The running joke at IBM’s site in East Fishkill, N.Y., is that engineers who have been working there for 30 years still have trouble locating their own offices. And sure enough, finding your way through the low hedge of cubicles in the 300 Building is very nearly impossible. That’s because the architect designed it to reproduce the tortuous wiring scheme inside a microprocessor. That’s the rumor, anyway.
Now imagine having to insulate such a tangle, but in microcosm: many kilometers of nanometer-scale copper conductors in a sliver of semiconductor the size of a child’s thumbnail. That was the challenge facing IBM Fellow Dan Edelstein, who 10 years ago led an industry-wide switch from aluminum to copper chip wiring that has enabled every microprocessor since 1998. Now Edelstein is hatching his next revolution. With his colleague Satya Nitta, he is surrounding the conductors in IBM’s bleeding-edge microprocessors with holes. He thinks his competitors will have little choice but to follow his lead in the infinitesimal realms he is staking out.
Those holes—IBM calls them air gaps—are actually cavities of vacuum embedded in the insulation that surrounds the chips’ wiring. Air gaps may well be the solution to a problem that has been tripping up chip manufacturers for almost a decade: when you cram nearly 10 kilometers of wiring into a space smaller than a postage stamp, the signal on one wire is felt by its neighbors. The electric field between them can then impede the flow of current through the wires, and that slows down the signals they carry.
IBM is a bit cagey about saying exactly when the new technique will go into production, other than that it will be in chips slated for production in 2009. Sources familiar with the technology, however, say there’s a chance that it might go into production sometime this year.
The advance, like so many in the industry, comes not a minute too soon to prop up Moore’s Law, which insists that transistor density will double about every 20 months. That’s how Intel got from its 33-megahertz 486 processor in 1989 to its 2.9-gigahertz Xeon processor in 2007.
Ever since the microchip was invented, the basic rule of thumb has been that transistor size is the limiting factor on chip speed. “Transistors are the fluke of nature,” Edelstein explains. “They get faster when they get smaller, but nothing else does.” A microprocessor may have hundreds of millions of transistors, but no matter how fast those transistors get, they depend on wires, which get much slower when they get smaller. That’s a problem for the transistors, which must compensate by using more power. It’s also a problem for the wires, which must radiate heat from the extra power the transistors are using.
As it turns out, the source of the signal lag is not so much the metal interconnects themselves but rather the insulation between the wires. So the question of the moment is, what can you put between those wires to prevent the signal from leaking?
Vacuum is the best insulator known. Since the 1990s, many chip manufacturers besides IBM, including Infineon Technologies, in Munich, and STMicroelectronics, in Geneva, have experimented with vacuum cavities, and some have even built prototype chips. But two problems have kept the technology from entering production. A chip needs insulation to shield its wires from one another, but it also depends on that insulation for structural support to survive what can be a rough manufacturing process, as well as the often high temperatures on a printed circuit board. Fill the insulation with holes, and the whole chip might collapse. The second problem is making air gaps compatible with standard chip-fabrication techniques. Despite the performance gains that companies have realized on their test chips with air gaps, added equipment and exotic materials have canceled out the performance gain with a money drain.
But the state-of-the-art chip IBM unveiled back in May could usher in the era of vacuum. “The technology is quite impressive and innovative,” says James Meindl, a director of the Microelectronics Research Center at Georgia Tech. It’s structurally sound, and IBM’s design has reduced the signal lag enough to “buy back” between 10 and 15 percent of chip speed that would otherwise be lost. That performance boost is about what you get from doubling the density of a chip’s transistors, and IBM did it without exotic materials, new tools, or costly redesigns. “It’s a very straightforward process,” says James Watkins, codirector of the MassNanoTech Institute at the University of Massachusetts in Amherst. “It stands a good chance of being mainstreamed.”