BMFB 3263Material Characterisation Scanning Electron Microscope (SEM) 1

BMFB 3263Material Characterisation

Scanning Electron Microscope (SEM)

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TEM

•Light microscope – limited by wavelength of light.•Electron microscope – much lower wavelength allows resolution a thousand times better.

SEM

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Topic outcomes

By the end of this topic, students should be able to List down the differences between optical

microscope and electron microscope Acknowledge the importance of electron microscope

for materials engineers and scientists Describe most important parts of SEM, and their

roles in obtaining high quality image Explain typical sample preparation for metals,

ceramic, polymers and films

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250x, Al alloy 356 casting showing primary α and eutectic phase.

SEM micrograph of same alloy, shows shrinkage cavity (other microstructure features like grain boundaries is not shown).

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Al alloy reinforced with SiC particles imaged with OM and SEM .

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Typical images can be obtained using SEM 1m

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Powders

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Thin Film-surface morphology

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TiO2 Nanotubes

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ZnO Nanowires by Anodisation at Our Lab

Anodisation of Zn

NanoPorous ZnO

Nanoflowers ZnO

Anodisation of Zn

Nanoneedles ZnO

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ZnO

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Cross-section of thin films

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Optical microscope vs SEM

Magnification limit is a function of wavelength of visible light, 2000 Å. Wavelength of electrons < 0.5 Å gives theoretical max magnification of 800,000x.

Resolving power OM is limited by wavelength of light.

In light microscope, quality of objective lens also plays major role in determining resolving power.

Resolving power – ability to optically separate 2 objects.

Limiting conditions for transmitting info using electromagnetic radiation – key parameter is ratio /d. 13

Electron Microscope EM is developed because wavelength has

an effective role on theoretical resolution. Green light used for light microscope has

wavelength of 0.5 µm, & thus theoretical resolution of 0.2 µm. Followed by blue, violet & ultraviolet. But ultraviolet is absorbed by glass.

X-ray ? Cannot be easily refracted to form image. Inability to focus the rays.

Electron – could easily be focused by magnetic field. De Broglie postulate ‘dual nature of electrons – particle & wave’. Also can be accelerated by electrical potential.

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Wavelength of light depends on colours

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Factors influence key parameters of performance for SEM1. Depth of field

2. Imaging noise

3. Resolution

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2 general types of SEM

Typical SEM Samples must be dried before hand Must be coated with conductive coating e.g. gold,

Pt Environmental SEM

For biological samples Do not have to dry the samples

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(a) Schematic diagram of SEM, (b) Ray paths in SEM-standard arrangement for image formation 18

Schematic diagram of Environmental SEM

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Electron Microscope

Another limit of light m/scope is depth of field – distance from nearest to farthest part of subject that is in focus when picture is taken.

Depth of field – ability to maintain focus across a field of view regardless of surface roughness. SEM can maintain 3D appearance of textured surface.

Advantage of electron m/scope – increase in depth of field and depth of focus.

SEM also usually equipped with EDX or X – ray mapping to determine chemical composition at a particular spot.

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Electron Microscope – basic principle

Focusing is possible because of dual-nature of electrons.

De-Broglie relationship : = h/mv where m – mass, v is velocity & h is Planck constant.

Electrons are deflected by both electrostatic & magnetic fields and can be brought to a focus by engineering the field geometry.

EM – virtual source of electrostatic field by anode & subsequent focus by electromagnetic lenses.

Cathode filament generates electrons and accelerated by potential difference between anode & filament. Shield serves to collimate electrons & direct them. 21


Study topography of solid samples, resolution around 0.3 µm to 0.15 nm.

A source of high energy electrons, condenser system and probe lens to focus the electron beam into fine probe that impinges on specimen.

Image is obtained by scanning the electron probe over surface and collect image signal – display after suitable amplification and processing.

Sample : electrically conductive, nonconductive materials require thin conductive coating to prevent electrical charging of the specimen.

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An energetic electron penetrating solid sample undergoes both elastic & inelastic scattering. Inelastic predominates, reducing energy in beam to kinetic energy kT.

Release of secondary electron emission from surface generate electron current in sample due to impact of high energy incident beam.

Si-dioxide nanoparticle

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In SEM, a fine beam of electrons is scanned across surface of

sample, a detector counts the number of low energy secondary

electrons, or other radiation, emitted from each point of the surface.

The brightness of each image pixel is modulated according to the output current of the detector for each point of surface and an image is build up in this way as the beam scans.

Because secondary electrons come from near surface region, the brightness of signal depends on surface area that is exposed to the primary beam.

