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442
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Area selective deposition using alternate
deposition and etch super-cycle strategies
Marceline Bonvalot,*a,b Christophe Vallée, *a,c Cédric Mannequin,b
Moustapha Jaffal,a Rémy Gassilloud,d Nicolas Posséméd and Thierry Chevolleaua
Area selective deposition (ASD) is a bottom-up process leading to a uniform deposition in only desired
areas of a patterned substrate, avoiding the use of photolithography for patterning. However, whatever
the strategy used to develop selective deposition by atomic layer deposition, there always comes a time
when selectivity becomes defective and growth in undesired substrate areas must be corrected. This
leads to the design of ASD by super-cycle alternating deposition and etch. Recent examples from the literature show a great diversity in the design of the etching step and indicate that the optimization of selective deposition by super-cycles is only possible through a careful optimization of the etching step parameters (chemistry, frequency, duration, etc.). In this paper, we discuss how to optimize this step and we
Received 13th October 2021,
Accepted 16th November 2021
DOI: 10.1039/d1dt03456a
rsc.li/dalton
show that different approaches can be developed to optimize the overall ASD process throughput, while
simultaneously limiting process drift and contamination. We also show that complementary selective properties can prove a valuable leverage enabling ASD processes based on super-cycles, such as structure
selective deposition, whereby a difference in thin film morphology in growth and non-growth areas can
be smartly taken advantage of during the etching step.
Introduction
For the past 50 years, miniaturization of microelectronic
devices has been continuously following Moore’s law and has
generated multiple innovations in fabrication processes. One
of the obvious examples is the successful implementation of
atomic layer deposition (ALD) in CMOS technologies, for the
mass production of HfO2 metal gate transistors at the 45 nm
technological node by Intel, back in 2007. From then on, ALD
has gradually become an invaluable technique for downscaling, both in the Front End and Back End of Line, providing
deposits with improved conformality and atomic-scale thickness control, and enabling the introduction of new exotic
materials, such as rare earth oxides and nitrides in, so far, all
silicon devices.
As of today, current semiconducting devices are quickly
reaching nanometer-scale dimensions, and further downscaling essentially relies on the extensive use of multiple patterning technologies. However, the associated processing complexity induces technical difficulties to accurately align various
a
University Grenoble Alpes, CNRS, LTM, Grenoble INP, F-38054 Grenoble Cedex,
France. E-mail: marceline.bonvalot@cea.fr
b
Institute of Applied Physics, Faculty of Pure and Applied Sciences, University of
Tsukuba, Tsukuba 305-8573, Japan
c
SUNY POLY, CNSE, Albany, New York 12203, USA. E-mail: valleec@sunypoly.edu
d
Univ. Grenoble Alpes, CEA, LETI, 38000 Grenoble, France
442 | Dalton Trans., 2022, 51, 442–450
mask layers, in turn leading to nanometer-scale misalignments, known as Edge Placement Errors (EPEs). Moreover, the
miniaturization approach based on multi-patterning photolithography contributes to exponentially increasing fabrication
costs and thus, a general agreement among scientists has
emerged about a major change in the conventional miniaturization paradigm.
In this regard, area selective deposition (ASD) has been the
focus of intense research activities for the past few years,
because it is a bottom-up thin film deposition strategy that
does not require any subsequent technically challenging
multi-patterning photolithography steps.1 ASD provides a good
control of where a deposit forms, depending on the chemistry
of a 2D patterned substrate. As such, ASD also prevents the
occurrence of EPE. When dealing with 3D patterned profiles,
ASD is referred to as topographically selective deposition
(TSD), whereby the subsequent deposit forms on specific surfaces, depending on their orientation with respect to the
substrate.
