Photolithography

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Photolithography
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Photolithography

• Photolithography or optical lithography is a
  process used in semiconductor device fabrication
  to transfer a pattern from a photomask (also
  called reticle) to the surface of a substrate.

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Photolithography involves a combination of

• Substrate preparation
• Photoresist application
• Soft-baking
• Exposure
• Developing
• Hard-baking

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Photoresist (??)

• Photoresist is a chemical substance that becomes
  soluble (negative) or become insoluble (positive)
  by exposure to ultraviolet light and developed.

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• A positive resist is a type of photoresist in
  which the portion of the photoresist that is
  exposed to light becomes soluble to the
  photoresist developer and the portion of the
  photoresist that is unexposed remains insoluble
  to the photoresist developer.
• A negative resist is a type of photoresist in
  which the portion of the photoresist that is
  exposed to light becomes relatively insoluble to
  the photoresist developer. The unexposed portion
  of the photoresist is dissolved by the
  photoresist developer.

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Spin coater (???)

• Spin coating is a procedure used to apply uniform
  thin films to flat substrates.

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Exposure - Mask Alignment

• The mask aligner uses UV light as an exposure
• light source to lithographically transfer mask
  patterns
• onto the resist-coated substrate (minimum
  feature size
• is 2 µ m).
• In contact printing, the mask is pressed against
  the resist
• coated wafer during exposure
• In order to reduce the contact gap, usually the
  pressures
• ranging from 0.05 - 0.3atm are used to push
  the mask into
• more intimate contact with the wafer.

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Stepper

• A stepper is a device, used in the manufacture of
  integrated circuits (ICs), that is similar in
  operation to a slide projector or a photographic
  enlarger.
• The term "stepper" comes from the
  "step-and-repeat" action of moving the wafer on
  its x and y axes to align the reticle with each
  individual device position

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The ASML PAS 5500/950B Step and Scan System
Reticle (Mask)
193 nm Excimer Laser Source
Exposure Column (Lens)
Wafer
Computer Console
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Photoresist Developing

• The process of allowing developer to interact
  with photoresist to form an image.
• Developing of negative photoresist is most
  commonly by spraying developer onto the wafer
  surface.
• Positive photoresist may be developed by
  immersion or spray. Temperature is very critical.