As you read this, two of our most advanced fabs here at Intel are gearing up for the commercial production of the latest Core 2 microprocessors, code-named Penryn, due to start rolling off the lines before the year is up. The chips, based on our latest 45-nanometer CMOS process technology will have more transistors and run faster and cooler than microprocessors fabricated with the previous, 65-nm process generation. For compute-intensive music, video, and gaming applications, users will see a hefty performance increase over the best chips they are now using.
A welcome development but hardly big news, right? After all, the density of transistors on chips has been periodically doubling, as predicted by Moore’s Law, for more than 40 years. The initial Penryn chips will be either dual-core processors with more than 400 million transistors or quad-core processors with more than 800 million transistors. You might think these chips don’t represent anything other than yet another checkpoint in the inexorable march of Moore’s Law.
But you’d be wrong. The chips would not have been possible without a major breakthrough in the way we construct a key component of the infinitesimal transistors on those chips, called the gate stack. The basic problem we had to overcome was that a few years ago we ran out of atoms. Literally.
To keep on the Moore’s Law curve, we need to halve the size of our transistors every 24 months or so. The physics dictates that the smallest parts of those transistors have to be diminished by a factor of 0.7. But there’s one critical part of the transistor that we found we couldn’t shrink anymore. It’s the thin layer of silicon dioxide (SiO2 ) insulation that electrically isolates the transistor’s gate from the channel through which current flows when the transistor is on. That insulating layer has been slimmed and shrunk with each new generation, about tenfold since the mid-1990s alone. Two generations before Penryn, that insulation had become a scant five atoms thick.
We couldn’t shave off even one more tenth of a nanometer—a single silicon atom is 0.26 nm in diameter. More important, at a thickness of five atoms, the insulation was already a problem, wasting power by letting electrons rain through it. Without a significant innovation, the semiconductor industry was in danger of encountering the dreaded “showstopper,” the long-awaited insurmountable problem that ends the Moore’s Law era of periodic exponential performance gains in memories, microprocessors, and other chips—and the very good times that have gone with it.
The solution to this latest crisis involved thickening the insulator with more atoms, but of a different type, to give it better electrical properties. This new insulator works well enough to halt the power-sucking hail of electrons that’s plagued advanced chips for the past four years. If Moore’s Law crumbles in the foreseeable future, it won’t be because of inadequate gate insulation. Intel cofounder Gordon Moore, of Moore’s Law fame, called the alterations we made in introducing this latest generation of chips ”the biggest change in transistor technology” since the late 1960s.
As difficult as finding the new insulator was, that was only half the battle. The point of the insulator is to separate the transistor’s silicon gate from the rest of the device. The trouble is, a silicon gate didn’t work with the new insulator material. The initial transistors made with them performed worse than older transistors. The answer was to add yet another new material to the mix, swapping the silicon gate for one made of metal.
It may not seem like such a big deal to change the materials used in a transistor, but it was. The industry went through a major upheaval several years ago when it switched from aluminum interconnects to copper ones and—at the same time—from SiO2 cladding for those interconnects to chemically similar “low-k ” dielectrics. And those changes had nothing to do with the transistor itself. A fundamental change to the composition of the transistor is pretty much unheard of. The combination of the gate and the insulator, the gate stack, hasn’t changed significantly since Moore, Andrew S. Grove, and others described it in this magazine back in October 1969!
So when you boot up your next machine and you’re surprised by how fast it rips through some video coding, remember: there’s more new under its hood than in any computer you’ve ever owned.
The story of how we and our co-workers solved the gate-insulation problem may seem esoteric, and in a literal way it is. But it is also emblematic of how Moore’s Law, the defining paradigm of the global semiconductor industry, is being sustained against often-daunting odds by the swift application of enormous intellectual and material resources to problems that, increasingly, are forcing engineers to struggle in realms until recently occupied only by physicists.