Although conventional growth techniques like CVD can
also lead to ASD, it is most achieved by ALD. All commercially
available ALD precursors naturally exhibit varying affinities to
different substrate chemistries. This in turn can induce a
natural nucleation delay before growth finally starts. This time
lag in spontaneous nucleation is however effective at a relatively low number of ALD cycles only, and the resulting
inherent selectivity remains quite low (<1 nm). The selectivity
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of an ALD process is quantified by comparing the thickness of
a deposit formed on a high affinity substrate surface (usually
referred to as growth area) with that formed on a low affinity
surface (non-growth area).2,3
To overcome this low selectivity, most ASD processes developed in the literature implement a surface functionalizing pretreatment to preferentially either activate or deactivate a
specific material surface with respect to its subsequent
exposure to metallic precursor flux from the ALD process. Selfassembled molecules (SAMs), hydrophobic CFx polymer deposits, and small inhibitors are usually used to inhibit ALD
growth,4–10 whereas electrons or metallic seed layers catalysing
ALD surface reactions can be used to promote growth.11,12
Fig. 1 illustrates these various schemes leading to ASD. In any
case, this surface functionalization pretreatment raises several
issues in nanoelectronic device fabrication, because the associated preparation processes are usually complex, time-consuming (>24 hours for SAM deposition using chemical bath), with
a relatively low throughput, and hardly compatible with largescale production. Moreover, SAM polymer brushes may also
display poor surface coverage, where defects can act as unexpected nucleation sites on non-growth areas.13
One way to solve this problem is to insert corrective etching
steps within ALD cycles to remove unwanted deposits from
non-growth areas and to reset their surface chemistry to a deactivated chemical initial state. In this paper, we will first
address the status of recent ASD development using alternate
deposition and etch strategies. Then we will discuss strategies
to optimize this process.
Area selective deposition using supercycles of deposition and etch:
experimental results from the
literature
Alternating deposition and etching steps can be carried out to
lead to ASD with improved selectivity as illustrated in Fig. 2. In
2015, Hashemi et al.14 first proposed a self-correcting etching
process for ASD involving surface deactivation by SAMs.
Perspective
Fig. 2 Inserting an etch-back step in an ASD process to improve its
overall selectivity.
OctaDecylPhosphonic Acid (ODPA) was first selectively deposited on Cu regions, creating a resist layer which prevented
subsequent Al2O3 ALD growth on this surface but not on SiO2
surfaces. Selectivity was further increased by coupling this deactivation pre-treatment with a chemical etching step to selectively remove any residual dielectric film deposited on Cu surfaces, while simultaneously leaving the dielectric film on SiO2
unaffected. The authors reported a selectivity enhancement by
a factor of ten in the dielectric film thickness, by performing
ten successive selective deposition and selective etching steps.
In the last few years, our group at the LTM laboratory and
CEA-LETI (Grenoble, FRANCE) has proposed an original solution avoiding the use of SAM functionalizing pre-treatments to
enhance selectivity in ASD processes. It consists of regularly
implementing corrective etching steps to remove deposits
formed on non-growth surfaces after the nucleation delay and
leads to multi-step processes alternating deposition and
etching.15,16 We have shown that depending on the targeted
feature where ASD is required, the etching modes can be of
choice either anisotropic or isotropic and include reactive ion
etching (RIE) with synergistic physical and chemical contributions, pure physical surface sputtering, pure radicalenhanced chemical etching and quasi-atomic layer etching,
whereby etching involves surface-limited chemical reactions
leading to atomic layer-by-layer removal. Both deposition and
etching steps can be thermally driven or plasma-based and
some examples of area selective deposition and topographical
selective deposition are given in the next sections.
Deposition and etch super-cycles for ASD
Fig. 1 Illustration of various ASD strategies for selective deposition on
surface A only (no deposition on surface B): by using the inherent
selectivity of the ALD process (here the precursor only reacts with
surface A), or by deactivating or activating surface A before the ALD
process.
This journal is © The Royal Society of Chemistry 2022
This section focuses only on ASD processes using super-cycles
of ALD or Plasma Enhanced ALD (PEALD) steps and etching
steps. The use of a plasma and its radicals or ions in the coreaction step of an ALD process has many advantages compared to a thermal ALD process in which the necessary energy
for the surface reaction is provided entirely by thermal energy
from the heating of the substrate. Examples include improved
GPC, improved density of the deposited layers, and a wider
selection of ALD precursors.17–23 The main drawback of this
assistance is the degradation of the compliance in high aspect
ratio structures, especially for the deposition of conductive
layers requiring a reducing plasma.24,25 The etching step can
be plasma assisted or not. To be able to quickly compare these
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processes, we tried to provide the same information when
available: chemistries, defects induced by the etching step
(contamination, density, and roughness), and change in the
process time compared to the time to deposit by ALD with
only the same film thickness on the growth area.
