IEEE Spectrum November, 2008 - 36

the industry over until the arrival of EUV
lithography in four or five years. Doublepatterning lithography, to borrow a phrase
from Winston Churchill, is the worst
method that's out there right now, except
for all the others.
ou b l i n g t r a n s i st o r
d e n s it y on a chip
means shrinking its
dimensions by about
30 percent. The industry is understandably
desperate to see the pace of Moore's Law
continue, and that pace is dependent on
the technology that can create those evershrinking transistors: optical lithography, also known as photolithography.
Photolithography literally prints microchips layer by layer. The technique's most
basic parameters are resolution and cost,
and they are in more or less direct conflict. To print the billions of tiny individual features that make up a modern chip,
you need extremely fine resolving power.
And because that modern chip with nearly
a billion transistors sells for only a few dollars, the printing method has to be stupendously cheap. Chip makers are constantly
jockeying for advantage by trying to introduce new technologies ahead of their competitors, but for the most part they all
move in lockstep between what are called
technology nodes.
A "node" loosely refers to the width of
the smallest features of an integrated circuit-for example, the length of a transistor's gate. In 1971, those 2300-transistor
Intel 4004s were manufactured using technology that could create features measuring 10 000 nanometers (10 micrometers).
Today's most advanced chips are at a
45-nm node, ostensibly because the smallest features in the pattern measure 45 nm.
Intel expects to begin producing 32-nm
node chips in 2009. Chips based on 22-nm
node processes are already under development and slated for production from 2011
through 2012. Using smaller wavelengths
and larger lenses, the semiconductor
industry has done a stunning job of scaling
down transistors. Consider that if the transistors in the Intel 4004 had been the size
of Humvees and had been scaled down to
the extent that they have, they would today
be as small as sesame seeds.
Every chip starts its life as a tiny
patch on a gleaming round wafer of silicon about the size of a dinner plate. This
wafer moves in and out of a series of
machines through a fabrication plant the
size of a football stadium. The result is a
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NA * iEEE SpEctrum * NovEmbEr 2008

wafer imprinted with patterns of hundreds of identical microchips, which are
then sliced and diced and go out into the
world to populate routers, coffeemakers,
ATMs, laptops, and fighter jets.
Optical lithography, which imprints
the patterns onto the wafer, is a lot like
old-style film and chemistry photography.
It actually works a lot like a slide projector,
in which a light source shines through a
pattern to beam an image onto a surface.
First, the wafer is covered with a lightsensitive material known as a photoresist,
which is like a more sophisticated version
of the emulsion used on photographic
paper. Next, light is streamed through a
photomask-an opaque "master pattern"
plate with holes that let light through to
form a pattern below; this mask is analogous to the negative in film photography.
The pattern is projected onto the photoresist-coated wafer using extremely
sophisticated optics. Where the photoresist is exposed, its chemical properties
are changed by the light. The parts that
are masked, and therefore unexposed,
retain their integrity, but the photoresist
under the illuminated areas becomes
chemically "weak." That exposed photoresist is washed away by a developer solution, revealing the material underneath.
This optical system is called a stepper or a scanner because it projects postage stamp-size chip patterns onto the
wafer one at a time, exposing the silicon patch and then rapidly moving on
to the next one, until the entire wafer is
covered with identical microchip patterns, hundreds to thousands of them
per wafer. Last, a corrosive plasma easily eats away the exposed wafer material, transferring the photoresist pattern
onto the semiconductor wafer below. A
wafer will cycle through these photolithographic steps, each cycle producing what
eventually will be a single layer of the finished microchip, up to 40 times.
This basic process has gotten more
complicated with each successive generation of chip because, according to the fundamental laws of optics, in order to produce smaller and smaller features on chips,
lithography tool manufacturers have had
to repeatedly reduce the wavelengths of
light used to project the chip patterns. And
as the wavelength becomes shorter, the
light source and optics become more complex and expensive, which is why lithography tools are subject to their own version
of Moore's Law: tool prices reliably double
every 4.4 years. That's partly a result of the
journey down the wavelength ladder from

the big, easy wavelengths of visible light
in the 1960s, to shorter-wavelength mercury lamps in the 1970s, to the even shorter
wavelengths of krypton-fluoride lasers in
the late 1990s, and finally to the punishingly short-wavelength argon-fluoride
laser light used today. The projected use
of incredibly small 13.5-nm light has been
hampered by the fact that you can barely
design a lens for it-light of wavelengths
that short is absorbed by everything in its
path, including the lens and the air itself.
But even using today's wavelengths, we
need to do more to achieve good resolution.
When light shines through a photomask, it
diffracts, spreading out as it travels away
from the lens. That diffraction causes
the features projected onto the silicon to
blur, rendering the finished chip unusable. Because each diffracted light beam
contains important information about the
chip's pattern, as much diffracted light as
possible must be collected if you want a
satisfactory image. The lens between the
light source and the photomask is there
to make sure that this diffracted light is
caught and used in the image.
Thus, how fine you can get your
resolution-how small you can make your
features-depends on the two most fundamental characteristics of an imaging system: the wavelength of the light and the
size of the lens aperture-the opening-
through which you're shining that light.
Wavelength and aperture are related in a
fundamental equation, called Rayleigh's
resolution criterion, that governs all lithography: resolution is proportional to the
wavelength divided by the size of the lens
opening. So to print smaller features, you
need shorter wavelengths, a bigger lens, or
ideally, some combination of the two.
Making the aperture bigger means
that more light can be captured. More
captured light means that smaller
features can still be "seen" by the lens.
But in optics, as in life, there is no free
lunch: while a larger-aperture lens yields
better resolution, it also requires a more
complicated and expensive stepper.
Numerical apertures have increased
steadily over the years, from 0.167 in 1973
to 1.0, long considered a barrier because 1.0
is the refractive index of air. In 2005, Nikon
and ASML broke that 1.0 barrier with a
fantastic cheat called water-immersion
lithography. The idea is simple: boost resolution by replacing the standard air gap
between the lens and the wafer surface
with water, a medium with a refractive
index greater than 1.0. The mythical 1.0
barrier was vanquished, and three years
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Table of Contents for the Digital Edition of IEEE Spectrum November, 2008