CTEM/SEM principles - CIME | EPFL · • Dark field • (Absorbed current) 13 - 4. MSE-603 Autumn...

MSE-603 Autumn 2011 STEM Marco Cantoni

14. STEM

Scanning Transmission Electron Microscopy

a) STEM, principle, basics,TEM / STEM with the same instrument

Transmission Electron Microscopya Textbook for Materials ScienceDavid B. Williams and Barry Carter

ISBN 0-306-45247-2

b) Analytical Electron Microscopy (AEM)Practical Analytical Electron Microscopy in Materials Science

David B. WilliamsISBN 0-9612934-0-3

c) High angle annular dark-field, HAADFZ-contrast

Handbook of Microscopy, Methods IIS. Amelinckx, D. van Dyck, J. van Landuyt, G. van Tendeloo

ISBN 3-527-27920-2

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CTEM/SEM principles

Conventional TransmissionElectron Microscope

ScanningElectron Microscope

Slide Projector TV

What you see is what

the detector sees !!!

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TEM-SEMinteraction of electrons with the sample

Specimen

Inci

dent

bea

m

Auger electrons

Backscattered electronsBSE

secondary electronsSE Characteristic

X-rays

visible light

“absorbed” electrons electron-hole pairs

elastically scatteredelectrons

direct beam inelasticallyscattered electrons

BremsstrahlungX-rays

1-100nm

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Detectorsin S(T)EM

• Secondary Electrons

• Backscattered Electrons

• X-rays

• EELS

• Bright field

• Dark field

• (Absorbed current)

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Advantages and disadvantages of Scanning Beam Microscopy

TEM <-> STEM (SEM)

Advantages

• Parallel detection of different signals

• Easy positioning of the beam (EDX, EELS)

• Small interaction volume, High energy (EDX)

Disadvantages

• Longer acquisition times(line by line)

• Image distortions (deflection coils)

• More complicated alignment procedure

• More expensive…

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STEM <-> (C)TEM

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a) Principle

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Reciprocity

A B

Sou

rce

Det

ecto

r

Spe

cim

en

Lens

A B

CTEM

Cowley (1969): for the same lenses, apertures and system dimension the image contrast must be the same for CTEM and STEM

2STEM = 2CTEM

2STEM = 2 CTEM

STEM

Lens

22

22

Cowley, 1989

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Illumination / opticsTEM / STEM with the same instrument

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Source

Illumination

Scanning

“descanning” / Detection

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Beam current versus probe size

100keV electron beam: beam current:1nA, probe size: 1nm

150MW/mm2 !!!

Field emission guns• provide high emission current• from a small source (~1nm)• require small demagnification

->small probe size

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Diffraction pattern = stationary patternBF/DF detectors

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MSE-603 Autumn 2011 STEM Marco Cantoni 13 - 13


Au particles on a C filmSTEM BF:

Detection of transmitted electrons:

contrast similar to

CTEM BF image (objective aperture

selects only transmitted electrons)

STEM ADF:

Detection of diffracted electrons on the annular

DF detector:

(integration of multiple CTEM DF images)

Diffraction pattern

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Bright field/ AnnularDark field detectorinfluence of camera length and convergence angle

The selected camera-length (magnification of the diffraction pattern) determines what the detectors “sees”

Big camera length small camera length

ADF

BF

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Bright field TEM <->STEM

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Bright field TEM STEM

CTEMCTEM

Effect of increasing detection angle (decreasing camera length)on STEM BF image contrast: Loss of dynamical diffraction contrast

STEM

STEM

STEM

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ADF STEM

• The ADF image provides a signal which depends strongly on the bragg scattering (Al2O3).

• Single atoms scatter electrons incoherently to higher angles~ “z-contrast”

Single atoms (or small groups of atoms) of Pt on

crystalline Al2O3

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b) Analytical Electron Microscopy(EDS)

• Thin samples -> correction factors weak (A and F can be neglected), quantification “easy”

• Very weak beam broadening -> high spatial resolution ~ beam diameter (~nm)

• STEM: beam control and positioning

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Interaction volumeSEM (30KeV), bulk

versusTEM (300KeV), thin film

PZT ceramics

bulk

20nm thick PZT

Small interaction volume -> high spatial resolution for EDX Analysis!

TEM

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STEM point analysisPbMg1/3Nb2/3O3 (bulk)

Processing option : Oxygen by stoichiometry (Normalised)

Spectrum Mg Si Nb Pb O Total Spectrum 1 30.02 13.32 56.66 100.00 Spectrum 2 19.15 7.96 4.11 11.72 57.06 100.00 Spectrum 3 6.01 12.49 22.13 59.37 100.00 Spectrum 4 5.65 12.39 22.67 59.29 100.00 Spectrum 5 5.63 12.48 22.52 59.36 100.00 Spectrum 6 5.98 13.66 20.11 60.25 100.00 Spectrum 7 5.55 12.45 22.66 59.34 100.00 Spectrum 8 5.49 12.96 21.84 59.72 100.00 Spectrum 9 5.63 12.19 23.04 59.14 100.00 Max. 30.02 13.32 13.66 23.04 60.25 Min. 5.49 7.96 4.11 11.72 56.66

All results in Atomic Percent

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Pb(Zr,Ti)O3 Thick films for MEMS

SEM image of a wet etched (in HF/HCl solution) side-wall of 2 µm PZT film. All the 8 interfaces corresponding to the intermediate crystallization steps became visible indicating a compositional gradient (preferential etching) across the PZT layers.

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CTEM dark field image

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STEM dark field

STEM dark field images. The ramps in the gray level indicate changes of density or chemical composition (~atomic number).

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EDX Line-scan

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EDX point analysis

Quantitative EDX Analysis of the points indicated in the imageProcessing options : Oxygen by stoichiometry (normalised)results a Percent

SpectrumZr Ti Pb O Total Zr/Ti

1 8.43 12.60 18.45 60.52 100.0 40/60

2 9.92 9.98 20.15 59.95 100.0 49/51

3 12.07 7.61 20.49 59.84 100.0 61/39

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CTEM/SEM principles - CIME | EPFL · • Dark field • (Absorbed current) 13 - 4. MSE-603 Autumn...

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Transcript of CTEM/SEM principles - CIME | EPFL · • Dark field • (Absorbed current) 13 - 4. MSE-603 Autumn...