[FoRK] moar Moore

Eugen Leitl eugen at leitl.org
Wed Oct 30 09:37:13 PDT 2013


http://spectrum.ieee.org/semiconductors/devices/the-status-of-moores-law-its-complicated/?utm_source=techalert&utm_medium=email&utm_campaign=103113

The Status of Moore's Law: It's Complicated

Node names—the milestones of the chip industry—no longer mean what they used
to

By Rachel Courtland Posted 28 Oct 2013 | 20:21 GMT

Illustration: Harry Campbell

One chilly Tuesday evening last December, dozens of physicists and engineers
who dream up tomorrow’s transistors met in San Francisco to ponder the far
future. Would today’s state-of-the-art switch—a three-dimensional transistor
dubbed the FinFET—be able to carry chips “to the finish,” a distant, possibly
unreachable horizon where transistors are made up of just a handful of atoms?
Or would we need a new technology to get us there?

This may all sound like the tech world’s version of arguing over how many
angels can dance on the head of a pin, but it actually has enormous
real-world implications. The semiconductor industry pulled in revenues of US
$300 billion in 2012. After decades of fulfilling Gordon Moore’s prophesy of
steadily doubling transistor densities (these days every 18 to 24 months),
the industry is now delivering integrated circuits with transistors that are
made using what chipmakers call a 20- or 22-nanometer manufacturing process.
An IC fabricated with this process, such as a microprocessor or a dynamic RAM
(DRAM) chip, can have billions of transistors.

Nevertheless, there on the cutting edge, the business is troubled. Each new
generation of ultradense chips demands a new manufacturing process of
mind-boggling industrial and technological complexity. The struggle has
become so pitched that researchers are now often at a loss for words to
describe the metrics of their progress.

At the December meeting, for example, Chenming Hu, the coinventor of the
FinFET, began by mapping out the near future. Soon, he said, we’ll start to
see 14-nm and 16-nm chips emerge (the first, which are expected to come from
Intel, are slated to go into production early next year). Then he added a
caveat whose casual tone belied its startling implications: “Nobody knows
anymore what 16 nm means or what 14 nm means.”

It’s actually become a fairly common refrain among industry experts. The
practice of attaching measurements to chip generations has “been hijacked by
marketers to an enormous extent,” one chip-design expert told me. “A lot of
it’s really smoke and mirrors,” says analyst Dan Hutcheson of VLSI Research
in Santa Clara, Calif. It’s “spin,” he says, that’s designed to hide widening
technological gaps between chip companies.

The nanometer figures that Hu discussed are called nodes, and they are, for
want of a better term, the mile markers of Moore’s Law. Each node marks a new
generation of chip-manufacturing technology. And the progression of node
names over the years reflects the steady progress that both logic and memory
chips have made: The smaller the number, the smaller the transistors and the
more closely they are packed together, producing chips that are denser and
thus less costly on a per-transistor basis.

But the relationship between node names and chip dimensions is far from
straightforward. Nowadays, a particular node name does not reflect the size
of any particular chip feature, as it once did. And in the past year, the use
of node names has become even more confusing, as chip foundries prepare to
roll out 14-nm and 16-nm chips, custom-made for smartphone makers and other
customers, that will be no denser than the previous 20-nm generation. That
might be just a temporary hiccup, a one-time-only pause in chip-density
improvement. But it’s emblematic of the perplexing state of the field.

Moore’s Law, when reflected through the steady march of node names, might
seem easy and inexorable. But today a plague of intense manufacturing and
design problems is forcing compromises that are sometimes sobering. And some
analysts suggest that regardless of what we call the next generation of
chips, the transition from old to new no longer provides nearly the kind of
payoff—in cost or performance—that it used to.

“What do you mean by 14 nm?” When I asked An Steegen that question at an
industry conference in July, she smiled and let out a wry, knowing laugh.
“Ah…what’s in a name?” asked Steegen, senior vice president for process
technology development at Imec, the Belgian research center. “Actually, not
that much any more.”

It’s a state of affairs that has been nearly two decades in the making. Once
upon a time, the node name told you practically everything you needed to know
about a chip’s underlying technology. If you trained your microscope on
microprocessors made by a handful of different companies using a
0.35-micrometer process, you’d find that their products were all remarkably
similar.

what’s in a chart? roadmap chart  

Data Source: GlobalFoundries

What’s in a name? Key chip dimensions, such as the transistor gate length
[yellow] and the metal one half pitch [orange]—half the distance spanned by
the width of a wire and the space to the next one on the dense, first metal
layer of a chip—have decreased but not strictly tracked the node name [red].
These numbers, provided by GlobalFoundries, reflect the company’s plans to
accelerate the introduction of 14 nm chips in 2014, a good year early.  In
the mid-1990s, when such chips were the state of the art, 0.35 µm was an
accurate measure of the finest features that could be drawn on the chip. This
determined dimensions such as the length of the transistor gate, the
electrode responsible for switching the device on and off. Because gate
length is directly linked to switching speed, you’d have a pretty good sense
of the performance boost you’d get by switching from an older-generation chip
to a 0.35-µm processor. The term “0.35-µm node” actually meant something.

