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Wavelength Dependence of EUV-
Induced Optics Contamination Rates
Shannon B. Hill, N. Faradzhev, L. J. Richter, C. Tarrio, S. Grantham, R. Vest and T. Lucatorto
National Institute of Standards and Technology
Gaithersburg, MD USA
This work supported in part by Intel Corporation and ASML
EUVL Symposium, Miami, FL (Oct. 18, 2011)
1
Outline
•Pressure and intensity dependence of
contamination rates
•Resist-outgas testing for NXE at NIST
•Wavelength dependence of contamination rates
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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With in situ cleaning, carbon-deposition is no
longer potential “show stopper”
Still threat to productivity
Constraint on resist development
EUV-induced optics contamination
Images courtesy of SEMATECH
New optic
After limited use in MET
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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Metrology of optics lifetime
How does contamination vary with
• Contaminant species
• Partial pressure of contaminant
• Partial pressure of water vapor
• EUV intensity
• Duty cycle of pulsed EUV sources
• Wavelength
• Time (EUV dose)
How is contamination measured
• Reflectometry
• X-ray photoelectron spectroscopy (XPS)
• Spectroscopic ellipsometry (SE)
• Other techniques: NEXAFS, RBS, TEM…
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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Contamination Studies at NIST and Rutgers Univ.
Species potentially found in
stepper, resist outgassing
(RGA, Cryo-trapping + GC-MS)
Rutgers: Adsorption &
desorption without EUV
irradiation.
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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NIST: Contamination kinetics
under EUV irradiation
Contamination rates measured over large pressure range C
gro
wth
rate
at
1 m
W/m
m2 (
nm
/h)
EUV-induced contamination rates (NIST)
• Contamination rates vary with log of pressure below 10-5 Torr for every species tested.
• Linear pressure scaling reported elsewhere at much higher pressures.
• Essentially impossible to completely eliminate contamination by clean vacuum practices
alone.
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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Pressure scaling of EUV contamination rates
driven by fundamental surface physics
0
0.1
0.2
0.3
0.4
0.5
1.0E-10 1.0E-09 1.0E-08
Partial pressure, Torr
Co
vera
ge, M
L
TiO2(011)
C/Ru(1010)
TiO2(011)
C/Ru(1010)
Benzene
TPD
300K
Toluene
Methyl Methacrylate (MMA)
Mo
lecu
lar
co
vera
ge
(m
on
ola
ye
rs)
Partial pressure (Torr)
C g
row
th r
ate
at
1 m
W/m
m2 (
nm
/h)
Equilibrium coverage (Rutgers, no EUV) EUV-induced contamination rates (NIST)
• EUV contamination and equilibrium molecular coverage (non-irradiated) scale with
log of pressure
• Additional measurements at Rutgers show surface binding energy decreases with
coverage, leading to sub-linear pressure dependence of coverage.
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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• Transition from log(p) not observed: estimate extrapolation limits
px
tert-butylbenzene
C10H14
tert-butylbenzene
C10H14
log(p/p0)
Extrapolation from accelerated testing to HVM tool conditions
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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Extrapolation from accelerated testing to HVM tool conditions
• Transition from log(p) not observed: estimate extrapolation limits
px
Extrapolation from
only 1 decade of
high-pressure data
tert-butylbenzene
C10H14
tert-butylbenzene
C10H14
log(p/p0)
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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Extrapolation to lower pressures: potential errors
log(p/p0)
px
Extrapolation from
only 1 decade of
high-pressure data
tert-butylbenzene
C10H14
tert-butylbenzene
C10H14
• Transition from log(p) not observed: estimate extrapolation limits
• Linear extrapolation of a few high-pressure measurements will underestimate rates at
lower pressures by multiple orders of magnitude.
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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Extrapolation to lower pressures: potential errors
log(p/p0)
px
Extrapolation from
only 1 decade of
high-pressure data
tert-butylbenzene
C10H14
tert-butylbenzene
C10H14
• Mitigation/cleaning by EUV + ambient water vapor can play important role at low
partial pressures of contaminant species.
