Transcript Molecular Dynamics

Ion Implantation
CEC, Inha University
Chi-Ok Hwang
Ion Implantation
• Ion implantation (introduced in 1960’s) vs chemical
diffusion
- High accuracy over many orders of magnitude of
doping levels
- Depth profiles by controlling ion energy and
channeling effects
- Dopants into selected regions using masking material
- Both p- and n-type dopants
- Recovering implant-damaged Si crystalline via
thermal annealing
• Definition of ion implantation
• CMOS energy range: 0.2keV-2MeV
Ion Implantaion
• Aspects of ion implantaion: dose, dose uniformity,
profiles (depth distribution), damage, damage
recovery after annealing
• Dose
It

qi A
• Limitations
-damage to the material structure of the target
-shallow maximum implantation depth (1㎛)
-lateral distribution of implanted species
-throughput is typically lower than diffusion doping
processes
Ion Implantation
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Ion species and substrate
Tilt and rotation
Ion energy
Dose rate
Results: dopant distribution, defect
distribution
Ion implantation
Ion Implantation
• Limitations
-complex machine operations
-safety issue to the personnel
• Ion implantation profiles
-range, R
-projected range, Rp
-projected straggle, ΔRp
-projected lateral straggle, ΔR⊥
Ion Implantation
Simulation size: cascade size (10-25 cm3 (M.-J.
Caturla etc, PRB 54, 16683, 1996) )
- 1000 atoms (J.B. Gibson etc, PR 120(4), 1229, 1960)
- a few hundreds of thousands of atoms (J. Frantz etc,
PRB 64, 125313 , 2001)
• Time scales
- thermal vibration periods of atoms in solids: 0.1 ps
(10-13 sec) or longer
- cascade lifetime: 10 ps (M.-J. Caturla etc, PRB 54,
16683, 1996)
- ion implantation (secs; annealing time secs-mins)
• Si density: 5 x 1022 /cm3 (5.43Å unit cell, 8/unit cell)
• ion dose: 1014 -1018 ions/cm2
•
Stopping powers
• Electric fields; nuclear charge of the
silicon atoms (short range interatomic
force by screening effect, nuclear
stopping) and valence electrons of the
crystal (polarizational force, nonlocal
electronic force)
• exchange of electrons with the silicon
atoms (local electronic stopping)
Ion Implantation
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ion implantation Potential: BCA
- nuclear stopping power; elastic
collision
Vij(r) = Zi Zje2 /r Φ(r)
Φ(r); screening of the nuclei due to the
electron cloud
① Thomas-Fermi
② ZBL; universal screening potential
- electronic stopping power; frictional
force
③ Stillinger-Weber potential
Ion Implantation
• Simulations of ion implantation
- Full MD
- Recoil Interaction Approximation (RIA)
(1-100 keV)
- BCA: valid for low-mass ions at
incident energies from 1-15 keV (M.-J.
Caturla, etc, PRB 54, 16683, 1996)
Ion Implantation
Three phases of collision cascade
- collisional phase (0.1-1 ps)
- thermal spike (1 ns)
- relaxation phase (a few thousands of fs)
• Measurements of depth profiling
- Rutherford Backscattering Spectroscopy
(RBS)
- Secondary Ion Mass Spectroscopy (SIMS)
- (Energy-Filtered) Transmission Electron
Microscopy ((EF)TEM)
•
BCA
• Primary recoil atoms,
• Binary scattering tables: described by
specifying the species involved in the
collision, the impact parameter, and the ion
energy
• Assuming that the potential energy of the ion
at the start of the collision is negligible
compared to its kinetic energy
• Neglecting multi-body interactions
Kinchin-Pease Model
• Damage model: damage generation, damage
accumulation, defect encounters, amorphization
• Number of Frenkel pairs proportional to the nuclear
energy loss
• Nuclear energy loss is deposited locally and induces
local defects
• Holds only when the secondary ion’s energy is
relatively low
• The percentage of the interstitials and vacancies
surviving the recombination decreases as the implant
energy increases
Kinchin-Pease Model
Number of point defects
n
E
2 Ed
Ed; displacement threshold energy (15 eV)
Net increase of point defects
N
n  nfrec (1 
)
N
N; local defect density
Nα; critical defect density for amorphization
f; fraction of defects surviving the recombination within
one recoil cascade
Kinchin-Pease Model
Damage dechanneling; defect encounter probability
N
P 
N
Amorphization: the critical density is taken to be 10% of the lattice
Density for all implant species