In Search of Calcium Fluoride
Manufacturers face new production challenges as they attempt to meet rising need.
Anyone who has ever owned a PC is familiar with the problem of technological obsolescence.
Faster models hit the market almost before people have had the opportunity to turn
on their new computers. What many people don’t know is that rapid progress
in the fabrication of semiconductor circuit components can largely be attributed
to advances in optical microlithography, one of the key technologies for creating
transistors and memory modules on silicon wafers.
CaF2 crystal is a key material for microlithography in the production of high-end microchips.
Improved microchip performance requires optics
that can generate ever-finer features. Advances in miniaturization are thus closely
related to the spatial resolutions of the optical systems employed for microchip
fabrication. These finer details can be obtained in two ways: by using either larger
lenses or shorter wavelengths. The problem with larger optics is that the number
of defects typically increases when the overall size is increased. And with shorter
wavelengths, the traditional lens material — fused silica (glass) —
behaves poorly because of radiation effects.
Today, state-of-the-art microlithography
technology has progressed to the point that ever-shorter wavelengths are being used
to “write” features on microchips.
193 to 157 nm
In fact, the latest generation of microlithography
tools employs excimer lasers that operate at 193 nm, a wavelength that allows writing
features as narrow as 100 nm. Several manufacturers, including Schott Lithotec,
Corning and Nikon, are dedicated to making this technology available for mass-production
applications in the microchip industry. Implementation of 157-nm microlithography
will lower the limit on microchip features to as small as 70 nm. Fabrication of
the first semiconductor circuit components that use this technology will most likely
begin late this year.
Schott Lithotec produces CaF2 crystals with a disc diameter of up to 350 mm and a thickness
of more than 100 mm.
Microlithography at 157 nm represents
a milestone in closing the technology gap between conventional fabrication of semiconductor
circuit components, which employ photo?lithographic methods, and extreme-ultraviolet
microlithography, which uses wavelengths from 11 to 13 nm. However, neither glass
nor quartz is transparent enough for 157 nm. The preferred material here is extremely
pure, low-defect, single-crystal calcium fluoride (CaF2), also known as fluorite.
Although barium fluoride has been considered
as an alternative to calcium fluoride, a number of technological hurdles have not
yet been resolved. No broadly accepted material can withstand irradiation by short-wavelength
UV as well as CaF2. When used as lenses or prisms, this material can concentrate
and deflect UV radiation down to about 130 nm. Its refractive behavior should be
as uniform as possible to preserve the quality of chip structures. Another important
material requirement is low stress birefringence at application wavelengths.
Octahedrons and tetrahedrons are typical structures of crystals made of calcium fluoride.
Demand for CaF2 is expected to rise sharply over
the next three to four years. The large quantity and high quality of the material
required call for a good understanding of the crystal growth process. Present-day
crystal growth technologies are usually based on the phase transition from the liquid
to the solid state, known as melt growth. Only a few materials of industrial importance,
such as the semiconductor silicon carbide, are grown from the gas phase.
The challenge to using CaF2 is that
it cannot be used as a lens material in its initial form. To grow CaF2 in single
crystals, the raw material must be purified so that the contaminants are decreased
to the parts-per-billion level.
This extremely pure powder is the starting
point, and what follows is a multistep process. The synthetic CaF2 powder is molten
and compressed into a single crystal inside a crystal growth furnace. Finally,
the single crystal is subjected to an annealing process that results in even higher
The fundamental principles of the melt
growth technologies for the manufacture of single crystals were established in the
first half of the 20th century. The most important technique for the manufacture
of semiconductor and oxide crystals used today is what is known as the Czochralski
In this technique, a crucible of molten
material is prepared, and a seed crystal is touched to the surface of the liquid.
Surface tension causes the molten liquid to cling to the seed crystal as it is slowly
raised. As the liquid is pulled up, it cools and solidifies, forming a single crystal
with the seed crystal.
For CaF2 production, however, the polycrystalline
raw material is molten in a crucible and then directionally solidified from a single-crystalline
seed at the bottom of the crucible. The advantage of this technique over the Czochralski
method is that the temperature conditions can be better controlled during the growth
process. As a result, crystals can be grown with lower defect density. The drawback
is that the crystals grow in contact with the crucible wall and cannot be observed,
which can reduce the yield.
During the crystal growth process,
careful attention must be paid to the temperature distribution. As in glassmaking,
the molten mass must be cooled in a controlled manner to prevent areas of thermal
tension that could lead to defects. Yet another problem that often arises is “compaction,”
a change of transmitted light resulting from localized density variations that cause
spatial variations in refractive index.
Researchers and developers are using
what is known as defect engineering to help grow larger crystals and, at the same
time, improve their quality in the microscopic or even the atomic range. This is
achieved by precisely controlling the growing conditions. The solution strategy
takes two routes. First, the crystal growth process is carefully analyzed to discover
the relationship between the important growth conditions and to draw up a defect
model. Second, a process model is needed that places the growth parameters (i.e.,
those elements of the process that can be directly manipulated) in relation to the
conditions — mainly the temperature field. This is achieved by the combined
use of experimental analysis and computer simulation.
The Crystal Growth Laboratory in Erlangen,
Germany, has worked closely with Schott Lithotec toward developing furnaces for
growing CaF2 crystals. In addition, the laboratory has built a prototype furnace
that will provide a valuable learning experience regarding crystal growth.
The ongoing challenge for any calcium-fluoride
producer is to achieve the requirements of lens blanks with large dimensions, while
reducing the wavelength and increasing the numerical aperture.
Meet the author
Konrad Knapp is vice president of business development
at Schott Lithotec AG in Mainz, Germany, where he is responsible for next-generation
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