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![Page 1: Principles of Optical Lithography, 2002 November 20. Principles of Optical Lithography Layout: Motivation IC manufacturing Traditional method and trends.](https://reader038.fdocuments.us/reader038/viewer/2022102907/56649dde5503460f94ad7e54/html5/thumbnails/1.jpg)
Principles of Optical Lithography, 2002 November 20.
Principles of Optical Lithography
Layout:• Motivation• IC manufacturing• Traditional method and trends• Resolution enhancement techniques
University ofSzeged
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Principles of Optical Lithography, 2002 November 20.
„From Lithography to Optical Lithography”
lithography: Graphic Arts. any of several printing processes,such as offset lithography, in which the image areas of a plateare treated (photographically or directly by hand, as with alithographic crayon) to accept greasy inks and repel water, whilethe nonimage area accept water and repel ink.
/Dictionary of Science and Technology/
“Object” “Image”printing process
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Principles of Optical Lithography, 2002 November 20.
„From Lithography to Optical Lithography”
Optical lithography is the process used to transfer a Optical lithography is the process used to transfer a pattern to a layer of an integrated circuitpattern to a layer of an integrated circuit..
application: cameras, microscopes, manufacturing of integrated circuits (IC)
“Object” “Image”optical imaging process
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Principles of Optical Lithography, 2002 November 20.
Manufacturing of Integrated Circuits
Chip fabrication occurs as a cycle of steps carried out as many as 20 times!
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Principles of Optical Lithography, 2002 November 20.
Silicon crystal (ingot)
1
polished wafer
2
SiO2
Thermal oxidation3
Si - substrate
4
Si substrate
Light
PhotoresistSiO2
Manufacturing of Integrated Circuits
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Principles of Optical Lithography, 2002 November 20.
Photolithography - Equipment
To HouseVacuum
Hollow Shaft
VacuumChuck
Resist Dispenser
Spin Coater:
Used for the application of photoresist, primer, and developer.
Photoresist Thickness Control:•Photoresist Viscosity•Spin Speed•Temperature•Humidity
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Principles of Optical Lithography, 2002 November 20.
Spin Speed vs. Thickness:
Photolithography - Equipment
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Principles of Optical Lithography, 2002 November 20.
Positive vs. Negative Resist
• Positive Resist: Photo Mask
Resist
Silicon Substrate
Oxide
Resist Resist
Oxide
• Light areas on the mask => photoresist is removed during development.• Long-chained molecules in resist are broken down to smaller chains by the
UV light, which are easily dissolved by the developer solution.• Capable of smaller features, better resolution, but has poor adhesion and
costs much more.
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Principles of Optical Lithography, 2002 November 20.
Positive vs. Negative Resist
• Negative Resist:
Resist
Photo Mask
Silicon Substrate
Oxide
Resist
Oxide
• Dark areas on the mask => photoresist is removed during development.• Short-chain molecules in the resist are “cross-linked” by the UV light. Resultant longer chains are resistant to developer solution.• Better adhesion, but incapable of producing submicron features.• Only used (anymore) for certain specialty applications.
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Principles of Optical Lithography, 2002 November 20.
Threshold of Photoresists
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Principles of Optical Lithography, 2002 November 20.
4
Si substrate
Light
PhotoresistSiO2
Manufacturing of Integrated Circuits
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Principles of Optical Lithography, 2002 November 20.
„From Optical Lithography to Projection Optical Lithography”
Contact Printing
Proximity Printing
Projection Printing
•simple design (M=1x, no optics)•easy to operate•limited to ~1 m features (diffraction)•some resolution loss due to gap
•simplest design (M=1x, no optics)•easy to operate but requires mask cleaning•limited to ~1m features
•complex design (M=1x, no optics)•limited by field size, optical aberrations etc.
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Principles of Optical Lithography, 2002 November 20.
Clean Room Environment
“first line of defence”“2. line of defence”
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Principles of Optical Lithography, 2002 November 20.
Transistor miniaturization
The first transistor1948
Salt-size transistors1964
Early integrated circuit1973
DRAM chip1997
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Principles of Optical Lithography, 2002 November 20.
Transistor miniaturization
• Moore’s Law: the number of transistors quadruples every four years
• Rock’s Law: the cost of a fab doubles every four years
• Result 1: fewer advanced fabs and fewer fab operators, fabless/foundry partnerships
• Result 2: Extend the lifetime of every generation, by means of super resolution techniques.
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Principles of Optical Lithography, 2002 November 20.
Rayleigh resolution limit
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Principles of Optical Lithography, 2002 November 20.
Rayleigh resolution limit
Lateral ResolutionCritical Feature SizeCritical Dimension
Axial ResolutionDepth of FieldDepth of Focus
NAkCD
1
22:NA
kDOF
How can we enhance the resolution?
