Atomic Lithium Beam Spectroscopy for Ne and Te in Reactive Plasmas
M. Böke and J. Winter
Institut für Experimentalphysik II, Ruhr-Universität Bochum, D-44780 Bochum, Germany
1. Introduction
In a reactive plasma the species composition and the flows of particles to a substrate depend
upon the electron energy distribution and its spatial variation and time dependence. The
electron density has a large influence on the growth rate of thin films deposited from the
discharge gas. To control plasma assisted processes it is important to have a detailed
knowledge of these quantities during every phase of a process, preferably with high spatial
and time resolution. The emitted radiation of a thermal atomic lithium beam is used to obtain
the density of the electrons and their mean kinetic energy. Using an intensified CCD camera
in combination with interference filters the light emitted from neutral lithium atoms (excited
through collisions with plasma electrons) is detected. The plasma parameters are obtained
from appropriate line intensity ratios and will be compared with probe measurements in rare
gases. The characterization of the atomic lithium is done by LIF measurements with laser
diodes. Discharge geometry is an GEC reference cell with or inductive coupling at 13.56
MHz and at pressures from 0.1 to 50 Pa.
2. Experimental Setup
Using a CCD-camera, optionally in combination with a spectrographical system or with
interference filters for the observed lithium lines, the spectral line emission from a beam of
neutral lithium atoms after collisional excitation
IC C D w ith filters
is investigated perpendicularly to the beam
or sp ectrom eter
direction with spatial and temporal resolution.
The atomic beam is produced by a lithium oven
which consists in a Knudsen cell - filled with an
G
E
C
-C
ell
alloy of copper and lithium (2% lithium) - and
L an gm u ir
p rob es
an aperture system in order to collimate the
beam. By indirect resistive heating of the
Knudsen cell up to 950°C one receives a
dynamic balance between diffusion from lithium
L i-oven
atoms to the surface of the Cu-Li-alloy and
evaporation of atoms controlled by vapour
pressure in the cell. The advantage of this solid
state source in comparison to fluid sources and
ion beam sources is a simple construction, its
high operating time, optional orientation of
mounting and its insensitiveness to unexpected
air leakage. The outlet velocity is in the range of
Figure 1: Experimental Setup
103 ms-1 and the flux of lithium atoms out of the
oven at about 1017-1018m-2s-1. The diagnostics is
installed on a ICP GEC reference (5 turn pancake coil, L=1.2µH; lower electrode (on ground
or defined potential): diameter: 10cm, distance between electrode and quartz plate: 4cm,
pressure: 0.1-50Pa, TeL eV, NeL 1017m-3-1018m-3, f=13.56MHz).
3. Atomic Lithium Beam supported Emission Spectroscopy
Atomic beam supported Li emission spectroscopy is
F
D
P
based on the measurement of line intensities and E /eV S
intensity ratios of electron excited states of the neutral
4
4
4
4
4 6 0 .3
Li- atom. These are in particular the 2S-2P transition at
2 7 4 .1
4 9 7 .2
3
3
670.8nm, the 2P-3S transition at 812.6nm, and the 2P- 4
3D transition at 610.4nm, see fig.2.
Li-beam
3 3 2 3 .3 6 1 0 .4
8 1 2 .6
spectroscopy has already been used successfully for the 3
diagnostics of the edge region of fusion plasmas to
determine plasma density fluctuations [1]. In the case of
2
reactive low pressure plasmas (e.g. ICP or CCP) several 2
factors impede the application. One main obstacle
6 7 0 .8
comes from the fact that these low temperature plasmas 1
are usually operated in the 0.1-10Pa range and have a
low degree of ionization. Thus a large fraction of 0
2
neutrals has to be accounted for.
Figure 2: Term Scheme of Lithium
At low pressure (0.1Pa) one obtains a well defined
narrow lithium beam (see fig. 3) because the mean free path of lithium atoms is in the range
of 10 centimeters and it is possible to reach high spatial resolution in the measurement of the
plasma parameters. The width of the lithium beam is obtained by laser induced fluorescence
(LIF) at the 2S-2P transition (670.8nm) using a tunable diode laser in an external resonator
(Littman setup).
