[extropy-chat] crossbar architecture nanoswitches

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Reinventing the transistor Claire Tristram Technology Review Sept 03

Every Friday afternoon at Hewlett-Packard Labs in Palo Alto, CA, R. Stanley
Williams, one of the most respected thinkers in the field of molecular
electronics, gets his group of 25 research scientists together to talk shop.
One by one, they make their way to the conference room. Williams walks in
exactly on time, sits down in front, and leans back, frowning, his hands
steepled. He was hired by HP in 1995 to rethink the basics of computing and
has handpicked the team inside this room to do just that. Williams likes to
wear jeans, and his hair reaches halfway down his back, so he gives a first,
fleeting impression of quietude and informality. But he apparently never
smiles, and his people work 19-hour days to meet his deadlines. Williams
waits a few minutes for the habitual latecomers, then stands up. He speaks
in an efficient monotone.
"We're going to hear first from Gun-Young today," he says. "What he has
accomplished is magnificent. Everyone here owes him a lunch because his hard
work has paid for our salaries for the last several months."
Gun-Young Jung, a recent postdoc from South Korea, stands up and quietly
describes his work on nano imprint lithography, a process that uses a
physical mold to create features as small as six nanometers across on
silicon wafers. That's more than an order of magnitude smaller than the
finest features achievable using today's advanced photolithographic
processes. Sometimes things stick to the mold, though. It's like cake batter
sticking to a pan, he says. His presentation lasts about ten minutes and is
followed by two others.
Listening to these speakers, one after another, gradually conveys a sense of
the group's style. They enjoy self-deprecating humor and inject frequent
expressions of bewilderment into their scientific explanations, like "I
don't know" and "it's still a mystery" and "I still need to investigate,"
and even "I am still quite a novice." And despite their obvious expertise,
this isn't false modesty.
Williams's group faces a monumental task: trying to make computers whose
functionality rests on the workings of molecules. To do so will mean
reinventing the transistor. While silicon and other inorganic semiconductors
have always been the basic building blocks of microchips, it turns out that
organic molecules can also have some potentially useful electrical
properties. Indeed, over the last few years, researchers have learned to
synthesize molecules that can function as electronic switches, holding
binary 1s or Os in memory or taking part in logical operations. And
molecules have one significant advantage: they are really small. Such work
is critical to the future of computing, because conventional chip
fabrication technology is on a collision course with economics. Today's best
computer chips have silicon features as small as 90 nanometers. But the
smaller the features, the more expensive the optical equipment needed to
manufacture them. A state-of-the-art fabrication plant for silicon
microchips now costs some $3 billion to build. A chip in which silicon
transistors are replaced with molecular devices, on the other hand, could in
principle be fabricated through a simple chemical process as inexpensive as
making photographic film. A circuit with 10 billion switches could
eventually fit on a grain of salt; that's a thousand times the density of
the transistors in today's best computers. A computer built from such
circuits could search billions of documents or thousands of hours of video
in seconds, conduct highly accurate simulations and predictions of weather
and other physical phenomena, and do a much better job of imitating human
intelligence, perhaps even communicating with us through natural
conversation.
But no matter how tempting in theory, it's speculative, blue-sky research,
and investing in molecular electronics is a gamble few companies have been
willing to make. HP's confidence in Williams is a big reason it's one of the
exceptions, says Shane Robison, the company's executive vice president and
chief strategy and technology officer. "In addition to his ability to put
together a first-class team of cross-disciplinary experts and an emphasis on
how to turn science and technology into real products, Stan's best quality
is probably his eternal optimism," says Robison. Of course, there's also the
lure of immense profits, should Williams's technology ever displace
conventional silicon chips. "Projects this ambitious are always a long shot,
but we wouldn't be doing it if we didn't think there was a good chance of
succeeding," Robison says.
To be sure, the company has hedged its bet by being cautious with funding.
