Computational Materials Science:

1

• Computational Materials Science
• Multiscale Modeling of Atomic Layer Deposition of
  Thin Films
• Andrey Knizhnik
• Kinetic Technologies Ltd, Moscow
• RRC Kurchatov Institute, Moscow

2
Challenges for ultra-thin film deposition
Deposition of films with atomic scale precision
of film thickness
Catalysis Microelectronics Nanotechnology
Uniform deposition in high-aspect ratio features
? Atomic layer deposition (ALD), Suntola T 1989
Mater. Sci. Rep. 4 261
3
Principles of ALD technique
Self-termination of adsorption provides atomic
scale control of the film thickness and ensures
uniform coverage.
4
Application of ALD technique
Application of ALD for deposition of high-k metal
oxide films in microelectronics
ZrO2, HfO2, Al2O3, La2O3, etc

• Zr(Hf)O2 deposition from Zr(Hf)Cl4 and H2O
• Zr(OH)/s/ ZrCl4ZrOZrCl3/s/ HCl
• ZrCl/s/ H2OZrOH/s/ HCl
• Film properties depend significantly on film
  deposition conditions
• Kinetic mechanisms of film growth are required

Experiment (ZrO2 ALCVD)
5
Features of ALD technique
Main features of atomic layer deposition

• Maximum film growth rate
• Temperature dependence of film growth rate
• Residual impurities in as-deposited films
• Selection of precursors
• Film roughness
• Influence with initial support state

6
Maximum film growth rate of ALD technique
Geometric considerations on maximum surface
coverage
- not observed
Zr(Hf)O2 deposition from Zr(Hf)Cl4 and
H2O. Repulsion between ligands of metal precursor
results in sub-monolayer coverage of the
substrate. Experimental maximum film growth rate
is about 0.5 ML/ALD cycle for halide precursors
and about 0.1 ML/ALD cycle for organometallics.
Maximum surface coverage is 0.25 ML/ALD cycle. M.
Ililammi, Thin solid Films 279 (1996) 124.
7
Maximum film growth rate of ALD technique
Quantum chemical calculations of precursor on the
surface
ZrCl4/g/ ZrOH/s/ ? ZrClx/s/ HCl/g/ Quantum
chemical calculation of ZrClx adsorption energy
with respect to gaseous species and hydroxylated
surface. HCl/g/ is removed from reactor by purge
gas. Maximum 0.5 ML/ALD cycle can be achieved in
agreement with experimental data.
0.5 ML
0.25 ML
Iskandarova, et al, SPIE, 2003
8
Multiscale modeling of thin film deposition
Construction of chemical mechanism of film growth
from first-principles data
QC calculations of reaction pathway
Rate coefficients calculation from Statistic
Theory
Simulation of film growth by reactor model

• Rate of film growth
• Mass increment per pulse
• Adsorbed groups at the surface
• Concentration of impurities

Comparison with experimental data
Fitting of rate parameters
9
First-principles modeling of deposition reactions
Quantum chemical simulation of ZrCl4 and H2O
precursor interactions with ZrO2 surface
(1) Hydrolysis of chemisorbed MCl2 groups
Minimum-energy pathway
H2O
(2) Chemisorption of MCl4 (M Zr, Hf) on the
hydroxylated MO2 surface (model
gas-phase reaction)
Minimum-energy pathway
ZrCl4
M.Deminsky, A. Knizhnik et al, Surf. Sci. 549
(2004) 67.
.
10
First-principles modeling of deposition reactions
Quantum chemical simulation of Al(CH3)3 (TMA) and
H2O precursor interactions with Al2O3 surface
Y. Widjaja, C.B. Musgrave, Appl. Phys. Lett.,
80,3304 (2002)
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Estimation of kinetic parameters for thin film
deposition
Energy profiles of the most important gas-surface
reactions
ZrCl4Zr(OH)2/s/ ? Zr(OH)OZrCl3/s/HCl
H2OZrCl2/s/ ?ZrCl(OH)/s/HCl
direct reaction
direct reaction
HCl
decay to products
H2O
ZrCl(OH)/s/
Loose TS
desorption
ZrCl2/s/
adsorption
Rigid TS
H2O-ZrCl2/s/
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Estimation of kinetic parameters for thin film
deposition
Equilibrium or Dynamics?
ZrCl4
ZrCl4
?chem
?chemgtgt ?relax
Zr(OH)2/s/
ZrCl4-Zr(OH)2/s/
Zr(OH)2/s/
ZrCl4Zr(OH)2/s/
Zr(OH)OZrCl3/s/HCl
?relax
HCl
desorption
adsorption
decay to products
Zr(OH)OZrCl3/s/
ZrCl4-Zr(OH)2/s/
Bulk
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Estimation of kinetic parameters for thin film
deposition
Transitional State Theory Evaluation of Reaction
Rate Constants
Decomposition of the surface complex over the
potential barrier.
Decomposition of the surface complex without the
potential barrier
QC calculations are not sufficient to determine
the structure of the loose transition
complex. Canonical variation transition state
theory was used to calculate rate constants.
Transition complex is rigid. The structure is
provided by the QC calculations.
Canonical variation transition state theory was
used to calculate rate constants.
Standard transition theory was used to calculate
rate constants
Reaction adsorption ka, cm3/mole s desorbtion kd, s1 decay to products kf, s1
Zr(OH)4/s/ZrCl4 Zr(OH)4ZrCl4/s/ Zr(OH)3-OZrCl3/s/ HCl. 3.3?1012 1.5?1010 T 1013.6 ?exp(11623/T) 4.3?1010 T0.4?exp(8258/T)
Zr(OH)2Cl2/s/ H2O Zr(OH)2Cl2- H2O ZrCl(OH)3/s/ HCl. 2.7?1013 1.7?1011 T 1013.6 ?exp(7570/T) 1013.8 ?exp(9452/T)
Hf(OH)4/sHfCl4 Hf(OH)4HfCl4/s/ Hf(OH)3-OHfCl3/s/ HCl. 6.8?1012 2.6?1010 T 1013.5?exp(5962/T) 8.1?1010T0.2?exp(7352/T)
Hf(OH)2Cl2/s H2O Hf(OH)2Cl2- H2O HfCl(OH)3/s/ HCl. 2.8?1013 1.35?1011 T 1013.8? exp(8323/T) 1013.9?exp(7515/T)
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Development of kinetic mechanism
Calculation of reaction constants using CARAT
 
