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:
- The state of the switch does not change when logic signals are applied.
- 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.
- 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.