unit part iii

8/12/2019 Unit Part III

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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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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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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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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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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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DETECTING SECONDARY ELECTRONS

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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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unit part iii

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