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(萌妹社区Org.com) -- Computer chips that transmit data with light instead of electricity consume much less power than conventional chips, but so far, they've remained laboratory curiosities. Professors Vladimir Stojanovi膰 and Rajeev Ram and their colleagues in MIT's Research Laboratory of Electronics and Microsystems Technology Laboratory hope to change that, by designing optical chips that can be built using ordinary chip-manufacturing processes.

鈥淚 don鈥檛 see anyone else that鈥檚 doing that,鈥� says Michael Watts, a researcher at Sandia National Laboratories who鈥檚 also working on optical chips. 鈥淚f they鈥檙e successful at that, then convincing a major processor or memory manufacturer that this is a viable approach will be much, much easier.鈥�

Granted access to the same manufacturing facilities that Texas Instruments uses to produce cell phone chips and microprocessors, the MIT researchers have demonstrated that they can put large numbers of working optical components and electronics on the same chip. But so far, the electronics haven鈥檛 been able to control the optics directly. That鈥檚 something that Stojanovi膰 hopes to show with a new batch of chips due back from TI and another major semiconductor manufacturer this winter.

Optical data transmission could solve what will soon be a pressing problem in chip design. As chips鈥� computational capacity increases, they need higher-bandwidth connections to send data to memory; otherwise, their added processing power is wasted. But sending more data over an electrical connection requires more power.

Smaller transistors are more energy-efficient than larger ones, so over time, chips鈥� total power consumption has changed little. But 鈥渢he fraction of power that鈥檚 used for communications has grown,鈥� Watts says. 鈥淎t some point, you have to devote all your power to communications. And that point鈥檚 not too far off. And then what鈥檚 left for computation? Nothing.鈥� Future chips could simply draw more power, but then they would also be harder to cool, and the battery life of laptops and handheld devices would dramatically shorten.

So chip companies would welcome a more energy-efficient way to move data around -- if they were confident that it was cost-effective. And that鈥檚 why demonstrating compatibility with existing manufacturing processes would be so persuasive.

Manufacturers build chips by sequentially depositing layers of different materials 鈥� like silicon, silicon dioxide, and copper 鈥� on a wafer of silicon, and then etching the layers away to build three-dimensional structures. The problem with using existing processes to build optical components is that the deposition layers are thinner than would be ideal. 鈥淵ou would want a normal photonic device to be a little bit taller and thinner so that you can minimize the surface-roughness losses,鈥� Stojanovi膰 says. 鈥淗ere you don鈥檛 have that choice because the film thicknesses are set by fabrication.鈥�

Optical chips use structures called waveguides to direct light, and researchers trying to add optical components to a silicon chip usually carve the waveguides out of a single crystal of silicon, Stojanovi膰 says. But waveguides made from single-crystal silicon require insulating layers above and below them, which standard chip-manufacturing processes like TI鈥檚 and Intel鈥檚 provide no way to deposit. They do, however, provide a way to deposit insulators above and below layers of polysilicon, which consists of tiny, distinct crystals of silicon clumped together and is typically used in the part of a transistor called the gate. So the MIT researchers built their waveguides from polysilicon instead.

So far, TI has produced two sets of prototypes for the MIT researchers, one using a process that can etch chip features as small as 65 nanometers, the other using a 32-nanometer process. To keep light from leaking out of the polysilicon waveguides, the researchers hollowed out the spaces under them when they got the chips back 鈥� the sole manufacturing step that wasn鈥檛 possible using TI鈥檚 in-house processes. But 鈥渢hat can probably be fixed more elegantly in the fabrication house if they see that by fixing that, we get all these benefits,鈥� Watts says. 鈥淭hat鈥檚 a pretty minor modification, I think.鈥�

The MIT researchers鈥� design uses light provided by an off-chip laser. But in addition to guiding the beam, the chip has to be able to load information onto it and pull information off of it. Both procedures use ring resonators, tiny rings of silicon carved into the chip that pull light of a particular frequency out of the waveguide. Rapidly activating and deactivating the resonators effectively turns the light signal on and off, and bursts of light and the gaps between them can represent the ones and zeroes of digital information.

To meet the bandwidth demands of next-generation chips, however, the waveguides will have to carry 128 different wavelengths of light, each encoded with its own data. So at the receiving end, the ring resonators provide a bank of filters to disentangle the incoming signals. On the prototype chips, the performance of the filter banks was 鈥渢he most amazing result to us,鈥� Stojanovi膰 says, 鈥渨hich kind of said that, okay, there鈥檚 still hope, and we should keep doing this.鈥� The wavelength of light that the resonators filter is determined by the size of their rings, and no one 鈥� at either TI or MIT 鈥� could be sure that conventional manufacturing processes were precise enough to handle such tiny variations.

Stojanovi膰 hopes that the next batch of prototypes, which should give the chips鈥� electronics control over the optical components, will demonstrate that the resonators perform as well when loading data onto light beams. At the same time, the team is looking to extend its approach to memory chips. 鈥淭he memory鈥檚 a much tougher nut to crack, because it is such a cost-driven business, where every process step matters,鈥� Stojanovi膰 says. 鈥淭hings are a lot harder to change there, and optics really needs to be absolutely compatible with process flow.鈥� But if memory chips as well as processors sent data optically, Stojanovi膰 says, then in addition to saving power, they could make computers much faster. 鈥淚f you just focus on the processor itself, you maybe get a 4x advantage with photonics,鈥� Stojanovi膰 says. 鈥淏ut if you focus on the whole connectivity problem, we鈥檙e talking 10, 20x improvements in system performance.鈥�

Provided by MIT