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Laser-produced plasma for EUV lithography
M. S. Tillack
UCLA MAE Department
Thermo/Fluids Research Seminar Series
29 June 2007
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The end is in sight for semiconductor
lithography based on direct laser exposure
Current technology
(www.intel.com/technology/silicon/lithography.htm)

Efforts to drop the wavelength to 157 nm ended unsuccessfully

Innovations such as immersion (high n) or phase-shift masks
(interference) have pushed the limits of feature size at a given
wavelength
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EUV lithography has become the
frontrunner “next generation” technology

Discharge (DPP) and laser (LPP) alpha tools have been built

LPP has several advantages: higher collection efficiency, more
manageable thermal loads and debris, more scalable to HVM
mask
EUV source
laser
wafer
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At UCSD we are developing technologies
for a LPP light source using Sn targets

Why 13.5 nm? Why Sn?

Key challenges:
Maximize in-band emissions, Minimize debris damage

Research at UCSD:
* Low-density and mass-limited targets
* Wavelength and pulse length optimization
* Double-pulse irradiation
* Gas and magnetic mitigation
UCSD Laser Plasma & Laser-Matter Interactions Lab
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13.5 nm is a large, but credible next step

Transition to reflective system results in smaller NA (0.15 vs. 1),
requiring much smaller wavelength for increased resolution (NA~1/2f)

Multilayer mirrors as low as 13.5 nm are commercially available
Mo/Si, 6.9 nm period
http://www-cxro.lbl.gov/optical_constants/multi2.html
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
32 cm diameter
~70% reflectivity
Yulin et al, Microelectronic
Engineering, vol. 83, Issue
4-9, (2006) 692-694.
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The UTA of Sn is an efficient source at 13.5 nm
Sn
Gerry O’Sullivan, University College Dublin
Sb
Sn+8
Te
Sn+7
I
Sn+6
Xe
Konstantin Koshelev, Troitsk Institute of Spectroscopy

Light comes from transitions in Sn+6 to Sn+14,
between 4p64dn and 4p54dn+1 or 4dn-1(4f,5p)

Lighting up these transitions, and only these
transitions requires exquisite control of laser
plasma
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Optimum conditions for EUV light generation:
a plasma temperature of ~35 eV, …
… and laser heating where
EUV light can escape
Region of EUV
emission
Calculation of non-LTE ionization
balance using Cretin
Interferometry data
840 ns after pre-pulse
Good News: These conditions can be achieved using
relatively “ordinary” Joule-class pulsed (10 ns) lasers
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Laser plasma is notoriously difficult to control
 Steep spatial gradients
 Strong time dependence
T
(eV)
Hyades: double-sided
illumination of foam
t (ns)
ne
20
(10 /cm3)
t (ns)
 Intimate dependence on temperature
 Non-LTE behavior (rate-dependent
R
(cm)
atomic populations)
t (ns)
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… and difficult to measure
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





Nomarski interferometer
Transmission grating EUV
spectrometer
In-band energy monitor
Faraday cup
2 ns visible imaging
Spatially-resolved visible
spectroscopy (2 ns)
EUV imaging at 13.5 nm
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Challenge #1: Maximize conversion
of laser light to in-band EUV output
Maximum in-band emission with
minimum out-of-band saves $$
on the laser and cost of ownership,
and reduces optic damage.
Intel’s EUV MicroExposure Tool
Our work:



Techniques to narrow the UTA
Optimized pulse length & wavelength
Avoidance of reabsorption
Cymer’s HVM EUVL source concept
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Low density targets produce a narrower UTA
Targets provided by Reny Paguio,
General Atomics
•
•
•
100 mg/cc RF foam
0.1-1% solid density Sn
e.g., 0.5%Sn = Sn1.8O17.2C27H54

We attempted to reduce debris by
reducing the target density.

The optical depth at 13.5 nm is
only ~7 nm of full density Sn.
Beyond that, light is reabsorbed.

