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INSTRUMENTATION
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INTRODUCTION
First designed in Britainabout 50 years back. Unlikeother optical microscope
The SEM has a large depth of field, which allowsmore of a specimen to be in focus at one time
The SEM also has much higher resolution similar upto (2000), so closely spaced specimens can bemagnified at much higher levels
Can examine object up to 200mm in diameter,weighing up to 3kg
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WHY SEM
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ADVANTAGES OF USING
SEM OVER OM
The SEM has a large depth of field, which
allows a large amount of the sample to be infocus at one time and produces an imagethat is a good representation of the three-dimensional sample.
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CONSTRUCTIONAL DETAILS
ELECTRON GUN
Tungsten filament thermionic emission
Type. The electrons are accelerated usually
Between 1KeV-30KeV
CONDENSER LENS
Two or three in no. de magnify the electron
Beam Until, as it hits the specimen,it may have a diameter of only 2-10nm
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CONSTRUCTIONAL DETAILS
contd.CONDENSER LENS
For example with a thermionic gun, the diameter of the
first cross-over point ~20-50m.
If we want to focus the beam to a size < 10 nm on the
specimen surface, the magnification should be ~1/5000,
which is not easily attained with one lens (say, the
objective lens) only.
Therefore, condenser lenses are added to de magnify
the cross-over points.
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CONSTRUCTIONAL DETAILS
contd.SCANNING COIL
Fine beam of electron is scanned across the specimen by thescan
coils by changing the magnetic field strength
VACUUM SYSTEMS
Chamber which "holds" vacuum, pumps are used to
produce vacuum
Valves to control vacuum, gauges to monitor vacuumSIGNAL DETECTION
Detectors which collect the signal
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CONSTRUCTIONAL DETAILS
contd.CATHODE RAY TUBE
(CRT)
Accelerates electronstowards the phosphor
coated screen wherethey produce flashesof light upon hittingthe phosphor.
a)DEFLECTION COIL
Create a scan patternforming an image in apoint by point manner
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SEM OPERATION
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SEM OPERATION
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O O
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SEM OPERATIONcontd..
The SEM is an instrument that produces a largelymagnified image by using electrons instead of light to forman image.
A beam of electrons is produced at the top of themicroscope by an electron gun.
The electron beam follows a vertical path through themicroscope, which is held within avacuum.
The beam travels through electromagnetic fields andlenses, which focus the beam down toward the sample.
Once the beam hits the sample, electrons and X-rays areejected from the sample.
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SEM OPERATIONcontd..
The region in which the electron penetrates the specimenis known as interaction volume
Even though radiation generated within this volume it willnot be detected unless it escapes from the specimen
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OBTAINING SIGNAL IN SEM
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OBTAINING SIGNAL IN SEMcontd
Away from incident light lose moreenergy so less spacial resolution
Closer to incident light havinghighest energy more spacialresolution containscrystallographic information
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OBTAINING SIGNAL IN SEMcontd
Secondary electrons generated bothby primary electron entering thespecimen and by back scattered electrons.
Hence the diameter of secondary electron originating region is greaterthen the diameter of incident beam.
Spacial distribution of secondary electrons
Intensity decreaseswith increase indistance fromincident light
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OBTAINING SIGNAL IN SEMcontd
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DETECTING SECONDARY ELECTRONS
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OBTAINING SIGNAL IN SEMcontd
Energy of Secondaryelectrons are too low(10-50eV) to excite scintillator foraccelerating it, it is biased.
Purpose1.Prevents the high voltage ofscintillator affecting incidentelectron beam2.Improves collection efficiency
By attracting the electrons
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DETECTING SECONDARY ELECTRONS
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OBTAINING SIGNAL IN SEM contd
The signal is not purely consist of secondary electrons itcontains some back scattered electrons.
HOW TO DETECT BACK SCATTERED ELECTRONS ALONE
If the scintillator bias is switched off or the collector givennegative voltage secondary electrons are extruded andback scattered signal is obtained
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BACK SCATTERED ELECTRONS
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ELECTRON DETECTORS
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OBTAINING SIGNAL IN SEM
contd.
