NIST adds new twist to nanowires
Research shows promise for improvements in capacity and stability for flash memory
- By William Jackson
- Jun 22, 2007
Photo by the National Institute of Standards and Technology
Researchers at the National Institute of Standards and Technology have demonstrated a new type of nonvolatile memory device that combines silicon nanowires and traditional layered-storage technology to produce a stable, high-density chip with low power consumption.
The technology is years away from commercial implementation, but the proof of concept shows that the techniques are practical and worth further research, said Qiliang Li, a scientist in NIST's Semiconductor Electronics Division.
Li, along with NIST project lead Curt Richter and colleagues from George Mason University in Fairfax, Va., and Kwangwoon University in Seoul, did the fabrication at NIST labs in Gaithersburg, Md.
Nanowires are wires with a diameter of a nanometer or less. For many purposes, they act essentially as one-dimensional objects, because they have little breadth or depth. They can be made of metals or semiconducting material such as silicon, which was used by the NIST team. Because electrons are constrained in a narrow area, they display different electrical qualities from larger objects.
The idea of using nanowires for memory is not new, but the NIST team's contribution was a technique to grow and self-assemble the wires on semiconductor-oxide-nitride-oxide-semiconductor (SONOS) chips. This hands-off approach could make manufacturing more practical, Richter said.
'This approach is engaged to provide a smooth transition for integrating [silicon nanowire] into viable memory devices with the well-known SONOS structure,' the researchers wrote in an article published in the June issue of Nanotechnology. 'In addition, the'device exhibits reliable write-read-erase operations with a large memory window.'Open windows
The memory window refers to the range of voltages that can be used to read or write to the device.
'Having a large window means it is easier to write to it, and it has a very long storage time,' Richter said.
The NIST program produced silicon nanowires about 20 nm in diameter. Charges pass through the nanowire tunnel into the stacked device and are stored in the oxide-nitrite interface. Positive and negative charges can be read as 1s and 0s and used to store data. Because the data remains stored when no power is applied, it is nonvolatile memory.
Li said the technology, if commercially developed, could be applied to devices such as flash drives and USB memory sticks to provide greater capacity and stability. Just how much more capacity or stability is not known. Researchers have not quantified advantages of the prototype nanowire technology compared to traditional flash memory devices and, in fact, are not yet claiming any advantages, Richter said.
'By no means should the results of this research be compared with an optimized operating technology,' he said. But the results are good enough to warrant further work, he added.
'We have a proof of concept,' Li said. The next steps would be to optimize the new technology and integrate it with existing applications.
Development could take several more years for the basic research, and creating a commercial product could take anywhere from an additional several years to 'a long time,' depending on the complexities of the fabrication process, Richter said.
'It's primarily a manufacturing issue,' rather than a physics problem, he said. How quickly development occurred would depend largely on economic drivers for changing the manufacturing process companies now have in place. 'Current technology works so very well' that economic drivers might not emerge for quite a while.
'This isn't a truly radical change, but it is a substantial change,' Richter said. The technique of growing nanowires in place demonstrated by the NIST team could offer a practical method of fabricating new devices, but not necessarily an easy one. 'It's still hard,' Richter said, 'but not as hard.'