet Lithography offers a wavelength of 13 nanometers
— 10 times shorter than today's best optical alternatives —
and will enable the patterning of chip features smaller than 50
nanometers. EUVL works by using a series of mirrors, rather than
lenses, to bounce the extreme ultraviolet light off a "reflective
mask" and onto the wafer.
"The precision with which you have to transfer that pattern onto
the wafer is much different than making an image that looks good
to your eye," Bokor says. "Your eye and brain are very forgiving
but integrated circuits are not."
Because the wavelength of EUVL is 13 nanometers, the surface of
the mirrors must be fabricated and aligned to sub-nanometer tolerances.
"You can't possibly make something that precise without having the
capability to measure it," Bokor adds.
Using a beam from LBL's Advanced Light Source (ALS), one of the
world's brightest sources of ultraviolet light, the researchers
designed and built an optical measuring device known as an interferometer.
The groundbreaking device is capable of detecting deviations in
EUV optics smaller than the radius of a hydrogen atom.
"We measured the first real EUV optics developed out of a huge industry-wide
EUV lithography program and determined that it had very high performance,"
says Bokor says. "We established the gold standard for EUV lithography
optics."
Also thanks to the ALS, Bokor and his colleagues developed a method
to identify defects in the masks used to pattern the chips. The
EUV masks' susceptibility to defects, caused, for example, by dust,
is a huge industry concern; one defect can ruin every chip patterned
by that mask. So far though, Bokor says this theoretical concern
hasn't proven to be a "show stopper" for EUV lithography.
And most recently, the researchers began focusing on the light-sensitive
photoresist used to coat the silicon. The goal, Bokor says, is to
help commercial vendors evaluate whether their resist is capable
of responding at the resolution necessary for the EUV fabrication
of nano-scale transistors. To this end, his research group constructed
an experimental EUV lithography system — again the only one
of its kind in the world — that runs off the ALS and is capable
of producing 30 nanometer features on silicon.
"The microchip industry doesn't like to take risks," he says. "But
we can help give the industry the confidence to make the big leap
to EUV lithography."
Jeffrey Bokor's Semiconductor Research Group
LBL Advanced Light Source
Lab Notes is published online by the Public Affairs Office of the UC Berkeley College of Engineering. The Lab Notes mission is to illuminate groundbreaking
research underway today at the College of Engineering that will dramatically change our lives tomorrow.
Editor, Director of Public Affairs: Teresa Moore
Writer, Researcher: David Pescovitz
Designer: Robyn Altman
Subscribe or send comments to the Engineering Public Affairs Office: lab-notes@coe.berkeley.edu.
© 2003 UC Regents.
Updated 2/28/03.
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