半导体光刻工艺介绍

发布于:2021-12-07 18:53:06

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Objectives
? Basic concepts for photolithography, including process overview, critical dimension generations, light spectrum, resolution and process latitude. ? Difference between negative and positive lithography. ? 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
Wafer fabrication (front-end) Wafer start Thin Films Polish

Unpatterned wafer Completed wafer Diffusion Photo Etch

Test/Sort

Implant

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Photolithography Concepts

? Patterning process – Photomask – Reticle ? ? ? ? ? Critical dimension generations Light spectrum and wavelengths Resolution Overlay accuracy Process latitude
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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
1:1 Mask 4:1 Reticle

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Three Dimensional Pattern in Photoresist
Linewidth Space Photoresist

Thickness

Substrate

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Section of the Electromagnetic Spectrum
Visible Microwaves
12

Gamma rays

X-rays

UV

Infrared

Radio waves
10

f (Hz) (m) ?

10 10

22

10 10

20

10 10

18

10 10
-8

16

10 10
-6

14

10 10
-4

10 10
-2

10 10 0

8

10 10 2

6

10 10 4

4

-14

-12

-10

? (nm)

157

193

248

365 i

405 436 h g

VUV DUV DUV

Common UV wavelengths used in optical lithography.

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Light Sources

Decreasing feature sizes requires shorter λ.

? Hg vapor lamps: Hg plasma inside glass lamp
– – – – Produces multiple wavelengths Limited in intensity “g” line: λ = 436 nm (used to mid 1980s) “I” line: λ = 365 nm (early 1990s, >0.3 μm)

? Deep UV by excimer lasers
– Kr + NF3 + (energy) → KrF + (photon emission) ? KrF: λ = 248 nm (used for 0.25 μm) ? ArF: λ = 193 nm (used for 0.12 μm)

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Important Wavelengths for Photolithography Exposure
U V W a v e le n g t h (n m ) 436 405 365 248 193 157 W a v e le n g t h N am e g - lin e h - lin e i- lin e D eep U V (D U V ) D eep U V (D U V ) V acu u m U V (V U V ) U V E m issio n S o u r c e M e r c u r y a r c la m p M e r c u r y a r c la m p M e r c u r y a r c la m p M e r c u r y a r c la m p o r K r y p t o n F lu o r id e ( K r F ) e x c im e r la s e r A r g o n F lu o r id e ( A r F ) e x c im e r la se r F lu o r in e ( F 2 ) e x c im e r la se r

Table 13.1

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Importance of Mask Overlay Accuracy
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 provide much room for error.
Cross section of CMOS inverter
Figure 13.4

PMOSFET

NMOSFET

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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 soluble – Developer removes exposed resist

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Negative Lithography
Areas exposed to light become crosslinked and resist the developer chemical. Island Exposed area of photoresist
Photoresist

Ultraviolet light Chrome island on glass mask

Window

Shadow on photoresist

Photoresist Oxide Silicon substrate

Oxide Silicon substrate

Resulting pattern after the resist is developed.

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Positive Lithography

Ultraviolet light Areas exposed to light are dissolved. Chrome island on glass mask Shadow on photoresist

Island
photoresist Photoresist

Window

Exposed area of photoresist
photoresist Photoresist oxide Oxide silicon substrate Silicon substrate oxide Oxide silicon substrate Silicon substrate

Resulting pattern after the resist is developed.

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Relationship Between Mask and Resist
Desired photoresist structure to be printed on wafer Island of photoresist

Substrate

Chrome Window

Quartz 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
Clear Field Mask Dark Field Mask

Simulation of metal interconnect lines (positive resist lithography)

Simulation of contact holes (positive resist lithography)

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Eight Steps of Photolithography
S te p 1 . V a p o r p r im e 2 . S p in c o a t 3 . S o ft b a ke 4 . A lig n m e n t a n d e x p o s u re 5 . P o s t-e x p o s u re b a k e (P E B ) 6 . D e v e lo p 7 . H a rd b a k e 8 . D e v e lo p in s p e c t C h a p te r 13 13 13 14 15 15 15 15
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Eight Steps of Photolithography
UV Light

HMDS

?

