2. electron source and vacuum system - electron microscopy and diffraction

8/9/2019 2. Electron Source and Vacuum System - Electron Microscopy and Diffraction

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Do Minh Nghiep

Materials Science Center

Electron MicroscopyElectron Microscopyand Di ractionand Di raction

2. Electron source2. Electron source


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ontentontent

Electron sourceElectron source

Classification erm on c em ss on gun

Field emission gun

Correction

01/01/2009 2Handouts-MSE4346-K51KHVL


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ContentContent

Vacuum systemVacuum system

Vacuum system for EMs Application and selection of vacuum

SEM with low vacuum

01/01/2009 Handouts-MSE4346-K51KHVL 3


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Electron sourceElectron source



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e n ons an ypese n ons an ypes

LaB6 Electron gun: whereelectrons aregenerated Elements of the un:

- Emitter (cathode)- Wehnelt cilynder or cap

- Anode

W

Gun types:- Thermionic emitter (W, LaB6)- Field emitter (W crystal)

amen: ungs en, a 6 w re or ro Cathode: negative electrode Anode:positive electrode



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Electron gunElectron gun

The filament/cathode:- thermionic emission

,- field emission gun

Wehnelt cap:negative bias

Anode:positive bias



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aracter st csaracter st cs

Filament current (FC)Filament current (FC)/curren

running through the emitter

emission current: current

enerated b the emitter



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FCFC--BC curveBC curveIncreasing the filament current will increase the beam current but

filament current will only shorten the life of the emitter.



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Emission mechanismEmission mechanism

Work function:Work function:== --ww

Energy (or work) required towithdraw an electron completely

from a metal surface.

s energy s a measure o

how tightly a particular metalholds its electrons.



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Optical brightness Optical brightness

Described by equation:

= (current) / [(area).(= (current) / [(area).(illumination angle)] = 4i)] = 4ibb/ (d/ (d2222)) ax ma r g ness o erm on c em ss on a g vo age

maxmax= J= JCC.e.V.e.Voo/.k.T/.k.T

C -e - electron charge = 1.59 x 10-19 CVo - accelerated voltagek - Boltzmann constant = 8.6 x 10-5 eV/K

-d - beam diameter- beam angle of coherenceib - beam current



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Field emitterThermionic emitter



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Thermionic emitters utilize heat to

Lanthanum hexaboride (LaB6)Tungsten filament (W)

01/01/2009

12Handouts-MSE4346-K51KHVL


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Thermionic emittersThermionic emittersWW filamentfilament: a wire of diameter

sharp tip of diameter 100 mand emission area of 100 m x150 m.

LanthanumLanthanum HexaborideHexaboride (LaB(LaB66)):: a rod/single crystal LaB6 with sharp tip

of diameter 50 m and length 0,5 mm is mounted and heated bycarbon/rhenium resistance, insensitive to LaB6.



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W wire bent into a loop of various dimensions (Tm=3410 degrees C)



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66

Heat is a lied b wa of separate resistance wires

or by ceramic mounts

and by ribbons: Filamentcurrent is separate from



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Similar in design to atungsten filament



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LaBLaB66 filamentfilament



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Field emission: The emission of electrons that are

high electric fieldThreeThree typestypes ofof fieldfield emittersemitters::

FE - cold emitter b electric fieldTE - thermal emitter (by electric field and heat)SE- Schottky emisison (by electric field, heat,

coating (EW decreased)

Flashing the filament (FE vs. TE)Filaments EW: W - 4,5 eV; LaB6 - 2,5 eV;

W/ZrO - 4,52,8 eVcoa ng r on < >



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e em erse em ers

ConfigurationConfiguration

Filament is single orientedcrystal of tungsten etched to a

fine tip of diameter 100 nm or ZrO coating

sma er, spo we e on awire and coated with ZrO. Electric field is concentrated at

.reduced and electron emissionwill be easier to realized withoutTungsten wire



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Field emittersField emitters

Emission beamEmission beam

A Field Emissiontip can beco or erma y ass s e

to help overcome the workfunction, but ultimately it is

a high voltage field of 3keV that is needed.



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compar soncompar son

Other factors to consider ?Cost: W= $15 LaB6 = $400 F.E. = $6000Lifetime: 100 hr. 1000 hr 5-8 000 hr.



