Out of vacuum characterisation of surfaces: a possible approach?

Transcription

1 Out of vacuum characterisation of surfaces: a possible approach? Lorenzo Giuntini Dipartimento di Fisica dell Università and Sezione INFN Firenze, Italy XVIII Congresso Nazionale sulla Scienza e Tecnologia del Vuoto, Firenze, 2-4 April 2007

2 1 - Recipe for Ion Beam Analysis measurements Accelerator Particle beam Characteristic radiation Sample 1600 Energy spectrum Cl Detector(s) conteggi/canale Counts Si Na Al Mg S Ca Energy (kev) E (kev) K Ca Signal

3 2 The icing on the cake: IBA Imaging For each detector signal we record: energy E of the detected radiation (e.g. X ray energy) beam position (x, y) Cu Si Ti Cu 1000 K Zn 500 µm conteggi Counts Copper grid on glass dark = copper light = glass Energy (kev) energia (kev) Max. Si Cu Min.

4 1 - accelerator and beam transport to the sample

5 Accelerator hall + IBA dual source injection

6 3 MV tandetron

7 External Scanning Microbeam Line

8 External Scanning BPM Proton Microprobe Line of Firenze QFC X-Y steerers X-Y steerers BPM coll. slits QD + scan coils Beam object slits

9 2 Techniques of analysis

10 IBA techniques in our laboratory X-ray (PIXE, Particle Induced X-ray Emission) electron Elastically backscattered particle (BS, Backscattering Spectrometry) γ-ray (PIGE, Particle Induced Gamma-ray Emission)

11 PIXE Main features of the techniques Multi-elemental, very fast, quantitative Z > 11 BS High sensitivity, trace level/tens of nm Low beam current intensity (down to pa) Z > 2 ( proton beams) Depth profiling Layer structure PIGE Low Z elements Negligible self-absorption of radiation

12 Problems for in vacuum measurements Sampling Damage: Heating (thermal damage) Drying Charging (Breaking) Analysis of objects of limited size Difficult sample handling/viewing

13 A simple solution (whenever possible) To measure in an external setup!

14 Problems for out-of of-vacuum measurements 1. Window! rupture (air inrush into beam-lines and accelerator: disaster!) 2. Probe (exit window + path in atmosphere):! spatial resolution degradation! energy straggling! energy loss (? ) 3. Emitted X rays:! absorption of low energy X rays 4. Backscattered particles (external path + detector entrance window):! energy straggling

15 Is it nevertheless possible to do IBA analyses in an external setup, possibly with good spatial resolution?

16 Problem nr.1: Window rupture commercial fast acting valve, 8 m upstream of the exit window actuated by a cold cathode gauge close to the window less than 8 ms to seal the beam line

17 Problem nr. 2: Si 3 N 4 Probe Exit Windows Microbeam: membrane: 100 nm thick 1.0 x 1.0 mm 2 wide For wide area samples: membrane: 200 nm thick 2.0 x 2.0 mm 2 wide For sub-millibeam applications: membrane: 500 nm thick 3.0 x 3.0 mm 2 wide 1.0 x 1.0 mm 2 window (close up of the exit snout) 2.0 x 2.0 mm 2 window

18 Problem nr. 2: Probe Path in external external beam path as short as possible, 2 mm in our set-up target beam helium atmosphere 2 mm

19 Problem nr : Emitted X Rays and Backscattered Particles He atmospehere: beam path backscattered particles path low energy X rays path

20 Results - 1: Probe Characteristics Proton beam energy Beam current FWHM on sample in vacuum (µm)( 3 MeV 1 na 5 7 FWHM on sample in helium (µm) 8 9 FWHM in air on sample (µm)( 15 15

21 Results 2 Absorption of X rays in the external path

22 Results 3 - Energetic resolution of the external BS system Statistical Noise External path straggling Detector dead layer 3 kev FWHM 10 kev 13 kev 6 kev (3 mev p beam on Au thick sample) Ε riv 18 kev

23 imultaneous PIXE - PIGE - BS measurements PIXE BS Beam PIXE PIGE Current

24 Examples of applications of IBA techniques at the external scanning microprobe in Firenze 1. Testing the structure of a SiC detector: comparison between in vacuum and external measurements 2. Discovering the structure of an or nué artefact

25 500 µm 1 - The SiC detector Au (thin) + Ni (thin) + SiC (bulk) Au (thin) + Ni (thin) + SiC (bulk) Au (thicker 5 ~ 3 µm) 4 3 SiC bulk Au t h i c k Bulk SiC

26 Proton and alfa particle spectra from the same SiC detector Counts Energy [kev] Channel

27 Comparison of BS measurements Au layer Ni 2 Si Interface (between Au and SiC) α - vacuum (Zagreb/Melbourne ) (10 15 at/cm 2 ) t (Au) = 470 ± 50 t (Ni2Si) = 1600 ± 160 proton - external (Firenze) (10 15 at/cm 2 ) t (Au) = 480 ± 50 t (Ni2Si) = 1700 ± 170

28 2 - Or nué or gold shaded paintings Or nué or gold lace embroideries made of silk threads, spiral-wrapped by an extremely thin golden tape XVII century, by a painting of Raffaellino del Garbo XIII century, scapolare of San Gregorio X

29 The Circumcision of the Baptist The Baptist's Head Presented to Herod XV century, by a painting of Antonio del Pollaiolo

30 2 - Study of an or nué artefact of the XIII century Why: production techniques materials of the golden tape (restorations ) discriminating original embroideries from mends Problems: material spatially non homogeneous roughness of the surface How: scanning microprobe (~2( 2 square millimetres areas )

31 Min. PIXE elemental maps Ag Au 3 MeV proton beam 50 pa 15 minutes run Cu 100 micron Fe Max.

32 Results Fe e Ca originate from uncovered silk Ag, Au e Cu originate from the metal tape Au constitutes the very first layer, ~ 80 nm Underneath the Au layer, the tape is made by an Ag+Cu alloy, very rich in Ag (from 97 up to 99.7%) From both PIXE and BS analyses, we found a tape thickness of ~ 10 micron

33 Conclusions At the external proton scanning microbeam facility: external set-up: easy target positioning and viewing no risks of sample damaging probe features: " < 10 µm m FWHM on sample (He) " 1 na maximum current layers down to tens of nm minimum detection limit down to ppm all the elements with Z>2 detectable! simultaneous PIXE-PIGE PIGE-BS

34 thank you for your attention and see you all, right here, for the ECAART conference september 2007!

35 Thanks to: Mirko Massi, Silvia Calusi Novella Grassi, Alessandro Migliori, Piero Mandò Maria Elena Fedi,, Luca Carraresi,, Francesco Taccetti Marco Manetti, Marco Montecchi,, Roberto Ciaranfi