SMALL MEASURES
Researchers Work to Extend Semiconductor Metrology to
Features of 40 nm and Below
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In this
false-color image, the large purple rectangle is a chip
feature about 40 x 150 nm. The magnified section shows
the planes of silicon atoms used to calibrate the
measurement.
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The features on integrated circuits are getting ever
smaller, but exactly how big they are is hard to say. As
feature sizes shrink below 100 nm, they have passed the
ability of current metrology methods to measure them
accurately. "The requirements for metrology as outlined in the
roadmap are not being met," says Vladimir Mancevski, chief
technology officer of Xidex Corp. (Austin, TX), a developer of
carbon nanotube technology. Indeed, the 2004 edition of the
International Technology Roadmap for Semiconductors shows a
wide swath of red, meaning "manufacturable solutions are not
known" for current and future metrology needs.
Several groups are tackling the next-generation metrology
problem. Xidex, working with the University of Texas at Austin
and the semiconductor manufacturers' consortium SEMATECH
(Austin, TX), recently showed that carbon nanotubes can act as
the tips of atomic force microscopes (AFM) to scan the
surfaces of computer chips for high- resolution metrology. The
work is under the auspices of the Advanced Materials Research
Center, a cooperative effort between SEMATECH and the state of
Texas.
The team started with a conventional silicon tip and,
through electro-deposition, placed a catalyst like nickel or
cobalt in holes in the tip. They then placed the tip in a
chemical vapor deposition chamber and grew the nanotubes.
Because they made the holes parallel to each other and made
sure the catalyst sat in the holes and nowhere else, they were
able to get a fixed array of carbon nanotubes, instead of the
clusters that other processes produce.
The multiwalled carbon nanotubes, essentially concentric
cylinders of nanotubes, have diameters of around 10 nm and
lengths of a few microns. That means they can fit into the
deep trenches and small holes in chip designs where standard
silicon-tipped AFM probes cannot. They also last much longer.
They would have to be changed once a month, as opposed to once
a day for a silicon tip, Mancevski says. He sees no reason
researchers should not be able to make single-wall nanotubes,
which would have a diameter of 1 nm.
Still Using Optics
Another group, at the National Institutes of Standards and
Technology (NIST; Gaithersburg, MD), is working on extending
optical microscopy down to the same levels. They use a
combination of brightfield microscopy and scatterometry to
measure features. Using low-numerical-aperture illumination
from a 436-nm source, they produce a plane wave that hits the
wafer and is scattered by the features. The intensities from
various plane waves, which create patterns of constructive and
destructive interference in the space above the wafer, give
them data that adds up to an image.
Rick Silver, leader of NIST's scatterfield microscopy
project, says the method has proved able to tell the
difference between lines 38-, 39-, and 40-nm wide. Simulations
show that lines 10- to 20-nm wide should not be a problem,
either. Because the approach combines two methods used
currently, it will be easier for semiconductor manufacturers
to adopt and will probably enter wafer fabs in the next few
years, Silver says.
Working with SEMATECH, NIST has also developed a set of
standards to measure features down to 40 nm. The standards
would be used to calibrate the tools used in metrology.
"Integrated circuit features have shrunk so much over the last
20 to 30 years that we've got to the point that there are no
standards available," says Michael Cresswell, a physicist in
NIST's semiconductor electronics division.
To create their ruler, NIST scientists used a selective
etch material that removes silicon in all but one crystal
lattice orientation, producing lines that are almost
atomically smooth. The spacing of silicon lattice planes is
known, so it is just a question of illuminating the etched
silicon with a high- energy beam of electrons and counting the
planes to know how thick the materials are.
"They came out nice and straight and relatively smooth,"
Cresswell says.
—Neil Savage
SOLAR NANOTECH
Nanotechnology Makes Photovoltaics More Attractive
Climate change and high oil prices are boosting interest in
alternative energies. The latest research could finally bring
photovoltaics out of the background of the energy debate,
based on recent findings that show nanotechnologies can trap
light in solar cells, potentially reducing the cost of solar
energy.
