Presentation on theme: "Material transport study with IBA and Role of vibrationaly"— Presentation transcript:
1 Material transport study with IBA and Role of vibrationaly excited hydrogen in erosionP. Pelicon, I. Čadež, S. Markelj, Z. Rupnik, P. VavpetičMicroanalytical Centre, Department for Low and Medium Energy PhysicsJožef Stefan Institute, Association EURATOM-MHESTJamova 39, SI-1000 Ljubljana, SloveniaReport 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 beams3. Study of the role of vibrationally excited hydrogen molecules in erosion of a-C:H4. Conclusion
3 1. Introduction: tandetron, in-situ ERDA, focused ion beam
4 Elastic Recoil Detection Analysis (ERDA) for hydrogen depth profiling in PFCsRBS/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 integratorPelicon 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 lensis focusing the beam at theanalyzing object.Example: beam envelopefor the focusing of 7Li2+ beamto 3x3 µm2Blue: horizontal planeRed: vertical plane
9 Thin H:D:C layer on silicon (TEXTOR pump duct sample,C:H:D soft film on Si)HApplication of crescent-shapedslits at the detector to diminishangular energy spread and maximizesolid angle: isotope resolution in thespectra is preserved: resolvedD and H signalDCFC NB31HD
10 Model castellated structure has been exposed in the erosion-dominated conditions of tokamak fusion reaktor TEXTOR at Forschungszentrum Jülich1MeasuredcastellatedGraphitesection1A. Litnovsky et al, J. Nucl. Mater , 917 (2005).
11 Micro-ERDA: lateral mapping of hydrogen at plasma-exposed surface of castellated limiterMo LαAreal distributionof molybdenum (islands)measuredby Li-beam excitedX-ray emmision (LIXE)and simultaneously measuredhydrogen lateral distributionby Li-beam ERDA over anarea of 1240 x 450 µm2.Sensitivity of the method is0.1 at. %.450 µm1240 µmHydrogenDistribution of hydrogen is anti-correlated with the distribution of molybdenum. Hydrogen retention is higher in the surface of uncoated graphite. Litnovsky et al, J. Nucl. Mater , 917 (2005).
12 Microbeam analysis of flake deposits Flakes may be individually analyzed for their elemental compositionwith 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 embeddedat -30° C in Tissuetek (liquid vax) and in a frozen water droplet. Both mediaare 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 surroundingTissuetek resulted in slice destruction.
13 Flakes from TEXTOR RF antenna (Collaboration with FZJ): Fe, Si, Cr, Nidominant traces160 x 160 µm2FeSiCrNiTiSTi, S above LOD
14 3. Study of the role of vibrationally excited hydrogen molecules in erosion of a-C:Hcollaboration with Thomas Schwarz-Selinger, IPP, GarchingA: ObjectivesB: Comments on previous resultsC: New resultsD: Workplan for the rest of 2009
15 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. ). Recent model calculations  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 . ERDA and RBS spectra are analysed by SIMNRA . Küppers J., Surf.Sci.Reps. 22 (1995) 249 Krstić P. et al., EPL 77 (2007) 33002 Schwarz-Selinger Th. et al., J. Vac. Sci. & Technol. A. 18 (2000) 995 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 2008 bcBeam characteristics – density [mol s-1 cm-2]:h [cm]Ib1(R=0)Ib2IbIc (R=0)Ib/IgIb /Ic1.72.7x10151.71x10162 x 10164.7x10160.730.4241.16x10153.17x10154.34x10153.15x10188.8.131.52x10141.7x10152.5 x10152.97x10160.090.085Markelj et al., 18th PSI, Toledo 2008 (P1-06).
18 Example of results and conclusions: Sample: S2-3Time of exposure: sTemperature of sample: 235 oCDriving pressure: 179 mTorrSample: S2-5Time of exposure: sTemperature of sample: 104 oCDriving pressure: 179 mTorrSample: S2-4Time of exposure: sTemperature of sample: 23 oCDriving pressure: 188 mTorra-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 resultsExperimental 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 methodS2-7 (SE-1)a-12C:H (64nm)H32760/128300S2-8 (SE-2)14790/123570S3-1 (SE#4p1)a-13C:H (20nm)35830/124S3-2 (SE#4p2)None – only bckg30400/0S3-3 (SE#5)D26160/138Li-ERDA&RBSp-RBSS3-4 (SE#1)a-13C:H (64nm)12900/144573For 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-1SE-2SE-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 exposureSE#1SE#5SE#4p1SE #1 D exposure 300°CSE #5 D exposure 27°C
25 SE #1 D exposure at 300°CTime 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°CUsing 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°CIt 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°CThe 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.