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Research persons: Mark Tillack

Location: University of California - San Diego, United States

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A breakthrough discovery at University of
California - San Diego may help aid the semiconductor industry’s quest to
squeeze more information on chips to accelerate the performance of electronic
devices. So far, the semiconductor industry has been successful in its
consistent efforts to reduce feature size on a chip. Smaller features mean
denser packing of transistors, which leads to more powerful computers, more
memory, and hopefully lower costs.

In an effort to help create faster, better and cheaper light sources for chips,
UC San Diego researchers, in collaboration with Cymer, Inc., are developing
laser-produced light sources for next generation Extreme Ultraviolet Lithography
(EUVL).

The researchers, led by mechanical and aerospace engineering scientist Mark
Tillack, filed a patent in May 2008 for their latest discovery indicating that
longer pulse lengths can provide similar performance as short pulse lengths.
Tillack and his team found that employing a long pulse in a CO2 laser system
used in an EUVL source could make the system significantly more efficient,
simpler, and cheaper compared to that using a shorter pulse.

Today’s semiconductor companies are diligently working on developing EUVL as the
leading candidate for next generation lithography tools to produce microchips
with features of 32 nanometers or less. While great progress has been made in
this field, several challenges still exist to cost effectively field EUVL in
high volume manufacturing. Nowadays, the light source in semiconductor
lithography is applied directly from a laser through a mask to a wafer. In EUVL,
a laser is used to produce extreme ultraviolet light that is sent to a mask and
then the wafer. This indirect process is more inefficient, and could require a
very large and very expensive laser source, Tillack said.

“CO2 lasers, which we use in our lab, have two advantages – they are inherently
cheaper to build and operate, and they give better conversion efficiency from
the laser to EUV light,” he said. “Our discovery that long pulses work well
enough means that the CO2 laser system can be built and operated more cheaply.”


Tillack pointed to possible future applications for EUVL, such as flash memory
chips, which will become denser and denser. “Imagine in the future being able to
make a 200 gigabyte flash disk memory stick cheaply,” he said. “EUVL could make
hard disks obsolete”.

“We didn’t know how to make a powerful source of light in this part of the
spectrum before,” added Tillack, also an associate director of the UC San Diego
Jacobs School of Engineering’s Center for Energy Research. “We might be opening
new avenues for advanced light sources. We need to continue our research and
begin to look at other possible applications.”
Note for Extreme Ultraviolet Lithography
Extreme ultraviolet lithography is a next-generation lithography technology
using the 13.5 nm wavelength. EUVL is a significant departure from the deep
ultraviolet lithography used today. All matter absorbs EUV radiation. Hence, EUV
lithography needs to take place in a vacuum. All the optical elements, including
the photomask, must make use of defect-free Mo/Si multilayers which act to
reflect light by means of interlayer interference; any one of these mirrors will
absorb around 30% of the incident light. This limitation can be avoided in
maskless interference lithography systems. However, the latter tools are
restricted to producing periodic patterns only.

The pre-production EUVL systems built to date contain at least two condenser
multilayer mirrors, six projection multilayer mirrors, and a multilayer object
(mask). Since the optics already absorbs 96% of the available EUV light, the
ideal EUV source will need to be sufficiently bright. EUV source development has
focused on plasmas generated by laser or discharge pulses. The mirror
responsible for collecting the light is directly exposed to the plasma and is
therefore vulnerable to damage from the high-energy ions and other debris. This
damage associated with the high-energy process of generating EUV radiation has
precluded the successful implementation of practical EUV light sources for
lithography.

The wafer throughput of an EUVL exposure tool is a critical metric for
manufacturing capacity. Given that EUV is a technology requiring high vacuum,
the throughput is limited (aside from the source power) by the transfer of
wafers into and out of the tool chamber, to a few wafers per hour.

Another aspect of the pre-production EUVL tools is the off-axis illumination (at
an angle of 6 degrees) on a multilayer mask. The resulting asymmetry in the
diffraction pattern causes shadowing effects which degrade the pattern fidelity.

Tillack's EUVL work was supported by
Cymer Inc. and by the
University of California under the UC Industry-University Cooperative Research
Program.
In figure 1, Mirrors are used in scientist Mark Tillack's UC San Diego
photonics lab to guide light into laser amplifiers.

In figure 2, Mark Tillack, a UC san Diego mechanical and aerospace engineering
scientist, is developing laser-produced light sources for next generation
Extreme Ultraviolet Lithography.