But around that same time, the link between performance and node name began
to break down. In pursuit of ever-higher clock speeds, chipmakers expanded
their tool kit. They continued to use lithography to pattern circuit
components and wires on the chip, as they always had. But they also began
etching away the ends of the transistor gate to make the devices shorter, and
thus faster.

After a while, “there was no one design rule that people could point to and
say, ‘That defines the node name,’” says Mark Bohr, a senior fellow at Intel.
The company’s 0.13-µm chips, which debuted in 2001, had transistor gates that
were actually just 70 nm long. Nevertheless, Intel called them 0.13-µm chips
because they were the next in line. For want of a better system, the industry
more or less stuck to the historical node-naming convention. Although the
trend in the measurements of transistors was changing, manufacturers
continued to pack the devices closer and closer together, assigning each
successive chip generation a number about 70 percent that of the previous
one. (A 30 percent reduction in both the x and y dimensions corresponds to a
50 percent reduction in the area occupied by a transistor, and therefore the
potential to double transistor density on the chip.)

The naming trend continued as transistors got even more complex. After years
of aggressive gate trimming, simple transistor scaling reached a limit in the
early 2000s: Making a transistor smaller no longer meant it would be faster
or less power hungry. So Intel, followed by others, introduced new
technologies to help boost transistor performance. They started with strain
engineering, adding impurities to silicon to alter the crystal, which had the
effect of boosting speed without changing the physical dimensions of the
transistor. They added new insulating and gate materials. And two years ago,
they rejiggered the transistor structure to create the more efficient FinFET,
with a current-carrying channel that juts out of the plane of the chip.

Through all this, node name numbers continued to drift ever downward, and the
density of transistors continued to double from generation to generation. But
the names no longer match the size of any specific chip dimension. “The
minimum dimensions are getting smaller,” Bohr says. “But I’m the first to
admit that I can’t point to the one dimension that’s 32 nm or 22 nm or 14 nm.
Some dimensions are smaller than the stated node name, and others are
larger.”

The switch to FinFETs has made the situation even more complex. Bohr points
out, for example, that Intel’s 22-nm chips, the current state of the art,
have FinFET transistors with gates that are 35 nm long but fins that are just
8 nm wide.

That is, of course, the view from a chip manufacturer’s side. For his part,
Paolo Gargini, the chairman of the International Technology Roadmap for
Semiconductors, says the node is and always has been defined by the proximity
of wires on the first metal layer on the back of the chip, a dimension that
was reflected well in DRAM and, later, flash memory, but not in logic.

Two transistors image

Illustrations: Emily Cooper

Two Transistors: Chipmakers are in the process of moving from traditional
planar transistors [left] to ones that pop out of plane [right]. Intel
introduced these 3-D transistors in 2011, and they are now shipping widely.
The leading foundries, such as GlobalFoundries, Samsung, and Taiwan
Semiconductor Manufacturing Co., are in the process of ramping up production
of 20-nanometer planar transistors. They will make the switch to 3-D with the
next generation.

Regardless of definition, numbers in node names have continued to decline.
Along with them, the distance between transistor gates and that between the
closest copper wires on the back of the chip have also decreased. Both of
those features help define how dense a chip can be and thus how many more you
can produce on a single silicon wafer to drive down costs.

But the difficulty inherent in printing ever-finer features has now taken its
toll. “When we got to around 28 nm, we were actually pushing the limits of
the lithographic tools,” says Subramani Kengeri, vice president of advanced
technology architecture at GlobalFoundries, the world’s second-biggest
chipmaking foundry after Taiwan Semiconductor Manufacturing Co.

To deal with this, Kengeri and his colleagues were forced to adopt a
lithographic technique called double patterning. It lets technicians pattern
smaller features by splitting a single patterning step into two, relying on a
slight offset between the two steps.

Intel used the technique to form transistors on its 22-nm chips, but it stuck
to single patterning to make the densest metal layer. Pushing the technique
to its limits, the company made wires with a pitch of 80 nm, which
encompasses the width of one wire and the space to the next. By adopting
double patterning, GlobalFoundries and others could push the pitch down to
about 64 nm for their 20-nm chips. But that move came with a significant
trade-off: Double-patterned chips take longer to make, adding significantly
to the cost.

