PORTLAND, Ore.—The holy grail of low-k dielectrics—air-gap interconnections—will migrate from chip-level to board-level, according to the Semiconductor Research Corp. (SRC), a technology research consortium based in Research Triangle Park, N.C.
Ultra-low capacitance air-gaps on printed circuit boards—along with new solder-less copper connections—will allow higher frequency operation while simultaneously lowering power requirements, according to SRC's Focus Center research program and the Georgia Institute of Technology .
"Continued improvements in computer performance require greater off-chip bandwidth—that's the big bottleneck right now to building bigger systems," said Steve Hillenius, executive vice president at SRC. "These two advancements promise to increase the bandwidth of off-chip interconnects and enable those systems to be built."
Air-gap technology for chips was pioneered at IBM, where trenches are etched to provide the ultimate dielectric—air—to isolate signal lines from each other and the ground plane. Japan's Matsushita has also reported that it has air-gap technology for chips.
Georgia Tech, too, has done air-gap research for on-chip interconnections, but by using a MEMS-like sacrificial layer that is later removed after the copper wire is deposited. Now under a cooperative development agreement with SRC, Georgia Tech has repurposed its air-gap technology for printed circuit boards. The intellectual property for the technology is owned by Georgia Tech, but any SRC member has access to it for transfer to manufacturing. SRC members include AMD, IBM, Intel, Texas Instruments, Freescale and Global Foundries.
How's it work?
Air-gap technology for circuit boards adds an organic polycarbonate material to the layer of the PC board where the air gaps are supposed to reside. The whole surface of the PC board is coated with the polycarbonate, then heated, until it evaporates from everywhere except the copper traces. Later, after the rest of the PC board is assembled, the whole board is heated again to a higher temperature so that the organic polycarbonate evaporates from the copper traces, escaping as a vapor and leaving the air gaps behind.
"This particular polycarbonate goes directly to a gas from its solid phase, and has been made photosensitive so you can apply it and pattern it just like photo resist, but it is self-patterning on copper, because its decomposition temperature goes up by about 50 degrees on copper," said professor Paul Kohl, director of the Interconnect Focus Center at Georgia Tech. Kohl collaborated with University of Florida professor Rizwan Bashirullah on the work.
Sacrificial organic polycarbonate (grey) is patterned using traditional photolithography atop the copper wires (orange), then evaporated away after the PC boards assembly (bottom).
The second related breakthrough jointly announced by SRC and Georgia Tech, and also available to all SRC members, is solder-less connections between circuit boards and chips. Instead of solder, the new method uses electroless plating to grow a tiny billet of copper in-between the chip and the board, essentially creating a single continuous copper wire, thus avoiding all the problems associated with soldering.