Download Observation of fast hydrogen atoms formed by ion bombarding of

Observation of fast hydrogen atoms formed by ion bombarding of
surfaces
T. Gans, V. Schulz-von der Gathen, U. Czarnetzki and H.F. Döbele
2 Experimental setup
University Essen, Institute for Laser- and Plasma Physics,
45117 Essen, Germany
Measurements are performed in an asymmetric capacitively
coupled RF discharge at 13.56 MHz in hydrogen with a 1
% argon admixture. The setup is described in detail elsewhere [1]. Two plane circular electrodes of 100 mm diameter are separated by a discharge gap of 25 mm. The RFAbstract
power ranges between 10 W and 100 W and the gas pressure
between 10 Pa and 150 Pa. The discharge axis is imaged
We report on the observation of fast hydrogen atoms in a onto the entrance slit of a 2 m grating spectrograph (Jenopcapacitively coupled RF reactor by optical emission spec- tik PGS). A special gateable CCD-camera (Picostar , LaVtroscopy. For the analysis we use the prominent H emission ision GmbH, 576 x 384 pixels, 13.2 x 8.8 mm ) samples
line of atomic hydrogen in combination with other lines from spectral intervals of about 4.5 nm with a spectral resolution
molecular hydrogen and argon. Several characteristic emis- of 0.04 nm and a spatial resolution of about 0.5 mm. The
sion structures can be identified. One of these structures is optical lines-of-sight are parallel to the electrode surfaces.
related to fast hydrogen atoms traveling from the surface of At each of these positions a ’classical’ emission spectrum
the powered electrode to the plasma bulk. From the appear- is recorded. The temporal resolution is 6 ns. The camera
ance time within the RF period we conclude that this feature gate can be fixed at a defined phase position of the RF-cycle.
is originated from ion bombardment of the electrode surface. From a series of such measurements at different phase posiMeasured pressure dependencies and a simple model for the tions the temporal development of specific emission lines at
ion dynamics support this assumption.
the selected wavelength can be reconstructed. The lifetime
of the excited states causes a blurring of the temporal evolution and a delay of the emission maximum with respect to
the excitation. In case of the H -line the delay is of the order
1 Introduction
of 5 ns.
Atomic hydrogen is an essential element in many low- and
high temperature plasmas. In fusion plasmas as well as in
technical plasmas surface interactions of the atomic or ionic
component are of great importance. Open questions are e.g.
the mechanisms leading to the generation of fast hydrogen
atoms or highly vibrationally excited hydrogen molecules
which are of great importance for technical discharges and
for the production of negative ions, respectively. Here the
interaction of ions with surfaces is observed at the powered electrode in a capacitively coupled RF-discharge. Typical ion energies at the cathode are of the order of several
hundred eV. Sheath expansion heating of the electrons, secondary electron emission and ion bombardment of the cathode are modulated temporally. This leads to complicated
temporal and spatial emission structures. These structures
are observed by a fast gated CCD camera which allows the
measurement even of faint molecular lines. The emission
structures of various atomic and molecular lines are compared. By this comparison the excitation channels can be
identified.
3 Experimental observations
Figure 1 displays a space-time evolution of the H line emission. The abscissae comprises 2 periods of 74 ns of the excitation. The transverse axis gives the distance from the powered electrode located at the bottom of the figure. The voltages across the plasma sheath in front of this electrode have
been measured by Czarnetzki et al. [2]. These measurements
show that there is a temporal reversal of the sheath electric
field and potential. During this short phase of positive voltage, electrons are attracted towards the electrode. The time
behaviour of the sheath potential is sketched in the lower part
of fig. 1.
In the asymmetric discharge the discharge chamber is on
the same potential as the grounded electrode. The different areas of the powered and the effective grounded electrodes lead to a higher current density close to the surface of
the powered electrode. Therefore, also the plasma emission
exhibits an asymmetric spatial structure, especially at high
1
15
II
III
10
5
IV
ABE656
I
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Distance from the powered electrode [mm]
20
SH EA T H
Distance from the powered electrode [mm]
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Time [ns]
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40
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80
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Time [ns]
Usheath
Figure 2: Space and time resolved emission profile of a
Fulcher- (v=2, Q=0) emission line; discharge conditions
as in Fig. 1
Figure 1: Space and time resolved emission profile of H
within a hydrogen discharge at 113 Pa and a transmitted
power of 100 W. Sketched is the time development of the
sheath voltage as measured by Czarnetzki et al. [2]
sion in the region of structure III, expanding from the sheath
edge to the grounded electrode (fig. 2). A comparison with
argon transitions with similar excitation cross sections as H
exhibits the same characteristic.
