Last week Intel and IBM both announced that they had figured out a way to further shrink the size of transistors, the tiny on-off switches that power computers. The trick, according to Intel, is introducing the metal hafnium into the mix—an addition that marks the first major change in transistor materials in four decades. Hafnium-based computer circuits would likely be denser, faster and consume less power than existing microprocessors.
"It's a very, very significant event," says electrical engineer Carlton Osburn of North Carolina State University, member of a research team that studied hafnium and other advanced transistor materials. "This directly addresses one of those grand challenges" in semiconductor manufacturing.
Although the companies have yet to release design details, Osburn and other experts were able to make some informed guesses about their inner workings and the challenges in manufacturing them.
Intel's demonstration consisted of a hafnium-based microprocessor capable of running three different computer operating systems. In its transistors, hafnium oxide plays the role of the so-called gate dielectric, an insulating layer that separates the transistor's electrode from its silicon channel for carrying current. A voltage emanating from the electrode switches the transistor on or off by controlling the flow of electrons across that channel. The key is making the insulator as thin as possible in order to switch the channel faster and pack more transistors onto a chip.
Over the past decade, Intel and other microchip makers had increasingly bumped up against a fundamental problem: electricity would begin leaking from the glasslike silicon dioxide insulating layer as its width shrank to nearly a nanometer. Consequently, the transistors required inordinate amounts of power.
To overcome this obstacle, chipmakers had to determine how to replace silicon dioxide with so-called high-k materials like hafnium and zirconium. A material's performance as a gate dielectric depends on its thickness and its k-value, or dielectric constant, which reflects its ability to store a charge. Because hafnium has a higher k-value than silicon dioxide, it should be able to do the same or better job at a thickness that prevents leakage. That advance would allow Intel to shrink the smallest dimension of its transistors from today's 65 nanometers to a svelte 45 nanometers, keeping the furious pace of transistor miniaturization on its expected track.
The beauty of silicon dioxide was that manufacturers could grow it simply by placing a silicon wafer into a vessel filled with oxygen, Yale University electrical engineer Tso-Ping Ma says. Producing hafnium oxide transistors would require chipmakers to add multiple new steps to the manufacturing process—in part because the electrodes must be fashioned from metal, instead of from a form of silicon, to remain compatible with the hafnium. Initial production costs would probably be higher and early chips likely to contain more defects, Ma says, because the materials would be more sensitive to heat and other influences.
Osburn says that a hafnium transistor would still need a thin layer of silicon dioxide at the bottom of the gate insulator to dissipate excess charges that would otherwise accumulate there and interfere with the device. Ma, who says he has worked with both the Intel and IBM research groups but is not privy to either's design, adds that the presence of silicon dioxide would require chipmakers to add nitrogen to the hafnium oxide as well. Without it, he says, the insulator would only have a modestly improved k-value that would be insufficient for the next two or three reductions in transistor size.
The only scientific question Ma sees is in the metal electrodes. There are actually two varieties of transistors used in computer circuits, and each requires its own type of metal. Ma says that one compound would most likely be a stable material such as titanium nitride or tantalum carbide, but that he does not know what the other metal would be.