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ENSC-E131 NanoFabrication and NanoAnalysis
Lithography
Jiangdong (JD) Deng, Ph.D
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9/26/2011 Jiangdong Deng 2
Photolithography (Talk 1) -
Fundamentals, Processes
• Overview of Photolithography
– Micro/Nano fabrication processes
– Basic steps of photolithography
• Fundamentals of photolithography – Photomask
– UV exposure (light source, exposure optics, equipment) – Photoresist and related processes
• Resolution Limits and Profile Control
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Photolithography (Talk 2)
Advanced Lithography for Nanofabrication – E-beam lithography
– Imprint Lithography
– X-ray Lithography
– Focus Ion-beam (FIB) Lithography
– SPM lithography
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5
Trend of Nanofabrication
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Metrology
6
Thin Films
(Photo)-lithography
Cleaning
Front-End
Processes
EtchIon
Implantation
Planarization
Test & Back End
DesignWafer
Preparation
Design
Wafer Preparation
- Material Growth, cutting, polishing Front-end Processes
- LPCVD, MOCVD, MBE, ALD, wafer-bonding
Lithography (photo-, E-Beam, FIB…)
Etch- RIE, wet etching,
Cleaning- Wet, plasma, O-zone
Thin Films
- PVD, PECVD, ALD, LPCVD Ion Implantation
Planarization- Spin coating, CVD,
Test and Back-end- Dicing/cleaver, Wire bonding, assembly
Metrology
Micro/Nano Fabrication Processes
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• Photo-litho-graphy: light-stone-
writing
• Photolithography is a technique that is
used to define the shape of nano/micro-
machined structures on a wafer
– Purpose: Transfer same circuit patterns
onto a large number of wafers.
• Key role in Nanofabrication
– Critical dimension generations
• Major Concerns in Photolithography
– Resolution
– Profile control
– Overlay accuracy
– Process latitude
Photolithography
•• 3 key parts/components in3 key parts/components inphotolithographyphotolithography
•• MaskMask
•• PhotoresistPhotoresist
•• UV exposure systemUV exposure system
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Photolithography used for Pattern Formation
Beginning of Integrated Circuits in 1959
Kilby (TI) and Noyce (Fairchild Semiconductors)
Substrate
Film deposition Photoresist application
Deposited Film
Photoresist
Exposure
Development Etching Resist removal
Mask
Etch mask
Light
• Basic lithography process
which is central to today’s
chip fabrication.
• Sensitive to light
• Durable in etching
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8 Steps in Photolithography
8) Developinspect
5) Post-exposurebake (option)
6) Develop 7) Hard bake(option)
UV Light
Mask
4) Alignment
and Exposure
Resist
2) Spin coat 3) Soft bake1) Surface preparation
HMDS
•Photoresist and Process
•PhotoMask
•UV Exposure
Ph li h h Ph i l
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• Resolution
• Contrast (profile control)
• Sensitivity
• Viscosity
• Adhesion• Etch resistance
• Surface tension
• Surface roughness• Storage and handling
• Contaminants and particles
•Photoresist and Process
•PhotoMask
•UV Exposure
10
Photolithography Physical
Characteristics
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8) Developinspect
5) Post-exposurebake (option)
6) Develop 7) Hard bake(option)
UV Light
Mask
4) Alignment
and Exposure
Resist
2) Spin coat 3) Soft bake1) Surface preparation
HMDS
Photolithography Processes
Ph li h h (T lk 1)
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Photolithography (Talk 1) -
Fundamentals, Basic Process
• Overview of Photolithography
– Micro/Nano fabrication processes – Basic steps of photolithography
• Fundamentals of photolithography
- Photomask – UV exposure (light source, exposure optics, equipment)
– Photoresist and related processes
• Photolithography limits
• Pattern profile control in Photolithography
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Photomasks
• Master patterns that will be transferred to wafers
• Transparent substrate coated with patterned, UV-opaque material
• Types: – Laser-plotting on plastic film (cheapest)
– Cr on soda lime glass
– Cr on quartz glass (most expensive, needed for deep UV litho)
• Polarity: – “light-field” = mostly clear, drawn feature = opaque
– “dark-field” = mostly opaque, drawn feature = clear
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Photomasks
Mask MaterialsWriting
Method
Smallest
Feature
Pattern
QualityPrice Vendors Others
Hard' MaskCr on glass
plate (Soda-
lime or Quartz )
Laser Writing, or
E-beam
Lithography
(EBL)
1um (EBL
can reach
sub-um)
Smooth
edge
expensive
($80/hour
in CNS)
CNS,
Advance
Reproductions
…
good for
alignment
Transparent
FilmInk on plastic
film
High-precision
laser printer (5k -
20k dpi)
>10umRough
edge
cheap
(<$35)
Outputcity,Advance
Reproductions
…
not good for
alignment
Two Types of Photomasks in CNS
I t f M k O l
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Importance of Mask Overlay
Accuracy
PMOSFET NMOSFET
Cross section of CMOS inverter
Top view of CMOS inverter
The masking layers
determine the accuracy by
which subsequent
processes can be
performed.
