Microtools for the nanoworld
AXSUN Technologies has sponsored the construction of an ALS beamline dedicated to LIGA, a lithography process for machining microdevices. Click here for more photos.
"If you're going to manipulate small
things, you need small tools," says
Keith Jackson. To build those, you
have to start with tools that are a
Jackson, a physicist in the
Materials Sciences Division's
Center for X-Ray Optics (CXRO),
heads the LIGA facility at the
Advanced Light Source. LIGA (a
German acronym for lithography,
electroplating, and molding) burns
patterns deep into a Plexiglas
resist with hard x-rays; the resists
are electroplated, then chemically
removed, leaving master molds or
individual devices-real 3-D
machines and parts, not flat pieces
like typical integrated circuits.
Although much larger than the nanoscale, micromachines produce nanoeffects. "The laws
of physics don't change as you go to smaller structures, of course, but as they scale
down, their interaction changes," Jackson says.
Nanoeffects are important, for example, in the kind of microchannel arrays used for gene
sequencing and other biological analyses. "Fluid flowing through a wide canal is only
grossly affected by the containing walls, but in a microchannel -a thin capillary-sidewall
chemistry, plus electrofluidic and osmotic effects, may dominate."
"The driving application" of micromachine technology these days, says Jackson, is the
race to build optical switchers for fiber networks. Light waves-typically a few hundred
nanometers in length-carry gigabytes of data, but the flow is interrupted when light has
to be converted to electrons for processing and switching, then converted back. "A big
electronic router costs tens of millions of dollars, so there's an economic push to go to
AXSUN Technologies, based in Massachusetts, recently
sponsored the construction of a new LIGA beamline at the
ALS to build subsystems of optical switchers, such as lenslets
and flippable micromirror arrays, for dividing and steering
streams of data-carrying photons. "It's almost like a free
beamline for the scientific community," Jackson says, because
it will often be available for noncommercial work.
To tap into this new world of startling possibilities,
researchers must start with the palpable. Says Jackson, "To
study nanophenomena, you need microtools."
Electrons, with effective wavelengths two orders of
magnitude shorter than hard x-rays, can expose nanoscale
"In the mid-1990s we saw the need for advanced
electron-beam lithographic capability," says Erik Anderson,
head of CXRO's Nanowriter facility, "and we were at the right
place at the right time." The Defense Advanced Research
Projects Agency supported the acquisition of the world's first Leica Microsytems
nonprototype Nanowriter, whose electron beam can directly expose a resist-coated
substrate on a moving surface controlled by laser interferometry.
Hardware includes a 100-kilovolt power supply
that accelerates energetic electrons off the
Nanowriter's thermal-field-emission gun and an
optics column that can focus the beam to a
width of from 5 to 2.5 nanometers. Anderson
and his colleagues added custom electronics
and software to control the operation of the
system, such as the movement of the stage
through laser feedback; they also built a unique
pattern generator that controls the position of
The Nanowriter can create quantum "dot"
electronics, magnetic thin-film devices, and
structures such as Gabor Somorjai's catalytic
nanoarrays. To meet the demands of Moore's
"law," which dictates a doubling in the
electronic devices per chip every 18 months,
the continually-improving Nanowriter draws
ever-finer patterns for
ever-more-densely-packed circuits. An
important feature is the machine's ability to
"stitch" together adjacent areas of circuitry
Meanwhile the Nanowriter keeps busy making
zone plates for experiments at the ALS. Concentric rings, the smallest of them as small
as 25 nanometers wide, are fabricated from massive materials like gold, producing a wide
variety of microscopic planar structures to focus x-rays that would pass unaffected
through conventional lenses.
The increased demand for nanodevices of every description means that "we collaborate
with a wide community, working together to achieve the smallest features," says
Anderson. "It's a never-ending challenge."
MAKING THE MOLECULES DANCE
When a small voltage is applied, a current flows between the tip of a scanning tunneling
microscope (TEM) and the electrons surrounding the atoms on a nearby surface.
Scanning back and forth in raster fashion while adjusting the voltage to keep the current
uniform (or vice versa), the TEM graphs the surface's ups and downs to create a
topographical image, atom by atom.
"Scanning tunneling microscopes are sensitive enough to follow atomic profiles-that's the
easy part," says Miquel Salmeron of the Materials Sciences Division. "Our goal is to study
the behavior of individual atoms and molecules."
Among many other systems, Salmeron and his colleagues have studied acetylene in such
detail that they have learned just how much energy it requires to nudge these tiny
molecules-two atoms of carbon and two of hydrogen-from one orientation to another as
they nestle in the lattice of a palladium surface.
At normal temperatures an atom is in constant frenetic motion and can't be imaged. "By
immersing the TEM in liquid helium, we immobilize the atom," Salmeron says. "We send
electricity through the tip to confirm that we have it; once we've found it, we can move
How to do this "is not written anywhere," he says, "but for each particular atom or
molecule we have our tricks. We can shoot electrons at it. Or kick it into rotation. Or it
may jump to the tip. If it's a molecule, we can smash it."
Salmeron's interest in catalysis led to his studies of acetylene on palladium, which
selectively catalyzes benzene (six atoms of carbon and six of hydrogen) from three
acetylene molecules. Attempts to trimerize the acetylene molecules by nudging them
with the TEM tip have not succeeded, but Salmeron has succeeded in making the
molecules rotate. "It takes just one electron to make the acetylene molecule rotate 120
degrees among three atoms of palladium." Changes in position reflect the molecule's
attempts to accomodate extra charge. Repeated kicks make the molecule spin.
A remarkable set of movies shows acetylene in action: the molecules look like little
Mickey Mouse heads spinning merrily on the palladium surface-the two carbon atoms form
a single white blob with hydrogen "ears," corresponding to what Salmeron calls "black
holes" (on the web at http://stm.lbl.gov/research/cryostm/c2h2/).
"Most of what we call nanotechnology involves hundreds or thousands of atoms," says
Salmeron, "but in a nanometer there's enough room for three atoms. If we are going to
achieve real nanotechnology, we are going to have to learn how to put atoms together
one at a time."