• Reducing the wavelength ()
• Increasing the numerical aperture (NA)
• Manipulating the k1 factor
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Principles of Optical Lithography, 2002 November 20.
Lithography Roadmap (reduction of )
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Principles of Optical Lithography, 2002 November 20.
Lithography Roadmap - Why Excimers
• It is difficult to design and manufacture anachromatic projection lens for excimerlasers in deep UV due to the small differencein chromatic dispersion between fused quartzand fluorite, in addition to other materialrelated problems, such as UV-induced defectformation.
< 0.6 pm
• Beam shaping
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Principles of Optical Lithography, 2002 November 20.
Development of Projection Optics (increasing of NA)
Lithography represents the greatestcapital expense in an advanced fab
Strategies to stretch machinecapabilities and extend the lifetimeof every generation
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Principles of Optical Lithography, 2002 November 20.
Development of Projection Optics (evolution of the stepper)
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Principles of Optical Lithography, 2002 November 20.
Stepper
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Principles of Optical Lithography, 2002 November 20.
Importance of DOF
DOF is limited by
• thickness of the resist• surface smoothness (although the wafer is polished, aftert several fabrication cycles the surface has a complex topology)
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Principles of Optical Lithography, 2002 November 20.
Chemical-Mechanical Polishing (CMP)
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Principles of Optical Lithography, 2002 November 20.
Chemical-Mechanical Polishing (CMP)
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Principles of Optical Lithography, 2002 November 20.
Importance of DOF
The required minimum feature size reduces
from 0.24 micron to 0.04 micron
The required DOF is 0.5 micron in 2010,
/limited by the thickness of the resist/
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Principles of Optical Lithography, 2002 November 20.
Mask writing process(e-beam or laser)
Stepper(NA, M, , , etc.)
Wafer reflectionThin film effects
PhotochemistryDiffusion
Development
Pattern transfer
Images in lithography
Designed Pattern
Latent Image
Device Layer
Mask
Real Image
Resist Image
Aerial Image
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Principles of Optical Lithography, 2002 November 20.
Aerial versus Resist image simulated by Solid-C
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Principles of Optical Lithography, 2002 November 20.
Image formation based on Fourier Optics
)},()},({{),( 1yx ffPyxmyxE FF
m(x,y): electric field transmittance of the mask
P(fx,fy): transfer function of the optical system (different interpretations are possible)
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Principles of Optical Lithography, 2002 November 20.
Image formation Manhattan Geometry
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Principles of Optical Lithography, 2002 November 20.
Image formation
Image quality
Number of diffracted orders(Spatial Fourier components)
Conventional imaging requiresminimum 3 diffraction orders
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Principles of Optical Lithography, 2002 November 20.
Fourier series of a grating
f x( )1
2
2
0
k
i
1k sin 2 i 1( ) x( )
2 i 1
=
k=0
k=1
k=2
k=40
f(x)=1/2
0--2-3 2 30
1
0
1
0
1
0
1/2
mask
threshold of the photoresist
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Principles of Optical Lithography, 2002 November 20.
Resolution Enhancement Techniques
Light Source
Filter
Condenser Lens
Mask
Projection Lens System
Wafer withPhotoresist
Mirror
Pupil-PlaneFilter
* Bandwidth
* Filter (annular, quadrupole, etc.)
* Spatial coherence ()
* Binary or PS mask (Manhattan geometry)* Magnification (M)* Numerical Aperture (NA)
* Defocus * Pupil-plane filter/Aberrations
* Image model
* Wavelength ()
* Image flare
* Resist system* Thickness* Absorp. Param. A, B* Rate Constant C* Refractive Index* Development Model* etc.
Input Parameters
* Exposure
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Principles of Optical Lithography, 2002 November 20.
Resolution Enhancement Techniques - OPC
The light intensity profile is not significantly different for the isolated1.0 micron line as for the 1.0 micronline/1 micron spacing grating.
Significantly different light intensityprofiles depending upon whether thelines are isolated or part of a gratingstructure
Isolated and grouped lines are expectedto print approximately the same with.
Linewidth variation
The size of a line is dependent uponits proximity to other geometries
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Principles of Optical Lithography, 2002 November 20.
Resolution Enhancement Techniques - OPC
Original Layout0.18 m
Conventional (no OPC)
OPC Layout
Silicon Image w/o OPC
Silicon Image with OPC
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Principles of Optical Lithography, 2002 November 20.
Resolution Enhancement Techniques - OPC
Solution:
OPC (Optical proximity correction):is the technique of predistorting mask patterns such that the printed patterns are as close to the desiredshape as possible.