The photoresist mask
pattern prepares individual
layers for proper
placement, orientation, and
size of structures to be
etched or implanted.
Small sizes and low
tolerances do not providemuch room for error. Figure 13.4
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Alignment Marks
2nd Mask
1st Mask
2nd mask layer
1st mask layer
RA, Reticle alignment marks, L/R
GA, Wafer global alignment marks,
L/R
FA, Wafer fine alignment marks, L/R
+ +
++
RAL
RAR
+ GA
+ FAL
+ FAR
+ GAR
+ GAL
Notch, coarsealignment
FAL
FAR
FAL/R +
+FAL/R
+
For 2ndmask
+From
1stmask
{
Ph t lith h (T lk 1)
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Photolithography (Talk 1) -
Fundamentals, Basic Process
• Overview of Photolithography
– Micro/Nano fabrication processes
– Basic steps of photolithography
• Fundamentals of photolithography
- Photomask – UV exposure (light source, exposure optics, equipment) – Photoresist and related processes
• Resolution limits
• Pattern profile control in Photolithography
Wavelengths of Exposure
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UV
Wavelength
(nm)
Wavelength
NameUV Emission Source Energy (eV)
436 g-line Mercury arc lamp 2.84
405 h-line Mercury arc lamp 3.1
365 i-line Mercury arc lamp 3.4
248Deep UV
(DUV)
Mercury arc lamp or
Krypton Fluoride (KrF)
excimer laser4.96
193Deep UV
(DUV)
Argon Fluoride (ArF) excimer
laser
6.42
157Vacuum UV
(VUV)Fluorine (F2) excimer laser
7.9
13.4EUV (Extreme
UV, soft-X-ray)
Plasma from Xe gas excited by
high power laser
92.6
0.5 X-Ray X-ray 2480
0.062 Electron 20 keV
0.012 Ion 100 keV
Decreasing feature sizes requires shorter λ.
