Microscopy 2009 Wk14 TEM

7/25/2019 Microscopy 2009 Wk14 TEM

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ICROSCOPY

THE TRANS ISSION ELECTRON ICRO

SCOPE

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Contents

Basic Systems Making Up a Transmission Eectron Microscope I!minating System Specimen Manip!ation System Imaging System "ac!!m System

"ISIBLE LI#HT$ ELECTRONS$ AN% LENSES Eectromagnetic Ra&iation an& t'e %i((raction P'enomenon E((ect o( %i((raction on Reso!tion Eectrons$ )a*es$ an& Reso!tion #enera %esign o( Lenses %esign o( Eectromagnetic Lenses

%E+ECTS IN LENSES

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Contents

MA#NI+ICATION %ESI#N O+ THE TRANSMISSION ELECTRON MI

CROSCOPE Comparison o( Lig't Microscope to Transmission Ee

ctron Microscope Aignment T'eory Ma,or Operationa Mo&es o( t'e Transmission

Eectron Microscope

Hig' Co ntrast

Hig' Reso!tion

%ark(ie&

%i((raction

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INTRO%UCTION

A TRANSMISSION ELECTRON MICROSCOPE$ orTEM$ 'as magni(ication an& reso!tion capa-iities t'at are o*er a t'o!san&times -eyon& t'a

t o((ere& -y t'e ig't microscope. It is an instr!ment t'at is !se& to re*ea t'e !t

rastr!ct!reo( pant an& anima ces as /e as*ir!ses an& may pro*i&e an image o( t'e *ery

macromoec!es t'at make !p t'ese -ioogicaentities.

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INTRO%UCTION

T'e TEM is a compe0 *ie/ing system e1!ippe& /it' a set o( eectromagnetic enses!se& tocontro t'e imaging eectronsin or&er to gener

ate t'e e0tremey (ine str!ct!ra &etaist'at are !s!ay recor&e& on p'otograp'ic (im. Since t'e i!minating eectrons pass t'ro!g't

'e specimens$ t'e in(ormation is sai& to -e a t

ransmitte&image. T'e mo&ern TEM can ac'ie*e magni(ications o( one miion times /it' reso!tions o( 2.3 nm.

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Basic Systems aking Up a Transmiss

ion Eectron icroscope

T'e transmission eectron microscope 4+ig!res 5.62A$B an& C7 is ma&e !p o( a n!m-er o( &i((erent systemst'at are integrate& to (orm one (!nctiona !nitcapa-eo( orienting an& imaging e0tremey t'in specimens.

T'e i!minating systemconsists o( t'e eectron g!n an& con&enser ensest'at gi*e rise to an& contro t'e amo!nt o( ra&iation striking t'e specimen.

A specimen manip!ation systemcompose& o( t'especimen stage$ specimen 'o&ers$ an& reate& 'ar&/areis necessary (or orienting t'e t'in specimen o!tsi&e an& insi&e t'e microscope.

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Basic Systems aking Up a Transmiss

ion Eectron icroscope

T'e imaging systeminc!&es t'e o-,ecti*e$interme&iate$ an& pro,ectorenses t'at are in*o*e& in (orming$ (oc!sing$ an& magni(ying t'e im

age on t'e *ie/ing screen as /e as t'e camerat'at is !se& to recor& t'e image. A *ac!!m systemis necessary to remo*e inter(

ering air moec!es (rom t'e co!mn o( t'e ee

ctron microscope. In t'e &escriptions t'at (oo/$ t'e systems /i -e consi&ere& (rom t'e topo( t'e microscope to t'e -ottom.

See Ta-es 5.8 an& 5.9.

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Page 167

TABLE 6.3 Major Column Components of the TEM*

Component Synonyms Function of Components

lluminationSystem Electron Gun Gun, Source Generates electrons and provides first

coherent crossover of electron beam

Condenser Lens 1 C1, Spot Size etermines smallest illumination spot sizeon specimen !see Spot Size in "able 6#$%

Condenser Lens & C&, 'rightness (aries amount of illumination on specimenin combination )ith C1 !see 'rightness in"able 6#$%

Condenser*perture

C& *perture +educes spherical aberration, helps controlamount of illumination striing specimen


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SpecimenManipulationSystem

Specimen E-changer Specimen *ir Loc Chamber and mechanism for inserting specimen

holder

Specimen Stage Stage .echanism for moving specimen inside column ofmicroscope

ma!in! System /b0ective Lens orms, magnifies, and focuses first image !see

ocus in "able 6#$%

/b0ective *perture Controls contrast and spherical aberration

2ntermediate Lens iffraction Lens 3ormall4 used to help magnif4 image fromob0ective lens and to focus diffraction pattern

2ntermediate *perture iffraction*perture, ieldLimiting*perture

Selects area to be diffracted

Pro0ector Lens 1 P1 5elps magnif4 image, possibl4 used in somediffraction )or

Pro0ector Lens & P& Same as P1

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"#ser$ationan% Camera

Systems

(ie)ingChamber

Contains vie)ing screen for final image

'inocular.icroscope

ocusing Scope .agnifies image on vie)ing screen foraccurate focusing

Camera Contains film for recording

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I!minating System

TABLE 6.& Major Components on Control 'anels of the TEM*

Component Synonyms Functions of Component

ilament Emission Effects emission of electrons upon heating

'ias *d0usts voltage differential bet)een filamentand shield to regulate 4ield of electrons

5igh (oltage +eset 5(, ( +eset *ctivates high voltage to gun

5igh (oltage Select 5(, ( Select Selects amount of high voltage applied to gun

.agnification Control .*G Controls final magnification of image b4activating combinations of imaging lenses

'rightness C& Controls current to second condenser lens

Gun "ilt Electronicall4 tilts electron beam beneath gunGun 5orizontal Electronicall4 translates electron beam beneath

gun

Spot Size C1 Controls final illumination spot size onspecimen

/b0ective Stigmator /' S"2G Corrects astigmatism

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ocus obbler ocus *id 5elps focus accuratel4 at lo) magnifications

E-posure .eter .onitors illumination for accurate e-posures

(acuum .eter (*C .onitors vacuum levels in various parts ofscope

ocusing Control ocusfine, medium,coarse

Controls current to ob0ective lens for accuratefocusing of image

'rightness Center 2llumination Centration "ranslates entire illumination s4stem ontoscreen center

Condenser Stigmator C/3 S"2G Corrects astigmatism in condenser lenses

2ntermediateStigmator

23" S"2G Corrects astigmatism in intermediate lens

'right8ar Selects brightfield or darfield operating mode

.ain .ain po)er s)itch to console

.ain Evac E(*C .ain s)itch to vacuum s4stem

5( obbler 5( .odulate obbles high voltage to locate voltage centerfor alignment

/b0ective obbler /' ./9L obbles current to ob0ective lens foralignment

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I!minating System

T'is system is sit!ate& at t'e top o( t'e microscope co!mn an& consists o( t'e eectron g!n4compose& o( t'e (iament$ s'ie&$ an& ano&e7

an& t'e con&enser enses.

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I!minating System

Eectron #!n. )it'in t'e eectron g!n 4+ig!re 5.637$ t'e (iamentser*

es as t'e so!rce o( eectrons. T'e stan&ar& (iament$ or cat'o&e4+ig!re 5.667$ is com

pose& o( a ":s'ape& t!ngsten /ire appro0imatey 2.3mm in &iameter4a-o!t t'e t'ickness o( a '!man 'air7.

Being a meta$ t!ngsten contains positi*e ions an& (reeeectronst'at are strongy attracte& to t'e positi*e ions.

+ort!natey$ it is possi-e to entice t'e o!termost or-ita$ or *aence$ eectrons o!t o( t'e t!ngsten -y (irst appying a 'ig' *otage to t'e (iament an& t'en 'eatingt'e meta -y r!nning a sma amo!nt o( %C eectrica c!rrent t'ro!g' t'e (iament /'ie operating /it'in a *

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Page 16:

Figure 6.21

(A) Diagram of an electron gun showing

filament, shield, and anode.

The shield is connected directly to the

high oltage, whereas the high oltage

leading to the filament has a aria!le

resistor ("#)to ary the amount of high

oltage.

The out$ut from the aria!le resistor is

then $assed through two !alancingresistors (%#)which are attached to the

filament.

(%) Actual electron gun from T&'

showing filament (f), shield (s), and

anode (a).

om$are to line drawing in 6.21(A).


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Figure 6.22

tandard "*sha$ed tungsten filament (f) used in most

electron microsco$es. The filament is s$otwelded to

the larger su$$orting arms, which $ass through the

ceramic (c) insulator and $lug into the electrical leads of

the gun.


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I!minating System

In ot'er /or&s$ a certain amo!nt o( energym!st -e p!t into t'e system to ca!se t'e eectrons to ea*e t'e (iament.

T'e amo!nt o( energy necessaryto -ring a-o!t eectron emission is terme& t'e /ork (!nctiono( t'e meta.

At'o!g' t!ngsten 'as a reati*ey 'ig' /ork (

!nction$ it 'as an e0ceent yie& o( eectrons,!st -eo/ its rat'er 'ig' meting pointo( 8$5;8; or 322 k"7 an& t'en so/y increases t'e amo!nt o( &irect c!rrentr!nning t'ro!g' t'e (iament to 'eat it to ac'ie*e t'e emi

ssion o( eectrons4t'ermionic emission7. As one appies more 'eat to t'e (iament$ t'e yie& o(

eectrons increases !nti t'e (iament -egins to met an& e*aporate in t'e 'ig' *ac!!m o( t'e microscope4+ig!re 5.687.

At some optima temperat!re$ t'e g!n ac'ie*es goo&eectron emission as /e as an accepta-e (iament i(e? t'is is terme& t'e sat!ration point4&isc!sse& ater7.

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Figure 6.2+

The im$ortance of $ro$er saturation of the filamentis

shown in these two cures. (a) olid line shows therelationshi$ !etween electron emission from a filament as

a function of the tem$eratureof the filament. The

o$timal tem$erature is 2,6- . (!) /hen one e0ceeds

2,6- filament tem$erature, the filament life dro$s

dramatically (dashed line).


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I!minating System

By e0amining +ig!re 5.68$ it -ecomes apparent t'at a goo& eectron yie&is ac'ie*e& at 6$522< =/'ere t'e (iament i(e is aro!n& 322 'o!rs.

O*ersat!ratingt'e g!n -y 'eating t'e (iament e*en 622< =-eyon& 6$522< = res!ts in a &ramatic &ecrease in (iament i(e.

