Lecture 16 –
Introduction to Optical Lithography
EECS 598-002 Winter 2006
Nanophotonics and Nano-scale Fabrication
P.C.Ku
Optical Lithography
„
An optical system that transfers the image from the mask
to the resist layer + the process of forming an etching
mask (i.e. the resist development and etc.)
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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Resolution limits for imaging
„
„
Small features correspond to large (kx, ky) components.
In traditional optical microscopes, the detector sees the
k
light in the far field
region.
k 2 = ω 2 µ0ε = k x2 + k y2 + k z2
⇒ k x2 + k y2 < ωn / c ⇒ k&,max = 2π n / λ
17.5
15
Resolving power
= λ / ( 2n ) ≡ λ / 2
12.5
k-space
real-space
10
eff
7.5
= diffraction limit
5
2.5
−2π n / λ
2π n / λ
k&
-2
-1
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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2
λ /n
3
Finite-size lens
„
In a real system, the cutoff spatial frequency is often
limited by the size of the lens which is quantitatively
described by a numerical aperture (NA).
NA ≡ n sin θ
k&,max
2π
NA
⇒
= sin θ ⇒ k&,max =
k
λ
θ
Resolving power Æ
λ / ( 2NA ) ≡ λeff / 2
where λeff = λ / NA
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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Patterning process
Dissolution
rate
resist
x
I
aerial image
x
Dissolution
rate
+
I
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Some clarifications
„
The minimum feature size:
„
„
Process window:
„
„
The fundamental limit of optical lithography is not determined by
the optical system alone but rather is an overall contributions from
the optics, resist, develop and etching processes.
Capability of printing small features does not always guarantee a
good quality and a repeatable and controllable patterning.
Alignment:
„
Alignment to the underlying layer is equally as important as the
optics.
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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How was our prediction in the past?
1.0 µm
0.7 µm
0.5 µm
0.35 µm
0.25 µm
0.18 µm
0.13 µm
0.10 µm ?
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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ITRS prediction in 1998
ITRS 1998:
193 DUV litho cannot
produce 65 nm process.
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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ITRS 1999
157 nm appears on the map.
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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ITRS 2005 report
Note: 157 nm
off the chart
now.
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Major challenges (at this moment…)
Data from ENIAC.
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Evolution of optical lithography
Contact and proximity printing
1:1 projection printing
Defects, gap control
Overlay, focus,
mask cost
Step-and-repeat projection
printing
Reduction possible
Step-and-scan projection
printing
Easier focus;
better usage of lens
area
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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A step-and-scan system (stepper or scanner)
Mask
Wafer
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Step-and-repeat vs step-and-scan
Step-and-repeat
Step-and-scan
scan
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Evolution of optics
From Introduction to Microlithography
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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An example of the optics (NA=0.6, 4X
reduction)
US Patent 5969803
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Challenges in lens design
„
Larger lens (required by better resolution) Æ aberration
„
„
„
„
„
Suitably rotating the lens in the step-and-scan system can
minimize the aberration
Finite linewidth of laser source Æ dispersion
Aspheric lens Æ more expensive
Tighter spec on surface quality of lens
Shortening the wavelength Æ more expensive raw
materials
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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Resolution vs minimum linewidth
„
Resolution often refers to the smallest pitch of a dense
line/space pattern. It is limited by the diffraction limit.
„
„
Important for DRAM/flash.
Minimum linewidth is the minimum line or space that we
can resolve. It has no fundamental limit.
„
Important for logic chips (e.g. the gate length of a transistor)
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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There’s no fundamental limit to optical lithography!
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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Fundamentals of lithographic optics
„
„
„
„
„
Diffraction
Partial coherence
Depth of focus
Reflection and interference
Polarization dependence
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Fraunhofer diffraction (scalar; far-field)
η
y
ξ
x
z
Mask plane
U ( x, y ) =
ikz
e e
Image plane
i
k 2 2
(x +y )
2z
iλ z
F [U (ξ ,η )] f x = x / λ z
fy =x/λz
EM field
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
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Diffraction from an aperture
F [U (ξ )] f
a
x =x /λz
Intensity
⎛ ax ⎞
= a sin c ⎜ ⎟
⎝ λz ⎠
⎛ ax ⎞
∝ a 2 sin c 2 ⎜ ⎟
⎝ λz ⎠
1
0.9
0.8
0.7
Before the lens
0.6
0.5
0.4
0.3
0.2
0.1
0
-4
-3
-2
-1
0
1
2
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4
λz/a
22
Diffraction of a line/space (N spaces) pattern
s
N π px ⎞
⎛
sin
⎜
⎟
λ
I ( x) ∝ ⎜
π px ⎟
⎜ sin
⎟
λ ⎠
⎝
p
2
π sx ⎞
⎛
sin
⎜
λ ⎟
⎜ π sx ⎟
⎜
⎟
⎝ λ ⎠
2
1
1
1
0.9
0.9
0.9
0.8
0.8
0.8
0.7
0.7
0.7
0.6
0.6
0.6
0.5
0.5
0.5
0.4
0.4
0.4
0.3
0.3
0.3
0.2
0.2
0.2
0.1
0.1
0.1
0
-5
0
N=5
5
0
-5
0
N=10
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0
-5
0
5
N=100
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Basic lithographic optics configuration
illumination
mask
projection lens
EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku
photoresist
(image plane)
24
Image formation
„
Need to have at least the 0-th and the 1st diffraction
orders being collected to recover the pitch information.
+1
0
0
-1
-1
Oblique incidence can improve
the minimum pitch but result
in a less image contrast.
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