Recently Intel
and IBM both announced that they had figured out a way to further shrink
the size of transistors. 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.
Although the
companies have yet to release design details, 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, 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 45 nanometers.
The beauty of
silicon dioxide was that manufacturers could grow it simply by placing a
silicon wafer into a vessel filled with oxygen. 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, because the materials would be more
sensitive to heat and other influences.
The presence of silicon
dioxide would require chipmakers to add nitrogen to the hafnium oxide as
well. Without it, the insulator would only have a modestly improved
k-value that would be insufficient for the next two or three reductions in
transistor size.