Williams's group has a four-year, $12.5 million grant from the U.S. Defense
Advanced Research Projects Agency (DARPA), and HP provides matching funds,
but about half of the DARPA funding goes to university research partners.
Signs of economizing are everywhere in the lab, from a shortage of supplies
in the coffee room to jury-rigged equipment. Nonetheless, the group has made
one breakthrough after another-most notably, by proving that a "crossbar"
design once common in conventional electronics can be resurrected on the
molecular scale. In a demonstration last year, the group trapped molecules
in the junctions between titanium and platinum nanowires arranged in an
eight-by-eight, one-micrometer-square grid, and showed that the molecules
can be switched "on" and "off" at specific junctions-a first step in
building a working memory or logic device.
Williams was first to come up with an architecture for molecular computing
that "worked," says Meyya Meyyappan, director of nanotechnology research at
NASA Ames Research Center at Moffett Field, CA. "I can't think of anyone
else who has that kind of forward thinking." But despite these
breakthroughs, the challenges in making molecular electronics are many and
baffling. How do you make nanocomponents interact with the conventional
electronics needed to get information in and out? How do you program a
computer made from molecular electronics? But most fundamentally, what
materials make the best molecular switches, and how can they be arranged
with the precision and reliability required for mass manufacturing?
That last question is one of the major concerns of Williams's lab these
days. And as time spent with the researchers reveals, things at this scale
are so difficult to observe and measure that conclusions can easily crumble
back into speculation-as when seemingly important results turn out to be
anomalies in experimental procedure or mistakes in interpretation. Even
successful, verifiable results still present more questions than answers.
When electrons travel through a macroscopic wire, they behave more or less
like fluid in a pipe running downhill. But what happens when a billion or so
electrons per second are passing through a single molecule? To borrow a
favorite phrase from those in Williams's group, it's a mystery.
It turns out that a little humility is a very good thing in this field, and
serious researchers are reluctant to supply even rough predictions about
when molecular electronics will meet the same rigorous standards of
manufacture and testing that today's computers do.
"If you were to ask me when we'll start seeing these things commercialized,
my answer is that I don't know," says Williams. "We don't understand the
fundamental physics of why molecules switch. Until we do, we can't build a
factory to produce them. It might take us decades to understand it. Or we
might figure it out tomorrow."
Spend some time in Williams's lab and you start to understand why a lot
about molecular electronics is still a mystery, beginning with the
relatively simple question of what exactly the researchers are building.
Yong Chen, a native of China and a member of Williams's group since 1998,
spends a lot of his time sitting in a stuffy, windowless, nine-square-meter
room padded with thick foam. It's the home of a delicate electron
microscope, which uses electron beams to create a rough picture of the
structures Chen creates in the laboratory down the hall.
Chen is the leader of the team that has given the group its biggest public
success to date, the 64-bit crossbar memory. His team first imprinted eight
parallel nanowires made of titanium and platinum on a silicon substrate, and
covered these wires with a one-molecule-thick layer of a synthetic chemical
called rotaxane. They then deposited a second set of titanium wires
perpendicular to the first, creating the possibility of an electrical
connection between the wires at any junction in the grid.
Each molecule of rotaxane-which was invented by chemist Fraser Stoddart at
the University of California, Los Angeles-consists of a long axle with two
lumps of atoms at each end, and a ring of atoms circling the axle. Stoddart
and Williams's groups theorize that when a voltage is applied through a
specific, intersecting pair of nanowires, the rings on the rotaxane
molecules between the wires "jump" from one end of the axle to the other and
stay there until another voltage is applied. This could raise or lower the
molecules' resistance to electrical current, and these two states of
conductivity would represent digital 1s or Os. Now Chen, eager to see how
small he can make such a device, is trying to print the individual wires
even closer together. It's painstaking labor, where you never know if you're
making progress until the moment it works.