T'e a*erage i(e o( a (iament ranges (rom 6; 'o!rs in o&er eectron microscopes to o*er 622 'o!rs in microscopes /it' goo& *ac!!m systems a

n& scr!p!o!sy maintaine& g!n areas. T'e ma,or ca!ses o( premat!re (iament (ai!reare?

o*ersat!ration$ 'ig' *otage &isc'arge ca!se& -y &irt in t'e g!n region$ poor *ac!!m$ an& air eaks or o!tgassing (rom contaminants in t'e g!n c'am-er

. T'e t/o contros on t'e pane o( t'e microscope t'at are !se& to initiate

t'e (o/ o( eectrons are !s!ay a-ee& acceerating *otage4or possi-yk"$ H" or HT7 an& emissionor sat!ration4or sometimes (iament7.

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I!minating System

I( one in&!ces t'e emission o( eectrons(rom a (iament as &escri-e& a-o*e$ t'is /i res!t in t'e emanationo( eectrons in a &irections.

)it'o!t a mec'anism (or g!i&ing t'em$ most o( t'e e

ectrons /o!& not enter t'e i!minating system. A secon&part o( t'e eectron g!n$ t'e s'ie&4aso ca

e& )e'net cyin&er$ -ias s'ie&$ or gri& cap7$ is in*o*e& in ass!ring t'at t'e ma,ority o( t'e eectrons go in t'e proper &irection.

T'e s'ie& is a capike str!ct!re t'at co*ers t'e (iament an& is maintaine& at a sig'ty more negati*e *otagepotentia t'an t'e (iament.

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I!minating System

Beca!se it is se*era '!n&re& *ots more negati*et'an t'e ;2 to 322 k" eectrons$ t'e s'ie& s!rro!n&s t'eeectrons /it' a rep!si*e (ie&t'at is -reac'a-e onyt'ro!g' a 6 to 8 mm apert!re&irecty in (ront o( t'e (iament tip.

Eectrons e0it t'e s'ie& apert!re an& are &ra/n to/ar& an apert!re& &isc$ or ano&e$ t'e t'ir& part o( t'e eectron g!n 4+ig!re 5.637.

T'e ano&e is connecte& to gro!n& so t'at t'e 'ig'y negati*e eectrons are strongy attracte& to it.

T'!s it is positi*e /it' respect to t'e g!n. In (act$ t'e 'ig'y attracti*e p!o( t'e ano&e in com-i

nation /it' t'e negati*e s!r(aceo( t'e s'ie& act as aneectrostatic @ens@ to generate a crosso*er imageo( t'e eectron so!rce near t'e ano&e 4+ig!re 5.697.

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Page 16:

Figure 6.21

(A) Diagram of an electron gun

howing filament, shield, and

anode.The shield is connected directly

to the high oltage, whereas the

high oltage leading to the

filament has a aria!le resistor

("#) to ary the amount of high

oltage.The out$ut from the aria!le

resistor is then $assed through

two !alancing resistors (%#)

/hich are attached to the

filament.

(%) Actual electron gun from T&'showing filament (f), shield (s),

and anode (a).

om$are to line drawing in

6.21(A).


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Figure 6.2 The self*!iased electron

gun.

The shield (/ehnelt cylinder) is slightly

more negatie than the filament tocontrol the release of electrons from

the gun.

A aria!le !ias resistor (see

Figure 6.21A) regulates the degree of

negatiity of the filament.

The anode seres as a $ositie

attracting force and seres as an

electrostatic lens (in com!ination with

the shield) to hel$ focus the electrons

into a crossoer s$ot a$$ro0imately ;m across.


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I!minating System

!st as eectromagnetic enses 'a*e t/o (orcesor poes$ eectrostaticenses 'a*e positi*ey an& negati*eyc'arge& s!r(aces to attract or rep

e an&$ t'ere-y$ (oc!s eectrons. T'e term crosso*erre(ers to t'e point /'ere t

'e eectrons (oc!s or con*ergean& cross o*ereac' ot'ers pat's.

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I!minating System

"aria-e Se( :Biase& # !n. It /as state& earier t'at t'e 'ig' *otage s'ie&is sig'ty morenegati*et'an t'e (iament.

T'is &i((erence in negati*e potentia$ or -ias$isesta-is'e& -y connecting t'e s'ie& &irecty tot'e negati*e 'ig' *otage ine /'ie pacing a *aria-e resistor in t'e 'ig' *otage ine to t'e (i

ament 4+ig!re 5.63A7.

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I!minating System

By *arying t'e *a!e o( t'is resistor$ t'e (iament may -e ma&e ess negati*e t'an t'e s'ie&4!s!ay -y 322 to 622 *ots7. T'e greater t'e *a!e o( t'e *aria-e -ias resistor$ t'e ess nega

ti*e t'e (iament /i -ecome. As t'e (iament -ecomes ess negati*e$ (e/er e

ectrons /i -e a-e to pass t'ro!g't'e s'ie&apert!re since t'ey are no/ rep!se& to a grea

ter &egree -y t'e s'ie&. T'e o*era e((ect o( t'e *aria-e -ias$t'ere(ore$ is to reg!ate t'e escape o( eectrons t'ro!g' t'e s'ie& apert!re.

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I!minating System

In a&&ition to 'ig' *otage$ one appies a certain amo!nt o( &irect c!rrent to t'e (iamentin or&er to 'eat !pt'e (iament an& en'ance eectron emission.

As t'is c!rrent passest'ro!g' t'e *aria-e -ias resis

tor$ a certain amo!nt o( *otage is generate& an& appie& to t'e s'ie&in or&er to make it more negati*e.

T'ere(ore$ as one contin!es to increase t'e 'eating c!rrent to t'e (iament$ t'e n!m-ers o( eectrons coming o(( t'e (iament /i increase.

B!t since t'e s'ie& is -ecoming progressi*ey negati*e$ t'e tota n!m-er o( eectronsact!ay passing t'ro!g' t'e s'ie& apert!re&oes not increase signi(icanty.

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I!minating System

T'e so:cae& sat!ration point o( t'e g!n is t'e point /'ere t'e n!m-er o( eectrons emitte& (rom t'e g!nno onger increasesas t'e (iament is 'eate&.

T'e g!n is$ t'ere(ore$ sai& to -e se(:-iasing$ since it t'rottes -ack on eectron emissionas t'e 'eat is increase&.

It is important t'at t'e operator reaie t'at increasing t'e

'eat o( t'e (iament -eyon& t'e sat!ration point/i not increase t'e -rig'tness o( t'e g!n-!t /i consi&era-y s'orten t'e (iament i(e.

On t'e ot'er 'an&$ !n&ersat!rationo( t'e (iament may ea& to insta-iities in t'e i!minationo( t'e specimen an& ca!se pro-ems i( anaytica proce&!res 4s!c' as D:ray anaysis7 are to -e attempte&.

T'e arrangement (or controing eectron emissionin mo&ern eectron microscopes is terme& t'e *aria-e se(:-iase& g!n.

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I!minating System

Con tro ing t'e Am o!n t o( I !m ination Str iking t'

e Specimen. It is possi-e to make practica !se o( t'e*aria-e -ias to reg!ate t'e amo!nt o( i!mination t'at strikes t'e specimen.

+or e0ampe$ /'en operating at 'ig' magni(ications/it' sma con&enser spot sies$ it may -e necessary to ater t'e -iasto e((ect greater g!n emissions.

O( co!rse$ t'e (iament i(e /i -e s'ortene&$ -!t t'ismay -e necessaryin or&er to criticay *ie/ an& (oc!s t

'e specimen. It is aso important to remem-er t'at t'e greater t'e

-eam c!rrent$ t'e greater t'e specimen &amage.

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I!minating System

Mo*ing t'e (iament coser to t'e s'ie& apert!re/ipermit more eectronsto pass t'ro!g' to t'e con&enser enses.

Ho/e*er$ i( t'e (iament is pace& too cose to t'e aper

t!re$ t'e -ias contro -y t'e s'ie& /i -e ost$ an& t'eemission /i -ecome e0cessi*e. +iaments pace& too (ar a/ay (rom t'e s'ie& apert!re$ on t'e ot'er 'an&$may ne*er yie& s!((icient n!m-erso( eectrons (rom t'e g!n.

T'ere(ore$ care(! pacement o( t'e (iament reati*e to t'e s'ie& apert!re is *ery important an& s'o!& -e in accor&ance /it' t'e man!(act!rers speci(ications

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Figure 6.2 The self !iased

electron gun.

The shield (/ehnelt cylinder) is

slightly more negatie than thefilament to control the release of

electrons from the gun.

A aria!le !ias resistor (see Figure

6.21A) regulates the degree of

negatiity of the filament.

The anode seres as a $ositie

attracting force and seres as an

electrostatic lens (in com!ination

with the shield) to hel$ focus the

electrons into a

crossoer s$ot a$$ro0imatelym across.


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I!minating System

T'e &istance o( t'eano&e(rom t'e (iament an& s'ie& is aso important.

As one mo*es t'e ano&e coserto t'e (iament$ moreeectrons/i -e e0tracte& (rom t'e g!n.

T'is -ecomes a consi&eration /'en !sing o/er acceerating *otages/'ere it may -e necessary to mo*e t'eano&e coser to assist in t'e e0traction o( t'e o/er energy eectrons 4;2 k"$ (or e0ampe7.

Some eectron microscopes 'a*e an e0terna a&,!stment scre/t'at /i mec'anicay a&,!st t'e 'eig't o( t'eano&e$ /'ie ot'er mo&es 'a*e a pne!maticay act!ate& @ano&e i(ter@ t'at c'anges in response to t'e kio*ot seecte& -y t'e operator.

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I!minating System

T'e c'oice o( k" s'o!& -e consi&ere& care(!y.

Lo/er k"s s!c' as ;2 k" /i generate an imag

e /it' 'ig'er contrast -!t o/er reso!tion$ /'ie 'ig'er k"s 4322336; k"7impro*e reso!tion-!t o/er o*era contrast.

Less specimen &amage/i res!t at t'e 'ig'er

k"s since t'e spee&ier eectronsinteract (ora s'orter perio& o( time /it' t'e specimen

.

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I!minating System

Ot'er #!n %esigns. T'e (iament s'ape may -e atere& as i!strate& in +ig!re 5.6;-$ /'ere t'e tip /as (irst (attene& an& t'en s'arpene& to a point.

It is aso possi-e to p!rc'ase a pointe& (iament ma&e -y /e&ing a singe crysta o( t!ngsten onto t'e c!r*e& tip o( a stan&ar& (iament 4+ig!re 5.6;c7.

Bot' types o( pointe& (iaments'a*e a consi&era-y s'orter i(etimet'an &o stan&ar& (iaments.

Ho/e*er$ since t'e initia g!n crosso*er imageis m!c' smaeran& t'e -eam is 'ig'y co'erent$ t'ey are necessary (or 'ig' reso!tion st!&ies/'ere -eam &amage may -e a consi&eration 4e.g.$ *ie/ing crystaine attice panes7.

