EuroCVD-16-2007_Puurunen

Understanding the surface chemistry
of ALD:
Achievements and challenges
Riikka Puurunen
VTT Technology Development Centre of Finland

Riikka Puurunen, EuroCVD-16, 17 Sept 2007
2
• “The overall mechanism of deposition is
extremely easy to understand.
Surface O-H groups react with TMA … This
leaves a methylated Al atom behind, free
to react with H2O during the next vapor pulse …”
• “A detailed understanding of growth process, however,
requires that one explain the anomalously low growth rate …
1.1 Å … approximately half of a monolayer of Al2O3 ...
This result is not understood at the present time.”
Higashi & Fleming, 1989
Growth of Al2O3 by the AlMe3/H2O ALD process
Appl. Phys. Lett. 55 (1989) 1963-1965.
Al
MeMe
Me
H2O
Al2O3

3
Outline
1. Introduction
definition of ALD, ALD vs. CVD, growth per cycle, growth mode
2. Achievements: the AlMe3/H2O ALD process
growth per cycle in the constant growth regime
in the light of reaction mechanism studies
3. Challenges
six open questions on the surface chemistry of ALD
4. Conclusion

4
Outline
1. Introduction
3. Challenges
4. Conclusion

5
ALD cycles
“Definition of ALD”
“A film deposition technique that is based on the sequential
use of self-terminating gas−solid reactions”
Puurunen, 2005, J. Appl. Phys.
ALD cycle
Substrate
before ALD
Step 2 /4
Step 4 /4
Step 1 /4
Step 3 /4
purge
purge
Mass
deposited

6
ALD vs. the general scheme of CVD
• ALD, additional requirement:
self-terminating reactions
x
in ALD
no gas phase reactions allowed
separate pulsing
of precursor
vapors
General CVD:
saturating
irreversible
pulse purge

7
Growth per cycle in ALD
• GPC almost always < monolayer
as function of temperatureas function cycle number
ALD cycles
GPC
ALD cycles
GPC
ALD cycles
GPC
ALD cycles
GPC
Temperature
GPC
Temperature
GPC
Temperature
GPC
Temperature
GPC
Type I Type II

8
Different growth modes in ALD
• layer-by-layer growth (a)
• island growth (b)
• random growth (c)
• island + layer-by-layer
• layer-by-layer + island
• …
BeslingBesling et al., J. Nonet al., J. Non--CrystCryst. Solids 303 (2002) 123. Solids 303 (2002) 123--133133
(InAs)1 – (GaAs)5 superlattice
UsuiUsui, Proc. IEEE, 80 (1992) 1641, Proc. IEEE, 80 (1992) 1641
on H-terminated Si

9
Outline
1. Introduction
3. Challenges
4. Conclusion

10
AlMe3/H2O to Al2O3:
growth per cycle vs. temperature
• Al2O3 3.5 g cm-3 1 monolayer = 0.29 nm = 12.0 Al nm-2
0
0.05
0.1
0.15
0 100 200 300
Al2O3 ALD temperature (°C)
Growthpercycle(nm)
Ott1997a
Matero2000
Putkonen2004
VTT, 2005
0
0.05
0.1
0.15
0 100 200 300
Growthpercycle(nm)
Ott1997a
Matero2000
Putkonen2004
VTT, 2005
VTT repeats
saturatedunsaturated
0
1
2
3
4
5
6
0 100 200 300
Al2O3 ALD temperature (°C)Growthpercycle(Alnm
-2
)
Ott1997a
Matero2000
Puurunen2001a
Jensen2002
Putkonen2004
Puurunen2004c
VTT, 2005
all saturated

11
Qualitative: reaction pathways, AlMe3 on oxides
(data: IR, NMR, MS, QCM, element analysis, modelling)
methane
released
all parts
bonded
CH4
ligand exchange reaction
dissociation, association
M
OH
Me3Al
M
O
Me Me
Al
Me3Al
M
O
M M
O
Me Me
Al
M
Me
M
Me
Me
Al
M
Me
O
M
O
Me
Al
M
O

0
1
2
3
4
5
6
0 100 200 300
TMA reaction temp. (°C)
Content(nm
-2
)
Me Al
12
Quantitative:
AlMe3 on oxides with controlled OH concentration
• Reaction temperature has
no effect
• Substrate heat-treatment temperature
affects strongly
560°C alumina,
3.4 OH/nm2
0
2
4
6
8
10
0 500 1000
Heat treatment (°C)
Content(nm
-2
)
OH Al
0
2
4
6
8
10
0 500 1000
Heat treatment (°C)
Content(nm
-2
)
OH Al
alumina silica
Puurunen, J. Appl. Phys. 97 (2005) 121301, and the refs. therein

0
2
4
6
8
0 2 4 6 8 10
OH concentration (nm
-2
)
Meadsorbed(nm
-2
)
13
Quantitative:
AlMe3 on oxides with controlled OH concentration
• 5-6 Me nm-2, non-ordered
rather tightly packed
y = 0.37x + 1.68
R
2
= 0.97
0
2
4
6
0 2 4 6 8 10
-2
)
Aladsorbed(nm
-2
)
theor. max
7.2 nm-2
5.0 nm-2

