Ultra-high densities would enable processing and memory power to reach unprecedented levels.

A program to develop ultra-dense integrated digital data-processing systems and circuits based on nanowires involves utilization of hybrid top-down and bottom-up assembly techniques to implement designs representative of a highly reliable defect- and fault-tolerant architecture. This program has included fabrication and assembly of molecular- scale logic blocks based on arrays of overlapping semiconducting nanowires, using novel wafer-scale assembly techniques. On the basis of breakthrough addressing techniques, these logic blocks have been connected to ultradense memory blocks and to external complementary oxide/semiconductor (CMOS)-process lithographic interfaces for testing. One of the main underlying ideas is to construct highly reliable components out of high-defect-density logic and memory elements, using recently developed sublithographic-scale programmable- logic-array architectures that incorporate novel reliable-circuit concepts and higher-level redundancy mechanisms. Using state-of-the-art modeling techniques and computational simulations, test designs have been optimized, various defect-tolerance approaches have been developed, and development and optimization of larger systems are continuing.

The accomplishments of this program thus far include the following:

Demonstration of Basic Logic Structures Without Feedback for Two-Bit Words

• A two-bit adder consisting of AND1/ inverter/AND2 stages was designed and simulated.
• It was demonstrated that two-bit adder structures could be consistently fabricated following a hybrid assembly-andlithography- based approach.
• Basic properties and statistics of device elements were characterized.
• Two-bit adder structures were assembled and validated by demonstrating them to be capable of performing an essential product-of-sums function.

Demonstration of Local Device Pitch of ≈ 100 nm

• It was shown that following the assembly- and-lithography approach, one can fabricate reliably crossed arrays of nanowires for logic circuits having local device pitches of the order of 100 nm. Demonstration of Two to Five Logic Tiles on One Die
• Hybrid assembly and lithography were demonstrated for fabrication of two or more logic tiles per standard die or chip, using a scalable process that is readily extensible to fabrication of devices for performing more-complex logic and computation.

Demonstration of State-of-the-Art Nanowire Devices

• A core/shell germanium/silicon nanowire heterostructure transistor having scaled performance parameters that exceed those of the best corresponding CMOS transistors by a factor of about 4 was demonstrated.
• A core/shell silicon/barium titanate structure that functions as the first nonvolatile and reversibly switchable nanowire transistor was demonstrated.

Continuation of this program promises to lead to development of computer hardware offering unprecedented levels of processing and memory power. Such hardware could enable satisfaction of requirements for cheap, highly dispersed, sensor-processor-memory communication systems for military applications. After further development, such hardware might be useful for massive direct digital electronic interfaces to nerves, scalable emulation engines, and other applications yet to be imagined.

This work was done by Charles M. Lieber of Harvard University for the Air Force Research Laboratory. For further information, download the free white paper at www.defensetechbriefs.com under the Electronics/Computers category. AFRL-0013

This Brief includes a Technical Support Package (TSP).

Assembly of Nanowire-Based Computing Systems (reference AFRL-0013) is currently available for download from the TSP library.

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