Wavelengths of Exposure
Sources
Light source:
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Spectrum image from http://zeiss-campus.magnet.fsu.edu/
I H G
Light source:
Mercury Arc Lamp
Th B i E M h d
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2b
Three Basic Exposure Methods
1:1 Exposure 1:1 Exposure ~5:1 Exposure
K 1,2
~0.3-0.9
Resolution (b):
d, the thickness of photoresist, s, the gap of mask-resit, k~3
Light Profile Comparisons of
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Light Profile Comparisons of
three exposure modes
b
Contact/Proximity exposure: Fresnel diffraction (near-field)
Projection exposure: Frounhoffer diffraction (far-field)
Comparisons of three exposure
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Comparisons of three exposure
modes
Exposure System Advantage Disadvantage Application
High resolution (Near-filed)Mask contamination anddamage
R&D
Low cost (<200K) Sensitive to wafer bowing
High throughput Defects impact
Proximity Low mask/samplecontamination
Poor resolution R&D
Hight resolution
Low mask contamination Expensive (>$5M)
High throughput (50 wafer/h)
Contact
ProjectionDominates inproduction
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Exposure Equipments
• Contact/Proximity Aligner
• Direct write lithography (Heidelburg, DWL66)
• Scanning Projection Aligner (scanner)
• Step-and-Repeat Aligner (stepper, production tool)• Step-and Scan System
Contact/Proximity Aligner Systems
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Contact/Proximity Aligner Systems
in CNS
Suss-MJB4 Suss-MA6
Suss-MJB3
AB-M
Illuminator
Alignmentscope (split
vision)
Mask
Wafer
Vacuumchuck
Mask stage
(X, Y , Z ,θ
Wafer stage
(X, Y, Z,θ
Mercuryarc lamp
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Heidelburg, DWL66Heidelburg, DWL66
Wafer stepper / wafer scanner Wafer stepper / wafer scanner
Exposure Procedure
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p
(Contact Mask Aligner)
Mask loading
Wafer loading
WEC contacting
Separation
Alignment
Exposure Contacting
Alignment/contact
Checking
Exposure
NO
Yes
Unload wafer/mask
• WEC- Wedge Error Compensation,
-to make the sample surfaceconformably contact the mask without
‘wedge’.
• Exposure Contact Control
•Proximity•Soft-contact
•Hard Contact
•Vacuum Contact
• Alignment/Contact Check•Shadow effect
Two Keys for a goodexposure:
1, Right exposure dosage
2. Good contact!!!
SU-8 Photolithography
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g p y
Process Conditions
Jiangdong Deng, “Soft-Lithography-summary-2-1-06-user-meeting-02”, SLF at Harvard CNS
Photolithography (Talk 1) -
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9/26/2011 Jiangdong Deng 28
Photolithography (Talk 1)
Fundamentals, Basic Process
• Overview of Photolithography
– Micro/Nano fabrication processes – Basic steps of photolithography
• Fundamentals of photolithography
– Photomask – UV exposure (light source, exposure optics, equipment)
– **Photoresist and related processes
• Lithography limits and Profile Control
Photoresist Basic
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-Positive tone:
•Exposure increases solubi lity
•Low molecular-weight (<10,000) polymer•Pre-cross linked before exposure. exposure
weakens polymer by rupture or scission of polymer
chains
•Mask image is same as wafer image
•Typical Resist, Shipley 1800, AZ series
-Negative tone
•Exposure decreases solubil ity
•High molecular-weight (~65,000) polymer
•Exposure causes cross-linking of polymer chains
•Wafer image is opposite of mask image
•Typical Resist, SU-8,
(Positive and Negative resist)
Components of Conventional
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Additives:•chemicals that control specific
aspects of resist material
Solvent:•gives resist its flowcharacteristics
•PGMEA (Propylene Glycol MethylEther Acetate-C6H12O3))
Sensitizers: (inhibitor, PAC)•photosensitive component of
the resist material
Resin:
•mix of polymers used as binder;
•gives resist mechanical andchemical properties
p
Photoresist
Positive Photoresist
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-(DNQ-novolak resist)
Resin in photoresist (positive)
W.S. Deforest, ‘Photoressit’, McGraw Hill (1975)
Photo Chemical Reaction in P-PR
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(DNQ-novolak resist)
Photo Reaction in P-PR
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(DNQ-novolak resist)
Photo Reaction in Positive resist
DiazoNaphtoQuinone
(DNQ)- sulfonate• The position of SO3-R
determine the
photoreaction wavelength
(I,g,h line)
• Lower solubility
Indene Carboxylic Acid
• Higher solubility (1000times higher) in alkaline
than DNQsulfonate
Release N2,
absorpt H2O,
Photo Recation in Negative PR
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(KTFR resist)
Sensitizer,-bis arylazide…
Sensitizer reacts with light.
