Lecture 16 - litho introduction
Transcript of Lecture 16 - litho introduction
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Lecture 16 –Introduction to Optical Lithography
EECS 598-002 Winter 2006Nanophotonics and Nano-scale Fabrication
P.C.Ku
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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.)
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Resolution limits for imaging
Small features correspond to large (kx, ky) components.In traditional optical microscopes, the detector sees the light in the far field region.
2 2 2 2 20
2 2,max/ 2 /
x y z
x y
k k k k
k k n c k n
ω µ ε
ω π λ
= = + +
⇒ + < ⇒ =
-2 -1 1 2
2.5
5
7.5
10
12.5
15
17.5
k
2 /nπ λ2 /nπ λ− k
Resolving power=
= diffraction limit
/ nλ
( )/ 2 / 2effnλ λ≡real-spacek-space
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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).
θ
,max,max
NA sin2sin NA
nk
kk
θπθλ
≡
⇒ = ⇒ =
Resolving power
( )/ 2NA / 2effλ λ≡
where / NAeffλ λ=
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Patterning process
resist
x
I aerial image
+
I
Dissolutionrate
Dissolutionrate
x
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Some clarifications
The minimum feature size: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.
Process window: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 theoptics.
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How was our prediction in the past?
1.0 µm0.7 µm0.5 µm0.35 µm0.25 µm0.18 µm0.13 µm0.10 µm ?
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ITRS prediction in 1998
ITRS 1998:
193 DUV litho cannotproduce 65 nm process.
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ITRS 1999
157 nm appears on the map.
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ITRS 2005 report
Note: 157 nmoff the chartnow.
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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
Step-and-repeat projection printing
Step-and-scan projection printing
Defects, gap control
Overlay, focus,mask cost
Reduction possible
Easier focus;better usage of lensarea
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A step-and-scan system (stepper or scanner)
Wafer
Mask
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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
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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) aberrationSuitably rotating the lens in the step-and-scan system can minimize the aberration
Finite linewidth of laser source dispersionAspheric lens more expensiveTighter spec on surface quality of lensShortening the wavelength more expensive raw materials
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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)
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There’s no fundamental limit to optical lithography!
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Fundamentals of lithographic optics
DiffractionPartial coherenceDepth of focusReflection and interferencePolarization dependence
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Fraunhofer diffraction (scalar; far-field)
[ ]2 2( )
2
//
( , ) ( , ) xy
ki x yikz z
f x zf x z
e eU x y F Ui z
λλ
ξ ηλ
+
==
=
ξ
η
Mask plane
x
y
Image plane
EM field
z
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Diffraction from an aperture
[ ] /( ) sin cxf x z
axF U azλ
ξλ=
⎛ ⎞= ⎜ ⎟⎝ ⎠
a Intensity 2 2sin c axazλ
⎛ ⎞∝ ⎜ ⎟⎝ ⎠
-4 -3 -2 -1 0 1 2 3 40
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
λz/a
Before the lens
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Diffraction of a line/space (N spaces) pattern
ps
2 2
sin sin( )
sin
N px sx
I x px sx
π πλ λ
π πλ λ
⎛ ⎞ ⎛ ⎞⎜ ⎟ ⎜ ⎟
∝ ⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠
-5 0 50
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
-5 0 50
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
-5 0 50
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
N=5 N=10 N=100
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Basic lithographic optics configuration
maskillumination projection lens photoresist(image plane)
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Image formation
Need to have at least the 0-th and the 1st diffraction orders being collected to recover the pitch information.
+1
-1
0
0
-1
Oblique incidence can improvethe minimum pitch but resultin a less image contrast.