Resist

Mask

?

1) Vapor prime

2) Spin coat

3) Soft bake

4) Alignment and Exposure

5) Post-exposure bake

6) Develop

7) Hard bake

8) Develop inspect
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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 Coat
Process 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
Photoresist dispenser

Vacuum chuck To vacuum pump Spindle connected to spin motor

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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 Off Most of Solvent in Photoresist
? Typical Bake Temperatures are 90 to 100°C
– For About 30 Seconds – On a Hot Plate – Followed by Cooling Step on Cold Plate

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Alignment and Exposure
UV light source Process Summary: ? Transfers the mask image to the resistcoated wafer ? Activates photo-sensitive components of photoresist ? Quality measures: – linewidth resolution – overlay accuracy – particles and defects

Mask

?

Resist
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Post-Exposure Bake
? Required for Deep UV Resists
? Typical Temperatures 100 to 110°C on a hot plate ? Immediately after Exposure ? Has Become a Virtual Standard for DUV and Standard Resists

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Photoresist Development
Process Summary: ? Soluble areas of photoresist are dissolved by developer chemical ? Visible patterns appear on wafer - windows - islands ? Quality measures: - line resolution - uniformity - particles and defects
To vacuum pump Spindle connected to spin motor
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Develop dispenser

Vacuum chuck

Hard Bake
? A Post-Development Thermal Bake ? Evaporate Remaining Solvent ? Improve Resist-to-Wafer Adhesion ? Higher Temperature (120 to 140°C) than Soft Bake

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Develop / Inspect
? 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

? 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 Adhesion Due to Surface Contamination
Resist liftoff

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HMDS Puddle Dispense and Spin
Spin wafer to remove excess liquid

Puddle formation

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HMDS Hot Plate Dehydration Bake and Vapor Prime
Process Summary: ? Dehydration bake in enclosed chamber with exhaust ? Hexamethyldisilazane (HMDS) ? Clean and dry wafer surface (hydrophobic) ? Temp ~ 200 to 250?C ? Time ~ 60 sec.
Chamber cover

Wafer

Hot plate

Exhaust
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Purpose of Photoresist in Wafer Fab

? To transfer the mask pattern to the photoresist 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 wafer surfaces. ? Better uniformity characteristics. ? Increased process latitude (less sensitivity to process variations).

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Spin Coat
? Photoresist
– Types of Photoresist – Negative Versus Positive Photoresists

? Photoresist Physical Properties ? Conventional I-Line Photoresists
– Negative I-Line Photoresists

– Positive I-Line Photoresists

? Deep UV (DUV) Photoresists

? Photoresist Dispensing Methods

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Types of Photoresists
? Two Types of Photoresist
– Positive Resist – Negative Resist

? CD Capability
– Conventional Resist – Deep UV Resist

? Process Applications
– Non-critical Layers – Critical Layers

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Negative Versus Positive Resists ? 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 soluble – Developer removes exposed resist

? Resolution Issues ? Clear Field Versus Dark Field Masks
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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 Good Resist Contrast ? Sharp walls ? No swelling ? Good contrast

Resist

Resist

Film

Film

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Surface Tension
Low surface tension from low molecular forces High surface tension from high molecular forces

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Components of Conventional Photoresist

Solvent: gives resist its flow characteristics

Resin: mix of polymers used as binder; gives resist mechanical and chemical properties

Sensitizers: photosensitive component of the resist material

Additives: chemicals that control specific aspects of resist material
Figure 13.18

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Negative Resist Cross-Linking
Unexposed areas remain soluble to developer chemical.
Photoresist

UV

Areas exposed to light become crosslinked and resist the developer chemical.