-Exposure
-releasing N2 and
-very active
Polymer(resin)-sensitizer-
cross link
-Post exposure baking (PEB)
- Higher insolubility
KTFR – Kodak Thin Film Resist,
-invented in 1957, dominated in the early years of semiconductor
Cyclized poly-isoprene
Photoresist Processes
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8) Developinspect5) Post-exposurebake (option) 6) Develop 7) Hard bake(option)
UV Light
Mask
4) Alignmentand Exposure
Resist
2) Spin coat 3) Soft bake1) Surface preparation
HMDS
Photoresist Processes
Surface Preparation
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Surface Preparation
• Cleaning:
- Remove any contaminants on the wafers prior to photoresist coating
• Dusts/particles, oil, residue, grease, wax…
• Cleaning approaches: solvent cleaning, TCA (1,1,1-trichloroethane), TCE
(trichloroethylene), Piranha cleaning (H2SO4 & H2O2), RCA (I, II), acid-
based Nanostriper, O2 plasma…
• Dehydration:
– remove water prior to priming and coating
• 200 C hotplate >5 min, or 15 min 80C oven
• Priming:
– form a polar (electrostatic) surface to isolate moisture adhesion on
wafer surface and
– Increase the adhesion between wafers and photoresist coating layer
Surface Preparation
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• For Si, SiO2, and other dielectric material: – primers form bonds with surface and produce a polar
(electrostatic) surface
– most are based upon siloxane linkages (Si-O-Si)• 1,1,1,3,3,3-hexamethyldisilazane (HMDS), (CH3)3SiNHSi(CH3)3
• trichlorophenylsilane (TCPS), C6H5SiCl3
• bistrimethylsilylacetamide (BSA), (CH3)3SiNCH3COSi(CH3)3
• For GaAs and other III-V materials: – GaAs already has a polar surface
• monazoline C
• trichlorobenzene
• xylene
• Priming approach:
1) Vapor coating (Industry)
2) Spin-coating (R&D, 2000rpm for 20s)
-Surface Priming (1)
Surface Preparation
S f (2)
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HMDS adhesion promotion on SiO2
-Surface Priming (2)
Surface tension (dyne/cm) H2O on surface (molecule/um^2)
Pre-HMDS 78 >35
Post-HMDS 21 <1
Photoresist Processes
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8) Developinspect5) Post-exposurebake (option) 6) Develop 7) Hard bake(option)
UV Light
Mask
4) Alignmentand Exposure
Resist
2) Spin coat 3) Soft bake1) Surface preparation
HMDS
Photoresist Processes
PR Spin Coat
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Process Summary
40
:• Wafer is held onto vacuum chuck
• Typical coating steps: Dispensing (low speed ~500rpm for
4-10s) Level out (high speed-1000-to
5000rpm, 30-60s)
• Quality measures:
– thickness – uniformity – particles and defects
Vacuum chuck
Spindle
connected to
spin motor
To vacuumpump
Photoresistdispenser
Time
Speed
dispensing
Spin
down
Level out
p
SU-8 2000 Photoresist Spin Curve
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SU 8 2000 Photoresist Spin Curve
0
50
100
150
200
250
300
350
500 1000 1500 2000 2500 3000 3500
T h i c k n e s s ( u m )
Coating Speed (RPM)
Su-8 Spin Curve (MicroChem and CNS)
2100-CNS
2100-MicroChem
2050-CNS2050-MicroChem
2025-CNS
2025-MicroChem
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
500 1000 1500 2000 2500 3000 3500
T h i c k n e s s ( u m )
Coating Speed (RPM)
SU-8 Spin Curve (MicroChem and CNS-LISE)
2015-CNS
2015-MicroChem2010-CNS
2010-MicroChem
2005-CNS
2005-MicroChem
-Resist coating performance depends on:
a) Solvent Concentration, b) Speed control, c) Coating environment
(air flow, T, Huminity), d) surface conditions (adhesion, energy…)
Photoresist Processes
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8) Developinspect5) Post-exposurebake (option) 6) Develop 7) Hard bake(option)
UV Light
Mask
4) Alignmentand Exposure
Resist
2) Spin coat 3) Soft bake1) Surface preparation
HMDS
Photoresist Processes
Soft baking
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• Drives Off Most of Solvent in Photoresist
– For positive resist, pre-crosslink happens between resin and sensitizer
• Impacts to post fabrication processes
– Photoresist-to-Wafer Adhesion
– Thermal stress within PR-film
– Resist Uniformity on Wafer
