transmission electron microscopy and diffraction - … · transmission electron microscopy ... 2)...

$: Transmission Electron Microscopy and Diffraction - … · Transmission Electron Microscopy ... 2) electron 9) Spec 11) ... 1965 Crewe: first scanning transmission electron microscope$
Aïcha Hessler-Wyser 1 CiMe

MSE637: introduction to TEM

Transmission Electron Microscopy and Diffraction

Dr Aïcha Hessler-Wyser, Dr Marco Cantoni, Dr Duncan Alexander

Centre Interdisciplinaire de Microscopie Electronique (CIME) Photovoltaics and Thin-Film Electronics Laboratory (PV-lab)



Outline

a.  Planning and ogranization b.  A little about diffraction c.  What can be done with a TEM? d.  Why electrons? e.  Electron-matter interaction



a. Planning MSEͲ637 TEM�Ͳbasics1st 2nd 3rd

COͲ121 COͲ12108:15 7)�Diffraction�techniques 10)�examples

CBED,�Kikuchi,�HOLZ,�map Imaging�with�diffraction�09:00 DA contrast��DA

09:15 8)�HRTEM�ͲI 10)�examplesindexing�of�dif�Pat

10:00 COͲ121 MC DA10:15 1)�Intro HRTEMͲII 11)�Indexing�practicals

Basics�of�TEM,�eͲ�scattering11:00 AHW MC DA

11:15 2)�electron�optics 9)�Spec�prep 11)��Application�examplessources,�Lenses inͲsitu

12:00 MC DA AHW

COͲ121 COͲ121�/�mxc30/mxc03313:15 3)�diffraction�basics DEMO

crystallography,�Bragg�law14:00 DA OSIRIS�mxcͲ030

14.15 4)�electron�diffractionintensities�and�imaging CM12�mxcͲ033

15:00 DA15:15 5)�TEM�basic�imaging Spec�Prep�browser

contrast,��BF/DF (laptop�required)16:00 AHW

16:15 6)�TEM�image�and�diffSAED

17:00 AHW



Slides available at http://cime.epfl.ch Please sign the presence sheet to get your credits Demo will be organised according to separated schedule, M. Cantoni will explain Exam: written, XX.YY.14, 3 pages report to hand in at the exam.

a. Administrative points: ED MS-636



a. Training

TEM

–  Theory MS637 mandatory –  Practical training: to be arranged with CIME's staff

after application (forms available on http://cime.epfl.ch)



b. Little history

1665 2007



b. Little history: optics

Optical microscopy Antiquity: first etch of convex lenses

XII-XIIIth centuries: magnification power of convex lenses, magnifier, glasses

1590 Janssen, first composed microscope

1609 Galilei: occhiolino

1665 Hooke: first cell image

1801 Young: wave nature of light

1872 (~) Abbe: the resolution limit is linked to wave length of the used beam



b. Little history: electrons?

Electron microscopy 1923 De Broglie: concept of wavelength associated to

particles, confirmation by Young's experiment 1927 Busch: focalisation low for magnetic fields

Davisson, Germer, Thomson: electron diffraction 1931 Ruska, Knoll: first images by electron microscopy




Electron microscopy 1923 De Broglie: concept of wavelength associated to

particles, confirmation by Young's experiment 1927 Busch: focalisation low for magnetic fields

Davisson, Germer, Thomson: electron diffraction 1931 Ruska, Knoll: first images by electron microscopy




1936 Scherzer: main electromagnetic lens aberrations cannot be avoided

1938 Von Ardenne: first microprobe scanning electron microscope

1939 Siemens: first industrial electron microscopes

1948 Gabor: holography invention

1951 Castaing: first X-ray micro-analyser

1960 XX: first MV microscope, competition for resolution

1965 Crewe: first scanning transmission electron microscope

1982 Binnig et Rohrer: scanning tunnelling microscope

1986 Ruska, Binnig et Rohrer: Prix Nobel Physics

1990 Rose: proposes the Cs corrector principle

1995 Haider: first realisation of the Cs corrector



b. Little history: resolution?

