Microlithography for IC Production

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Microlithography for IC Production

• by John Broky
• OPTI 422
• 9/22/03

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What is Microlithography?
It is a form of printmaking in the micron range,
predominantly used for circuits on silicon
wafers.
It creates highly accurate 2-D patterns in
photosensitive resist material, which is used to
build integrated circuits.

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Integrated Circuits

• Complex circuitry that fits on semiconductor
  microchips the size of a fingernail. The
  circuitry consists of several layers stacked upon
  each other where each layer contains an elaborate
  network of connections just microns in size.

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Components of the Exposure System of a Typical
Stepper

• Lithographic Projection Lens
• Illumination Subsystem
• Wafer Positioning Subsystem

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Lithographic Projection Lens

• Contains many simple lens
  elements
• Images photomask with
  demagnification 5x
• Wavefront aberration must be
  less than 1/10 the source
  wavelength
• Corrected for chromatic
  aberration at 365nm

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Illumination Subsystem

• 1000 watt mercury arc lamp
• Layers of dielectric filters allow only
  365nm light
  through
• Light passes through relay optics and is
  projected through the
  photomask and onto
  the entrance pupil of the lithographic lens
• Non-uniformity must be less than 1
• Photoresist is exposed to the image
  for only a few
  hundredths of a second

light
photomask
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Wafer Positioning Subsystem

• The position of the silicon wafer must be as
  precise as the optics
• Each wafer contains around 100 semiconductor
  devices, or chips
• Less than one second to move between successive
  exposure sites
• It detects and corrects tilt errors because the
  wafer surface is not flat enough to guarantee
  focus everywhere
• A stepper can expose about 60 wafers in one hour

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Close-ups of Integrated Circuits
Pictures courtesy of www.charfaq.umn.edu
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Other Lithography Processes

• Optical Proximity Printing
• X-Ray Proximity Lithography
• Scanners
• Step-and-Scan
• Immersion Lithography

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References

• Sheats, James R., and Bruce W. Smith.
  Microlithography Science and Technology. New
  York Marcel Dekker, Inc, 1998.
• http//www.charfac.umn.edu/MMS/ProjectMicro/Explor
  ations/Jun1998.html
• http//www.geocities.com/beatles_forever_plus_some
  /Silicon_information.html
• http//www.siliconstrategies.com/story/OEG20030422
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