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Figure 6.2Drawing of three different filament ti$s. (a) tandard "*sha$ed

filament ti$. (!) tandard filament ti$ that was flattened and then

shar$ened to a fine $oint. (c) Filament ti$ where a crystal of tungsten

was s$otwelded onto the cured end.


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I!minating System? LaB

5

Besi&es -eing ma&e o( t!ngsten$ (iaments may aso -e constr!cte& o( ant'an!m 'e0a-ori&e$/'ic' 'as a o/er /ork (!nction.

Typicay$ t'ese (iaments operate at temperat

!res 3$222< = o/er t'an t!ngsten an&'a*e a-rig'tness se*era times greatert'an a stan&ar& t!ngsten so!rce.

T'e i(etime o( s!c' (iaments ranges (rom >22

to 6$222 'o!rs. T'is type o( (iament may -e ma&e (rom a singe LaB5crysta/it' one en& 'a*ing a point meas!ring ony se*era micrometers across4+ig!re 5.657.

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Figure 6.264anthanum he0a!oride cathode. The crystal () is held in $lace !y

means of $yrolytic gra$hite (5) !locs with com$ressie force

generated !y moly!denum (') alloy $osts designed to withstand

e0tremely high tem$eratures.


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I!minating System

LaB5(iaments are coming into !se so/y$ since t'ey are consi&era-y more e0pensi*e t'an t!ngsten (iamentsan& are e0tremey c'emicay reacti*e /'en 'ot. +ort'e atter reason$ *ac!!ms greater t'an 32:;Pa are ess

entia4see section @"ac!!m System@7$ an& specia (iament mo!nts m!st -e constr!cte& (rom s!c' nonreacti*e eements as r'eni!m or *itreo!s car-on.

LaB5(iaments are !se(! /'en sma -eam crosso*ersiescontaining arge n!m-ers o( eectrons are necess

aryas in 'ig' magni(icationreso!tion st!&ies$ (or eementa anaysis$ or in 'ig' reso!tion scanning eectron microscopy.

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I!minating System?

co& ( ie & em ission g !n

A totay &i((erent g!n$ neary a t'o!san& times -rig'ter t'an t'e stan&ar& g!n$ may aso -e !se& !n&er certain con&itions.

In t'e co& (ie& emission g!n$t'e (iament is a singe

crysta o( t!ngsten/it' its atomic crystaine attice precisey oriente&to ma0imie eectron emission. Eectrons are notgenerate& -y t'ermionic emission 4'

eating7$ -!t are act!ay &ra/n o!t o( t'e t!ngsten crysta -y a series o( positi*e 'ig' *otage ano&est'at act

as eectrostatic ensesto (oc!s t'e g!n crosso*er to aspot sie o( 32 nm4+ig!re 5.6>7.

44


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45

Figure 6.27

The field emission gun. &lectrons are e0tractedfrom a single crystal of tungsten !y a series of

anodes that are made seeral thousand olts

$ositie. 8t is not necessary to heat this ty$e of

filament.


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I!minating System?

co& ( ie & em ission g!n

A ma,or &isa&*antage o( t'e co& (ie& emission g!n is t'e !tra'ig' *ac!!m re1!ire& 4greater t'an 32:F Pa7an& t'e e0treme s!scepti-iity o

( t'e (iament to contaminants. Co& (ie& emission g!ns are *ery !se(! in 'ig

' reso!tion scanningan& scanning transmissioneectron microscopes an& are no/ -eing in

corporate& into 'ig' reso!tion transmission eectron microscopes.See Ta-e 5.;(or a comparison o( t'e t'ree ma,or (iaments.

46


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47

Page 17&TABLE 6.( Comparison of the Three Major Filaments in Terms of Bri!htness)Sie of the Source Crosso$er) Ener!y Sprea%) Ser$ice Life) an% +acuum ,e-uire%

Col% Fiel% Emission Lanthanume/a#ori%eTun!stenFilament

'rightness !*8cm&1= 1=$ ?1=$

Energ4 Spread !e(% =#&3=#@ 1#=3= 1#=3=

Service Life !hours% ?&,=== 1,===3&,=== $=31==

(acuum +eAuired !Pa% 1=B 1=BD 1=B@

!Courtesy D. Rathkey, FEI Company.)


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I!minating System: Con&enser Lenses

Con&enser Lenses.T'is secon& ma,or part o( t'e i!minating system gat'ers t'e eectrons o( t'e (irst crosso*er image (rom t'e g!n an& (oc!ses eectrons onto t'e specimen.

Mo&ern transmission eectron microscopes 'a*e t/o con&enser enses$ !nike t'e (irst microscopes t'at 'a&ony one. T'e (irst con&enser ens 4&esignate& C37 is a&emagni(ying enst'at &ecreases t'e sie o( t'e ;2 Gmg!n crosso*erto generate a range o( spot sies(rom 62 Gm &o/n to 3 Gm.

T'e secon& con&enser ens 4C67$on t'e ot'er 'an&$ enarges t'e C3 spot. T'e o*era e((ect o( -ot' ensesis to contro precisey t'e amo!nt o( eectron irra&iationor i!mination striking t'e specimen.

48


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I!minating System:Con&enser Lenses

T'e operating principe (or !sing C3 an& C6 is togenerate a spot on t'e specimeno( t'e proper sie to i!minate ony t'e area -eing e0amine&.

T'ere(ore$ at 'ig'er magni(ications smaer spot sies s'o!& -e (oc!se& on t'e specimen4+ig!re 5.6FB7$ /'ie arger spotsmay -e !se& ato/er magni(ications 4+ig!re 5.6FA7.

Beca!se spot sies are controe&$ -eam &amage can -e minimie& to parts o( t'e specimennot -eing *ie/e&.

T'is o((ere& a great a&*antage /'en TEMs /ere ma&e /it' t/o con&enser enses rat'er t'an

one. 49


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50

Figure 6.29

The condenser lens system.

(A) 8n this mode, the m guncrossoer is reduced to m !ycondenser lens 1, 1,

and then slightly enlarged !y

condenser lens 2, 2, to yield a

1 :m s$ot on the s$ecimen

that is fie times !righter than

the initial gun crossoer.


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51

Figure 6.29

(%) At higher magnifications,

the m gun crossoer isreduced to 1. m !y ahighly energi;ed 1. This

refracts the $eri$heral

electrons to such a great

angle that they cannot enter2 and are therefore lost.

After 2 slightly enlarges the

1 s$ot, the resulting 2:ms$ot is rather dim.


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S!ppose one is /orking at a magni(ication o( ;2$222D. At t'is 'ig' magni(ication$ t'e C3 ens s'o!& -

e 'ig'y energie& to &emagni(y t'e ;2 Gm i!

mination spot(rom t'e g!n &o/n to 3 to 6 Gm. Ne0t$ t'e C6 ens s'o!& -e !se& to a&,!st t'e

sie o( t'e C3 i!mination spot to co*er ony t

'e specimen area -eing *ie/e&. Since t'e a*erage *ie/ing screen is a-o!t 322 mm across$ a6 Gmspot o( i!mination enarge& ;2$222D /o!& ,!st co*er t'e screen 46 Gm D ;2$222 322

mm7. 52


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T'ere(ore$ t'e C6 ens s'o!& aso -e 'ig'y energie& to generate a 6 Gm spot on t'e specimen.

At a magni(ication o( 32$222D$ it is possi-e to k

eep C3 'ig'y energie& -!t to !se C6 tomagni(y or sprea& o!tt'e 6 Gm spot an a&&itiona ;D to ,!st co*er t'e 322 mm screen.

Ho/e*er$ t'e i!mination /i -e a-o!t ; time

s &immer. It is important$ 'o/e*er$ to keep t'e C6 ens sprea& o!t a/ay (rom crosso*er tominimie specimen &amage -!t /it' eno!g' i!mination to (oc!s.

53


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I( one st!&ies +ig!re 5.6F$ it is a

pparent /'y smaer spot siesare necessariy &immer. I( C3 is 'ig'y energie& in or&e

r to generate a sma spot 4+ig!re 5.6FB7$ t'e (oca engt' is ma&e so s'ortan& t'e apert!re ange so greatt'at many eectrons are re(racte& to s!c' an e0tent t'at t'ey &o not enter C6.

On t'e ot'er 'an&$ i( C3 is /eakene& to generatea arger spot$t'e (oca engt' is onger an& t'e apert!re ange is smaer so t

'at e((ecti*ey a eectrons mayno/ enter C6 4+ig!re 5.6FA7. T'ere(ore$ as t'e C3 spot is ma

&e progressi*ey smaer$ o*era i!mination ten&s to &iminis'.

54


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T'is poses a pro-em at 'ig'er magni(ications /'ere *ery sma spot sies are nee&e&.

I!mination may -ecome so &imt'at microscopists m!st ao/ 82 to 52 min!tes(or t'eir eyes to a&apt to /

orking !n&er s!c' &ark con&itions. Ho/e*er$ it may -e possi-e to increase t'e i!minati

on on t'e specimen!sing t'e tec'ni1!es &escri-e& pre*io!sy in t'e c'apter section entite& Controing t'e Amo!nt o( I!mination Striking t'e Specimen$@ or -y

!sing 'ig' sensiti*ity eectronic cameras to *ie/ &imyi!minate& specimens.

55


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Apert!res in Con &enser Lenses .%epen&ingon t'e &esign o( t'e transmission eectron microscope$ one or -ot' con&enser enses may 'a*e apert!res o( *aria-e sies.

#eneray$ t'e C3 apert!re is an interna apert!re o( a (i0e& sie$ /'ie t'e C6 apert!re is *aria-e-y inserting into t'e eectron -eam pat'/ay apert!reso( &i((erent sies attac'e& to t'een& o( a s'a(t.

A pop!ar met'o& is to !se a moy-&en!m (oistrip containing 8 or 9 'oes o( ;22$ 822$ 622$ an& 322 Gm in &iameter 4+ig!re 5.67.

56


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57

Figure 6.2*ring seals the a$$aratus from the atmos$here.


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Ast igm atism Correct ion

St igmatorsare ocate& -eneat' not ony t'e o-,ecti*e-!t aso t'e con&enser an& interme&iate enses.

T'ey (!nction to correct t'e ra&ia ens asymmetriest'at pre*entone (rom (oc!sing t'e image in a &irectionsan& generating circ!ar i!mination spots.

Since an astigmatic ens is stronger in one &irection 4nort':so!t'$ (or instance7 t'an anot'er$ one creates a compensating (ie& o( e1!i*aent strengt'in t'e opposite &irection 4east:/est7.

T/o parameters m!st -e consi&ere&? &irection o( t'e

astigmatism4aim!t'7an& strengt' o( t'e astigmatism4ampit!&e7. One m!st -e a-e to a&,!st -ot' *aria-es to s!it t'e p

artic!ar sit!ation.

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Ast igm atism Correct ion

O&er stigmators /ere compose& o( pairs o( magnetics!gst'at co!& -e mec'anicay rotate& into positionto compensate (or astigmatism.

