22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Reactive adsorption of molecules and radicals on surfaces
D. Marinov, O. Guaitella, J.-P. Booth and A. Rousseau
Ecole Polytechnique, LPP, CNRS, UPMC, Université Paris-Sud, Palaiseau, France
Abstract: Chemisorbed atoms are believed to play a key role in surface-catalysed
reactions in plasmas. Heterogeneous losses and formation of new molecules on surfaces
are controlled by the coverage and the reactivity of species on the surface. Usual plasma
diagnostics are mainly focused on the kinetics of gas phase species interacting with surfaces
and very little is known about the properties of atoms on the surface. In this work we
summarize results obtained using an original approach allowing in-situ characterization of
stable chemisorbed atoms grafted to surfaces under N 2 /O 2 plasma exposure. The role of
these atoms in surface reactivity of silica under plasma exposure is investigated. The
importance of weakly-bonded atoms is demonstrated.
Keywords: adsorption and recombination of atoms, production of molecules on surfaces,
isotopic exchange
1. Introduction
Surface kinetics plays a central role in many plasma
applications.
Low pressure technological plasmas,
plasma-catalyst systems and thermal protection for space
re-entry are just few examples. Regardless of the great
number of works devoted to the study of plasma-surface
interactions, there is still a lot of uncertainty. For
example, the recombination probabilities of atoms on
nominally the same materials determined in different
studies can scatter over orders of magnitude [1, 2].
Uncertainties of the rates of surface processes limit the
accuracy and the predictive capability of the kinetic
models.
In this paper, surface processes occurring in N 2 /O 2
plasmas in contact with dielectric surfaces will be
discussed. Previous experimental and modelling studies
[3-5] resulted in a well established model of surface
recombination mechanisms. It is widely accepted that the
recombination of atomic species on dielectric surfaces
proceeds in two steps. First, atoms from the gas phase are
captured by surface active sites. In the second step, these
adatoms recombine either with impinging gas phase
atoms (Eley-Rideal mechanism) or with physisorbed
atoms diffusing along the surface (Langmuir–
Hinshelwood mechanism). It is usually supposed that
active sites represent only a small fraction of surface
atoms (~1%) and adsorbed atoms are irreversibly trapped
(chemisorbed) by the active sites. The aim of this study is
to challenge the existing models and to provide a realistic
picture of the processes taking place on the surface.
The properties (coverage, binding energy, reactivity) of
chemisorbed atoms on real surfaces remain barely
studied. One of the reasons for this is that classical
techniques of the surface science are not applicable in the
harsh plasma environment. While the plasma science
approach is more general and it is mostly focused on the
kinetics of species in the gas phase. In this work, we
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present results obtained using an original experimental
technique that allows in-situ probing of the coverage and
the reactivity of atoms adsorbed on real surfaces exposed
to a low pressure plasma [6-9].
2. Experimental
The main idea of the experimental approach used in this
work is based on the assumption that chemisorbed atoms
are stable and they remain on the surface even after the
end of the plasma exposure. Thus we perform a
pretreatment of the inner surface of a silica discharge tube
with a flowing discharge in normal N 2 (or O 2 ) at
p = 0.53 mbar. The schematic of the setup and the
experimental sequence are shown in Fig. 1. After the
pre-treatment, the reactor is evacuated and the pre-treated
surface is probed with a pulsed plasma in a corresponding
heavy isotope 30N 2 (38O 2 ) in static (no gas flow)
conditions.
Fig. 1. Schematic of the set-up (upper panel) and the
experimental sequence of the N 2 isotopic study (lower
panel).
1
Nads (cm-2)
1E16
XPS
14
1E15
1E14
0.1
1
10
100
1000
28
N2 pretreatment time (s)
Fig. 2. Adsorption kinetics of N on silica discharge tube.
Discharge in 28N 2 p = 0.53 mbar, P = 17 W. Results of
XPS measurements of the N surface coverage are shown
for comparison.
A question may arise, if nitrogen atoms grafted to the
surface by N 2 plasma do indeed play a role of active sites
for surface recombination. In order to get an insight in
the reactivity of N ads the duration of the probe discharge
2
1016
Ndes (cm-2)
3. Results and discussion
3.1. Adsorption and reactivity of N on silica
Fig. 2 shows the density of 14N ads as a function of the
Excellent
duration of 28N 2 plasma treatment.
sub-monolayer sensitivity of the isotopic exchange
technique (LOD ≈ 2·1013 cm-2) allowed us to follow the
adsorption kinetics for pretreatment times as short as
100 ms. The maximum 14N ads coverage reached after 1 h
treatment time is 6·1015 cm-2, which is close to a
monolayer coverage on silica. The N ads density obtained
by the ex-situ XPS agrees reasonably well with the
isotopic study. Both techniques indicate that a SiO x N y
layer is formed on clean SiO 2 surface under N 2 plasma
exposure. Surface treatment with a flowing afterglow
doesn’t result in noticeable nitrogen incorporation, which
means that low energy (~ 15 eV) ion bombardment is
essential for nitridation and nitrogen atoms alone are
inefficient. We have found that the surface can be
restored to pure SiO 2 state after a 1 h oxygen plasma
treatment. One can conclude that the effect of the N 2
plasma on the silica surface is not limited to a mere
occupation of a few surface active sites; the chemical
composition of the surface is modified.
was varied in the range of 5·10-3 - 103 s. Fig. 3 shows the
evolution of the number of adsorbed atoms picked up
from the surface [14N des ] as a function of the probe 30N 2
discharge duration. These measurements indicate that
groups of 14N ads having different reaction probabilities
exist on the SiO x N y surface. The most reactive N ads with
the coverage of 2·1015 cm-2 are exchanged with a
characteristic time of 10 seconds, while the reactivity of
the remaining group of atoms (4·1015 cm-2) is two orders
of magnitude smaller. Observed distribution of reactivity
may be explained by a distribution of binding energies of
atoms on the surface; strongly-bonded surface species
have a smaller probability to react with atoms from the
gas phase. Fig. 3clearly demonstrates that different
groups of N ads have different “efficiency” as active site
for recombination.
