Presentation at the AVS 20th International Conference on Atomic Layer Deposition (ALD 2020) featuring the 7th International Atomic Layer Etching Workshop (ALE 2020), online, 29.6.-1.7.2020. Authors: Riikka L. Puurunen and J. Ruud van Ommen Abstract text: Atomic layer deposition (ALD) has become of global importance as a fundamental building block for example in semiconductor device fabrication, and also gained more visibility (e.g., the Millennium Technology Prize 2018). In recent years, the number of ALD processes has increased, new groups have entered the field, and fundamental insights have been gained. At the same time, significantly varying views exist in the field related to the description and meaningfulness of some core ALD concepts. Open, respectful but critical scientific discussion would be needed around these concepts - for example at this AVS ALD/ALE conference, the world’s largest conference on ALD. The discussion on terminology of ALD that started in the 2005 surface chemistry review [1] is continued in this contribution, taking into account recent progress reported in leading reviews such as Ref. 2. We start by considering the concept of “ideal ALD”. How should it be defined so that the well-recognized practical benefits of ALD are maintained, while no unnecessary utopian requirements are created? We propose that the repetition of well-separated saturating, irreversible chemisorption reactions (which by definition saturate at a monolayer of the chemisorbed species) is sufficient to reproduce the benefits of ALD. A requirement of “full monolayer growth” (of the ALD-grown material), progressed e.g. in numerous cartoons of ALD, is not needed. There should also be no reason to expect a constant growth per cycle (GPC) within the ALD window (the saturating chemistry is typically weakly temperature dependent), although such a scheme is repeatedly reproduced in the literature. Other fundamental concepts will be pointed out, where mix-ups have been created. For example, although the GPC (or etch per cycle in Atomic Layer Etching) is a saturation-related concept and not a time-related kinetic parameter, Arrhenius plots have been sometimes created to extract “activation energies” of some process from these “growth/etch rates (per cycle)”. Also, “Langmuir adsorption” has been adopted as a way to model ALD in a simplified, lumped way. Notably, Langmuir adsorption assumes no interaction between adsorbed species, contrasting some recent discussions of “cooperative effects” in ALD. Also, concepts of “adsorption isotherm” and amount adsorbed vs. time (“saturation curve”), although fundamentally different, have been mixed. We hope that the discussion on the fundamentals of ALD will be intensified, and that the discussion will help the field progress and flourish in the future. [1] Puurunen, J. Appl. Phys. 97 (2005) 121301. [2] Richey, de Paula, Bent, J. Chem. Phys. 152 (2020) 040902.

1. Puurunen & Van Ommen, ALD 2020 online conference 29.6.-1.7.2020, pre-recorded poster talk (18.6.2020)
Riikka L. Puurunena
J. Ruud van Ommenb
aAalto University School of Chemical
Engineering;
bDelft University of Technology
On the fundamentals
of ALD: The
importance of getting
the picture right
ALD 2020 online
conference
29.6.-1.7.2020
Goulas, Puurunen, van Ommen 2020

2. Puurunen & Van Ommen, ALD 2020 online conference 29.6.-1.7.2020, pre-recorded poster talk (18.6.2020)
2
EuroCVD 22 - Baltic ALD
16, in Luxembourg,
June 24-28, 2019. Virtual
Project on the History of
ALD (VPHA).
Abstract Poster
2018 Millennium
Technology Prize
Dr. Tuomo Suntola
1974 Suntola Atomic
Layer Epitaxy (ALE)
http://vph-ald.com
aldhistory.blogspot.fi
1965 1st record of
Molecular Layering (ML)
1990 name Atomic
Layer Deposition (ALD)
introduced (?)
2007 Intel
announces ALD
high-k logic chips
1985 ALE-EL display
production
2013 Virtual Project on the
History of ALD launched
2001 first AVS
ALD conference

3. Puurunen & Van Ommen, ALD 2020 online conference 29.6.-1.7.2020, pre-recorded poster talk (18.6.2020)
Critical discussion
on ALD fundamentals
…
Puurunen, Journal of Applied Physics 97 (2005)
121301; https://doi.org/10.1063/1.1940727
II.B Early experiments
(ALE, ML) 
II.C ALD processes 
Citation to the J. Appl. Phys. 2005 review added in a recent erratum on a review article on the history of ALD:
“Erratum: “History of atomic layer deposition and its relationship with the American Vacuum Society” [J. Vac. Sci. Technol. A 31, 050818 (2013)]”
Journal of Vacuum Science & Technology A 38, 037001 (2020); https://doi.org/10.1116/6.0000143
Richey, de Paula, Bent,
Understanding chemical and
physical mechanisms in atomic
layer deposition,
Journal of Chemical Physics 152
(2020) 040902;
https://doi.org/10.1063/1.5133390
Sønsteby, Yanguas-Gil, Elam,
Consistency and reproducibility in
atomic layer deposition,
Journal of Vacuum Science &
Technology A 38 (2020) 020804;
https://doi.org/10.1116/1.5140603
https://www.atomiclimits.com/ald
database/


