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

Material transport study with IBA and Role of vibrationaly excited hydrogen in erosion

Report on WP09-PWI-04-03/MHEST projects, SEWG 04 (Material migration) meeting, JET-Culham, 7-8 July 2009

Contents:

1. 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

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

RBS/ERDA :Beam: 4230 keV 7Li2+, Sample tilted 75°RBS detector at 160°, ERDA detector at 30°ERDA detector equipped with 11 µm Al foilDose controlled by mesh charge integrator Pelicon et al., NIM B 227, 591 (2005)

Elastic Recoil Detection Analysis (ERDA) for hydrogen depth profiling in PFCs

20 40 60 80 100 120

0

50

100

150

200

C

ou

nts

Channels

Measurement Three-layer simulation Single layer simulation

0 50 100 150 200 250

0,00

0,02

0,04

0,06

0,08

0,10

0,12

H a

t. co

nc

Depth [nm]

Li-ERDA, three-layer model Li-ERDA, one-layer model BAM reference value Round-robin average value

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))

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)

High Energy Focused Ion Beam

Focused beam formation:Magnet quadrupole triplet lensis focusing the beam at the analyzing object.Example: beam envelope for the focusing of 7Li2+ beam to 3x3 µm2

Blue: horizontal planeRed: vertical plane

Data acquisition:All detectors (incl. Chopper RBS) Connected to the sameMultiplexing ADCDetector event:(E,i,Ux,Uy)

On-line datasorting

Off-linedata sorting(list modemeasurements)

PIXE De t. RBS De t.

Protonskižarek

Pre isko va nid e l vzo rc a

Prem iki žarka

SE De t.

STIM Thin samples

AnnularRBS Det.

Deflecting coils (x,y)

Sample positioned in the focal plane

Ion beam

RBSdetector

Chopper(dose normalization)

Quadrupole lens

ERDA With 75º tilted sample

Foil

Data acquisition:All detectors (incl. Chopper RBS) Connected to the sameMultiplexing ADCDetector event:(E,i,Ux,Uy)

On-line datasorting

Off-linedata sorting(list modemeasurements)

Data acquisition:All detectors (incl. Chopper RBS) Connected to the sameMultiplexing ADCDetector event:(E,i,Ux,Uy)

On-line datasorting

Off-linedata sorting(list modemeasurements)

PIXE De t. RBS De t.

Protonskižarek

Pre isko va nid e l vzo rc a

Prem iki žarka

SE De t.

STIM Thin samples

AnnularRBS Det.

Deflecting coils (x,y)

Sample positioned in the focal plane

Ion beam

RBSdetector

Chopper(dose normalization)

Quadrupole lens

ERDA With 75º tilted sample

Foil

Application of crescent-shaped slits at the detector to diminish angular energy spread and maximize solid angle: isotope resolution in thespectra is preserved: resolved D and H signal

Thin H:D:C layer on silicon(TEXTOR pump duct sample,C:H:D soft film on Si)

D

H

CFC NB31

D

H

Region of interest in

ITER:Divertor

vertical target close to the strike zone

CFCs

W

Region of interest in

ITER:Divertor

vertical target close to the strike zone

Region of interest in

ITER:Divertor

vertical target close to the strike zone

CFCs

W

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. 337-339, 917 (2005).

Areal distribution of molybdenum (islands)measuredby Li-beam excited X-ray emmision (LIXE)

and simultaneously measuredhydrogen lateral distribution by Li-beam ERDA over an area of 1240 x 450 µm2. Sensitivity of the method is 0.1 at. %.

1240 µm

450

µm

Mo Lα

Hydrogen

Distribution of hydrogen is anti-correlated with the distribution of molybdenum. Hydrogen retention is higher in the surface of uncoated graphite.

Micro-ERDA: lateral mapping of hydrogen at plasma-exposed surface of castellated limiter[1]

[1] Litnovsky et al, J. Nucl. Mater. 337-339, 917 (2005).

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.

