A frantic race
is under way in the microelectronics industry to integrate more
and more capability onto computer
chips. Yet conventional optical lithography—the current
practice of directing light through a mask, or reticle, to print
integrated circuits on chips—is being pushed to its limits.
When beams of light cannot be made thinner, the number of circuits
that can be written on a given chip can go no higher. Extreme
ultraviolet lithography (EUVL), which has emerged as the lithographic
method of the future, is expected to be capable of reducing the
feature size from 130 nanometers to less than 50 nanometers.
“One of the highest risk areas for EUVL technology was the
development of the reflective reticle,” says materials scientist
Paul Mirkarimi. Mirkarimi’s team has not only overcome that
concern but also won an R&D 100 Award for the ion-beam thin-film
planarization process, which generates nearly perfect surfaces
for reticles and
other critical components for EUVL. With this novel deposition
and etching process, surfaces that have been contaminated with
particles piled up to 70 nanometers high can be made almost perfectly
smooth. The resulting profile of the thin-film coating is less
than 1 nanometer high. Imagine “smoothing” a 70-story
skyscraper to the height of a single-story house.
Ion-beam thin-film planarization
can be used to fabricate nearly defect-free reticles and
projection optics for extreme ultraviolet lithography.
(a) With the deposition and etching process, a 50-nanometer
particle is smoothed until it is less than 1 nanometer high, or about
the height of a single atom. (b) Overall surface roughness
is dramatically reduced as well.
Almost Perfect Surfaces
EUVL reticle blank consists of a substrate coated with a molybdenum–silicon
(Mo/Si) multilayer film designed to have optimal reflectivity at
extreme ultraviolet (EUV) wavelengths of 13 to 14 nanometers. Reflectivity
is critical because EUVL uses strongly attenuated EUV light directed
at reflective optical components to create minute features. The
film is coated with a buffer layer and an absorber layer and is
processed with an electron-beam lithographic tool to form a patterned
EUVL reticle. The finished reticle absorbs EUV light at specific
locations and reflects it everywhere else.
challenge in developing this reticle technology is to manufacture
reticle blanks that are
virtually defect-free. The allowable defect density is about 0.0025
defects per square centimeter for defects of approximately 50 nanometers
and larger. This density corresponds to just one defect for every
two 15-centimeter-square reticle blanks—or a single defect
the size of a basketball on a flat surface slightly larger than
the states of Oklahoma and Texas combined. Says Mirkarimi, “To
our knowledge, these are the most stringent defect specifications
ever required for a coating process.”
Livermore team’s technology smooths, or “planarizes,” substrate
particles during the multilayer coating process. A primary ion source
sputters material off a target onto the substrate, and a second ion
beam etches, assisting in the formation of a smooth, uniform film
with remaining defects less than 1 nanometer high. Defects that small—just
a few atomic layers thick—are considered to be benign according
to EUVL printability modeling.
deposition processes—magnetron sputtering and ion-assisted
electron-beam evaporation—can also be used to print computer
chips. But magnetron sputter deposition results in larger substrate
particles. And at least for Mo/Si coatings, there are no data to
suggest that ion-assisted electron-beam evaporation smooths out defects.
Mirkarimi’s team also demonstrated that their planarization
process smooths rough substrates, making it applicable to projection
optics, another critical EUVL component. The figure and finish specifications
of these optics are about 0.1 nanometer, which are extremely challenging
and expensive to achieve simultaneously. “There is the risk
that sufficient quantities of these optics won’t be produced
because of the difficulty in fabricating them,” says Mirkarimi.
the planarization process, coatings with EUV reflectivities of about
67 percent can be obtained on substrates with roughness
of approximately 0.4 nanometer, which is sufficient for projection
optics. Thus, finish specification for the optics could be relaxed,
significantly reducing the production costs and increasing the availability
of these optics. Livermore’s process is equally effective for
smoothing homogeneous films. By successively depositing and etching
thin silicon layers, the team achieved a level of particle smoothing
with homogeneous silicon films similar to the level accomplished
with Mo/Si multilayer films.
team for the ion-beam thin-film planarization process (left
to right): Dan Stearns, Victor Sperry, Sherry Baker, Eberhard
Spiller, and Paul Mirkarimi.
Putting EUVL to Use
the smaller feature size that’s possible with EUVL, many
more transistors can be placed on an integrated circuit. Desktop
computer microprocessor chips will operate at more than 10 gigahertz,
and random access memory chips can have gigabyte capacities. Such
powerful, affordable computers are expected to make a variety of
computationally intensive applications practical. Real-time, multilanguage
voice recognition and translation are just two examples.
the Semiconductor Industry Association reported that the industry
was annually manufacturing about 60 million transistors
for every man, woman, and child on earth. By 2010, this number is
expected to be 1 billion transistors, as integrated circuits make
their way into even more devices used in our daily lives. If we think
our lives are computerized now, we obviously haven’t seen anything
Key Words: extreme ultraviolet lithography (EUVL), ion-beam thin-film
planarization, R&D 100 Award, reticle.
For further information contact Paul Mirkarimi (925) 423-4848
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