– Etching rate and Linewidth Control During Development
• Typical Bake Temperatures are 90 to 115°C
– Shipley 1800, 115°C; SU-8, 65/95 °C
– Baking time (>2min) varies with the thickness
– On a Hot Plate (better uniformity than oven baking)
Photoresist Processes
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8) Developinspect5) Post-exposurebake (option) 6) Develop 7) Hard bake(option)
UV Light
Mask
4) Alignmentand Exposure
Resist
2) Spin coat 3) Soft bake1) Surface preparation
HMDS
Post-Exposure Bake (PEB)
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p ( )
• Required for negative PR (like SU-8), to ensure
cross-link in the resist• Typical Temperatures 90 to 110°C on a hot
plate
– SU-8, 65/95 °C
Photoresist Processes
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8) Developinspect5) Post-exposurebake (option) 6) Develop 7) Hard bake(option)
UV Light
Mask
4) Alignmentand Exposure
Resist
2) Spin coat 3) Soft bake1) Surface preparation
HMDS
Photoresist Development
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Process Summary:
• Soluble areas of photoresistare dissolved by developerchemical
• Developer:
• For Positive Resist(Shipley 1800) KOH (CD-351),
NaOH (CD-30), or
(CH3)4NOH (TMAH, CD-26,
MF319) Time: 30-90s
• For negative resist (SU-8) PGMEA (SU-8 developer,
or BTS-220) Time: 1-20min
Vacuum chuck
Spindle
connected to
spin motor
To vacuumpump
Developdispenser
•Spinning option ( bath option in CNS)
p
Photoresist Processes
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8) Developinspect5) Post-exposurebake (option) 6) Develop 7) Hard bake(option)
UV Light
Mask
4) Alignmentand Exposure
Resist
2) Spin coat 3) Soft bake1) Surface preparation
HMDS
Hard Bake
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• An Optional Post-Development Thermal Bake to
– Evaporate Remaining Solvent
– Stabilize and harden the developed photoresist
• Impacts to the post processes
– Improve Resist-to-Wafer Adhesion
– Improve the resistance for dry or wet etching – Improve the edge roughness due to plastic flowing
– Not easy to be removed, thus not good for lift-off process
– May cause extra thermal stress• Higher Temperature than Soft Bake, for >5min
– Shipley 1800, 140-160°C
– SU-8, 150°C
Photoresist Processes
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8) Developinspect
5) Post-exposurebake (option)
6) Develop 7) Hard bake(option)
UV Light
Mask
4) Alignmentand Exposure
Resist
2) Spin coat 3) Soft bake1) Surface preparation
HMDS
Inspection
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Resist liftoff
• Inspect to Verify a Quality Pattern
– Identify Quality Problems (Defects)
– Characterize the Performance of the Photolithography Process
– Prevents Passing Defects to Other Areas
• Etch
• Implant
– Rework Mis-processed or Defective Resist-coated Wafers
Photolithography (Talk 1) -
Fundamentals Basic Process
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• Overview of Photolithography
– Micro/Nano fabrication processes – Basic steps of photolithography
• Fundamentals of photolithography
- Photoresist and related processes
- Photomask
– UV exposure (light source, exposure optics, equipment)
• Resolution limits
• Pattern profile control in Photolithography
Fundamentals, Basic Process
Resolution Limit and Profile Control
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Linewidth Space
Thickness
Substrate
Photoresist
Three Dimensional Pattern in Photoresist
Criterias for ‘good photolithography’
- Line/space size -Profile control - Etching resistance
- Adhesion -Sidewall roughness - Surface tension- Uniformity -Defects
Exposure Systems
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2b
1:1 Exposure 1:1 Exposure ~5:1 Exposure
K 1,2
~0.2-0.8Resolution (2b):
d, the thickness of photoresist, s, the gap of mask-resit, k~3
Photolithography-Diffraction
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g p y
• At smaller dimensions,
diffraction effectsdominate
• If the aperture is on the
order of λ, the lightspreads out after passing
through the aperture. (The
smaller the aperture, the
more it spreads out.)
