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Center for Nanolithography Research
Solid Immersion andEvanescent Wave Lithographyat Numerical Apertures > 1.60
Bruce SmithY. Fan, J. Zhou, L. Zavyalova, M. Slocum, J. Park, A. Bourov,
E. Piscani, N. Lafferty, A. Estroff
Rochester Institute of TechnologyCenter for Nanolithography Research
Center for Nanolithography Research
Outline
The imaging limits of materials
Pushing the limits of immersion lithography
The solid immersion lens
Solid immersion lithography (SIL)
Evanescent wave lithography (EWL)
Imaging 26nm at 1.85NA
Center for Nanolithography Research
“glass”
media
photoresist
substrate
“glass”
media
photoresist
substrate
Material and Optical Limitations
NA = ni sin θ
1. Sin θ increases slowly at large angles (sin 68°=0.93)
2. Hyper-NA will be forced upon material refractive index
3. Resolution will become a function of the lowest index (fluid, optics, photoresist).
nmn62to
n52
(0.93)nm)0.30)(193nto(0.25
sinθnλkhp
iiii
1min ===
Center for Nanolithography Research
Technology Limits in Media
Fused silica (n=1.56) Sapphire (n=1.92)
TIR @θc = sin-1(nL/nH)
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90
Angle (degrees)
Ref
lect
ance
32°
TM
Air (1.00)H20 (1.44)
HIF (1.54)SIL (1.70)
67° 58°49°
TE
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90
Angle (degrees)
Ref
lect
ance
32°
TM
Air (1.00)H20 (1.44)
HIF (1.54)
67° 58°49°
TE
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90
Angle (degrees)
Ref
lect
ance
TM
Air (1.00) H20 (1.44)HIF (1.54)
SIL (1.56)
90°84°68°
TE
40°
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90
Angle (degrees)
Ref
lect
ance
TM
Air (1.00) H20 (1.44)HIF (1.54)
90°84°68°
TE
40°
TIR from Snells’ Law:
θc = sin-1(nL/nH)
Center for Nanolithography Research
Numerical Aperture1.3 1.4 1.5 1.6 1.7 1.8 1.9 2
Half-Pitch (nm) k1=0.25 37 34 32 30 28 27 25 24k1=0.30 45 44 39 36 34 32 30 29
Angle in media Water (1.44) 65° 76°HIF (1.55) 57° 65° 75°HIF2 (1.65) 52° 58° 65° 76°Photoresist (1.70) 50° 55° 62° 70°HI PR (1.85) 45° 49° 54° 60° 67° 77°Fused silica (1.54) 58° 65° 77°Sapphire (1.92) 43° 47° 52° 56° 62° 70°
Technology Limits in Media
TIR
Center for Nanolithography Research
Impact of Angle in Photoresist Simple Approximations
0
0.2
0.4
0.6
0.8
1
1.2
0 20 40 60 80
Angle in resist (degrees)
Unp
olar
ized
mod
ulat
ion
0
1
2
3
4
5
6
7
Abs
orpt
ion
scal
ing
0
5
10
15
20
25
30
35
0 20 40 60 80Angle in resist (degrees)
DO
F sc
alin
g ⎥⎦⎤
⎢⎣⎡
θsin1
2⎥⎦⎤
⎢⎣⎡
θcos 1
AbsorptionModulation
Oblique absorption requires low k photoresistParaxial DOF scales with 1/sin2(θ)Angles above 30° (0.85 NA) require attentionOblique reflection becomes an issue > 30°
Polarization and absorption Depth of focus
0.85
NA
1.20
NA
0.85
NA
1.20
NA
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A Solid Immersion Lens
znnsin
n/
upper
lowerupper
e)z(A⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜
⎝
⎛
+⎥⎥
⎦
⎤
⎢⎢
⎣
⎡
⎟⎟⎠
⎞⎜⎜⎝
⎛−−
=αθ
λπ
21222
- A high index solid immersion lens is placed in close proximityto an image plane
- “Dry imaging for NA values > 1.0- Used in optical storage applications
- Energy coupled into the thin film decays exponentially:
nupper = lensnlower = airz = gap
Center for Nanolithography Research
Solid Immersion LithographySapphire SIL Breadboard
Sapphire Properties:- Hexagonal, single-crystalline Al2O3- n = 1.92, birefringence ~8x10-3
- Equilateral prism at 60° is 1.67NA- Designed for NA 1.05~1.92- MgF2 is ideal AR layer
Polarized ArF beam
Grating mask
Zero-order block
Turning mirrors
Sapphire prism
prism
Challenges:- Gap and gap control- Birefringence- CAR resist diffusion length limit- Resist/BARC process optimization
cylinder
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Laser Detector Laser
Optical Coupling in the Prism
(a) Baseline (no wafer). (b) Reflection (with wafer).
