2. Electron Source and Vacuum System - Electron Microscopy and Diffraction
Transcript of 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
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ContentContent
Vacuum systemVacuum system
Vacuum system for EMs Application and selection of vacuum
SEM with low vacuum
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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)
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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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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)
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(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
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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
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- change of sample state and structure in the process
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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
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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
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