Semiconductor Manufacturing Technology

1
Lecture 14
Lithography

Taken in part from Chapters 13-15 Semiconductor
Manufacturing Technology by Michael Quirk and
Julian Serda
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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
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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
11 Exposure
11 Exposure
51 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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(No Transcript)
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Photomask and Reticle for Microlithography
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Three Dimensional Pattern in Photoresist
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Section of the Electromagnetic Spectrum
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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, gt0.3 µm)
• Deep UV by excimer lasers
• Kr NF3 (energy) ? KrF (photon emission)
• KrF ? 248 nm (used for 0.25 µm)
• ArF ? 193 nm (used for 0.12 µm)

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Important Wavelengths for Photolithography
Exposure
Table 13.1
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Importance of Mask Overlay Accuracy
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 soluble
• Developer removes exposed resist

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Negative Lithography
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Positive Lithography
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Relationship Between Mask and Resist
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Clear Field and Dark Field Masks
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Eight Steps of Photolithography
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Eight Steps of Photolithography
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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
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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 100C
• For About 30 Seconds
• On a Hot Plate
• Followed by Cooling Step on Cold Plate

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Alignment and Exposure
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Post-Exposure Bake

• Required for Deep UV Resists
• Typical Temperatures 100 to 110C on a hot plate
• Immediately after Exposure
• Has Become a Virtual Standard for DUV and
  Standard Resists

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Photoresist Development
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Hard Bake

• A Post-Development Thermal Bake
• Evaporate Remaining Solvent
• Improve Resist-to-Wafer Adhesion
• Higher Temperature (120 to 140C) 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
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HMDS Puddle Dispense and Spin
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HMDS Hot Plate Dehydration Bake and Vapor Prime
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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
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Surface Tension
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Components of Conventional Photoresist
Figure 13.18
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Negative Resist Cross-Linking
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PAC as Dissolution Inhibitor in Positive I-Line
Resist
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Good Contrast Characteristics of Positive I-line
Photoresist
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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.
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Chemically Amplified (CA) DUV Resist
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Exposure Steps for Chemically-Amplified DUV Resist
Table 13.5
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Steps of Photoresist Spin Coating
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Automated Wafer Track for Photolithography
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Photoresist Dispense Nozzle
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Resist Spin Speed Curve
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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

Figure 13.28
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Solvent Content of Resist Versus Temperature
During Soft Bake
Figure 13.29