Ne/er microscopes !se primariy eectromagnetic stigmatorssince t'ey are ess e0pensi*e to -!i&$ easier to!se$ an& some/'at more precise in t'eir correction.

Eectromagnetic stigmatorsmay consist o( eig't tiny eectromagnets encircing t'e ens (ie&.

By *arying t'e strengt' an& poarity o( *ario!ssets o(magnets$ one can contro -ot' ampit!&e an& aim!t'in or&er to generate a symmetrica magnetic (ie&4+ig!re 5.8;7.

)'en stigmators -ecome &irty$ t'ey /i no onger e((ecti*ey compensate (or astigmatisman& m!st -e /it'&ra/n (rom t'e microscope an& ceane&.

95

Figure 6.+


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96

Figure 6.+

(A) once$tual drawing of electromagnetic stigmator

showing orientation of eight electromagnets around the

lens a0is. trength and direction are controlled !y

ad?usting a$$ro$riate com!inations of magnets togenerate a symmetrical field. The stigmator is located

under the condenser and the o!?ectie lens $ole$ieces.

(%) Actual stigmator a$$aratus taen from an electron

microsco$e. The large arrow indicates one of the eight

electromagnetic iron slugs oriented around the central

a0is. The entire a$$aratus fits u$ into the !ore of the

o!?ectie lens so that the area indicated in the large

arrow is $ositioned ?ust under the s$ecimen.

The smaller arrow $oints out indiidual electrical

contacts through which current flows to energi;e the

electromagnets. The close*u$ $hotogra$h (!ottom)

shows some of the electromagnets that are $ositioned

near the s$ecimen (arrow).


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Interme&iate 4%i((raction7 Lens

Interme&iate 4%i((raction7 Lens.As one procee&s &o/n t'e co!mn$ t'is ens imme&iatey (oo/s an& is constr!cte& simiary to t'e o-,ecti*e ens.

In o&er$ simper microscopes$ magni(ication isatere& -y *arying t'e c!rrent to t'is ens$ /'ie in ne/ermicroscopes t'e pre(erre& met'o&

is to !se com-inations o( se*era enses toao/ a /i&er$ &istortion:(ree magni(ication range.

97


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Interme&iate 4%i((raction7 Lens

T'e ma,or (!nction o( t'is ens is to assist in t'e magni(icationo( t'e image (rom t'e o-,ecti*e ens.

At *ery o/ magni(ications$ t'e o-,ecti*e ens is s'!t o((an& t'e interme&iate ens !se& in its pace to generate t'e primary image.

At'o!g' t'e image pro&!ce& -y t'e *ery ong (oca engt' interme&iate ensis poorcompare& to t'at generate& -y !sing a t'ree enses$ it is a&e1!ate (or o/ magni(ication /ork.

T'e interme&iate ens may -e e1!ippe& /it' an apert!re t'at is !se& /'en operating t'e microscope in t'e&i((raction mo&e 4C'apter 3;7.

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Pro,ector Lens

Most mo&ern transmission eectron microscopes 'a*e t/opro,ector enses4P3 an& P67 t'at (oo/ t'e interme&iate ens.

Bot' P3 an& P6 are !se& to (!rt'er magni(y images(rom t'e interme&iate or &i((raction ens.

E0cept (or *ery 'ig' magni(ications$ ony t'ree o( t'e (o!ri

maging enses are normay energie& at any one time$ an&*ario!s tripet com-inationsare !se& to ac'ie*e t'e magni(ication range &esire&.

In a microscope /it' (o!r imaging enses$ t'e (irst pro,ectorens can aso -e !se& as a &i((raction ens$ an& it may -e possi-e to insert a specimeninto a speciay mo&i(ie& 'o&er

ocate& eit'er -et/een P3 an& P6 or -eo/ P6 (or speciaie&$ o/ ange &i((raction st!&ies.

As /it' interme&iate enses$ pro,ector enses s!((er (rom &istortionst'at 'a*e ess e((ect on reso!tiont'an &o a-errations occ!rringin t'e o-,ecti*e ens.

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Pro,ector Lens

Pro,ector enses are sai& to 'a*e great &ept' o( (oc!s$meaning t'at t'e (ina image remainsin (oc!s (or a ong &istance aong t'e optica a0is. T'is is &etermine& -y E1!ation 5.

E1!ation 5.? %ept' o( +oc!s

here1 M 4 total ma!nification ,' 4 resol$in! poer of instrument #ein! use%

4 aperture an!le esta#lishe% #y o#jecti$e lens

100


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Pro,ector Lens

At a magni(ication o( 322$222J$ in an instr!ment /it' reso*ing po/er o( 2.6 nman& 'a*ing an apert!re ange o( 32:6ra&ians$ t'e &ept' o( (oc!s o( t'e pro,ector ens may -e cac!ate& to -e 622 meters.

T'is -ecomes important /'en one reaies t'at t'e p'

otograp'ic (im is not in t'e same paneas t'e *ie/ingscreen. +or t'e same reason$ it is possi-e to ocate m!tipe i

mage recor&ing&e*ices at *ario!s points -eyon& t'e pro,ector ens$ since t'ey /i a -e in (oc!s.

Ho/e*er$ t'e magni(ication /i increaseas one mo*es(art'er a/ay(rom t'e pro,ector ens. T'e reations'ip -et/een &ept' o( (ie& an& &ept' o( (

oc!s reati*e to apert!re ange is s'o/n in +ig!re 5.85.

101


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102

Figure 6.+6

De$th of field (Dfi) occurs in the o!?ect $lane, whilede$th of focus (Dfo) refers to the de$th in the image $lane that is in

focus.

8n the !ottom figure, note that an a$erture

increases !oth the de$th of field and de$th of focus.


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"ie/ing System an& Camera

T'e (ina image is pro,ecte& onto a *ie/ing screencoate& /it' a p'osp'orescent inc:acti*ate& ca&mi!m s!(i&e po/&erattac'e& to t'e screen /it' a -in&er s!c'as ce!ose nitrate.

Most eectron microscopes pro*i&e (or an incination o( t'e *ie/ing screen so t'at t'e image may -e con*enienty e0amine& eit'er /it' t'e !nai&e& eye or /it' a stereomicroscopecae& t'e -inoc!ars.

)it' t'e stereomicroscope$ at'o!g' t'e image may a

ppear to -e ro!g' &!e to t'e 322 Gm:sie&grains o( p'osp'orescent partices making !p t'e screen$ it is necessary to *ie/ a magni(ie& image in or&er to (oc!s acc!ratey.

103


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A s'!tter is pro*i&e& to time t'e e0pos!re so t'at t'e proper negati*e &ensity4as &etermine& -y t'e pre*io!s cai-ration7 may -e o-taine&.

Most eectron microscopes'a*e timers t'at *ary (rom a (raction o( a secon& to @'o&@ positionsin /'ic' a timer may -e !se& (or *ery ongman!a e0pos!res.

Eectron micrograp's are e0pose& (or 2.; to 6secon&sin or&er to recor& a &ensity e*esan& to minimie image s'i(t or &ri(t4i.e.$ so/ mo*ement o( t'e image a(ter e0pos!re to t'e -ea

m7. 104


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Once t'e time 'as -een seecte&$ t'e i!mination e*e is a&,!ste& /it' t'e C3 an& C6 ens contros !nti t'ee0pos!re meter reac'es t'e cai-ration point.

T'e (im is t'en a&*ance& !n&er t'e *ie/ing screen$ an& t'e screen is mo*e& to permit eectrons to pass onto t'e (im.

As one -egins to raise t'e *ie/ing screen$ t'e -eam is-ocke& -y t'e s'!tter !nti t'e screen is totay raise&.

T'e s'!tter is t'en opene& (or t'e proper inter*a$ a(ter /'ic' t'e -eam is again -ocke& !nti t'e screen isrepositione&.

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AOR OP ERATION AL O %ES O+

THE TRANS ISSION ELECTRON

ICROSCOPE

106

High ContrastHigh ResolutionDark Field

Difraction

a,or Operat iona o&es o( t'e Trans


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m ission Eectron icroscope

%!ring t'e aignment proce&!re$ one s'o!& -e a/are t'at t'e con*entiona transmission eectron microscope may -e set !p (or operationin se*era &i((erent operationa mo&es.

%epen&ing on t'e &esign o( t'e microscope$ t'is may in*o*e reati*ey (e/ or many m!t!aye0c!si*e a&,!stments.

In a&&ition$ certain specimen preparation tec'

ni1!es may -e !tiie& to (!rt'er en'ance t'ese operationa mo&es.

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Hig' Contrast

A constant pro-em /it' -ioogica specimensis t'eir o/ contrast.

In t'e 'ig' contrast mo&e$ t'e instr!ment is a&,!ste& to gi*e contrast at t'e e0pense o( '

ig' reso!tion. As a res!t$ t'is mo&e is generay !se& at mag

ni(ications !n&er ;2$222D. T'e con&itions t'at may -e c'ange& to en'anc

e contrast are s!mmarie& -eo/

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Ho/ to O-tain Hig' Contrast

3. T'e (oca engt' o( t'e o-,ecti*e ens is increase&.

T'is necessitates !sing s'orter specimen 'o&er cartri&ges 4+ig!re 5.82$ e(t7 in a top entry st

age to position t'e specimen 'ig'er in t'e o-,ecti*e ens. In a si&e entry stage$ a&,!stment o( t'e :a0is

or specimen positioning may aso -e nee&e& i(a specia 'o&er is not pro*i&e&.

It may -e recae& t'at onger (oca engt's res!t in narro/er apert!re anges$ a /orsening o( c'romatic a-erration$ an& a oss o( reso!tion.

109


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110

Figure 6.+

(left) hort, to$*entry grid holder for high contrast,

low*magnification wor. #esolution is not as good with

this ty$e of grid holder since the s$ecimen is $laced

higher in the o!?ectie lens, necessitating a longer focallength of the lens. (right) tandard to$*entry s$ecimen

grid holder for high resolution wor. The s$ecimen grid

is $laced on the end of the grid holder shaft and held in

$lace with a sleee that is sli$$ed oer the shaft (arrow).

These holders are $laced in the s$ecimen stage with the

grids in the downward $osition in the $ole$iece.


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6. Lo/er acceerating *otages are !se&.T'e res!ting o/er energy eectrons are more rea&iy a((ecte&-y&i((erences in specimen &ensity an& t'ickness$ an& contrast /i -e t'ere-y increase&.

Un(ort!natey$ t'is interaction /it' t'e specimen generates a pop!ation o( imaging eectrons /it' a /i&e range o( energies$ res!ting in an increase in c'romatic a-erration.

Lo/er acceerating *otages are aso more &amaging t

o t'e specimen$ since t'e eectrons are so/e& &o/nmore an& trans(er more energy to t'e specimen$ res!ting in e0cessi*e 'eating.

