30th ICPIG, August 28th - September 2nd , 2011, Belfast, UK
Topic number B5
Influence of helium admixture on charged particle dynamics in an
electronegative oxygen capacitively coupled plasma
A. Greb1 , K. Niemi1 , D. O’Connell1 , T. Gans1
1
Centre for Plasma Physics, Queen’s University Belfast, BT7 1NN, Belfast, Northern Ireland, UK
The influence of helium admixture on an electronegative oxygen capacitively coupled plasma
is investigated by means of a one-dimensional numerical fluid model with semi-kinetic treatment of electrons. This model is operated at 100 Pa with a sinusoidal driving voltage at a
frequency of 13.56 MHz and is solved self-consistently for each individual RF-cycle. The
−
change in the spatial profiles of dominant charged particles (O+
2 , O , e) as well as the electronegativity are analysed under variation of the helium admixture. Discharge excitation
dynamics, in particular of the atomic oxygen line λ = 844 nm (3 P −3 S ) are investigated
and compared to those observed from pure oxygen capacitively coupled plasma experiments.
The simulations show that additional helium in the oxygen discharge acts as inhibitor for
excitation mechanisms during the phase of sheath collapse.
solid: 0% helium
dashed: 99% helium
1.0
Normalised densiity
1. Introduction
To model the capacitively coupled plasma a
hybrid model was applied. There, the electron
transport coefficients and reaction rates for electron impact collisions are simulated kinetically
in advance [1], whereas the densities of the accounted species as well as the electric field are
simulated with a fluid approach. The simulations are performed in O2 gas with variable He
admixture at a constant pressure of 100 Pa. For
simplicity reasons, the simulations are done in an
one-dimensional domain across the discharge gap
of the capacitively coupled plasma.
A total of 9 species are accounted for in the
model. These species are electrons (e), the background gases oxygen (O2 ) and helium (He), oxygen molecular ions (O2+ ) and atomic oxygen negative ions (O− ), helium metastable atoms (He∗ ),
helium ions (He+ ), helium molecular ions (He+
2)
and helium excimers (He∗2 ). The incorporated
oxygen ion species were particularly chosen because of experimental results [2, 3]. There, their
densities have been measured by a photo detachment technique and were found as the most dominant species under the given conditions.
A comparable chemical reaction scheme for the
helium species used in this work can be found in
the literature [4].
The self-consistent model is closed by the
boundary conditions for each considered species,
by the potential and mean electron energy.
+
O
0.8
2
0.6
e
0.4
0.2
-
O
0.0
0
10
20
30
40
Position (mm)
Fig. 1: Time averaged spatial profiles of O− , O+
2 and
electrons for two different helium admixtures at U0 ≈
300 V and p = 100 Pa.
2. Influence of He admixture
2.1. Spatial profiles
The spatial profiles of the dominant charged
−
particles (O+
2 , O , e) are obtained from a converged solution along the symmetry axis of the
discharge and are shown in figure 1. The profiles of the heavy particles remain static for all
times, however, the electron density profile shows
a temporal variation due to their higher mobility, which was then time averaged. In figure 1 all
density profiles were normalised to the maximum
density of the dominant positive ion O+
2 for each
corresponding helium admixture.
It can be observed that the atomic oxygen
negative ion spatial profile exhibits a maximum
within the centre of the bulk for a high helium
admixture (99%), whereas it shows a broader
30th ICPIG, August 28th - September 2nd , 2011, Belfast, UK
Topic number B5
0.70
electronegativity
0.65
0.55
n
O
-
/n
e
0.60
0.50
0.45
0.40
0
20
40
60
80
100
He fraction (%)
Fig. 2: Electronegativity as a function of the helium
admixture at constant power P = 0.1 W/cm2 and p
= 100 Pa.
double peaked profile near the electrodes for less
He admixture. Furthermore, the O− density
decays with a higher gradient in the electrode
region than the electrons but never exceeds the
electron density. This is due to the dominant
production and destruction mechanisms. Generally, the spatial variation of electrons entails a
corresponding variation of the O− density, since
the latter is only generated by electron collisions
in the model. However, in the bulk region the
destruction of O− exceeds the production due
to collisions with O2 (1 ∆), which is assumed
to be the main collision partner leading to
destruction [5]. This leads to the observed double
peaked structure in the O− spatial profile.
