[Engineering Feature]
Back To Nature For Next-Gen Semis
While the fight to scale down rages on, semiconductor manufacturers look more to nature for new technologies.
TECHNOLOGY RESEARCH With the cost of new state-of-the-art fabs approaching $4 billion and rising, semiconductor companies are pouring billions into research to find new methods to create lower-cost semiconductors and perhaps even replace CMOS altogether some day. One of the thought processes used by researchers involves turning to nature for the answer to scaling and cost woes:
Carbon nanotubes: By its very nature, carbon-nanotube (CNT) technology is ideally suited to create electric circuits at the nano scale. CNTs are a form of carbon that take on cylindrical shapes (Fig. 2) and exhibit outstanding bond strength (stronger than bonding in diamonds) and unique electrical properties, such as extremely high current densities.
CNTs have many applications outside of electric circuits, but to date, researchers haven't found a reliable method to arrange them into a complete circuit that's manufacturable on a large scale. CNTs naturally tend to align themselves into intersecting hexagons that form into tubes, which doesn't lend itself to forming complex semiconductor logic. However, the predictable patterning of memory structures in crosspoint formations may be a more ideal starting point for the use of CNTs. Researchers are looking for ways to create more complex CNT circuits by placing one end of the nanotube down and growing the rest into desired patterns using a catalyst in combination with a chemical vapor deposition process. That's because CNTs tend to grow along field lines from negative to positive polarity.
The last major hurdle researchers face in using CNTs in electric circuits involves controlling the actual type of CNT produced, from metallic, semiconducting, single-walled, and multiwalled. Currently, there's no reliable way to do this. So, researchers are turning to chemical engineering for the answers.
Superconducting circuits: Since Heike Kamerlingh Onnes discovered the phenomena of zero resistance in certain materials in 1911, other elements and compounds have been found to become superconducting at temperatures ranging from less than one degree K to 120 K.
In addition to being perfect conductors, the interiors of superconductors have zero magnetic field (the Meissner effect). Furthermore, another key property is that the magnetic penetration depth of superconductors is frequency-independent, unlike normal metals.
Superconductors also offer the following additional benefits over traditional metals and semiconductor devices:
Very low operating power
Zero dc electrical resistance
Orders of magnitude lower RF resistance
Virtually lossless and dispersionless transmission lines
Ultra-low power dissipation.
With these advantages, it's easy to understand why superconductors are ideal for high-speed RF and mixed-signal devices, such as data converters, phaselocked loops (PLLs), memories, and signal-processing applications.
Hypres is using superconducting microelectronic (SME) technology to create the industry's first all-digital transceiver by moving electrons right at the antenna—a process known as direct digitization. Signals are then processed on receive and transmit at ultra-high speed and accuracy using SME, thereby making the transceiver all-digital.
"When applied to wireless communications, 'Digital RF' offers profound improvements in wireless operating efficiency, data-signal strength and speed, power conservation, and equipment cost reduction," says Dick Hitt, CEO of Hypres.
The primary disadvantage to using superconducting materials is the requirement to cool them to cryogenic temperatures. (The Hypres SME technology becomes superconductive at 4 K.) However, advances in refrigerator technologies allow for much smaller refrigerators, with some being around the size of a breadbox or thermos.
Molecule cascade: If you like setting up dominos and knocking them over, you'll find IBM's new molecular cascade technology interesting. It employs the same domino-toppling effect.
When several carbon monoxide (CO) molecules are properly aligned, moving a specific CO molecule initiates a cascade of molecular movements. Scientists have combined these molecules to form very small logical AND and OR functions, data storage, and interconnect so they actually function as mini circuits (Fig. 3). In fact, these circuits are 260,000 times smaller than today's smallest semiconductor technology.
To date, scientists have built circuitry as complex as a three-input sorter that occupies a mere 12 by 17 nm. To give you an idea of scale, you could fit about 190 billion of these sorters on top of a standard pencil-top eraser (about 7 mm in diameter), give or take a few billion.
The basic idea is to arrange CO molecules on a copper surface in a metastable configuration that, under certain conditions, will cascade into a lower energy configuration similar to toppling dominos. By their nature, CO molecules exhibit weak repulsion when placed one lattice spacing apart and are monostable in this configuration.
If you think of an un-toppled set of dominos representing a logic zero and a toppled set as logic one, the same concept can be applied to a molecular cascade. IBM researchers Heinrich and Lutz found that if the intersections of such cascades were cleverly designed, logical ANDs and ORs could be created.
Heinrich and Lutz designed molecular arrangements that acted as crossovers (allowing two cascade paths to cross over each other) and fanouts (splitting one cascade into two or more paths).
One issue researchers must resolve before these cascades can be used as circuit elements is how to "reset," so they can perform their function more than once. Yet they do believe it's possible.
Reprints
