Monday, March 14, 2011
Non-volatile memory bits can be made by applying phase-change material to previously created nanometer-scale gaps in carbon nanotube filamentary conductors, according to a paper published by researchers at the University of Illinois, Urbana-Champaign, Illinois.
The configuration achieves programming currents as low as 0.5 microamps for set and 5 microamps for reset operations, two orders of magnitude lower than state-of-the-art devices, the authors claim in their ScienceExpress paper, published March 10.
The researchers, led by Professor Eric Pop of University of Illinois, grew single-wall and multi-wall carbon nanotubes with diameters of between 1 and 6 nanometers and then used electrical breakdown, e-beam writing or an atomic force microscope to form a gap of less than 100 nanometers in each wire. The devices than received a sputtered layer of germanium-antimony-tellurium (GST) so that the gaps in the CNTs are filled with the phase-change material. Although amorphous GST is laid down over the entire device, the switching occurs only in the nanogap of the CNT, which is the location of highest electric field and heating effect.
The authors claim their results address the scaling of both the size and power reduction that is possible with programmable PCM bits. The paper reports on reversible switching with programming currents between 1 and 8 microamps, two orders of magnitude lower than state-of-the-art PCM devices. The PCM material switches between an off resistance of 50-megaohm to an on resistance of 2-Mohm but which can be as low 500-kohm, depending on sample prepation.
The authors report hundreds of switching cycles. Conventional PCM devices have been reported at more than 1 million cycles endurance.
The authors go on to suggest that 5-nm GST bits in a CNT gap could operate at 0.5-V and less than 1 microamp, such that nanosecond switching times would result in sub femtojoule-per-bit energy consumption. There is also the option to use GeSb which has a lower switching threshold. The authors claim the results are "encouraging for ultra-low power electronics and memory based on programmable PCM with nanoscale carbon interconnects."
However, the paper does not address such issues as physical density – the reported experiments are based on long lateral CNT structures – or the introduction of additional manufacturing complexity, so the development is a long way from being proven as a useful commercial option.
Micron Technologies Inc. and Samsung Electronics Co. Ltd. are the two companies most active in commercial phase-change memory. Both have 1-Gbit designs but neither company is boasting of high volume sales into commercial designs and concerns have been raised over the ability of conventional PCM-on-silicon memories to scale and compete with flash memory which is already at higher than 1-Gbit capacity implemented on processes with finer minimum dimensions.
By: DocMemory
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