Spring 2010 Experimental Methods in Physics Marco Cantoni 1 Advanced Techniques 1. High-Resolution TEM 2. Analytical EM 3. 3D Microscopy, Special Techniques, Trends Spring 2010 Experimental Methods in Physics Marco Cantoni Field emission (electron, ion) microscopy +/- HV FEM: vacuum HV<0 FIM: low pressure rare gas HV>0 Sharp tip (R radius of curvature): Electric field E=V/R (E≈10 9 V/m) V≈1 kV R≈0.1 μm Magnification M=D/Rβ (M≈ …10 6 ) D≈0.3 m β≈1.5 (tip shape) D
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Advanced Techniques09-10... · Atom probe Tomography • Use the sample atoms as imaging ions….! – APFIM: Atom Probe Field Ion Microscopy • Measure the Time Of Flight (TOF),
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Spring 2010 Experimental Methods in Physics Marco Cantoni 1
Advanced Techniques
1. High-Resolution TEM
2. Analytical EM
3. 3D Microscopy, Special Techniques, Trends
Spring 2010 Experimental Methods in Physics Marco Cantoni
Field emission (electron, ion) microscopy
+/- HV FEM: vacuumHV<0
FIM: low pressure rare gasHV>0
Sharp tip(R radius of curvature):Electric field E=V/R
(E≈109 V/m) V≈1 kVR≈0.1 μm
Magnification M=D/Rβ(M≈ …106) D≈0.3 m
β≈1.5 (tip shape)
D
Spring 2010 Experimental Methods in Physics Marco Cantoni 3
Field Ion MicroscopyGas Field Ionisation Source (GFIS)
•• atoms (molecules) are trapped by atoms (molecules) are trapped by polarizations forcespolarizations forces
•• Trapped atoms hop on the surface until Trapped atoms hop on the surface until they are ionisedthey are ionisedIonisation: tunnelling process withprobability D:
I : Ionisation potentialΦ : Work function of emitterV : El. Potentialc : constant
•• Ions are ejectedIons are ejectedfrom the surfacefrom the surface
-c(I- )
VD eαΦ
Spring 2010 Experimental Methods in Physics Marco Cantoni 4
Field ion micrograph from a [100] orientedspecimen of Pt 17 at.% Rh catalyst materialimaged in Ne gas with an applied voltage of approx. 10 kV at 80K. Each of the bright spots in the left picture is the image of an individual atomon the specimen surface.
Ball model . The prominent sites, representing kink site atoms on the surface are shown in white. These forma serie of concentric rings similar to those seen in the field-ion picture
Field emission (electron, ion) microscopy FEM, FIM
Spring 2010 Experimental Methods in Physics Marco Cantoni 5
Atom probe Tomography
• Use the sample atoms as imaging ions….!– APFIM: Atom Probe Field Ion Microscopy
• Measure the Time Of Flight (TOF), to determine the mass of the ion…!– elemental analysis on atomic level
• Use Laser to assist ablation of ions– LAWATAP: Laser Assisted Wide Angle
Tomographic Atom Probe– Insulating samples become possible
Spring 2010 Experimental Methods in Physics Marco Cantoni 6
Wide Angle position sensitive detector
Sample needs to have tip shape: metals: etching, insulators: FIB
Spring 2010 Experimental Methods in Physics Marco Cantoni 7
Annu. Rev. Mater. Res. 2007.37:127-158
Spring 2010 Experimental Methods in Physics Marco Cantoni 8
Detection speed
• Electrical pulsingThe pulse repetition rate is variable in discrete steps from 1 kHz to 250 kHz, and a detection rate of up to 2×106 ion min−1 (120×106 ion h−1) can be achieved. This implies that a data set containing 109 atoms can be obtained in 8 1/3 h from a single cooperative specimen. For electrical pulsing, the full-width half-maximum (FWHM) value of m/Δm is 500.
