1 introduction to em 2011 · autumn 2011 experimental methods in physics, marco cantoni electron...

Autumn 2011 Experimental Methods in Physics, Marco Cantoni

Electron Microscopy

An Introduction

1


Outline

1. Introduction, types of microscopes, some examples,

2. Electron guns, Electron optics, Detectors3. SEM, interaction volume, contrasts4. TEM, diffraction theory, image formation,

defect analysis

2


Microscopes…

http://www.ucmp.berkeley.edu/history/hooke.html

1665

2007

3


Optical Microscopy

• Antique: first convex lenses• XII-XIII century: magnifying

power of convex lenses: magnifying glass, looking glasses

• 1590: Janssen, first comound microscope

• 1609 Galilei: occhiolino• 1665 Hooke: first image of

biological cell• 1801 Young: wave character of

light• 1872 (~) Abbe: resolution limit

linked to the wave length of the illuminating wave

4


a microscope is…

• a light source• an illumination system• a sample• a magnifying lens

system• a detector (eye,

photographic plate, camera…)

in principle…

http://micro.magnet.fsu.edu/primer/index.html

5


magnification

• Ratio between real size of an object and its apparent size on the image (paper, screen, eye)

6


Which magnification…?

Who is lying ? The scale bar or the indicted magnification……?

Measure the size of a grain on the screen and calculate the magnification !

7


resolution

8


Why electrons…?

9


electrons

10


History

1897: J.J. Thomson: predicts the existence of electrons

1923: De Broglie: concept of wavelength associated to particles, confirmation by Young’s experiment

1927: Busch: focalisation low for magnetic fields, Davisson, Gremer, Thomson: electron diffraction

1931: Ruska, Knoll: first images by electron microscope

11


History

FEI Titan2009

Siemens 1939

12


Comparison of different microscopes

13


Types of electron microscopes

TransmissionElectron Microscope

ScanningElectron Microscope

Slide ProjectorTV

What you see is what the

detector sees !!!

Scanning beam

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Autumn 2011 Experimental Methods in Physics, Marco Cantoni 15

Types of microscopes

SEM• 0.3-30keV• Inelastic scattering: surface

information: topography• Chemical composition (EDX)• Pseudo-elastic (back-)

scattering: Compositional information (diffraction, EBSD)

• TEM• 60-300keV (up to 3 MeV)• “Projection” of the THIN

sample• Elastic scattering dominant:

Diffraction and diffraction contrast

• Interference between transmitted and diffracted beams: high-resolution (atomic resolution)

• Inelastic scattering: chemical composition (EDX)


SEM, signals

X-rayselasticinelastic

Backscattered electrons

e-beam

Secondaryelectrons

photons Auger electrons

IR,UV,vis.

Absorbed current, EBIC

• Secondary electrons(~0-30eV), SE

• Backscattered electrons (~eVo), BSE

• Auger electrons

• Photons: visible, UV, IR,X-rays

• Phonons, Heating

• Absorbtion of incident electrons (EBIC-Current)

16


SEM, secondary electrons

• Electrons with low energy (0-50eV) leaving the sample surface

• Intensity depends on inclination of the surface

• topography

17


SEM, backscattered electrons

• Electrons with high energy (~Eo) backscattered from below the surface

• Intensity depends on density (atomic weight)

• ~compositionNb3Sn in Cu matrix

18


SEM, characteristic X-rays

• Incident electrons ionize atoms in the sample:Emission of characteristic X-rays when atoms relax

• Detection and analysis of X-ray allows determination of chemical composition Extraction of element maps

Acquire X-ray spectrum in each image point

19


Depth of field: photons and SEM

LM

~10-20°

SEM

~10-3 rad

10mm

1mm

100µm

10µm

1µm

0.1µm

10µm 1µm 100nm 10nm

10 102 103 104

1nm

105

SEMLM

=500nm

10mrad

1mrad

0.1mrad

Résolution

Pro

fond

eur

de c

ham

p h

Grandissement (grossissement) G

LM=0.5m

dept

h of

fiel

d h

resolution

magnification M

20


Some typical SEM

At CIME:Zeiss NVision40

(FIB)

Zeiss UltraHR-SEM

Resolution

1.0 nm @ 15 kV, 1.7 nm @ 1 kV, 4.0 nm @ 0.1 kV

Magnification12 - 900,000x in SE mode Acceleration Voltage0.02 - 30 kV

Probe Current4 pA - 10 nA

Standard Detectors:EsB Detector with filtering gridHigh efficiency In-lens SE DetectorEverhart-Thornley Secondary Electron Detector

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Interaction e – matter in a TEM

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Signals from a thin sample

Specimen

Inc

ide

nt b

ea

m

Auger electrons

Backscattered electronsBSE

secondary electronsSE Characteristic

X-rays

visible light

“absorbed” electrons electron-hole pairs

elastically scatteredelectrons

dire

ct

be

am

inelasticallyscattered electrons

BremsstrahlungX-rays

1-100 nm

23


Some “typical” TEMs

EPFL: Philips CM300

Japan: HITACHI H-1500

1’300’000V !!!

