Tokyo -- As CPU performance continues its rapid climb, engineers everywhere face a similar challenge: how to speed signals from ever-faster central-processing units to peripheral large-scale integration chips. Fully expecting electrical transmission to max out eventually, Toshiba has responded with optoelectronics that can zip signals at a trillion bits a second.
A Toshiba engineering team presented its technology at the 56th Electronics Components and Technology Conference in San Diego last week.
Other companies have reported similar speed breakthroughs. In technical papers, claims have included data rates of 640 Gbits/second (10 Gbits/s/channel) and 800 Gbits/s (12.5 Gbits/s/channel).
While different optoelectronic package structures have specific advantages and disadvantages, Toshiba said its package is good for high data rates and suitable for reflow processes, while offering stable optical coupling. "We hope that the . . . package will accelerate the emergence of high-performance CPUs that require such a package," said Hideto Furuyama, senior specialist in the advanced back end of the line (BEOL) technology unit of Toshiba's Center for Semiconductor R&D.
"We'll be ready for volume production around 2008 to 2009, when MPUs [multiprocessing units] requiring Tbit/s-level transmission start to emerge," he said.
Toshiba said its optical package does not require special processes peculiar to optical connections, allowing it to go through current volume production lines.
In the optoelectronics package architecture--called the post-reflow optical interface--an optical interface module is placed on a conventional CPU mounted on a standard printed-circuit board. High-speed signals are drawn out to the optical interface for transmission via optical fibers. Meanwhile, other signals are wired onto the pc board. By using a reflow process, an LSI is first mounted on an intermediate layer--an interposer--after which the interface goes on top of the interposer. Toshiba said it will supply samples on request.
"We aimed at developing a simple optoelectronic package that can be produced in volume," said Furuyama. When CPUs need Tbit/second I/O data rates, a conventional package like a ball-grid array can't get the job done, he said. That's when an optoelectronic LSI package is called on to replace traditional packages.
And to control costs, Toshiba looks to build in bulk. "We think that the new optoelectronic package should be made in volume, more than 1 million units a month," Furuyama said. "We want to make it drastically inexpensive compared to other forms of optoelectronic LSI packages."
For volume production, the optoelectronic package fits into present manufacturing processes. An interposer mounted with high-speed LSI undergoes solder reflow so it can be put on a glass epoxy substrate--the most popular pc board.
After reflow, the optical interface module is mechanically attached to the upper surface of the interposer. That allows the optical interface module to be fabricated apart from the standard reflow process, without exposure to high temperatures.
The optical interface module consists of optoelectronic ferrules (fiber-optic holders) with optical lasers and photo diodes, buffer ICs, optical fibers and electrical contact pads for the interposer.
Digital signals flow between the LSI on the interposer and the buffer ICs integrated into the optical interface module. Because the distance between LSI and buffers is very small, signals pass without a drastic loss of quality. The buffer ICs are also arranged close to the optical semiconductors, to keep analog signal paths short and further minimize degradation.
For the compact interface module, Toshiba developed an optoelectronic ferrule--4.4 x 4.5 x 1 mm--to hold 12 fiber-optic channels. The fiber-optic cable was deemed smooth enough to eliminate polishing, thus reducing costs. Each optic fiber's edge and a set of lasers and photo diodes are fixed with resin, further lowering the interface's cost, Furuyama said.
The package has 24 optical lines, each with 10.7-Gbit/s bandwidth. If all four sides of the LSI interface package have optical connections, the total data rate is 10.7 x 24 x 4 (all sides) = 1,027.2 Gbits/s.
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