Nano bridge builds logic
Technology Research News
from the Japanese National Institute for Materials Science have given an
old technology -- the mechanical electric switch -- a quantum update.
The researchers have devised a nanoscale mechanical switch that
works by rapidly creating and destroying a minuscule metal bridge between
a pair of wires positioned just one nanometer apart. A nanometer is one
millionth of a millimeter, or the span of 10 hydrogen atoms.
The researchers' device, dubbed the quantized conductance atomic
switch, could one day be used to form electronic circuits and memory devices,
and could be manufactured on silicon computer chips, said Hasegawa.
Unlike earlier generations of mechanical switches, the device does
not have moving parts. "Our switch works by simply applying... voltage,
same as semiconductor devices," said Tsuyoshi Hasegawa, an associate director
at the National Institute for Materials Science in Japan.
The atomic bridge between wires grows in the presence of a low positive
voltage and shrinks in the presence of a low negative voltage. It works
in a dry environment at room temperature, Hasegawa said.
The researchers' prototype switches between on and off states at
1 megahertz, or million times per second, using a voltage of 600 millivolts,
or thousandths of a volt. With thinner wires, the switching speed could
reach 1 gigahertz, or billion times per second, which is comparable to the
switching speed of today's computer chips, said Hasegawa.
The researchers made the switch by depositing a 1-nanometer thick
layer of silver on a silver sulfide-coated silver wire, positioning a platinum
wire on the silver layer, and applying a negative voltage. The voltage caused
the atoms in the silver layer to be incorporated into the silver sulfide
coating, leaving a one-nanometer gap between the silver sulfide and platinum
A positive voltage turns the switch on because it draws enough silver
atoms to the surface of the silver sulfide to bridge the gap between the
two wires. A subsequent negative voltage causes the silver atoms to return
to the silver sulfide.
Many switches can be packed into a small area by positioning the
silver sulfide wires perpendicular to the platinum wires. If each switch
were used as a memory element, such a configuration would allow a memory
chip made from the switches to store 2.5 gigabits per square centimeter,
according to Hasegawa. Today's state-of-the-art memory chips store about
1 gigabit per square centimeter. Smaller wires would yield even higher capacities,
Because the switches are so tiny, they operate in the realm of quantum
physics, which opens the possibility of using the switch to make a multi-bit
memory device, according to Hasegawa. In the realm of atoms and subatomic
particles, the rules of physics are different from the everyday world. Unlike
that of larger switches, the electrical conductance through the researchers'
tiny switch is quantized, meaning it increases or decreases by discrete
amounts. A pair of the switches can represent as many as 16 values, or 4
bits, according to Hasegawa.
The researchers also used the switches to form the basic binary
logic gates required to make computer processor chips. They made an AND
gate using two switches formed from a single silver sulfide wire and two
platinum wires combined with a resistor that restricts electric currents
to specific voltages. An AND gate produces a 1 only if both inputs are 1.
They made an OR gate using two switches formed from two silver sulfide wires
and a single platinum wire combined with a resistor. An OR gate produces
a 0 only if both inputs are 0. They made a NOT gate using one switch combined
with two resistors and a capacitor, which briefly stores electric charge.
A NOT gate turns an input of 1 into 0 and vice versa.
The researchers' next step is to make the switch more reliable and
to develop practical methods of fabricating large numbers of the devices,
according to Hasegawa.
The switch could be used in commercial products within ten years,
according to the researchers.
Hasegawa's research colleagues were Kazuya Terabe of the Japanese
National Institute for Materials Science and the Japan Science and Technology
Agency, and Tomonobu Nakayama and Masakazu Aono of the Japanese Nation Materials
Science, Japan Science and Technology Agency, and Riken. The work appeared
in the January 6, 2005 issue of Nature. It was funded by the Japanese
Ministry of Education, Culture, Sports, Science and Technology.
Timeline: 10 years
TRN Categories: Integrated Circuits; Nanotechnology
Story Type: News
Related Elements: Technical paper, "Quantized Conductance
Atomic Switch," Nature, January 6, 2005
January 26/February 2, 2005
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