10 R&DMagazine August 2014
Driving Back Defects
As semiconductor features grow smaller, inspection equipment developers race to find
wafer errors before they hinder production.
Traditional lithography is based on a sim- ple principle: Oil and water don’t mix. The method, first developed by an actor in Bavaria in 1796, used a smooth piece
of limestone on which an oil-based image was
drawn and overlayed with gum arabic in water.
During printing, the ink was attracted to the oil,
and was repelled by the gum.
Today, a new principle guides a very different
lithography: Defects and wafers don’t mix.
Though lithography has advanced far beyond
the simple technology used to create billboards,
the same goal applies, that the finished product must be at least a reasonable facsimile of
the original. For semiconductor foundries and
chipmakers, the goal is more elusive each year.
As Moore’s Law states, the number of transistors
in an integrated circuit doubles every two years.
Even if this pace is slowing, as some experts
suggest, the challenge is high to ensure that
nanoscale features operate as designed.
Part of this effort is wafer inspection. Even
though a typical 300-mm-dia wafer is made
from an almost entirely pure, crystalline semiconductor, usually silicon, it will contain defects.
These defects can have a deleterious effect on
chip performance, quality and consistency.
Wafer inspection is difficult for several rea-
sons. First, feature size is extremely small. Elec-
tron-beam lithography (EBL) has a practical size
limit of 10 nm. But to ensure nanoscale devices
work properly and reliably, top-down process
control at much less than 10 nm is almost man-
datory. Second, chip development involves arbi-
trary patterning capabilities and fast prototyping.
Inspection strategies must therefore be generalist
in nature, able to cope with a steady succession
of new designs. Finally, defects aren’t few and
far between. Even in the exacting world of semi-
conductor fabrication, thousands of defects may
exist on a single wafer. The goal for manufactur-
ers is to minimize those defects. Or, at the very
least, eliminate the defects that may have the
worst effects on production.
KLA-Tencor Corp., Milpitas, Calif., has
addressed this need for semiconductor and
nanoelectronics industries with a variety of
surface analysis tools that incorporate advanced
imaging technologies. In July, at the Semicon
West conference in San Francisco, the company made a substantial revision to its core
set of wafer inspection technologies. The new
“Inspection and Review portfolio” includes four
systems designed to provide advanced defect
inspection and review capability for the development and production of integrated circuit
devices at the 16-nm node and smaller.
Coping with size changes
The semiconductor industry is in the midst of
a transition to 16-nm, 14-nm and even smaller
design nodes, and chipmakers are experiencing
increased process control challenges. KLA-Tencor’s latest innovations reveal the attention paid
to this progression in the market.
As lithography technology continuously
shrinks, the defect number becomes a signifi-
cant issue in wafer manufacturing inspection.
A certain number of defects results in what the
industry refers to as “review and classification”
problems. Defects are numerous enough (in
the thousands) and occur with such variety
that they have specific family names, including
systematic, random and nuisance. Defects can
result from any stage of the fabrication, from
deposition to etching, and can include pro-
trusions, polish marks, pits and scratches. The
most effective way to reduce the defect numbers
is to inspect the potential risk area instead of
the entire chip. This so-called “inspection care
area” is typically accomplished with the aid of
an electron-beam inspection (EBI) system. A
highly capable EBI system carefully pre-selects
this care area to get the most benefit from an
inspection. This can dramatically reduce defects
and enhance throughput. But EBI can take
much longer than traditional optical methods.
KLA-Tencor’s 2920 Series EBI defect inspec-
tion system encapsulates many of KLA-Tencor’s
recent innovations in optics, built around a
third-generation broadband plasma illumination
source. This platform delivers twice the light of
the previous iteration, the 2910, enabling the
use of a new deep ultraviolet (DUV) wavelength
band. The instrument is also equipped with
the industry’s smallest optical inspection pixel,
which provides increased resolution for detecting
tiny defects like subtle protrusions.
The combination of high-resolution optics
and a low-noise sensor allows the 2920 Series to
deliver sub-0.5-μm defect coordinate accuracy,
allowing it to accurately identify and target care
areas. One standout innovation that allows the
instrument to reach this level of accuracy while
providing a 30% improvement in throughput
over the 2910 is a technology called NanoPoint.
During chip development, this suite of detec-
tion technologies helps find the wafer defects that
most immediately threaten yield, accelerating
the identification of design issues that reveal the
need for mask design. According to Keith Wells,
VP and GM of the Wafer Inspection (WIN) Div.
at KLA-Tencor, NanoPoint’s value has already
been demonstrated on early metal layers, where
line-edge roughness on dense patterns had pre-
viously limited the ability to detect “yield-killing
defects” inline at advanced nodes.
“Our challenge is to design equipment that
can discover defects whose size is further and
further below the inspection wavelength,” says
Wells. “In the past we have offered various
improvements to the light source, optics and
other subsystems, but NanoPoint addresses the
issue from a new angle.”
Part of KLA-Tencor’s recently introduced
“Inspection and Review” instrument portfolio,
the 2920 Series electron-beam wafer defect
inspection system uses third-generation
broadband plasma illumination technology to
enhance sensitivity and increase throughput.
Image: KLA-Tencor Inc.