14 lithography
TRANSCRIPT
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Lecture 14Lecture 14
Taken in part from Chapters 13-15
Semiconductor Manufacturing Technology
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Objectives
Basic concepts for photolithography, includingprocess overview, critical dimension generations,light spectrum, resolution and process latitude.
Difference between negative and positivelithography.
Eight basic steps to photolithography. Wafer surface preparation for photolithography.
Photoresist physical properties.
Applications of conventional i-line photoresist.
Deep UV resists
Photoresist application techniques
Soft bake processing
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Wafer Fabrication Process Flow
Implant
Diffusion
Test/Sort
Etch
Polish
PhotoCompleted wafer
Unpatterned
wafer
Wafer start
Thin Films
Wafer fabrication (front-end)
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Patterning processPhotomask Reticle
Critical dimension generations
Light spectrum and wavelengths
Resolution
Overlay accuracy
Process latitude
Photolithography Concepts
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Three Basic Exposure Methods
1:1 Exposure 1:1 Exposure ~5:1 Exposure
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Contact printing capable of high resolution
but has unacceptable defect densities. May be
used in Development but not manufacturing.
Proximity printing cannot easily print
features below a few mm in line width. Used in
nano-technolgy.
Projection printing provides high resolution
and low defect densities and dominates today.
Typical projection systems use reduction optics(2X - 5X), step and repeat or step and scan.
They print 50 wafers/hour and cost $5 - 10M.
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Steps in Lithography Process
Lithography has three parts:(1) Light source, (2) Wafer exposure (3) Resist
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Photomask and Reticle for Microlithography
4:1 Reticle1:1 Mask
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Three Dimensional Pattern in Photoresist
LinewidthSpace
Thickness
Substrate
Photoresist
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Section of the Electromagnetic Spectrum
Visible
Radio wavesMicro-wavesInfraredGamma rays UVX-rays
f (Hz) 1010101010101010 10104681012141622 1820
P(m) 420-2-4-6-8-14 -10-12
1010101010101010 1010
365 436405248193157
ghiDUVDUVVUV
P (nm)
Common UV wavelengths used in optical lithography.
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Light Sources
Decreasing feature sizes requires shorter .
Hg vapor lamps: Hg plasma inside glass lamp
Produces multiple wavelengths
Limited in intensity
g line: = 436 nm (used to mid 1980s) I line: = 365 nm (early 1990s, >0.3 m)
Deep UV by excimer lasers
Kr + NF3 + (energy) KrF + (photon emission)
KrF: = 248 nm (used for 0.25 m)
ArF: = 193 nm (used for 0.12 m)
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Important Wavelengths forPhotolithography Exposure
UV Wavelength
(nm)
Wavelength
NameUV Emission Source
436 g-line Mercury arc lamp
405 h-line Mercury arc lamp
365 i-line Mercury arc lamp
248 Deep UV (DUV)Mercury arc lamp or
Krypton Fluoride (KrF) excimer laser
193 Deep UV (DUV) Argon Fluoride (ArF) excimer laser
157 Vacuum UV (VUV) Fluorine (F2) excimer laser
Table 13.1
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Importance of Mask Overlay Accuracy
PMOSFET NMOSFET
Cross section ofCMOS inverter
Top view ofCMOS inverter
The masking layers determine
the accuracy by which
subsequent processes can be
performed.
The photoresist mask patternprepares individual layers for
proper placement, orientation,
and size of structures to be
etched or implanted.
Small sizes and low tolerances
do not provide much room for
error.
Figure 13.4
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Photolithography Processes
Negative Resist
Wafer image is opposite of mask image
Exposed resist hardens and is insoluble
Developer removes unexposed resist
Positive Resist
Mask image is same as wafer image
Exposed resist softens and is solubleDeveloper removes exposed resist
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Negative Lithography
Ultraviolet light
Island
Areas exposed to light becomecrosslinked and resist thedeveloper chemical.
Resulting pattern afterthe resist is developed.
Window
Exposed area ofphotoresist
Shadow onphotoresist
Chrome island onglass mask
Silicon substrateSilicon substrate
PhotoresistPhotoresistOxideOxide
PhotoresistPhotoresist
OxideOxide
Silicon substrateSilicon substrate
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Positive Lithography
photoresist
silicon substrate
oxide oxide
silicon substrate
photoresist
Ultraviolet light
Island
Areas exposed tolight are dissolved.
Resulting pattern afterthe resist is developed.
