Solid Immersion and
Evanescent Wave Lithography
at Numerical Apertures > 1.60
Bruce Smith
Y. Fan, J. Zhou, L. Zavyalova, M. Slocum, J. Park, A. Bourov,
E. Piscani, N. Lafferty, A. Estroff
Rochester Institute of Technology
Center for Nanolithography Research
Center for Nanolithography Research
Outline
¾ The imaging limits of materials
¾ Pushing the limits of immersion lithography
¾ The solid immersion lens
¾ Solid immersion lithography (SIL)
¾ Evanescent wave lithography (EWL)
¾ Imaging 26nm at 1.85NA
Center for Nanolithography Research
Material and Optical Limitations
NA = ni sin θ
1. Sin θ increases slowly at large
angles (sin 68°=0.93)
“glass”
media
2. Hyper-NA will be forced upon
material refractive index
3. Resolution will become a
function of the lowest index
(fluid, optics, photoresist).
hpmin
photoresist
substrate
k1 λ
(0.25 to 0.30)(193n m) 52
62
to
=
=
=
nm
ni sinθ
ni (0.93)
ni
ni
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Technology Limits in Media
TIR from Snells’ Law:
θc = sin-1(nL/nH)
SIL (1.70)
HIF (1.54)
0 (1.44)
H2TIR
@
Air (1.00)
-1
SIL (1.56)
HIF (1.54)
Air (1.00) H2 0 (1.44)
100
40°
68°
84°90°
90
49°67° 58°
90
TE
80
Reflectance
Reflectance
θc = sin (nL/nH)
32°
100
TM
70
60
TM
70
60
50
50
40
40
0
10
20
30
40
50
60
70
Angle (degrees)
Fused silica (n=1.56)
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80
90
TE
80
0
10
20
30
40
50
60
70
Angle (degrees)
Sapphire (n=1.92)
80
90
Technology Limits in Media
1.3
37
45
Half-Pitch (nm) k1=0.25
k1=0.30
Angle in media Water (1.44)
HIF (1.55)
HIF2 (1.65)
Photoresist (1.70)
HI PR (1.85)
Fused silica (1.54)
Sapphire (1.92)
65°
57°
52°
50°
45°
58°
43°
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Numerical Aperture
1.5
1.6
1.7
32
30
28
39
36
34
1.4
34
44
76°
65°
58°
55°
49°
65°
47°
75°
65°
62°
54°
77°
52°
1.8
27
32
1.9
25
30
TIR
76°
70°
60°
67°
77°
56°
62°
70°
2
24
29
Impact of Angle in Photoresist
Simple Approximations
⎡ 1 ⎤
⎢⎣ cos θ ⎥⎦
Modulation
5
Absorption
0.6
0.4
4
3
2
0
0
20
40
1.20NA
0.2
60
Angle in resist (degrees)
80
DOF scaling
0.8
⎡ 1 ⎤
⎢⎣ sin 2 θ ⎥⎦
30
6
Absorption scaling
1
25
20
15
10
1
5
0
0
0
1.20NA
7
0.85NA
Unpolarized modulation
1.2
Depth of focus
35
0.85NA
Polarization and absorption
20
40
60
Angle in resist (degrees)
¾ Oblique absorption requires low k photoresist
¾ Paraxial DOF scales with 1/sin2(θ)
¾ Angles above 30° (0.85 NA) require attention
¾ Oblique reflection becomes an issue > 30°
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80
A Solid Immersion Lens
- A high index solid immersion lens is placed in close proximity
to an image plane
- “Dry imaging for NA values > 1.0
- Used in optical storage applications
- Energy coupled into the thin film decays exponentially:
A( z ) = e
⎛
⎜ 2πnupper
−⎜
λ
⎜⎜
⎝
⎡
⎛
⎢ sin 2 θ −⎜ nlower
⎜ nupper
⎢
⎝
⎣
2 ⎤1 / 2
⎞
⎟ ⎥
⎟ ⎥
⎠ ⎦
⎞
⎟
+α ⎟ z
⎟⎟
⎠
nupper = lens
nlower = air
z = gap
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Solid Immersion Lithography
Sapphire 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
prism
cylinder
Challenges:
- Gap and gap control
- Birefringence
- CAR resist diffusion length limit
- Resist/BARC process optimization
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Sapphire
prism
Turning
mirrors
Zero-order
block
Grating mask
Polarized ArF beam
Optical Coupling in the Prism
Laser
Detector
(a) Baseline (no wafer).
