IBM Spin Coating Process

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IBM Spin Coating Process

• Greg Burtt
• Greg Hewitt
• Dave Valente
• Contact Engineer Kevin Remillard
• Faculty Mentor Jeff Marshall

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Typical Photolithography Process
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IBMs Problem

• During the spin coating process, defects
  sometimes appear on the wafer.
• Damage to the bowl from handling and cleaning
  have also been spotted as a cause of the defects.
• A cleaning process must be run a few times a day
  to remove built up resist from the bowl set (a
  3-part set of plastic pieces that houses the
  silicon wafer being coated and collects the
  excess applied chemicals) in order to reduce the
  probability of defects.
• This cleaning process involves the use of a
  cleaning disk and a solvent to attempt to remove
  as much resist as possible from the bowl, but it
  is incapable of removing all of the build-up.
• Consequently, the bowl set must be removed every
  two weeks and trucked to a company that cleans
  them. This is a very costly process for IBM for a
  few reasons.
• There is a lot of down-time involved in removing
  and replacing the bowls, this is time translates
  directly to lost money because of decreased
  productivity.
• They must also pay to have the bowls trucked to a
  facility and to have them cleaned there.
• Some bowl sets come back with damage and need to
  be replaced.

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Defect Description

• Wafer defects are thought to be caused by the
  loss of proper air flow through the bowl setup.
• Very small particles (ball bearings) of resist
  become suspended in the air during the normal
  application process, the flow ideally carries all
  these particles away from the wafer surface into
  a drain.
• In some cases, the flow becomes disrupted
  allowing these particles to return from the air
  to the wafer surface. Alternatively, the flow
  picks up new ball bearings from the sides of the
  bowl and deposits them on the wafer surface.
  Either instance results in a defect on the wafer.
• IBM believes, and we will assume their conclusion
  is correct, that in an otherwise undamaged bowl,
  these defects are primarily caused by a build-up
  of dried resist on the bowl surfaces.

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Goal

• Our goal is to reduce the cost to IBM of
  maintenance and upkeep of the bowl sets by
  altering the coating or cleaning process. Thereby
  allowing bowls to be changed less often, reducing
  cleaning costs and the risk of damage during
  handling. Two fundamentally different approaches
  can be taken.

1. Airflow To decrease the sensitivity of the
   spin-coating process to imperfections in the bowl
   such as dried resist.
2. Material and Cleaning To reduce the ability of
   the resist to collect on the sides of the bowl in
   volumes that have a detrimental effect on the
   spin-coating process.

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Possible Solution Avenues
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Two Testing Options

• Airflow

• Buildup of Resist

• If the airflow path on the flow chart is taken, a
  full experimental setup must be created. The flow
  velocity, angular velocity of the wafer, and
  internal geometry of the photoresist application
  machine must be accurately replicated. This would
  allow us to make alterations to the velocity,
  knife edge, roughness, and bowl geometry and test
  their effects. This would be done with a mix of
  computer simulation and visual observation using
  suspended particles such as what would be found
  in smoke or the vapors of dry ice.

• If the resist buildup path is chosen, the
  experimental setup can be greatly simplified. The
  bowl set and chamber would be used mainly to
  characterize the resist buildup and test cleaning
  disk designs and process changes. Material and
  coating ideas would be tested on samples obtained
  from manufacturers. Parameters such as contact
  angles could be measured and compared. Resist
  could also be applied and dried onto the samples,
  then the ease of removal could be assessed
  experimentally.

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Constraints

• Process chemicals cannot be changed
• Wafer spin speed is variable, so a solution must
  accommodate any angular velocity up to a maximum
  of several thousand RPMs
• IBM prefers that the bowl geometry remain
  unchanged
• Exhaustive testing would have to result
• Cost to IBM can not increase
• This includes the cost associated with bowl
  manufacture and cleaning as well as equipment
  down-time for cleaning and bowl changes