Homework:%20CMOS%20Fabrication

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• Homework CMOS Fabrication
• Digital IC Design Martin Page 62
• 2.1 Discuss briefly the relationships between an
  ion beams acceleration potential, the beam
  current, and the time of implantation on the
  resulting doping profile.
• Ans Acceleration potential determines depth of
  implantationbeam current and exposure time
  determine dosage
• 2.2 Place the following processing steps in their
  correct order metal deposition and patterning,
  field implantation, junction implantation, well
  implantation, polysilicon deposition and
  patterning, field-oxide growth.
• Ans well implantation, field implantation,
  field-oxide growth, polysilicon deposition and
  patterning, junction implantation, metal
  deposition and patterning
• 2.3 What are major problems associated with a
  single threshold-voltage-adjust implant?
• Ans NWELL and psubstrate generally have
  different doping levels, may be difficult to
  achieve required threshold voltages by a single
  adjustment step for optimal well doping adjust
  thresholds separately

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Homework CMOS FabricationDigital IC Design
Martin Page 62

• 2.4 What is the reason for using a field implant
  and why is it often not needed in the well
  regions?
• Ans Implant guarantees that Si under FOX will
  never invert when large voltages are in vicinity
  better device isolation
• 2.5 What are the major trade-offs in using a wet
  process or a dry process when growing thermal
  SiO2?
• Ans Wet process faster since water vapor
  diffuses in Si faster than oxygen gas but lower
  quality porous oxide results. Dry process gives
  higher density oxide
• 2.6 Why is polysilicon used to realize gates of
  MOS transistors rather than metal?
• Ans Poly gates (as opposed to metal) permit a
  self-aligned process, eliminating need for
  another mask.Poly mask defines transistor
  channel area
• 2.7 Why can a microcircuit not be annealed after
  metal has been deposited?
• AnsAnnealing is a high temp process, metal
  distribution may be adversely affected

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