111

Ho/ to O-tain Hig' Contrast 6. Lo


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/er acceerating *otages are !se&

Lo/er energy eectrons are more s!scepti-eto poor *ac!!m con&itions$ /it' t'e e0acer-ation o( c'romatic a-erration.

Cean$ 'ig' *ac!!ms are nee&e& to minimie e

ectron energy osses$ an& t'e microscope itse( s'o!& -e cean$ since t'ese eectrons are more easiy a((ecte& -y astigmatism.

Lasty$ it /i -e recae& t'at o/er energy eec

trons 'a*e onger /a*eengt's$ so t'at t'e reso*ing po/er /i -e &egra&e&.

112


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8. Smaer o-,ecti*e apert!res s'o!& -e !tiie&.

T'ese apert!res /i remo*e more o( t'e perip'eray &e(ecte& eectrons(rom t'e specimen$

so t'at t'e s!-tracti*e image (rom t'e o-,ecti*e ens /i -e accent!ate& in contrast 4i.e.$ t'esigna:to:noise ratio is increase&7.

Sma apert!res are more prone to astigmatis

m pro-ems$ making cean *ac!!ms an& specimen anticontaminators essentia.

113


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9. P'otograp'ic proce&!res may -e empoye&. Most images generate& in t'e transmission eectron m

icroscopeare en'ance& (or contrast !sing p'otograp'ic tec'ni1!es.

%!ring e0pos!re o( t'e eectron micrograp'$ t'e sensiti*ity o( t'e e0pos!re meter may -e a&,!ste& to sig'ty o*ere0pose t'e (im.

Un&er&e*eopment /i t'en en'ance t'e contrast rangein t'e (ina negati*e.

%etais /i necessariy -e ost in t'e interme&iate &ensity ranges. O( co!rse$ &!ring t'e printing o( t'e negati*e$ one may !se 'ig'er contrast p'otograp'ic papers4see C'apter F7.

114


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;. T'e specimen may -e prepare& to en'ance contrast.

Stan&ar& (i0ation an& staining tec'ni1!es /i increase&ensity -y &epositing t'e 'ea*y metasaong *ario!s organees.

Certain em-e&&ing me&ia 4poyet'eene gyco7t'at may -e &isso*e& or etc'e& a/ay /i 'ep -oost contrast$ or one may !tiie staine&$ (roen sections /it'o!t any em-e&&ing me&ia.

T'e easiest approac' is simpy to c!t t'icker sectionsK'o/e*er$ t'e res!ting c'romatic a-errationan& s!perimposition o( str!ct!re /i &egra&e reso!tion.

115


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Hig' Reso!tion

Most o( t'e con&itions !se& to ac'ie*e 'ig' reso!tionin t'e eectron microscopeare t'e opposite con&itions&isc!sse& a-o*e (or t'e 'ig' contrast mo&e.

Since contrast/i -e acking in t'ese specimens$ e((orts s'o!& -e ma&e to -oost contrast !sing appropriate specimen preparation an& &arkroom tec'ni1!es$ as &escri-e& in t'e pre*io

!s section.

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Ho/ to O-tain Hig' Reso!tion

3. T'e o-,ecti*e ens s'o!& -e a&,!ste& togi*e t'e s'ortest possi-e (oca engt'an&t'e proper specimen 'o&ers !se&.

In some systems$ t'is is simpy a matter o( pressing a singe -!tton$ /'ereas$ in certain microscopes se*era ens c!rrents m!st -e c'ange& concomitanty.

Per'aps it may e*en -e necessary to insert a &i((erent poepiece in t'e o-,ecti*e ens.

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6. A&,!stments to t'e g!n$ s!c' as t'e !se o( 'ig'er acceerating *otages$/i res!t in 'ig'er reso!tion (or t'e reasons area&y mentione& in t'e &isc!ssion on 'ig' contrast.

C'romatic a-errationmay -e (!rt'er essene& -y !sing (ie& emission g!nssince t'e energy sprea& o( eectrons generate& (rom s!c' g!ns is consi&era-y narro/er. 4T'e energy sprea& (or t!ngsten 6 e" /'ie (ie& emission 2.632.; e".7

In an eectron microscope e1!ippe& /it' a con*entiona g!n$ a pointe& t!ngsten (iament /i generate a more co'erent$ point so!rce o( eectrons /it' -etter reso!tion capa-iities.

118


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8. Use apert!res o( appropriate sie. +or most specimens$ arger o-,ecti*e ens apert!ress'o!&

-e !se& to minimie &i((raction e((ects. I( contrast is too o/ &!e to t'e arger o-,ecti*e apert!re$ sma

er apert!res may -e !se&-!t reso!tion /i -e &iminis'e&.

In a&&ition$ t'ey m!st -e kept cean since &irt/i 'a*e a more prono!nce& e((ect on astigmatism. Sma con&enser ens apert!res /i &iminis' sp'erica a-errat

ion$ -!t t'is /i -e at t'e e0pense o( o*era i!mination. T'e i!mination e*es may -e impro*e& -y atering t'e -ias t

o e((ect greater g!n emissionsK 'o/e*er$ t'is may t'ermay &amage t'e specimen.

119


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9. Specimen preparation tec'ni1!esmay aso en'ance t'e reso!tion capa-iity.

E0tremey t'in sections$ (or instance$ /i &iminis' c'romatic a-erration.

)'ene*er possi-e$ no s!pporting s!-strates s'o!& -e !se& on t'e gri&. To ac'ie*e a&e1!ate s!pport$ t'is may re1!ire

t'e !se o( 'oey (ims /it' a arger t'an norma n!m-er o( 'oes 4'oey nets$ see C'apter 97.

T'e areas *ie/e& are imite& to t'ose o*er t'e'oes.

120


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;. isceaneo!s con&itionss!c' as s'orter *ie/ingan& e0pos!re times/i minimie contamination$ &ri(t$an& specimen &amage$ an& 'ep to preser*e (ine str!ct!ra &etais.

Some o( t'e ne/est microscopes 'a*e specia accesso

ries (or minima eectron &ose o-ser*ation o( t'e specimen an& may e*en !tiie eectronic image intensi(iersto en'ance t'e -rig'tness an& contrast o( t'e image.

Anticontaminators o*er t'e &i((!sion p!mps an& specimen area /i &iminis' contamination an& reso!tion

oss. Hig' magni(ications /i -e necessary$ so care(! a&,!stment o( t'e i!minating system is important.

121


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It may take neary an 'o!r (or t'e eyes to totay a&aptto t'e o/ ig't e*es$ an& t'is a&aption /i -e ost i(one m!st ea*e t'e microscope room.

Aignment m!st -e /e &one an& stigmation m!st -ec'ecke& perio&icay&!ring t'e *ie/ing session.

T'e circ!itry o( t'e microscope s'o!& -e sta-iie& -yao/ing t'e ens c!rrents an& 'ig' *otage to /arm !p (or 3 to 6 'o!rs -e(ore !se.

Bent specimen gri&s s'o!& -e a*oi&e&since t'ey may pace t'e specimen in an improper (oca pane (or op

tim!m reso!tion. In a&&ition$ t'ey pre*ent acc!rate magni(ication &etermination an& are more prone to &ri(t since t'e s!pport (ims are o(ten &etac'e&.

122


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%ark(ie&

In t'e norma operating mo&e o( t'e transmission eectron microscope$ t'e !nscattere& rays o( t'e -eam are com-ine& /it' some o( t'e &e(ecte& eectrons to (orm a -rig't(ie& image.

As more o( t'e &e(ecte& or scattere& eectronsare ei

minate& !sing smaer o-,ecti*e ens apert!res$ contrast /i increase. I( one mo*es t'e o-,ecti*e apert!re o(( a0is$ as s'o/n

in +ig!re 5.;2$ e(t$ t'e !nscattere& eectrons are no/eiminate& /'ie more o( t'e scattere& eectrons enter

t'e apert!re. T'is is a cr!&e (orm o( &ark(ie& i!mination. Un(ort!natey$ t'e o((:a0is eectrons 'a*e more a-erra

tions an& t'e image is o( poor 1!aity.

123


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124

Figure 6.chematic diagram showing two ways of setting u$

microsco$e for darfield imagingB (left) dis$lacement

of o!?ectie a$erture off*a0isC (right) tilt of illumination

system into on*a0is o!?ectie a$erture.

%ark(ie&


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Hig'er reso!tion &ark(ie& may -e o-taine& -y titingt'e i!mination system so t'at t'e -eam strikes t'e specimen at an ange.

I( t'e o-,ecti*e apert!re is e(t normay centere&$ it /i no/ accept ony t'e scattere&$ on:a0is eectrons an&

t'e image /i -e o( 'ig' 1!aity 4+ig!re 5.;2$ rig't7. Most microscopes no/ 'a*e a &!a set o( -eam tit co

ntros t'at /i permit one to a&,!st t'e tit (or eit'er -rig't(ie& or &ark(ie& operation.

A(ter aignment o( t'e tit (or -rig't(ie& (oo/e& -y a

&ark(ie& aignment$ one may rapi&y s'i(t (rom one mo&e to t'e ot'er /it' t'e (ip o( a s/itc'. Bot' sets o( contros aso pro*i&e (or separate stigmat

ion contros to correct (or any astigmatism intro&!ce&-y t'e titing o( t'e -eam to arge anges.

125

%ark(ie&


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T'e &ark(ie& mo&e can -e !se& to en'ance contrast in certain types o( !nstaine& specimens4t'in (roen sections7 or in negati*ey staine&specimens.

An e0ampe o( a &ark(ie& image is s'o/n in +ig!re 5.;3.

126


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127

Figure 6.1

(to$) Darfield image o!tained !y tilting illumination system.

(!ottom) ame s$ecimen iewed in standard !rightfieldmode. $ecimen consists of inorganic salt crystals.

%i((raction


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In specimens t'at contain crystas o( !nkno/n composition$ t'e &i((raction tec'ni1!e may -e !se& to meas!re t'e spacing o( t'e atomic crystaine attice an& &etermine t'e composition o( t'ecrysta$ since &i((erent crystas 'a*e !ni1!e spacings o( t'eir attices.

T'e &i((raction p'enomenonis -ase& on t'e re(ectionor &i((raction o( t'e eectron -eam to certain anges -ya crystaine attice.

Instea& o( (oc!sing a con*entiona image o( t'e crysta

on t'e *ie/ing screen !sing t'e o-,ecti*e ens$ one !ses t'e interme&iate or &i((raction ens to (oc!s on t'e-ack (oca paneto see t'e seecte& area &i((raction 4SA%7 on t'e screen.