14
Quantitative: [Al] vs. [OH]
mass balance:
[Me] = 3 x [Al] - ∆[OH]
3
]Me[
]OH[
3
1
]Al[ +∆=⇒
experiment:
[Al] = 0.37 [OH] + 1.68
Agreement with (1) complete reaction with OH’s
(2) additionally dissociation/association
(3) until reaction stops by steric hindrance
5-6 nm-2
y = 0.37x + 1.68
R
2
= 0.97
0
2
4
6
0 2 4 6 8 10
-2
)
Aladsorbed(nm
-2
)

15
AlMe3/H2O to Al2O3:
growth per cycle vs. temperature
• trend & absolute values explained, first time for ALD (?)
• published in 2005, not once cited
not noticed / disagreed on / regarded insignificant???
Puurunen, Appl. Surf. Sci. 245 (2005) 6
0
1
2
3
4
5
6
0 100 200 300
Growthpercycle(Alnm
-2
)
Ott1997a
Matero2000
Puurunen2001a
Jensen2002
Putkonen2004
Puurunen2004c
VTT, 2005
[Al] = 1.68 + 0.37[OH]

16
Outline
1. Introduction
3. Challenges
4. Conclusion

(a) Which surface sites react with a given reactant?
17
Challenge 1.
Qualitative understanding of the reactions
OH O NH2 N
(b) Surface species after the reaction?
OH almost always assumed, are there others?
SH S HNH
etc
L L
M L
L
L
M L L L LL
LM
L
M
etc

(a) What is the real value of the GPC at different temperatures?
Data by different groups
(b) How is the GPC related to the no. of reactive sites on the
surface
Experiments on
controlled substrates
Temperature
GPC
18
Challenge 2.
Quantitative understanding of the reactions
Temperature
HfO2
GPC
?
Temperature
Al2O3
GPC
OH
GPC
Al2O3
GPC
OH content

19
Challenge 3.
Kinetics of ALD reactions
• Description of reaction rates
- classical chemical kinetic reaction rate models?
• ALD reactions regarded “fast”, and mostly, mass transport
defines cycle times
• Rapidity & (assumed) irreversibility make performing kinetic
measurements difficult
• Even relative data on the rates of parallel/succeeding
reactions would be relevant
e.g., AlMe3 ligand exchange vs. dissociation

20
Challenge 4.
Growth mode in ALD processes on given substrates
• “Two-dimensional growth” almost never demonstrated
although often assumed
• Monolayer-sensitive techniques (e.g. LEIS) need to be
employed for studying the first cycles & tens of cycles
Puurunen et al., J. Appl. Phys. 96 (2004) 4878
ZrO2 ALD on H-terminated Si
0
0.5
1
0 50 100
ALD cycles
Surfacefraction
2-d
RD
LEIS
ZrO2
surface
fraction

21
Challenge 5. Demonstrating quantitative relationship
between computational chemistry and experiments
• lots of ALD computational chemistry studies have been
contributed to qualitative understanding
• before computational chemistry can forecast the future,
it must be able to reproduce the present
0
2
4
6
0 100 200 300
ALD temperature (°C)
GPC(Alnm
-2
)
Al2O3
GPC
(Al nm-2)

22
Challenge 6.
Calculating the sizes of complex ligands
• many simple models can
assist in interpretations of
ALD growth data
• info needed: ligand sizes
Model II
Chemisorbed MLz
M
L
MLn
Model III
Ligand L
L
Model I
Reactant MLn
Puurunen, J. Appl. Phys. 97 (2005) 121301 ( a review)
N
N
M
O
M
M
O
M
O
O
M
O
M
O
O
M
M
O
NO
M
O
M M M M M
MM
M
M
MM
Organometallic
M M
Si
M
Metalorganic
O
M
O
N
M
O
H
O
M
O
F
FF
F
F
F
O
M
O
O
M
O
N
M
N
M
N
M
N
M
N
Si
M
Si M N
N
O
H
O
S N
S
M
O
M
O
O
O
N
NM
N
NM

23
Outline
1. Introduction
3. Challenges
4. Conclusion

24
Conclusion
• Higashi, Fleming (1989) questions on the surface chemistry of
AlMe3/H2O process to deposit Al2O3: now answered
• With current techniques, we should be able to answer many
similar questions -- are we still asking the questions?
My wish list:
• The 6-point list of open questions helps one to ask the relevant
questions and to find answers
• Ligand sizes calculated & published within 1-3 years?

25
Acknowledgements
• VTT Technical Research Center of Finland
• Tekes Finnish Funding Agency for Technology and Innovation
• Picosun
• IMEC
• TKK Helsinki University of Technology
• Fortum Oil & Gas (Neste Oil)
• ASM Microchemistry

EuroCVD-16-2007_Puurunen

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