• Laser (picosecond) PulsingFor the LEAP 3000X Si XTM, the laser pulse repetition rate is variable in steps from 1 kHz to 500 kHz, and a detection rate of up to 5×106 ion min−1 (300×106
ion h−1) can be obtained. Therefore, a data set containing 109 atoms is attainable from a single very cooperative specimen in 3 1/3 h, which is a factor of 2.5 faster than with electrical pulsing.
Spring 2010 Experimental Methods in Physics Marco Cantoni 9
Al–2.2 at.% Mg–0.12 at.% Sc alloy
Spring 2010 Experimental Methods in Physics Marco Cantoni 10
Spring 2010 Experimental Methods in Physics Marco Cantoni 11
Spring 2010 Experimental Methods in Physics Marco Cantoni 12
Spring 2010 Experimental Methods in Physics Marco Cantoni 13
Introduction to Tomography
• Tomography is imaging by sections or sectioning. A device usedin tomography is called a tomograph, while the image produced isa tomogram.
• The method is used in medicine, archaeology, biology, geophysics, oceanography, materials science, astrophysics and other sciences.
• In most cases it is based on the mathematical procedure calledtomographic reconstruction.
• The word "tomography" is derived from the Greek tomos (slice) and graphein (to write).
Wickipedia
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Introduction to Tomography
• Tomography is a method in which a 3-D structure isreconstructed from a series of 2-D projections (images) acquired at successive tilts (Radon 1917).
• First developed for use in medical imaging (1963, Nobel Prize for Medicine in 1979) using X-rays, ultrasound and magnetic resonance (e.g. ‘cat-scans’)..
• Found further application in geology, astronomy, materials science, etc…
P. Midgley, tomo workshop in Berlin
Spring 2010 Experimental Methods in Physics Marco Cantoni 15
Introduction to Tomography
Recording
• Series of 2D images• Destructive: serial
sectioning, FIB, LEAP• Non-destructive:
X-rays, TEM
Reconstruction and « viewing »• Registration (alignment of
images)• Back-projection, reconstruct
(tilt-series)• Tomogram• Segmentation (image
processing), extraction of the desired information
Spring 2010 Experimental Methods in Physics Marco Cantoni 16
3D imaging in medicine
• Non-invasive methods are preferred!
• The disadvantage of conventional X-radiographs is its inability to discriminatebetween organs of close absorptivity or overlapping organs in the viewingdirection.
Spring 2010 Experimental Methods in Physics Marco Cantoni 43
The principle: field ion microscope as a source
Pyramidal tip
round tip
Spring 2010 Experimental Methods in Physics Marco Cantoni 44
30kV Electrons vs He Ions
Spring 2010 Experimental Methods in Physics Marco Cantoni 45
5kV Electrons vs 30kV He
Spring 2010 Experimental Methods in Physics Marco Cantoni 46
(top) Secondary electron SEM image of alignment cross.(bottom) Secondary electron ALIS image of same alignment cross
•• smallsmall interaction volume for SEinteraction volume for SE--II
•• almostalmost no SEno SE--IIII
•• highhigh SESE--YieldYield ((currentscurrents in the in the fAfA--pApA range)range)
•• SESE--YieldYield dependsdepends on on atomicatomicnumbernumber z, z, materialmaterial contrastcontrast
Spring 2010 Experimental Methods in Physics Marco Cantoni 47
Rutherford back-scattered ions
(left) Secondary electron image of a solder bump showingtopography, but not muchmaterial difference.(right) RBI image of same solder bump clearly showing the difference between areas of tin (dark) and lead (bright).
Laser shot on coated material
SESE RBIRBI
SESE RBIRBI
RBI are not affected by charging effects
Spring 2010 Experimental Methods in Physics Marco Cantoni 48
ALIS RBI Images: No Charging Artifacts even on Uncoated Biological Samples
RBI images show no signs of charging on insulating materials. Sample is Uncoated Benthic Foraminifera courtesy of WHOI.