Calculation of the rate constant for the reaction
Zr(OH) ZrCl4 in the framework of the CARAT
module. The parameters of the reaction,
reactants, and result dependence of the reaction
rate on temperature.

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Reactor scale modeling of thin film deposition
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Kinetic mechanism generation for thin film
deposition
Kinetic mechanism for ZrO2 film deposition for
CWB code
 

List of gas-surface reactions for description of
film growth in ALD reactor.
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Reactor scale modeling of thin film deposition
Macro-scale simulation of ZrO2 film ALD process
Variation of the film mass increment during one
ALD cycle
Experimental results from J. Aarik et al. / Thin
Solid Films 408 (2002) 97.
M.Deminsky et al, Surf. Sci. 549 (2004) 67.
18
Improving kinetic parameters
Dependence of reaction kinetic parameters on
local environment
Experimental data on temperature dependence of
film growth rate can not be fitted with given
mechanism. The smooth experimental temperature
dependence can be explained by dependence of
water desorption energy from MO2 surface on the
surface hydroxylation degree.
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Improving kinetic parameters
Quantum chemical simulation of local effects
forwater adsorption on the Zr(Hf)O2 surface
Dependence of water adsorption energy on the
t-Zr(Hf)O2 (001) surface hydroxylation from DFT
calculations
50 surface hydroxylation
I. Iskandarova et al, Microelectron. Eng. 69
(2003) 587.
25 surface hydroxylation
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Reactor scale modeling of thin film deposition
Temperature dependence of ZrO2 and HfO2 film
growth rate
Relative increment of HfO2 film mass and
thickness per cycle as a function of the
process temperature
Relative increment of ZrO2 film mass and
thickness per cycle as a function of the
process temperature
J. Aarik et al. / Thin Solid Films 408 (2002) 97.
J. Aarik et al,Thin Solid Films 340 (1999) 110.
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Reactor scale modeling of thin film deposition
Sensitivity analysis of kinetic mechanism of ZrO2
and HfO2 film growth
Relative increment of HfO2 film mass and
thickness per cycle as a function of the
process temperature
Relative increment of ZrO2 film mass and
thickness per cycle as a function of the
process temperature
The dashed areas correspond to the variation of
the pre-exponential factors by one order of
magnitude and the variation of the activation
energies of dehydroxylation reactions over the
range 3 kcal/mole.
22
Reactor scale modeling of thin film deposition
Simulation of Al2O3 film growth rate from TMA and
H2O
Low temperature reduction of film growth rate is
reproduced correctly using derived kinetic
mechanism.
The dashed areas correspond to the variation of
the pre-exponential factors by one order of
magnitude and the variation of the activation
energies of dehydroxylation reactions over the
range 3 kcal/mole.
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Reactor scale modeling of thin film deposition
Low temperature reduction of film growth rate
At low temperatures ALD precursors are trapped in
stable adsorption complex and do not react. This
results in reduction of film growth rate in ALD
process. Precursors with smaller deep of
potential well are required, e.g. alkylamide
HfN(CH3)24 (Musgrave et al, MRS 2005), or
plasma assisted ALD (e.g. O3 instead of H2O).
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Residual Impurities in deposited ALD film
Cl impurity in ZrO2 film
Probability of Cl atom to survive
1 ALD cycle
Since steady-state film growth rate is 0.4
layer/cycle several ALD cycles are required to
capture chlorine atom gt Residual chlorine
concentration should be quite small
2 ALD cycle
3 ALD cycle
N ALD cycle
gt
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Residual Impurities in deposited ALD film
Lattice kinetic Monte Carlo modeling of ZrO2 film
composition

