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Microscopy

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Stereoscopy creates the illusion of three-

dimensional depth from images on a

two-dimensional plane

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Scanning Electron Microscopy (SEM) 

SEM is easy to use:

 Routinely used in both research and industry

and not just in materials science –  geology, archaeology,forensics, biology, ……………………..

Image interpretation is natural and simple (unlike TEM)

SEM (1950s) is a much younger technique than TEM

(1930s)

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The transmission electron microscope (TEM) was the first type

of Electron Microscope to be developed and is patterned exactlyon the light transmission microscope except that a focused beam

of electrons is used instead of light to "see through" the

specimen. It was developed by Max Knoll and Ernst Ruska in

Germany in 1931. The first scanning electron microscope

(SEM) debuted (first appearance of something) in 1938 ( VonArdenne) with the first commercial instruments around 1965. Its

late development was due to the electronics involved in

"scanning" the beam of electrons across the sample.

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SEM-Characteristic Information: 

Topography: The surface features of an object or "how it

looks", its texture; direct relation between these features andmaterials properties

Morphology: The shape and size of the particles making up

the object; direct relation between these structures and

materials propertiesComposition: The elements and compounds that the object is

composed of & the relative amounts of them; direct

relationship between composition and materials properties

Crystallographic Information: How the atoms are arranged

in the object; direct relation between these arrangements and

material properties

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OM SEM

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How Fine Can You See?

Can you see a sugar cube? The thickness of a sewing needle? The

thickness of a piece of paper?The resolution of human eyes is of the order of 0.1 mm, 100μm ≈ 

4 mils. However, something vital to human beings are of sizes

smaller than 0.1 mm, e.g. our cells, bacteria, microstructural

details of materials, etc. 

Microstructural Features which concern Us

• Grain size: from < μm to the cm regime

• Grain shapes

• Precipitate size: mostly in the μm regime• Volume fractions and distributions of various phases

• Defects such as cracks and voids: < μm to the cm Regime Scale

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SEMElectron/Specimen Interactions

When the electron beam strikes a sample,

both photon and electron signals are emitted.Incident Beam

Specimen 

X-rays

- composition info

Auger electrons

- Surface sensitive

compositional

Backscattered

electrons

- Atomic number

and topographical

Cathodoluminescence (light)

- Electrical

Secondary electrons

- Topographical 

Specimen CurrentElectrical

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Bremsstrahlung X-Rays

Bremsstrahlung" means "braking radiation" and is retained from the original

German to describe the radiation which is emitted when electrons are decelerated or

"braked" when they are fired at a metal target. Accelerated charges give off

electromagnetic radiation, and when the energy of the bombarding electrons is highenough, that radiation is in the x-ray region of the electromagnetic spectrum. It is

characterized by a continuous distribution of radiation which becomes more intense

and shifts toward higher frequencies when the energy of the bombarding electrons is

increased. The curves below are from the 1918 data of Ulrey, who bombarded

tungsten targets with electrons of four different energies.

The bombarding electrons can also eject

electrons from the inner shells of the atoms of

the metal target, and the quick filling of those

vacancies by electrons dropping down from

higher levels gives rise to sharply definedcharacteristic x-rays. 

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Auger Electrons:

Auger electrons are electrons ejected by radiation-less

excitation of a target atom by the incident electron beam.When an electron from the L shell drops to fill a vacancyformed by K-shell ionization, the resulting X-ray photonwith energy EK   - EL may not be emitted from the atom. Ifthis photon strikes a lower energy electron (e.g., an M-shell

electron), this outer electron may be ejected as a low-energyAuger electron. Auger electrons are characteristic of the finestructure of the atom and have energies between 280 eV(carbon) and 2.1 keV (sulfur). By discriminating betweenAuger electrons of various energies, a chemical analysis of

the specimen surface can be made.

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Electron-specimen interaction

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Specimen Interaction Volume 

The volume inside the specimen in which interactions occur whileinteracting with an electron beam. This volume depends on the

following factors:

• Atomic number of the material being examined; higher atomic

number materials absorb or stop more electrons , smaller interaction

volume.

• Accelerating voltage: higher voltages penetrate farther into the

sample and generate a larger interaction volume

• Angle of incidence for the electron beam; the greater the angle

(further from normal) the smaller the interaction volume.

