Stojanovich, Ram & Watts advance future of US optoelectronic chip production

February 15, 2012

EECS faculty members Vladimir Stojanovic, Rajeev Ram and Michael Watts are collaborating to build the case for integrating optoelectronic and electronic chip components to create the next generation of energy efficient and high performing chips to move data using light instead of electricity. Read more about this work and why the US stands to lead in the manufacture of these future opto-electronic chips in the

A new test chip developed by Vladimir Stojanovic, Rajeev Ram and their colleagues, which monolithically integrates electrical and optical components and was produced on an existing IBM manufacturing line.Image: Vladimir Stojanovic, Rajeeve Ram and Milos PopovicAs the United States seeks to reinvigorate its job market and move past economic recession, MIT News examines manufacturing’s role in the country's economic future through this series on work at the Institute around manufacturing.

The most recent of those to open, however, is in China, and while that may have been a strategic rather than economic decision — an attempt to gain leverage in the Chinese computer market — both the Chinese and Indian governments have invested heavily in their countries’ chip-making capacities. In order to maintain its manufacturing edge, the United States will need to continue developing new technologies at a torrid pace. And one of those new technologies will almost certainly be an integrated optoelectronic chip — a chip that uses light rather than electricity to move data.

Optoelectronic chips could drastically reduce future computers’ power consumption. But to produce the optoelectronic chips used today in telecommunications networks, chipmakers manufacture optical devices — such as lasers, photodetectors and modulators — separately and then attach them to silicon chips. That approach wouldn’t work with conventional microprocessors, which require a much denser concentration of higher-performance components.

In a 2010 paper in the journal Management Science, Erica Fuchs, an assistant professor of engineering and public policy at Carnegie Mellon University, who got her PhD in 2006 from MIT’s Engineering Systems Division, and MIT’s Randolph Kirchain, a principal research scientist at the Materials Systems Laboratory, found that monolithically integrated chips were actually cheaper to produce in the United States than in low-wage countries.

During the telecom boom of the late 1990s, Fuchs says, telecommunications companies investigated the possibility of producing monolithically integrated communications chips. But when the bubble burst, they fell back on the less technically demanding process of piecemeal assembly, which was practical overseas. That yielded chips that were cheaper but also much larger.

Making the case

To try to get U.S. chip manufacturers to pay more attention to optics, Stojanovic and professor of electrical engineering Rajeev Ram have been leading an effort to develop techniques for monolithically integrating optical components into computer chips without disrupting existing manufacturing processes. They’ve gotten very close: Using IBM’s chip-fabrication facilities, they’ve produced chips with photodetectors, ring resonators (which filter out particular wavelengths of light) and waveguides (which conduct light across the chip), all of which are controlled by on-chip circuitry. The one production step that can’t be performed in the fabrication facility is etching a channel under the waveguides, to prevent light from leaking out of them.

Loyal opposition

This approach falls somewhere between monolithic integration and the piecemeal-assembly technique used today. Because it involves several additional processing steps, it could prove more expensive than fully realized monolithic integration — but in the near term, it could also prove more practical, because it allows the performance of the optics and electronics to be optimized separately. As for why the researchers would collaborate on two projects that in some sense compete with each other, Watts says, “Sometimes the best policy is: When you come to a fork in the road, take it.”

Getting on board

The lasers used in telecommunications networks, however, are made from exotic semiconductors that make them even more difficult to integrate into existing manufacturing processes than photodetectors or waveguides. In 2010, Lionel Kimerling, the Thomas Lord Professor of Materials Science and Engineering, and his group demonstrated the first laser built from germanium that can produce wavelengths of light useful for optical communication.

But the real advantage of on-chip lasers, Kimerling says, would be realized in microprocessors with hundreds of cores, such as the one currently being designed in a major project led by Anant Agarwal, director of MIT's Computer Science and Artificial Intelligence Laboratory (CSAIL). “Right now, we have a vision that you need one laser for one core, and it can communicate with all the other cores,” Kimerling says. “That allows you to trade data among the cores without going to [memory], and that’s a major energy saving.”