Wednesday, September 20, 2006

Bringing Light to Silicon

Intel has announced a new type of silicon laser that can transfer data on a beam of light--and could make computers many times faster.

By Kate Greene

A hybrid silicon laser could speed up computers by replacing the bottleneck of copper wires that route electrons between transistors. The yellow strips in this image are metal contacts that allow current to flow into indium phosphide (orange), the light-emitting material. Photons are collected and concentrated by the silicon cavities (gray), where laser beams (green) are emitted. (Credit: Intel)

Researchers at Intel and the University of California, Santa Barbara announced on Monday that they've succeeded in building a silicon-based laser that could be easily fabricated using the same manufacturing tools as those used to make microprocessors. They believe that the light source, dubbed a hybrid silicon laser, is the device that will finally allow engineers to integrate photonics inexpensively into computer chips.

The advantages of adding lasers to microprocessors are evident in the fiber optics industry: by encoding data in light, it's possible to pipe information through fiber at a speed of gigabytes per second. The catch is that optical devices, such as lasers, modulators, and detectors, are relatively expensive and complicated to make; hence, the computer industry hasn't been able to take advantage of this high-bandwidth technology.

Instead, today's microprocessors rely on copper wires to route electrons between transistors. With billions of transistors in each processor, and multiple processors built into computers, copper creates a significant bottleneck.

The hybrid laser would let data zip between transistors and chips at unprecedented speeds--it might allow engineers to rethink computer architecture, says Mario Paniccia, director of Intel's Photonics Technology Labs. "It could really change the way you look at computing," he says. "We've found a way to integrate a light source into silicon in a volume manufacturing sort of way," says Paniccia, "and the performance is good."

By engineering a new type of laser that combines the light-emitting properties of a material called indium phosphide, and the light-routing properties of silicon, Paniccia and John Bowers, professor of electrical and computer engineering at UCSB, have overcome earlier challenges that kept silicon-based lasers from being feasible.

While silicon is not naturally a good light emitter, it does have the ability to confine and route light. This makes it an ideal material for the laser's cavity, where photons bounce back and forth, building up enough intensity to eventually produce a laser beam.

Some researchers have tried to affix external light sources to silicon cavities. The problem with this approach, says Paniccia, is that it is prohibitively expensive and difficult to perfectly align an external light source with nanometer-scale silicon cavities in the manufacturing process.

To solve this problem, the researchers built their light source directly onto the cavity. They first etched laser cavities in silicon, using the same lithography process used to produce Intel's microprocessors. Separately, they built an indium phosphide light emitter. Next, the silicon and the indium phosphide were bonded together in a unique process that uses a thin layer of "glass glue" only 25 atoms thick. The glue is needed, explains Paniccia, because the atoms of silicon and indium phosphide don't naturally line up when directly bonded together, resulting in a nonfunctioning device.