Historically, the first ASD process using super-cycles of ALD
and etching without surface inhibition by SAM was developed
by Vallat et al. at LTM and CEA/Leti for the selective deposition
of Ta2O5 on TiN (growth area) versus Si or SiO2 (non-growth
area).15 For this Ta2O5, ASD, the etch-back step was carried out
after every 8 PEALD cycles and consisted of adding NF3 to O2
in the plasma step of the last CCP PEALD cycle, thus promoting isotropic chemical etching. This way, the 7 nm selectively
deposited thin film was etched four times faster than Si-rich
surfaces, resulting in an estimated etching selectivity of 4.
However, it exhibited significantly degraded properties, such
as a quite large ∼9% residual fluorine contamination and a
24% lower density (6.1 g cm−3 vs. 8 g cm−3 for standard Ta2O5
thin film). The growth per cycle (GPC) of one super-cycle (8
PEALD + 1 etching step) was 0.3 nm per super-cycle, resulting
in a processing time at least three times larger than that of the
standard PEALD process.
A similar deposition/etch process was later further developed
by Vallat et al. for TiO2 ASD, but with the O2/NF3 isotropic
plasma etch-back step being introduced every 20 PEALD cycles
only.16 A similar etching selectivity of 4 was also obtained, but a
lower fluorine contamination level of ∼4% was determined by
XPS, along with an improved 11 nm thin film density of 3.6 g
cm−3, as compared to 3.9 g cm−3 for TiO2 deposited without
super-cycles (ie 8% density drop induced by etch-back steps).
The GPC for one super-cycle (20 PEALD + 1 etching step) was
∼0.45 nm per super-cycle, resulting in a processing time
approximately twice as long as the standard PEALD process.
In a recent paper, a defect-free ASD process was developed
by Krishtab et al. using a cyclic halogen plasma-induced passivation of amorphous carbon, for the ASD of TiO2 in a SiO2
trench.26 In this ASD process, a prior He/H2 plasma was used
to remove the oxygen-rich layer from the amorphous carbon
surface and to create a hydrogenated a-CH surface layer
instead.27,28 The obtained a-C:H surface was then exposed to
Cl2 or CF4 plasmas to achieve surface halogenation. Then TiO2
thermal ALD was carried out using a TiCl4 metallic precursor
and H2O co-reactant. Direct TiO2 growth was observed on SiO2
surfaces without any nucleation delay, attributed to the spontaneous hydrolysis of Si–Cl or Si–F bonds during the first H2O
ALD pulse. The global ASD process consisted of 60 standard
PEALD cycles followed by an etch-back step using either Cl2 or
CF4 plasma, for 10 or 15 seconds respectively. The overall performance of the ASD process was quantitatively assessed by
expressing the deposition selectivity, S, as:29
S ¼ ðV film V grain Þ=ðV film þ V grain Þ
ð1Þ
where Vfilm and Vgrain correspond to the total volumes of the
TiO2 film and nuclei respectively over a 2 μm2 area on growthand non-growth areas.
444 | Dalton Trans., 2022, 51, 442–450
The best results were obtained for CF4-based super-cycles
that allowed the deposition of 8 nm of TiO2 with a selectivity
of 0.998, as compared to 0.812 obtained with identical ALD
cycle numbers in a single run. Although no etch selectivity was
observed between a-C:H and TiO2 with CF4 plasma, TiO2
grains on non-growth areas were found to etch twice as fast as
TiO2 films on growth areas. Finally, a very low increase in the
TiO2 film roughness was reported, from 0.144 nm RMS for the
standard ALD process to 0.155 nm for the ASD process using a
Cl2 plasma and 0.153 nm in the case of CF4 plasma,
respectively.
Vos et al.30 also developed an ASD process relying on etchback steps for Ru deposition on Pt vs. SiO2: a standard Ru ALD
process was combined with etching steps involving O2 plasma
(30 seconds) followed by H2 gas (15 seconds) exposure steps.
The O2 plasma was used to etch away unwanted Ru nuclei
formed on SiO2 surfaces, thanks to the formation of volatile
RuO4, while H2 gas exposure was used to reduce oxidized Ru
on the Pt surface, to promote subsequent Ru growth by ALD.