Informationregardingshape ofspecimen
Chemicalconstituentsof the
specimen
Collidedelectron, on
detectiongives atomicno. contrast.Irregularitiescan be
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PERFORMANCE OF SEM
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PERFORMANCE OF SEM
PIXELS
Minimum spot obtained on the CRT is 0.1mm(100m)
The size of the specimen pixel is given by
Where ,
M-magnification
a)If electron probe>specimen pixelResolution is degraded
b)If electron probe
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PERFORMANCE OF SEMcontd
RESOLUTION
The ultimate resolution of the SEM as being that of the smallest probe
which can provide adequate signal from the specimen
PROBE SIZE
Decreases with increasing the strength of the condenser lens anddecreasing the working distance
When probe dia current in the beam
Relation between these two is given by
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EFFECT OF BEAM TILT
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Facet fractured surface
viewed in SEM with
secondary electron .Imagetaken at same condition but
exposure at different angle
TOPOGRAPHIC IMAGES
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CHARACTERISTIC
INFORMATION: SEM
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CHARACTERISTIC INFORMATION: SEM
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TOPOGRAPHYThe surface features of an object or "how it looks", its texture; directrelation between these features and materials propertiesMORPHOLOGYThe shape and size of the particles making up the object; direct relation
between these structures and materials propertiesCOMPOSITIONThe elements and compounds that the object is composed of and therelative amounts of them; direct relationship between composition andmaterials properties
CRYSTALLOGRAPHIC INFORMATIONHow the atoms are arranged in the object; direct relation between thesearrangements and material properties
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TOPOGRAPHIC IMAGES
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COMPOSITIONAL IMAGE
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CRYSTALLOGRAPHIC
INFORMATION FROM SEM
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TRANSMISSION ELECTRON
MICROSCOPE
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INTRODUCTION
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INTRODUCTION A TRANSMISSION ELECTRON MICROSCOPE, or TEM, has magnification
and resolution capabilities that are over a thousand times beyond that offered
by the light microscope at a order of 105
10
6.
The TEM is a complex viewing system equipped with a set of electromagneticlenses used to control the imaging electrons in order to generate theextremely fine structural details that are usually recorded on photographicfilm.
In the electron gun, the electrons emitted from a cathode, a solid surface, areaccelerated by high voltage (Vo) to form a high energy electron beam withenergy E = eVo. Because electron energy determines the wavelength ofelectrons and wavelength largely determines resolution of the microscope.
To achieve a high resolution, the TEM is usually operated under an
acceleration voltage of greater than 100 kV. In practice, 200 kV is commonlyused and meets most resolution requirements.
Since the illuminating electrons pass through the specimens, the informationis said to be a transmitted image.
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TEM Vs OM
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Scheme of a Transmission Electron Microscope
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Electron gun: The electrons are generated andaccelerated.
Condenser system: A set-up of different magnetic lensesand apertures.
Objective lens: Important lens in the microscope since itgenerates the first intermediate image.
Intermediate lens: Switching between imaging anddiffraction mode.
Projective lenses: Further magnification of secondintermediate image.
Image observation: Images and diffraction pattern cancan directly be observed on theviewing screen.
Vacuum system: Because of strong interactions of
electron with matter, gas particlesmust be absent in the column.
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Electron Source The general structure of an electron gun is
composed of three main parts: cathode or
electron source, Wehnelt electrode andanode.
Electrons are emitted from the surface of thecathode and accelerated by an electric fieldtoward the cathode. The Wehnelt electrode is
placed between the cathode and the anode. It is biased a few hundred volts negative with
respect to the cathode in order to stabilizethe electron beam against voltage fluctuationby reducing the electron beam current
whenever necessary. There are two basictypes of electron guns: thermionicemission(tungsten filament) and fieldemission (applying a very high
electric field to a metal surface).
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The Sample
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The Sample Samples are typically 3mm in diameter and
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The Sample For a non metallic sample:-
Cut\slice a section of material
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Image Formation
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g
All rays from a point in the object are
gathered by the lens and converge to a point
in the image.
All parallel rays are focused in the focal
plane.
The back focal plane of the objective lens
contains groupings of rays that have left the
object at the same angle.
The back focal plane contains the diffraction
pattern of the sample.
Diffraction pattern and image are both
formed in the imaging process
The intermediate lens is then focused on
either the image plane (for the image), or the
back focal lane for the diffraction attern .
sample
Objective
lens
Imaging Modes
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g g
Two principle modes of TEM operation, A Projecting the diffraction pattern,
B Projecting the image.
The intermediate lens selects either the Back Focal Plane or the image plane of the
objective lens.
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Factors affecting TEM Image
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Working Principle
The "Virtual Source" at the top represents the electrongun, producing a stream of monochromatic electrons.
This stream is focused to a small, thin, coherent beamby the use of condenser lenses 1 and 2.
The first lens (usually controlled by the "spot sizeknob") largely determines the "spot size"; the generalsize range of the final spot that strikes the sample.
The second lens (usually controlled by the "intensity or
brightness knob" actually changes the size of the spoton the sample; changing it from a wide dispersed spotto a pinpoint beam.
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The beam is restricted by thecondenser aperture(usually user selectable),knocking out high angleelectrons (those far from theoptic axis, the dotted linedown the center).
The beam strikes the specimenand parts of it are transmitted. This transmitted portion is
focused by the objective lensinto an image.
