Extreme printing
Next-generation ultraviolet lithography technique could make it possible to create nano-scale
integrated circuits with double the processsing speed. Siobhan Wagner reports
A NEW way of printing circuit boards
using advanced lithography could
produce semiconductors with double the
amount of processing speed.
Extreme ultraviolet lithography
(EUVL) would create circuit patterns on
nano-scale chips using a beam of light
with a wavelength 14 times shorter than
conventional techniques. Current
lithography uses a light beam with a
193nm wavelength, but EUVL would use a
13.5nm wavelength beam.
The aim is to produce semiconductor
components that are more than half to
possibly even a quarter of the size of
current 65nm-sized circuitry. ‘You’ll be
able to put in more transistors so you get
faster speed,’ said Samir Ellwi,
vice-president of strategic technology at
Powerlase, a Crawley-based laser
manufacturer that has been working with
EUVL research groups around the world
for the past 10 years.
The technique presents some unique
technical requirements for semiconduc-
tor manufacturers. While current
lithography processes take place in the
open, EUVL needs to be carried out in a
vacuum because all matter absorbs
extreme UV radiation. Also, there are no
materials that can be used to make
refractive lenses that operate at the
13.5nm wavelength, so the light must be
focused using only specially-shaped
reflective mirrors.
One major challenge facing the
industry has been deciding on the most
appropriate source for a UV light beam
with such a short wavelength. Powerlase
is exploring two potential sources: laser
produced plasma (LPP) and discharge
produced plasma (DPP). The company is
developing the LPP approach with the
University of Central Florida (UCF).
The LPP process begins by heating a
droplet of liquid tin suspended in a
vacuum with an intense laser pulse. A
diode-pumped laser from Powerlase is
aimed at the target. As the droplet is
heated it creates an environment known
as the plasma medium, which emits light
at 13.5nm.
This computer-
generated graphic
shows extreme
ultraviolet light as
a beam (coloured
purple) being
generated from a
plasma source in
the top right-hand
side of the
apparatus
DPP, on the other hand, works on a
different principle. The process creates
an electric field between two
conductive electrodes, a cathode and an
anode. The field between the two
electrodes will pinch the EUV target,
which can be a tin droplet, a vapour of
tin or ablated tin, creating a high
temperature plasma medium that
emits the 13.5nm wavelength light. A
special mirror collects this light and
focuses it for use.
Powerlase is supplying lasers
to Japan’s Extreme Ultraviolet
Lithography System Development
Association (EUVA), which is
working to perfect the use of DPP as a
light source.
Ellwi said that LPP appears to have
the greatest potential. ‘With DPP you
can be limited with the collection of the
light,’ he said. ‘You are restricted
because you have to take the light out
the EnGIneeR 19 MAY–1 JUNE 2008 35
of the interaction chamber between the
two electrodes.’
A second challenge to the industry
is ensuring potential throughput is
high enough. Ellwi said that current
EUVL laboratory demonstrations only
produce 10 wafers/hr. ‘One critical
point that relates directly to the
throughput is having an EUV source
with sufficient output power,’ he said.
The 13.5nm EUV light accounts for
just two and a half per cent of total light
generated. For successful EUV lithogra-
phy to be possible, the power at the
EUV target needs to reach 100W. This
will ensure sufficient light is captured
by the stepper (lithography machine)
for successful semiconductor printing.
Should the EUVL work continue to
progress, Ellwi is confident 32nm scale
components can be achieved. ‘I believe
by 2012 we will have full production
machines worldwide,’ he said.
LITHOGRAPHY
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