Sem vasu

8/10/2019 Sem vasu

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8/10/2019 Sem vasu


8/10/2019 Sem vasu


CONTENT

• INTRODUCTION

• CONSTRUCTIONAL DETAILS

• SEM OPERATION

• OBTAINING SIGNAL IN SEM

• PERFORMANCE OF SEM

• CHARACTERISTIC INFORMATION: SEM

• SEM IMAGES IN DIFFERENT FIELDS

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INTRODUCTION

• First designed in Britain about 50 years back.

Unlike other optical microscope

• The SEM has a large depth of field, which allows more of aspecimen to be in focus at one time

• The SEM also has much higher resolution similar up to (×2000), so

closely spaced specimens can be magnified at much higher levels

• Can examine object up to 200mm in diameter, weighing up to 3kg

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COMPARISON OF OM,TEM AND SEM

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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 ofthe sample to be in focus at one time and produces an image that is

a good representation of the three-dimensional sample.

• The combination of higher magnification, larger depth of field,

greater resolution, compositional and crystallographic informationmakes the SEM one of the most heavily used instruments in

academic/national lab research areas and industry

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CONSTRUCTIONAL DETAILS contd….

CONDENSER LENS

• For example with a thermionic gun, the diameter of the

first cross-over point ~20-50µm.

• 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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SCANNING COIL

• Fine beam of electron is scanned across the specimen by the scan

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 vacuum

SIGNAL DETECTION

• Detectors which collect the signal

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CATHODE RAY TUBE (CRT)

• Accelerates electrons towards the phosphor coated screen where

they produce flashes of light upon hitting the phosphor.

a)DEFLECTION COIL

• Create a scan pattern forming an image in a point by point manner

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SEM OPERATION

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SEM OPERATION

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SEM OPERATION contd…..

• The SEM is an instrument that produces a largely magnified image

by using electrons instead of light to form an image.

• A beam of electrons is produced at the top of the microscope by an

electron gun.

• The electron beam follows a vertical path through the microscope,which is held within a vacuum.

• The beam travels through electromagnetic fields and lenses, which

focus the beam down toward the sample.

• Once the beam hits the sample, electrons and X-rays are ejected

from the sample.

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SEM OPERATION contd…..

• The region in which the electron penetrates the specimen is known

as interaction volume

• Even though radiation generated within this volume it will not be

detected unless it escapes from the specimen

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OBTAINING SIGNAL IN SEM

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OBTAINING SIGNAL IN SEM contd…

Away from incident light lose moreenergy so less spacial resolution

Closer to incident light havinghighest energy more spacial

resolution contains

crystallographic information

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SECONDARY

ELECTRONS

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• Secondary electrons generated both by primary electron entering

the specimen and by back scattered electrons.

• Hence the diameter of secondary electron originating region is

greater then the diameter of incident beam.

Spacial distribution of secondary electrons

Intensity decreases

with increase in

distance from

incident light

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DETECTING SECONDARY

ELECTRONS

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Energy of Secondary

electrons are too low(10-

50eV) to excite scintillator for

accelerating it, it is biased.

Purpose

1.Prevents the high voltage of

scintillator affecting incident

electron beam

2.Improves collection efficiency

By attracting the electrons

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DETECTING SECONDARY ELECTRONS

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• At present separate detectors are used for detecting back scattered

electrons

ROBINSON DETECTOR

ADVANTAGE

Rapid response timeDISADVANTAGE

bulky

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DETECTING BACK SCATTERED ELECTRONS

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SOLID STATE SILICON DETECTOR

when electron impregnate on the

semi conductor .It results in

electron-hole formation(voltage)

Which can be further amplifiedDISADVANTAGE

slow response time

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DETECTING BACK SCATTERED ELECTRONS

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ELECTRON DETECTORS

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OBTAINING SIGNAL IN SEM contd….

Information

regarding

shape of

specimen

Chemical

constituents

of the

specimen

Collided

electron, on

detectiongives atomic

no. contrast.

Irregularities

can be

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PERFORMANCE OF SEM

PIXELS

• Minimum spot obtained on the CRT is 0.1mm(100µm)

• The size of the specimen pixel is given by

Where ,

M-magnification

a)If electron probe>specimen pixel

Resolution is degraded

b)If electron probe<specimen

pixel

Signal will be weak

c)If electron probe=specimen pixel

Optimum performance

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PERFORMANCE OF SEM contd……

DEPTH OF FIELD

The depth of field (DOF) is the portion of a

scene that appears acceptably sharp in the image

FOR FINDING DEPTH OF FIELD (H)

Where ,

WD-aperture diameter A -working distance

M-magnification

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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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MINIMUM USABLE BEAM CURRENT

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MINIMUM USABLE BEAM CURRENTRelation between critical current and contrast is given by

Where,

Q is the product of detector efficiency and electron yield

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EFFECT OF BEAM TILT

• One of the principal use of SEM is for studying surface features or

topography

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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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TOPOGRAPHY

The surface features of an object or "how it looks", its texture;direct

relation between these features and materials properties

MORPHOLOGY

The shape and size of the particles making up the object; direct relation

between these structures and materials properties

COMPOSITION

The elements and compounds that the object is composed of and the

relative amounts of them; direct relationship between composition and

materials propertiesCRYSTALLOGRAPHIC INFORMATION

How the atoms are arranged in the object; direct relation between these

arrangements 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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SEM IN DIFFERENT FIELDS

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BIOLOGY

SEM (scanning electron microscopy) image of E.coli

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MICRO MACHINING

SEM IMAGES

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REFERENCES

• Electron microscopy and analysis by

• www.purdue.edu

• http://en.wikipedia.org/wiki/Scanning_electron_microscope

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http://www.purdue.edu/

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

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

http://www.purdue.edu/

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