Neither the Fulcher nor the argon lines exhibit structure
IV of the H emission. Petrovic et al. [4] observed a similar phenomenon in a hydrogen DC discharge and attributed
it to fast neutral hydrogen atoms created at the surface by
the impact of hydrogen ions. These fast atoms are excited
by collisions with hydrogen molecules of the background
gas. The cross sections for collisional interaction between
fast hydrogen atoms and molecules indicate, however, that
the main channel for collisional losses is vibrational excitation of molecules [7]. The dissociation cross section changes
by orders of magnitude between the different vibrational levels. Therefore, the production of these vibrationally excited
hydrogen molecules can be of great importance for the discharge behaviour.
The Doppler-shift of the H radiation is a direct measure
of the particle velocity. However, here observation is parallel to the surface while the fast atoms move perpendicular
to the surface. In the collision process which causes excitation the fast atom can be scattered with a component in
the direction of observation. Therefore, two conditions for
the observation of fast atoms by the Doppler-shift have to be
met: Firstly, the atom must have enough energy so that the
excitation cross section is sufficiently large. This requires
energies in excess of about 75 eV. Secondly, the atom has to
be scattered with a component in the direction of observation. In general, scattering can occur in all directions and the
velocity distribution is blurred. The initial velocity distribution of the fast atoms can be reconstructed from the measure-
RF-powers. Emission maxima appear close to the powered
electrode. Here the evolution of several structures (I-IV) can
be distinguished. Structures I and II can be explained from
E-field measurements done by Czarnetzki et al. [2].
Structure I is caused by the field reversal across the space
charge sheath, typical for RF-discharges in hydrogen. Electrons are accelerated towards the electrode and the discharge
current is at its maximum. When the potential becomes negative again, electrons are pushed out of the sheath towards
the bulk. The second emission structure (II) is related to
this sheath expansion heating. Both maxima are delayed
with respect to the maximum excitation by about 5 ns due
to the lifetime of the excited state. Structure III results from
fast secondary electrons created by ion impact at the time of
maximum ion bombardment of the electrode. The emission
maximum is delayed substantially by more than 5 ns with
respect to the maximum negative voltage across the sheath
due to the ion inertia [8]. The secondary electrons are accelerated to energies beyond the maximum of the excitation
cross section already within a distance shorter than the mean
free path for collisions. They can contribute to the excitation
of H only after loosing part of their energy in collisions
with the background gas in the plasma bulk where no further
acceleration occurs. This hypothesis is supported by comparison with emission lines from the molecular hydrogen
triplet system (Fulcher lines). The excitation cross section
of the Fulcher lines is comparable to H at low energies between the threshold and about 25 eV but falls off much more
rapidly at higher energies. The Fulcher lines show no emis2
600
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kinetic energy [eV]
Distance from the powered electrode [mm]
Kinetic energy of f ast atoms
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Time [ns]
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pressure [Pa]
Figure 3: Intrusion of fast exciting neutrals into the plasma;
observation near H (
nm); p= 28 Pa; P=100 W
Figure 4: Pressure dependence of the kinetic energy of those
particles which result in excitation of fast hydrogen atoms
ment only, if the angle dependent scattering cross section is
known. In this work no attempt was made to determine the
velocity distribution. With the CCD-camera the entire spectrum of the Doppler-broadened H -line and its spatial distribution at a fixed phase of the RF is recorded simultaneously.
Each wavelength within the spectrum is related to a specific
Doppler-shifted velocity in the direction of observation.
Figure 3 shows the emission profile of atoms at a Doppler
nm corresponding to an energy of about
shift of
28 eV. The pressure was 28 Pa at an RF-power of 100 W.
Most remarkably is the propagation of the maximum of the
emission intensity into the plasma. This shows that the emission of fast neutrals from the surface is strongly temporally
modulated and occurs basically as a burst around t=20 ns in
the first period shown in fig. 3. From the slope a velocity of
or an energy of 220 eV results. This propagation velocity is believed to be close to the average velocity of
the emitted atoms perpendicular to the surface. The energy
of the fast atoms compares well with the expected energy of
the hydrogen ions at the applied peak-to-peak voltage of 950
V and the relatively large mean free path at the pressure of
28 Pa. Measurements at other Doppler-shifts under identical
conditions give the same result. With increasing Dopplershift, the signal amplitude decreases rapidly and measurements can be performed with a reasonable signal-to-noise
ratio up to shift corresponding to an energy of about 120 eV.
The propagation velocity was measured at several pressures between 10 Pa and 150 Pa and constant RF-power of
100 W. The result is shown in fig. 4.With increasing pressure, also the number of collisions of ions with the background gas increases and the mean ion energy decreases.