3

4. Puurunen & Van Ommen, ALD 2020 online conference 29.6.-1.7.2020, pre-recorded poster talk (18.6.2020)
Outline
• Introduction: ALD growth & fundamental discussion
• ALD cartoons over the years
• What is ideal ALD?
• ALD (temperature) window?
• Adsorption isotherm vs. saturation with time
• Growth per cycle (GPC) is not a rate
• Conclusion
4

5. Puurunen & Van Ommen, ALD 2020 online conference 29.6.-1.7.2020, pre-recorded poster talk (18.6.2020)
ALD cartoons over the years
https://openlearning.aalto.fi/mod/page/view.php?id=7632; ALD-cartoons-evolving-file
Koltsov, 1971 Suntola et al., 1980 Puurunen, 2005
Parsons et al., 2011
Leskelä & Ritala, 1995
George, 2010 Wikipedia, 2014 Knoops et al., 2015
5

6. Puurunen & Van Ommen, ALD 2020 online conference 29.6.-1.7.2020, pre-recorded poster talk (18.6.2020)
Ideal ALD – one monolayer per cycle?
https://commons.wikimedia.org/wiki/Category:Tuomo_Suntol
a#/media/File:Reconstruction_of_the_first_atomic_layer_epit
axy_experiment_by_Tuomo_Suntola.jpg
6
Puurunen, Chem. Vap. Dep. 20 (2014) 332;
https://doi.org/10.1002/cvde.201402012

7. Puurunen & Van Ommen, ALD 2020 online conference 29.6.-1.7.2020, pre-recorded poster talk (18.6.2020)
https://en.wikipedia.org/wiki/Atomic_layer_dep
osition#/media/File:ALD_schematics.jpg
“Full monolayer growth”, progressed e.g. in
numerous cartoons of ALD, is an exception.
Ideal ALD – one monolayer per cycle?
7
Puurunen, Chem. Vap. Dep. 20 (2014) 332;
https://doi.org/10.1002/cvde.201402012

8. Puurunen & Van Ommen, ALD 2020 online conference 29.6.-1.7.2020, pre-recorded poster talk (18.6.2020)
https://en.wikipedia.org/wiki/Atomic_layer_dep
osition#/media/File:ALD_schematics.jpg
“Full monolayer growth”, progressed e.g. in
numerous cartoons of ALD, is an exception.
Ideal ALD – one monolayer per cycle?
8
Puurunen, Chem. Vap. Dep. 20 (2014) 332;
https://doi.org/10.1002/cvde.201402012

9. Puurunen & Van Ommen, ALD 2020 online conference 29.6.-1.7.2020, pre-recorded poster talk (18.6.2020)
Ideal ALD - saturating reactions sufficient?
Adapted from: Puurunen, J. Appl. Phys. 97 (2005) 121301;
https://doi.org/10.1063/1.1940727
Saturating, irreversible
reaction (chemisorption)
Fully reversible
(chemisorption or
physisorption)
Saturating, irreversible
reaction, not saturated
Combination of (1) saturating,
irreversible reaction and (2)
reversible physisorption
Partly reversible chemisorption
“ideal ALD”
Not saturating
(continuous CVD)
amount
time
pulse purge
9

10. Puurunen & Van Ommen, ALD 2020 online conference 29.6.-1.7.2020, pre-recorded poster talk (18.6.2020)
What is an ALD (temperature) window?
Growth/cycles
Process temperature
ALD window
L1
L2
H1
H2
See also: Sønsteby, Yanguas-Gil, Elam, JVSTA
38 (2020) 020804; https://doi.org/10.1116/1.5140603
10

11. Puurunen & Van Ommen, ALD 2020 online conference 29.6.-1.7.2020, pre-recorded poster talk (18.6.2020)
Growth/cycles
Process temperature
ALD window
L1
L2
H1
H2
GPC
Temperature Temperature
GPC
What is an ALD (temperature) window?
11
Adapted from: Suntola, ”Atomic Layer Epitaxy”, Mater. Sci. Reports 4 (1989) 261-362;
https://doi.org/10.1016/S0920-2307(89)80006-4
Puurunen, J. Appl. Phys. 97 (2005) 121301; https://doi.org/10.1063/1.1940727