(a) Before pressure is applied. (b) After pressure is applied.
Detector
Center for Nanolithography Research
NA=1.66
0
0.2
0.4
0.6
0.8
1
0 10 20 30 40 50Air gap thickness (nm)
Ref
lect
ance
TheoreticalMeasurement
Immersion solid (sapphire), N0=1.92 Air gap, N1=1.00, d1=0~50 nm
Resist, N2=1.71-0.399i, d2=78 nm BARC, N2=1.70-0.1i, 92 nm
Substrate Nsub=0.87-2.76i
Resist assembly
Estimation of Gap Thickness- Reflectance measurement
used to estimate gap thickness.
- Gap controllable from 0-50nm- 12nm air gap utilized.
Center for Nanolithography Research
1.42NA, 34nm 1.60NA, 30nm 1.66NA, 29nm
Solid Immersion Lithography at the Resist Limit
ILSim simulations
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Beyond the Resist LimitEvanescent Wave Coupling
nuppersinθ = NAmax
nlowersinθ
NA = nisinθHomogeneous
propagation
Evanescent region
higher n
θc
nlower- low
Homogeneous propagation
Evanescent region
nupper- hi
thin film
znnsin
n22/12
upper
lower2upper
e)z(A⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜
⎝
⎛
α+⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎟⎟⎠
⎞⎜⎜⎝
⎛−θ
λ
π−
=
Energy coupled into the thin film decays exponentially:
Center for Nanolithography Research
1.85NA, 26nm
Evanescent Wave LithographyBeyond the Resist Limit - 26nm hp at 1.85NA
NA (1.85) has been pushed higher than the index of the resist (1.70).Image pattern depth of <10 nm.Sets the stage for new material
development toward 25nm.Potential with TSI and hard-
mask imaging layers.
Center for Nanolithography Research
Gap Requirements / TolerancesAssume 50% intensity loss across the image – no loss in modulation
1% gap ∆ results in ~0.5-1% intensity ∆ at 1.70NA – dose control issue
znnsin
n22/12
upper
lower2upper
e)z(A⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜
⎝
⎛
α+⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎟⎟⎠
⎞⎜⎜⎝
⎛−θ
λ
π−
=
010
203040
506070
8090
1.65 1.70 1.75 1.80 1.85 1.90
Numerical ApertureG
ap (n
m) f
or 5
0% in
tens
ity
10
20
30
40
50
60
70
80
1.0 1.1 1.2 1.3 1.4 1.5 1.6
Gap Index
Gap
(nm
) for
50%
inte
nsity
28nm hp at 64° in sapphire HIF2 gap for 0.25k1
Water
HIF2 28nm hp
27nm hp
Center for Nanolithography Research
Implications of SIL andEvanescent Wave Lithography
1. SIL / EWL is useful for determining the ultimate limits of optical lithography in the 25nm regime.
2. NA possible beyond the fluid index.
3. Higher index photoresists may not be necessary if top-surface imaging (TSI) can be employed.
4. SIL may be feasible if small fluid gaps can be maintained.
Center for Nanolithography Research
Numerical Aperture1.3 1.4 1.5 1.6 1.7 1.8 1.9 2
Half-Pitch (nm) k1=0.25 37 34 32 30 28 27 25 24k1=0.30 45 44 39 36 34 32 30 29
Angle in media Water (1.44) 65° 76°HIF (1.55) 57° 65° 75°HIF2 (1.65) 52° 58° 65° 76°Photoresist (1.70) 50° 55° 62° 70°HI PR (1.85) 45° 49° 54° 60° 67° 77°Fused silica (1.54) 58° 65° 77°Sapphire (1.92) 43° 47° 52° 56° 62° 70°
Can be achieved with immersion lithography
May be possible with SIL / EWL
Not likely
Technology Limits in Media
Acknowledgements
SRC, DARPA/AFRL, Sematech, ASML, Photronics, TOK, JSR, Rohm and Haas, Brewer, NYSTAR, Corning Tropel
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