Basics of SEM

8/8/2019 Basics of SEM

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Basics of

Scanning Electron Microscopy


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SEM

Scanning Electron Microscope (SEM)

is a type of electron microscope capable ofproducing high-resolution images of asample surface

Due to the manner in which the

image is created, SEM imageshave a characteristic three-dimensional appearance


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History of Microscopy

Zacharias Janssen

1595


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Anton van

Leeuwenhoek



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Max Knott and Ernst Ruska


Createdthe TEMin 1931


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M. von Ardenne (1938)

added scanning coils toTEM and created the firstScanning ElectronMicroscope


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1951:The First Modern SEM

D. McMullan in the CambridgeUniversity Engineering Laboratory



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1939: First commercialTEM by Siemens(Ruska, von Borries)

1964: First commercialSEM by CambridgeInstruments


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1986 Nobel prize for E.Ruska(together with G. Binningand H.Rohrer, who developed the ScanningTunneling Microscope)

Binning Rohrer



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The Compound MicroscopeDissected

Eyepiece

ObjectiveLenses

Sample Stage

Coarse and FineAdjustments

Mirror


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HOW DOES A MICROSCOPE WORK?


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Lens System

Magnifies theImage bybending LIGHT!


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Image Formation in Microscope


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Focus vs Out of Focus

Focus

Out of

Focus


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The Function of a MICROSCOPE

Microscopes have two important functions:

Magnify the Image

Resolve the Image

Magnification is

the increasing insize of an object

Resolution is the

ability to distinguishbetween two

separate points.


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MAGNIFICATIONMAGNIFICATION W/

RESOLUTION


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Why is Illumination Important?

For the human eye, seeing objects, inmore and more detail, depends on light.No light means no seeing!

Resolution is afunction of theQUALITY of the

LENS and alsoof theWAVELENGTH

of LIGHT USED


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The Light Microscope is limited to amaximum of only 2000X magnification witha resolution of 200 nm

Why is IlluminationImportant?

WHY?


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ANSWER:BECAUSE it USESWHITE LIGHT which

has a wavelength of 400to 700 nm!

The Importance

of Illumination


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Electro Magnetic Spectrum


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How does one improveMAGNIFICATION &

RESOLUTION?

ANSWER:USE an ILLUMINATION withSMALLER WAVELENGTH!

USE an ELECTRON!!!


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DUAL NATURE OF ELECTRON

PARTICLE

WAVE

Since it moves in awavelike manner,then this meansthat it acts like

white light!!


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Using Electrons as Light Source

WAVE

Electron Wavelength may even be

made smaller simply by increasingthe energy of the electron!

THIS MEANS BETTER

RESOLUTION!!!


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The Electron Microscope

The ELECTRON

MICROSCOPE is simply aMICROSCOPE which usesELECTRONS to

ILLUMINATE the SAMPLE!

Resolution using

Electron Beam:

1 to 20 nm!


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Parts of the Electron MicroscopeElectron

Gun

Sample

chamber

Vacuum

PUMP

CRT Screen

Column

Operations

Console

Column

Console


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INSIDE theSEM


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ELECTRON GUN

Produceselectrons thatwill illuminate

the sample

Speed of Electronsis controlled by

accelerating voltage!


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Wehnelt cap

ELECTRON GUN


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Incandescent Bulb vs Gun Filament

Produces White

Light

Produces Electron

Beam


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An Electron GUNenclosed in a

glass jar

BEAM of

ELECTRONS

GUN


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Tungsten

Used as Filamentdue to Its HighMelting Point

(3695 K, 3422 C, 6192 F)


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Lanthanum Hexaboride, LaB6

BRIGHTERbut very

REACTIVE


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FIELD EMISSION GUNS

Field Emission is the ability ofmaterials to emit electronswhen a huge amount of electric

POTENTIAL is applied

TUNGSTEN

SCHOTTKY

EMITTER


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SEM Cathode Comparison

10-9 Torr10-8 Torr10-6 Torr10-5 TorrVacuumRequired

100-1000 A/cm2

steradian100-500 A/cm2

steradian20-50 A/cm2

steradian1 A/cm2

steradianBrightness


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Why is there a need to use HIGHVACUUM in the CHAMBER?

