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
1 - Recipe for Ion Beam Analysis measurements Accelerator Particle beam Characteristic radiation Sample 1600 Energy spectrum Cl Detector(s) 1400 1200 conteggi/canale Counts 1000 800 600 400 Si Na Al Mg S Ca 200 0 0 1 2 3 4 Energy (kev) E (kev) K Ca Signal
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) 10000 Cu Si Ti Cu 1000 K Zn 500 µm conteggi Counts 100 10 Copper grid on glass dark = copper light = glass 1 0 1 2 3 4 5 6 7 8 9 10 11 12 Energy (kev) energia (kev) Max. Si Cu Min.
1 - accelerator and beam transport to the sample
Accelerator hall + IBA dual source injection
3 MV tandetron
External Scanning Microbeam Line
External Scanning BPM Proton Microprobe Line of Firenze QFC X-Y steerers X-Y steerers BPM coll. slits QD + scan coils Beam object slits
2 Techniques of analysis
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)
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
Problems for in vacuum measurements Sampling Damage: Heating (thermal damage) Drying Charging (Breaking) Analysis of objects of limited size Difficult sample handling/viewing
A simple solution (whenever possible) To measure in an external setup!
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
Is it nevertheless possible to do IBA analyses in an external setup, possibly with good spatial resolution?
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
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
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
Problem nr. 3 + 4: Emitted X Rays and Backscattered Particles He atmospehere: beam path backscattered particles path low energy X rays path
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
Results 2 Absorption of X rays in the external path
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
imultaneous PIXE - PIGE - BS measurements PIXE BS Beam PIXE PIGE Current
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
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
Proton and alfa particle spectra from the same SiC detector Counts 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 220 210 200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 950 1.000 1.050 1.100 Energy [kev] 1.150 Channel 1.200 1.250 1.300 1.350
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
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
The Circumcision of the Baptist The Baptist's Head Presented to Herod XV century, by a painting of Antonio del Pollaiolo
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 )
Min. PIXE elemental maps Ag Au 3 MeV proton beam 50 pa 15 minutes run Cu 100 micron Fe Max.
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
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
giuntini@fi.infn.it thank you for your attention and...... see you all, right here, for the ECAART conference september 2007!
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