HU Berlin Plasmaphysik The Scavenger Effect - does it work? W. Bohmeyer* a,d, F.L. Tabares b, M. Baudach a,g, A. Cwiklinski e, T. Schwarz-Selinger g, A. Markin c, J.A. Ferreira b, G. Fussmann a,d and A. Loarte f a AG Plasmaphysik der Humboldt Universität, Newtonstr. 15, D Berlin, Germany b Euratom Ciemat Association, Madrid, Spain c Institute of Physical Chemistry, Russian Academy of Sciences, Moscow, Russia d Max-Planck-Institut für Plasmaphysik, EURATOM Ass. Wendelsteinstr. 1, Greifswald, e Fachbereich für Physik, FU Berlin, Arnimallee 14, D Berlin f EFDA closed support unit,D Garching,Boltzmannstr.2 Germany g Max-Planck-Institut für Plasmaphysik, EURATOM Ass.Boltzmannstr.2 D 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.

The Scavenger Effect - does it work? I ntroduction E xperimental set up and results for the pump duct E xperimental set up and results for the main chamber S ummary

gas phase reactions of nitrogen or nitrogen containing species with the growth precursors. Formation of non-reactive species that can be pumped away NX NX C x H y stable non-reactive species + F.L.Tabarés, V. Rohde and the AUG Team Nucl. Fusion 45 (2005) L27-L31. Scavanger technique

Si wafer window Thickness measurement Langmuir probe no uncontrolled C x H y losses at cold walls the behavior of re-eroded species is studied ions can not reach the collector plates QMS turbopump CH 4 /N m QMS Experimental setup

highly sticking species pump duct deposition profile CH 4 :C 2 H 2 :C 2 H 6 QMS:

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 !

Pump duct: Collector temperature 310 and 330 K nitrogen effect: -reduction of deposition -shift to from deposition to erosion

Pump duct : deposition profile ex situ thickness measurement (T coll not fixed, about 320 K) strong influence of nitrogen

Nitrogen injection at different positions 2. into the hot tube: strong effect 1. into the pump duct: weak effect hot tube. 5.2 pump duct Φ N 2 [sccm] min nm 6

Neon injection Neon has no influence !

Protection of the collector area against ions by an additional limiter Ions are not included in the deposition process vacuum chamber wall Optical system: in-situ film thickness H 2 /D 2 /Ar plasma Si collector Injection of CH 4 /N 2 /H 2 Main chamber: Experimental setup QMS

Main chamber, He-plasma no atomic hydrogen → no HCN and NH x Deposition is not influenced → no volume process No erosion → no surface effect

Main chamber, H 2 discharge, methane, nitrogen enhanced erosion surface reaction reduced deposition surface and volume process

Main chamber, He-discharge Bayessian data analysis HCN CH 3 CN CH 3 NH 2 NH 3

1. CH 3 + NH 2 ->CH 3 NH 2 (Methylamine) not stable 2. CH 3 + HCN->CH 3 CN (Acetonitrile)+H -> C 2 H 2 + H 2 + XN bzw.->C 2 H 4 + XN 3. C 2 H + HCN->C 2 HCN (Cyanoacetylene)+H 4. C 2 H 3 + NH 2 ->C 2 H 3 NH 2 (Ethenamine) C 2 H 4 NH(Ethylenimine) C 2 H 4 +NH 4 important reactions (Bayessian data analysis) surface reaction volume reaction volume reaction s <<1 volume reaction s ≈ 1 volume reaction, s≈1 [M.Baudach, thesis, HU Berlin, 2008]

Summary 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. Nitrogen molecules are stable and dont influence film formation, but they are decomposed in the plasma..

Thank you! PSI-2

HU Berlin Plasmaphysik Pumping duct experiments No uncontrolled C x H y 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 CH 4 :C 2 H 2 :C 2 H 6 =3:4:2 (traces of C 3 Hy) CH 4 C2H2C2H2 C2H6C2H6

HU Berlin Plasmaphysik Evaluation of the sticking behavior in the duct Most of the particles stick at the inner wall of the tube. Only 5% are found at the bottom The sticking probability is high hydrogen plasma for ≈ 6 hours collector temp. : ≈310 K CH 4 :C 2 H 2 :C 2 H 6 =3:4:2QMS: +1 nm / 6 h discharge +20 nm/ 6 h discharge 10 mm 30 mm deposition erosion

HU Berlin Plasmaphysik duct, summary Nitrogen: volume reaction, scavenger effect Hydrogen : surface reaction, erosion

HU Berlin Plasmaphysik ion+stable molecule k[cm 3 /s]

HU Berlin Plasmaphysik

HU Berlin Plasmaphysik

HU Berlin Plasmaphysik

HU Berlin Plasmaphysik

HU Berlin Plasmaphysik