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NEC unveils nano switch with adjustable operational voltage

Posted: 14 Jun 2007     Print Version  Bookmark and Share

Keywords:nanometer-scaled switch  programmable logic devices  NanoBridge 

NEC Corp. in collaboration with the National Institute of Materials Science (NIMS) and the Japanese Science and Technology Agency (JST) has developed improved nanometer-scaled switch, NanoBridge with enhanced turn-on voltage, which enables the operation voltage of the switch to be adjusted to that of CMOS.

Programmable logic devices employing NanoBridge can achieve reduced chip size and increased performance as NanoBridge realises a switch of small size, potentially 4F?, as well as a low on-resistance of <100Ω, said NEC.

In NanoBridge, tantalum oxide is being used for the solid electrolyte as the diffusion rate of copper ions, which form a conduction path, is extremely small, allowing a significant improvement in the turn-on voltage.

Moreover, the state of the switch does not change when logic signals are applied. The switch achieves high stability against electromigration. Estimations indicate that an ON state will be maintained for more than 10yrs when a constant current of a few milliamperes is present at room temperature. The NanoBridge features a tantalum oxide that has excellent durability against high temperatures required in the LSI fabrication process.

New technology
Previously, copper sulfide was used for the solid electrolyte in NanoBridge. However, several issues remained through its use. First, as turn-on voltage is below the operation voltage of CMOS when copper sulfide is used, the on/off states of NanoBridge changed when logic signals were applied. Second, the retention time for on/off states was shorter than three months at room temperature. And thirdly, the film had poor durability against the high temperatures present during the LSI fabrication process.To solve the first issue, the turn-on voltage must 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 the second issue, 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 was chosen for the solid electrolyte as its diffusion rate for copper ions is much smaller than that of copper sulfide. Thus, the turn-on voltage is expected to be higher than the operation voltage of CMOS (up to 1V). 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 over ten years, even when there is continuous current flow in the on state.

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, thus realising 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 in the future.

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