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1. Depth of field

e- beam converging to the image plane with semi-angular aperture , where d is the resolution required and D is the depth of field for SEM

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Depth of field

DoF- represent the distance along microscope axis over which the specimen can be displayed within the blurring image

For a beam of a fixed divergence angle , blurring is measured by the diameter of the ‘disc of confusion’, d, which is related to to the axial shift, D, where

(D/2) tan =d/2

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Depth of field and resolution of the SEM (final aperture 5 x 10-3 rad) compared with that achieved by the optical light microscope

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Noise

Noise within image present in all forms Related to collecting efficiency of the imaging

system Noise influences the resolution In SEM, resolution is improved if

the signal is collected for longer periods of time Increase the contrast

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Electron sources

In SEM, incident e- is focused on the specimen, but sometimes it is also scanned over the specimen

Resolution is set by the diameter of incident beam and mode of operation

Important thing: minimise energy spread within the electron beam, to minimise chromatic aberration

Electron sources influence: Brightness

Quality of image being obtained

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Electron beam source – tungsten or lanthanum hexaboride LaB6 cathode filament. Alternative – via field emission (FE).

Beam is focused by two successive condenser lenses into beam with very fine spot size (~ 5nm).

Condenser aperture – eliminate high angle electrons.

Objective aperture – further eliminate high-angle electrons.

Beam then passes thru objective lens where pairs of coils scan the beam in grid or raster fashion over rectangular area of surface.

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Beam diameter / spot size – one of the most important parameters, and it is controlled directly by operator electromagnetic probe lens or objective lens. Focus achieved by varying current passing thru objective lens.

Charging - manifested as bright streaks or flashes across width of photograph, observed when spot size is excessively large. Results when specimen accumulates a net negative charge.

Non-conductive samples examined at excessive accelerating voltage, contaminants particles or films.

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Reduce charging – coat with gold or carbon, connect to ground with conductive adhesive, clean properly.

Inelastic collision (electron – electron) secondary electrons, X-ray, phonon (heat).

Elastic collision (electron – nucleus) produce backscattered electrons (BSE).

If mean atomic wt (Z) of specimen is low, e.g plastic, probability of backscattering event is lower than if Z is higher. So high-Z metal release greater number of BSE than low Z specimens.

BSE imaging distinguish zones of different Z material. 32

Comparison of the performance for different electron sources

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Filament SEM Filament is much smaller than this

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Resolution

Resolution- critical parameters which governs the performance of a SEM

Resolution is a balance between the effects of aberration of the final lenses and diffraction effects

Most SEM has at least 5nm resolution Ultra high resolution SEM: provide smallest

diameter and brightest electron source combine with ability to the detect secondary the emitted secondary electrons with the highest efficiency

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Imaging modes

3 main methods to collect emitted electron signals

1. Secondary electron

2. Backscattered electrons

3. Absorbed specimen current

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Methods of detecting e- in SEM; (a) Secondary electron, (b) Backscattered electron, scintillation counter

Photomultiplier system to detect the secondary

electrons

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Backscattered electrons

scintillation counter

Absorbed electron current

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3 most commonly used SEM imaging modes together with resolution attainable

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Secondary electron emission at surface irregularities: contributions from specimen collection are indicated

•Specimen with pronounced surface roughness, contrast is modified by surface collection contributions

•Any improved contrast is usually accompanied by degradation of image details

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Secondary electrons images, (a) fracture of steel, (b) low alloy ferritic steel steam pipe after operation, (c) rapidly cooled polyethelene where morphology in the centre is revealed as a consequence of damage by incident beam

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Scanning Electron Microscope (SEM) Back-scattered electron detection gives rise to

images presenting a compositional contrast (BSE yield being a function of atomic number).

Different atomic number gives different contrast. As primary electron strike the surface they are

inelastically scattered by atoms in sample. Interactions lead to emission electrons – detected to

produce image. X-rays, which also produced by this interaction may

also be detected in SEM equipped for Energy Dispersive X-ray Spectroscopy (EDX).

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Scanning Electron Microscope (SEM) Factors directly affect SE emission from surface :

Work function of surface (energy supplied for electron to escape), depends on both composition & atomic packing.

Beam energy & intensity. Density of sample (limited influence). Surface topography (most pronounced effect) or local

curvature of surface. FE SEM – capable of nm resolutions at beam

energies as low as 200 V, and give excellent contrast based on either atomic number or work function.

Alternative imaging – cathodoluminescence & electron beam image current. 43


Working distance (WD) – distance separating surface from final pole piece.

Short WD – decreases depth of field, raises lower limit of magnification, may reduce image clarity by interfering with SE collection, & may limit sample movement.

Long WD – enhances DoF, permits low mag, but also reduces image clarity coz electron signals have to travel farther for detection.

Energy dispersive Spectroscopy (EDS) – detect X-rays emitted by specimen.

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Scanning Electron Microscope (SEM) Every elements has their own characteristic energy

and wavelength pattern. Limitation not able to detect - light elements (C, N,

O), specifically identify compounds or ionic state of detected element.

But new tech able to detect light elements down to boron (Z = 5) for quantitative.

X-ray signal mode of display : spectrum, line scan & X-ray dot image or elemental map.

X-ray dot – characteristic photon recorded as white dot.