2.2. Electronegativity
In this work, the electronegativity is defined as
the density ratio of negative ions and electrons
(nO− /ne ). This density ratio is obtained by averaging the corresponding densities in space and
time. Figure 2 shows the dependence of the electronegativity on the helium admixture for a constant power (0.1 W/cm2 ) and pressure (100 Pa).
Over a long range of He gas fractions the
electronegativity increases over-linearly until it
reaches its maximum at 0.66 which means that
the time and space averaged O− density never
exceeds the electron density. Experiments show
typically slightly higher electronegativity but this
may be explained due to the neglect of other possible negative ion species in the applied model.
Furthermore, it is observed that for very high
admixtures of helium (> 95%) the electronegativity drops rapidly to an even lower value.
Fig. 3: Emission from atomic oxygen line λ = 844 nm
for a) pure oxygen and b) 1 % helium admixture at
same parameters as figure 2.
This is due to the rapid increase in the electron
density density with additional helium, whereas
the atomic oxygen negative ion does not decrease
at the same rate.
2.3. Excitation dynamics
The influence of He admixture on the excitation
dynamics is shown exemplarily by means of the
emission from the atomic oxygen line λ = 844 nm
(3 P −3 S) through dissociative detachment of oxygen molecules. Figure 3 illustrates the observed
emission features as phase resolved optical emission spectroscopy (PROES) image in grayscale for
(a) a pure oxygen discharge and (b) oxygen with
50% helium admixture at same conditions as in
figure 2.
For a pure oxygen discharge, clearly three excitation features can be observed (figure 3a). These
features agree very well with experimental results
for a comparable set of parameters [6] and can
be identified as sheath expansion (I), sheath collapse (II) and excitation due to secondary electrons (III).
As soon as helium is added to the plasma the
emission features change drastically (figure 3b).
30th ICPIG, August 28th - September 2nd , 2011, Belfast, UK
The additional helium clearly amplifies the emission from the sheath expansion process (I) and at
the same time leads to a slightly more pronounced
emission from the sheath collapse phase (II). Furthermore, the sheath collapse shows a phase shift
of the maximum to later times. The emission
from secondary electron collisions (III) decreases
and can be explained by means of the higher secondary electron emission coefficient of helium.
Moreover, the helium in the discharge promotes
a rise of the mean electron energy due to its higher
ionisation and excitation energy thresholds.
Further investigations need to be conducted
with regard to experimental benchmarks and
implementations of required modifications to
the simulation model to understand and predict
better the influence of helium on charged particle
dynamics in an electronegative oxygen capacitively coupled plasma.
References
[1] G.J.M. Hagelaar and L.C. Pitchford, Plasma
Sources Sci. Technol. 14 (2005) 722
[2] E. Stoffels, W.W. Stoffels, D. Vender, M.
Kando, G.M.W. Kroesen and F.J. de Hoog,
Phys. Rev. E 51 (1995) 2425
[3] D. Vender, W.W. Stoffels, E. Stoffels, G.M.W.
Kroesen and F.J. de Hoog, Phys. Rev. E 51
(1995) 2436
[4] K. Niemi, S. Reuter, L.M. Graham, J.
Waskoenig, N. Knake, V. Schulz-von der Gathen and T. Gans J. Phys. D: Appl. Phys. 43
(2010) 124006
[5] H.M. Katsch, T. Sturm, E. Quandt and H.F.
Döbele, Plasma Sources Sci. Technol. 9 (2000)
323
[6] S. Nemschokmichal, K. Dittmann, J. Meichsner, IEEE Trans. Plasma Sci. 36 (2008) 1360
Topic number B5