128

%i((raction


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Since t'e crystaine attice &i((racts eectrons to (orm -rig't spotson t'e *ie/ing screen 4simiar to t'e mirrore& rotating sp'ere sometimes!se& in -arooms to re(ect a ig't so!rce onto

t'e /as7$ t'e image /i consist o( a centra$ -rig't spot s!rro!n&e& -y a series o( spots$ /'ic' are t'e re(ections.

T'e centra -rig't spot represents non&i((racte&rays /'ie t'e perip'era spots represent rays &i((racte& at *ario!s anges.

129

%i((raction


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T'e &istance o( t'ese spots (rom t'e -rig't centra spot is in*ersey proportionato t'e spacing o( t'e crystaine attice.

A crysta /it' sma attice spacings/i &i((ract t'e eectrons to greater angesto gi*e spots t'at are space& (ar (rom t'e centra spot.

T'is is !n(ort!nate (or -ioogists$ since organic crystas$s!c' as protein$ /it' arge attice spacings/i &i((ract t'e -eam so itte t'at t'e spots /i -e cro/&e& aro

!n& t'e centra -rig't spotan& eng!(e& -y its -riiance. )it' organic crystas$ t'e speciaie& tec'ni1!e o( 'ig'

&ispersion eectron &i((ractionm!st -e !se&.

130

%i((raction Practices


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A(ter t'e crysta is ocate& 4!sing t'e stan&ar&-rig't:(ie& imaging mo&e7$ t'e crysta is centere& on t'e *ie/ing screen an& t'e o-,ecti*e apert!re remo*e&.

A(ter pacing t'e TEM in t'e seecte& area &i((raction4SA%7 mo&e$ an SA% apert!re o( t'e appropriate sie is inserte& to seect t'e area o( t'e crysta one /is'es to &i((ract.

+oc!s s'arpy on t'e e&ge o( t'e SA% apert!re!sing t'e SA% 4interme&iate ens7 contro.

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Re(oc!s t'e image !sing t'e o-,ecti*e ens (oc!s contros to -ring t'e image into t'e same pane as t'e interme&iate apert!re. 4I( contrast is ina&e1!ate at t'is point$ temporariy reinsert t'e o-,ecti*e apert!re to c'eck (oc!s an& t'en remo*e it -e(ore procee&ing.7

Pace t'e TEM into t'e &i((raction mo&e 4!s!ay a -!tton a-ee& %@ or @%I++@7 an& ens!re t'at t'e secon& con&enser 4C67 ens is sprea& to pre*ent -!rning o( t'e*ie/ing screen.

+or p'otograp'y$ a&,!st t'e sie o( t'e &i((raction pattern !sing t'e camera engt' contro$ rea&,!st t'e C6 ens so t'at t'e pattern is *ery &im$an& (oc!s t'e centra -rig't spot as sma as possi-e!sing t'e interme&iate ens.

132



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In or&er to c!t &o/n on t'e gare (rom t'e -rig'tcentra spot$ a p'ysica -eam stopper is inserte& to co*erit.

E0pos!res are !s!ay ma&e (or 82 to 52 secon&sin t'e man!a mo&e since t'e i!mination e*es /i -e *e

ry o/. Singe crystas /i generate separate spots /'ie poycrystaine specimens /i pro&!ce so many spots aro!n& t'e centra pointt'at t'ey /i -en& to (orm a series o( concentric rings4+ig!re 5.;67.

Some -ioogica appications o( &i((raction may -e to con(irm t'at a crysta present in '!man !ng tiss!e is a(orm o( as-estos$ or to i&enti(y an !nkno/n crysta ina pant or -acteria ce. See aso C'apter 3; an& t'e re(erence so!rces at t'e en& o( t'is c'apter.

133


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134

Figure 6.2

Diffraction $attern o!tained from $olycrystalline

s$ecimen showing characteristic ring $attern.


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135


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136


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ELECTRO N S$ )A"ES$ AN % RES

OLUTION

137

Eectrons$ ) a*es$ an& Reso !t ion


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P'ysicists 'a*e &emonstrate& t'at$ -esi&es -eing &iscr

ete partices 'a*ing a negati*e c'arge an& a mass o( .3D32:68kg$ eectrons aso 'a*e /a*e properties.

In (act$ t'e /a*eengt' 47o( an eectron is e0presse&-y t'e e1!ation o( t'e +renc' p'ysicist &e Brogieas s'o/n in E1!ation 5.8.

E1!ation 5.8? &e Brogie E1!ation (or )a*eengt'

o( an Eectron

'm*

/'ere ' Pancks constant 45.565 D 32:68

ergssec7 m mass o( t'e eectron * eectron *eocity

138

Eectrons$ )a *es$ an& R eso!t ion


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A(ter appropriate s!-stit!tions associating kinetic energy to mass$ *eocity$ an& acceerating*otage$ t'e e1!ation may -e e0presse&?3.684"736

)'ere? " acceerating *otage T'ere(ore$ i( one /ere operating a transmission eectron microscope at an acceerating *otage o( 52 k"$ t'e /a*eengt' o( t'e eectron /o!& -e 2.22; nm$ an& t'e reso*ing po/er o( t'

e systema(ter s!-stit!tion o( t'ese *a!es into E1!ation 5.6s'o!& -e appro0imatey 2.228 nm.

139

%etermine reso*ing po/er


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Ra&i!s o( Airy %isc? t'e ra&i!s o( t'e Airy &iscas meas!re& to t'e (irst &ark ring is e0press -yE1!ation 5.3?

r 2.536 n 4sin 7

140



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In (act$ t'e act!a reso!tion o( a mo&ern 'ig'reso!tion transmission eectron microscope iscoser to 2.3 nm.

T'e reason /e are not a-e to ac'ie*e t'e near

y 322:(o& -etter reso!tion o( 2.228 nm is &!e to t'e e0tremey narro/ apert!re anges 4a-o!t 3$222 times smaer t'an t'at o( t'e ig'tmicroscope7 nee&e& -y t'e eectron microscop

e enses to o*ercome a ma,or reso!tion imiting p'enomenon cae& sp'erica a-erration.

141



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In a&&ition$ t'e &i((raction p'enomenonas /e as c'romatic a-erration an& astigmatism4to-e &isc!sse& ater7 a &egra&e t'e reso!tion capa-iities o( t'e TEM.

To appreciate t'ese pro-ems$ it is necessary to !n&erstan& 'o/ enses (!nction.

142

%esign o( E ectrom agnet ic Lenses


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Since eectrons are partices /it' s!c' sma masst'att'ey /i -e stoppe& e*en -y gas moec!es present int'e air$ gass enses are o( no *a!e in an eectron microscope.

Ho/e*er$ since eectrons 'a*e a c'arge$ t'ey can -e a(

(ecte& -y magnetic (ie&s. +or e0ampe$ an eectron acceerate& t'ro!g' a *ac!!

m /i (oo/ a 'eica pat' /'en it passes t'ro!g' amagnetic (ie&generate& -y a coi o( /ire /it' a &irect

c!rrent 4%C7 r!nning t'ro!g' it4see +ig!re 5.337. S!c' simpe eectromagnetic cois are terme& soenoi&s.

143



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144

Figure 6.11

ingle electron $assing throughelectromagnetic lens. 8nstead of traeling in a straight

line along the a0is of the lens, the electron is forced !y

the magnetic field to follow a helical tra?ectory that will

conerge at a defined focal $oint after it emerges from

the lens. Therefore, electromagnets, which are D

$owered, !ehae similar to conerging glass lenses.



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S!ppose one i!minates a specimen 4t'e arro/ s'o/nin +ig!re 5.367 /it' a -eam o( eectrons s!c' t'at some o( t'e eectrons t'at interact /it' a specimen point4A in+ig!re 5.367 are transmitte& t'ro!g' t'e specimen an& enter t'e eectromagnetic ens.

%epen&ing on t'eir precise tra,ectories as t'ey enter t'e magnetic (ie&$ t'ey /i ass!me *ario!s 'eica pat'sas t'ey spee& t'ro!g' t'e ens.

A(ter ea*ing t'e ens$ t'e eectrons /i (oc!s at pointAto generate an image point o( t'e specimen.

T'e &istance (rom t'e center o( t'e ens to /'ere t'eeectrons con*erge at A represents t'e (oca engt'o(t'e eectromagnetic ens

145



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+ig!re 5.36A gro!p o( eectrons originating (rom point A in t'e specimen pane pass t'ro!g' an eectromagnetic ens to a -e (oc!se& at an a

ppropriate point 4A7 in t'e image pane. T'e specimen is represente& as t'e 'ea*y arro/ in t'is &ra/ing. T'e eectromagnetic ens -e'a*es as a t'in -icon*e0 gass ens as

s'o/n in +ig!re 5..

146



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It is possi-e to c'ange t'e (oca engt' o( an eectromagnetic ens -y c'anging t'e amo!nt o( %C c!rrent r!nning t'ro!g' t'e coi o( /ire. T'is reations'ip is e0presse& in E1!ation 5.9?

E1!ation 5.9?

+oca Lengt'o( Eectromagnetic Lens

( = 4"i67 /'ere = constant -ase& on n!m-er o( t!rns

in ens coi /ire an& geometry o( ens " acceerating *otage i miiamps o( c!rrentp!t t'ro!g' coi

147



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As t'e acceerating *otageo( t'e eectron is increase&$ t'e (oca engt'is aso increase&since t'e eectrons pass m!c' more rapi&y t'ro!g' t'e ens an& ass!me ooser 'eica ro!tes.

An increase in c!rrent p!t t'ro!g' t'e ens coi$ 'o/e*er$ res!ts in a s'orter (oca engt'-y (orcing t'e eectrons to ass!me tig'ter 'eica t

ra,ectories.

148



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Being a-e to c'ange t'e (oca engt'o( a ens is o( practica importance$ -eca!se t'is is 'o/ one can (oc!s an image (orme& -y a ensas /e as c'ange t'e magni(ication.

In t'e ig't microscope$ /'ere t'e gass enses are o(

a (i0e& (oca engt'$ (oc!ssing is &one -y p'ysicay mo*ing t'e specimen into t'e proper pane o( (oc!s (or eac' o-,ecti*e ens or *ice *ersa.

Simiary$ magni(ications are c'ange& -y remo*ing ano-,ecti*e ens o( one (i0e& (oca engt' an& repacing it/it' anot'er.

149



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O-*io!sy$ t'e eectromagnetic enses o( t'e eectron microscope are a&*antageo!s -eca!set'ey permit one to c'ange (oca engt's 4e.g.$ c'ange (oc!s an& magni(ication7 -y *arying t'e

c!rrent r!nning t'ro!g' t'e ens coi/it'o!t'a*ing to mo*e t'e specimen or p'ysicay c'ange enses.

150



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T'e e((iciency o( t'e eectromagnetic enscan-e greaty impro*e& -y concentrating t'e magnetic (ie& strengt'cose to t'e pat' o( t'e eectrons.