• Scaling at lowest pressures determined by intensity…
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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Growth rate of base vacuum
Intensity scaling: saturation & mass-limited growth
• C growth rate is mass limited for I > Isat : every adsorbed molecule photo-reacts.
• Isat increases with pressure and varies with species.
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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Growth rate of base vacuum
Intensity scaling: saturation & mass-limited growth
• C growth rate is mass limited for I > Isat : every adsorbed molecule photo-reacts.
• Isat increases with pressure and varies with species.
• Pressure scaling of rates changes from logarithmic to linear for I > Isat .
• I > Isat is required for resist qualification testing to ensure maximum contamination rate.
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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Wafer Witness sample
(MLM)
Relay MLM
Intensity saturation in resist outgas testing
Spectroscopic ellipsometry map Line profile through spot center
Position (mm)
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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Study contamination at different wavelengths
• Grazing-incidence mirror & Zr filter used on resist-outgas beamline for higher intensity.
• Contamination rates were lower than expected with broadband light below 13.5 nm.
• Compare contamination rates over range of wavelength bands using normal-incidence
multilayer mirror with Be, Sn or In filters.
Calculated power spectra at sample
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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Wavelength scaling of contamination rates
• Compare ratio of rates at different
wavelengths to in-band rates (MLM
+ Be filter).
• Horizontal bars contain 80% of
power for each filter
• Rates for different species over wide
range of pressures display same
dramatic increase between ~10 nm
and ~60 nm. (note log scale).
• Consistent with previous
measurements (Denbeaux, SPIE
2010) and calculations (Jindal, SPIE
2009.)
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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Calculated power spectra at sample
Wavelength scaling of C deposition per photon
Horizontal bars contain 80% of
power for each filter • Contamination per photon also
shows significant increase.
• Higher rates not just due to more
photons.
• Measurements with (110-200) nm
filter underway.
• Optics closest to source likely to
have highest contamination rate in
tool.
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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Calculated power spectra at sample
Summary
• Pressure scaling of contamination rates is highly sub-linear
(logarithmic)
• Intensity dependence saturates at species and pressure
dependent Isat
• Estimating contamination risks at tool conditions requires
measurements over range of pressure and intensity.
• Out of band light may pose greater contamination threat than
in-band for some optics.
• NXE3100/3200B resist qualification up and running at NIST.
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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Ongoing work
• Resist-outgas testing and qualification
• Effect of EUV intensity/dose on cleanability of C
• Correlation of metrologies: ellipsometry, XPS, reflectometry
• Cleaning with H-atom hot-filament and plasma, as well as
EUV + H2O2, NO
• Non-C contamination: S, F, Cl, etc.
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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Thank you!
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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EUVL Symposium, Miami, FL (Oct. 18, 2011)
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Supplemental Slides
Initial wavelength dependence measurements (Albany: Denbeaux, et al, SPIE 2010)
• Comparing rates using broadband source through different filters
• Contamination per unit dose is ~7x higher at wavelengths above 13.5 nm
11 – 17 nm
17 – 80 nm
17
– 4
0 n
m
57
– 8
0 n
m
wavelength (nm)
average rate (nm/J/cm2)
11 – 17 0.9 + 0.4
17 – 80 6.7 + 1.1
17 – 40 4.3 + 0.7
57 – 80 4.5
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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Calculated wavelength dependence (SEMATECH: Jindal, SPIE 2009)
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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Influence of duty cycle on contamination: pulsed-vs-CW source (Collaboration with Sergiy Yulin, Fraunhofer Institute IOF, Jena, Germany)
• Identical EUV dose, average intensity & pressures of tert-butylbenzene
NIST quasi-CW synchrotron: duty ≈ 0.1 (114 MHz , 1ns pulse width)
IOF Xe pulsed plasma: duty ≈ 6×10-4 (4 kHz , 150 ns pulse width)
• Deposited C thicknesses similar despite ≈ 170x difference in peak EUV intensity
TiO2-capped optic
Pulsed (IOF)
Synchrotron (NIST)
Ru-capped optic
Pulsed (IOF)
Synchrotron (NIST)
EUVL Symposium, Miami, FL (Oct. 18, 2011)
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