ANSWER: Electrons are so small that these can be easilySCATTERED by gas atoms or molecules

Filament will

BURN out if AIR ispresent!


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INSIDE theSEM


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LENS SYSTEM

Magnetic

Lenses

OpticalLenses


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ELECTROMAGNETIC LENSES

Lenses made ofmagnet to control the

size of the electronspray


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LENSESFOCUSES theELECTRONBEAM to a

SPOT SIZE of2 to 200 nm

How Does theEM Lens Work?


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ElectroMagnetic Lens

OBJECTIVE

LENS


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INSIDE theSEM


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Sample Chamber

SOME SAMPLESTAGES can actuallyaccommodateMULTIPLE SAMPLES!

X=125

mm

Y=100

mm

Z=80

mm

26 Pin Stub Holder


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INSIDE theSEM

H i th IMAGE d d i


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How is the IMAGE produced in

the SEM?

The electron beam is produced in the Gun

It is focused on the sample by Lenses

Beam is SCANNED over the sample surface


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Wh t h h B Hit


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What happens when Beam Hits

Sample?

The electrons interactwith the material toproduce different

products!


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Interaction Products

1. Secondary Electron

- electron is knockedout of the atom

2. Backscattered

Electron

- incident electron isknocked back


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Interaction Products

3. Characteristic X-ray

- primary electronknocks out electron,loses energy andreleases x-rays

4. Auger Electrons

- emission of anelectron from an atomcauses the emission ofa second electron


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InteractionVolume

I F i


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Image Formationin SEM

These products are

then COLLECTED bya DETECTOR and

processed to produce

the IMAGE in theCRT Screen!


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Secondary Electrons

SE Signals are best forSURFACE

TOPOLOGYEXAMINATIONS


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BACKSCATTERED ELECTRONS

BSE is best for

Compositional Difference

Imaging


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SE vs BS Imaging

BackscatteredElectronImage

SecondaryElectronImage

CHARACTERISTIC X-RAYS


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CHARACTERISTIC X-RAYS

WDS Spectrum ofSalt Crystal


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Interaction of Products Flash Presentation

on SEI, BEI, X-ray


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Advantages of Using SEM High depth of field: 3-D imaging

Image using CompoundMicroscope

Image using SEM


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3-D or STEREO IMAGING


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Advantages of Using SEM Bulk examination is possible (no

thickness limitation) Different detectors could allow different

analytical techniques (compositional,

phase contrast, etc)


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Limitations Inability to reproduce color

Sample must be stable in vacuum (wet orsamples that emit gases are not possibleto examine)

Sample must be conductive

Salt inWater

Fly Proboscis

Helicobacter

What Type of Samples can one


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What Type of Samples can one

view with the SEM? All Samples may be viewed in the SEM!

The only requirement is that sampleshould be electrically conductive!

METALS

CERAMICSPLASTICS

What if a Sample is an


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p

INSULATOR?

LIVING

THINGS

TO VIEW THESE, THEN ONE NEEDS TO MAKEIT CONDUCTIVE!!!

CERAMICS &PLASTICS


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Charge-up in SEM Imaging

Distortion in SEM Imaging caused by build

up of electrons in the non-conductive sample

Methods


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for non conductive specimen Metal Coating

Low Voltage Low Vacuum mode (in the specimen

chamber )


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METAL COATINGTo make a sample

conductive, the solution isto COAT it with a

METAL!!

Sputter Coatwith Gold or

Carbon


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Why is GOLD found

in this SALTsample?


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LOW VOLTAGE


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Flash presentation

on Low vacuum

Low vacuum Mode


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THANKS FOR LISTENING!

Basics of SEM

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