Design handles iffy nanocircuits
By
Kimberly Patch,
Technology Research NewsScientists are getting better at growing molecular-scale nanotubes and nanowires, which is paving the way for packing trillions of ultrasmall circuits on computer chips.
These tiny circuits pose challenges that don't show up at larger scales, however. One of the biggest has to do with the number of defects in a device.
Nanoscale circuits are more sensitive to temperature changes, cosmic rays and electromagnetic interference than today's circuits. "For such a densely-integrated circuit to perform a useful computation, it has to deal with the inaccuracies and instabilities introduced by fabrication processes and the tiny devices themselves," said Jie Han, a research assistant at Delft University of Technology in the Netherlands.
If it is difficult to make a defect-free device that has several million electrical circuits, it is orders of magnitude more difficult to manufacture devices with trillions of circuits. Today's Pentium 4 computer chips contain about 42 million transistors.
Rather than having to discard rising numbers of devices that don't make the grade, researchers are exploring ways to build defect tolerance into electronics so the hardware will work even when it contains a sizable percentage of faulty circuits. "Future nanoelectronics architectures will have to be able to tolerate an extremely large number of defects and faults," said Han.
Han and a Delft University colleague have come up with an architecture that combines circuits that have redundant logic gates and interconnections with the ability to reconfigure structures at several levels on a chip. The combination of redundancy and reconfigurability allows the system to identify and discard minor or modest faulty signals and reconfigure to avoid more severe problems.
Redundant circuits allow one or a few circuits to produce errors without the logic gate as a whole giving the wrong result. "If modest errors occur, some components are carrying on faulty signals while others in the majority are still working properly," said Han.
The chip's reconfigurability compensates for errors that have a larger effect, said Han. "If the effect of errors is tremendous, some computing modules will fail," he said. In this case, the hierarchical reconfigurability compensates for the loss.
The researchers' simulations show that the architecture is capable of tolerating a relatively large number of faults, said Han. The architecture can tolerate a failure rate of 0.2 percent, which is much higher than today's chip designs can tolerate, said Han. A trillion-circuit chip based on the architecture should be able to work reliably if at least 100 billion of the circuits are functioning, he said.
The architecture has potential for correcting both permanent defects in a chip, and transient faults, which may be common in systems based on nanometer-scale devices, Han said.
The idea of redundant logic circuits goes back to the father of modern computing, John von Neumann, but relying only on redundancy to catch errors requires an impractical number of circuits. Adding reconfigurability to the architecture cut down considerably on the amount of redundancy needed, said Han.
Defect and fault tolerance is clearly a very important problem as devices get smaller, said Seth Goldstein, an associate professor of computer science at Carnegie Mellon University.
The reconfigurable nature of the researchers' approach is "absolutely essential" to solving this problem, said Goldstein. "If you can reconfigure around defects then you can have less waste -- you can figure out where the defects are and... route around them," he said.
The researchers are heading in the right general direction, and this is important for the field, but they may not have gone far enough in making the devices reconfigurable, said Goldstein. "They have some reconfigurable resources but their basic plan is to... present a defect-free circuit after they've done one step of post-manufacturing reconfiguration," said Goldstein.
It remains to be seen how feasible this approach is, versus an even more flexible approach that allows for more reconfiguration, according to Goldstein. The approach "seems to be fairly brute-force," he said. "If you look at the overhead that they have to pay for the end result it seems like it's not much better."
The researchers are working on further simulations in order to make the architecture more efficient, said Han. "The goal is to achieve maximum fault-tolerance by using minimum component redundancy," he said.
The architecture could be used for computers made with any type of circuit, including fantastically small devices made from molecules, nanowires or carbon nanotubes, and single electron tunneling circuits, said Han. Nanowires and carbon nanotubes can be as small as one nanometer in diameter, which is the size of a row of 10 hydrogen atoms. A nanometer is one millionth of a millimeter.
The architecture could be ready to be applied to practical devices in less than two years, according to Han.
Han's research colleague was Pieter Jonker. The work appeared in the January 16, 2003 issue of Nanotechnology. The research was funded by Delft University of Technology.
Timeline: < 2 years
Funding: University
TRN Categories: Nanotechnology; Integrated Circuits; Architecture
Story Type: News
Related Elements: Technical paper, "A Defect-and Fault-tolerant Architecture for Nanocomputers," Nanotechnology, January 16, 2003.
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March 26/April 2, 2003
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