1. Material transport study with IBA and Role of vibrationaly
excited hydrogen in erosion
P. Pelicon, I. Čadež, S. Markelj, Z. Rupnik, P. Vavpetič
Microanalytical Centre, Department for Low and Medium Energy Physics
Jožef Stefan Institute, Association EURATOM-MHEST
Jamova 39, SI-1000 Ljubljana, Slovenia
Report on WP09-PWI-04-03/MHEST projects,
SEWG 04 (Material migration) meeting, JET-Culham, 7-8 July 2009

2. Contents: Introduction: tandetron, ERDA
2. Material transport as seen with HE focused ion beams
3. Study of the role of vibrationally excited hydrogen molecules in erosion of a-C:H
4. Conclusion

3. 1. Introduction: tandetron, in-situ ERDA, focused ion beam

4. Elastic Recoil Detection Analysis (ERDA) for hydrogen depth profiling in
PFCs
RBS/ERDA :
Beam: 4230 keV 7Li2+, Sample tilted 75°
RBS detector at 160°, ERDA detector at 30°
ERDA detector equipped with 11 µm Al foil
Dose controlled by mesh charge integrator
Pelicon et al., NIM B 227, 591 (2005)

5. Results of the round robin measurements "Hydrogen in Silicon", organized by “Bundesastalt Für Materialforschung und –prüfung” (BAM), Berlin. The result of the IJS, obtained with the Elastic Recoil Detection Analysis (ERDA) with Li ions, is marked by red circle, average value by thick blue line. Result of the laboratory 1 is the result of BAM. (Source: U. Reinholz, H.P. Weise, BAM Berlin, Round robin test "Hydrogen in Silicon", Results sent to the participants of the round-robin, Pelicon et al., NIM B 227, 591 (2005))

6. In 2008, ERDA has been configured inside new measurement chamber at JSI for studies of hydrogen in surfaces, thin films, dinamic processes of interaction of hydrogen and surfaces.
Fig.1: Erda spectrum of a:C:D film for calibration of D detection methods, measured by 4.2 MeV 7Li beam (In collaboration with Th. Schwartz-Seliger, IPP Garching, sept. 2008)

7. High Energy Focused Ion Beam
Focused beam formation:
Magnet quadrupole triplet lens
is focusing the beam at the
analyzing object.
Example: beam envelope
for the focusing of 7Li2+ beam
to 3x3 µm2
Blue: horizontal plane
Red: vertical plane

9. Thin H:D:C layer on silicon
(TEXTOR pump duct sample,
C:H:D soft film on Si) H
Application of crescent-shaped
slits at the detector to diminish
angular energy spread and maximize
solid angle: isotope resolution in the
spectra is preserved: resolved
D and H signal D
CFC NB31 H D

10. Model castellated structure has been exposed in the erosion-dominated
conditions of tokamak fusion reaktor TEXTOR at Forschungszentrum Jülich1
Measured
castellated
Graphite
section
1A. Litnovsky et al, J. Nucl. Mater , 917 (2005).

11. Micro-ERDA: lateral mapping of hydrogen at plasma-exposed
surface of castellated limiter[1]
Mo Lα
Areal distribution
of molybdenum (islands)
measured
by Li-beam excited
X-ray emmision (LIXE)
and simultaneously measured
hydrogen lateral distribution
by Li-beam ERDA over an
area of 1240 x 450 µm2.
Sensitivity of the method is
0.1 at. %.
450 µm
1240 µm
Hydrogen
Distribution of hydrogen is anti-correlated with the distribution of molybdenum. Hydrogen retention is higher in the surface of uncoated graphite.
[1] Litnovsky et al, J. Nucl. Mater , 917 (2005).

12. Microbeam analysis of flake deposits
Flakes may be individually analyzed for their elemental composition
with PIXE (Proton Induced X-ray Emmission), if layered on adhesive supporting material.
Prepared flake crosscut slices may provide information on deposition sequence. Embedding of the flakes in the supporting material is required to enable mechanically stable cuts.
In our first experiments with flake cutting, the flakes were embedded
at -30° C in Tissuetek (liquid vax) and in a frozen water droplet. Both media
are used for biological tissue preparation. Cutting was succesfull with Tissuetek.
Both 35 and 15 micrometer thick slices were even and smooth in appearance.
However, during the Tissuetek drying, adhesion to the surrounding
Tissuetek resulted in slice destruction.

