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Laser-produced plasma for EUV lithography
UCLA MAE Department
Thermo/Fluids Research Seminar Series
29 June 2007
M. S. Tillack
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The end is in sight for semiconductor lithography based on direct laser
exposure
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
(www.intel.com/technology/silicon/lithography.htm
)
Current technology
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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
wafer
mask
EUV source
laser
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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 mitigationUCSD Laser Plasma & Laser-Matter Interactions Lab
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13.5 nm is a large, but credible next step
Mo/Si, 6.9 nm periodhttp://www-cxro.lbl.gov/optical_constants/
multi2.html
QuickTime™ and aTIFF (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.
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
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The UTA of Sn is an efficient source at 13.5 nm
Konstantin Koshelev, Troitsk Institute of Spectroscopy
Sn+6
Sn+7
Sn+8
Gerry O’Sullivan, University College Dublin
Sn
Xe
Sb
Te
I
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 data840 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
Intimate dependence on temperature
Non-LTE behavior (rate-dependent atomic populations)
t (ns)
Hyades: double-sided illumination of foamT
(eV)
ne
(1020/cm3)
R(cm)
t (ns)
t (ns)
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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
… and difficult to measure
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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
Cymer’s HVM EUVL source concept
Challenge #1: Maximize conversion of laser light to in-band EUV output
Techniques to narrow the UTA
Optimized pulse length & wavelength
Avoidance of reabsorption
Our work:
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Low density targets produce a narrower UTA
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.
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
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Conversion efficiency can be optimized by choosing the laser pulse length &
wavelengthConversion efficiency is higher at longer wavelengths
Laser absorption occurs at a lower density, allowing the EUV light to escape more efficiently.
0
0.5
1
1.5
2
2.5
10 10.5 11 11.5 12
Intensity (Log10 W/cm2)
CE7 ns
15 nsAndo 2.3 ns
Ando 5.6 nsAndo 8.5 ns
0.0
0.5
1.0
1.5
2.0
2.5
0 5 10 15 20
Pulse Duration (ns)
CE Sequoia
Ando
2 ns gives better CE, but longer pulses may be more cost-effective
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A spectral dip can occur due to reabsorption, degrading the conversion
efficiencye.g., spot size is one of several factors that contribute to opacity control
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
CE vs. laser intensity(I=21011W/cm2 )
Emission spectrum
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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
Gas stopping
Magnetic stopping
Gas plus magnets
Double-pulsing Full density Mass-limited Mass-limited plus
gas
Our work:
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Gas stops ions, but also stops EUV photons
Calculated
Conversion efficiency at 45˚, 78 cm
E-mon vs. attenuation calculation
Ion yield at 10˚, 15 cmFaraday cup vs. SRIM estimate
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
free expansion velocity v=6x106 cm/s
aluminum
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A magnetic field produces a synergistic effect in combination with background
gas
photoionization peak appears when gas is present
Faraday cup time-of-flight measurements
at 10˚, 15 cm from target
100% dense
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We recently discovered a technique that dramatically reduces the ion emission
energyCamera gate
Pre-pulse:
Wavelength 532, 1064 nm
Pulse length 130 ps
Energy 2 mJ
Intensity ~1010 W/cm2
Spot size 300 µm
Main pulse:
Wavelength 1064 nm
Pulse length 7 ns
Energy 150 mJ
Intensity 21011 W/cm2
Spot size 100 µm
Low-energy short pre-pulse forms
target; main pulse interacts with pre-
plasma.
Degrees of freedom to control
performance.
2 mm Sn slab
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Ion energy was reduced by a factor of 30!
v=L/t, E=1/2 mv2
5.2 keV
Energy spectra of ions vs. time delay
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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
130 ps pre-pulse,2 mJ, 532 nm
840 ns 1840 ns
440 ns10 ns
Thermalplasma
coldplume
500 m
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The location of absorption of the main pulse is clearly displaced away from the
surface
Region of EUV generation
Nomarski interferometer
lasers
single pulse double pulse
pre-plasma mainplasma
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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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300 mTorr Hydrogen TOF spectrum
10 mTorr Argon TOF spectrum
Gas is more effective at stopping ions that already have their energy
degraded
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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MCP
Pre-pulse + gas + mass-limited target could satisfy requirements of a practical EUVL
source
We need better diagnostics to measure vanishingly small yields
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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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Center for Energy Research
Laser-matter interactions at UC San Diego
Thermal, mechanical and phase
change behavior
Relativistic laser plasma
(fast ignition)
optics damage
Laser plasmas:• EUV lithography• HED studies
(XUV, electron transport)
Laser ablation plume dynamics,
LIBS, micromachining