Today Chen is open mouthed, rapt, focusing absolute attention on the monitor
in front of him, while also trying to carry on a conversation. He is not
entirely successful. Several minutes pass quietly as a question hangs in the
air, unanswered. He increases the microscope's magnification as he searches
through a series of fuzzy, gray-on-gray images that look like satellite
photos of a desert.
"After we finish the fabrication process, we come in here to check what kind
of thing we have got," he says. "I want to see if the wire is grounded to
the substrate or suspended above it. There's one. Oops, I lost it."
Eventually he finds something that looks like a length of rebar on a pile of
charcoal dust but is actually a wire, 35 nanometers in width, resting on the
silicon base. He takes a picture, silent again, holding his breath since
sound waves will affect the quality of the image.
"We can talk now," he says. "Here, in fact, you can see this wire is broken.
Too bad. This is a routine experiment, frankly." Chen's goal is to find a
combination of materials-a "recipe," if you will-that will impart a
Teflon-like non-stickiness to the mold that deposits the wires on the
substrate; otherwise they bulge and twist when the mold is removed. But
sitting in this hushed, foam-covered room, watching one of the leading
scientists in the field searching through grainy images, you realize just
how difficult it is to work on this scale. Three weeks later, after five
months of painstaking experiment and observations, Chen and Gun-Young Jung
find the result they were looking for, bringing the possibility of
molecular-sized circuits a small step closer.
"I miscalculated several things," Chen says simply.
Now he can move on to the next problem.
Observing results, of course, is the last step in a train of events that
traditionally begins with a theory about how things should behave. In the
case of molecular electronics, though, very little has run a straight course
from theory to experiment to result. Theories can languish for years waiting
for tools precise enough to test them. In fact, chemists first proposed the
idea of molecular electronics in the mid-1970s, but another 20 years would
pass before anyone could begin to put it into practice. Lately, though,
experimental results have begun to leapfrog the ability of theorists to
explain them.
One puzzle is the lack of consistency in measuring experimental results,
from lab to lab and even from experiment to experiment. Alex Bratkovsky, a
theoretical physicist and native of Moscow who joined HP in 1996, says he
was one of the first to realize that a molecule's orientation between metal
electrodes is critical to understanding its switching properties. "The
current depends tremendously on how the molecule connects with the
substrate," Bratkovsky says. "The signal may go away, then come back,
depending on the position of the molecule. We disregarded that fact for
quite a while." Since controlling the orientation of the molecule is still
beyond current experimental tools, results vary widely from lab to lab, and
scientists need to judge in many instances whether differences between their
results have real meaning or can be explained by effects still outside of
experimental control.
To understand the switching phenomenon, the HP researchers are studying a
range of new molecules that might be controlled more easily than rotaxane,
Bratkovsky says. Some of these are already being designed, but progress is
slow. It can take more than two years to design, simulate, synthesize, and
finally test a molecule for its electronic properties-after which
researchers may find themselves beginning all over again.
Across the hallway from Bratkovsky, Duncan Stewart, an experimental
physicist recently hired by Williams's lab, spent more than six months on a
contrarian experiment to help investigate why some molecules can act as
molecular switches, changing their conductivity in response to an applied
voltage. Instead of designer molecules like rotaxane, Stewart used a simple
hydrocarbon molecule consisting of a chain of 18 carbons surrounded by
hydrogen atoms. Stewart calls it the "Plain Jane of the molecular world."
It's stable, inert, and theoretically should have no interesting electronic
properties. But it switched anyway.
"I have heaps of data, and the story is that the data do not fit any model,
or any existing theory. So even in the simplest case, we don't understand
how electrons are traveling through a molecule," he says. "At times it's
extremely frustrating. You have to be very pigheaded, beat your head against
a wall for six months, and eventually a single brick budges, and eventually
the whole wall crumbles and you see another wall."
If the materials studied by these researchers seem baffling and
unpredictable, the machinery they use is even more so. Progress in molecular
electronics is often at the mercy of unpredictable glitches in the
experimental equipment. This is, after all, laboratory science and not
engineering.
