The RLE researchers can also control the collapse of nanoscale ‘walls,’ imprinting straight lines on a chip—or, as in this case, reproducing the MIT logo. |
The
manufacture of nanoscale devices—the transistors in computer chips, the optics
in communications chips, the mechanical systems in biosensors and in microfluidic
and micromirror chips—still depends overwhelmingly on a technique known as
photolithography. But ultimately, the size of the devices that photolithography
can produce is limited by the very wavelength of light. As nanodevices get
smaller, they’ll demand new fabrication methods.
In
a pair of recent papers, researchers at Massachusetts Institute of Technology’s
(MIT’s) Research Laboratory of Electronics and Singapore’s Engineering Agency
for Science, Technology and Research (A*STAR) have demonstrated a new technique
that could produce chip features only 10 nm—or about 30 atoms— across. The
researchers use existing methods to deposit narrow pillars of plastic on a
chip’s surface; then they cause the pillars to collapse in predetermined
directions, covering the chip with intricate patterns.
Ironically,
the work was an offshoot of research attempting to prevent the collapse of
nanopillars. “Collapse of structures is one of the major problems that lithography
down at the 10-nm level will face,” says Karl Berggren, the Emanuel E. Landsman
(1958) Associate Professor of Electrical Engineering and Computer Science, who
led the new work. “Structurally, these things are not as rigid at that length
scale. It’s more like trying to get a hair to stand up. It just wants to flop
over.” Berggren and his colleagues were puzzling over the problem when, he
says, it occurred to them that “if we can’t end up beating it, maybe we can use
it.”
Status quo
With photolithography, chips are built up in layers, and after each layer is
deposited, it’s covered with a light-sensitive material called a resist. Light
shining through an intricately patterned stencil—called a mask—exposes parts of
the resist but not others, much the way light shining through a photographic
negative exposes photo paper. The exposed parts of the resist harden, and the
rest is removed. The part of the chip unprotected by the resist is then etched
away, usually by an acid or plasma; the remaining resist is removed; and the
whole process is repeated.
The
size of the features etched into the chip is constrained, however, by the
wavelength of light used, and chipmakers are already butting up against the
limits of visible light. One possible alternative is using narrowly focused
beams of electrons—or e-beams—to expose the resist. But e-beams don’t expose
the entire chip at once, the way light does; instead, they have to scan across
the surface of the chip a row at a time. That makes e-beam lithography much
less efficient than photolithography.
Etching
a pillar into the resist, on the other hand, requires focusing an e-beam on
only a single spot. Scattering sparse pillars across the chip and allowing them
to collapse into more complex patterns could thus increase the efficiency of
e-beam lithography.
The
layer of resist deposited in e-beam lithography is so thin that, after the
unexposed resist has been washed away, the fluid that naturally remains behind
is enough to submerge the pillars. As the fluid evaporates and the pillars
emerge, the surface tension of the fluid remaining between the pillars causes
them to collapse.
Getting uneven
In the first of the two papers, published last
year (2010) in Nano Letters, Berggren and Huigao Duan, a visiting
student from Lanzhou University in China, showed that when two pillars
are very close to each other, they will collapse toward each other. In a follow-up paper, appearing in Small, Berggren,
Duan (now at A*STAR) and Joel Yang (who did his PhD work with Berggren, also
joining A*STAR after graduating in 2009) show that by controlling the shape of
isolated pillars, they can get them to collapse in whatever direction they
choose.
More
particularly, slightly flattening one side of the pillar will cause it to
collapse in the opposite direction. The researchers have no idea why, Berggren
says: When they hatched the idea of asymmetric pillars, they expected them to
collapse toward the flat side, the way a tree tends to collapse in the
direction of the axe that’s striking it. In experiments, the partially
flattened pillars would collapse in the intended direction with about 98% reliability. “That’s not acceptable from an industrial perspective,”
Berggren says, “but it’s certainly fine as a starting point in an engineering
demonstration.”
At
the moment, the technique does have its limitations. Space the pillars too
close together, and they’ll collapse toward each other, no matter their shape.
That restricts the range of patterns that the technique can produce on chips
with structures packed tightly together, as they are on computer chips.
But
according to Joanna Aizenberg, the Amy Smith Berylson Professor of Materials
Science at Harvard
University, the
applications where the technique will prove most useful may not have been
imagined yet. “It can open the way to create structures that were just not
possible before,” Aizenberg says. “They’re not in manufacturing yet because nobody
knew how to make them.”
Although
Berggren and his colleagues didn’t know it when they began their own
experiments, for several years Aizenberg’s group has been using the controlled
collapse of structures on the micrometer scale to produce materials with novel
optical properties. But “particularly interesting applications would come from
this sub-100-nm scale,” Aizenberg says. “It’s a really amazing level of control
of the nanostructure assembly that Karl’s group has achieved.”