Problems:
•Proximity effects•Nonlinearity•Line shortening•Corner rounding
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Principles of Optical Lithography, 2002 November 20.
Resolution Enhancement Techniques - OPC
OPC processes1. Catastrophic OPC: The goal is to guarantee presence of a pattern without feature-size control worries.Precise dimension control is not critical. An example is to ensure a wire is continuous and that no open circuit will result.
2. One dimensional OPC (Linewidth variation minimization):Vary the size of each feature on the photomask depending on its nominal dimension and environment /For example, dense and sparse lines are made smaller on the mask whereas lines of intermediateperiodicities are made larger/
3. Line shortening method:
4. Corner rounding:
biasing if there is room
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Principles of Optical Lithography, 2002 November 20.
Resolution Enhancement Techniques - OPC
Limited by the pixel size of the mask writing process!!!
Uncorrected Test PatternVertex Count=94
Super Aggressive CorrectionVertex Count=1106
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Principles of Optical Lithography, 2002 November 20.
Resolution Enhancement Techniques/Off-Axis Illumination/
0. +1. +2.-1.-2.
Photo-Mask
Projection Lens
Photo-Resist
Intensity in the Pupil-plane
+1.
-1.
0.
ConventionalIllumination
Off-AxisIllumination
TWO BEAM IMAGING!
NApitchCD
2
1
2
1
pitchNA
maxsin
%1.974
421
20
10
aa
aaC
NACD
4
1
pitchNA
2
%6.902
21
20
10
aa
aaC
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Principles of Optical Lithography, 2002 November 20.
Resolution Enhancement TechniquesPhase Shifting Techniques
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Principles of Optical Lithography, 2002 November 20.
Resolution Enhancement TechniquesPhase Shifting Techniques
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Principles of Optical Lithography, 2002 November 20.
Resolution Enhancement TechniquesPhase Shifting Techniques
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Principles of Optical Lithography, 2002 November 20.
Resolution Enhancement TechniquesPhase Shifting Techniques
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Principles of Optical Lithography, 2002 November 20.
Resolution Enhancement TechniquesPhase Shifting Techniques
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Principles of Optical Lithography, 2002 November 20.
Resolution Enhancement TechniquesPhase Shifting Techniques
Phase-shifting masks are classified as “strong” and “weak”, according to theirability to suppress the zero order diffraction component.
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Principles of Optical Lithography, 2002 November 20.
Resolution Enhancement TechniquesPhase Shifting Techniques
Phase Shifting LayerChrome
Alt
erna
ting
/Lev
enso
n P
SMA
tten
uate
d P
SM
Phase Shifting Layer (T0.04-0.15)
Phase Shifting Layer Dark Field Grating(PS Layer)
Chr
omel
ess
PSM
Phase Shifting LayerChrome
Out
rigg
er P
SMR
im P
SM
Phase Shifting Layer
Chrome
Inte
rfer
omet
ric
PSM
?
Phase Shifting Method without Phase Shifting Layer.
a.
b.
c.
d.
e.
f.
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Principles of Optical Lithography, 2002 November 20.
Resolution Enhancement TechniquesPhase Shifting Techniques
tE0tE1
rE1rE0
BackIllumination
FrontIllumination
2·
Fused Silica
Substrate
Reflective ChromeLayer
x
z
Four-beam imaging
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Principles of Optical Lithography, 2002 November 20.
Resolution Enhancement TechniquesPhase Shifting Techniques
CCD
L3 L1L2
Mask
Attenuator
PiezoTranslator
M2
Optical Axis
M1
Beam Splitter
Ar+-ion laser(=457.9) nm
Image Plane
1°0.7°
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Principles of Optical Lithography, 2002 November 20.
Resolution Enhancement TechniquesPhase Shifting Techniques
1
00 3.5
Inte
nsi
ty
X(m)
1
0
Inte
nsi
ty
0 3.5X(m)1
0
Inte
nsi
ty
1
0In
ten
sity
0 3.5X(m)0 3.5X(m)1
0
Inte
nsi
ty
1
0
Inte
nsi
ty
0 3.5X(m)0 3.5X(m)1
0
Inte
nsi
ty
1
0
Inte
nsi
ty
0 3.5X(m) 0 3.5X(m)
Calculated Measured CCD Image
a
b
c
d
e
f
g
h
R im
age
T im
age
Cor
rect
Ph
ase
Wro
ng
Ph
ase
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Principles of Optical Lithography, 2002 November 20.
What’s next for Optical Lithography
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Principles of Optical Lithography, 2002 November 20.
What’s next for Optical Lithography
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Principles of Optical Lithography, 2002 November 20.
Lithography Roadmap
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Development of Projection Optics (increasing of NA)
Catadioptric and immersion objectives
NANO Fórum 2006 December 13.