An unexpected advantage is a
narrower spectrum.
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Conversion efficiency can be optimized by
choosing the laser pulse length & wavelength
2.5
Conversion efficiency is higher
at longer wavelengths
2.0
CE
1.5
Sequoia
1.0
Ando
0.5
0.0
0
5
10
15
20
Pulse Duration (ns)
2 ns gives better CE, but longer
pulses may be more cost-effective
2.5
2
CE
1.5
1
7 ns
15 ns
Ando 2.3 ns
Ando 5.6 ns
Ando 8.5 ns
0.5
0
10
10.5
11
11.5
2
Intensity (Log10 W/cm )
12
Laser absorption occurs at a lower
density, allowing the EUV light to
escape more efficiently.
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A spectral dip can occur due to reabsorption,
degrading the conversion efficiency
e.g., spot size is one of several factors that contribute to opacity control
Emission spectrum
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
(I=21011W/cm2 )
CE vs. laser intensity
CE depends on a balance between emissivity and opacity
In a smaller spot:
 Lateral expansion wastes laser energy  less emissivity
 Lateral expansion reduces plasma scale length  less opacity
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Challenge #2: Minimize optic damage from
high-energy particles

Laser-produced plasma generates debris
and energetic ions

Optic lifetime must be >30,000 hours

Debris can be cleaned, but…

Energetic ions damage multilayer mirrors
Our work:
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Gas stopping
Magnetic stopping
Gas plus magnets
Double-pulsing
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Full density
Mass-limited
Mass-limited plus gas
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Gas stops ions, but also stops EUV photons
Ion yield at 10˚, 15 cm
Faraday cup vs. SRIM estimate
Conversion efficiency at 45˚, 78 cm
E-mon vs. attenuation calculation
Calculated
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Hydrogen is best, but not good enough (and nasty)
Collisionality in plumes also affects their range
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Magnetic diversion is partially effective,
but not sufficient
5 GW/cm2
aluminum
free expansion velocity
v=6x106 cm/s
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A magnetic field produces a synergistic
effect in combination with background gas
Faraday cup time-of-flight measurements
at 10˚, 15 cm from target
photoionization peak
appears when gas is
present
100% dense
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We recently discovered a technique that
dramatically reduces the ion emission energy

Low-energy short pre-pulse forms target;
main pulse interacts with pre-plasma.
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Degrees of freedom to control performance.
Pre-pulse:
Wavelength
Pulse length
Energy
Intensity
Spot size
Main pulse:
Wavelength
Pulse length
Energy
Intensity
Spot size
532, 1064 nm
130 ps
2 mJ
~1010 W/cm2
300 µm
1064 nm
7 ns
150 mJ
21011 W/cm2
100 µm
2 mm Sn slab
Camera gate
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Ion energy was reduced by a factor of 30!
Energy spectra of ions vs. time delay
5.2 keV
v=L/t, E=1/2 mv2
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The main pulse interacts with a pre-formed
gas target on a gentle density gradient
Pre-plume density profile
840 ns after the pre-pulse
Thermal
plasma
500 m
cold
plume
10 ns
440 ns
840 ns
1840 ns
130 ps pre-pulse,
2 mJ, 532 nm
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The location of absorption of the main pulse
is clearly displaced away from the surface
single pulse
double pulse
Region of EUV generation
lasers
pre-plasma
main
plasma
Nomarski interferometer
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At the optimum delay time, no loss of
conversion efficiency is observed
CE
Particle energy
reduction factor
Delay (ns)
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Gas is more effective at stopping ions
that already have their energy degraded
300 mTorr Hydrogen TOF
spectrum
10 mTorr Argon TOF
spectrum
In both cases, the predicted transmission of 13.5 nm light is >95%
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“Mass-limited” targets should reduce the
debris loading, without loss of light output
 Thin films were fabricated using e-beam
evaporative coating of Sn on plastic and glass
 Film thicknesses from 20 to 100 nm, as well
as foils from 1 to 10 µm were tested
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Unlike single-pulse results, ion energy was
reduced using a pre-pulse on thin coatings
 Acceleration with single pulse likely due to low-Z substrate
 Double-pulse pump beam never reaches the substrate
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Pre-pulse + gas + mass-limited target could
satisfy requirements of a practical EUVL source
We need better diagnostics to
measure vanishingly small yields
MCP
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Acknowledgements

Contributors: Yezheng Tao, S. S. Harilal, Kevin Sequoia

Financial support: General Atomics, William J. von Liebig Foundation,
Cymer Inc., LLNL and the US Department of Energy
http://cer.ucsd.edu/LMI
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Laser-matter interactions at UC San Diego
Laser plasmas:
•
•
EUV lithography
HED studies (XUV,
electron transport)
Relativistic
laser plasma
(fast ignition)
Thermal,
mechanical and
phase change
behavior
Laser ablation plume
dynamics, LIBS,
micromachining
optics damage
Center for Energy Research