Puurunen_invited-talk_ALD-modelling_ALD2005_050805

Atomic scale modeling of
atomic layer deposition processes
Riikka Puurunen
MEMS Technology Group
VTT Technical Research Centre of Finland

2
Riikka Puurunen, 09 Aug 2005
ALD is fantastic !
Sneh et al.,
Thin Solid Films 402 (2002) 248.
Si Al2O3/Ta2O5 nanolaminate
Al2O3: 1.4 nm (~15 cycles)
Ta2O5: 2.7 nm

3
But fantastic ≠ perfect
electrical performance of
field effect transistors
14
20 25
30,
40
down-scaling
successful
down-scaling fails
50
60
80
HfCl4/H2O
cycles
Down-scaling
stops — why?
figure courtesy Schram et al., IMEC
data Ragnarsson et al., VLSI2005, p. 234
turning point:
3-5 nm HfO2

4
Many historical assumptions on ALD are invalid.
Atomic scale models help to understand why
temperature
mono-
layer
growth per cycle
cycles temperature cycles
growth per cycle
mono-
layer

5
There are many types of atomic-scale models
Computationally demanding (high-
performance computers required)
Computationally easy (pen, paper,
pocket calculator needed)
So far at best semi-quantitatively
related to growth characteristics such
as growth per cycle
Target: model quantitatively related
to growth characteristics such as
growth per cycle
A restricted system calculated in
great detail
Relevant system chosen?
A well selected, simplified
phenomenon treated.
Simplifications reasonable?
Not accessible to everyoneAccessible to anyone
Based on solving the Schrödinger
equation highly sophisticated
Based on simple geometrical
assumptions crude
Ab initio models“Ball models”
discussed not discussed

6
Outline
Introduction
Existing models
• Growth per cycle (GPC) models
• Growth mode models (random deposition, island growth)
• Other recent models
What is the use of the models?
• Case 1. Mechanism of Al2O3 ALD (GPC model)
• Case 2. Density of nanometer-thin HfO2 films (growth mode models)
Conclusion

7
Growth per cycle (GPC):
characteristic parameter of an ALD process
Growth per cycle
• defined by the choice of process: reactants,
temperature, substrate
• reactor-independent
Mass
increment
Time
Aarik et al.,
Thin Solid Films 340, 110 (1999).
cycles
growth
per
cycle
Classification:
Puurunen and Vandervorst, J. Appl. Phys. 96, 7686 (2004).

8
(2004), in press.
Growth per cycle (should be) highly reproducible
Al2O3 growth temperature (°C)
AlMe3/H2O
GPC by different groups
within ~10% excellent !
Al atoms
per cycle
[nm-2]
HfO2 growth temperature (°C)
HfCl4/H2O
Hf atoms
per cycle
[nm-2]
Puurunen, J. Appl. Phys. 97, 121301 (2005) - a review
sometimes, scatter (>100%) in
results of different groups.
origin of scatter ???

9
“Ball models” for the growth per cycle
M
L
MLn L
Model 1
further description: Puurunen, J. Appl. Phys. 97, 121301 (2005) - a review
Model 2 Model 3
Ritala et al*, Morozov et al.
*Chem. Mater. 5, 1174
(1993)
Ylilammi
Thin Solid Films 279, 124
(1996)
Siimon and Aarik, Puurunen*
*Chem. Vap. Deposition 9, 249
(2003)

10
Growth per cycle typically
small fraction of a monolayer (ML)
Growth per cycle
• typically ~5-50% of ML
• limited by:
Reactant A,
chemisorbed monolayer
ALD-grown material
number
of reactive
sites?
• Less than ML growth has
consequences to growth mode and
layer characteristics
steric
hindrance?
other
factors?

11
Growth mode defines
how material gets arranged during growth
Two-
dimensional
growth (2d)
Random
deposition
(RD)
cycles
Island growth
(IG)

12
Growth mode models:
“shower model” for random deposition
AlN coverage (monolayers)
Substrate surface fraction
2d
RD
Measured
(LEIS)
Deposition probability
=
growth per cycle (in ML)
Model: Puurunen, Chem. Vap. Deposition 10, 159 (2004).
Application example: Puurunen et al., J. Appl. Phys. 96, 4878 (2004).
Data: Puurunen et al.,
Chem. Mater. 14, 720
(2002).