1,6
1,4
1,2
FWHM / cm
1,0
0,8
0,6
FWHM Li beam
aperture: 5mm
distance: 4cm
0,4
0,2
0,0
0,0
0,5
1,0
pressure / Pa
Figure 3: LIF Intensities:
Beam Width
1,5
2,0
T / [eV ]
e
—
Figure 4: Crosssections for e -Excitation
and Ionization
At higher pressure (a few 10Pa) the situation changes: The beam becomes broader and
degenerates to a lithium cloud due to collisions with the discharge gas. The discolouration of
the discharge stays usable but the spatial resolution is limited and the modelling effort
becomes higher.
The other problem is due to the low electron temperature in the reactive plasmas. Whereas for
fusion edge conditions with Te values above 10eV energy independent constant cross sections
may be used (see fig. 4), the threshold region with significant dependence on energy applies
for reactive plasmas with Te of about 1eV. A collisional radiative model in consideration of all
states in Figure 2 is required.
4. Probe Measurements and α−γ−Hysteresis
To prove and to characterize the diagnostics is installed on plasmas in rare gases and
compared to probe measurements. The results of these probe measurements in the ICP GEC
cell are shown in the following figures. The central electron density (see fig 5) increases with
the discharge pressure. Increasing the power (fig. 6) leads the growth of the electron density
into a saturation area and the radial density profiles (see fig. 7) become flatter. The zone of
high electron density is enlarged.
60
50
50
40
m
N e / 10 m
-3
-3
40
30
16
N e / 10
16
30
20
30sccm Ar, 1 00W
10
20
30sccm Ar, 5Pa
10
0
0
0
1
2
3
4
5
6
7
8
9
60
80
100
120
140
160
180
200
Power / W
pressure / Pa
Figure 5: Central Electron Density as a
Function of the Power
Figure 6: Central Electron Density as
a Function of the Pressure
1
100
90
Power / W
N e (nor malized)
80
rad ial pr ofile
P=100W, Γ =30sccm Ar
2 Pa
5 Pa
8 Pa
5 Pa 200 W
10
20
α
γ
60
50
40
0.1
0
5 0s ccm A r
70
30
r / mm
Figure 7: Electron Density
Profiles
40
50
0
2
4
6
8
10
12
14
16
pressure / Pa
Figure 8: α−γ−Ηysteresis
Figure 7 also shows flatter radial profiles at lower pressures. The α−γ−hysteresis of the
discharge one can see from figure 8: Increasing the power at a given pressure the discharge
switches at the dotted curve from α- into γ-regime (transition from mainly capacitive to
mainly inductive coupling). Decreasing the power the transition back into the α-mode takes
place when the power reaches the solid curve.
5. Résumé and Outlook
From the measured line intensities and line intensity ratios it is possible to obtain locally the
electron energy and density at low pressure. For that purpose a detailed collisional radiative
model is in preparation. At higher pressure the spatial resolution of the diagnostics is reduced
due to beam broadening and degeneration to a cloud of lithium atoms. In addition in this case
the density reduction of the neutral lithium atoms by collisional ionization has to be taken into
account.
In order to enlarge the efficiency of the diagnostics and to reach a sufficient beam collimation
and penetration depth even at higher pressures the transition to pulsed and fast atomic beams
(also with heavier atoms, e.g. potassium) provided by laser ablation [3] is in preparation. Also
a comparison to Thomson scattering at the same discharge geometry is in preparation.
This work is supported by Deutsche Forschungsgemeinschaft within SFB 191 (B8).
6. References
[1]
A. Dinklage, T. Lokajczyk, H.J. Kunze, B. Schweer and I.E. Olivares; Rev. Sci.
Instrum., 69 (1), 321-322 (1998)
[2]
M. Böke, G. Himmel, B. Schweer and J. Winter "Atomic Lithium Beam Spectroscopy
for Ne and Te in Reactive Plasmas" in: Lausanne Report LRP 629/99 "Workshop on
Frontiers in Low Temperature Plasma Diagnostics III", Switzerland ( 1999)
[3]
Y.T. Lie, A. Pospieszczyk and J.A. Tagle; Fusion Technology, 6, 447-452 (1984)