These results demonstrate the
advantage of the isotopic exchange technique compared
to classical surface diagnostics that do not provide
information about the reactivity of species on the surface.
τ=0.42 s
τ=316 s
τ=6.7 s
1015
14
Molecules 14N15N (16O18O) produced on the surface in
the probe discharge are detected using a QMS. The
coverage of atoms picked up from the surface during the
probe discharge is determined from the known surface-tovolume ratio of the discharge tube. By varying the
duration of the probe discharge the kinetics of surface
reactions catalysed by adsorbed 14N ads (16O ads ) is
investigated. In-situ isotopic exchange measurements are
compared with the result of ex-situ XPS diagnostics of the
pre-treated silica surface.
1014
0.1
30
1
10
100
1000
N2 plasma duraτion (s)
Fig. 3. Evolution of the density of 14N ads atoms picked up
from the surface as a function of the probe 30N 2 discharge
duration. Silica discharge tube was pre-treated by 28N 2
plasma during 60 minutes. Multi-exponential fit with is
shown by a solid line.
By using short dc pulsed in 30N 2 have found that the
rate of 15N recombination on the surface was much
smaller than the rate of 14N15N release [6]. This means
that even the most reactive 14N ads are not the main sites
for 15N recombination. One can conclude that in spite of
assumptions usually made in mesoscopic models [3-5],
surface recombination of N on silica takes place on
weakly bonding active sites. Atoms adsorbed on these
sites are not stable and therefore they are not detectable in
our experiments.
3.2. Adsorption and reactivity of O on silica
Similar isotopic exchange experiments were performed
with oxygen plasmas containing 36O 2 . Fig. 4 shows the
evolution of the density of 16O atoms picked up by a
discharge in 36O 2 from the surface of the discharge tube
made of silica. The saturation is reached at a level close
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Odes [cm-2]
16
6.0x1014
O3
3.0x1014
p=6.7 mbar
E=0.16 J/pulse
0
50
t [ms]
4x1015
Fig. 5. Time evolution of the absolute number density of
O and O 3 measured in the afterglow of a pulsed dc
discharge in O 2 at p = 6.7 mbar in the presence of high
specific surface silica material on the reactor walls.
2x1015
0
0
100
200
300
400
500
36
duration O2 plasma [s]
Fig. 4. Evolution of the density of 16O atoms picked up
from the surface as a function of the probe discharge
duration in 36O 2 . Silica discharge tubes were initially
cleaned by Ar plasma during 30 minutes.
3.3. Ozone production from surface recombination of O
The important role of weakly bonded O atoms is
confirmed by observation of an efficient O 3 production by
surface recombination (O+O 2 ) wall [10]. This reaction
occurs spontaneously if the binding energy of the species
is below 1 eV. Fig. 5 shows the decay of atomic O
concentration in the post discharge of a pulsed discharge
in O 2 in the presence of high specific surface silica
material on the reactor walls. The removal of O is
accompanied by a rapid production of O 3 which can only
be explained by the surface mechanism because the
3-body gas phase recombination is too slow. In the
conditions shown in the graph below, ozone productions
accounts for 25% of the oxygen atom loss rate on the
silica surface.
4. Conclusions
In this work, interaction between N and O atoms and
silica surface was investigated using isotopic exchange
technique.
In addition to an excellent sensitivity
(~1013 cm-2), the advantage of this technique compared to
the standard surface analysis is that it provides
information about the reactivity of adsorbed species. The
point of departure of our study was based on the concept
of surface kinetics proposed in the mesoscopic models
such as [3-5]. The experimental results obtained in the
present work demonstrate that the realistic picture of
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O
0.0
Silica
6x1015
9.0x1014
O, O3 [cm-3]
to a full monolayer as in the SiO 2 -N 2 case shown in
Fig. 3. These experiments demonstrate that under O 2
plasma exposure O atoms on the outmost layer of oxide
materials are continuously replaced by oxygen atoms
coming from the gas phase. This observation points to a
fact that from the viewpoint of plasma-surface
interactions there is no fundamental difference between
adsorbed O-atoms and surface atoms of the material itself.
The parameter that controls the reactivity is the binding
energy of these atoms to the surface.
interaction between N 2 /O 2 plasmas and silica surfaces is
much more complex. The major findings of this study
can be summarized as follows:
1. Dynamic nature of silica surface under plasma
exposure. The surface in contact with the plasma is
not static. Atoms of the material are continuously
exchanged due to the bombardment by ions and
radicals.
2. Distribution of reactivity. Atoms adsorbed on real
surfaces have different reactivities depending (most
probably) on their binding energy and local adsorption
configuration.
3. The important role of weakly bonded atoms. We have
found that unstable weakly-bonded N and O atoms are
the main recombination sites on silica. These weekly
bonded species may be either physisorbed or
chemisorbed with the binding energy smaller than
approximately 1 eV.
We believe that the relevance of above observations is
not limited to the case of the N 2 /O 2 -SiO 2 system and
similar behaviour is expected for different surfaces and
plasma chemistries. These general phenomena have to be
taken into account in models for a realistic description of
surface kinetics in plasmas.
5. Acknowledgement
Financial support from the French National Research
Agency (ANR) and from Ecole Polytechnique is
gratefully acknowledged.
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[5]
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