Chapter 8 Ion Implantation Instructor: Prof. Masoud Agah

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Chapter 8Ion ImplantationInstructor Prof.
Masoud Agah
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ION IMPLANTATION

• Two problems associated with diffusion especially
  for IC fabrication
• High-temperature process
• Unable to provide shallow junction depths
• Ion implantation is a relatively simple means to
  place a known number of atoms in a wafer.
• Ion implantation process
• Ionization of the dopant source to form positive
  ions
• Acceleration of ions through a high voltage field
  to reach the required energy
• Projection of high-energy ions towards the wafer
  surface (target)
• Collision of ions with silicon atoms resulting in
  energy loss
• End of penetration of ions in the substrate
  (coming to rest)

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SYSTEM REQUIREMENTS

• May achieve better control of distribution of
  dopants versus depth with ion implantation
• Process can be faster
• Process does not require as much thermal
  processing

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ION IMPLANTATION SYSTEM

• Ion implanter is a high-voltage accelerator of
  high-energy impurity ions
• Major components are
• Ion source (gases such as AsH3 , PH3 , B2H6)
• Mass Spectrometer (selects the ion of interest.
  Gives excellent purity control)
• HV Accelerator (voltage up to 1 MeV)
• Scanning System (x-y deflection plates for
  electronic control)
• Target Chamber (vacuum)

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ION IMPLANTATION SYSTEM

• Cross-section of an ion implanter

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ION IMPLANTATION SYSTEM

• Cross-section of an
• ion implanter

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ION IMPLANTATION

• High energy ion enters crystal lattice and
  collides with atoms and interacts with electrons
• Each collision or interaction reduces energy of
  ion until it comes to rest
• Interactions are a complex distribution. Models
  have been built and tested against observation

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ION IMPLANTATION

• To prevent channeling, implantation is normally
  performed at an angle of about 8 off the normal
  to the wafer surface.
• An annealing step is required to repair crystal
  damage and to electrically activated the dopants.
• The implanted dose can be accurately measured by
  monitoring the ion beam current.
• Complex-doping profiles can be produced by
  superimposing multiple implants having various
  ion energies and doses.
• Lateral scattering effects are smaller than
  lateral diffusion.
• Expensive

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ION IMPLANTATION

• Projected range (RP) the average distance an ion
  travels before it stops.
• Projected straggle (?RP) deviation from the
  projected range due to multiple collisions.

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MODEL FOR ION IMPLANTATION

• Distribution is Gaussian
• Np peak concentration
• Rp range
• ?Rp straggle

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MODEL FOR ION IMPLANTATION

• The implanted impurity profile can be
  approximated by the Gaussian distribution
  function.
• For an implant contained within silicon
  Q (2p)0.5 NP ?RP

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MODEL FOR ION IMPLANTATION

• Model developed by Lindhard, Scharff and Schiott
  (LSS)
• Range and straggle roughly proportional to energy
  over wide range
• Ranges in Si and SiO2 roughly the same
• Computer models now available at low cost for PCs

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MODEL FOR ION IMPLANTATION

• Range of impurities in Si

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MODEL FOR ION IMPLANTATION

• Straggle of impurities in Si

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SiO2 AS A BARRIER

• SiO2 serves as an excellent barrier against
  ion-implantation

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SiO2 AS A BARRIER

• The minimum oxide thickness for selective
  implantation
• Xox RP ?RP (2 ln(10NP/NBulk))0.5
• An oxide thickness equal to the projected range
  plus six times the straggle should mask most ion
  implants.
• A silicon nitride barrier layer needs only be 85
  of the thickness of an oxide barrier layer.
• A photoresist barrier must be 1.8 times the
  thickness of an oxide layer under the same
  implantation conditions.
• Metals are of such a high density that even a
  very thin layer will mask most implantations.

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ADVANTAGES

• Advantages over diffusion
• low temperature process
• allows wider range of barrier materials
• permits wider range of impurities
• better control of dose
• wider range of dose
• can control impurity concentration profile
• can introduce very shallow layers

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PROFILE CONTROL

• Various shapes of profiles can be created by
  varying the energy of the incident beam

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RADIATION DAMAGE

• Impact of incident ions knocks atoms off lattice
  sites
• With sufficient dose, can make amorphous Si layer

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RADIATION DAMAGE

• Critical dose to make layer amorphous varies with
  temperature and impurity

• Radiation damage can be removed by annealing at
  800-1000oC for 30 min. After annealing, almost
  all impurities become electronically active.

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Ion Implantation

• Implanting through a sacrificial oxide layer
• Large ions (arsenic) can be slowed down a little
  before penetrating into the silicon.
• The crystal lattice damage is suppressed (at the
  expense of the depth achieved).
• Collisions with the thin masking layer tends to
  cause the dopant ions to change direction
  randomly, thereby suppressing channeling effect.
• The concentration peak can be brought closer to
  the silicon surface.

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Ion Implantation

• For deep diffusion (gt1µm), implantation is used
  to introduce a certain dose, and thermal
  diffusion is used to drive in the dopants.
• The resulting profile after diffusion can be
  determined by

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Ion Implantation

• A boron implantation is to be performed through a
  50nm oxide so that the peak concentration is at
  the Si-SiO2 interface. The implant dose in
  silicon is to be 1013/cm2. What are the energy of
  the implant and the peak concentration at the
  interface?
• Peak at Si-oxide interface ? RP 0.05µm ? Energy
  15keV (?RP0.023µm)
• Implanted dose in silicon 1013 ? Q2x1013 ?NP
  Q/2.5?RP 3.5x1018/cm3
• How thick should the oxide layer be to mask the
  implant if the background concentration is
  1016/cm3?
• Xox 0.05 0.023(2 ln(10 x 3.5 x 1018/1016))0.5
  0.14µm
• If the oxide layer is 50nm, how much photoresist
  is required on top of the oxide to completely
  mask the implant?
• PR thickness 1.8 x (oxide thickness) 1.8 x
  (0.14 0.05) 0.16µm