14 Lithography

8/3/2019 14 Lithography

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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



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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


PhotoresistPhotoresistOxideOxide

PhotoresistPhotoresist

OxideOxide



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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

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