Chapter 8 Ion Implantation

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Chapter 8Ion Implantation
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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)
• HV Accelerator (voltage gt 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

m/q(B2R2)/(2V)
Or Faraday cup
Acceleration energy voltage x charge on ion
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http//www.bpc.edu/mathscience/chemistry/images/pe
riodic_table_of_elements.jpg
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ION IMPLANTATION

• High energy ion enters crystal lattice and
  collides with atoms and interacts with electrons
• Types of collisions Nuclear and electron
• Each collision or interaction reduces energy of
  ion until it comes to rest
• Amount of energy loss is dependent on ion, the
  energy it has at the time of the scattering
  event, and the type of scattering.

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From Handbook of Semiconductor Manufacturing
Technology by Yoshio Nishi and Robert Doering
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From Handbook of Semiconductor Manufacturing
Technology by Yoshio Nishi and Robert Doering
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Channeling

• Deep penetration by the ion because it traveled
  along a path where no semiconductor atoms are
  situated
• Process is used for materials characterization
  Rutherford backscattering
• To prevent channeling
• Implantation is performed at an angle of about
  8 off the normal to the wafer surface.
• The wafer surface is amorphorized by a high dose,
  low energy implantation of a nonelectrically
  active ion.
• Hydrogen, helium, and silicon are common ions
  used

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Determining the Dose

• The implanted dose can be accurately measured by
  monitoring the ion beam current using a Faraday
  cup
• The integrated current during the implant divided
  by the charge on the ion is the dose.

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Post Implantation Anneals

• An annealing step is required to repair crystal
  damage (recrystallization) and to electrically
  activated the dopants.
• Dislocations will form during the anneal so times
  and temperatures must be chosen to force
  dislocations disappear.
• If the anneal time is long and the temperature is
  high, a drive of the implanted ions may occur.

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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.

http//eserver.bell.ac.uk
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MODEL FOR ION IMPLANTATION

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

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MODEL FOR ION IMPLANTATION

• For an implant contained within silicon, the dose
  is

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ION IMPLANTATION MODEL

• 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

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Range of impurities in Si
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Straggle of impurities in Si
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Si
SiO2
AZ-7500 resist
Si3N4
http//www.iue.tuwien.ac.at/phd/hoessinger/node22.
html
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http//www.ensc.sfu.ca/glennc/e495/e495l7j.pdf
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http//www.ensc.sfu.ca/glennc/e495/e495l7j.pdf
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SiO2 AS A BARRIER

• The minimum oxide thickness for selective
  implantation
• Xox RP ?RP (2 ln(10CP/CBulk))0.5
• An oxide thickness equal to the projected range
  plus six times the straggle should mask most ion
  implants.

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Other Materials

• 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.
• Nickel is one of the most commonly used metal
  masks

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ADVANTAGES

• Low temperature process
• The wafer is cooled from the backside during high
  energy, high current diffusions are performed
• Less change of stress-induced dislocations due to
  thermal expansion issues
• Wider range of barrier materials
• Photoresist
• Wider range of impurities
• No concern about solid solubility limitations
• Implantation of ions such as oxygen, hydrogen,
  helium, and other ions with low solid solubility
  is possible.

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Advantages over Diffusion

• Better control and wider range of dose compared
  to predep diffusions
• Impurity concentration profile controlled by
  accelerating voltage
• Very shallow layers
• Lateral scattering effects are smaller than
  lateral diffusion.

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• Complex-doping profiles can be produced by
  superimposing multiple implants having various
  ion energies and doses.

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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

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Recrystallization

• Radiation damage can be removed by annealing at
  800-1000oC for 30 min. After annealing, a
  significant percentage of the impurities become
  electronically active.
• Point defects coalesce into line dislocations
• Line dislocations merge into loop dislocations
• Loop dislocations slowly disintegrate as
  interstitial Si atoms move on to lattice sites

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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