Resolution evolution with time and new technologies

0.0001

0.001

0.01

0.1

1

1800 1840 1880 1920 1960 2000 2040

Res

olut

ion

(An

g.-1)

Year

Electron Microscope

Light Microscope

Corrected EM

Ross

Amici

Abbe

Ruska

Marton

Dietrich(200keV)

Haider(200keV)

H. Rose, 1994 TEAM project, 2002



a detector (eye, photographic plate,

video camera...

an illumination section (lenses, apertures...)

a magnification section (lenses, apertures...)

a sample (+ a "goniometer")

A "light" source

c. What can be done with a TEM?




Different signals Different detectors

= Different informations!

1. Interaction of electrons with matter




Electron source

Lenses

Detectors

Pumping unit

2. Electron optics




Electron diffraction

3. Basics of diffraction

4. Diffraction 7. Diffraction

techniques




Conventional TEM BF, DF Basics of electron diffraction (SAED)

5. TEM principles 6. Diffraction

contrast




Phase contrast Atomic column imaging High resolution

8. HRTEM




3mm m

ax. 2.4

mm

.

3mm.

9. Sample preparation



d. Why electrons? Smallest visible objects….

- with eye : 0.1 mm = 10 -4 m (size of one eye « stick »)

- with light microscope : 10 -7 m (magnification max ~ 2000 ×)

Can we simply magnify the image of an object to observe every detail ?

NO ! There are fundamental limitations linked to :

- the type of probe …

-  observation techniques

With light microscopy : probe = visible light One anlayses the modification of visible light chacteristics after its interaction with matter.



d. Why electrons?

The resolution limit It is the smallest distance Y separating two distinct objects. Below, their image cannot be distinguished anymore. The smaller the wave length is, the smaller Y is… and the higher the number of details visible in the image.

- Visible light : λ ≈ 500 nm = 5.10 -7 m

resolution ≈ 300 nm (1000 atomic diameter)

- Wave associated to electrons with 100 keV acceleration : λ ≈ 4.10 -12 m

resolution ≈ 10 -10 m (interatomic distance )

The idea is then to use radiation with a wave length as small as possible… for example an electron wave:



d. Why electrons?

electrons

Electromagnetic radiation: E = hc/λ so if λ < 5 nm, E >1 keV (RX)

Sources? Lenses?

Electrons according wave length of de Broglie: λ = h/p

with p = mov = 2moeV

é non relativistic: é relativistic:

€

λ =h

2m0eV( )12

€

λ =h

2m0eV 1+eV2m0c

2

#

$ %

&

' (

)

* +

,

- .

12



d. Electrons? Somes numbers Electron charge (e) -1.602 x 10-19 C

1 eV 1.602 x 10-19 J

Electron rest mass (mo) 9.109 x 10-31 kg

Electron rest energy (moc2) 511 keV

Kinetic energy (charge x tension) 1.602 x 10-19 Nm (pour tension 1 volt)

Plank's constant (h) 6.626 x 10-34 Nms

1 ampère 1 C/sec

Light speed in vacuum (c) 2.998 x 108 m/sec

Accelerating voltage [KV]

Unrelativistic λ [nm]

Relativistic λ [nm]

Mass [x mo] Velocity [x 108 m/s]

100 0.00396 0.00370 1.196 1.644

120 0.00352 0.00335 1.235 1.759

200 0.00273 0.00251 1.391 2.086

300 0.00223 0.00197 1.587 2.330

400 0.00193 0.00164 1.783 2.484

1000 0.00122 0.00087 2.957 2.823



d. Radiation: generalities

Radiation choice

Radiations distinguish themselves by their properties: •  Wave length •  Particle energy •  Mechanisms of interaction with matter => different

information can be obtained

Experimental requirements: –  Radiation sources and their properties (brilliance,

monochromaticity, coherence)? –  Ways to control a beam (lenses, mirrors)? –  Detection systems (sensitivity, dynamic range, spatial

localisation, …)? –  Side effects (structure modification, radiation damages)?



d. Radiation: wave length



d. Radiation: generalities

Electron incident onto a bulk material Electron radiation

•  Secondary electrons (SE): electrons ejected from material at low energy (5 to 50 eV).