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Charge-up phenomena

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Conspicuous----easily seen

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Contamination in Image

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Conspicuous----easily seen

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The count ratio of backscattered electrons to the electron incident on

specimen surface is called electron reflectivity. A change in this reflectivity

renders a contrast to the backscattered image. They are used for visualization

of the compositional distribution & topography of the specimen.

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Secondary electrons

Generated from the collision between the incoming electrons and

the loosely bonded outer electrons. (SE1)

Secondary electrons have very low energy, possibly few ten eV

(often Low energy electrons (~10-50 eV)

and emitted from a very narrow area about 10 nm. So these are

used to study topography and is expected high resolution image.

Secondary electrons are also emitted when backscattered electron

spring out.

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What are the ad antage & disad antages of sample coating

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What are the advantage & disadvantages of sample coating

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SE2

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SE2• The secondary electrons that are generated by the

backscattered electrons that have returned to the surfaceafter several inelastic scattering events

• SE2 come from a surface area that is bigger than the spotfrom the incoming electrons  resolution is poorer than forSE1 exclusively

Sample surface

Incoming electronsSE2

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Specimen coating methods for SEM

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Electron Guns used in SEM

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Evacuation in SEM

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Explanation point wise

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Block-diagram for more understanding

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Signal Detection and Display • If you change the target material, the high and low energy peaks remain

(although their intensity may change) while the low intensity peaks change

 position and are characteristic of the sample.

• The reason we produce this type of profile is because the incident electrons

we send into the sample are scattered in different ways. There are two broad

categories to describe electron Scattering : –  Elastic Scattering : Backscattered electrons

 –  Inelastic Scattering : Secondary electrons

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Detectors

Secondary electron detector:

(Everhart-Thornley)

Backscattered electrondetector:

(Solid-State Detector)

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MENA3100

Why do we need vacuum?

• Chemical (corrosion!!) and thermal stability isnecessary for a well-functioning filament (gunpressure)

 –

A field emission gun requires ~ 10-10

 Torr – LaB6: ~ 10

-6 Torr

• The signal electrons must travel from thesample to the detector (chamber pressure)

 – Vacuum requirements is dependant of the type ofdetector

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MENA3100

Environmental SEM: ESEM

•Traditional SEM chamber pressure: ~10-6 Torr

• ESEM: 0.08 – 30 Torr

• Various gases can be used• Requires different SE detector

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Detection of Secondary electrons 

Remember, secondary electrons are low energy electrons. We can easily

collect them by placing a positive voltage (100 - 300V) on the front of our

detector. Since this lets us collect a large number of the secondaries (50 -100%), we produce a “3D” type of image of the sample with a large depth

of field. The type of detector used is called a scintillator / photomultiplier

tube.

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MENA3100

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 processesin-situ such as:

 – mechanical stress-testing

 – oxidation of metals

 – hydration/dehydration (e.g. watching paint dry)

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Backscattered electrons

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Backscattered electrons

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Backscattered & Secondary electrons (different diagram for more understanding)

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Backscattered & Secondary electrons (different diagram for more understanding)

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Operating parameter Magnification

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Resolution 

We can also improve the resolution by:• Increasing the strength of the condenser lens 

• Decreasing the size of the objective aperture 

• Decreasing the working distance (WD = the distance the 

sample is from the objective lens)

Depth of Field

Depth of field is improved by:

• Longer working distance 

• Smaller objective apertures 

• Lower magnifications The height over which a sample can be clearly focused is called the

Depth of Field. The SEM has a large depth of field which produces

the images that appear 3-dimensional in nature.

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Depth of Field vs. Resolution:

Depth of field and resolution have a reciprocal

relationship:

Improving resolution in conventional SEM’s  leads to asmaller depth of field While increasing depth of field

decreases resolution useful for each particular sample.

Some photographs are given below,

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Kapton is a polyimide film developed by DuPont which can remain stable in a wide range of

temperatures, from -273 to +400 °C (-459 - 752 °F / 0  –  673 K). Kapton is used in, among

other things, flexible printed circuits (flexible electronics) and thermal micrometeoroid

garments, the outside layer of space suits. The chemical name for Kapton K and HN is

 poly(4,4'-oxydiphenylene-pyromellitimide).78

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Elemental analysis

E Di i X t t (EDX EDS)

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Energy Dispersive X-ray spectrometer (EDX or EDS)

Below is characteristic x-rays, summery 

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