T'is is accompis'e& -y s'ro!&ing t'e coion top$ -ottom$ an& si&e /it' a so(t:iron casingsot'at t'e magnetism /i r!n t'ro!g' t'e s'ro!& 4+ig!re 5.38A7.

T'e strengt' o( t'e ens is t'ere-y increase&.

4T'e term so(t ironre(ers not ony to t'e 'ar&ness o( t'e meta$ -!t aso in&icates t'at t'e iron is magnetie& ony /'en t'e eectromagnetic (ie& is con&!cte& t'ro!g' it.7

151


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152

Figure 6.1+A

Diagram of electromagnetic lens showing

soft*iron casing (shroud) and soft*iron $ole$iece

that sli$s down inside !ore of lens.



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T'e strengt' o( t'e enscan -e (!rt'er increase&-y concentrating t'e magnetismto an e*en smaer area insi&e t'e ens -ore -y means o( a iner terme& a poepiece4so name&-eca!se it sits in t'e nort' an& so!t' poes o( t'e magnet7.

T'e cyin&rica poepiece 4+ig!re 5.38A an& B7 consists o( !pper an& o/er cores o( so(t iron 'e& apart -y a nonmagneti

c -rass spacer. T'e magnetic (ie& is no/ concentrate& -et/een t'e top an& -ottom 4nort' an& so!t'7 iron components o( t'e poepiece.

T'ese nort' an& so!t' coreso( t'e poepiece are -ore& m!c' smaer t'an t'e poepiece ineran& m!st -e as symmetrica as is mec'anicay possi-ein or&er to ac'ie*e 'ig' res

o!tion. In practice$ t'ey are rarey per(ect an& may possess a n!m-

er o( &e(ectst'at may &egra&e reso*ing po/er.

153


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154

Figure 6.1+%

(top) Photograph of a polepiece thatts into the electromagnetic lens coilshon on the left! "he soft#ironcasingof the lens coil is remo$e% tore$eal the ire in%ings aro&n% the

'rass spool!('ottom) n the polepiece the northpole isar'itraril on top* folloe% ' anonmagnetic

'rass spacer that hol%s north an%

%e(ects in Lenses


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A n!m-er o( imper(ections in enses may re&!ce reso!tion.

Astigmatism res!ts /'en a ens (ie& is not symmetrica in strengt'$ -!t is stronger in one pane 4nort' an&

so!t'$ (or e0ampe7 an& /eaker in anot'er 4east an& /est7 so t'at ony part o( t'e image/i -e in (oc!s at one time4+ig!re 5.397.

A point /o!& not -e image& as s!c'$ -!t /o!& appear eiptica in s'apeK a cross/o!& -e image& /it' eit'er t'e *ertica or 'orionta arm$ -!t not -ot'$ in (oc

!s at one time.

155


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156

+ig&re 6!14,stigmatism in a lens! -ince the lens el% isasmmetricall ea.er in the north/so&th plane* o'ectsoriente% along the north/so&th ais ill foc&s at a longer%istance! contrast* %&e to a stronger east/estlens el%* o'ects oriente% east/est ill come tofoc&s at a shorter %istance from the lens! "he eect isthat onl some portions of the image (eithernorth/so&th or east/est) ill 'e in foc&s at one time!'$io&sl* resol&tion ill 'e %egra%e% since the imageill 'e foc&se% in onl one plane!

%e(ects in Lenses


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Some ca!ses o( astigmatismare an imper(ectygro!n& poepiece -ore$ non'omogeneo!s -en&ing o( t'e poepiece metas$ an& &irt on parts o( t'e co!mns!c'as poepieces$ apert!res$ an& specimen 'o&ers.

Beca!se it is impossi-e to (a-ricate an& maintaina e

ns /it' a per(ecty symmetrica ens (ie&$ it is necessary to correct astigmatism-y appying a correcting (ie&o( t'e appropriate strengt' in t'e proper &irection toco!nteract t'e asymmetry.

S!c' a &e*ice is cae& a st igmatoran& can -e (o!n& in t'e con&enser$ o-,ecti*e$ an& interme&iate enseso(t'e eectron microscope 4see +ig!re 5.8;B7.

157

Figure 6.+

(A) once$tual drawing of electromagnetic stigmator

showing orientation of eight electromagnets around


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158

showing orientation of eight electromagnets around

The lens a0is. trength and direction are controlled

%y ad?usting a$$ro$riate com!inations of magnetsto generate a symmetrical field. The stigmator is

4ocated under the condenser and the o!?ectie lens

$ole$ieces.

(%) Actual stigmator a$$aratus taen from an

&lectron microsco$e. The large arrow indicates one

of the eight electromagnetic iron slugs orientedaround the central a0is. The entire a$$aratus fits u$

into the !ore of the o!?ectie lens so that the area

indicated in the large arrow is $ositioned ?ust under

the s$ecimen.

The smaller arro points o&t in%i$i%&al electrical

contacts thro&gh hich c&rrent os to energiethe electromagnets!"he close#&p photograph ('ottom) shos someof"he electromagnets that are positione% near the-pecimen (arro)!

%e(ects in Lenses


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Astigmatism in a gass ens co!& -e correcte&-y regrin&ing t'e c!r*at!re o( t'e ensso t'att'e strengt' is symmetrica$ or -y imposing anot'er ens (ie& o( t'e appropriate strengt' o*

er one o( t'e a-errant (ie&s o( t'e origina ensas is &one /it' correcting eyegasses.

159

%e(ects in Lenses


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C'rom atic a-erration res!ts /'en eectromagnetic

ra&iations o( &i((erent energies con*ergeat &i((erent (oca panes. )it' a gass ens$ s'orter /a*eengt' ra&iations are s

o/e&&o/n an& re(racte& more t'an are onger /a*eengt's o( ig't.

E((ecti*ey$ t'e s'orter$ more energetic /a*eengt's o(ig't come to a s'orter (oca pointt'an &o t'e onger/a*eengt's 4+ig!re 5.3;7.

In an eectromagnetic ens$ t'e re*erse is tr!e? s'orter /a*eengt'$ more energetic eectrons 'a*e a onger

(oca pointt'an &o t'e onger /a*eengt' eectrons. In -ot' cases$ 'o/e*er$ c'romatic a-erration res!ts in t'e enargement o( t'e (oca point4simiar to t'e Airy &isc p'enomenon ca!se& -y &i((raction e((ects7 /it'a conse1!entia oss o( reso!tion.

160


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161

Figure 6.1

hromatic a!erration in a glass lens. Different

waelengths do not come to focus at the same $oint.

ote how the iolet $art of the s$ectrum (gray area)

focuses at a shorter distance than does the red $art of thes$ectrum. This results in an enlarged, unshar$ $oint

rather than a smaller, focused one. #esolution of the

$oint will !e degraded.

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C'romatic a-errationcan -e correcte& -y !sing a monoc'romatic so!rceo( eectromagnetic ra&iation. )it' gass enses$ one /o!& !se a monoc'romatic ig

't 4possi-y -y !sing a s'orter /a*eengt' -!e (iter7. In an eectromagnetic ens$ one /o!& ins!re t'at t'e

eectrons /ere o( t'e same energy e*e -y care(!

y sta-iiing t'e acceerating *otage an& 'a*ing agoo& *ac!!m to minimie t'e energy osso( t'e eectronsas t'ey passe& t'ro!g' t'e co!mn.

T'icker specimensgi*e rise to a spectr!m o( eectrons/it' *arie& energy e*esan& conse1!enty /orsen c'romatic a-erration4+ig!re 5.35A7.

T'in specimensare t'ere(ore essentia (or 'ig' reso!tion st!&ies.

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163

+ig&re 6!16,hromatic a'erration in an o$erl thic. sectionis e$i%ence% ' an image that is 'l&rre% o$erall %&eto %egra%e% resol&tion!

+ig&re 6!16hromatic change of magnication occ&rs henan o$erl thic. specimen is $iee% at lo magnicationsith a lo accelerating $oltage! nl the central part ofthe image is sharp since the eect is maimal atthe peripher!

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C'rom atic c'ange in m agni( icat ion occ!rs /'en t'ick specimens are *ie/e& at o/ magni(ications !sing a o/ acceerating *otage.

T'e image appears to -e s'arp in t'e center$ -!t -ecomesprogressi*ey o!t o( (oc!s as one mo*es to/ar& t'e perip'ery4+ig!re 5.35B7.

T'is is -eca!se t'e o/er energy eectrons are image& at a&i((erent pane t'an t'e 'ig'er energy eectrons.

T'e e((ect is ma0ima at t'e perip'ery o( t'e image$ since t'ese eectrons are coser to t'e ens cois an&$ t'!s$ are more a((ecte& -y t'e magnetic (ie&.

T'is pro-em may -e minimie& -y !sing t'inner specimens$ 'ig'er acceerating *otages$ 'ig'er magni(ications$ an& -y correcting any ot'er &istortionst'at may -e present in t'e ens.

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Sp' erica a-erration is &!e to t'e geometry o( -ot'gass an& eectromagnetic ensess!c' t'at rays passing t'ro!g' t'e perip'ery o( t'e ens are re(racte& moret'an rays passing aong t'e a0is.

Un(ort!natey$ t'e *ario!s rays &o not come to a com

mon (oca point$ res!ting in an enarge&$ !ns'arp point4+ig!re 5.3>A7.

At some &istance$ 'o/e*er$ one s'o!& enco!nter t'es'arpest possi-e pointt'at /o!& constit!te t'e circe o( minim!m con(!sion4i.e.$ t'e smaest Airy &isc7 an& t'e practica (oca point o( t'e ens.

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Figure 6.17

(A) $herical a!errationin a lens.

=eri$heral rays are refracted more than


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=eri$heral rays are refracted more than

central rays, so that all rays do not

conerge to a common, small focal $oint.8nstead, an enlarged, diffuse s$ot lie the

Airy disc will !e generated.

The ertical line indicates the one $oint

where the $oint will !e smallest (i.e.,

haing the smallest circle of confusion).(%) orrection of s$herical a!errationwith

an a$erture (here shown inside the lens)

to cut out $eri$heral rays and there!y

$ermit remaining rays to focus at a

common small imaging $oint.

#esolution will !e im$roed since indiidual

8mage $oints in the s$ecimen will !e

smaller.

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Sp'erica a-erration may -e re&!ce& -y !sing an apert!re to eiminate some o( t'e perip'era rays4+ig!re 5.3>B7.

At'o!g' apert!res m!st -e !se& in t'e eectron microscope to re&!ce sp'erica a-erration as m!c' as pos

si-e$ t'ey &ecrease t'e apert!re ange an& t'ere-y pre*ent t'e eectron microscope (rom ac'ie*ing t'e !timate reso*ing po/er speci(ie& in t'e e1!ation (or reso!tion4E1!ation 5.37. r 2.536 n 4sin 7

In a&&ition$ t'e /orsening o( reso!tion as a res!t o(!sing a onger (oca engt' ens is s'o/n in E1!ation 5.;.

