Me. Bases Feb 2015

2

Electron Microscopy

• beams of electrons are used to produce images

• wavelength of electron beam is much shorter than light, resulting in much higher resolution

Figure 2.20

definition of resolution

dmin =0.61 n sin

n = refractive index = wavelength

d

note: resolving power independent of lens properties

: for green (500nm) light dmin = c. 0.2 µm

Microscopía Óptica vs Microscopía Electrónica de Barrido

m

OM SEM

Baja profundidad de campo Baja resolución

Alta profundidad de campo Alta resolución

radiolarian

http://www.mse.iastate.edu/microscopy/

Depth of focus

Optical microscopy vs SEM

• A SEM typically has orders of magnitude better depth of focus than a optical microscope making SEM suitable for studying rough surfaces

• The higher magnification, the lower depth of focus

Screw length: ~ 0.6 cm

Images: the A to Z of Materials

IMPERFECCIONES CRISTALINAS Y MICROSCOPIA ELECTRONICA






the electron gun

Bias (Wehnelt)Cylinder

Filament (20-100 KV)

Anode

stream of electrons originating from outer shell of filament atoms

wavelength & voltage

Wavelength () of electrons is determined by the accelerating voltage (V) on the filament from

which they are emitted

= 0.1*(150/V)0.5

. . . thus very high voltages (up to 100 kV) are used to produce small values of (<0.005 nm)

(de Broglie, 1924)

why high vacuum ?

• mean free path of electrons v short in air

- at least 10 -5 mbar usually aimed for

• also

- tungsten filaments burn out in air

- columns must be kept dust free

• achieved by 2-fold pumping:

rotary (mechanical) pump + diffusion pumpor + turbo pump

TEM - transmission electron microscopyTEM - transmission electron microscopy

ExamplesExamples

dislocationsin superalloydislocationsin superalloy

SiO2 precipitate particle in Si

SiO2 precipitate particle in Si



ExamplesExamples

Matrix - '-Ni2AlTi

Precipitates - twinned L12 type '-Ni3Al

Matrix - '-Ni2AlTi

Precipitates - twinned L12 type '-Ni3Al


DiffractionDiffraction

Cr23C6 - F cubica = 10.659 Å

Cr23C6 - F cubica = 10.659 Å

Ni2AlTi - P cubica = 2.92 Å

Ni2AlTi - P cubica = 2.92 Å


Polycrystalline regionsPolycrystalline regions

polycrystalline BaTiO3 spotty Debye rings

polycrystalline BaTiO3 spotty Debye rings




EL MICROSCOPIO ELECTRÓNICOEL MICROSCOPIO ELECTRÓNICO

Electron beam-sample interactions• The incident electron beam is scattered in the sample,

both elastically and inelastically• This gives rise to various signals that we can detect

(more on that on next slide)• Interaction volume increases with increasing acceleration

voltage and decreases with increasing atomic number

Detectors

Image: Anders W. B. Skilbred, UiO

Secondary electron detector:(Everhart-Thornley)

Backscattered electron detector:(Solid-State Detector)


MENA3100

How do we get an image?

• In brief: we shoot high-energy electrons and analyze the outcoming electrons/x-rays

Electrons inElectrons out

or: x-rays out

Secondary electrons (SE)• Generated from the collision

between the incoming electrons and the loosely bonded outer electrons

• Low energy electrons (~10-50 eV)• Only SE generated close to

surface escape (topographic information is obtained)

• Number of SE is greater than the number of incoming electrons

Topographical Contrast

Topographic contrast arises because SE generation depend on the angle of incidence between the beam and sample.

Bright

Dark

+200V

e-

lens polepiece

SE

sample

Everhart-ThornleySE Detector

Scintillator

light pipe

Quartzwindow

+10kVFaraday

cage

Photomultiplier tube

PMT

MENA3100

Backscattered electrons (BSE)

• A fraction of the incident electrons is retarded by the electro-magnetic field of the nucleus and if the scattering angle is greater than 180 ° the electron can escape from the surface

Backscattered electrons (BSE)• High energy electrons (elastic scattering)• Fewer BSE than SE

BSE vs SE

Effect of Atomic Number, Z, on BSE and SE Yield

SEM - scanning electron microscopySEM - scanning electron microscopySpecimen Specimen

Conducting - little or no preparation attach to mounting stub

for insertion into instrument may need to provide conductive path with Ag paint

Conducting - little or no preparation attach to mounting stub

for insertion into instrument may need to provide conductive path with Ag paint

Non-conducting - usually coat with conductive very thin layer (Au, C, Cr)

Non-conducting - usually coat with conductive very thin layer (Au, C, Cr)

The most versatile instrument for a materials scientist?