Shadow onphotoresist
Exposed areaof photoresist
Chrome islandon glass mask
Window
Silicon substrateSilicon substrate
PhotoresistPhotoresistOxideOxide
PhotoresistPhotoresist
OxideOxide
Silicon substrateSilicon substrate
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Relationship Between Mask and Resist
Desired photoresist structure to be
printed on wafer
Window
Substrate
Island of photoresist
QuartzChrome
Island
Mask pattern required when using
negative photoresist (opposite of
intended structure)
Mask pattern required when using
positive photoresist (same as
intended structure)
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Clear Field and Dark Field Masks
Simulation of contact holes
(positive resist lithography)
Simulation of metal interconnect lines
(positive resist lithography)
Clear Field Mask Dark Field Mask
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Eight Steps of Photolithography
Step Chapter
1.Vapor prime 132.Spin coat 133.Soft bake 134. Alignment and exposure 14
5.Post-exposure bake (PEB) 156.Develop 157.Hard bake 158.Develop inspect 15
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Eight Steps of Photolithography
8) Develop inspect5) Post-exposurebake
6) Develop 7) Hard bake
UV Light
Mask
P
P
4) Alignmentand Exposure
Resist
2) Spin coat 3) Soft bake1) Vapor prime
HMDS
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Photolithography Track System
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Vapor Prime
The First Step of Photolithography:
Promotes Good Photoresist-to-Wafer Adhesion
Primes Wafer with Hexamethyldisilazane,HMDS
Followed by Dehydration Bake
Ensures Wafer Surface is Clean and Dry
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Spin CoatProcess Summary:
Wafer is held onto vacuum chuck Dispense ~5ml of photoresist
Slow spin ~ 500 rpm
Ramp up to ~ 3000 to 5000 rpm
Quality measures:
time speed thickness uniformity particles and defects
Vacuum chuck
Spindle connected
to spin motor
To vacuum pump
Photoresistdispenser
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Soft bake
Characteristics of Soft Bake: Improves Photoresist-to-Wafer Adhesion
Promotes Resist Uniformity on Wafer
Improves Linewidth Control During Etch Drives OffMost of Solvent in Photoresist
Typical Bake Temperatures are 90 to 100rC For About 30 Seconds
On aHot Plate Followed by Cooling Step on Cold Plate
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Alignment and Exposure
Process Summary:
Transfers the mask image to the resist-coated wafer
Activates photo-sensitive components ofphotoresist
Quality measures:
linewidth resolution overlay accuracy particles and defects
UV light source
Mask
Resist
P
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Post-Exposure Bake
Required for Deep UV Resists
Typical Temperatures 100 to 110rC on a hotplate
Immediately after Exposure
Has Become a Virtual Standard for DUV andStandard Resists
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Photoresist Development
Process Summary:
Soluble areas of photoresistare dissolved by developerchemical
Visible patterns appear onwafer- windows
- islands
Quality measures:
- line resolution- uniformity
- particles and defectsVacuum chuck
Spindle connected to
spin motor
To vacuum pump
Developdispenser
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Hard Bake
APost-Development Thermal Bake Evaporate Remaining Solvent
Improve Resist-to-WaferAdhesion
Higher Temperature (120 to 140rC) than SoftBake
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Develop / Inspect
Inspect to Verify a Quality Pattern
Identify Quality Problems (Defects)
Characterize the Performance of thePhotolithography Process
Prevents Passing Defects to OtherAreas
Etch
Implant
ReworkMis-processed or DefectiveResist-coated Wafers
Typically an Automated Operation
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Vapor Prime
Wafer Cleaning
Dehydration Bake
Wafer Priming
Priming Techniques
Puddle Dispense and Spin
Spray Dispense and Spin
Vapor Prime and Dehydration Bake
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Effect of Poor Resist AdhesionDue to Surface Contamination
Resist liftoff
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HMDS Puddle Dispense and Spin
Puddle formationSpin wafer to remove
excess liquid
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HMDS Hot Plate Dehydration Bakeand Vapor Prime
Wafer
Exhaust
Hot plate
Chamber coverProcess Summary: Dehydration bake in enclosed
chamber with exhaust
Hexamethyldisilazane(HMDS)
Clean and dry wafer surface
(hydrophobic)
Temp ~ 200 to 250rC
Time ~ 60 sec.
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Purpose of Photoresist in Wafer Fab
To transfer the mask pattern to thephotoresist on the top layer of the
wafer surface
To protect the underlying material
during subsequent processing e.g. etch
or ion implantation.
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Progressive Improvements in Photoresist
Better image definition (resolution).
Better adhesion to semiconductor wafersurfaces.
Better uniformity characteristics.
Increased process latitude (less sensitivity toprocess variations).