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%etermine reso*ing po/er


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Ra&i!s o( Airy %isc? t'e ra&i!s o( t'e Airy &iscas meas!re& to t'e (irst &ark ring is e0press -yE1!ation 5.3?

r 2.536 n4sin 7

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E1!ation 5.;? Limit o( Reso!tion$ &s$ Impo

se& -y Sp'erica A-erration

%s ks( 28

/'ere k a constant reate& to ens c'aracteristics ( (oca engt' o( ens apert!re ange o( ens$ normay t'e o-,ecti

*e ens

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+rom E1!ation 5.;$ /e see /'y a s'ort (oca engt' ens com-ine& /it' a smaer apert!re4to generate a smaer apert!re ange7 /i 'ep to re&!ce t'e &egra&ation o( reso!tionca!se&

-y sp'erica a-erration. Conse1!enty$ smaer apert!resare genera

y more &esira-e t'an arger onesto impro*e reso!tionin spite o( t'e t'eoretica a&*antage o((ere& -y arge apert!res as in&icate& in E1!ation 5.3.

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Certain types o( image &istortionsmay arise /'en sp'erica a-erration occ!rs in t'e (ina imaging 4or pro,ector7 enses o( t'e transmissioneectron microscope.

Since perip'era eectrons are re(racte& to a greater e0tent t'an centra rays$ t'e image (orme& -y t'ese perip'era eectrons /i -e at a greater magni(icationan& in a &i((erent (oca panet'an t'e image generate& (rom more centray positione& eectrons.

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A gri& o( ines /o!& not -e image& as s1!are4+ig!re5.3FA7$ -!t /o!& ass!me t'e s'ape o( a s!nken pio/$ 'ence t'e name pinc!s'ion &istortion4+ig!re 5.3FB7.

T'is type o( &istortion may occ!r /'en attempting tooperate t'e transmission eectron microscopeat e0cessi*ey o/ magni(ications.

Anot'er type o( imper(ection$ -arre &istortion$occ!rs/'en one attempts to !se an eectromagnetic ens ina &emagni(ying mo&erat'er t'an t'e norma magni(ying mo&e.

In t'is case$ t'e centra part o( t'e image is magni(ie&more t'an t'e perip'ery so t'at t'e gri&/ork ass!mesa s/oen or -arre s'ape 4+ig!re 5.3FC7.

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Figure 6.19 Distortions in a lens.(A) ormal image of grid $attern.

(%) 8mage with $incushion distortion.

() 8mage with !arrel distortion.

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+ort!natey$ it ispossi-eto ne!traie one type o( &istortion /it' t'e ot'er. +or instance$ i( one pro,ector ensis &ispaying e0cessi

*e pinc!s'ion &istortion$ it is possi-e to operate anot'er pro,ector ens in t'e &emagni(ying mo&e to intro&

!ce an opposing -arre &istortion. T'e ens systems o( mo&ern eectron microscopes are

&esigne& to a!tomaticay co!nter-aancet'e *ario!stypes o( &istortions t'ro!g'o!t a /i&e magni(ication range.

In o&er microscopes$ 'o/e*er$ one m!st take care not to intro&!ce t'ese &istortions in t'e o/er magni(ication range.

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A c!rio!s p'enomenon cae& image rotation/i -e notice& as one c'anges magni(ication in o&er microscopes. T'is occ!rs -eca!se t'e eectrons (oo/ a spira pat't'ro!

g' t'e enses$ an& t'e spira s'i(ts as t'e strengt' o( t'e ens is *arie&.

Image rotation not ony res!ts in rotation o( t'e image on t

'e *ie/ing screen as one increases magni(ication$ -!t asoe0aggerates t'e e((ects o( &istortion. It is possi-e to minimie or eiminate image rotation entire

y-y ens!ring t'at a series o( enses'a*e opposing rotations rat'er t'an a 'a*ing rotations in t'e same &irection.

T'is is accompis'e& -y r!nning t'e ens c!rrent t'ro!g' t'e coiin t'e opposite &irection 4i.e.$ re*ersing poarity7 an& is a principe !tiie& in some o( t'e ne/er transmission eectron microscopes.

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agni(ication


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Besi&es (orming images /it' 'ig' reso!tion$ t'e enses o( t'e eectron microscope are a-e to (!rt'er magni(y t'ese images.

Magni(ication re(ers to t'e &egree o( enargemento( t'e &iameter o( a (ina image compare&to t'e origina.

In practice$ magni(ication e1!as a &istance meas!re& -et/een t/o pointson an image &i*i&e& -y t'e &istance meas!re& -et/een t'ese sa

me t/o points on t'e origina o-,ect$ or

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agni(ication


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Conse1!enty$ i( t'e image &istance -et/een t/o pointsmeas!res 6;.; mm /'ie t'e &istance -et/een t'ese same t/o points on t'e o-,ectmeas!res ; mm$ t'en t'e magni(ication is

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agni(ication


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As /i -e &isc!sse& ater$ t'ere are at east t'ree magni(ying enses in an eectron microscope? t'e o-,ecti*e$ interme&iate$an& pro,ector enses.

T'e (ina magni(ication is cac!ate& as t'e pro&!ct o( t'e in&i*i&!a magni(ying po/ers o( ao( t'e enses in t'e system as s'o/n in E1!ation 5.5.

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agni(ication


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E1!ation 5.5? Cac!ation o( Tota agni(ic

ation$

T

$ o( t'e TE

/'ere? MT tota magni(ication or mag

MO mag o( o-,ecti*eens

MI mag o( interme&iateens

MP mag o( pro,ector ens4es7

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agni(ication


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+or e0ampe$ i( t'e transmission eectron microscope is operating in t'e 'ig' magni(icationmo&e$ typica *a!es (or t'e respecti*e ensesmig't -e? 622 J ;2 J 62 622$222J.

I( one /ere to operate t'e microscope in t'e o/ magni(ication mo&e$ per'aps t'e *a!es /o!& -e? ;2 J 2.; J ;2 3.6;2J.

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agni(ication


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Interme&iate magni(ications may -e pro&!ce& -y *arying t'e c!rrent to t'e *ario!s enses. Sometimes$ it is &esira-e to *ie/ as m!c' o( t'e speci

men as possi-e in or&er to e*a!ate 1!icky t'e 1!aity o( t'e preparation or to ocate a partic!ar portion o(t'e specimen.

In t'is case$ an e0tremey o/ magni(ication is o-taine& -y pacing t'e microscope in t'e scan magni(icationmo&e$ /'ic' can -e accompis'e& -y s'!tting o(( t'eo-,ecti*e ens an& !sing t'e ne0t ens 4t'e interme&iate ens7 as t'e imaging ens as (oo/s? 3 J 2.; J 322 ;

2J. O( a t'e enses !se& to c'ange magni(ication$ t'e o-,

ecti*e ensis !se& t'e east. Normay it is maintaine& at aro!n& ;2 or 322J /'ie t

'e ot'er enses are *arie&.181

agni(ication


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At'o!g' it is t'eoreticay possi-e to increaset'e magni(ication in&e(initey$ t'e 1!aity o( t'e image magni(ie& is &epen&ent on t'e reso*ing po/er o( t'e enses in t'e system.

Conse1!enty$ t'e term !se(! magni(icationis!se& to &e(ine t'e ma0im!m magni(ication t'at s'o!& -e !se& (or a partic!ar optica system. It is &e(ine& -y t'e (orm!a in E1!ation 5.>.

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agni(ication


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E1!ation 5.>? Use(! agni(ication

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agni(ication


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In t'e case o( t'e ig't microscope$ a typica *a!e /o!& -e 3$222J-eca!se t'e reso*ing po/er o( t'e '!man eye is a-o!t 2.6 mm$ /'ie t'e reso*ing po/er o( t'e ig't microscope is appro0imatey 2.6 Gm.

An eectron microscope /it' a reso*ing po/er o( 2.6nmco!& -e e0pecte& to 'a*e a top magni(ication o( appro0imatey 3$222$222J$or a t'o!san& times greatert'an t'e ig't microscope.

In practice$ &!e to t'e &iminis'e& i!mination at s!c''ig' magni(ications$ microscopists /o!& pro-a-y take t'e micrograp' at a magni(ication o( 6;2$222J an& p

'otograp'icay enarge t'e negati*e to t'e nee&e& magni(ication.

Ho/e*er$ ony rarey &o -ioogists nee& s!c' 'ig' magni(ications.

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agni(ication


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%epen&ing on t'e mo&e o( TEM$ t'e magni(ication c'anges may occ!r as a series o( &iscrete steps or as an in(initey *aria-e or oom@ magni(ication series.

Mo&ern eectron microscopes 'a*e &igita &ispayst'at gi*e t'e appro0imate tota magni(ication /'en one *

aries t'e magni(ication contro. O&er microscopes !s!ay 'a*e an anaog ga!ge t'at

may eit'er rea& t'e c!rrent to one ens$ or t'ey may 'a*e a magni(ication kno- /it' a series o( cick stops t'at may -e correate& to a partic!ar magni(ication.

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agni(ication


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A microscopes 4inc!&ing ig't microscopes7m!st -e cai-rate& in or&er to &etermine moreacc!ratey t'e tota magni(ication$ since a n!m-er o( *aria-esmay ca!se t'e magni(ication t

o *ary -y as m!c' as 62 to 82o*er a s'ort perio& o( time.

E*en mo&ern instr!ments /it' &irect rea&ing&igita &ispays are g!arantee& to -e acc!rate to ony ; to 32o( t'e state& *a!es.

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%esign o( t'e Transmission Eectr

on icroscope


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Com parison o( L ig't icroscope to Transm

ission Eectron icroscope

Basic Systems ak ing Up a Transmiss ion E

ectron icroscope

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Com parison o( L ig't icroscope to Tr

ansm ission Eectron icroscope


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T'e transmission eectron microscope is simiar in many /ays to t'e compo!n& ig't microscope.

+or instance$ in -ot' microscopes$ eectromag

netic ra&iations originating (rom a t!ngsten (iament are con*erge& ontoa t'in specimen -ymeans o( a con&enser ens system.

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T'e i!mination transmitte& t'ro!g' t'e specimen is (oc!se& into an image an& magni(ie& (irst-y an o-,ecti*e ensan& t'en (!rt'er magni(ie& -y a series o( interme&iate an& pro,ector

enses!nti t'e (ina image is *ie/e& 4+ig!re 5.37.

Bot' kin&s o( microscope may recor& images!sing a si*er:-ase& p'otograp'ic em!sionsince it is sensiti*e to -ot' types o( ra&iations

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O( co!rse$ t'e enses o( ig't microscopes arecompose& o( gass or 1!art rat'er t'an t'e eectromagneti

Microscopy 2009 Wk14 TEM

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