Study of laserproduced plasma for an extreme ultraviolet lithography source

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Study of laser-produced plasma for an extreme
ultraviolet lithography source

• Yezheng Tao,
• M. S. Tillack, K. Sequoia, S. Yuseph, R. Burdt,
  and F. Najmabadi
• Center for Energy Research
• University of California, San Diego
• 3rd Workshop on PEARL
• May 6th-9th, 2009. Dublin City University,
  Ireland

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Overview

• EUV lithography and its requirement to EUV
  source.
• Our works on EUV source,
• Density and EUV emission profiles of NdYAG
  laser-produced Sn plasma
• Opacity impacts on the properties of EUV emission
  
• Reduction of concentration of Sn to control
  opacity
• Effect of CO2 laser pulse duration on in-band CE
  and EUV source size
• Density and EUV emission profiles of CO2 laser-Sn
  plasma
• Effect of plasma density profile on ion
  acceleration

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EUV Lithography requires a powerful, long
lifetime, cost effective in-band 13.5 nm EUV
source

• Requirements to EUVL source,
• Wavelength 13.5nm in 2BW
• Power 115W (at IF), 300W
• Etendue 1-3.3 mm2sr
• Repetition 7-10kHz
• Lifetime 30,000 hrs
• Stabilities lt? 0.3 (over 50 shots)

Line resolution and depth of focus
LPP
or DPP?
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Basic processes of a laser-produced plasma
From, Physics of Laser Plasma. ed. A. Rubenchik
and S. Witkowski. 1991 North-Holland.
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Experimental Facilities for EUV source at UCSD
Lasers 3 NdYAG and 3 CO2 NdYAG Pulse
duration 0.1 30 ns Intensity
up to 1014 W/cm2 Sync.
Jitter lt 0.5 ns CO2 Pulse duration 10 200
ns Intensity up to 8 1010
W/cm2 Sync. Jitter lt 5 ns
Diagnostics, EUV emission EUV energy monitor
TGS spectrometer
In-band EUV waveform Ions Electric
Energy Analyzer s t resolved visible
spectroscopy Faraday Cup Plasma Green
and IR interferometers In-band EUV
imaging Fast visible imaging
Targets Droplet (Sn solutions) 30 µm Sn
sphere 30-250 µm Sn coating 10 nm 100 nm
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Opacity plays a key role in the generation of EUV
emission from LP Sn plasma
M-EUV-PHC
Small EUV energy monitor

• Widely distributed EUV DER.
• Sn plasma is optically thick to 13.5 nm EUV
  emission

M-EUV image taken at top view
Y.Tao et al. Appl.Phys.Lett., 85, 1919(2004)
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Opacity limits the dominant emitting region of
efficient EUV emission in well underdense corona
ne(cm-3)
532nm 266nm
1 ?1021
EUV emission profile
1 ?1020
1-D simulation
1 ?1019
2 ?1018
0 100 200 300 400 500
Distance from target surface(?m)
Y.Tao et al. Appl.Phys.Lett., 86, 201501 (2005)
Y.Tao et al. Appl.Phys.Lett., 87, 241502 (2006)

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Opacity makes that in-band CE is not sensitive to
laser focal spot size
Y.Tao et al. Opt. Lett., 31, 2492(2006)
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Opacity results in a particular angular
distribution of EUV emission that depends on
individual exp. conditions
K. L. Sequoia, et al. Appl.Phys.Lett.,92, 221505
(2008)
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Composited target with low concentration of Sn
could reduce opacity

• 100 mg/cc RF foam
• 0.1-1 solid density Sn
• e.g., 0.5Sn Sn1.8O17.2C27H54

S. S. Harilal, et al. Opt. Lett. 31, 1549(2006)
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Opacity makes the transition of the driving laser
of EUVL source from NdYAG to CO2 laser
Soft x-ray spectra from Sn plasma
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Timing scheme for interferometry and EUV imaging
Interferometer, 130 ps EUV
imaging, time integrated
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Application of long CO2 laser pulse could
significantly enhance efficiency, simplify
structure, and reduce CoO of CO2 laser used in
EUV source

• In-band CE, 2.8 (2??), is constantly obtained
  with CO2 laser pulse with pulse durations from 25
  to 110 ns .
• Long pulse could significantly simplify and
  reduce the cost of the CO2 laser used in EUVL
  source.
• Larger pulse energy accompanying with long pulse
  makes it easier to realize mass-limited target
  for droplet target
• Long pulse may make it easier for alignment.

Y. Tao et al. Appl.Phys.Lett., 92, 251501(2008).
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High CE, 5 , was demonstrated by forming a
crater with a flat bottom on Sn plate
In-band CE as a function of shot number
Target shape to obtain higher CE
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Discussion (4)- Absorption of CO2 laser is
localized near the nc
The absorption length for 1/e intensity
attenuation of laser light in plasma is obtained
from D.Attwood, Soft x-ray and EUV radiation.

• For long pulse NdYAG laser, distributed laser
  absorption is significant.
• For CO2 laser, most of the laser energy is always
  locally absorbed around the nc.

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Discussion (5) effect of plasma density profile
on ion acceleration
the charge separation formed during the plasma
expansion, the maximum electric field associated
with the electron Debye sheath can be described,
Gentle plasma density profile modify the
acceleration electric filed
Courtesy of Physics News Graphics
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Interaction of main laser pulse with a gentle
density profile instead of infinite sharp boundary
200 ?m
pre-pulse
Shadowgraph of pre-plume taken at 840 ns after
pre-pulse
Density profile of pre-plume
Y.Tao et al. J Appl. Phys., 101, 023305 (2007)
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More than 30 times reduction in ion energy is
obtained while almost without loss of conversion
efficiency
CE
Energy spectrum of ions
Particle energy reduction factor
5.2 KeV
150eV

• More than 30 times reduction in ions kinetic
  energy at flux peak is obtained at delay around 1
  ?s.
• Almost the same CE as compared with that obtained
  with single pulse is also achieved at the same
  delay time. Enhancement of CE at small delay is
  observed.

Y.Tao et al. Appl.Phys.Lett., 89,111501 (2006)
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limited target gas could mitigate debris
efficiently

• More than 100 times reduction in ion energy was
  observed with a 30 nm Sn film irradiated by dual
  pulses while keeping high CE.
• 14 mTorr Ar is enough to stop all of the ions
  with a thin Sn film target driven by dual pulses.

TOFs of Sn ions from 30 nm Sn film
Y.Tao et al. Optics Letter, 39,1339 (2007)
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Summary

• Experimental investigations showed that Sn plasma
  is optically thick to 13.5 nm EUV emission.
• Widely distributed EUV emitting region in corona
  was observed in NdYAG laser-produced Sn plasma.
• Opacity effects properties of EUV emission and
  selection of laser parameters.
• Higher in-band CE was achieved with CO2 laser
  pulse with pulse durations from 15 to 200 ns as
  compared with that of NdYAG laser.
• Non-classical plasma expansion and slightly
  expanded EUV emission region were observed in CO2
  laser-produced Sn plasma.
• Plasma density also plays a key role in ion
  acceleration, and modifications of laser pulse
  shape could mitigate energetic ions from the EUV
  plasma while keeping high in-band CE.

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Acknowledgements
Cymer Inc., KLA-Tencor Inc., EUVA in Japan., and
University of California. and Thank You!