Flakes from TEXTOR RF antenna (Collaboration with FZJ):

Fe Si 160 x 160 µm2Cr Ni

STi Ti, S above LOD

Fe, Si, Cr, Ni dominant traces

A: Objectives

B: Comments on previous results

C: New results

D: Workplan for the rest of 2009

3. Study of the role of vibrationally excited hydrogen molecules in erosion of a-C:H

collaboration with Thomas Schwarz-Selinger, IPP, Garching

A. ObjectivesWe 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., http://www.rzg.mpg.de/~mam/

B. Comments on previous results

Results presented at 18th PSI, Toledo 2008 (Markelj et al., P1-06).

Beam characteristics:

Samples were exposed to the beam from the special source of vibrationally hot molecules ISPEC:

Profile:Composition:

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.

ba

Exposure geometries used for measurements in 2008

Markelj et al., 18th PSI, Toledo 2008 (P1-06).

Beam characteristics – density [mol s-1 cm-2]:

h [cm] Ib1

(R=0)

Ib2

(R=0)

Ib

(R=0)

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

c

0 2 4 6 8 10 12 14 16-18

-16

-14

-12

-10

-8

-6

-4

-2

S2-4

X [mm]

Y [m

m]

60,0061,0062,0063,0064,0065,0066,0067,0068,0069,0070,0071,0072,0073,0074,00

5 10 15 20 25

-20

-15

-10

-5

S2-5

X [mm]

Y [m

m]

64,00

64,50

65,00

65,50

66,00

66,50

67,00

5 10 15 20 25

-20

-15

-10

-5

X [mm]

Y [m

m]

50,00

52,00

54,00

56,00

58,00

60,00

62,00

64,00

66,00

68,00

70,00

S2-3

Sample: S2-3Time of exposure: 67220 sTemperature of sample: 235 oCDriving pressure: 179 mTorr

Sample: S2-4Time of exposure: 79310 s Temperature of sample: 23 oCDriving pressure: 188 mTorr

Sample: S2-5Time of exposure: 77340 sTemperature of sample: 104 oCDriving pressure: 179 mTorr

Example of results and conclusions:

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.

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.

Experimental arrangement used for November 2008 & January 2009 measurements.

15°

53°

14°

15°

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.

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.

Sample is mounted on a holder allowing active temperature control by resistive heater and water cooling.

C. New results

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)

a-12C:H (64nm)

H 14790/123 570

S3-1 (SE#4p1)

a-13C:H (20nm)

H 35830/124 300

S3-2 (SE#4p2)

a-13C:H (20nm)

None – only bckg

30400/0 300

S3-3 (SE#5)

a-13C:H (20nm)

D 26160/138 300 Li-ERDA&RBS

p-RBS

S3-4 (SE#1)

a-13C:H (64nm)

D 12900/144 573 Li-ERDA&RBS

p-RBS

For all measurements with HABS capillary temperature was 2000K (173W heating power (I=13A)).

SE-1 SE-2

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.

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.

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.

Samples after exposure

SE#1SE#5

SE#4p1

Results, analysis and discussion of measurements – samples SE #5 and SE #1

SE #1 D exposure 300°C SE #5 D exposure 27°C

Sample: S3 - 3 (a-13C:H #5)Exposure to D-beamExposure temperature: 300 KExposure time: 26160 sInitial depth: 20 nm

Sample: S3 - 4 (a-13C:H #1)Exposure to D-beamExposure temperature: 573 KExposure time: 12900 sInitial depth: 64 nm

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.

SE #1 D exposure at 300°C

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.

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

SE #1 D exposure at 300°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.

SE #5 D exposure at 27°C

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.

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.

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.

- 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.

D. Plan for the rest of year 2009

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)

a-12C:H (64nm)

H / 570 Erosion crater -31nm/14790s

= -2.10x10-3 nm/s

S3-1 (SE#4p1)

a-13C:H (20nm)

H / 300 Homogeneous layer build-up

52nm/35830s=

1.45x10-3 nm/s

S3-2 (SE#4p2)

a-13C:H (20nm)

None / 300 No change /

S3-3 (SE#5)

a-13C:H (20nm)

D / 300 Homogeneous layer build-up

27nm/26160s=

1.03x10-3 nm/s

S3-4 (SE#1)

a-13C:H (64nm)

D / 603 Erosion crater -23.5nm/12900s

= -1.82x10-3 nm/s