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Thermal diffusion, Ion implantation. Doping: modifying resistivity. Gas phase diffusion Solid source diffusion Ion implantation e.g. POCl 3 gas e.g. boron-doped film 20-200 keV ions at 1000 o C at 1000 o C at room temperature

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thermal diffusion ion implantation
Thermal diffusion,Ion implantation


doping modifying resistivity
Doping: modifying resistivity

Gas phase diffusion Solid source diffusion Ion implantation

e.g. POCl3 gas e.g. boron-doped film 20-200 keV ions

at 1000oC at 1000oC at room temperature

Oxide mask Oxide mask Resist mask

doping vs resistivity
Doping vs. resistivity

Single crystal silicon resisitivity is lower than that of polysilicon, for same doping concentration.

two modes of diffusion
Two modes of diffusion

Fixed supply of dopant:

when dopant atoms diffuse into silicon, surface concentration decreases

Infinite supply of dopant:

New supply of dopant keeps surface concentration constant when dopant atoms diffuse into silicon


fixed source vs infinite source
Fixed source vs. infinite source

Longest diffusion time

Longest diffusion time

Fixed number of dopantatoms “Infinite” initial dopant atoms

mos controlled thyristor
MOS-controlled thyristor

Multiple diffusions:

-deep p diffusion

-heavy n+ diffusion

-intermediate n diffusion

-shallow p+ diffusion

-heavy p+ diffusion on back


multiple diffusion 2
Multiple diffusion (2)

Take n-silicon wafer

Perform p-diffusion

p diffusion


3. Perform n-diffusion  p-diffusion becomes deeper

N-concentration must be higher than p; otherwise dopant type does not change.


p diffusion



diffusion profiles
Diffusion profiles

When n-dopant concentration is high enough,

p-type silicon turns n-type.

This n-type silicon can be turned to p-type again, by applying even enough boron concentration.

ion implantation chapter 15
Ion implantation(Chapter 15)
  • Ion-solid interactions
  • Dopant profiles
  • Implant damage and damage anneal
  • Implant equipment
  • Implant applications


basic implantation process





Basic implantation process

Accelerated ions (B+, P+, As+)

hit silicon, penetrate, collide randomly and come to rest.

Photoresist mask.

Peak concentration is inside silicon




implant parameters
Implant parameters

Ion energies 10-200 keV

Implant depths 10-500 nm

Doses 1011 to 1016 ions/cm-2.

Concentrations ca. 1015 cm-3 to 1020 cm-3.

5.1015 cm-2 ion implant dose and depth of ca. 200 nm translates to ca. 25 Ohm/sq sheet resistance


projected range and straggle
Projected range and straggle

Ion (P), dose (1014 cm-2), energy (100 keV)


Projected range


projected range r p in si
Projected Range (Rp) in Si

Rp (μm)

Rp depends on incident

and target atomic masses

  • In practise,
  • 200 keV max
  • Boron 500 nm
  • Phophorous 200 nm
  • Arsenic 100 nm

Ion energy (keV)


implant profiles
Implant profiles

100 keV; B, P, As into silicon

Implantation thru 200 nm thick oxide (50 keV vs. 150 keV)


implant profiles 2
Implant profiles (2)

As a function of energy:

P+: 50, 100, 150 keV energies

As a function of dose P+ doses:


implantation damage
Implantation damage

Light ions like boron, and low doses (<1014 cm-2) cause mainly point defects

High doses (>1015 cm-2) and heavy ions (As+, Sb+, Ge+) cause extended damage and amorphization


implant dose damage and anneal

implant damage

electrical activity

dopant solubility

dopant diffusivity

Implant dose, damage and anneal

Damage has to be annealed away.

Temperatures are ca. 1000oC ( implantation is a high temperature process !)

Crystalline defects annealed.

Dopants find their place in the crystal lattice.


annealing after implantation
Annealing after implantation
  • implantation is a room temperature process
  • silicon crystal is damaged by high energy ions
  • this damage is annealed away at ca. 1000oC
  • implantation always requires high temperature just like thermal diffusion (but easier process sequence by elimination of oxide)
main elements of an implanter

wafer chamber

Faraday cup

load lock

selection magnet

acceleration tube

ion optics


ion source

gas 1

gas 2

Main elements of an implanter


implanter equipment
Implanter equipment
  • Generation of ions

– dopant gas containing desired species

    • BF3, B2H6, PH3, AsH3, AsF5
  • – plasma provides positive ions
    • 11B+, BF2+, 31P+, 31P++
  • Ion extraction
    • ions are extracted from the source due to a high electric field
  • Ion selection
    • magnetic field mass analyzer selects the appropriate ion (mass & charge)
  • Ion acceleration
    • further accelerate ions giving the ions their final kinetic energy.
  • Beam scan / Disk scan
    • provides a uniform dose of ions over the wafer surface.
  • In-situ dose monitoring


diffusion vs implantation
Diffusion vs. implantation

Doping by diffusion:

Oxide mask for diffusion

MOS gate needs to be aligned to S/D junctions

 misalignment

Self-aligned gate by implantation:

polysilicon gate blocks ions, and MOS channel remains undoped


implantation vs diffusion
Implantation vs. diffusion
  • Implantation is more accurate in dose control
  • Implantation produces greater variety of profiles
  • Implantation is possible through the surface layers
  • Sideways spreading is 1/3 of the vertical range in implantation and isotropic in duffusion
  • Diffusion is high-temperature process, implantation is room temperature process
  • Damages after implantation are annealed at high temperature
  • Diffusion is the best for high doping level, deep junctions and double side doping


doping of epi films
Doping of epi films

Gas phase dopant molecules are mixed in silane flow:

2 PH3 2 P + 3 H2

B2H6 2 B + 3 H2

Epitaxial films come in all the same varieties as silicon wafers:



-high resisitivity (not intentionally doped)

Benefits of epi:

Oxygen and carbon of the substrate wafer are buried under epi.

Dopantuniformity is very good.

epi doping
Epi doping

Add gaseous dopants into the flow:

B2H6 for boron

PH3 for phoshorous

AsH3 for arsenic

Very small partial pressures enough:

10-10 bar 1015 cm-3

10-8 bar 1017cm-3

10-6 bar 1019cm-3


Epitaxy temperatures

Reactor cleaning before each wafer.

In-situ wafer cleaning to ensure the best possible cleanliness.

dopant diffusion during epi
Dopant diffusion during epi

Because epitaxy is a high temperature process, dopant atoms diffuse during epitaxy.

Diffusion is from high dopant concentration to low concentration.

Epi doping level is independent of substrate doping level, but the interface is not sharp due to diffusion.

Lightly doped epi Heavily doped substrate