Photolithography-NA
j ti t
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-projection system
NA-Numerical Aperture, represents
the light collected by the condenser
or objective lens
•If we want to image the
aperture on an image
plane (resist), we cancollect the light using a
lens and focus it on the
image plane.•But the finite diameter
of the lens means some
information is lost(higher spatial
frequency components).
Photolithography-Diffraction
projection system
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• Image formed by a small
circular aperture (Airy disk)
as an example
• Image by a point source
forms a circle with diameter1.22λf/d surrounded by
diffraction rings (airy
pattern)
-projection system
Photolithography-Diffraction
projection system
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-projection system
• Rayleigh suggested that areasonable criterion forresolution (b = distance betweenA and B) is that the centralmaximum of one point sourcelies at the first minimum of theAiry pattern of the other point (b
= diameter of circle)• The numerical aperture (NA) of
a lens represents the ability ofthe lens to collect diffracted lightand is given by NA = n sin α inthis expression n is the index ofrefraction of the mediumsurrounding the lens and α is theacceptance angle of the lens ( n= 1 for air)
NA f n
f
d
f b
λ
α
λ λ 61.0
)sin2(
22.122.1===
Reyleigh Resolution:
Practical resolution:
NAk b λ
•= 1 (0.25< k1 <0.8)
Photolithography Resolution
projection system
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-projection system
• Three ways to improveresolution bmin
– Reduce wavelength
– Increase NA
– Reduce k 1
NAk b λ
•= 1min
Electromagnetic Spectrum
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Visible
Radio wavesMicro-wavesInfraredGamma rays UVX-rays
f (Hz) 10 10 10101010101010 10 4681012141622 1820
(m) 420-2-4-6-8-14 -10-1210 10 10101010101010 10
365 436405248193157
g hi DUVDUVVUV (nm)
Common UV wavelengths used in optical lithography.
Decreasing feature sizes requires shorter λ.
Wavelengths of ExposureSources
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UVWavelength
(nm)
WavelengthName
UV Emission Source Energy (eV)
436 g-line Mercury arc lamp 2.84
405 h-line Mercury arc lamp 3.1
365 i-line Mercury arc lamp 3.4
248Deep UV
(DUV)
Mercury arc lamp or
Krypton Fluoride (KrF)
excimer laser4.96
193Deep UV
(DUV)
Argon Fluoride (ArF) excimer
laser
6.42
157Vacuum UV
(VUV)Fluorine (F2) excimer laser
7.9
13.4EUV (Extreme
UV, soft-X-ray)
Plasma from Xe gas excited by
high power laser
92.6
0.5 X-Ray X-ray 2480
0.062 Electron 20 keV
0.012 Ion 100 keV
Decreasing feature sizes requires shorter λ.
Photolithography-λ
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Photolithography λ
Photolithography-NA
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• Increasing NA
– Improvements in lens
design.• In the mid eighties,
NA ~0.4,
• In 2000, for 248nmexposure systems, NA> 0.8.
• Now, for 193nm (ArFsystem), 0.93 is possible in the air
– Immerse into the high
index medium (n>1,water n=1.47@193nm)
Photolithography-Immersion Litho
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52nm in the air!52nm in the air!
Photolithography-DOF
-projection system
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projection system
DOFDOF--Depth of FocusDepth of Focus
The range over which there are clear optical images
+
-
Photoresist
Film
Depth of focusCenter of focus
Lens2)(5.0~ NA DOF
λ
λ ΝΑ R DOF365 nm 0.45 486 nm 901 nm
365 nm 0.60 365 nm 507 nm
193 nm 0.45 257 nm 476 nm
193 nm 0.60 193 nm 268 nm
Why need to meet DOF Requirement?