•  Back scattered electrons (BSE): incident electrons that are successively diffused and leave the sample: they are at high energy, close to initial energy E0.

•  Auger Electrons: ejected electrons with an energy characteristic from the target atoms.

Electromagnetic radiation

•  X rays with continuous energy, resulting from desceleration of incident electrons ("Bremstrahlung" or "slowing down radiation").

•  X rays with energy characteristic from the target atoms. •  Cathodoluminescence: Visible radiation mainly emitted by

isolating or semi-conducting materials.



e. a little about diffraction

Interaction of electrons with the sample

Specimen

Inc

ide

nt

be

am

Auger electrons

Backscattered electrons BSE

secondary electrons SE Characteristic

X-rays

visible light

“absorbed” electrons electron-hole pairs

elastically scattered electrons

dire

ct

be

am

inelastically scattered electrons

Bremsstrahlung X-rays

1-100 nm



e. Electron-matter interaction

Elastic diffusion

Incident electron

Diffused electron

ϑ

No energy transfer Low angle diffusion: Coulomb interaction with the electron cloud. High angle diffusion, or back scattering: Coulomb interaction with nucleus. Atom is not ionised.

Incident electron

Back scattered electron

Rutherford diffusion




Inelastic diffusion

Incident electron Secondary

electron

Diffused electron

An incident electron ejects a bound electron then changes direction, with an energy lowered by the electron bound energy. The ejected electron, of low energy, is called secondary electron SE. The incident electron can be slowed down by Coulomb interaction with the nucleus. In the case of inelastic interaction, there is energy transfer, and the target atom can be ionised.

Incident electron

Diffused electron

ϑ

X ray (Bremstrahlung)




Electron-holes creation: An incident electron onto a semiconductor can excite a valence electron to the conduction band, creating an electron-hole pair.

Cathodoluminescence: The excited electron recombines with its hole, emitting a photon with the gap energy, in the visible. This technique allows to put forward the defects that modify the gap energy.

Egap

Bande de conduction

Bande de valence

Electron incident

Egap

Bande de conduction

Bande de valence

Bande de conduction

Bande de valence

Electron avec perte d'énergie

Eg =hν




Interaction with plasmons

Plasmons are collective oscillations of free electrons that occur when an electron goes through the free electron "gaz".

Plasmon energy depends on the free electron density, the latter changes with the atomic numer Z. The plasmon excitation induces thus an energy loss of the incident electron, which is chemically dependent on the sample.




Interaction with phonons

Phonons are collective atom oscillations in a cristal. These are equivalent to a sample heating.

The electrons that excite phonons loose energy < 0.1 eV, but can be diffused at high angle.

No usefull information can be exctracted from phonons.




Electron absorption –  atom displacement ("knock on") –  chemical bound breaking, –  charge collective oscillation excitation (plasmons) –  lattice atom vibrations (phonons) –  excitation of surface electronic level (transition valence/

conduction,...) –  core atomic level excitation (ionisation) –  Bremsstrahlung radiation –  the relative impact of these various interaction mechanisms varies

with the type of material (no simple modelling of absorption)




Thickness needed to absorb 99% of a radiation for different elements

Element (specific masse)

4-Be 1.84 g/cm3

13-Al 2.7 g/cm3

29-Cu 8.93 g/cm3

82-Pb 11.3 g/cm3

X rays Cu-Kα λ=1.54 Å Mo-Kα λ=0.71 Å

16 mm 83 mm

0.35 mm 3.3 mm

0.10 mm 0.10 mm

0.017 mm 0.034 mm

Thermal neutrons λ≈1.08 Å

89 m ??

