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ULE® Glass with Improved Thermal Properties for
EUVL Masks and Projection Optics Substrates
Ken Hrdina and Carlos Duran
Corning Incorporated
2012 International Symposium on Extreme Ultraviolet Lithography
Brussels, Belgium, Sept. 30 – Oct. 4, 2012
2 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
Introduction – Ultra Low Expansion ULE® Glass
• Corning has made ULE® Glass for 4 decades: – A high-purity, single-phase SiO2 glass doped with TiO2
– Used as a substrate for high performance mirrors:
• EUV Lithography driving material improvements: – “Polishability”
– Absolute CTE better than ±5 ppb/K
– Detailed metrology and uniformity requirements
– Push for lower thermal expansion
Hubble Telescope
2.4 m Primary Mirror
(1978)
Subaru Telescope
8.2 m Primary Mirror
(1994)
3 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
Introduction – ULE® Glass (continued)
• Most outstanding feature of ULE® Glass is its extremely low coefficient of
thermal expansion:
– Mean CTE5-35C = 0±30 ppb/K (more than 100x better than PYREX®)
• Single-Phase Glass enables other important properties:
– Long Term Dimensional stability
– No hysteresis when cycled up to ~300 °C • Room-Temperature shape and CTE are unchanged
• No specific thermal cycle or cooling rate is needed
• No temporal drift in properties is triggered
• Material is not affected by standard optical manufacturing and coating
techniques
– Mechanical strength • No delayed elastic effects have been observed (J.W. Pepi & D. Golini,
Adv. Optics 20, 3087 (1991)
4 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
ULE® Glass for EUVL – Why?
• High throughput in EUVL requires a bright light source (100’s of Watts).
• Mirrors are expected to heat up:
– Single mirror reflectivity is only ~70% at 13.5 nm.
– Vacuum makes heat removal difficult.
– In a 6 mirror + mask system, >90% of the light is absorbed before it
gets to the wafer.
– Ultra low expansion material in mask and mirrors is a must in order to
preserve sub-nm wavefront distortion.
• ULE® Glass can be polished to sub nm roughness.
• Temperature of CTE=0 (crossover temperature Tzc) can be tuned.
• Metrology capable of non-destructively certifying material compliance
within a narrow range.
5 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
Thermal Expansion in ULE® Glass
• CTE in ULE® Glass is controlled by composition: – Temperature dependence of expansivity curve is known.
– Curve shifts in a predictable manner with changes in TiO2 concentration.
– Manufacturing adjustments in a narrow range around room temperature are
understood, and compatible with production process.
– Non-destructive metrology allows us to define the actual CTE of all glass we
make (ultrasound velocity technique).
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-60
-40
-20
0
20
40
60
80
-10 0 10 20 30 40 50
Temperature in [°C]
Ex
pa
ns
ivit
y i
n [
pp
b/K
]
Less TiO2
Nominal ULE
More TiO2δ(ΔL / L)
δT
Th
erm
al e
xp
an
sio
n
ΔL / L
[p
pm
]
Temperature [°C]
Tzc = 20 C
6 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
Minimizing Distortion in EUVL Optics and Masks
CASE I
Standard Grade
ULE®Glass
7 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
Minimizing Distortion in EUVL Optics and Masks
Case II
EUVL Grade
ULE®Glass
With TZC set at ~20⁰C.
• Tune Tzc to expected temperature range in the optic:
• Done by adjusting TiO2 content during glass forming.
• It is difficult to control when Tzc is tightly specified.
8 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
Minimizing Distortion in EUVL Optics and Masks
Case III
EUVL Grade
ULE®Glass.
With TZC set at
~midpoint.
9 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
Minimizing Distortion in EUVL Optics and Masks
Case IV
EUVL Grade
ULE®Glass.
With TZC set at
~midpoint.
And
Reduced
expansivity
slope. •Reduce slope of expansivity curve:
• In addition to tuning Tzc
• Gain becomes more important as the optic temperature range grows.
10 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
Expansivity Improvement Definition
TEMPERATURE (°C)
EX
PA
NS
IVIT
Y (
ppb/K
) Current
behavior
Ideal behavior
(“100%” Improved)
Goal of this project was to optimize the expansivity through thermal
treatment only, without introducing changes to a well established ULE®
Glass production process.
11 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
Expansivity Improvement –
Required Metrology
• Characterization of glass structural state after annealing:
• We use Fictive Temperature (Tf) as parameter to describe the
structural state of the glass.
• Measure Tf through FTIR measurements (method by A. Agarwal,
K.M. Davis and M. Tomozawa, J. Non-Crystalline Solids 185 (1995),
p191-198). (J.E. Shelby, Phys. Chem. Glasses., 2005, 46 (5), 494-
499).
• Specific implementation and calibration for ULE® Glass submitted to
J. Non-Crystalline Solids – Tingley et al. (2012).
• Measurement of effect on expansivity:
• Sandwich Seal measurements
• Other production-compatible techniques
• Modeling of Tf
• Calibration of Ultrasound Velocity technique for improved material.
12 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
Glass Tf Characterization
FTIR Measurements and Empirical Modeling
• Quenched samples after isothermal
holds at various temperatures to
calibrate Tf vs. FTIR Peak Position.