300’000V

resolution < 1.7Å

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Some “numbers”

• “Speed”of a 300keV electron: 76% oflight-speed

• Wavelength of a 300’000V beam:0.0197 Å~2pm (0.002nm)

• Image resolution: ~1.7Å• Typical scattering

angles: 10-3 rad

Pb

Mg/Nb


Resolution SEM vs TEM

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Resolution SEM vs TEM

TEM dark field image g=(200)dyn

HRTEM zone axis [001] HRTEM zone axis [001]

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Optical analogy

Lamp

Slide

Magnified image

Magnifying lens

Condenserlens

LightFilament (Gun)

Specimen

Magnified image

Magnifying lens

Condenserlens

Electrons

Slide projector TEM

Light

Slide: Transparent for light

Glass lenses

ElectronsSpecimen: transparent for electrons -> thin: ~10-100nm

Magnetic lenses

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Some “typical” TEMs

EPFL: Philips CM300

Japan: HITACHI H-1500

1’300’000V !!!

300’000V

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Two basic operation modesDiffraction <-> Image

Diffraction Mode Image Mode

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TEM as a “multipurpose” machine

• Atomic structure– Diffraction

(lattice parameters, orientations) ~1pm

– High resolution TEM ~0.2nm

• Microstructure& Defects– Bright field image– Dark field imaging

~ m - nm

• Chemistry– Energy dispersive X-ray

microanalysisEDX, ~1nm

– Electron Energy Loss SpectroscopyEELS

In-situCooling/Heating

Elastic scattering

Inelastic scattering


elastic interaction of the electron beam with the sample

• Electrons don’t loose energy– Bright field/Dark field Images (BF/DF)– Diffraction patterns of selected areas

(SAED)– High resolution TEM (HRTEM) sin2 hkldn

Bragg’s law

DIFFRACTION PATTERN

sample

32


Bragg‘s lawTo obtain constructive interference, the path difference between the two incident and the scattered waves, which is 2dsinΘ, has to

be a multiple of the wavelength λ. For this case, the Bragg law then gives the relation between interplanar distance d and diffraction

angle Θ:

nλ = 2dsinΘ

L

R

d=L/R Example:

(300keV) = 1.97 pm

L (typ.) = 300mm

R=1.47mm

d = …nm ?

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(SAED)– High resolution TEM (HRTEM)

Objective- Aperture

BRIGHT FIELD IMAGE

sample

34


Objective- Aperture



(SAED)– High resolution TEM (HRTEM)

DARK FIELD IMAGE

sample

35


Shape, lattice parameters, defects, lattice planes

(K,Nb)O3Nano-rods

Bright field image

dark field image

Diffraction pattern

High-resolution image

36


High-resolution TEM

Pb

Ti O

Pb

Ti

• K. Ishizuka (1980) “Contrast Transfer of Crystal Images in TEM”, Ultramicroscopy 5,pages 55-65.

• L. Reimer (1993) “Transmission Electron Microscopy”, Springer Verlag, Berlin.• J.C.H. Spence (1988), “Experimental High Resolution Electron Microscopy”, Oxford

University Press, New York.

37


High resolution….?!

electron at 300keV:

• v = 2.33 10-8 m/s (0.78 c)

• = 0.00197nm

• diff = 10-3 rad

Precipitate in a PbTiO3 ceramic

Pb

Ti

Atomic resolution…?

38


Scherzer-defocus: “black-atom” contrast

39


Cu

O2

Hg Hg

“White-atom” contrast

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High resolution

• The image should resemble the atomic structure of the sample !

• Atoms…?Thin samples: atom columns: sample orientation (beam // atom columns)

• Contrast varies with sample thickness and defocus…!

• Comparison with simulation necessary !

41


experimental image compared to simulations

Space-charge Random-site

Ta

Mg

Pb

Pb

Cantoni, M; Bharadwaja, S; Gentil, S; Setter, N. 2004. Direct observation of the B-site cationic order in the ferroelectric relaxor Pb(Mg1/3Ta2/3)O-3.JOURNAL OF APPLIED PHYSICS 96 (7): 3870-3875.

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Electron microscopy

• A very powerful and versatile tool• “nice images”• High resolution• Different information (signals) from the same

sample• Understanding the image contrast requires

understanding of the interaction of electrons with matter and electron optics (especially in TEM).

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Bibliography, SEM

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Bibliography, TEM

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1 introduction to em 2011 · autumn 2011 experimental methods in physics, marco cantoni electron...

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