Oxide

Substrate Unexposed Exposed

Soluble

Crosslinks

Pre-exposure - photoresist

Post-exposure - photoresist

Post-develop - photoresist
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PAC as Dissolution Inhibitor in Positive I-Line Resist
Unexposed resist, containing PACs, remain crosslinked and insoluble to developer chemical.
Photoresist
Substrate Exposed Unexposed

UV

Resist exposed to light dissolves in the developer chemical.
Oxide

PAC

Pre-exposure + photoresist

Soluble resist

Post-exposure + photoresist

Post-develop + photoresist
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Good Contrast Characteristics of Positive I-line Photoresist
Positive Photoresist:
Resist

? Sharp walls ? No swelling ? Good contrast

Film

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DUV Emission Spectrum
KrF laser emission spectrum
100 80 60 40 Relative Intensity (%) 40 Relative Intensity (%) 20 0 20 0
DUV* 248 nm

Emission spectrum of high-intensity mercury lamp
120
100 80 60
h-line 405 nm g-line 436 nm i-line 365 nm

248 nm 200 300 400 Wavelength (nm)
* 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.

500

600

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Chemically Amplified (CA) DUV Resist
Unexposed resist remains crosslinked and PAGs are inactive.
Photoresist Substrate

UV

Resist exposed to light dissolves in the developer chemical.
Oxide

Exposed
PAG PAG H+ H+ H+ PAG

Unexposed

PAG PAG

PAG

PAG

PAG

PAG

Pre-exposure + CA photoresist

Acid-catalyzed reaction (during PEB)

Unchanged

Post-exposure + CA photoresist

Post-develop + CA photoresist
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Exposure Steps for ChemicallyAmplified DUV Resist
1 . R e s in is p h e n o lic c o p o ly m e r w ith p ro te ctin g g ro u p th a t m a k e s it in so lu b le in d e v e lo p e r. 2 . P h o to a c id g e n e ra to r (P A G ) g e n e ra te s a c id d u r in g e x p o su re . 3 . A c id g e n e ra te d in e x p o se d re s ist a re a s se rv e s a s c a ta ly st to re m o v e re s in -p ro te ctin g g ro u p d u rin g p o st e x p o su re th e r m a l bake. 4 . E x p o se d a re a s o f re s ist w ith o u t p ro te ctin g g ro u p are so lu b le in a q u e o u s d e v e lo p e r.

Table 13.5

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Steps of Photoresist Spin Coating
1) Resist dispense 2) Spin-up

3) Spin-off

4) Solvent evaporation

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Automated Wafer Track for Photolithography
Wafer stepper (Alignment/Exposure system) Transfer station

Load station

Vapor prime

Resist coat

Develop Edge-bead and removal Rinse

Wafer Transfer System

Soft bake

Cool plate

Cool plate

Hard bake

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Photoresist Dispense Nozzle
Z Y X q

Nozzle position can be adjusted in four directions. Resist flow

Resist dispenser nozzle Wafer Stainless steel bowl Bottom side EBR Air flow Vacuum chuck Air flow Exhaust

Spin motor
Vacuum

Drain

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Resist Spin Speed Curve
Spin Speed Curve of IX300 80000 70000

60000
Resist Thickness (?) 50000 40000
110 cP 70 cP

30000
20000 10000

21 cP

0

1000

2000

3000

4000

5000

6000

7000
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Spin Speed (RPM)

Soft Bake on Vacuum Hot Plate
Purpose of Soft Bake: ? Partial evaporation of photoresist solvents ? Improves adhesion ? Improves uniformity ? Improves etch resistance ? Improves linewidth control ? Optimizes light absorbance characteristics of photoresist
Chamber cover

Wafer

Hot plate Solvent exhaust
Figure 13.28

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Solvent Content of Resist Versus Temperature During Soft Bake
Residual Solvent (% w/w)

DNQ/Novolak resist

Bake Temperature (°C)
Figure 13.29

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