13
Growth mode models:
island growth model
b • Spherical islands in
square surface lattice
• Random deposition on
the islands
Cycles
Growth
per
cycle
Excel spreadsheet for playing with the parameters:
riikka.puurunen@vtt.fi
Model, application: Puurunen & Vandervorst, J. Appl. Phys. 96, 7686 (2004).
Phase I II III

14
Island growth explains “S-type” growth curves
(substrate-inhibited growth, Type 2)
Al2O3
Si substrate
Si cap
AlMe3 / H2O at 300°C on H-terminated Si
Model, application: Puurunen & Vandervorst, J. Appl. Phys. 96, 7686 (2004).
Data: Puurunen et al., J. Appl. Phys. 96, 4878 (2004).
2 0 0
0
p [
1 0 05 00
Al atoms
deposited
[nm-2]
Cycles
0
1
0 5 10 15
Al2O3 coverage (monolayers)
Substratesurfacefraction
experiment (LEIS)
2d growth
random deposition
island growth
Al2O3 coverage [ML]
Substrate
surface
fraction

15
Other recent models
Alam & Green
J. Appl. Phys. 94, 3403 (2003)
• Model to simulate especially
substrate-inhibited growth curves
of Type 2. Simulation in two parts.
• Follow-up discussion
J. Appl. Phys. 2004, 2005
Kim, Kim & Kang
J. Appl. Phys. 97, 093505 (2005)
• Model to calculate nonlinear
growth curves for multicomponent
thin films from data on binary films
• Similarities with the random
deposition model
Number of cycles
Hfcoverage,Hf/cm2

16
Outline
Introduction
Existing models
• Case 1. Mechanism of Al2O3 ALD (GPC)
• Case 2. Density of nanometer-thin HfO2 films (growth mode)
Conclusion

17
Case 1. Mechanism of Al2O3 ALD:
from where does the growth per cycle originate?
Decrease in GPC caused by
→ change in reaction mechanism
with temperature?
→ change in number of reactive
sites with temperature?
?
bulk reaction:
2 AlMe3 + 3 H2O Al2O3 + 6 CH4
(2004), in press.
Al2O3 growth temperature [°C]
Al atoms
per cycle
[nm-2]
150 250 300200
6
4
2
0
further description: R. L. Puurunen, Appl. Surf. Sci. 245, 6 (2005).
see also: Puurunen, J. Appl. Phys. 97, 121301 (2005) - a review
Al atoms
adsorbed
[nm-2]
AlMe3 reaction temperature [°C]

18
Case 1. Mechanism of Al2O3 ALD?
Effect of OH concentration on adsorbed species
Methyl
groups
adsorbed
[nm-2]
OH groups [nm-2]
• 5-6 nm-2 ~ constant
• theoretical maximum 7.2 nm-2 (Model 3)
self-termination by steric hindrance of
ligands
• Infrared: AlMe3 reacts with ~all OH

19
Case 1. Mechanism of Al2O3 ALD?
Effect of OH concentration on growth per cycle
Mass balance:
[Me] = 3 [Al] – ∆ [OH]
⇒ [Al] = 1/3 [Me] + 1/3 ∆ [OH]
Al2O3 growth temperature [°C]
Al atoms
per cycle
[nm-2]
[Al] = 1.68 + 0.37 [OH]
200°C 9 OH nm-2
300°C 7 OH nm-2
Surface OH group concentration
on alumina or silica [nm-2]
Al atoms
adsorbed
[nm-2]
y = 1.68 + 0.37 x

20
Case 2. Density of
nanometer-thin HfO2 films?
Density
(TEM-
RBS)
[g cm-3]
RBS thickness [nm]
10
6
2
86420
bulk
density
RBS thickness [nm]
Equivalent
oxide
thickness
[nm]
• Why the measured low
density is not reflected in
electrical properties of HfO2?
further description: Puurunen et al., Appl. Phys. Lett. 86, 073116 (2005).

21
Case 2. Origin of TEM-RBS
thickness difference?
HfO2
substrate
mixed layer
HfO2
substrate
substrate
HfO2
substrate
HfO2
ρ analysis ≈ 0 nm
XPS << 0.6 nm
HfO2
substrate
Cl,H
OH OH OH OH
AFM ≈ 2 x 0.2 nm
this case ≈ 0 nm
substrate
HfO2
2 filling layers: ≈ 0.6 nm
TXRF ≈ 0.05 nm
• expected minimum TEM-RBS difference ≈ 1 nm
HfO2
glue
substrate
10 nm
Olivier Richard / IMEC

22
Case 2. Density of
nanometer-thin HfO2 films?
10
8
6
4
2
0
Densityρ
obs
[gcm
-3
]
86420
HfO2 thickness h
RBS
[nm]
0.4
0.6
0.8
1.0
RBS thickness [nm]
Density
(TEM-
RBS)
[g cm-3]
Atomic-scale roughness
difference in TEM, RBS
“low density” always
measured for nanometer-thin
films

23
Outline
Introduction
Existing models
• Case 1. Mechanism of Al2O3 ALD (GPC)
• Case 2. Density of nanometer-thin HfO2 films (growth mode)
Conclusion

24
ALD is fantastic ! But fantastic ≠ perfect
• Atomic-scale ball models help to correlate
growth characteristics with performance
• Few models exist
space for many more models!
• Always needed: carefully obtained and
reproducible experimental data Temperature
GPC
?
• Near future: correlate experiments with atomic scale models
(both “ball models” and ab initio models)
• Predicting the features of new ALD processes—future or
utopia?
M
L
L

