Multi-Scale modelling of
atomic layer deposition
Presented by: Mahdi Shirazi
Supervised by: Dr. Simon Elliott
Outline
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Atomic layer deposition (ALD)
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Description of ALD
Goals
Obstacles
Findings from atomic-scale modelling
Strategy for Kinetic Monte Carlo
What is ALD?
ALD is based on self-limiting surface
reactions of two chemicals. For an
oxide, a metal precursor & an
oxygen precursor.
Process is cyclic :
1.
Pulse of metal precursor - a
monolayer of metal precursor
molecules chemisorbs onto surface .
2.
Purge - to remove unreacted
precursor and by-products from
chamber.
3.
Pulse of oxygen precursor – to
create a monolayer of chemisorbed
oxygen precursor on surface
4.
Purge – to remove unreacted
precursor and by-products from
chamber.
www.isr.umd.edu/~hennlec/images/ALD/ALD_reaction_475.jpg
The desired film thickness is reached by repeating the cycle.
Typical growth per cycle is about 0.1 nm/cycle and cycle time is
typically 1-4 s/cycle.
Goals
• Model interaction of precursors with surface and
growth by ALD.
• Go beyond the atomic length scale and the time
scale of individual reactions.
• Explain why amorphous or crystalline layers are
deposited?
Obstacles
• Reaction mechanisms consist of rare events.
• Need to evaluate system beyond picosecond
timescale.
• Using density functional theory (DFT) is time
consuming.(e.g. Cl-NEB calculation takes 3 hours for 300 atoms by 120
Intel-Xeon CPU).
• To find reaction events: how efficient are
Nudged Elastic Band (NEB), Conjugate Gradient
(CG), quasi-Newton algorithms and ab initio
Molecular Dynamics (MD)?
 We should take advantage of Kinetic Monte
Carlo (KMC) to describe ALD.
Outline
• Atomic layer deposition
• Findings from atomic-scale modelling
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ALD reactions
Non-ALD reaction
Evaluating barriers by NEB
Importance of coordination number
• Strategy for Kinetic Monte Carlo
Slab modelling
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Growth of HfO2 from Hf(N(CH3)2)4 and H2O
Monoclinic structure is stable phase in low
temperature.
Direction of growth (111)
Four layers have been regarded as slab
Extended surface 22
We used hydroxylated surface
VASP code.
Slab=yellow, oxygen=red, hydrogen=white
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VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html
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H2O dissociates at active Lewis acid
and base sites at surface
Cover the surface with hydroxyl
groups and water molecules
Rate of proton diffusion depends on
coverage of OH.
1-Charles B. Musgrave et al., Chem. Mater. 2006, 18, 3397-3403.
Hafnium =grey, oxygen=red, hydrogen=white
Adsorption and dissociation of H2O
at HfO2 surface1
Hafnium =blue, oxygen=red, hydrogen=white, Nitrogen= darkblue, carbon=grey
Sequence of ALD reactions
Barriers were calculated by Cl-NEB1
HfX4+2H2O 4HX+HfO2 X=N(CH3)2
We used DFT2,3 to calculate activation energies to implement them into the  KMC
1.
Graeme Henkelman, Hannes Jo´nsson et al J. Chem. Phys.113, 22, 9901 2000
2.
VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html
3.
Entropy calculated by TURBOMOLE http://www.turbomole.com/ T=500K
Hafnium =blue, oxygen=red, hydrogen=white, Nitrogen= darkblue, carbon=grey
Sequence of ALD reactions
Barriers were calculated by Cl-NEB1
HfX4+2H2O 4HX+HfO2 X=N(CH3)2
We used DFT2,3 to calculate activation energies to implement them into the  KMC
1.
Graeme Henkelman, Hannes Jo´nsson et al J. Chem. Phys.113, 22, 9901 2000
2.
VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html
3.
Entropy calculated by TURBOMOLE http://www.turbomole.com/ T=500K
Hafnium =blue, oxygen=red, hydrogen=white, Nitrogen= darkblue, carbon=grey
Sequence of ALD reactions
Barriers were calculated by Cl-NEB1
HfX4+2H2O 4HX+HfO2 X=N(CH3)2
We used DFT2,3 to calculate activation energies to implement them into the  KMC
1.
Graeme Henkelman, Hannes Jo´nsson et al J. Chem. Phys.113, 22, 9901 2000
2.
VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html
3.
