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8/19/2019 2006Optical Lithography
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1
Martina Ryssel
Indo-German Winter Academy 2006
Optical LithographyMartina Ryssel
8/19/2019 2006Optical Lithography
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Martina Ryssel
Optical Lithography - Outline
1. Introduction
2. Lithography Process
3. Photoresist
4. Mask Technology
5. Beyond the Basics
1
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Optical Lithography - Introduction
1 Lithography in the Integrated Circuit Production Process
new layer
lithography
etching implantation/diffusion
test
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Optical Lithography - Introduction
1 Lithography Classification by Wavelenght
Optical lithography = 100 - 450 nm
X-ray lithography = 0.5 – 2 nm
Electron beam lithography = 0.02 – 0.01 nm
Ion beam lithography
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Optical Lithography - Introduction
Optical Lithography Light Sources (I)1
Mercury arc lamp
400 500 600 700
Wavelength (nm)
I
n t e n s i t y
Spectrum of
an Hg-Lamp
I-Line
H-Line
G-Line
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Optical Lithography - Introduction
Optical Lithography Light Sources (II)1
Excimer laser
Typical gases: KrF 248 nmArF 193 nmF2 157 nm
transparency limit of optical materials
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Optical Lithography - Outline
1. Introduction
2. Lithography Process
3. Photoresist
4. Mask Technology
5. Beyond the Basics
2
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Optical Lithography - Process
2
Lithography Process
pattern transfer
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Optical Lithography - Process
Substrate Preparation
Cleaning: removes contaminations
Dehydration bake: evaporates any moisture present at the
surface of the wafer
Application of primer: promotes adhesion of the photoresist byproviding a hydrophobic surface
Most commonly used: Hexamethyldisilizane (HMDS)
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Photoresist Application
2
Optical Lithography - Process
Spin-on method : used to get resist onto the substratewith the required uniform thickness
The resulting thickness depends on:
Spinning speed and time
Resist viscosity and quantity
Nature and concentration of solvent
Humidity
Substrate material and topography ...
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Optical Lithography - Process
Prebake (soft bake)
Prebake is used to drive the solvent from the resist
film thickness is reduced
post-exposure bake and development properties are changed
adhesion is improved
the film becomes less sticky and thus less susceptible toparticulate contamination
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Exposure – Contact and Proximity
2
Optical Lithography - Process
N-Silicon
PR
UV Light~10 μm Mask
N-Silicon
PR
UV Light MaskContact Lithography
Proximity Lithography
better resolutioncheaper
defects
surface bow and warp can bend mask
diffraction effects
less problems with defects
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Optical Lithography - Process
Exposure – Projection
mirror
lightsource
filter
condenser lense
reticle
reduction lense
wafer
movable waferstage
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Optical Lithography - Process
Exposure - Numerical Aperture
NA = n · sinα = D/2f
αSi-wafer
mask
D = lens diameterf = focal length
Rayleigh criterion:(minimum resolvable feature size)
NAk x λ
1min =
k1 depends on several parameters(structures, mask, resist, illumination)
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Post-Exposure Bake
2
Optical Lithography - Process
standard photoresist:
PEB to reduce the standingwave effect
photoresist
λ /nP
R
substrate
overexposure
underexposure
photoresist
substrate
Also: important process stepfor chemically amplified
resists (see there)
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Development
2
Optical Lithography - Process
Puddle Spray
developerpuddle
wafer
Wafer is spin-rinsed after development
Developer (for example
tetramethyl ammonium
hydroxide, TMAH)
dissolves the softened
part of photoresist
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Postbake
2
Optical Lithography - Process
• Evaporation of all solvents in PR
• Improvement of etch and implantation resistance
• Improvement of PR adhesion with surface
• Polymerize and stabilize photoresist
PR
Substrate Substrate
normal baking over baking
PR
But : Overbaking can cause degradation of image by flowing resist
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Pattern Transfer
Optical Lithography - Process
subtractive transfer (etching)
additive transfer (selective deposition lift off)
and impurity doping (ion implantation)
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Lift-off
2
Optical Lithography - Process
photoresist
substrate andadditional layer
photomask
deposited layer
Steep resist flanks are required for this method!
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Optical Lithography - Process
Strip
2
After pattern transfer, the remaining resist has to be
removed.
This can be done by
- dissolving in acid (f.e. H2O2 and H2SO4)
- plasma stripping
21i l i h h li
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Optical Lithography - Outline
1. Introduction
2. Lithography Process
3. Photoresist
4. Mask Technology
5. Beyond the Basics
3
22O ti l Lith h Ph t i t
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Positive and NegativePhotoresist
3
Optical Lithography - Photoresist
Etching
Positive resist becomessoluble with exposure tolight.
Negative resist becomes
insoluble.