Researchers at the School of Electronics and Computer
Science (ECS) at the University of Southampton (Southampton,
UK) designed diffractive-nanostructure arrays on the surface
of solar cells to create optical asymmetries that prevent
light from escaping. "Our basic idea is that if we pattern on
a subwavelength scale (~100 nm), we can make light twist or
turn in ways that it would not ordinarily do using standard
materials," explains Darren Bagnall of the research group.
This light-trapping technology may reduce the thickness of
semiconductor materials needed in solar panels, which would
translate to lower-cost solar cells.
The next steps of the process will focus on proving the
technology in the field and developing a cost-effective method
for producing these nanopatterned layers. The ECS approach is
funded through the £4.5-million, four-year, "Photovoltaic
Materials for the 21st Century" project of the Engineering and
Physical Sciences Research Council, a group of six
universities and seven companies that hopes to reduce the cost
of generating solar-power electricity by 50% by 2008. The
project hopes to develop new "thin film" solar cells to
replace traditional single-crystal silicon solar cells, which
are produced in an expensive high-temperature process.
Although less efficient than the traditional solar cells, the
new thin films are potentially much cheaper to make.
While reductions in the cost of solar cells will offer
tangible benefits, other advances in the United States focus
on product efficiency. Carbon nanotube towers atop
photovoltaic cells could also help extract more power from the
sun, according to scientists at the Georgia Tech Research
Institute (GTRI; Atlanta, GA).
The group coated such nanometer-sized towers with special
p-type and n-type semiconductor (p/n) junction materials to
increase the surface area available to produce electricity.
"First, the nanotube 'towers' significantly increase the total
surface area available for a photon of light to impinge the
p/n-type materials," explains Jud Ready of GTRI. "This
impingement is critical as excitons generated by the light
dissociate into their separate charges and produce current
flow." The group is exploring cadmium telluride, cadmium
sulfide, boron/phosphorous-doped silicon, and
organic/polymeric materials and plans to assess more exotic
photovoltaic materials in the future.
"By appropriately tailoring the aspect ratio of the towers
and 'streets' between them, reflected light off of one tower
can be scavenged by an adjacent tower," Ready says. "With
planar photovoltaic cells, this reflected light, though
minimized via various antireflective coatings, is typically
reflected back into the atmosphere and wasted. The ballistic
conduction properties of the carbon nanotube can allow for
significantly more robust and efficient extraction of the
charge carriers from the photovoltaic materials. This results
in an increase in the electron mobility and improves charge
carrier transport to the electrodes."
—Phillip Espinasse
OLEDS/PLEDS
Barriers Still Prevent R2R Printing for Flexible
Substrates
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Here, a DVD player
prototype that was produced with CDT PLEP technology.
Toppan/CDT hopes to produce similar products with R2R
technology.
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Organic LEDs (OLEDs) offer important advantages over color
LCDs. They are brighter and use less power because only the
illuminated elements draw electricity, instead of the
continuous backlight draw necessary with LCDs. OLEDs are
limited to small sizes and low-volume production, however,
which keeps prices high and prevents broader use. Efforts
between two partnerships—one between Toppan Printing (Tokyo,
Japan) and Cambridge Display Technology (CDT; Cambridge, UK),
and the other between the U.S. Display Consortium (USDC; San
Jose, CA) and Vitex Systems Inc. (San Jose, CA)—hope to reduce
the cost of OLEDs through roll-to-roll (R2R) printing
processes for OLED production that beat the lifetime barrier
facing OLED and light-emitting polymer displays (PLEDs).
Kimberly Allen, director of display technology and strategy
at iSuppli Corporation (El Segundo, CA) says, "There is
currently quite a lot of interest in printed display
technology . . . Toppan/CDT are developing polymer OLED
displays, [while] Vitex is working on a third-party
encapsulation solution that it can sell to any small-molecule
module maker."
The Toppan/CDT partnership is moving into Phase II of its
program to develop R2R processing of PLEDs. During Phase I,
Toppan/CDT proved the feasibility of roll printing, according
to CDT officials. During Phase II, research will concentrate
on improving the performance of the roll- printed displays,
with the ultimate goal of producing roll-printed displays with
the lifetime, efficiency, and color fidelity of more costly
inkjet-printed displays. Toppan has invested ¥1 billion in a
pilot line for the roll processing of displays based on CDT
technology and intends to start with glass substrates but move
on to more flexible substrate alternatives such as plastic.