The resulting etching selectivity was infinite, because SiO2
remains inert under O2 plasma and H2 gas exposures. O contamination was estimated to be ∼5%, similar to that measured
in the standard ALD Ru thin film, and thus, this etch-back
step did not induce any additional contamination. Although
no density measurement of the ASD Ru thin film was mentioned, its surface roughness was reportedly considerably
smothered after the etch-back step, in agreement with a quasiatomic layer etching (ALE) process. Comparing the Ru thicknesses after 400 ALD cycles with or without additional etching
steps, this overall ASD process time was estimated to be
approximately twice as long as that of the standard ALD
process.
Song et al.31 have developed an ASD process for TiO2 on
SiO2 vs. Si surfaces, with super-cycles based on thermal ALD
and thermal ALE. WF6 and BCl3 gases were used separately at
170 °C for the thermal ALE of TiO2. When exposed to WF6, the
TiO2 surfaces form solid WOxFy/TiOxF and volatile TiF4. The
subsequent BCl3 exposure removes W species and forms solid
B2O3, which is removed during the next WF6 dose. The supercycle was defined as 30 ALD cycles followed by 5 ALE cycles
and enabled the selective deposition of 12 nm of the TiO2 thin
film, corresponding to ten times as large a thickness, compared to that of inherent TiO2 ASD.32 XPS measurements
showed that the ALE reactants lead to 3–5 at% B, 1–2 at% F
and less than 0.1 at% W incorporation in the ASD TiO2 film.
The reported GPC was ∼0.83 nm per super-cycle, resulting in a
process time ∼1.6 times longer than that of the standard
process.
Lee et al. also used corrective etching steps to enhance the
selectivity for the ASD of SiO2 on SiO2 vs. SiN.33 They showed
that with an HF treatment of SiO2 and SiN surfaces, a selective
ALD of SiO2 on SiO2 can be obtained using the di(isopropylamino)silane (DIPAS) precursor and ozone at 100 °C. However,
SiO2 starts to grow on SiN non-growth areas after 50 cycles,
which reduces the SiO2 growth (on SiO2) to 4 nm only. To
resolve this issue, the insertion of an etching corrective step
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Perspective
was put on trial, based on two distinct etching process steps: a
diluted HF (0.1 wt%) wet treatment and an ALE treatment
including a dry etching with HF-pyridine and H2O. Vallat
et al.15,16 reported that the etching gas also resets a new
nucleation delay by restoring inhibition mechanisms on nongrowth areas. The periodic HF treatment every 100 cycles
resulted in the removal of the unwanted SiO2 film and restored
–NH2 surface groups on SiN, preventing further chemical reaction with the aminosilane precursor. No data concerning the
impact of the etch-back process on SiO2 material properties
are available in ref. 33, but as deduced from published figures,
300 cycles with 3 etch-back steps appear necessary to selectively deposit 10.8 nm SiO2, whereas the ALD process has a
GPC of ∼0.8 Å per cycle. This resulted in a process duration
larger than twice the duration of the standard process.
The different ASD processes are summarized in Table 1.
The comparison between the various ASD processes clearly
highlights the role of the super-cycle design, more specifically
the etching chemistry, the etching time, and the etching frequency. This will be discussed in the following sections.
Deposition and etch super-cycles for TSD
Super-cycles of ALD and ALE processes can also be defined for
topographical selective deposition (TSD), as illustrated in
Fig. 3. However, since ALD or PEALD processes lead to confor-
Table 1
mal thin film formation, TSD requires an anisotropic etching
step to achieve selectivity. Such anisotropy cannot be obtained
in conventional ALD reactors equipped with plasma assistance, because they are mostly equipped with remote plasma
sources supplying radical-enhanced plasmas in the vicinity of
the substrate, thereby significantly limiting anisotropic chemical reactions during PEALD.
At LTM laboratory, we have an inductively coupled plasma
(ICP) remote PEALD reactor from Oxford Instruments
equipped with an additional RF substrate biasing system,
which enables the formation of a non-collisional sheath at low
pressures, thereby allowing to take advantage of directional
ionic flux for the promotion of anisotropic etching steps
within the PEALD reactor. A first proof of concept of this TSD
approach was published in 2019 by Chaker et al.34 using
super-cycles of PEALD combined with pure physical Ar sputtering, leading to the direct vertical coating of 3D features.
