...but didn’t dare to ask!

All you wanted to know about

Electron Microscopy...

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IntroductionThis booklet is written for those whoknow little or nothing about electronmicroscopy and would like to knowhow an electron microscope works,why it is used and what useful resultsit can produce.

"With a microscope you see the surface of things. It magnifies thembut does not show you reality. Itmakes things seem higher and wider.But do not suppose you are seeingthings in themselves."

Feng-shen Yin-Te (1771 – 1810)

In The Microscope 1798

A publication ofFEI Electron OpticsFEI Electron Optics, a division of FEICompany, is one of the world’s lead-ing suppliers of transmission andscanning electron microscopes.

Our commitment to electronmicroscopy dates back to the mid-1930s, when we collaborated in EMresearch programmes with universi-ties in the UK and the Netherlands.In 1949, the company introduced itsfirst EM production unit, the EM100transmission electron microscope.

Innovations in the technology andthe integration of electron optics,fine mechanics, microelectronics,computer sciences and vacuum engineering have kept FEI at the forefront of electron microscopy ever since.

ISBN nummer 90-9007755-3

What is ElectronMicroscopy?

The TransmissionElectron

Microscope

The ScanningElectron

Microscope

AdditionalTechniques c o n t e n t s

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Nobody knows for certain whoinvented the microscope. The lightmicroscope probably developed from the Galilean telescope duringthe 17th century. One of the earliestinstruments for seeing very smallobjects was made by the DutchmanAntony van Leeuwenhoek (1632-1723) and consisted of a powerfulconvex lens and an adjustable holderfor the object being studied (speci-men). With this remarkably simplemicroscope (Fig. 1), Van Leeuwenhoekmay well have been able to magnifyobjects up to 400x and with it hediscovered protozoa, spermatozoaand bacteria and was able to classifyred blood cells by shape.

The limiting factor in VanLeeuwenhoek’s microscope was thequality of the convex lens. The prob-lem can be solved by the addition ofanother lens to magnify the imageproduced by the first lens. This com-pound microscope – consisting of an objective lens and an eyepiece together with a means of focusing,a mirror or a source of light and aspecimen table for holding and positioning the specimen – is the basis of light microscopes today.

Why use electrons instead of light?A modern light microscope (oftenabbreviated to LM) has a magnifica-tion of about 1000x and enablesthe eye to resolve objects separatedby 0.0002 mm (see box A). In thecontinuous struggle for betterresolution, it was found that theresolving power of the microscopewas not only limited by the numberand quality of the lenses but also bythe wavelength of the light used forillumination. It was impossible toresolve points in the object whichwere closer together few hundrednanometres – see box B). Using lightwith a short wavelength (blue orultraviolet) gave a small improve-ment; immersing thespecimen andthe front of

The word is derived from the Greek mikros (small) and skopeo

(look at). Ever since the dawn of science there has been an interest

in being able to look at smaller and smaller details. Biologists have

wanted to examine the structure of cells, bacteria, viruses and

colloidal particles. Materials scientists have wanted to see

inhomogeneities and imperfections in metals, crystals and ceramics.

In the diverse branches of geology, the detailed study of rocks,

minerals and fossils could give

valuable insight into the origins

of our planet and its valuable

mineral resources.

What is ElectronMicroscopy?

the objective lens in a medium witha high refractive index (oil) gaveanother small improvement but these measures together onlybrought the resolving powerof the microscope tojust under 100 nm.

Resolution and Magnification (1)

Given sufficient light, the unaidedhuman eye can distinguish two points 0.2

mm apart. If the points are closer together,only one point will be seen. This distance is

called the resolving power or resolution of theeye. A lens or an assembly of lenses (a micro-scope) can be used to magnify this distance andenable the eye to see points even closer togetherthan 0.2 mm. Try looking at a newspaperphotograph or one in a magazine through amagnifying glass for example.

Box A

In the 1920s it was discovered thataccelerated electrons (parts of theatom – see box C) behave in vacuumjust like light. They travel in straightlines and have a wavelength which isabout 100 000 times smaller thanthat of light. Furthermore, it wasfound that electric and magneticfields have the same effect on elec-trons as glass lenses and mirrors haveon visible light. Dr Ernst Ruska at theUniversity of Berlin combined thesecharacteristics and built the firsttransmission electron microscope(often abbreviated to TEM) in 1931.For this and subsequent work on thesubject, he was awarded the NobelPrize for Physics in 1986. The firstelectron microscope used two mag-netic lenses and three years later headded a third lens and demonstrateda resolution of 100 nm (see box D),twice as good as that of the lightmicroscope. Today, using five magnetic lenses in the imaging system, a resolving power of 0.1 nmat magnifications of over 1 milliontimes can be achieved.

