Invited talk given at the Workshop on Fundamentals of Atomic Layer Deposition (ALD): Modelling and ValidationTU Delft, The Netherlands, July 3, 2019. Talk was recorded by TU Delft staff and is to be shared later. Website: https://www.tudelft.nl/en/faculty-of-applied-sciences/about-faculty/departments/chemical-engineering/scientific-staff/van-ommen-group/workshop-fundamentals-of-ald/. Twitter hashtag: #ALDfun

1. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Surface coverage
in ALD
Prof. Riikka Puurunen,
Aalto University, Finland
Workshop on Fundamentals of Atomic Layer
Deposition (ALD): Modelling and Validation
TU Delft, The Netherlands, July 3, 2019
aalto.fi/cmet/catalysis; @rlpuu

2. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
EuroCVD 22 - Baltic ALD
16, in Luxembourg,
June 24-28,
2019. Abstract. Poster.
2002 Doctoral thesis, Helsinki
University of Technology, 2002
  Cartoon model of ALD GPC,
Chem. Vap. Deposition
2005 ALD review: history, processes &
Me3Al-H2O surface chemistry case study,
J. Appl. Phys. (Appl. Phys. Rev.)
>1400 time cited
2015 Microscopic conformality test structures
 pillarhall.com;
2019 ALD conformality review App. Phys. Rev.
2013 ALD review, process update + crystallinity,
J. Appl. Phys. (Appl. Phys. Rev.)
2013 Virtual Project on the History of ALD
(VPHA),  vph-ald.com; review 2016
2018 Millennium Technology Prize
1974 Suntola ALE
http://vph-ald.com
Aldhistory.blogspot.fi
2

3. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Outline
1. ALD introduction
+ Why TMA + Al2O3?
2. Surface coverage
-- (so) many definitions
3. Growth-Per-Cycle (GPC) in ALD:
Cartoon model(s)
4. GPC in ALD: Model validation
5. Some terminology discussion
6. Conclusion
Contains
“Twitter portals”
to click!
3

4. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
ALD introduction
+ Why TMA & Al2O3?
4

5. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Atomic layer deposition (ALD)
ALD cycle
Reactant A
Reactant B
By-product
Substrate
before ALD
Step 2 /4
purge
Step 4 /4
purge
Step 1 /4
Reactant A
Step 3 /4
Reactant B
Reactant A
Reactant B
By-product
(Scheme: Puurunen)
George, Chem. Rev. 110 (2010) 111–131.
DOI: 10.1021/cr900056b
 Time 
Most cited ALD
review of all times
5

6. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
ALD
materials
by two-
reactant
processes
> 700 processes
”Periodic table of ALD processes”: 1st ed.: Puurunen, J. Appl. Phys. 97 (2005) 121301. DOI: 10.1063/1.1940727
2nd ed.: Miikkulainen, Leskelä, Ritala, Puurunen, J. Appl. Phys. 113 (2013) 021301. DOI: 10.1063/1.4757907.
Web-based voluntary updating in TU Eindhoven-led initiative in: https://www.atomiclimits.com/alddatabase/
https://www.atomiclimits.com/alddatabase/,
62005 Puurunen review2013 update+crystallinity

7. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
>700 ALD reactant - co-reactant pairs
7
H2O
NH3
H2S
Non-metal co-reactants, “thermal” ALD
Energy-enhanced ALD
O2
N2
H2
Metal precursor type
Elements
Halides
Alkyls
Cyclopentadienyls
Alkoxides
b-diketonates
Alkylamides and
silylamides
Amidinates
InorganicMetal-organic
Organo-
metallic
Class
N
NM
N
M
O
M
O
O
M
M
M
M
Cl
M
etc
etc
Puurunen, Appl. Phys. 97 (2005) 121301. https://doi.org/10.1063/1.1940727
Miikkulainen, Leskelä, Ritala, Puurunen, J. Appl. Phys. 113 (2013) 021301. http://doi.org/10.1063/1.4757907.
O3
…
Metal reactant type
2005 Puurunen review2013 update+crystallinity

8. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
In ALD, gas-solid reactions ideally:
saturating & irreversible*
amount adsorbed saturates
amount adsorbed stays
pulse purge
Definition of ”ideal ALD”, especially for modelling purposes:
Puurunen, J. Appl. Phys. 97 (2005) 121301. DOI:10.1063/1.1940727
*chemisorption
non-saturationunsaturationdesorption
NO:
YES:
?
partial reversibility?
82005 Puurunen review

9. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Concept of “ALD window”:
where ALD conditions fulfilled
92005 Puurunen review
Simplified
George, Chem. Rev. 110 (2010) 111; DOI: 10.1021/cr900056b. Adapted
from: Suntola, Atomic Layer Epitaxy. In Handbook of Crystal Growth, Vol.
3, Part B, Hurle, D. T. J., Ed.; Elsevier: Amsterdam, 1994; Chapter 14.

10. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
ALD: Cycle
dependency
Puurunen & Vandervorst, JAP 2004, https://doi.org/10.1063/1.1810193
Some growth modes
Steady regime: GPC ~constant
102005 Puurunen review

11. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Steady regime: GPC ~constant
“Monolayer” / (“atomic layer”) growth?
R. L. Puurunen, ” A
Short History of Atomic
Layer Deposition:
Tuomo Suntola's
Atomic Layer Epitaxy”,
Chem. Vap. Deposition
20 (2014) 332-344;
DOI:cvde.201402012
(open access)
Zn + S
 ZnS
Me3Al/H2O  Al2O3
112005 Puurunen review
~1/3 ML

12. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Prom Puurunen, Can. Chem. Conf., Ottawa, June 14, 2015 (in SlideShare)
2015
12

13. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
y = 0.37x + 1.68
R2
= 0.97
0
2
4
6
0 2 4 6 8 10
OH concentration (nm-2
)
Aladsorbed(nm
-2
)
Why TMA-water? Archetype & benchmark
• Near-ideal; surface chemistry
comprehensively reviewed (2005)
• Wide ALD window: r.t. to ~ 300C
• Recent development e.g. saturation profile
2005 Puurunen review2013 update+crystallinity2019 conformality review 13
Example image: Arts et al. J. Vac. Sci. Technol. A 37 (2019) 030908;
DOI:10.1116/1.5093620
Puurunen, Appl. Surf. Sci. 245 (2005) 6.
y = ax + b,
a ≠ 1, b > 0
1:1

14. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Surface coverage
- (so) many definitions
14

15. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Adsorption
Physisorption
• non-specific
• minimal electronic interaction
• chemical nature of the adsorbate not
altered
• adsorption energy similar to the energy
of condensation (exothermic)
• non-activated
• equilibrium is established
• multilayers may form
Chemisorption
• chemical specificity
• changes in electronic state
• reversible/irreversible
• chemisorption energy as for a chemical
reaction (exothermic/endothermic)
• often involves an activation energy
• for “large” activation energies (“activated
adsorption”), true equilibrium may be
achieved slowly
• monolayer adsorption
http://old.iupac.org/reports/2001/colloid_2001/manual_of_s_and_t/node16.html, accessed 2.7.2019
Ideal ALD: sequential use of self-terminating
gas–solid reactions (i.e., chemisorption)
152005 Puurunen review

16. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Coverage & monolayer:
so many definitions!
Chemisorbed ML, Q
Physisorbed ML, 
ML of the ALD-grown
material, 
*, , Q, …
* units? none; nm-2
162005 Puurunen review

17. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Different uses for the definitions
Chemisorption: coverage Q
at saturation unity by definition
Physisorption: Estimate steric hindrance
Material monolayer:
ALD sanity check
(GPC < 1 ML);
process throughput
172005 Puurunen review
Ideal ALD: irreversible chemisorption
(Model III)

18. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Coverage - one more definition:
GPC in saturation profile modelling
• Rose & Bartha 2009  “The density of reactive sites
equals the density of deposited <metal> atoms
divided by the number of deposition cycles.”
• Ylilammi et al. 2018  Adsorption density q defined
via gpcsat
• Arts et al. 2019: “… the surface coverage θ is defined
as the reacted fraction of available reaction sites, in
such a way that θ = 1 in saturation”… “… average
area … of an adsorption site … can be calculated
from the growth per cycle.”
Rose & Bartha, Appl. Surf. Sci. 255 (2009) 6620; DOI:10.1016/j.apsusc.2009.02.055
Ylilammi et al. J. Appl. Phys. 123 (2018) 205301; DOI:10.1063/1.5028178
Arts et al. J. Vac. Sci. Technol. A 37 (2019) 030908; DOI:10.1116/1.5093620
18
pillarhall.com

19. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Saturation profile modelling:
 lumped sticking probability
19
https://en.wikipedia.org/wiki/Langmuir_adsorption_
model#/media/File:Langmuir_Adsorption_Model.jpg
Ylilammi, Ylivaara, Puurunen, J. Appl. Phys. 123, 205301 (2018); DOI: 10.1063/1.5028178
Arts et al. J. Vac. Sci. Technol. A 37 (2019) 030908; DOI:10.1116/1.5093620
https://en.wikipedia.org/wiki/Langmuir_adsorption_model, accessed 13.9.2018
• Flat surface & isothermal conditions
• Surface sites are equal
• Adsorbed species do not interact
• Adsorption & desorption are elementary processes
Cremers, Puurunen, Dendooven, Appl. Phys. Rev. (2019) in press, DOI: 10.1063/1.5060967
AssociationAssumed: Langmuir adsorption

20. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Chemisorbed ML
Physisorbed ML
ML of the ALD-
grown material
(d) “Sticking coefficient and
GPC-related monolayer
& coverage” NEW!
Coverage & monolayer:
so many definitions!
… and there are more …
20(a)-(c): 2005 Puurunen review

21. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Growth-Per-Cycle (GPC) in
ALD: Cartoon model(s)
21

22. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
ALD GPC cartoons
FIG. 13. Schematic illustration for analyzing
sterically hindered chemisorption
• on the basis of the size of the MLn
reactant Model I by Ritala et al. Refs. 462
and 468 and Morozov et al. Ref. 133,
• the size and geometry of the chemisorbed
MLz species Model II by Ylilammi Ref.
1127, and
• the size and number of ligands L Model III
by Siimon and Aarik Ref. 432 and
Puurunen Ref. 1128.
Left: side view, right: top view.
222005 Puurunen review
Model III: Ligands L

23. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
The ALD Model:
background
Fig. 11, Puurunen, J. Appl.
Phys. 97 (2005) 121301.
https://doi.org/10.1063/1.19
40727 open access pdf
Dissociation
L
Reaction:
MLn + surface
L/M < n
L/M = n
L/M = n
2005 Puurunen reviewPuurunen, CVD 9 (2003) 249; DOI:10.1002/cvde.200306265

24. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
The ALD Model (part 1 of 2)
1. Mass balance for L for MLn
chemisorption
• ligands can be lost via ligand
exchange with “-a”:
24
2. Ligand size (nm2)
•  Maximum packing density
Puurunen, Chem. Vap. Deposition 9 (2003) 249; DOI:10.1002/cvde.200306265
 max

25. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
3. Know / assume the
reaction mechanism / cause of
saturation
• Fixed or flexible L/M ratio?
• Ligand exchange /
association-dissociation?
4. Know / assume
number of “-a” sites (nm-2)
• Which fraction reacts?
The ALD model (part 2 of 2)
Puurunen, CVD 2003; https://doi.org/10.1002/cvde.200306265
25

26. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Example 1: ML4 + –a  -ML3 + gas (L/M = 3)
• Ligand
exchange
with one -a
• Steric
hindrance
or –a
concentrati
on limits
Puurunen, Chem. Vap. Deposition 9 (2003) 249; https://doi.org/10.1002/cvde.200306265
GPC in ML
26

27. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Example 4: ML4 + –a  -MLx + gas (L/M varies)
• All –a
consumed
in ligand
exchange
• Further
association/
dissociation
• Steric
hindrance
limits
Puurunen, Chem. Vap. Deposition 9 (2003) 249; https://doi.org/10.1002/cvde.200306265
GPC in ML
27

28. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
GPC in ALD: Model validation
28

29. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Example: HfCl4
• Amount of Hf adsorbed is
used to measure (“low”)
surface –OH group
concentration
• Soethoudt/IMEC, at
#CVDALD2019
• Reaction with one -OH
• cL
maxthr = 8.8 Cl/nm2
(Cl ionic radius used?)
29
Nyns et al., J. Electrochem. Soc. 155 (12) (2008) G269; DOI: 10.1149/1.2980427
Analogy: Example 1

30. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Example: Me3Al + H2O  Al2O3
• ~All OH groups react
• Steric hindrance
cL
maxthr = 7.2 Me/nm2
(van der Waals radius),
 max = 0.7
L/M
Ligand
exchange
Dissociation/
association
302005 Puurunen review
Puurunen, Appl. Surf. Sci. 245 (2005) 6-10. DOI:10.1016/j.apsusc.2004.10.003
Analogy: Example 4

31. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Some terminology discussion
31

32. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Some “ALD window” discussion
• Barry, 2018: “I belive that some processes show an
#ALDep window, I just don't believe it is a necessary
condition for #ALDep to occur”
• Yanguas-Gil, 2018: “I object to the idea that
stability with temperature of the growth per cycle
is a property of self-limiting processes.”
• Elliott, 2018: “… the max coverage is 100%, so above
a sufficiently high temperature, GPC=const... ignoring
non-ALD reactions of course.”
• Puurunen, 2019: “... The sources … which presented
#ALDwindow as a region of constant GPC, pointed as
reference to https://doi.org/10.1021/cr900056b … -
now the most cited #ALDep review of all times …
George 2010, Chem. Rev., DOI:10.1021/cr900056b2010
Over-
simplified?
32

33. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
ALD window discussion:
already since 2005
RLP’s shared Google file: ALD-window-historical-evolution
332005 Puurunen review

34. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Surface chemistry of TMA-water process
Puurunen:
• Different works/reviews present substantially different conclusions on the surface
chemistry. See e.g. TSF 2014, DOI:10.1016/j.tsf.2013.11.112 98th CCC 2015,
SlideShare
Vandalon & Kessels, JVSTA 2017, DOI:10.1116/1.4993597:
• ” … the self-limiting behavior in the TMA half-cycle cannot be caused by steric
hindrance.” “The absolute –CH3 densities reached after the TMA half-cycle, <6 nm-2
…, are not high enough for steric hindrance””
Adomaitis, at EuroCVD-BalticALD (#CVDALD2019), unpublished:
34

35. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Saturation profile (TMA-water)
“ALD measurable” (Yanguas-Gil, 2017)
 Fingerprint saturation profile (?)
* Contains maximum information
Ylivaara, Yim, Ylilammi, Utriainen,
Puurunen, unpublished material,
manuscript in preparation
Example image: Arts et al. J. Vac. Sci. Technol. A 37 (2019) 030908;
DOI:10.1116/1.5093620
Normalized saturation profile ( coverage)
* Enables extraction of lumped sticking
coefficient, but:
* Effect of e.g. temperature on GPC lost
pillarhall.com
35

36. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Conclusion
36

37. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Conclusion
1. Surface coverage(s) in ALD
central for describing surface
chemistry. Many definitions
may cause confusion
2. “Cartoon” / “ball” models help
to see the big picture
3. TMA-water ALD process needs
better understanding of surface
chemistry
37

38. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Thank you! Discussion?
• Thanks to Dr. Fatemeh Hashemi for organizing the workshop &
Prof. Ruud van Ommen for inviting me to visit TU Delft
”All men by nature desire to know”
Tuomo Suntola Aristotle
#SuntolaQuote
38
2018
Millennium
Technology Prize

39. Puurunen, ALD fundamentals workshop, Delft 3.7.2019
Additional materials
Web of Knowledge
Puurunen RL citation analysis
with papers indicated
39
2005 Puurunen review
2013 update+crystallinity
2019 conformality review
2003 Cartoon model

40. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Web of Knowledge, 30.6.2019
Web of Knowledge citation analysis, 30.6.2019, for author: Puurunen RL
2003 Cartoon
model of ALD
GPC
Island growth model
40
2005 Puurunen
review (APR)
2013 update +
crystallinity
(APR)
2019
conformality
review (APR)
New!
Appl. Phys. Rev. :
https://doi.org/10.1063/1.50
60967

41. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Web of Knowledge, 30.6.2019
Island growth model: cases
Cartoon model GPC: cases
Cartoon model GPC: case
41

42. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Web of Knowledge, 30.6.2019
Cartoon model GPC: case
Surface chemistry case
TMA on alumina
TMA on silica
“non-growth ligand exchange”
42

43. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 Web of Knowledge, 30.6.2019
Random deposition model
43

44. Puurunen, ALD fundamentals workshop, Delft 3.7.2019 44
• Puurunen @rlpuu, 2017:
“Haukka's ending slide: great
#ALDep cartoon from years
ago. But who is the original
artist? Can Twitter find out?
#ALDALE2017
#RealTimeChem”
• Twitter found out!
 By: Nick Kim,
http://www.lab-initio.com
ALD cartoons!
& The power of Twitter!