IEEE Spectrum July, 2012 - 34

Electrical potential

Source

At least that's how it used to be.
Then, about 10 years ago, chipmakers
began to notice a problem: Even
state-of-the-art manufacturing processes couldn't produce chips with
consistent properties. Nowadays two
transistors, fabricated a few dozen
nanometers apart on the same piece
of silicon, will not have the same electrical properties. It's one
of the key barriers that the global chip industry-with sales
of US $300 billion-must overcome to keep producing better,
faster, cheaper, more energy-efficient chips.
The culprit is scaling. Chips have improved because their
transistors and connecting wires have kept getting smaller,
but now they're so small that random differences in the placement of an atom can have a big impact on electrical properties. Some batches vary so much that more than half will run
30 percent slower than intended or consume 10 times as much
power as they should when on standby.
Some of these defective chips can be sold at a discount,
but if they're for application-specific designs-say, for mobile
phone communication or video encoding-they might find no
better destination than the junkyard. And the defect rate will
only get worse as transistors continue to shrink.
Chip variability is what the International Technology
Roadmap for Semiconductors calls a "red brick" problem:
one of a handful of important issues that lack any clear solution, forming a red brick wall that prevents forward progress.
But just because variability is here to stay doesn't mean
we can't mitigate its effects. We could accomplish much by
changing the way we design chips. This has traditionally
been done by introducing a margin of error to account for
the worst-case scenario. Now that ridiculously small defects
have entered the mix, that approach no longer works, and
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Drain

Channel

chipmakers must overcompensate for the problem. The result
is pessimistically designed chips that operate far slower and
consume far more energy than they should.
Fortunately, a new family of design techniques promises to
predict not only the worst-case scenario for a chip but also the
likelihood that the scenario will happen. These approaches
use statistical methods to make informed trade-offs between
how fast the chips will run and how many good chips a given
batch is likely to yield. Some makers of high-end microprocessors like IBM and foundries like the Taiwan Semiconductor
Manufacturing Co. are already using some of these statistical
techniques in their design flows. Although statistical tools are
still far from being widely adopted, if we can push them along,
these tools will help us make affordable chips that are as fast
and efficient as those the semiconductor road map calls for-
and perhaps then some.

Variation has been present since the first inte-

grated circuit was created. But its nature and scope-and the
way it shows up in the behavior of digital circuits-have changed.
For a long time, most variation was global, caused by
slight alterations in the manufacturing process. Such
changes differentiate one chip from another or all the chips
on one wafer from those on another. Global variations tend
to affect the electrical characteristics of all transistors in
spectrum.ieee.org

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Today's chips are marvels
of mass production. Tens of
thousands of silicon wafers
can move through a single fab
in a month, each one carted
from tool to tool, blasted by
heat, bombarded by ions,
immersed in vapor, coated
with chemicals, hit with
radiation, and exposed to
acid. It can take months and
hundreds of steps to transform a plain piece of silicon
into an array of chips. But
at the end of this elaborate
assembly line, chipmakers
finally get a pile of identical
devices that perform just as
their designers intended.

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Table of Contents for the Digital Edition of IEEE Spectrum July, 2012