Tuesday, December 29, 2009

CWRU Self-Welded Metal-Catalyzed Carbon Nanotube Bridges Create Solid Electrolytic Non-Volatile Memories with 10 Billion Memory Elements per Square Centimeter of Chip Area


Case Western Reserve University (Cleveland, OH) Massood Tabib-Azar reveals how to grow self-welded metal-catalyzed carbon nanotube bridges that can be used to form solid electrolytic non-volatile memories in U.S Patent  7,640,226.

Tabib-Azar developed systems and methods that simultaneously grow a plurality of carbon nanotubes on substrates and across large wafers via employing vapor deposition of material on the surface of the substrate and fluid flow to aid in and direct the growth of the nanotubes in pre-specified locations and directions. In addition, the nanotubes created can be used as gas and chemical sensors, electronic switches, resonators, and non-volatile memory devices.

The innovation provides for systems and methods that enhance CNT growth techniques resulting in self-welded single-walled and multi-walled CNTs grown between silicon or other appropriate posts. To achieve self-welding, a functionalized surface of one of the posts can allow the CNT to grow into that surface forming a very strong bond. The functionalized surface can be a carbon-covered post that can allow the carbon atoms in the CNT to attach themselves to the carbon atoms on the surface of the post.

Additionally or alternatively, the functionalized surface can comprise disparate active layers that support growing CNTs, forming a strong bond, and/or creating a junction for a variety of active devices. This bonding process can be demonstrated and confirmed by growing CNTs on an amorphous carbon layer. For example, bond strength measurements using AFM load-displacement measurements and related electrical measurement symmetry can be utilized for the samples. These samples show that the bond strength identified via employing the AFM technique can be quite large and can be higher than self-welded silicon wires.

An aspect of Tabib-Azar’s innovation is it addresses a difficulty in CNT-based electronics and electromechanical bridges: namely, simultaneously growing self directed nanotubes across large wafers that can weld themselves to the opposite posts without the aid of an electric field or other localized and externally applied forces.

According to Tabib-Azar, a copper layer can be deposited upon a generated CNT, and thereafter, the copper can be sulfidized to produce a thin layer of copper sulfide that forms a junction with the CNT. The CNT-CuS junction exhibits switching characteristics applicable in non-volatile memory devices.

For instance, upon injection of electrons into the CuS by the negatively-biased CNT, a region with neutral Cu atoms can be formed that has metallic conductivity. Further, when voltage polarity is reversed, the Cu-rich region can become depleted and the conductivity can decease such that the device can be switched off.

Thus, CNTs can be utilized to form junctions with nano-meter scale areas with a solid-electrolyte (e.g., CuS, . . . ), and the formation can be utilized in non-volatile memory devices, gas sensors, and the like. By employing CNTs, small devices can result due to small contact areas associated with the CNTs (e.g., diameters in the range of 1-100 nm). CNT-wired solid-electrolytic memories can be utilized with computing devices due to their cross-bar geometry and nanometer scale since they can be manufactured with very high device densities in excess of 10 billion memory elements per square centimeter chip area, for example.

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