The same trend should be expected for the energy of the
fast neutrals. Qualitatively, fig. 4 supports this assumption.
However, at higher pressures, the kinetic energy approaches
a constant value of about 80 eV. The cross section for collisional excitation of H rises steeply by two orders of magnitude around this energy [7]. Therefore, only those atoms
within the velocity distribution with an energy above this effective threshold can be observed. If the mean energy is below this effective threshold at high pressures, not the average
velocity but part of the high energy tail of the distribution
function is observed.
4 Simple Model
The emission measurements show that the flux of fast neutral
atoms has a strong temporal variation. The maximum has a
phase shift of about 10 ns with respect to the maximum negative sheath potential. This phase shift is essentially the same
as for the secondary electrons which have a similar temporal
variation. These results suggest that also the ion bombardment at the electrode surface is modulated temporally. In
RF-discharges it is believed generally that ions due to their
inertia are drifting mainly in a phase averaged potential and
show only weak temporal variations. However, hydrogen
ions are relatively light and the temporal variations can be
much stronger.
In order to investigate whether the ion bombardment at
the cathode can be modulated temporally for light ions we
use a simple analytic model which takes into account only
charge exchange collisions with a velocity independent cross
section. This simplification ignores completely the complex
ion chemistry in the sheath and the details of the scattering
processes. Therefore, the results can be expected to give only
a qualitative picture. The temporal variation of the mean
energy is shown in fig. 5. The ion energy is measured in
units of
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3
Further, the dissociation cross section increases by orders of
magnitude at higher vibrational levels. Therefore, fast neutral atoms can influence the discharge behaviour strongly.
3.5
average ion energy /0 E
3.0
K =∞
2.5
2.0
K=1
K = 0.1
References
K=0
1.5
[1] T.Gans, V.Schulz-von der Gathen, and H.F.Döbele,
Plasma Sources Sci. Technol. 10 (2001) 17-23
1.0
0.5
0
1
2
3
4
RF phaseφ
5
[2] U.Czarnetzki, D.Luggenhölscher and H.F.Döbele,
Plasma Sources Sci. Technol. 8 (1999) 230-248
000628a.opj
0.0
6
[3] S.Djurovic and J.R.Roberts, J.Appl.Phys., 74 (1993)
6558
)
(
( '#
Figure 5: Average ion energy as a function of the RF phase.
The sinusoidal sheath voltage is zero at
and
.
The energy is measured in units of . K is a dimensionless
parameter depending on sheath voltage, mean free path, and
ion mass. Details are given in the text.
[4] Z.Lj.Petrovic, B.NM.Jelenkovic,
Phys.Rev.Lett. 68 (1992) 325
The parameter K is
$
* "'# , + '% & $ &
and
A.V.Phelps,
[5] S.B.Radovanov, K. Dzierzega, J.R.Roberts,
J.K.Olthoff, Appl.Phys.Lett. 66 (1995) 2637-2639
and
[6] S.B.Radovanov, J.K.Olthoff, R.J.VanBrunt,
S.Djurovic, J.Appl.Phys. 78 (1995) 746-757
and
[7] A.V.Phelps, J.Phys.Chem.Ref.Data 19 (1990) 653-675
[8] U.Czarnetzki, D.Luggenhölscher and H.F.Döbele, Appl.
Phys. A (2001) accepted for publication
Here
is the maximum sheath width, the mean free
path, and
the time averaged sheath potential. With decreasing K the temporal variation becomes less pronounced
but the phase shift increases. For our conditions, K varies
typically in the range between 0.1 and 1. Therefore, phase
shifts of 1 (corresponding to 10 ns) or more can be expected.
This is in good agreement with the observed phase shift in
fig 1. According to the model, the phase shift should increase at lower pressures. This is also in agreement with the
experimental observations. For ions with higher mass, e.g.
argon, K would be substantially smaller
and
the temporal variation would vanish practically.
*.-0/1 2
23
5 Conclusions
Temporally and spatially resolved emission spectroscopic
investigations of a hydrogen RF discharge have been performed. The emission characteristics of several atomic and
molecular lines have been compared. From this comparison the excitation mechanisms of various emission structures
in the discharge can be identified. Most remarkable is the
observation of fast neutral hydrogen atoms created by ion
bombardment of the cathode. The energy of these atoms is
comparable to the energy of the impinging ions. It can be
expected that the neutral flux is of the same order as the incoming ion flux [4]. An inspection of the cross sections for
the interaction of fast hydrogen atoms with molecular hydrogen shows that the dominant channel for energy loss is
vibrational excitation[7]. Vibrationally excited hydrogen is
regarded as one of the precursors of negative ion formation.
4