NEC Corporation, the National Institute of Materials Science
(NIMS), and the Japanese Science and Technology Agency (JST), have
together succeeded in improving the turn-on voltage of a new
nanometer-scaled switch, NanoBridge(R) *1, which enables the
operation voltage of the switch to be adjusted to that of CMOS.
Programmable logic devices 2* employing NanoBridge(R) can achieve
reduced chip size and increased performance as NanoBridge(R)
realizes a switch of small size, potentially 4F2, as well as low
on-resistance (less than 100ohm).
In NanoBridge(R), tantalum oxide is being used for the solid
electrolyte 3* as the diffusion rate of copper ions, which form a
conduction path, is extremely small, allowing a significant
improvement in the turn-on voltage.
The features of NanoBridge(R) are as follows:
----------------------------------------------------------
(1)The state of the switch does not change when logic signals are
applied.
(2)The switch achieves high stability against electromigration 4*.
Estimations indicate that an on state will be maintained for more
than 10 years when a constant current of a few milliamperes is
present at room temperature.
(3)Tantalum oxide has excellent durability against high
temperatures required in the LSI fabrication process.
Previously, copper sulfide was being used for the solid
electrolyte in NanoBridge(R); however the following issues remained
through its use.
(1) As turn-on voltage is lower than the operation voltage of
CMOS when copper sulfide is used, the ON/OFF states of
NanoBridge(R) changed when logic signals were applied.
(2) The retention time for ON/OFF states was shorter than three
months at room temperature.
(3) The film had poor durability against the high temperatures
present during the LSI fabrication process.
To solve (1), the turn-on voltage needed to be higher than the
operation voltage of CMOS. Further research showed that the turn-on
voltage is determined by the diffusion rate of copper ions and is
improved by reducing the diffusion rate. For (2), it was
anticipated that retention would also be improved by reducing the
diffusion rate. Durability against high temperatures was also taken
into consideration. Accordingly, tantalum oxide, a kind of binary
metal, was chosen for the solid electrolyte as its diffusion rate
for copper ions is much smaller than that of copper sulfide.
Therefore, the turn-on voltage is expected to be higher than the
operation voltage of CMOS (up to1V). Moreover, tantalum oxide is
widely used for the insulator in DRAM and has good durability
against the high temperatures in the LSI fabrication process. Joint
research has confirmed a turn-on voltage of more than 1V, and
expected retention of more than ten years, even when there is
continuous current flow in the ON state.
NEC, NIMS, and JST have jointly succeeded in the development of
new NanoBridge(R) technology. The new compact
circuit-reconfiguration switch can be stacked on the logic block,
reducing chip area to a fraction of that of a conventional chip,
thereby realizing lower costs. Its main advantage is its low
resistance during ON states, which will enable a reduction in the
price and an improvement in the performance of programmable devices
5* in the future.
This research breakthrough will enhance the development
efficiency and performance of many electronic products such as
mobile phones and digital televisions, as NanoBridge(R) is expected
to enable every conceivable kind of function, even for portable
devices like mobile phones, owing to its ability to reconfigure
circuitry.
The results of this joint research project will be presented on
June 12 at the 2007 Symposium on VLSI Technology in Kyoto,
Japan.
Notes*
---------
1.NanoBridge is a registered trademark of NEC Corporation in
Japan.
2.LSIs whose circuitry can be reconfigured manually by the user,
allowing reduced device development time and dramatic cuts in
initial development costs. The programmable-logic market is
expanding rapidly due to intense competition in device
development.
3.A solid body in which ions can move about freely.
4.When current flows through metallic wiring, metal atoms are
driven by the electron flow, making resistance high, which can lead
to the breaking of wires. In cases where the metallic wiring is
extremely fine, this breakdown becomes more evident.
5.To reduce the number of switches used in conventional
programmable logic, logic cells with a large number of transistors
were being used. This is because the surface area of the
semiconductor switch that is being used for reconfiguration is
large, making resistance high. As a result, in addition to
degrading the usage efficiency ratio of the created circuits, this
set up reduced the degree of freedom of pairing logic cells and the
processing performance of parallel-arithmetic operations. On the
other hand, NanoBridge(R) enables the use of logic cells with fewer
transistors as it is one thirtieth the size of a conventional
semiconductor switch. As a result, usage efficiency of the
circuitry is improved by one digit, and the chip size for the same
function performed by NanoBridge(R) is one tenth that of
conventional programmable logic. Moreover, performance (namely,
power consumption and operation speed) are improved. Meanwhile,
under the assumption of the same chip size, multiple functions can
be packed into a chip with programmable logic by the two-terminal
NanoBridge(R), thereby enabling large-scale applications.