1. Max-Planck-Institut für Plasmaphysik, EURATOM Association
M. Schlüter, W. Jacob, C. Hopf, and T. Schwarz-Selinger
Energy, Temperature, and Flux Dependence of the Chemical Sputtering of
Carbon Surfaces by Low-energy Ions in the Presence of Atomic Hydrogen
P 2-05
100 200 300 400 500 600 700 800 900 1000
10
12
10
13
0.1
1
10
10
-4
10
-3
10
-2
10
-1
50 eV
rate(cm
-2
s
-1
)
temperature (K)
Ar+
+ H°, R=jH° / jAr
+:
800 eV (R=350)
400 eV (R=340)
200 eV (R=340)
100 eV (R=360)
50 eV (R=350)
H°
(right scale)
yield(C/Ar
+
)
800 eV
r
0 1 10 100 1000
0
1
10
100
0.0
0.5
1.0
Ar
+
+ H°
700 K
300 K
E = 50 eV,
jAr
+= 4 10
12
cm
-2
s
-1
R = j H° / j Ar
+
yield(C/Ar
+
)
rx10 (Tmax) CH

PS (1) CS (2) CE (3) IECE (4)
C
CH
r
particles coverages
H°
Ar+
C
C-H-complex
(1, 2) temperature independent (3, 4) 340 K < T < 950 K
10 100 1000
0.1
1
10
Ar
+
+ H°
Ne
+
+ H°
He
+
+ H°
N2
+
+ H°
yield(C/ion)
energy (eV)
0 100 200 300 400 500
0
1
2
3
Ar
+
+ H°
Ne
+
+ H°
200 eV,
jion = 4  10
12
cm
-2
s
-1
yield(C/ion)
R = jH° / jion
 Understanding erosion of amorphous hydrogenated carbon thin films (a-C:H) is a prerequisite to model
carbon transport and hydrogen retention in future fusion experiments and to develop methods to
remove codeposited layers.
 Use of a particle beam experiment for isolated investigation of the ion–H synergism in order to develop
a microscopic picture.
 Attempt to calculate and predict erosion rates of carbon thin films due the ion–H synergism for different
ion-energies, temperatures and particle-fluxes.
Particle-beam experiment MAJESTIX
Modelling chemical sputtering
out diffusion of
volatile species:
EROSION
ion
atomic hydrogen
carbon
a-C:H hydrogen
ion bond breaking build up of volatile species
under the surface
x
a-C:H
Ellipsometry
H2, D2
ion source
H°, D° Ar+, Ne+, He+, N2
+
10 - 1000 eV
H°,D° source
Load lock &
preparation chamber
cold N2 gas
filament
the sample
many erosion
“craters”
Principle process of chemical sputtering:
Integral model of chemical sputtering:
1. Ions break C–C bonds.
2. Atomic hydrogen (H°) passivates the broken bonds.
3. Repetition of steps (1) and (2) leads to
subsurface formation of erosion precursors.
4. The erosion precursors desorb thermally activated
or ion-induced.
• UHV experiment with load lock / preparation chamber
• Quantified H atom beam source (about 1015 cm-2s-1)
• Ion source for low energy ions (20 - 800 eV, 1012 cm-2s-1)
• Samples: hard a-C:H films (H content approx. 30%)
• Temperature variation: 110 – 950 K
• Diagnostics: ellipsometry, 632.8 nm
10 100 1000
0.1
1
10
T = 300 K
R = 350
Ar
+
+ H°
yield(C/Ar
+
)
energy (eV)
Results
Model predictions and new data
Conclusions
 Temperature (T) dependence:
yields constant in temperature
range 110 – 350 K,
increase for T>350 K,
Tmax=700 K
 Energy (E) dependence:
yield increases with energy,
threshold < 20 eV
 Flux dependence:
yield increases with
neutral-to-ion flux ratio R,
saturation for R > 1000.
Model predictions for other projectile ions using the fit parameters from Ar+ + H° and comparison
to new data: He+ + H°, Ne+ + H° and N2
+ + H°.
 New integrated model for chemical sputtering describes E, T and R (neutral-to-ion flux ratio)
dependence for simultaneous bombardment of a-C:H films with ions and atomic H.
 The ion-induced processes, which depend on energy, mass and nuclear charge, are calculated using
TRIM.SP. The successful transfer to other ion species (He+, Ne+, N2
+) indicates that this description is
appropriate.
 Ion bombardment enhances the thermal chemical erosion yield at high temperatures through creation
of additional reaction centers.
For the case N2
+ + H° see Poster P1-14
Erosion rate:
* Ref.: Rev. Sci. Instrum., 74: 5123, 2003
* Ref.: J. Nucl. Mater. 33-37: 376, 2008
Introduction
Y are calculated using
TRIM.SP code. Ansatz:
Erosion yield:  solution of a rate equation system
for the 3 coverages in steady state:
bond breaking (TRIM.SP) limitation to surface
ion + H°
model
process terms
H° build up
creation of reactive sites
H° loss
CS erosion process
IECE erosion process
1. PS: physical sputtering.
2. CS: chemical sputtering.
3. CE: chemical erosion.
4. IECE: ion-induced chemical erosion.
Fit of model parameters to all
data for Ar+ + H°
(9 parameters, 86 data points).