Carrying this technique over from the 20-nm node to 14 nm would mean that
chipmakers would have to double-pattern even more layers of the chip. So last
year, Kengeri and his colleagues announced a chip industry first: They would
put a stop to the shrink. GlobalFoundries’ line of 14-nm chips, which are
slated to begin production in 2014, may be the foundry world’s first FinFET
transistors. But the company will build the new chips with the same wiring
density used in its 20-nm chips. “The first-generation FinFET is basically
reusing all of that and plugging a FinFET into that framework,” Kengeri says.
“It’s really a 20-nm FinFET, in a way.” Nevertheless, the company refers to
these as 14-nm chips because they offer roughly a generation’s-worth jump in
performance and energy efficiency over its 20-nm chips.

Kengeri hopes that by putting a one-generation pause on shrinking chips and
focusing on introducing 3-D transistors, GlobalFoundries will catch up with
Intel, which is already shipping 3-D devices in its 22-nm chips.
GlobalFoundries’ 14-nm chips aren’t any denser than—and therefore cost just
about as much as—the previous generation, but they’re still a big
improvement, Kengeri says. “Our point—and our customers agree—is that as long
as they see that value, they don’t care what the technology is called or what
is inside.”

“It is quite a controversial move,” says William Arnold, chief scientist at
ASML, the world’s largest maker of semiconductor-fabrication equipment. “The
customers of the foundries, the people who are making cellphone parts, are
very skeptical of not being able to get a shrink along with a performance
improvement. They’re pretty vocal about saying that they’re not happy about
that.”

The foundries’ latest move aside, chips are still more or less doubling in
density from node to node, says Andrew Kahng, a professor at the University
of California, San Diego, and an expert on high-performance chip design. But
for Kahng, the steady progression of node names masks deeper problems. There
is a difference, he says, between “available density” (how closely you can
pack circuits and wires on a chip) and “realizable density” (what you can
actually put into a competitive commercial product).

The sheer density and power levels on a state-of-the-art chip have forced
designers to compensate by adding error-correction circuitry, redundancy,
read- and write-boosting circuitry for failing static RAM cells, circuits to
track and adapt to performance variations, and complicated memory hierarchies
to handle multicore architectures. The problem, Kahng says, is that “all of
those extra circuits add area.” His group has been scouring company specs and
deconstructing images of chips for years, and they’ve come to an unsettling
conclusion: When you factor those circuits in, chips are no longer twice as
dense from generation to generation. In fact, Kahng’s analysis suggests, the
density improvement over the past three generations, from 2007 on, has been
closer to 1.6 than 2. This smaller density benefit means costlier chips, and
it also has an impact on performance because signals must be driven over
longer distances. The shortfall is consistent enough, Kahng says, that it
could be considered its own law.

This might be a recoverable loss. So far, Kahng says, the chip industry has
made it a priority to keep up the pace of Moore’s Law, ensuring that
manufacturers can continue to build and release new product families while
using a new process every 18 to 24 months. This means there hasn’t been time
to explore a number of design tricks that could be used to cut down on power
or boost performance. “When you’re on that kind of schedule, you don’t have
time to optimize things,” he says. As the value of the simple shrink
decreases, he says, chipmakers should then be able to revisit their designs
and find chip-improving approaches they may have missed or else left on the
cutting-room floor.

When will the scaling stop? Today’s patterning technology, which relies on
193-nm laser light, is becoming an ever more costly challenge, and its
natural successor, shorter-wavelength extreme ultraviolet lithography, has
been long delayed.

Kahng says chipmakers may face a more immediate struggle with wiring in just
a few years as they attempt to push chip density down past the 10-nm
generation. Each copper wire requires a sheath containing barrier material to
prevent the metal from leaching into surrounding material, as well as
insulation to prevent it from interacting with neighboring wires. To perform
effectively, this sheath must be fairly thick. This thickness limits how
closely wires can be pushed together and forces the copper wires to shrink
instead, dramatically driving up the resistance and delays and drastically
lowering performance. Although researchers are exploring alternative
materials, it’s unclear, Kahng says, whether they will be ready in time to
keep up with Moore’s Law’s steady pace.

Many people in the industry, who have watched showstopper after showstopper
crop up only to be bypassed by a new development, are reluctant to put a hard
date on Moore’s Law’s demise. “Every generation, there are people who will
say we’re coming to the end of the shrink,” says ASML’s Arnold, and in “every
generation various improvements do come about. I haven’t seen the end of the
road map.”

But for those keeping track of the road, those mile markers are starting to
get pretty blurry.

A version of this article originally appeared in print as “The End of the
Shrink.”


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