Photolithography-DOF
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• The defocus tolerance (DOF)
• Much bigger issue in miniaturization
science than in ICs
A small aperture (NA) was used to ensure
the foreground stones were as sharp as the
ones in the distance.
What you need here is a use a telephoto
lens at its widest aperture.
How about contact/proximity system ?
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2b
1:1 Exposure 1:1 Exposure ~5:1 Exposure
K 1,2
~0.2-0.8Resolution (2b):
d, the thickness of photoresist, s, the gap of mask-resit, k~3
Light Profile Comparisons ofthree exposure modes
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b
Contact/Proximity exposure: Fresnel diffraction (near-field)
Projection exposure: Frounhofer diffraction (far-filed)
Light intensity profile
– contact/proximity system
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Resolution Limit and Profile Control – Mask-resist Gap
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Gap between mask and resist
0
1
2
3
4
5
6
7
0 10 20 30 40 50 60 T h e o r e t i c a l M a x . R
e s o l u t i o n ( u m )
Effective Mask-resist Gap (um)
Resoulution for Contact/ Proximity Exposure
I-line, 365nm
H-line, 405nm
G-line, 435nm
Effective mask-resist gap:
gd sb λ λ 5.1)5.0(5.1 =+=
d sg 5.0+=
Resolution for
Contact/proximity
exposure:
d, the thickness of photoresist,
s, the gap of mask-resit,
Resolution Limit and Profile Control- PR Contrast (1)
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Mask
P-PR
- γ , Photoresist Contrast.
- E0- disolusion dose, Ef - Resist sensitivity, 100% solution dose
- Ideal: a) E0 ~Ef step function, γ~ ∞, b) Ef small
- Reality, γ ~ 2-8
Mask
N-PR
IntensityIntensity
Exposure Dose (mW/cm2)= Lamp Intensity (mJ/cm2) x exposure timeExposure Dose (mW/cm2)= Lamp Intensity (mJ/cm2) x exposure time (s)(s)
Resolution Limit and Profile Control- PR Contrast (2)
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Ideal Exposure - Ideal Resist Real Exposure - Ideal Resist
maskmask
Resolution Limit and Profile Control- PR Contrast (3)
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Real Exposure - Real Resist (positive)
P-PR
mask
γ↓ D↑
N-PR
Su-8-2025
Resolution Limit and Profile Control- Light Reflections Effects (1)
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Polysilicon
Substrate
STISTI
UV exposure light
Mask
Exposedphotoresist
Unexposed
photoresist
Notched photoresist
Edgediffraction
Surfacereflection
Resolution Limit and Profile Control- Light Reflections Effects (2)
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Planarization
Trick: Planarrization
Resolution Limit and Profile Control- Standwave Effects
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Standing waves cause non-
uniform exposure along thethickness of the photoresist film.
Incident wave
Reflected wave
PhotoresistFilm
Substrate
Solution Trick: B-ARC (Bottom Anti-Reflection Coating)
polymer type, dielectric multilayer…
Resolution Limit and Profile ControlSummary
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•Challenge Photolitho process:High resolution, with
High aspect ratio (>5:1), andDense patterns
•Keys to ‘good exposure’
1)Right photoresist (type, contrast, thickness)2)Right Dosage
Thickness, substrate, feature size and density
3)Good contact!!! (for contact/proximity system)Uniform PR film, particle/defect control, pre-patterns,
right exposure contact control…
Photolithography Track System
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Automated Wafer Track forPhotolithography
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Wafer Transfer System
Load station Transfer station
Vaporprime
Resistcoat
Develop andRinse
Edge-beadremoval
Softbake
Coolplate
Coolplate
Hardbake
Wafer stepper(Alignment/Exposure
system)
Illuminatoroptics
Excimer laser(193 nm ArF )
Step and Scan Exposure System
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Beam line
Operatorconsole
4:1 Reduction
lensNA = 0.45 to 0.6
Wafer transportsystem
Reticle stage
uto-alignment system
Wafer stage
Reticle library(SMIF pod interface)