What can we study in a SEM?

• Topography and morphology

• Chemistry

• Crystallography

• Orientation of grains

• In-situ experiments:– Reactions with atmosphere– Effects of temperature

“Easy” samplepreparation!!

“Big” samples!

Low-voltage micrograph (300 V) of distribution of adhesive droplets on aPost-it note. No conductive coating was applied: such a coating would alter this fragile specimen.

http://en.wikipedia.org/wiki/Post-it_note

Image Magnification

Example of a series of increasing magnification (spherical lead particles imaged in SE mode)

Low-temperature SEM magnification series for asnow crystal. The crystals are captured, stored, and sputter-coated with platinum at cryogenic temperatures for imaging.

http://en.wikipedia.org/wiki/Snow

Topography and morphology

• High depth of focus

Image: Camilla Kongshaug, UiOImage: Christian Kjølseth, UiO

SE Images - Topographic Contrast

The debris shown here is an oxide fiber got stuck at a semiconductor device detected by SEM

1m

Defect in a semiconductor device Molybdenumtrioxide crystals

Scanning electron microscope (SEM) image showing corrosion products on hot-dip galvanized steel after immersion in sodium chloride solution

Comparison of SEM techniques:Top: backscattered electron analysis – compositionBottom: secondary electron analysis – topography

Backscattered electron (BSE) image of an antimony-rich region in a fragment of ancient glass. Museums use SEMs for studying valuable artifacts in a nondestructive manner.

http://en.wikipedia.org/wiki/Antimony

Typical SEM/EDX of Ipetumodu clay showing the morphology of the clay and its chemical composition (x500) de: Effects of Alumina Cement on the Refractory Properties of Leached Ipetumodu Clay Davies Oladayo FOLORUNSO1,2*, Peter Apata OLUBAMBI2,3, and Joseph Olatunde BORODE 1,2

MENA3100

X-rays• Photons not electrons• Each element has a

fingerprint X-ray signal• Poorer spatial resolution

than BSE and SE• Relatively few X-ray signals

are emitted and the detector is inefficient

relatively long signal collecting times are needed

Analytical Methods in Electron Microscopy

• a: Energy-Dispersive Spectrometry (EDS)

Analytical Methods in Electron Microscopy

• b: Electron Energy Loss Spectroscopy (EELS)

By examining energy losses at high resolution (about 30 meV), as in HREELS, data concerning the vibrations of molecules on surfaces can be determined

A typical x-ray spectrum for an alloy (5p coin)

In-situ imaging• A modern SEM can be equipped with various

accessories, e.g. a hot stage

In-situ imaging: oxidation of steel at high temperatures

• 800 °C, pH2O = 667 Pa• Formation of Cr2O3

Images: Anders W. B. Skilbred, UiO

2 min 10 min 90 min

Environmental SEM: ESEM

• Traditional SEM chamber pressure: ~ 10-6

Torr

• ESEM: 0.08 – 30 Torr

• Various gases can be used

• Requires different SE detector

Why ESEM?

• To image challenging samples such as:– insulating samples– vacuum-sensitive samples (e.g. biological samples)– irradiation-sensitive samples (e.g. thin organic films)– “wet” samples (oily, dirty, greasy)

• To study and image chemical and physical processes in-situ such as:– mechanical stress-testing– oxidation of metals– hydration/dehydration (e.g. watching paint dry)

Piece of a crystallized polystyrene latex, SE image with ElectroSscan 2020 ESEM.

The world's first ESEM prototype

Wool fibers imaged in an ESEM by the use of two symmetrical plastic scintillating backscattered electron detectors. Pseudocolor.

Fungal spores in lemon grass leaf, SE image, ElectroSscan E3 ESEM

Orchid pollen viewed in an ElectroScan 2020 ESEM, with GSED, 23 kV and 4.9 torr (=653 Pa).

Hydration of NaCl crystals on Teflon, as water vapor pressure rises, at room temperature, in an ESEM by the use of two symmetrical plastic scintillating backscattered electron detectors. Field width 300 µm, 10 kV

Summary• Signals:

– Secondary electrons (SE): mainly topography

• Low energy electrons, high resolution• Surface signal dependent on curvature

– Backscattered electrons (BSE): mainly chemistry

• High energy electrons• “Bulk” signal dependent on atomic number

– X-rays: chemistry• Longer recording times are needed

Documents

Me. Bases Feb 2015