12. Puurunen & Van Ommen, ALD 2020 online conference 29.6.-1.7.2020, pre-recorded poster talk (18.6.2020)
Equilibrium vs. accumulation
“… were fit using modified Langmuir adsorption isotherm
expressions where the pressures are replaced with exposures.”
J. Phys. Chem. A 114 (2010) 114; http://doi.org/10.1021/jp9049268
https://youtu.be/KddVM4nxgwIDerivation of the Langmuir
adsorption isotherm - video 12

13. Puurunen & Van Ommen, ALD 2020 online conference 29.6.-1.7.2020, pre-recorded poster talk (18.6.2020)
Growth per cycle (GPC) is not
a (kinetic) reaction/growth rate
http://aldhistory.blogspot.com/2016/10/term-growth-per-cycle-gpc-gaining-use.html, accessed 16.6.2020
https://en.wikipedia.org/wiki/Arrhenius_plot, accessed 16.6.2020
J. Appl. Phys. 97
(2005) 121301;
https://doi.org/10.
1063/1.1940727
GPC
ln(k)
1 / T
Arrhenius equation & plot:
13

14. Puurunen & Van Ommen, ALD 2020 online conference 29.6.-1.7.2020, pre-recorded poster talk (18.6.2020)
Conclusion: The importance
of getting the picture right
“Everything should be
made as simple as possible,
but not simpler”
A. Einstein
14
Goulas, Puurunen, van Ommen 2020

15. Puurunen & Van Ommen, ALD 2020 online conference 29.6.-1.7.2020, pre-recorded poster talk (18.6.2020)
Acknowledgements
• Aris Goulas for patient improving of the ALD cartoon
• Many people in the ALD community for critical discussion
Riikka Puurunen
• https://research.aalto.fi/en/persons/riikka-
puurunen/
• Twitter: @rlpuu
• https://www.slideshare.net/RiikkaPuurunen
Ruud van Ommen
• https://www.tudelft.nl/tnw/over-
faculteit/afdelingen/chemical-engineering/principal-
scientists/ruud-van-ommen/
• Twitter: @JRvanOmmen
15

16. Puurunen & Van Ommen, ALD 2020 online conference 29.6.-1.7.2020, pre-recorded poster talk (18.6.2020)
Title: On the fundamentals of ALD: the importance of getting the picture right,
Authors: Riikka L. Puurunen and J. Ruud van Ommen
Session: Precursors and Chemistry: Simulation, Modeling, and Theory of ALD
Abstract Text:
Atomic layer deposition (ALD) has become of global importance as a fundamental building block for example in semiconductor device fabrication, and
also gained more visibility (e.g., the Millennium Technology Prize 2018). In recent years, the number of ALD processes has increased, new groups have
entered the field, and fundamental insights have been gained. At the same time, significantly varying views exist in the field related to the description
and meaningfulness of some core ALD concepts. Open, respectful but critical scientific discussion would be needed around these concepts - for
example at this AVS ALD/ALE conference, the world’s largest conference on ALD.
The discussion on terminology of ALD that started in the 2005 surface chemistry review [1] is continued in this contribution, taking into account recent
progress reported in leading reviews such as Ref. 2. We start by considering the concept of “ideal ALD”. How should it be defined so that the well-
recognized practical benefits of ALD are maintained, while no unnecessary utopian requirements are created? We propose that the repetition of well-
separated saturating, irreversible chemisorption reactions (which by definition saturate at a monolayer of the chemisorbed species) is sufficient to
reproduce the benefits of ALD. A requirement of “full monolayer growth” (of the ALD-grown material), progressed e.g. in numerous cartoons of ALD, is
not needed. There should also be no reason to expect a constant growth per cycle (GPC) within the ALD window (the saturating chemistry is typically
weakly temperature dependent), although such a scheme is repeatedly reproduced in the literature.
Other fundamental concepts will be pointed out, where mix-ups have been created. For example, although the GPC (or etch per cycle in Atomic Layer
Etching) is a saturation-related concept and not a time-related kinetic parameter, Arrhenius plots have been sometimes created to extract “activation
energies” of some process from these “growth/etch rates (per cycle)”. Also, “Langmuir adsorption” has been adopted as a way to model ALD in a
simplified, lumped way. Notably, Langmuir adsorption assumes no interaction between adsorbed species, contrasting some recent discussions of
“cooperative effects” in ALD. Also, concepts of “adsorption isotherm” and amount adsorbed vs. time (“saturation curve”), although fundamentally
different, have been mixed.
We hope that the discussion on the fundamentals of ALD will be intensified, and that the discussion will help the field progress and flourish in the future.
[1] Puurunen, J. Appl. Phys. 97 (2005) 121301.
[2] Richey, de Paula, Bent, J. Chem. Phys. 152 (2020) 040902.
16