23O ti l Lith h Ph t i t
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Optical Lithography - Photoresist
Chemistry of Standard Resists
- long molecule chains are dissolved by exposure to light
(positive resist)-Example: DNQ (Diazonaphthoquinone) + Novolack
- long molecule chains are produced by exposure to light(negative resist)
24Optical Lithography Photoresist
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Chemically Amplified Resist
Optical Lithography - Photoresist
- Resist molecules are photoacid generators (PAG)
- PAGs produce acid when exposed to light
- Acid acts like a catalyst in dissolving the molecule chains
Incident light Post exposure bake
25Optical Lithography Photoresist
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Photoresist Characteristics
3
Optical Lithography - Photoresist
T o d a y :
Line edge roughness: molecular structure of resist becomes important
positive resistnegative resist
log(dose) log(dose)
Sensitivity : Exposure dose where resist is totally dissolved (positive)
Contrast :
Film Thickness after Develop
100
0log
1
D
D
=γ
(inverse of curve slope)
26Optical Lithography - Photoresist
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Optical Lithography - Photoresist
Photoresist Characteristics (secondary)
-Low particle content
-Low ion content
-Good substrate adhesion
-Good thermal stability
-Easy removability
27Optical Lithography - Outline
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Optical Lithography - Outline
1. Introduction
2. Lithography Process
3. Photoresist
4. Mask Technology
5. Beyond the Basics4
28Optical Lithography – Mask Technology
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Example
Optical Lithography Mask Technology
29Optical Lithography – Mask Technology
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Production
Optical Lithography Mask Technology
Quartz substrateCr/CrO2 layer (sputtering)
PR coating
Pattern writing byE-beam lithography
Development and etching PR removal
Source: PKL
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Optical Lithography Mask Technology
Pellicles
Protection for photomasks (projection lithography)
193nm: Organic pellicles
Below 193 nm:Transparency and durability problems
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Phase Shift Masks
Opt ca t og ap y as ec o ogy
Alternating PSM Attenuated PSM
Phase shift
without PS
with PS
Amplitude(wafer)
Intensity(wafer)
Amplitude(mask)
Source: ASML
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p g p y gy
Optical Proximity Correction (OPC)
Mask modifications to reduce image imprecisions
Serifs/Hammerheads improved precision at corners
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p g p y gy
Optical Proximity Correction (II)
Dummy structures below resolution limit improved result due to more homogeneous
distribution of etchant
34Optical Lithography - Outline
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1. Introduction
2. Lithography Process
3. Photoresist
4. Mask Technology
5. Beyond the Basics
5
35Optical Lithography – Beyond the Basics
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Immersion Lithography
α
λ λ
Δ sinnk
NAkx
⋅⋅=⋅= 11
Advantages:
- Higher numericalaperture can be achieved
- smaller patterns without
smaller wavelengths But: - expensive technique- defects in the liquid- complex process
36Optical Lithography – Beyond the Basics
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EUV Lithographymask station
wafer station
optics
EUV radiation
collector optic Optics and masksneed to be reflective
Needs vacuum
high absorption ofEUV radiation
Source:Plasma (laser or gas discharge stimulated)
37Optical Lithography – Beyond the Basics
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X-ray Lithography
Source: Synchrotron X-rays
Problems: expensive, complex masks required (f.e. gold on SiC,produced by electron beam writing)
38Optical Lithography – Beyond the Basics
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Electron Beam Lithography
5
Scaleresolution of ~20nm
MasksNo physical mask-plates are needed
Cost and Maintenance
TimeThe electron beam must be scanned across patterned areas pixel bypixel
FabricationConventional fabrication techniques such as metal lift-off andetching can become difficult at sub-micron length scales
39Optical Lithography – Beyond the Basics
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Ion Beam Lithography ion source
focussing
ion beam
ion separator
unwanted ions
aperture
objective lenses
aperture
control electrodes
substrate
table
similar to electron beam lithography,but using a focused ion beam
Scales below 100nm
No masks needed
Ion implantation
Ion/Atom scattering
40Optical Lithography - Sources
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Görtler, Strowitzki: Kompakte, hochrepetierende Excimerlaser für industrielle Anwendungen, TuiLaser AG
MICROLITHOGRAPHY From Computer Aided Design (CAD) to Patterned Substrate, Cornell Nanofabrication Facility,Microlithography Manual
pi.physik.uni-bonn.de/hertz/bilingual/Vorlesungen/Atomphysik_WS0607/AtomphysikKB2.pdf
http://www.lpm.u-nancy.fr/webperso/nanomag/download/Cours%20Micro-Nano/Techno%20CMOS_Chihiwu/ch06%20rev1.ppt
Thomas Zell: Lithography, 3. Dresdner Sommerschule Mikroelektronik
Prof. Dr. Herberger (FHM), Praktikum Mikroelektronik, WS 06/07
Uwe Stamm, Heinrich Schwoerer und Rainer Lebert, Strahlungsquellen für die EUV-Lithographie
H. Kück, Übersicht über gängige Belichtungsverfahren der Mikrotechnik, izfm, Uni Stuttgart
Vorlesung Nanobiotechnologie, Zentrum für NanoBiotechnologie, Universität für Bodenkultur Wien
Prof. Heiner Ryssel, Vorlesung Technologie Integrierter Schaltungen, Lehrstuhl Elektronische Bauelemente, FAU Erlangen
Andreas Erdmann, Lithography Course, Part 1, Fraunhofer Institut IISB, Erlangen
Prof. Dr. Roland Zengerle, Vorlesung Mikrosystemtechnik 1
Kurt Ronse, Optical Lithography, The Microelectronics Training Center, IMEC
Prof. Heiner Ryssel und Andreas Erdmann, Optik für die Mikro- und NanoelektronikReiser, Huang, The molecular mechanism of novolak–diazonaphthoquinone resists, European Polymer Journal
http://www.lithoguru.com/scientist/lithobasics.html
http://sst.pennnet.com/Articles/Article_Display.cfm?Section=ARCHI&Subsection=Display&ARTICLE_ID=205024&p=28
Andrew Wagner: Resist Sensitivity and Contrast Experiment, Rochester Institute of Technology
41Indo-German Winter Academy 2006
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Optical Lithography
Thank you for your kind attention!
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