After a decade of research, conversion of electrical signals to light for high speed transmission at the die level, the module level, and the board level, is edging into production. Suppliers say integration of photonics on silicon—with conventional CMOS technology—is what's needed to meet data center demands for next generation performance.
"Optics is now a key bottleneck for the challenges of size, cost and power in the networking space, and CMOS photonics is the likely solution to those challenges," suggests Mark Nowell, Cisco Systems senior director of engineering, CTO in the Shared Services and Transceiver Module Group, who will speak about the recent progress in the technology at SEMICON West in San Francisco in July. "After years of R&D, it's now making the transition to commercial products."
Making the optical modulators, multiplexers and detectors all in CMOS instead of assembling separate optical components greatly reduces size, cost and power usage. First applications have been active optical cables between server units and pluggable transceivers at the front panel, but moving the optical connections to the board level may be next. Analyst firm Yole Développement sees the silicon photonics market reaching some $215 million by 2017.
IBM takes a die-level monolithic approach with its 25-Gbps per channel WDM optical transceiver on single CMOS chip, announced in December, and now in qualification. The device integrates the optical modulator, photo detector and passive photonics features all in 90-nm CMOS.
"We looked at 3-D integration, too, but decided monolithic integration was the most cost effective in the near term," notes Yurii Vlasov, manager of the silicon nanophotonics project at IBM, who will discuss IBM's approach. He suggests that monolithic integration addresses packaging and testing costs at the same time as die costs, and also, importantly, allows semiconductor-like inline testing early in production.
Vlasov notes as well that the goal was to use the existing fab facilities, and not spend any additional capital. The company uses a germanium-first process on a high-resistivity SOI substrate to build a germanium photo detector for optical- to- electrical conversion; then makes passive waveguides, filters and couplers in shallow trench isolation; adds a modulator for electrical- to- optical conversion in shallow silicon etch; and then continues with the analog and mixed-signal amplifiers and control circuits. This single transceiver chip with optical and electrical I/Os can be packaged either for a pluggable module at the server, or for mounting on a circuit board, or for putting in a socket with a processor.
"The choice is all just a question of maturity—and especially reliability," says Vlasov.