Entropy calculated by TURBOMOLE http://www.turbomole.com/ T=500K
Hafnium =blue, oxygen=red, hydrogen=white, Nitrogen= darkblue, carbon=grey
Sequence of ALD reactions
Barriers were calculated by Cl-NEB1
HfX4+2H2O 4HX+HfO2 X=N(CH3)2
We used DFT2,3 to calculate activation energies to implement them into the  KMC
1.
Graeme Henkelman, Hannes Jo´nsson et al J. Chem. Phys.113, 22, 9901 2000
2.
VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html
3.
Entropy calculated by TURBOMOLE http://www.turbomole.com/ T=500K
Hafnium =blue, oxygen=red, hydrogen=white, Nitrogen= darkblue, carbon=grey
Sequence of ALD reactions
Barriers were calculated by Cl-NEB1
HfX4+2H2O 4HX+HfO2 X=N(CH3)2
We used DFT2,3 to calculate activation energies to implement them into the  KMC
1.
Graeme Henkelman, Hannes Jo´nsson et al J. Chem. Phys.113, 22, 9901 2000
2.
VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html
3.
Entropy calculated by TURBOMOLE http://www.turbomole.com/ T=500K
Hafnium =blue, oxygen=red, hydrogen=white, Nitrogen= darkblue, carbon=grey
Sequence of ALD reactions
Barriers were calculated by Cl-NEB1
HfX4+2H2O 4HX+HfO2 X=N(CH3)2
We used DFT2,3 to calculate activation energies to implement them into the  KMC
1.
Graeme Henkelman, Hannes Jo´nsson et al J. Chem. Phys.113, 22, 9901 2000
2.
VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html
3.
Entropy calculated by TURBOMOLE http://www.turbomole.com/ T=500K
Barrier to H+ diffusion to amide group
Cl-NEB
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Our calculations showed that H+ diffusion barrier varies
between 0 to 2eV.
3.71eV
Barrier to H+ diffusion to amide group
Cl-NEB
Test whether amide ligand can desorb without combining with H+?
No: barrier increases from 1.6 eV to 3.7 eV in absence of H+.
Discovery of reaction events
(MD superior to optimisation)
Densification1
Ligand transfer
show role of coordination number
1.
A. Este`ve, M. Djafari Rouhani et al, J. Chem. Theory Comput. 2008, 4, 1915–1927
Non-ALD reaction
(MD superior to optimization)
• Ligand decomposition
We find that activation
energies are tuned by
coordination number
Rate catalogue
Reactions
Barrier (eV)
HfX4(g)HfX4(s)
-
HfX4(s)HfX3(s)
2.80
HfX3(s)HfX2(s)
2.96
Densification
0<0.5
HfX2(s)HfX1(s)
1.29
HfX1(s)HfX0(s)
1.64
HfX0(s)HfX1(s)
0.69
HfX1(s)HfX2(s)
2.24
HfX2(s)HfX3(s)
5.26
HfX3(s)HfX4(s)
4.92
Ligand decomposition
<0.5
Ligand transfer
<0.5
H2OOH-+H+
Depends on coordination number
of hafnium atoms at surface
H+ diffusion to amide group
0-2eV
Outline
• Atomic layer deposition
• Findings from atomic-scale modelling
• Strategy for Kinetic Monte Carlo1
• Call reaction catalogue
• Stick to on-site KMC
• Tie rates to coordination number of atoms at
surface
• Implementation of new application into the
SPPARKS2 code in progress
1.
2.
Arthur F. Voter, Introduction to the Kinetic Monte Carlo Method
SPPARKS http://www.sandia.gov/~sjplimp/spparks.html
BKL algorithm1
1.
A. Bortz, M. Kalos, and J. Lebowitz, J. Comput. Phys. 17, 10 1975
Conclusions
• New mechanisms of ALD reactions were
found and quantified.
• Ab initio MD superior to optimisation
methods in identifying global basins.
• Role of coordination number is important
in growth of complex material.
• Introduce new application for KMC.
Acknowledgement
We are grateful for funding by Science Foundation Ireland under the
FORME project, http://www.tyndall.ie/forme/ and acknowledge a generous
grant of computing time from the SFI and HEA-funded Irish Centre for High
End Computing (ICHEC). We also thank A. Esteve & M. D. Rouhani in
LAAS and Steve Plimpton & Corbett Battaile at Sandia National Laboratory
for their collaboration.
Thank you for your attention