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EDS X-ray spectrum from small Ti carbonitride particle in Al. (a) Al metal, (b) TiCN particle

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Wavelength-dispersive spectrometry : The line profiles for nickel, chromium, silicon, and manganese in a specimen of cast iron. One can see that the silicon and nickel are associated, as are the chromium and manganese. Line analyses can then be calibrated with quantitative analyses. 47

An energy dispersive (EDX) spectrum of NiO

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Sample preparation Surfaces of the bulks of metals, polymers,

ceramics Cutting, etching or fracturing- depends on what

kind of information we want to observe. Examples:

If we want to observe grains, grains boundaries, open pores structure: Similar with sample preparation for optical microscope

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Sample preparation – electrostatic charging should be avoided.

Coating – sputtering of heavy metal such as gold-palladium or carbon.

Carbon is usually more desirable when EDS is used so as not to interfere with chemical analysis.

Fractography and failure analysis – surface should NOT be damaged or altered in any way by any prior specimen preparation procedure.

Spatial & orientational relationship to component must be known, images recorded over full range.

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How to prepare SEM microstructure of thin films?

2 microstructures to consider Surface microstructure Cross-section microstructure

Barium strontium titanate thin film produced using RF magnetron sputtering

Surface Cross-section

Bare in mind, normally thin films are prepared on brittle silicon surface, SiC, glass, ITO glass etc. Therefore, it is possible to obtain a brittle fracture!

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Surface microstructure of thin film Cut to the desired size Place it on sample holder Eliminate any dust or loose impurities e.g.

using air gun For non-conductive sample: sputter a thin

layer of conductive layer such as gold or platinum

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Cross-section of thin film

2 options to consider on how to prepare a brittle fracture Make a line of scratch on the sample Break the sample

Froze the sample in liquid nitrogen Break the sample

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What can you do if you have film on non-brittle substrate?? Cut the sample to the desired size Mount in resin for protection Polish the surface

Almost impossible to get fractured information

Can also try to peel off the film

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Polymer samples

Depends on what we want to know. For composites, worthwhile to soak in liquid

nitrogen, and fracture the sample Observe distribution of matrix and reinforcement Wettability of the components

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Ductile failure – typical fracture surface shows dimple

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Brittle failure – typical fracture surface shows facets & cleavage.

Surface looks rough and shiny at lower magnification.

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Ductile – brittle failure, fracture surface consists of combination of dimple and facets / cleavage.

Fracture surface of Alumina sample – brittle material

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Fatigue striation indicates cyclic crack propagation, 700x.

‘Beach marks’ indicative of progressive fatigue failure. However, absence of this does not means failure is not because of fatigue. Fatigue initiation is marked by arrow. 180x 59

Intergranular fracture indicative of hydrogen embrittlement, 400x

Surface of casting void in fracture surface of Aluminum casting – dendrite structures. 1000x

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Backscattered electrons – topographical & compositional detail. Contrast between areas with different chemical composition can be observed. Photo courtesy of Dr Zainovia.

Outer columnar NiO layer

Cr2O3 layer

Porous region

Ni-10%Cr

Voids

(d)

Figure 6.13. Cross sectional morphology for oxide formed after (a) 30 min. (SEM-BEI) `

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Surface of textured NiO (100) – NaCl structure.

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Electron Microscope - limitation

Electrons are high energy particle which will easily be affected by any matter they encounter. When they do, interaction results in emission of all the lower forms of energy – X-rays, secondary electrons, ultraviolet, heat, etc.

Therefore, electrons cannot penetrate very deeply. Cannot even pass thru air.

Microscope has to be kept in high vacuum. Specimens always dead. Also require

proper preparation to get good image.

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Energy Dispersive X-Ray Spectroscopy

Energy dispersive X-ray spectroscopy (EDS or EDX) is an analytical technique used for the elemental analysis or chemical characterization of a sample.

As a type of spectroscopy, it relies on the investigation of a sample through interactions between electromagnetic radiation and matter, analyzing x-rays emitted by the matter in response to being hit with the electromagnetic radiation.

EDS systems are most commonly found on scanning electron microscopes (SEM-EDX).

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Its characterization capabilities are due in large part to the fundamental principle that each element has a unique atomic structure allowing x-rays that are characteristic of an element's atomic structure to be identified uniquely from each other.The incident beam may excite an electron in an inner shell, ejecting it from the shell while creating an electron hole where the electron was. An electron from an outer, higher-energy shell then fills the hole, and the difference in energy between the higher-energy shell and the lower energy shell is released in the form of an x-ray. The x-ray released by the electron is then detected and analyzed by the energy dispersive spectrometer. These x-rays are characteristic of the difference in energy between the two shells, and of the atomic structure of the element form which they were emitted.

Energy Dispersive X-Ray Spectroscopy

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Energy Dispersive X-Ray

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EDS Spectrum

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Documents

BMFB 3263Material Characterisation Scanning Electron Microscope (SEM) 1