• Empirical model to calculate Tf after
anneal and obtain correlation
between model predictions and
measurements.
Model Predictions vs. Measurements
y = 1.000x
R2 = 0.976
860
880
900
920
940
960
980
860 880 900 920 940 960 980
SIMULATED TfM
EA
SU
RE
D T
f
13 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
Measuring Changes in Expansivity -Sandwich Seal Differential Expansion Measurement
Incoming Light
Center – reference glass
Top & Bottom – unknown glass
Outgoing Light (Elliptically Polarized)
Seal with stress (due to difference in CTE)
(Linearly Polarized)
• Use polarized light to measure stress, and evaluate thermal expansion.
• Technique by H. Hagy and collaborators, J. Opt. Soc. Am. A3, P83 (1986), and
references therein.
• New setup incorporates computer control and data analysis: • Can detect small differences in thermal expansion.
• Requires smaller samples than other techniques.
• Temperature range from ~200 to 425 K (-70 to 150 °C).
14 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
Sandwich Seal Data Analysis (Conceptual) - Seals made with standard production ULE® Glass -
-600
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0
200
400
600
0 10 20 30 40
TEPERATURE (C)
EX
P D
IFF
(p
pb
)
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0
20
40
0 10 20 30 40
TEMPERATURE (C)
EX
PA
NS
IVIT
Y (
pp
b/C
)
Seal Signals Material Expansivity
• Expansivity curves in production ULE® Glass have a constant shape vs. T
• Only effect of changes in TiO2 concentration is to shift the curves
vertically.
• Stress measured in sandwich seals made with this glass will be linear: • Sign of stress indicates whether “bread” expands more or less than “meat”.
• Slope is a direct measurement of difference in CTE between bread and meat.
15 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
Sandwich Seal – Correlation to Ultrasound
(Actual Measurements)
• Set of six sandwich seal samples were made using ULE® Glass with different amounts of TiO2 doping.
• System has excellent sensitivity to small expansion differences.
• Expansion Difference vs. Temperature plots are straight lines:
– Explicitly shows that Expansivity is independent of CTE offset.
• Uncertainty in CTE difference from Sandwich Seal is ~±2-3ppb/K – Excellent tool for evaluation of new materials.
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0
500
1000
1500
2000
2500
200 250 300 350 400
TEMPERATURE (K)
Exp
an
sio
n D
iff
(pp
b)
Seal#6
Seal#5
Seal#4
Seal#3
Seal#2
Seal#1
R2 = 0.98
-20
-10
0
10
20
30
-20 -10 0 10 20 30
Ultrasound CTE difference (ppb/K)
Seal C
TE
diff
ere
nce
(ppb/K
)
16 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
Sandwich Seal Data Analysis (Conceptual)
- Seals made with non-Standard ULE® Glass -
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0
25
50
75
0 10 20 30 40
TEMPERATURE (C)
EX
PA
NS
IVIT
Y (
pp
b/C
)
-600
-400
-200
0
200
400
600
0 10 20 30 40
TEMPERATURE (C)
EX
P D
IFF
(p
pb
)
Seal Signals Material Expansivity
• Seals are prepared using Standard ULE® Glass for the “meat”, and non-
standard for the “bread”
• If expansivity curves are not parallel, measured stress is no longer linear
with temperature.
• Fit measured seal stress adding a quadratic term:
• Sign and magnitude of quadratic term indicates whether the bread
expands slower than or faster than standard ULE® Glass.
17 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
Sandwich Seal Measurement Results
(Actual Measurements)
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0
500
1000
1500
2000
2500
200 250 300 350 400
TEMPERATURE (K)
Exp
an
sio
n D
iff
(pp
b)
Seal#6
Seal#5
Seal#4
Seal#3
Seal#2
Seal#1
S1S1
S1S2
S1S5
Seals made with “bread” slices of non-Std Anneal ULE®
• Curved lines indicate expansivity slope change.
• Measurement of curvature allows quantification of
improvements in expansivity (next slide).
18 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
Change in Expansivity Slope
(Quadratic Term in Sandwich Seal Fits)
• Samples prepared from different Boules of ULE® Glass.
• Consistent behavior seen for all seals measured.
• US Patent Applications 2011/0207592 and 2011/0207593
19 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
Thermal Expansion in Improved ULE® Glass
Improvement in
Thermal Expansion
• Improvement shown in this figure is ~ 24%
• US Patent Applications 2011/0207592 and 2011/0207593
20 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
Summary
• Until now, Tzc of ULE® Glass has been determined during the glass
forming process
• Using controlled annealing, we can simultaneously improve the
expansivity and fine tune the Tzc of ULE® Glass (US Patent Application
2011/0048075) – Process can be tuned to meet specific customer requirements.
– We have demonstrated improvements in expansivity greater than 30%.
– Tzc tuning will allow us to supply glass within narrower specification ranges
• Corning ULE® Glass is an enabler for EUV Lithography today – We are committed to continue to improve material properties and
manufacturing processes towards future needs
21 Hrdina/Duran / ULE® Glass for EUV Lithography © Corning Incorporated - 2012
ACKNOWLEDGMENTS
• D. Sears
• J. Tingley
• T. Carapella
• B. Tuttle
• J. Maxon
• B. Whispell
• D. Gauthier