13. Flakes from TEXTOR RF antenna (Collaboration with FZJ):
Fe, Si, Cr, Ni
dominant traces
160 x 160 µm2 Fe Si Cr Ni Ti S
Ti, S above LOD

14. 3. Study of the role of vibrationally excited hydrogen
molecules in erosion of a-C:H
collaboration with Thomas Schwarz-Selinger, IPP, Garching
A: Objectives
B: Comments on previous results
C: New results
D: Workplan for the rest of 2009

15. A. Objectives
We are interested in different processes involving vibrationally excited neutral hydrogen molecules (VEHMs) that are, or potentially might be, important for edge plasma. Within present TA we are interested in potential role of VEHMs in chemical erosion of carbon layers.
Chemical erosion of carbon materials by neutral hydrogen atoms is well established and studied (e.g. [1]). Recent model calculations [2] have indicated possible role of vibrationally excited D2 molecules for explanation of some experimental results on chemical erosion of deuterated carbon.
Objective of our effort is to elucidate the possible role of VEHMs having thermal kinetic energy on chemical erosion of carbon films. Present study is focused on explanation of previously observed increase of the thickness of carbon film at room temperature when it is exposed to hydrogen atom flux.
Target samples used in the present experiments were amorphous hydrogenated carbon thin films (a-C:H). Layer thickness was measured before and after sample exposure to hot neutral hydrogen beam by ellipsometry and total erosion was determined [3]. ERDA and RBS spectra are analysed by SIMNRA [4].
[1] Küppers J., Surf.Sci.Reps. 22 (1995) 249
[2] Krstić P. et al., EPL 77 (2007) 33002
[3] Schwarz-Selinger Th. et al., J. Vac. Sci. & Technol. A. 18 (2000) 995
[4] Mayer M.,

16. B. Comments on previous results
Samples were exposed to the beam from the special source of vibrationally hot molecules ISPEC:
Beam characteristics:
Composition:
Profile:
Vibrational temperature of effusing gas beam was determined by vibrational spectrometer DTVE-B to be between 2800 K and 3400 K for different experiments. Only H2 was used in these measurements.
Gas beam has characteristic shape: In central region molecules flowing directly from dissociation chamber are more important while only molecules from recombination chamber are present in the wings of distribution. Background pressure contributes progressively more and more when sample is further from the source.
Results presented at 18th PSI, Toledo 2008 (Markelj et al., P1-06).

17. Exposure geometries used for measurements in 2008b c
Beam characteristics – density [mol s-1 cm-2]:
h [cm]
Ib1
(R=0)
Ib2 Ib
Ic (R=0)
Ib/Ig
Ib /Ic
1.7
2.7x1015
1.71x1016
2 x 1016
4.7x1016
0.73
0.42 4
1.16x1015
3.17x1015
4.34x1015
3.15x1016
0.16
0.14
5.4
8x1014
1.7x1015
2.5 x1015
2.97x1016
0.09
0.085
Markelj et al., 18th PSI, Toledo 2008 (P1-06).

18. Example of results and conclusions:
Sample: S2-3
Time of exposure: s
Temperature of sample: 235 oC
Driving pressure: 179 mTorr
Sample: S2-5
Time of exposure: s
Temperature of sample: 104 oC
Driving pressure: 179 mTorr
Sample: S2-4
Time of exposure: s
Temperature of sample: 23 oC
Driving pressure: 188 mTorr
a-C:H layer thickness modification after exposure to the beam of hot hydrogen for three sample temperatures. Exposure conditions were similar and exposure geometry was the same, “c”. Characteristic erosion is observed for higher temperature (darker is deeper) while apparent layer thickness increase is obtained for RT.

19. Conclusions from previous results:
Observed erosion indicated possible presence of atoms in the beam of hot hydrogen what was subsequently confirmed by measurements with DTVE-B.
Previously unobserved apparent layer thickness increase observed at RT having same characteristic shape of the gas beam.
No any effect was possible to be attributed to VEHMs.
Work was continued along two lines:
Attempt to eliminate atoms in hot hydrogen beam from ISPEC to a negligible amount. Different inserts were used in order to decrease H concentration without much success. Even effusing gas beam is not in thermodynamic equilibrium, the observed rate of dissociation might be an inevitable consequence of energy redistribution by surface collisions in the source.
Elucidation of observed apparent layer thickness increase at room temperature. Efforts since 2008 EU TF PWI annual meeting in Frascati was mainly devoted to this, second problem.

20. C. New results
Experimental arrangement used for November 2008 & January 2009 measurements.
In plane ERDA and RBS methods are used: 15o incidence angle of ion beam on the sample; ERDA detector at 30o and RBS at 165o with respect to the ion beam.
High energy ion beam (4.2 MeV 7Li2+ or 1.5 MeV 1H+) is used for real time in situ observation of surface processes induced by sample exposure to H or D beam from atomic source HABS.
Sample is mounted on a holder allowing active temperature control by resistive heater and water cooling.

21. Exposures performed: Sample Incident particle
Exp. time [s] / <Pd> [mTorr]
Sample temperature [K]
IBA method
S2-7 (SE-1)
a-12C:H (64nm) H
32760/128
300
S2-8 (SE-2)
14790/123
570
S3-1 (SE#4p1)
a-13C:H (20nm)
35830/124
S3-2 (SE#4p2)
None – only bckg
30400/0
S3-3 (SE#5) D
26160/138
Li-ERDA&RBS
p-RBS
S3-4 (SE#1)
a-13C:H (64nm)
12900/144
573
For all measurements with HABS capillary temperature was 2000K (173W heating power (I=13A)).