Tan Ha, a native of Vietnam, is in charge of the equipment used in the lab's
clean room. Two or three times a day he dons a clean-room suit and goes into
the room to test, adjust, and modify equipment for what are in many cases
first-of-a-kind experiments. We suit up. "Now we're ready for chemical
warfare," he says. The mask over his face makes it difficult to judge
whether he is joking.
Once inside we make a beeline for a machine called a chemical vapor
deposition reactor. It looks like a big steel cylinder on its side, encased
in glass. "I have a special relationship with this machine," he says, and
touches the glass with a gloved hand.
This type of reactor is standard fare in semiconductor fabrication
facilities, but Ha has modified the machine to perform the ultraprecise
experiments required by Ted Kamins, a member of Williams's group since 1995.
Kamins has worked for years on the ultimate dream of nano research: making
devices "grow" in desired structures rather than building them piece by
piece. His goal is to grow the nanowires required by molecular electronics,
as an alternative to using nano imprint lithography. So far, Kamins has
synthesized wires as small as 10 nanometers in diameter by exposing
"nanoparticles" of various materials to a mixture of gases in the deposition
reactor. In the ensuing reaction, long chains of silicon grow up around the
particles, producing what looks under the electron microscope like a forest
of needles.
Growing the wires required for molecular electronics is exciting stuff, but
Kamins's particular experiments almost didn't happen. Ha tells me that he
spent over a year of his life trying to make the machine work. "Every time
we ran an experiment, contamination would destroy the process," he says. It
wasn't that the machine was broken; it's just that no one had ever needed to
do the experiments that Kamins wanted to do. "It got to be a spiritual
agenda for me," says Ha. "Ted was frustrated. So was I. I'd be in here on my
knees all day long, modifying things screw by screw. I'd go to bed at night
and close my eyes and see the plumbing diagram on my eyelids. It turned out
to be a problem in the exhaust system. I went home and told my wife, 'That's
it; I am a proven equipment engineer.' That's how happy I was."
Much to Duncan Stewart's disappointment, Williams asked him to publish his
results with the hydrocarbon molecule after six months and concentrate on
other work. Yet Williams encouraged Ha to keep working on his knees and
dreaming about plumbing diagrams for a year, for experiments that Williams
estimates are at least six years from fruition and may never yield a
practical result. In a sea of competing theories and possibilities, and with
the budget pressures he complains about with some regularity, how does he
decide?
"It's a matter of experience," Williams says. "I've been down many blind
alleys many times in my career. They're so enticing. You can get into these
things and think, okay, just one more step, just one more step. Other things
feel like they are in the right direction, and I can see where we're going."
In other words, he has learned to trust his intuition, because it's all he
has. "I've been through the cycle many times."
Williams's longest commitment to any idea in molecular electronics is to the
crossbar architecture. But he admits that even this idea might be a blind
alley. Will it ever be possible, for example, to cleanly trap molecules at
the junction of two wires with complete confidence in their orientation?
Then there's the practical problem of gain, or turning a weak electrical
input into a strong output; this is a critical capability needed both to
carry out logic operations and to amplify the tiny currents crossing the
molecular switches so that conventional silicon systems can detect them. And
it's a problem with no demonstrated solution.
"Stan is a smart guy, God bless him, and if anyone can solve these things,
it's going to be his team," says James Tour, a Rice University chemist who
is working on a competing approach to molecular computing. "But he's got a
tough problem. At every crosspoint the molecules need to be stable. Then
they need to interface with all the wires coming out. There's an enormous
cost to that. They have a steep hill to climb."
"It's certainly possible that we are wrong," admits Williams. Then he shakes
his head and stops being humble for a brief moment.
"I don't think so," he says. "I think we've picked the winner, something
that will allow this thing we call Moore's Law to continue on for another 50
years. I used to think it was impossible. Now I think it's inevitable."