ctem/sem principles - cime | epfl · advantages and disadvantages of scanning beam microscopy tem ...

MSE-603 Autumn 2013 STEM Marco Cantoni

15/16. STEM, EDX & HAADF

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


CTEM/SEM principles

Conventional TransmissionElectron Microscope

ScanningElectron Microscope

Slide Projector TV

What you see is what

the detector sees !!!


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


Detectorsin S(T)EM

• Secondary Electrons

• Backscattered Electrons

• X-rays

• EELS

• Bright field

• Dark field

• (Absorbed current)


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…


a) Principle


STEM <-> (C)TEM


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


Illumination / opticsTEM / STEM with the same instrument


Source

Illumination

Scanning

“descanning” / Detection


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


Diffraction pattern = stationary patternBF/DF detectors



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


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


Bright field TEM <->STEM


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


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


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


Interaction volumeSEM (30KeV), bulk

versusTEM (300KeV), thin film

PZT ceramics

bulk

20nm thick PZT

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

TEM


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


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.


CTEM dark field image STEM dark field

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

TEM dark field images. Strong diffraction contrast


EDX Line-scan


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


STEM Element MappingPMN/PT 90/10 (bulk)


Artifactshow to recognize/minimize them


Analytical TEM of multifilamentNb3Sn superconducting wires

Prof. R. Flükiger, V. Abächerli, D. Uglietti, B. SeeberDept. Condensed Matter Physics (DPMC),University of Geneva

Typical cable:1 x 1.5mm cross-section121x121 filaments of Nb3Snin a bronze (Cu/Sn) matrix

0.5 mm

Superconducting Nb3Sn cables for high magnetic fields 10-20T:increase current density, lower costPotential Applications:NMR, Tokamak fusion reactorsLarge Hadron Collider (LHC), CERN


Processing„bronze route“

Nb3Sn

Nb

Cu,Sn

Nb

Cu,Snbronze

Hea

t tre

atm

ent

SEM: reacted filament (1 out of 14‘000)

Ti

Ti

Ta

“Nano”-engineering: controlled creation of “imperfections” of nm scale (coherence length)

Cu and Ti are believed to play an important role at the grain boundaries: „dirty“ grain boundaries = pinning

• Is it possible to detect Cu and Ti at the grain boundaries ?• What is the difference between the grain boundaries

depending on where the additives are added to the unreacted material ?


Typical problems:thinning of heterogeneous specimens:

selective thinning

Cross-section, polished mechanicallyto 30 um, ion milled until perforation

STEM, Dark field:core of filament too thick, preferential etching of bronze matrix

Nb3Sn filament

bronze


Specimen preparation by focused Ion Beam (FIB):large areas with uniform thickness ideally for EDX Analysis

in the TEM (STEM mode)

SEM (FIB)

STEM, Bright field

Ion milling

FIB

ED

S, e

lem

ent

map

s

STEM-DF

Sample #21

15um

thickness:40-50nm


Spot analysisLine profile

Point Ti%at

Nb%at

Sn %at Ta%at

1 0.1 79.7 17.1 2.9

2 0.4 79.2 17.8 2.4

3 0.8 77.8 18.5 2.7

4 1.8 75.1 20.8 2.1

5 0.5 76.5 20.9 1.9

6 0.2 74.3 23.1 2.2

7 1.6 73.1 23.4 1.7

8 1.2 73.7 22.8 2.1

9 0.9 70.4 26.4 2.1

Sample #21

Tc/Jc„useful“

bronzeNb

Sn

„Nb3Sn“


grain boundaries ? Ti/Cu

Sample #21

Cu

Sn

TaTi

Nb

Cu and Ti at the grain boundaries:

width ~ coherence lenght (4nm)

possible pinning centers !!

EDX line-scan


grain boundary without Ti

Sample #24

Cu

Sn

TiTa

Nb

Quantitative Line-scan


New possibilities due toSDD (silicon drift detector) technology

X-rays

SDD Si(Li)Thickness 300-400µm 3mm

Area 30,50,80mm2 30mm2

Det. Interval 4-10µsec. 50-100µsec.Speed: 100’000cps 10’000cps

Cooling Peltier Liq. N

SDD


TECNAI OSIRISAnalytical TEM2012 @ CIME 5x

10x

electrons

Det. area


400x400 pixels (5umx5um)160’000 spectra4msec., (10min.)

2.5nA

Nb

Cu

Sn


• 400x400 pixels (500nmx500nm)

• 160’000 spectra

• 4msec., (10min.), 2.5nA

CuSTEM DF Sn


Synthetic sapphire: Al2O3

Al2O3with La (250ppm) in the grain boundaries

Paul Bowen, M. StuerEPFL-LPT


HAADF STEM

La at grain boundaries

Cl (from synthesis)


QUALITATIVE EDX Mapping:

GREAT!


c) High Angle Annular Dark Fieldz-contrast

(atomic resolution)

Ultramicroscopy 30 (1989) 58-69North-Holland, AmsterdamZ-CONTRAST STEM FOR MATERIALS SCIENCES.J. PENNYCOOK

Ultramicroscopy 37 (1991) 14-38;North-HollandHigh-resolution Z-contrast imaging of crystalsS.J. Pennycook and D.E. Jesson



High Angle Annular Dark field detector

Big camera length small camera length

ADF

BF

HAADF


High angle incoherent scattering

The annular DF detector is placed beyond the bragg-scattered electrons…

Small camera length and large diameter of the detectors inner diameter

The image is formed by high angle incoherently

scattered electrons-> Rutherford scattering at

the nucleus of the atoms

z2

Z-Contrast Si nano-crystals in SiO2 formed by implantation


HAADF <-> HRTEM

Pt catalyst on Al2O3

Pt particles become visible in the HAADF image

HAADF BF


HRTEM STEM-HAADF

HRTEM <-> STEM HAADF


Defocus…?

Also for High Resolution STEM-HAADF there is a

defocus value which gives the highest resolution ( =

sharpest peak of maximum intensity).


No contrast reversal as in CTEM HRTEM images !!!!no repeated contrast patterns at different defocus settings


HAADF-STEM study of β′-type precipitates inan over-aged Al–Mg–Si–Ag alloy



Chemical analysis on atom columns


Cs-corrected STEM

ADF images of Si[112]recorded with

aberration corrected STEMProbe size = 60pm (0.06nm)

Resolution 78pm

Atomic resolutionEELS analysisof defects and interfacesR.F. Klie*, Y. ZhuMicron 36 (2005) 219–231

EELS spectrum ofsingle atom columns

Ti-L edgein SrTiO3 8° grain

boundary

ctem/sem principles - cime | epfl · advantages and disadvantages of scanning beam microscopy tem ...

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