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A com arisonA com arison



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erm on c vs e em er erm on c vs e em er

Thermionic emitterThermionic emitterPros:

Field emitterField emitterPros:

- direct and rapid heating

- chip technology

- narrow electron beam fromsource

- good brightness

- relative low brightness

- evaporation of cathode

- big depth of field (DOF)CONS:

- ultra high vacuum (UHV >10-

- thermal drift duringoperation

torr)- high gradient of electric field- tip sensitive to shape,

- roa e ec ron eam romsource

sur ace, eros on, r



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us men o e ec ron gunus men o e ec ron gun



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Filament centering

Gun horizontal

Gun tilt



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Vacuum systemVacuum system



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InIn SEMSEM thethe pressurepressure PP mustmust bebe < (10-4) P a ( 10-6 torr)

- for thermionic W emitter: P < 10-4 Pa (10-6 torr)

- for more sensitive LaB6 emitter: P < 10-5 Pa (10-7 torr)

- Elimination of interaction between electron beam and gas

molecules, which reduce intensity of primary beam and scatter it- If the pressure over 1 Pa (10-2 torr) may be a discharge between

scintilating crystal (+10.000 V) and Faraday cup (+250 V)

-

(dissociation of hydrocacbon compound dirties sample)

LimitationsLimitations:: escapeescape ofof gasgas impossibleimpossible; drying and freezing of, -



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Why vacuum in EM column ?Why vacuum in EM column ?

In order to allow passage of the electronbeam through the microscope withoutinterference from gas molecules, the

pressure within the instrument has to bere uce o e po n , w ere ere s verysmall probability, that an electron will

.

Pressure should be lower than 10-5 torr.



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VacuumVacuum s stems stem:: low ver hi h 1 Pa = 10-2 torr

Rough vacuum: 105

-102

Pa (103

-1 torr)Medium vacuum: 101-10-1 Pa (10-1-10-3 torr)

High vacuum: 10-1-10-5 Pa (10-3-10-7 torr)

Ultra-high vacuum: < 10-5

Pa (10-7

torr) PumpPump typestypes:: rotary pump, diffusion pump, turbo-pump

- Rotary pump: for rough to medium vacuum (from air -

- Diffusion pump: for high vacuum (10-2 to < 10-9 torr)

- - - -3 < -10



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Diffusion pumpDiffusion pump



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Vacuum system is

devided into various



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Pa 10-5 10-1 102 105



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Environmental SEMEnvironmental SEM



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Vacuum is variableVacuum is variable

Different names: Environmental (E)/

Elevated Pressure (EP) SEM

- -

Various pumps: for sample stage, column

Media: O2, Ar, N, H2O, air



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SE ionizes gas molecules insample chamber (only about

ionization). Formed ions will neutralize

any electric charge built upabove the surface of non-conductive sample. Specific SE detectors are

used for ESEM because of high pressure nearby samplesurface.



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O eratin rinci les of ESEMO eratin rinci les of ESEM

VPSE detectors of LEO allow measurementof SE in both cases of low and high vacuum.



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Imaging of wet samples

- by replacing air with water vapour in sample

Following of hydration and dehydration process

- -

- change of sample state and structure in the process

-

surface



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Max resolution only 2 nm, much lower than that,

Low density of incident electrons because of

Background artefacts caused by electronscatterin

Low contrast



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ESEM overcomes limitation of SEM: coating is required in the case ofelectric charge on non-conductive sample surface, moist

.

ESEM allows to see samples (solid/liquid) in its original state: mediamay be water, argon, nitrogen, oxigen, air.

ESEM allows to analyze moist/wet samples at high pressure (50 torr),without vacuum.

In-situ study of specimen with video-capture ofdissociation, hydration,

dehydration, crystallization, melting, absorbtion, corrosion processes(agglomeration of polymer colloids; expansion, dissolvation, separationmechanism of medicine; kinetic physico-chemical behavior )



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Illustrations: crystallizationIllustrations: crystallization

15 min.

10 min. 20 min.

VPSE detector of LEO used



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Illustrations: metal meltingIllustrations: metal melting



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In presence of air and under controlled pressureof sample chamber, the charged region doesntexist and quality of image is better

Charged region on surface ofCaCO3 sample under high vacuum


Illustrations: water lossIllustrations: water loss


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Illustrations: water lossIllustrations: water loss

Image of dehydrated (link, bottom) and hydrated(right, top) samples observed in vacuum andw ou vacuum


2. electron source and vacuum system - electron microscopy and diffraction

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