(a) Before pressure is applied.
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Laser
Detector
(b) Reflection (with wafer).
(b) After pressure is applied.
Estimation of Gap Thickness
- Reflectance measurement
used to estimate gap
thickness.
1
NA=1.66
0.8
Theoretical
Measurement
- 12nm air gap utilized.
Reflectance
- Gap controllable from 0-50nm
0.6
0.4
0.2
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
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0
0
10
20
30
40
Air gap thickness (nm)
50
Solid Immersion Lithography
at the Resist Limit
1.42NA, 34nm
1.60NA, 30nm
ILSim simulations
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1.66NA, 29nm
Beyond the Resist Limit
Evanescent Wave Coupling
nuppersinθ = NAmax
Homogeneous
propagation
Evanescent
region
thin film
θc
higher n
nupper- hi
nlower- low
nlowersinθ
Evanescent
region
Homogeneous
propagation
NA = nisinθ
Energy coupled into the thin film decays
exponentially:
A( z ) = e
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⎛
⎜ 2 πn upper ⎡
⎛
⎢ sin 2 θ −⎜ n lower
−⎜
⎜ n upper
⎢
λ
⎜⎜
⎝
⎢⎣
⎝
⎞
⎟
⎟
⎠
2⎤
⎥
⎥
⎥⎦
1/ 2
⎞
⎟
+α ⎟ z
⎟⎟
⎠
Evanescent Wave Lithography
Beyond the Resist Limit - 26nm hp at 1.85NA
1.85NA, 26nm
¾ NA (1.85) has been pushed
higher than the index of the
resist (1.70).
¾ Image pattern depth of <10 nm.
¾ Sets the stage for new material
development toward 25nm.
¾ Potential with TSI and hardmask imaging layers.
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Gap Requirements / Tolerances
Assume 50% intensity loss across the image – no loss in modulation
1% gap ∆ results in ~0.5-1% intensity ∆ at 1.70NA – dose control issue
90
HIF2
70
60
50
28nm hp at 64° in
sapphire
Water
40
30
20
10
1.0
Gap (nm) for 50% intensity
Gap (nm) for 50% intensity
80
28nm hp
80
70
HIF2 gap for 0.25k1
60
50
40
27nm hp
30
20
10
0
1.1
1.2
1.3
1.4
1.5
1.6
1.65
1.70
Gap Index
A( z ) = e
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⎛
⎜ 2 πn upper ⎡
⎛
⎢ sin 2 θ −⎜ n lower
−⎜
⎜ n upper
⎢
λ
⎜⎜
⎝
⎣⎢
⎝
1.75
1.80
Numerical Aperture
2⎤
⎞
⎟ ⎥
⎟ ⎥
⎠ ⎦⎥
1/ 2
⎞
⎟
+α ⎟ z
⎟⎟
⎠
1.85
1.90
Implications of SIL and
Evanescent 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 topsurface imaging (TSI) can be employed.
4. SIL may be feasible if small fluid gaps can be
maintained.
Center for Nanolithography Research
Technology Limits in Media
1.3
37
45
Half-Pitch (nm) k1=0.25
k1=0.30
Angle in media Water (1.44)
HIF (1.55)
HIF2 (1.65)
Photoresist (1.70)
HI PR (1.85)
Fused silica (1.54)
Sapphire (1.92)
65°
57°
52°
50°
45°
58°
43°
Numerical Aperture
1.5
1.6
1.7
32
30
28
39
36
34
1.4
34
44
76°
65°
58°
55°
49°
65°
47°
75°
65°
62°
54°
77°
52°
1.8
27
32
76°
70°
60°
67°
77°
56°
62°
70°
1.9
25
30
Can be achieved with immersion lithography
May be possible with SIL / EWL
Not likely
Acknowledgements
SRC, DARPA/AFRL, Sematech, ASML, Photronics, TOK, JSR, Rohm and
Haas, Brewer, NYSTAR, Corning Tropel
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