However, significant silicon recess was observed, along with
sputtered Si redeposition in the vertical coating material. To
improve these points, we later developed a TSD process using
super-cycles of PEALD and Plasma ALE steps.35 The two ALE
sequential steps involved an isotropic CF4/H2 plasma to induce
surface fluorination, followed by a low energy directional Ar
sputtering process to selectively remove horizontal fluorinated
coatings. Consequently, Si lateral redeposition mechanisms
Summary of various ASD processes using ALD and etch super-cycles
ASD process
Nature of the etching
Etching chemistry
Ta2O5 on TiN vs. Si/SiO2
TiO2 on TiN vs. Si/SiO2
TiO2 on SiO2 vs. Si
Isotropic plasma etching
Isotropic plasma etching
Isotropic thermal ALE
O2/NF3
O2/NF3
WF6 and BCl3
TiO2 on SiO2 vs. a-C:H
Ru on Pt vs. SiO2
SiO2 on SiO2 vs. SiN
RIE plasma
Isotropic plasma
Isotropic wet etching
Cl2 or CF4
O2 + H2
Diluted HF (0.1 wt%)
Contamination from
etching chemistry
Etching
selectivitya
Throughputb
Ref.
9 at% F
4 at% F
3–5 at% B
1–2 at% F
—
No
—
4
4
—
1/3
1/2
2/3
15
16
22
1
∞
∞?
—
1/2
<1/2
17
21
24
a
Etching selectivity = etching rate of selectively deposited film/etching rate of non-growth area. b Throughput is defined as: [time ALD process]/
[time ASD process for equivalent ALD thickness]. An ideal ASD process will give a value of 1.
Fig. 3 (a) ASD by super-cycles of deposition and etch. The blue line corresponds to growth on the growth area and the grey line corresponds to
growth (not desired) on the non-growth area. A = growth area; B = non-growth area. (b) TSD by super-cycles of deposition and etch. The blue line
shows the ALD growth on all surfaces (top, bottom and sidewalls) assuming a conformal deposition. The dotted grey line corresponds to the anisotropic etching of the film on top and bottom surfaces.
This journal is © The Royal Society of Chemistry 2022
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were suppressed and the silicon substrate recess was strongly
limited. However, 8 at% Fluorine contamination was measured
in the selectively deposited film, while no contamination was
measured when using super-cycles with sputtering only.
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Area selective deposition using supercycles of deposition and etch:
requirements, optimization, and
drawbacks
As seen from previous literature results, the success of an ASD
process using deposition and etch super-cycles relies on the
optimization of many parameters, one of them being the
etching step (chemistry, duration, frequency). This step must
be optimized with the following objectives in mind:
- Large etching selectivity: the etch rate of ALD nuclei on
non-growth surfaces must be larger than that on non-growth
surfaces and if possible than that of the ALD film on growth
surfaces.
- If possible, passivation of non-growth surfaces by etching
chemistry, to promote additional nucleation delay.
- No alterations of the selective thin film physical and
chemical properties, induced by the etching process: e.g.,
surface contamination, density, induced roughness, etc.
- Optimize the duration of the whole ASD process, so as not
to affect the overall process throughput.
- If possible, make sure that the ASD process can be easily
transferred to topographical selective deposition processes in
3D patterned substrates (considering isotropic vs. anisotropic
etching steps).
Optimization of the etch-back chemistry
The etch-back chemistry must be selective, long enough to
completely remove unwanted materials without damaging the
selectively deposited film, in terms of crystallinity, roughness
and density for instance, and non-damaging to non-growth
surfaces (for instance via amorphization or increased roughness, creation of voids, dangling bonds, etc.).
Some of these points are illustrated in Fig. 4. If the kinetics
of the etch back-step is too fast, it may not only remove
Dalton Transactions
unwanted materials from non-growth areas, but also etch the
whole selectively deposited film. If the etching selectivity is
low, the non-growth surfaces will be gradually etched away
during every etch-back step.
These examples show that the etching kinetics must be slow
enough to be controlled at the atomic scale and simultaneously
exhibit a high selectivity. It thus seems straightforward that the
appropriate etch-back process should be of ALE-type, for which
infinite selectivity can be obtained, thanks to self-limiting reaction mechanisms at the atomic scale. ALE processes involving
chemical reactions for both surface modification and removal
steps may be the best option for this purpose, as illustrated in
ref. 30 and 31 while plasma ion-based processes, such as Ar
sputtering should be limited to cases where etching anisotropy
is mandatory. In this regard, the process developed by Vos
et al.30 is optimal in the sense that the same chemistry (O2 and
H2) is used for both deposition and etching steps, which considerably limits contamination induced in the selectively deposited material by the etching chemistry.