25
Thanks to:
• All coauthors
• Past and present colleagues on ALD
• (ASM) Microchemistry
• Fortum Oil and Gas ( Neste Oil)
• Helsinki University of Technology (HUT)
• Interuniversity Microelectronics Centre (IMEC)
• VTT Technical Research Centre of Finland
• Special thanks to
• Tom Schram, electrical data (IMEC)
• Olivier Richard, TEM figure (IMEC)
• Additional financing: TEKES, ALD 2005
RIIKKA PUURUNEN
Research Scientist, Dr.
MEMS Technology
email: riikka.puurunen@vtt.fi
VTT TECHNICAL RESEARCH CENTRE
OF FINLAND
VTT Information Technology
Visiting address: Micronova, Tietotie 3,
Espoo
P.O. Box 1208, FIN-02044 VTT, Finland

26
Appendix 1: Some recent work on ALD modelling (1/2)
• Film growth model of atomic layer deposition for multicomponent thin films
Kim et al., J. Appl. Phys. 97, 093505 (2005).
• Hafnium oxide films by atomic layer deposition for high-κ gate dielectric applications: analysis of the
density of nanometer-thin films
Puurunen et al., Appl. Phys. Lett. 86, 073116 (2005).
• Correlation between the growth-per-cycle and the surface hydroxyl group concentration in the atomic
layer deposition of aluminium oxide from trimethylaluminium and water
Puurunen, Appl. Surf. Sci., 245, 6 (2005).
• Island growth as a growth mode in atomic layer deposition: a phenomenological model,
Puurunen and Vandervorst, J. Appl. Phys. 96, 7686 (2004).
• Island growth in the atomic layer deposition of zirconium oxide and aluminum oxide on hydrogen-
terminated silicon: growth mode modeling and transmission electron microscopy,
Puurunen et al., J. Appl. Phys. 96, 4878 (2004).
• Random deposition as a growth mode in atomic layer deposition,
Puurunen, Chem. Vap. Deposition 10, 159 (2004).
[Correction: Chem. Vap. Deposition 11, 234 (2005).]
• Analysis of hydroxyl group controlled atomic layer deposition of hafnium dioxide from hafnium
tetrachloride and water,
Puurunen, J. Appl. Phys. 95, 4777 (2004).
[Comment: Alam and Green, J. Appl. Phys. 98, 016101 (2005),
Reply: Puurunen, J. Appl. Phys. 98, 016102 (2005).]

27
Appendix 1: Some recent work on ALD modelling (2/2)
• Growth per cycle in atomic layer deposition: real application examples of a theoretical model,
• Growth per cycle in atomic layer deposition: a theoretical model,
[Correction: Chem. Vap. Deposition 10, 124 (2004).]
• Mathematical description of atomic layer deposition and its application to the nucleation and growth of
HfO2 gate dielectric layers
Alam and Green, J. Appl. Phys. 94, 3403 (2003)
[Analysis and application of the model: Puurunen, J. Appl. Phys. 95, 4777 (2004),
Further discussion: J. Appl. Phys. 98, 016101 (2005); J. Appl. Phys. 98, 016102 (2005).]

28
Appendix 2: Selected reviews on ALD (1/2)
• Surface chemistry of atomic layer deposition: a case study for the trimethylaluminum/water process,
Puurunen, J. Appl. Phys. 97, 121301 (2005).
• Formation of metal oxide particles in atomic layer deposition during the chemisorption of metal chlorides:
a review,
Puurunen, Chem. Vap. Deposition, 11, 79 (2005).
• Some recent developments in the MOCVD and ALD of high-k dielectric oxides,
Jones et al., J. Mater Chem. 14, 3101 (2004).
• Advanced electronic and optoelectronic materials by Atomic Layer Deposition: An overview with special
emphasis on recent progress in processing of high-k dielectrics and other oxide materials,
Niinistö et al., Phys. Stat. Solidi A 201, 1443 (2004).
• Atomic layer deposition of metal and nitride thin films: Current research efforts and applications for
semiconductor device processing,
Kim, J. Vac. Sci. Technol., B 21, 2231 (2003).
• Atomic layer deposition chemistry: Recent developments and future challenges,
Leskelä and Ritala, Angew. Chem. Int. Ed. 42, 5548 (2003).

29
Appendix 2: Selected reviews on ALD (2/2)
• Atomic layer deposition,
Ritala and Leskelä, Handbook of Thin Film Materials, Vol. 1, Ed. H.S. Nalwa, Academic Press (San
Diego) 2002, pp. 103-159.
• Adsorption controlled preparation of heterogeneous catalysts,
Haukka et al., Stud. Surf. Sci. Catal. 120A, 715 (1999).
• The chemical basis of surface modification technology of silica and alumina by molecular layering
method,
Malygin et al., Stud. Surf. Sci. Catal. 99, 213 (1996).
• Surface chemistry for atomic layer growth,
George et al., J. Phys. Chem. 100, 13121 (1996).
• Atomic layer epitaxy,
Suntola, Handbook of Crystal Growth, Vol. 3, Ed. D. T. J. Hurle, Elsevier (Amsterdam) 1994, pp.
601-663.
• Atomic layer epitaxy,
Goodman and Pessa, J. Appl. Phys. 60, R65 (1986).

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