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HU Berlin Plasmaphysik
The Scavenger Effect - does it work?
W. Bohmeyera,d, F.L. Tabaresb, M. Baudacha,g, A.
Cwiklinskie, T. Schwarz-Selingerg , A. Markinc ,
J.A. Ferreirab, G. Fussmanna,d and A.
Loartef a  AG Plasmaphysik der Humboldt
Universität, Newtonstr. 15, D-12489 Berlin,
Germanyb Euratom  Ciemat Association, 28040
Madrid , Spainc Institute of Physical
Chemistry, Russian Academy of Sciences, Moscow,
Russiad Max-Planck-Institut für Plasmaphysik,
EURATOM Ass. Wendelsteinstr. 1, 17491 Greifswald,
e Fachbereich für Physik, FU Berlin, Arnimallee
14, D-14159 Berlinf EFDA closed support
unit,D-85748  Garching,Boltzmannstr.2  Germanyg
Max-Planck-Institut für Plasmaphysik, EURATOM
Ass.Boltzmannstr.2 D-85748 Garching   This work
has been carried out within the research
programme of the European Task Force on Plasma
Wall Interaction and was partially supported by
TW4-TPP-TRIDEP.
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The Scavenger Effect - does it work?
Introduction Experimental set up and
results for the pump duct Experimental set
up and results for the main chamber
Summary
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Scavanger technique
gas phase reactions of nitrogen or nitrogen
containing species with the growth precursors.
Formation of non-reactive species that can be
pumped away

stable non-reactive species
F.L.Tabarés, V. Rohde and the AUG Team Nucl.
Fusion 45 (2005) L27-L31.
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Experimental setup
Thickness measurement
Langmuir probe
QMS
window
Si wafer
CH4 /N2
1.5 m
no uncontrolled CxHy losses at cold walls
the behavior of re-eroded species is studied
ions can not reach the collector plates
QMS turbopump
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pump duct deposition profile

highly sticking species
QMS
CH4C2H2C2H6
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Pump duct / nitrogen
strong influence of nitrogen during
methane injection no influence of nitrogen
for clean plasma (not enhanced
erosion by nitrogen)
0.02nm/min
-
-
Scavenger effect yes, it works !
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Pump duct Collector temperature 310 and 330 K
nitrogen effect -reduction of deposition
-shift to from
deposition to erosion
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Pump duct deposition profile
ex situ thickness measurement (Tcoll not fixed ,
about 320 K)
strong influence of nitrogen
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Nitrogen injection at different positions
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hot tube .5 .2
pump duct 2 1 .5
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FN2 sccm
nm
2
0
0 50 100
150 min
2. into the hot tube strong
effect
1. into the pump duct weak
effect
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Neon injection
Neon has no influence !
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Main chamber Experimental setup
QMS
Injection of CH4/N2/H2
Protection of the collector area against ions
by an additional limiter Ions are not
included in the deposition process
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Main chamber, He-plasma
no atomic hydrogen ? no HCN and NHx Deposition
is not influenced ? no volume process No erosion
? no surface effect
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Main chamber, H2 discharge, methane, nitrogen
reduced deposition surface and volume process
enhanced erosion surface reaction
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Main chamber, He-discharge
Bayessian data analysis
HCN CH3CN CH3 NH2
NH3
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4 important reactions (Bayessian data analysis)
1. CH3 NH2-gtCH3NH2 (Methylamine) not
stable
surface reaction volume reaction
2. CH3 HCN-gtCH3CN (Acetonitrile)H
-gt C2H2 H2 XN bzw.-gtC2H4 XN
volume reaction s ltlt1
3. C2H HCN-gtC2HCN (Cyanoacetylene)H
volume reaction s 1
4. C2H3 NH2-gtC2H3NH2(Ethenamine) lt-gtC2H4NH(Ethy
lenimine)lt-gtC2H4NH
volume reaction , s1
M.Baudach, thesis, HU Berlin, 2008
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Summary
Nitrogen molecules are stable and dont influence
film formation, but they are decomposed in the
plasma.
Close to the plasma- where energetic neutrals
are included- enhanced erosion is detected
(surface process) as well as a reduction of the
deposition rate by a volume process. Far away
from the plasma we detected only a volume effect,
the conversion of (high sticking) hydrocarbons
to inert ones, the so called scavenger
effect. Enhanced erosion was not found.

.
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PSI-2
Thank you!
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HU Berlin Plasmaphysik
Pumping duct experiments
No uncontrolled CxHy losses at cold walls The
behavior of re-eroded species is studied Ions
can not reach the collector plate plasma density
in the tube
Turbo pump QMS
C2H2
CH4
C2H6
CH4C2H2C2H6 342 (traces of C3Hy)
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HU Berlin Plasmaphysik
Evaluation of the sticking behavior in the duct
hydrogen plasma for 6 hours
collector temp. 310 K
deposition
erosion
Most of the particles stick at the inner wall of
the tube. Only 5 are found at the bottom The
sticking probability is high
20 nm/ 6 h discharge
CH4C2H2C2H6 342
QMS
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HU Berlin Plasmaphysik
duct, summary
Nitrogen volume reaction, scavenger
effect Hydrogen surface reaction , erosion
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HU Berlin Plasmaphysik
kcm3/s
10-19
10-12
10-10
10-16
10-11
ionstable molecule
10-10
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HU Berlin Plasmaphysik
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HU Berlin Plasmaphysik
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HU Berlin Plasmaphysik
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HU Berlin Plasmaphysik
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HU Berlin Plasmaphysik