Phase II of the R2R project will focus on improving
patterning technology, manufacturing technology based on
printing, barrier technology for packaging, and materials such
as dendrimers and polymers that the company expects to come
from CDT, Toppan says. "At the moment, we are not at a stage
where we can speak about specific progress," says Takao
Taguchi, spokesman for Toppan Printing, "but we expect
significant results in three to four months."
Across the Pacific Ocean, USDC has contracted with Vitex to
use its flexible glass substrate technology to develop
high-volume R2R capability that can produce flexible OLEDs.
Vitex's glass is thin, clear, flexible, and reportedly
exhibits barrier properties similar to a sheet of glass. The
substrate is actually plastic and relies on Vitex's Barix
technology, which uses alternating layers of polymer and
ceramic thin films to resolve the moisture and oxygen
sensitivity problems that limit OLED lifetimes.
Allen of iSuppli says, "Flexible OLEDs cannot reach the
market until there is adequate barrier technology to protect
the organic materials from water vapor in the air. Vitex is
hoping to sell its coated substrates, which should provide
this barrier to OLED makers, but its technology is not
adequate. Most OLED companies, including Toppan/CDT, are
working on their own barriers, but they have not succeeded yet
either."
Michael L. Kleper, a full professor and member of the
senior faculty at Rochester Institute of Technology
(Rochester, NY) says, "The commercial printing industry is at
the threshold of the door of opportunity presented by printed
electronics. It offers great promise for not only revitalizing
the traditional print industry, but enabling the mass
production of inexpensive electronic components and devices.
The materials science work that is being undertaken by
Toppan/CDT and USDC/Vitex is evidence that a printed
electronics world is definitely a part of the future."
Despite Kleper's enthusiasm, analysts caution that price is
always a factor in the end. So-called engineered plastic
substrates—layers of coatings and barriers—cost much more than
simple glass. "The display market has long proved that a
premium price for premium qualities like flexibility sounds
good on paper but doesn't hold up in the market," Allen says.
"OEMs will want the same price as glass. While there may be
some compromise at a slightly higher price, flexible display
makers will still have to work hard to reduce costs."
—Charles Whipple
LASERS
New Class of Raman Lasers Offers High Gain and Compactness
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The figure shows
the MBE-grown semiconductor injection Raman laser.
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Based on the nonlinear optical process, stimulated by Raman
scattering, traditional Raman lasers typically exhibit small
gains and require external pumping with powerful lasers, which
limits their application. In a joint collaboration between
Harvard University (Cambridge, MA) and Texas A&M
University (College Station, TX), researchers fabricated a
semiconductor injection Raman laser operating with a gain of
10-3 cm/W, 4 to 5 orders of magnitude higher than
in most existing Raman lasers (see figure).
"It is this high value of the gain that allowed us to
achieve Raman lasing in such a small device and with such a
low optical pump power: 40 mW," explains Alexey Belyanin of
Texas A&M. Based on an indium gallium arsenide/indium
aluminum arsenide heterostructure, the device was grown by
molecular beam epitaxy and lattice-matched to an indium
phosphide substrate. The optical source and the Raman
generation are realized monolithically in the same material by
appropriately engineering the band structure of a quantum
cascade laser. "This allows us to use intracavity excitation
instead of excitation from an external source and, more
importantly, allows us to regenerate the exciting radiation
all along the cavity, therefore getting around the problem of
absorption of the fundamental radiation," explains Mariano
Troccoli of the research team.
The group is focusing on achieving broad tunability by
applying the electric field and operating in the THz range at
room temperature. While achieving tunability is quite feasible
via design changes, THz generation may not be as trivial. This
novel laser will find use in applications similar to those of
mid- and far-IR lasers, particularly in areas of sensing,
imaging, bio/chemical analysis, and national security and
defense, with the advantage that these lasers are capable of
larger tunability.
—Phillip Espinasse
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