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PhotoresistTypes of Photoresist
Negative Versus PositivePhotoresists
Photoresist Physical Properties
Conventional I-Line Photoresists
Negative I-Line Photoresists
Positive I-Line Photoresists
Deep UV (DUV) Photoresists
Photoresist Dispensing
Methods
Spin Coat
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Types of Photoresists
Two Types of PhotoresistPositive Resist
Negative Resist
CD CapabilityConventional Resist
Deep UV Resist
Process Applications
Non-critical Layers
Critical Layers
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Negative Versus Positive Resists
Negative ResistWafer image is opposite of mask image
Exposed resist hardens and is insoluble
Developer removes unexposed resist
Positive Resist
Mask image is same as wafer image
Exposed resist softens and is soluble
Developer removes exposed resist Resolution Issues
Clear Field Versus Dark FieldMasks
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Photoresist Physical Characteristics
Resolution
Contrast
Sensitivity
Viscosity Adhesion
Etch resistance
Surface tension
Storage and handling
Contaminants and particles
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Resist Contrast
Poor Resist Contrast
Sloped walls
Swelling
Poor contrast
Resist
Film
Good Resist Contrast
Sharp walls
No swelling
Good contrast
Resist
Film
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Surface Tension
Low surface tension High surface tensionfrom low molecular from high molecularforces forces
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Components of Conventional Photoresist
Additives:chemicals that control specificaspects of resist material
Solvent:gives resist its flowcharacteristics
Sensitizers:
photosensitive component ofthe resist material
Resin: mix of polymers used asbinder; gives resist mechanical
and chemical properties
Figure 13.18
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Negative Resist Cross-LinkingAreas exposed to lightbecome crosslinkedandresist the developerchemical.
Unexposed areasremain soluble todeveloper chemical.
Pre-exposure- photoresist
Post-exposure- photoresist
Post-develop- photoresist
UV
OxidePhotoresist
Substrate
Crosslinks
Unexposed Exposed
Soluble
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PAC as Dissolution Inhibitorin Positive I-Line Resist
Resist exposed tolight dissolves in thedeveloper chemical.
Unexposed resist, containingPACs, remain crosslinked andinsoluble to developer chemical.
Pre-exposure+ photoresist
Post-exposure+ photoresist
Post-develop+ photoresist
UV
OxidePhotoresist
Substrate
Solubleresist
Exposed Unexposed
PAC
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Good Contrast Characteristics ofPositive I-line Photoresist
Positive Photoresist:
Sharp walls
No swelling
Good contrast Film
Resist
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DUV Emission Spectrum
* Intensity of mercury lamp is too low at 248 nm to be usable in DUV photolithography applications.
Excimer lasers, such as shown on the left provide more energy for a given DUV wavelength.
100
80
60
40
20
0
248 nm
RelativeIntensity(%
)
KrF laser emission spectrum Emission spectrum of high-intensity mercury lamp
120
100
80
60
40
20
0
200 300 400 500 600
Wavelength (nm)
RelativeIntensity(%)
g-line436 nm
i-line365 nm
h-line405 nm
DUV*248 nm
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Chemically Amplified (CA) DUV Resist
Resist exposed to lightdissolves in thedeveloper chemical.
Unexposed resist remainscrosslinked and PAGs areinactive.
Pre-exposure+ CA photoresist
Post-exposure+ CA photoresist
Post-develop+ CA photoresist
UV
OxidePhotoresist
Substrate
Unchanged
Exposed Unexposed
Acid-catalyzedreaction (during
PEB)
PAG
PAG
PAGPAG
H+
PAG
PAG
PAG
H+
H+ PAG
PAG
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Exposure Steps for Chemically-
Amplified DUV Resist
1. Resin is phenolic copolymer with protecting group thatmakes it insoluble in developer.
2. Photoacid generator (PAG) generates acid during exposure.3. Acid generated in exposed resist areas serves as catalyst to
remove resin-protecting group during post exposure thermalbake.
4. Exposed areas of resist without protecting group are solublein aqueous developer.
Table 13.5
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Steps of Photoresist Spin Coating
3) Spin-off 4) Solvent evaporation
1) Resist dispense 2) Spin-up
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Wafer Transfer System
Load station Transferstation
Vaporprime
Resistcoat
Developand
Rinse
Edge-beadremoval
Softbake
Coolplate
Coolplate
Hardbake
Wafer stepper(Alignment/Exposure system)
Automated Wafer Track for Photolithography
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Photoresist Dispense NozzleZ
Y
X
U
Resist dispenser nozzle
Bottom side EBR
Vacuum
VacuumchuckVacuumchuck
Spin motor
WaferWafer
Exhaust
Drain
Resist flowResist flow
Stainlesssteel bowl
Air flowAir flowAir flowAir flow
Nozzle position can be adjustedin four directions.
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Resist Spin Speed Curve
80000
70000
60000
50000
40000
30000
20000
10000
0
1000 2000 3000 4000 5000 6000 7000
Spin Speed (RPM)
Spin Speed Curve of IX300
ResistThickn
ess()
21c
P
110 cP
70 cP
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Soft Bake on Vacuum Hot Plate
Purpose of Soft Bake:
Partial evaporation ofphotoresist solvents
Improves adhesion
Improves uniformity
Improves etch resistance Improves linewidth control
Optimizes light absorbancecharacteristics of photoresist
Hot plate
Wafer
Solventexhaust
Chamber cover
Figure 13.28
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Solvent Content of Resist VersusTemperature During Soft Bake
DNQ/Novolak
resist
Bake Temperature (C)
ResidualSolven
t(%w
/w)
Figure 13 29