IEEE Spectrum March, 2007 - 36

ing processes, known as photolithography, are the heart of chip
making and they are carried out repeatedly, up to 30 to 40 times
for an advanced chip like a 64-bit Itanium microprocessor.
Each time a wafer goes through one of these photolithography
cycles, a liquid polymer called a photoresist is applied to it. Then
it goes into a step-and-repeat projection camera, better known
as a "stepper," which exposes a pattern onto the resist-coated
wafer. For each exposure, the stepper moves to a new position,
each one corresponding to a single chip.
The pattern is projected onto the wafer in ultraviolet light
through a photomask, a thin plate of transparent quartz covered
with the pattern defined in chrome of a particular chip layer.
After the wafer has been exposed, it is washed with a solvent,
removing the undesired resist and leaving the image of the photomask on the wafer.
Each time the wafer is patterned, one of three different processes is performed. In some cases, the wafer will be etched
to remove material. This can involve a liquid acid or a hightemperature plasma, both of which will eat away any surface
not covered by the resist. During other steps, the wafer is put
through another diffusion or deposition process that adds material to it. In this case, the resist acts as a barrier, preventing the
new material from adhering to parts of the wafer.
The third process involves changing the concentration of
ions in parts of the wafer to adjust the conductivity of the semiconductor. Here the resist again acts as a barrier, preventing
the ions from penetrating into the protected parts of the wafer.
After the wafer has been processed, all the remaining resist is
removed with a special solvent.
This process is repeated many times to create all the components of a transistor. Then, the process is repeated several more
times, adding layers of metal interconnects to the components
to create functional circuits. Afterward, insulating material is
added to the wafer, the wafer is patterned for interconnects,
metal is deposited and polished smooth, and the next layer of
insulation is added.
While the basic chip-making processes have been around
since the early days of semiconductor manufacturing, more
than 35 years ago, several significant changes have occurred.
For instance, there are a lot more photolithography cycles today
to create the layers of wires needed to connect the proliferation
of transistors mandated by Moore's Law.
Also, shorter wavelengths of light are used to resolve smaller
features, new materials are always being introduced, new processes such as plasma etching and silicon straining have come
into vogue, and optical tricks of remarkable complexity are
employed to produce features even smaller than the wavelengths
that are resolving them.
And, last but not least, the whole process is now controlled
by software.

THE AMT SuITE has four major components: the Manufacturing
Execution System, the Process Control Automation Framework,
the Engineering Analysis Framework, and the Material Handling
and Tool Control. Each is composed of several programs, and
each of those programs controls a different part of the chipmaking process.
One program in the Manufacturing Execution System that's
called Fab-Wide Scheduling, for example, acts like the world's most
sophisticated traffic cop: it tracks all the wafers through the fab to
ensure that they pass in and out of different machines in sync with
all the other wafers that are simultaneously moving from machine
to machine. Another program, in the Process Control Automation

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Framework called Advanced Process Control, automatically detects
random variations in the processing equipment-maybe a chemical
vapor deposition machine has deposited too much metal to make a
wire, for instance-and adjusts the recipe accordingly.
On each manufacturing line, as well as on the main technology
development line where we refine recipes for new chips, the AMT
system takes thousands of readings from hundreds of machines.
There are defect-density and film-thickness readings from quality
inspection tools, temperature and pressure readings from deposition and etch tools, and even readings of the velocity of the gas
flowing to diffusion furnaces. All this information feeds into
the fab's database, which typically contains tens of trillions of
bytes of data, or five to 10 times the amount of information you'd
find in the entire print collection of the U.S. Library of Congress.
These data are crucial to determining the merits of the different
recipes our researchers experiment with while creating a new
chip. Automated engineering programs, part of the Engineering
Analysis Framework, evaluate transistor performance, wafer

HANdS OFF: A robotic wafer handler inserts a wafer into a thin-film-deposition
machine while a technician looks on.

yields, and manufacturing processes related to the chip-making
experiments run on the technology development line. These proprietary programs identify recipes that improve transistor performance and the yield of chips, reduce power consumption and
heat dissipation, and otherwise help produce better chips faster.
This automated analysis of hundreds of thousands of data points
enables fast tuning of the manufacturing recipes in the fab.
How fast can these recipes be put into use? In the blink of an
eye, basically. Once we determine that a new recipe works for, say,
an etching bath, the system feeds it back to the chip-making tools
on the technology development line and starts using it right away.
So a batch of wafers that is already in the midst of photolithography
can benefit immediately from these revised instructions.
For example, suppose a wafer lot has just been exposed in a
stepper and is awaiting a cleansing chemical bath in an etcher.
By the time it gets to that etching bath, there might be a better
recipe awaiting the wafers than the one used for the batch that
just exited the machine. These experiments continue until the
microprocessor recipe is fine-tuned for use in all the company's
high-volume manufacturing lines.
The AMT suite of programs has improved tremendously
over the last couple of decades. The most recent innovation,
developed by Intel's Logic Technology Development group, is

March 2007 | IEEE Spectrum | NA 41

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Table of Contents for the Digital Edition of IEEE Spectrum March, 2007