Laser-produced plasma for EUV lithography

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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

Laser-produced plasma for EUV lithography

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