Monday, January 01, 2007

World's Fastest Optical Chip

Continued from page 1

By Kate Greene

Photonic Fabrication

Making the Infinera chips is no simple task. Optical devices are three-­dimensional structures, far more challenging to manufacture than two-dimensional silicon transistors. Making the lasers, detectors, modulators, and other components of the finished chip requires repeatedly depositing and etching away many thin layers of different materials, such as indium gallium arsenide and indium phosphide.

Infinera's process starts with a wafer of indium phosphide. The wafer moves along an assembly line, where it is coated with a syrupy chemical called photoresist. Ultraviolet light shines through a mask with stencil-like designs and irradiates the photoresist, effectively "developing" intricate patterns that allow some semiconductor material to stay on the wafer and some to be etched away.

At a high level, it's the same as the photolithography that companies like Intel use to make silicon microprocessors for your PC. But there's an important difference. "In an Intel chip, it's all silicon. In optics you use various semiconductors with various functions," Welch says. And the indium phosphide wafers go through many more rounds of deposition and etching than silicon wafers do. Infinera is tight-lipped about the details of its manufacturing process, which was designed with the help of engineers experienced in such tasks as manufacturing silicon microchips and mass-producing light-emitting diodes. Welch says the company has exclusive patents on key aspects of the technology for placing large numbers of devices on indium phosphide wafers.

The 1.6-terabit chip differs from the 100-gigabit version largely in the number of devices patterned onto it. Each 100-gigabit chips contains, among other components, 10 lasers, 10 detectors, 10 modulators (which encode data by switching light on and off), and 10 waveguides that direct photons into a multiplexer. The 1.6-terabit chip's 240 components include 40 lasers, 40 detectors, 40 modulators, and 40 channels. And each modulator encodes data four times as fast.

After the wafers come off the line, they are sliced into chips--several hundred of them. Finally, the chips are tested for potential malfunctions, combined with electronic chips built by Infinera on a device called a line card, and installed in optical networking units for shipment.

Demand for Internet video and voice services is exploding, threatening to overwhelm the typical broadband connection, which transmits between one and six megabits per second. "We're all thinking that people will need 25, 50, or 100 megabits," Welch says. To meet that demand, Internet companies will have to pack more equipment into already overcrowded switching stations. "With Internet traffic growing at 60 to 100 percent per year, you can't keep installing ­refrigerator-size racks in the basement," Welch says. "Photonic integration becomes the technology that enables the Internet to grow."