• Chemical mechanism in lattice model
• Adsorption of MCl4 groups
• Hydrolysis of M-Cl groups
• Surface and bulk diffusion

• At each time step one and only one chemical
  reaction is chosen based on it rate and total
  rate of all chemical reactions

Cl
O
Lattice kinetic Monte Carlo model
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Residual Impurities in deposited ALD film
Lattice kinetic Monte Carlo modeling of ZrO2 film
composition
Lattice kinetic Monte Carlo model Temperature
dependence of chlorine atoms concentration in
zirconia film
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Roughness of ALD films

• ALD is not atomic layer deposition, it is
  sub-monolayer deposition due to
• Steric hindrance of metal precursors
• Small concentration of the active sites for
  adsorption (dehydroxylation of the surface).
• How submonolayer coverage influence on the film
  roughness?

Sub-monolayer coverage can result in increasing
of roughness of ALD films and non-uniform
coverage.
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Diffusion of precursors on the surface
H diffusion
H
H
O
Zr
O
Zr
29
Diffusion of precursors on the surface
Final
Initial
HfCl4 diffusion
Zr
Zr
HfCl4 molecule on the fully hydroxylated surface
30
Diffusion of precursors on the surface
Summary of precursor diffusion properties

• Diffusion of H atoms is rather rapid
• Diffusion of OH groups over t- and m-MO2(001)
  surfaces is very slow
• Diffusion of HfCl4 molecules over the fully
  hydroxylated t-HfO2(001) surfaces is rapid
• Diffusion of HfCl4 molecules over the bare
  surface is slow
• Diffusion of chemically adsorbed HfCl3 molecules
  over the bare surface is slow, only local
  relaxation of HfCl3 molecules can take place.

31
 
   

1 Ref. 5 gives an unreasonable value of the
threshold (more than 90 kcal/mol), which we think
is in error. Instead, we use the difference
between initial and final state (17 26
kcal/mol), like our lowest estimation.  
2 Formally this reaction describes the
formation of a bulk phase. But the formula
AlO(OH)2(S) presumes that the surface complex
still consists of AlO and 2(OH), so that this
virtual reaction was introduced in order to
have formation of a bulk phase in the model. To
avoid any effect on the overall mechanism rate,
we assumed the largest possible rate coefficient
for this reaction.  

i HIKE deliverable D3.1 (Oct 2002).
Roughness of ALD films
Lattice kinetic Monte Carlo modeling of HfO2 film
roughness
Surface profile with local relaxation at T100 C
Steric hindrance of precursors does not in
increasing of film roughness. Only dehyroxylation
of the surface results in growth of film
roughness with film thickness.
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Roughness of ALD films
Nucleation kinetics of HfO2 on Si, deposited by
ALD
M.L. Green and M. Alam.
Roughness is mainly due to non-uniform nucleation
at surface with low concentration of active
adsorption cites (OH groups).
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First-principles modeling of deposition reactions
Quantum chemical simulation of ZrCl4 precursor
interactions with Si(001) surface
(1) Chemisorption of MCl4 (M Zr, Hf) as inter-
and intra-dimer structures on the hydroxylated
oxidized and unoxidized Si(001) surface
Calculated minimum-energy pathways


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Conclusions

• ALD is a promising tool for deposition of
  uniform ultra thin films with atomic scale
  precision.
• Steric hindrance of precursors in a ALD process
  reduces film growth rate, but not increase
  significantly film roughness.
• Temperature dependences are generally smooth due
  to dependence of rate constants on local chemical
  environment.
• Low temperature growth is restricted by formation
  of stable intermediate complex.
• More reactive precursors are needed to reduce
  temperature of an ALD process plasma enhanced
  ALD can be used.
• Nucleation of the film determines mainly film
  roughness.

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Acknowledgements

• Anatoli Korkin
• Ed Hall
• Marius Orlovski
• Matthew Stoker
• Leonardo Fonseca
• Jamie Schaeffer
• Bill Johnson
• Phil Tobin

• Boris Potapkin
• Alexander Bagaturyants
• Elena Rykova
• Alexey Gavrikov
• Andrey Knizhnik
• Maxim Deminsky
• Ilya Polishchuk
• Mikhail Nechaev
• Inna Iskandarova
• Elena Shulakova

• Vladimir Brodskii
• Stanislav Umanskii
• Andrey Safonov
• Dima Bazhanov
• Ivan Belov
• Ilya Mutigullin
• Anton Arkhipov
• Evgeni Burovski
• Maxim Miterev