6 m

0.26 m

14 m

Electrons λ=0.037 Å à 100 kV λ=0.020 Å à 300 kV

39 µm

42 µm ~330 µm

11 µm

0.6 µm




Mean free path –  It is the distance an electron travels between interactions with

atoms:

Scatter from isolated atoms –  The interaction cross section represents the chance of a particular

electron to have any kind of interaction with an atom. –  The total scattering cross section is the sum of all elastic and

inelastic scattering cross sections:

–  If a specimen contains N atoms/vol, it has then a thickness t, the probability of scattering from the specimen is given by QTt:

with QT the total cross section for scattering from the specimen in units of cm-1, N0 the Avogadro's number (atoms/mole), A the atomic weight (g/mole) and ρ the atomic density

€

λ =1QT

=A

N0σTρ

€

σT =σ él +σ inél

€

QT = NσT =N0σTρA

€

QT t =N0σT (ρt)

A




The atomic scattering factor An incident electron plane wave is given by:

€

ψ( r ) =ψ0 e2πi k ⋅ r

€

€

ψsc ( r ) =ψ0 f (ϑ ) e

2πi k ⋅ r

r

When it is scattered by a scattering centre, a spherical scattered wave is created, which has amplitude ψsc but the same phase: where f(θ) is the atomic scattering factor, k the wave vectors of the incident or scattered wave, and r the distance that the wave has propagated.




The atomic scattering factor The incident electrons wave has a uniform intensity. Scattering within the specimen changes both the spatial and angular distribution of the emerging electrons. The spatial distribution (A) is indicated by the wavy line. The change in angular distribution (B) is shown by an incident beam of electrons being transformed into several forward-scattered beams.




The atomic scattering factor The atomic scattering factor is related to the differential elastic scattering cross section by

–  f(θ) is a measure of the amplitude of an electron wave scattered from an isolated atom.

– ⏐f(θ)⏐2 is proportional to the scattered intensity.

–  f(θ) can be calculated from Schrödinger's equations, and we obtain the following description:

f(θ) depends on λ, θ and Z

€

f (ϑ ) 2 =dσdΩ

f (ϑ ) =1+ E0

m0c2

!

"#

$

%&

8π 2a0λ

sinϑ2

!

"

###

$

%

&&&

2

Z − fX( )




The atomic scattering factor fn = 10+14 m fé 10+14 m

fX 10+14 m

(sinθ)/λ 0.1 0.5 0.1 0.5 0.1 0.5

1H -0.378 -0.378 4'530 890 0.23 0.02

63Cu 0.67 0.67 51'100 14'700 7.65 3.85

W 0.466 0.466 118'000 29'900 19.4 12

Atomic scattering factors for neutrons (independent of θ!), electrons and X rays, are a function of scattering angle and wave length λ [Å].

Tiré de L.H. Schwartz and J.B. Cohen, Diffraction from Materials fn : fé fX = 1 : 104 : 10




[ ]∑=

⋅=

mailleatomesi

iimaille ifirrA rKπθπ 2exp)(]2exp[/1

with ri th position of an atom i: ri = xi a + yi b + zi c

and K = g: K = h a* + k b* + l c*

[ ]∑=

++=

mailleatomesi

iiiih lzkyhxif )(2expF kl π

The structure factor The amplitude (intensity) of a diffracted beam depends on the lattice structure and its atom positions: The structure factor is given by the sum of all scattering centres (the atoms) of the crystal that can scatter the incident wave:



e. Resulting emission

Relaxation of an excited atom Characteristic X ray

Fluorescence X X ray energy caracteristic of the transition and thus of the element

Visible photon

Fluorescence Low transition energy, visible or UV photon is emitted

Auger electron

Emission Auger The relaxing process interacts with an elctron with a caracteristic energy




Relaxation of an excited atom




Emission RX: EDS spectrum (Energy Dispersive Spectroscopy)

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