Optimization of the super-cycles
The addition of etching steps will undoubtedly lengthen the
process time to deposit a material of thickness equivalent to a
classical ALD process, especially considering that the etching
must be slow to ensure atomic scale control. Incorporation of
etching steps will also contribute to limit the net deposited
thickness. A trade-off thus needs to be defined, so that etching
provides a good selectivity along with a notable deposition
thickness. It is therefore necessary to optimize the frequency
and duration of etching steps, to limit the throughput drop, in
view of potential process industrialization.
Such a super-cycle optimization can be found in the recent
paper from Vos et al.,36 in which they compared two extreme
etch-back strategies: in a first set of experiments, they tested a
high frequency-low duration corrective etch-back step, whereas
in a 2nd set, a low frequency-high duration corrective etch-back
step was tested. The 2nd set of experiments was found to be
preferred in terms of the net deposition rate. According to
their work, the net deposition rate (Dnet) of the ASD process
can be written as a function of the growth per cycle on growth
and non-growth areas (GPCG and GPCNG) as:
Dnet ¼ αN½GPCG GPCNG Fig. 4
ASD.
Illustration of the impact of a non-optimized etch-back step on
446 | Dalton Trans., 2022, 51, 442–450
ð2Þ
where N is the number of super-cycles and α the number of
deposition cycles in one super-cycle.
This equation shows that lowering the GPC on non-growth
areas increases the net deposition rate. To a first order approximation, it indicates that the lower GPC on non-growth areas
are inherent to the ALD process. However, additional strategies
to further reduce this value can be developed, such as adding
a passivation step during or after the etch-back process. In the
papers from Vallat et al.,15,16 the fluorine plasma chemistry
used for Ta2O5 and TiO2 etching was also used with the aim to
create hydrophobic Si–F surface bonds on Si surfaces, thus
inducing new nucleation delays of the order of 10 cycles.
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It must be underlined at this point that the relation (2) was
obtained based on the assumption that the material on growth
and non-growth areas is removed at similar etch rates.36 There
may however exist experimental cases for which the corresponding etch rates are different. In such cases, the following
relation should be used instead:
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Dnet ¼ αN½GPCG GPCNG ðEPCG =EPCNG Þ
ð3Þ
where EPCG and EPCNG are respectively the etching rates of
the material deposited on growth and non-growth areas.
This equation shows that an ideal throughput can be
obtained when EPCNG ≫ EPCG. In their experiments, Vos
et al.30 found that the Ru nuclei deposited on non-growthareas etched twice as fast as selectively deposited Ru thin
films. One likely explanation might be that the Ru nuclei are
more efficiently etched away, due to their smaller size. Thus,
increasing the frequency of the etching steps should in this
case increase the net deposition rate.
Another explanation accounting for an increased etching
rate of nuclei from non-growth areas may be linked to their
morphology (number of facets and surface area, and crystallinity). Coffey et al.37–39 showed that an ALE vacuum ultraviolet
(VUV) oxidation process led to higher etch rates of Pd particles
as compared to Pd thin films. They also showed how facets
and surface areas of Ru films impacted their oxidation and
etching kinetics. Krishtab et al.26 also showed larger plasma
etching rates of TiO2 grains, which were attributed to the
shape-selective nature of the etching chemistry, which induces
a local enhancement of the neutral-to-ion flux in TiO2 grains,
as compared to TiO2 thin films. From these reports, we can
assume that all etching processes based on surface modification (such as those induced by diffusion or oxidation)
should be more efficient for grains and/or nuclei as compared
to plain thin films, due to the local enhancement of their relative surface area. This is illustrated in Fig. 5, wherein we
assume that (1) the same material thickness is modified
during the initial surface-modifying ALE process (oxidation or
halogenation for example), and (2) the whole modified thickness is removed during the second ALE process removal step.
Considering the relative surface areas of a smooth thin film
and that of a spherical material, it is therefore of no surprise
that the nuclei are etched away twice as fast as their uniform
film counterpart. Similarly, it can also be expected that rough
films are etched much faster than uniform films.
