|Optical and scanning electron microscope
images, each one-tenth the size of the previous one, show
the wafer on which the 1-mm
square memories were imprinted (a), individual memory
chips with their test connections (b), a single memory,
invisible at the center of the test structure (c), nanowires
connected to the test pins (the memory is at the intersection
of the lines) (d), the crossed-wire structure of the memory
(e), and the entire 64-bit memory with a bit stored at
each of the intersections of the eight vertical and eight
horizontal wires (f). (Hewlett-Packard)
A Hewlett-Packard research team has advanced efforts to develop
practical chips based on molecular electronics by creating
the first molecular-electronic memory chip. The experimental
device stored only 64 bits but achieved a storage density
10 times that of conventional silicon electronics. In addition,
the chip's memory was nonvolatile (preserved with the power
off) and was combined with logic elements, both of which provide
advantages that are not available with silicon-based random
During the past three years, several groups have succeeded
in producing individual electronic devices, including transistors,
from organic molecules such as rotoxane and from carbon nanotubes.
But until now, no one had succeeded in making usable multiple-device
chips or in developing techniques with the potential for mass
production. Conventional lithography cannot produce the nanoscale
devices needed, and no one has yet perfected self-organizing
techniques that can enable molecules to form the circuits
The Hewlett-Packard group, led by senior scientist Yong
Chen, accomplished its feat by using a new imprinting process
recently developed by Princeton University physicists (see
Briefs, October/November 2002, pp. 10-11). The work, which
has not yet been published in a technical journal, is described
in U.S. Patent 6,432,740. The method creates a mold out of
quartz, which is then pressed into a layer of photoresist.
Where the mold compresses the resist, it can be etched away.
Where the resist is not compressed, it protects the underlying
layer. This process allows the use of the same lithography
techniques employed by the chip industry, but it yields narrower
It takes a couple of days to create the molds using electron-beam
and optical techniques, Chen explains. But this does not matter
because using the mold to create the chip only takes a few
minutes, and potentially could be done in seconds or less.
In making the nanocircuits, a layer of wires is first laid
down using 40-nm linewidths, which is one-third the width
of the finest lines available in commercial chips. On top
of this is laid a two-dimensional crystal layer of rotoxane
molecules and then another layer of wires perpendicular to
the first. The result is a crossbar switch consisting of about
1,000 molecules wherever the upper and lower wires cross.
When a potential is applied to two selected wires, the rotoxane
molecule switches from a conducting to nonconducting state
or back again. The large difference in conductance between
the two states, a factor of 10,000, is not fully understood,
Chen says. A much smaller potential can then be applied to
read the switch.to detect whether it is in a conducting or
nonconducting state. Because the molecule remains in one state
until switched out of it, the memory is nonvolatile and is
unaffected by turning the power off.
Because each switch can work as a nonvolatile memory or
as part of a logic device, the circuitry for multiplexing
and demultiplexing can be built into the same chip as the
memory cells. This is crucial in molecular electronics, in
which many cells must feed into a few output wires. We see
the next steps as finding better molecules and combining them
more effectively with existing silicon circuits, says Chen.
He expects the first commercial uses of molecular electronics
to be specialized memory chips, which could reach the market
in as little as five years. Such a development would eliminate
the existing limits to shrinking today¡'s silicon circuits.
Eventually, molecular crossbar switches could be based on
a single molecule for each switch, allowing trillions of bits
to be packed into a square centimeter.