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Optional Objective and Selected Area
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Optional Objective and Selected Areametal apertures can restrict the beam; theObjective aperture enhancing contrast byblocking out high-angle diffracted electrons,
the Selected Area aperture enabling the userto examine the periodic diffraction ofelectrons by ordered arrangements of atomsin the sample.
The image strikes the phosphor image screen
and light is generated, allowing the user to seethe image. The darker areas of the imagerepresent those areas of the sample that fewerelectrons were transmitted through (they arethicker or denser). The lighter areas of theimage represent those areas of the samplethat more electrons were transmitted through(they are thinner or less dense).
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Imaging
Diffraction
Field Imaging
Bright field
Dark Field
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Diffraction Pattern
By adjusting the intermediate lens diffraction pattern can begenerated.
The incoming plane electron wave interacts with the atoms, andsecondary waves are generated which interfere with each other. This
occurs either constructively or destructively. For thin crystalline samples, this produces an image that consists of
a pattern of dots in the case of a single crystal, or a series of rings inthe case of a polycrystalline or amorphous solid material. For thesingle crystal case the diffraction pattern is dependent upon the
orientation of the specimen and the structure of the sampleilluminated by the electron beam.
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Bright Field Image
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Bright Field Image
In the bright field (BF) mode of
the TEM, an aperture is placed inthe back focal plane of theobjective lens which allows onlythe direct beam to pass.
In this case, the image results
from a weakening of the directbeam by its interaction with thesample.
Therefore, mass-thickness and
diffraction contrast contribute toimage formation: thick areas,areas in which heavy atoms areenriched, and crystalline areasappear with dark contrast.
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Dark Field Image
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Dark Field Image
In dark field (DF) images, the direct beam is blocked by the aperture
while one or more diffracted beams are allowed to pass the objectiveaperture. Since diffracted beams have strongly interacted with thespecimen, very useful information is present in DF images, e.g., aboutplanar defects, stacking faults or particle size.
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Bright Field and Dark Field
ItTransmitted Beam
IdDiffracted Beam
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Atomic Force Microscope
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Basic principles
The AFM consists of a cantilever with a sharp tip (probe) at its endthat is used to scan the specimen surface. The cantilever is typicallysilicon or silicon nitride with a tip radius of curvature on the order ofnanometers.
When the tip is brought into proximity of a sample surface, forces
between the tip and the sample lead to a deflection of the cantileveraccording to Hooke's law.
As well as force, additional quantities may simultaneously bemeasured through the use of specialized types of probe.
Typically, the deflection is measured using a laser spot reflected fromthe top surface of the cantilever into an array of photodiodes. Othermethods that are used include optical interferometry, capacitivesensing or piezoresistive AFM cantilevers.
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AFM Operating Modes
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AFM Operating ModesContact Mode
Laser beam measures the deflection of the tip
Feedback to a piezoelectroc scanner keeps force (cantileverdeflection) constant
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Tapping Mode
Tip oscillates with the amplitude of several nm
Typical frequency 50 400 kHz Touches the surface at a fixed amplitude
Sample is moved up/down, so that amplitude is constant
Non Contact Mode
Tip oscillates with the amplitude of several nm
Typical frequency 50 400 kHz
Remains 5-10 nm from the surface
Sample is moved up/down, so that amplitude is constant
Good for soft materials
Scanner
Scanner
Scanner
Operation: Contact Mode
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Operation: Contact Mode
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Electrostatic repulsive forces are caused
by electrons at the surface atoms
The force on the tip is kept constant byadjusting the z-position of the piezoscanner. This gives a topographic image
Adhesion forces between differentmaterials could be studied usingdifferent tip materials
Hardness/elasticity of the surface canbe studied by varying the force at each
point
Practical problems
Water or liquid layer
Particles on surface
Surface damage
Contact-mode at7.8K
Superconductingfilm is composedof screwdislocations
Imaged area= 800nm x 800 nm
YBa2Cu3O7-d sputter-deposited ona SrTiO3 substrate.
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Operation: Tapping Mode In tapping mode, the cantilever is driven to oscillate up and
down at near its resonance frequency by a small piezoelectricelement mounted in the AFM tip holder.
The amplitude of this oscillation is greater than 10 nm, typically100 to 200 nm.
Due to the interaction of forces acting on the cantilever whenthe tip comes close to the surface, Van der Waals force or dipole-dipole interaction, electrostatic forces, etc cause the amplitudeof this oscillation to decrease as the tip gets closer to the sample.
A tapping AFM image is therefore produced by imaging the
force of the oscillating contacts of the tip with the samplesurface.
This is an improvement on conventional contact AFM, in whichthe cantilever just drags across the surface at constant force andcan result in surface damage.
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Operation: Non-Contact Mode
In this mode, the tip of the cantilever does not contact the samplesurface. The cantilever is instead oscillated at a frequency slightlyabove its resonance frequency where the amplitude of oscillation istypically a few nanometers (
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