22. First exposures of a-C:H to H-beam from HABS brought similar results as previous with ISPEC with one distinct difference – layer increase at RT did not correspond to the atomic beam profile.
SE-1
SE-2
SE-1: ellipsometry data modelling showing a 67 nm thick dense film with a 50 nm thick carbon polymer top layer - deposition! Modelling ruled out the possibility of tungsten contamination also considered as a possibility.
SE-2: ellipsometry data modelling showing a dense film with modified top surface within the crater and no modification outside the crater.
In order to elucidate the nature of apparent layer increase new samples with 13C isotope were produced at IPP and exposure to H as well as D were performed. In situ ERDA and RBS were performed for detailed diagnostics.

23. Results, analysis and discussion of measurements – samples SE #5 and SE #1
Samples after exposure
SE#1
SE#5
SE#4p1
SE #1 D exposure 300°C
SE #5 D exposure 27°C

24. Sample: S3 - 3 (a-13C:H #5)
Exposure to D-beam
Exposure temperature: 300 K
Exposure time: s
Initial depth: 20 nm
Sample: S3 - 4 (a-13C:H #1)
Exposure to D-beam
Exposure temperature: 573 K
Exposure time: s
Initial depth: 64 nm

25. SE #1 D exposure at 300°C
Time evolution of erosion process; [H] and [D] from Li-ERDA; [C] from Li-RBS Si edge shift and p-RBS. [H] and [C] are steadily decreasing due to layer thinning but [D] is constant (17x1015 at/cm2) after an equilibrium is attained in some four minutes.
Proton RBS recorded before and after sample exposure to D. SIMNRA simulation (below) fitted well such spectra but due to unknown cross section for 7Li-13C, peak intensity could not be reproduced. Assuming same stopping power for 12C and 13C carbon surface concentration [C] could be deduced from the shift of Si edge in RBS spectra.

26. SE #1 D exposure at 300°C
Using literature values for the yield for chemical erosion (Schlüter m. et al., J. Nucl. Mater. 376 (2008) 33) it was possible to determine absolute flux of D-atoms for present exposure experiments.
Evaluated thickness variation with time as obtained from present ERDA/RBS measurements gives initial and final values in accordance with those obtained by ellipsometry

27. SE #5 D exposure at 27°C
It was proven by proton RBS that observed increase of the layer thickness is due to the co-deposition of carbon from background vacuum - 12C peak well distinguished from initially only 13C present. So, it is not a consequence of some layer structural change – also considered as a possibility. Furthermore, by only irradiating sample (without feeding D2 through hot HABS) under otherwise identical background vacuum conditions has shown that layer is not produced by some thermal cracking on the surface.
As shown by ellipsometry an homogeneous layer was deposited during exposure experiment at room temperature. The only observed variation of the layer thickness (besides intentionally left reference covered surfaces) in the centre of sample is due to the heating by the probing ion beam.

28. SE #5 D exposure at 27°C
The most surprising and still unexplained phenomenon is the homogeneous layer thickness when exposure experiment is performed with HABS as contrasted to the case of ISPEC. The most probable explanation to us is that co-deposition process is strongly dependent on atom energy and that atoms from HABS (170meV) are less reactive then atoms from ISPEC (presumably 30-40meV). Other explanations are also possible and more work is needed to understand this observation.
Time evolution of deposition process: Mainly D is incorporated in the newly formed co-deposit indicating fast isotope exchange of H atoms from hydrocarbons from background vacuum during layer growth. 12C concentration was determined from RBS edge displacement assuming 13C being constant.

29. D. Plan for the rest of year 2009
- Final data evaluation and analysis of previous measurements with both, ISPEC and ERDA-HABS experiments will be performed until September.
New experiments are planned in the second half of 2009 after present results are well “digested” – presumably before annual TF-PWI meeting.
All data evaluation will be performed before the end of 2009 and conclusions drawn.

31. Exposures results: Sample Incident particle / sample temperature [K]
Observed change
Rate of thickness change [nm/s]
IBA
S2-7 (SE-1)
a-12C:H (64nm)
H / 300
Homogeneous layer build-up
16nm/32760s=
0.49x10-3 nm/s
S2-8 (SE-2)
H / 570
Erosion crater
-31nm/14790s
= -2.10x10-3 nm/s
S3-1 (SE#4p1)
a-13C:H (20nm)
52nm/35830s=
1.45x10-3 nm/s
S3-2 (SE#4p2)
None / 300
No change /
S3-3 (SE#5)
D / 300
27nm/26160s=
1.03x10-3 nm/s
S3-4 (SE#1)
a-13C:H (64nm)
D / 603
-23.5nm/12900s
= -1.82x10-3 nm/s