Moreover, the surface chemical modification of non-growth
areas, induced either by the etching step or by an additional
surface treatment step, can also promote the subsequent
growth of small sized nuclei, which in turn are etched more
easily than a plain thin film. For example, an island growth
mode is commonly observed during the MCl4/H2O (M = Ti, Zr
or Hf ) ALD processes on the H-terminated Si substrates. The
lack of reactivity of hydrogen-terminated surfaces towards
these ALD reactants is thought to be responsible for such a
growth mode, and island growth most likely starts from
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Fig. 5 Illustration of a faster ALE process for spherical nuclei as compared to a uniform thin film, due to the higher relative surface area.
Yellow color: the material to be etched; purple color: surface modification followed by etching during the two ALE process steps.
oxygen-containing defect sites after a nucleation delay promoting defect activation.40,41
Consequently, hydrogen-based chemistry for the etch backstep can in turn favor the growth of small sized nuclei on nongrowth areas, as illustrated in Fig. 6. The ALD process is
started on two substrates A (growth-area) and B (non-growth
area), with a nucleation delay inherent to the chemical state of
surface B. An etch-back step is then applied to remove the
thinner uniform film formed on substrate B, once the nucleation delay has expired. In this specific case, EPCG = EPCNG.
However, the B surface becomes H-terminated after the dedicated etching step, restoring an additional nucleation delay,
but also promoting the island growth on this surface when
inhibition becomes defective (i.e., after the 2nd nucleation
delay has expired). The following etch-back step then gives
EPCG ≪ EPCNG, thus limiting the film thickness to be removed
from growth areas and improving the net deposition of the
super-cycle deposition/etch process.
In the case of selective deposition of metal oxides exposed
to a thermal ALE as the etch-back step, it has been shown that
amorphous metal oxides etch at a much faster rate than their
Fig. 6 Schematic of the deposition/etch process showing the increase
in the net deposited thickness as a function of the crystalline state of the
deposited material on growth and non-growth surfaces A = growth
area; B = non-growth area.
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Table 2 Etch per cycle for the ALE of crystalline and amorphous
HfO2 42–44
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ALE etch rates Å per cycle @ 250 °C
Material
HF +
TiCl4
HF + Al
(CH3)2Cl
SF4 +
TiCl4
SF4 + Al
(CH3)2Cl
c-HfO2
a-HfO2
0.02
0.36
0.08
0.68
0.08
0.70
No etch
0.50
crystalline counterpart.42,43 Table 2 shows the estimated ALE
etch rates for HfO2, as reported by R. Mullins et al.44
The crystallinity of ALD metal oxides, such as HfO2, is
strongly correlated to its thickness, deposition time, and chemistry of the substrate. As an example, we found that HfO2 deposited with HfCl4 and H2O at 200 °C is crystalline (mostly
monoclinic) down to 3 nm thickness when deposited on Pt,
while it exhibits an amorphous morphology up to 8 nm thickness when deposited on the TiN substrates.45 D. M. Hausmann
and R.G. Gordon provided a clear mechanism for the nucleation
and growth of the HfO2 crystallites during the thermal ALD
process:46 after x ALD cycles and the initial formation of nuclei,
the next ALD cycle contributes to enhance their mass resulting
in a growing crystallite. As the film grows thicker, it becomes
crystalline, thanks to the growth of crystallite nuclei. The crystalline film growth rate is also observed to be larger than that of
the amorphous material. This observation is in agreement with
the high diffusion rate of metallic adsorbates on a metallic
surface, contributing to an organized crystalline growth mode.
Conversely, the diffusion of metallic nuclei physisorbed on
nitride or oxide surfaces is in comparison partially hindered,
due to their relatively lower thermal conductivity, which in turn
favors amorphous phase formation.47
Fig. 7 shows an illustration of an optimized HfO2 ASD
process on two different substrates, for which thick crystalline
HfO2 is first deposited on growth areas. Successive ALE etchback steps with SF4 and DMAC (dimethylaluminum chloride)
should then only etch amorphous HfO2 on non-growth areas,
in other words EPCG = 0. In such an approach, no initial
nucleation delay is needed between the two substrates if the
crystalline properties of the thin film can differ in both growth
and non-growth areas. Finally, the deposition scheme as a
function of the number of ALD cycles is illustrated in Fig. 8.
Optimization of the process reactor – addressing crosscontamination issues and process drifts
If chemically induced, the etching rate is controlled by substrate temperature only, and thus, the temperature of the ASD
process is dictated by that of the etching process, as long as
this temperature remains within the ALD window of the metallic precursor.
The possibility for PEALD processes to provide a large ALD
operating temperature window is an interesting advantage
giving relative flexibility in terms of temperature adjustment of
ASD processes with etch-back steps. However, etching conditions do not solely depend on temperature set points
448 | Dalton Trans., 2022, 51, 442–450
Fig. 7 Illustration of etch-back strategies leading to ASD, when dealing
with various morphological states of a thin film. A = growth area; B =
non-growth area.
Fig. 8 Film thickness as a function of ALD cycles. In this example, only
the amorphous material is etched, and no passivation step or initial
nucleation delay is needed to achieve a high selective thickness. A =
growth area, B = non-growth area, a = amorphous, and c = crystalline.
Solid line = ALD crystalline film; dotted line = ALD amorphous film.
leading to volatile product formation. When a plasma etching
step is needed for an ASD process development, then the
corresponding plasma parameters must be adjusted carefully
to the reactor design as well. Table 3 lists some PEALD reactor
designs provided by suppliers, along with various types of
etch-back steps that can be successfully developed in such
reactors.
Reactors used for ASD processes developed at the LTM laboratory were respectively, a 300 mm CCP tool from PlasmaTherm working at high pressure, for the incorporation of an
isotropic etch-back step in the ASD process,15,16 and a 200 mm
Table 3 Etch-back steps that can be developed in conventional PEALD
tools (CCP = capacitively-coupled plasma, ICP = inductively coupled
plasma, MW = microwave, and ALE = atomic layer etching)
PEALD tool
Etch-back step
CCP discharge with mesh or
grid
CCP discharge without
mesh
ICP or MW discharge used
as a remote system
ICP or MW discharge
operating at low pressure
Isotropic only
Isotropic mainly/anisotropic in some
specific cases only
Isotropic only
Anisotropic or isotropic, depending on
the nature of the plasma etching gas
and material to be etched; a substrate
biasing is needed for accurate control
of ion energy (plasma ALE)
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prevent it in non-growth areas. When growth begins in nongrowth areas, then a corrective step must be applied, leading
to the design of ASD by super-cycle alternating deposition and
etch. The optimization of such processes relies mostly on the
optimization of the etching step itself (chemistry, frequency,
time, etc.). We have discussed the extent to which other complementary selective properties can also be used to optimize
ASD based on super-cycles, such as structure selective deposition, whereby a difference in thin film morphology in growth
and non-growth areas can be smartly taken advantage of
during the etching step.
Fig. 9 Illustration of a spatial ASD process with separated zones for the
co-reactant and etch plasmas to prevent any cross-contamination
(adapted from the schematic of the TEL NT333™ tool).
ICP tool from Oxford Instruments with a low power RF substrate biasing and working at low pressure to ensure a non-collisional sheath near the substrate.34,35,48–50 The low power
regime of the 13.56 MHz RF waveform biasing system is the
key to the development of ASD with super-cycles of PEALD and
Plasma ALE. Similarly, the optimization of super-cycles for
TSD process development presented earlier requires a selective
and anisotropic etch-back step, with an excellent nanometerscale control over the etch rate. In this case, a plasma ALE
process is the process of choice, with low energy ion flux to
enable a precise control of their energy, and most importantly
an ion energy distribution function as narrow as possible. In
this case, an additional polarizing system with a tailored waveform may be preferred over a conventional RF waveform.51
The addition of etching chemistries into an ALD chamber
can modify the global state of the reactor via gaseous adsorbates on reactor walls. In the case of a plasma based ASD
process, this can alter the plasma chemistry during the subsequent deposition process, via modifications of radicals (O,
H), recombination probability at reactor walls, and desorption
of etching adsorbates during deposition. This may in turn lead
to flows of radicals containing etching species towards the
surface of the material, raising non negligible cross-contamination and process drift issues. To circumvent such problems,
spatial ALD seems to be more appropriate, because it provides
separate reactant localization in space for both deposition and
etching steps, as suggested by A. Mameli et al. and P. Poodt
et al.52–55 Fig. 9 shows a possible design of a spatial ALD
process for ASD with etch-back steps, where plasmas for deposition (co-reactant) and etching are not localized in the same
region, thus considerably limiting process drift and cross-contamination issues.
Conclusions
Area selective deposition processes should ideally provide
uniform growth in desired growth areas, and simultaneously
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was partly supported by the French RENATECH
network.
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