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The NanometreAs distances become shorter, the

number of zeros after the decimalpoint becomes larger, so microscopists

use the nanometre (abbreviated to nm)as a unit of length. One nanometre is a

millionth of a millimetre (10 –9 metre). Anintermediate unit is the micrometre (abbreviated

to µm) which is a thousandth of a millimetreor 1000 nm.

Some literature refers to the Ångström unit (abbre-viated to Å) which is 0.1 nm and the micron formicrometre.

The ElectronOne way of looking at an atom

is to visualise it as a minute "solarsystem" in which electrons orbit like planets

round a central nucleus. The nucleus consistsof neutral and positively charged particles and the

electrons have a compensating negative charge so thatthe atom is neutral. The electrons which are about 1800x

lighter than the nuclear particles occupy distinct orbits eachof which can accommodate a fixed maximum number of

electrons.

When electrons are liberated from the atom, however,they behave in a manner analogous to light. It is this

behaviour which is made use of in the electronmicroscope, although we should not lose sight ofthe electron’s role in the atom, which will bemade use of later.

Resolution andMagnification (2)

The resolving power of a microscopedetermines its maximum magnification.

It is only necessary to magnify the resolvingpower to 0.2 mm, the resolving power of the

unaided human eye, for all the fine detail of anobject to be revealed. Higher magnification will notreveal any more information and is unnecessary.

The resolving power of a microscope is one of itsmost important parameters. The reason that mag-nifications are often quoted is that it gives an ideaof how much an image has been enlarged.

Fig. 1 Replica of one of the 550 light microscopes made by Antony van Leeuwenhoek.

Box B

Box C

Box D

Transmission ElectronMicroscope (TEM)The transmission electron microscopecan be compared with a slide projec-tor (Fig. 2). In the latter, light from alight source is made into a parallelbeam by the condenser lens; thispasses through the slide (object) andis then focused as an enlarged imageonto the screen by the objective lens.In the electron microscope, the lightsource is replaced by an electronsource (a tungsten filament heated invacuum), the glass lenses arereplaced by magnetic lenses and theprojection screen is replaced by afluorescent screen which emits lightwhen struck by electrons. The wholetrajectory from source to screen isunder vacuum and the specimen(object) has to be very thin to allowthe electrons to penetrate it.

Not all specimens can be made thinenough for the TEM. Additionally,there is considerable interest inobserving surfaces in more detail.Early attempts at producing imagesfrom the surface of a specimeninvolved mounting the specimennearly parallel to the electron beamwhich then strikes the surface at avery small angle. Only a very narrowregion of the specimen appears infocus in the image and there is con-siderable distortion. The techniquehas not found wide application inthe study of surfaces.

PenetrationElectrons are easily

stopped or deflected bymatter (an electron is nearly

2000x smaller and lighterthan the smallest atom). That is

why the microscope has to beevacuated and why specimens – forthe transmission microscope – have to

be very thin in order to be imagedwith electrons. Typically, the speci-

men must be no thicker than afew hundred nanometres.

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Scanning MicroscopyImagine yourself alone in an

unknown darkened room with only afine beam torch. You might start exploring

the room by scanning the torch beam system-atically from side to side gradually moving

down so that you could build up a picture of theobjects in the room in your memory.

A scanning microscope uses an electron beaminstead of a torch, an electron detector insteadof eyes and a fluorescent screen and camera asmemory.

Slide projector

Projector Screen Objective Lens

SlideCondenser Lens

Light Source

Fluorescent Screen

Aperture

ObjectiveLens

CondenserLens

Electron Beam

Electron Source

Specimen(thin)

TEM

Fig. 2 The transmission electron microscope compared with a slide projector

Fig. 3 A modern transmission electronmicroscope the Tecnai 12.

Box E

Box F

Scanning Electron Microscope(SEM)It is not completely clear who firstproposed the principle of scanningthe surface of a specimen with a finely focused electron beam to produce an image of the surface. The first published descriptionappeared in 1935 in a paper by theGerman physicist Dr Max Knoll.Although another German physicistDr Manfred von Ardenne performedsome experiments with what couldbe called a scanning electron micro-scope (usually abbreviated to SEM) in 1937, it was not until 1942 thatthree Americans, D. Zworykin, DrHillier and Dr Snijder first described a true SEM with a resolving power of50 nm and a magnification of 8000x.Nowadays SEMs can have a resolvingpower of 1 nm and can magnify over 400 000x.

Figure 4 compares light microscopy(using transmitted or reflected light)with TEM and SEM. A combinationof the principles used in both TEMand SEM, usually referred to as scanning transmission electronmicroscopy (STEM), was firstdescribed in 1938 by Dr Manfredvon Ardenne. It is not known whatthe resolving power of this instru-ment was. The first commercialinstrument in which the techniqueswere combined was a Philips EM200equipped with a STEM unit devel-oped by Dr Ong of Philips ElectronicInstruments in the U.S.A. (1969).At that time, the resolving power was 25 nm and the magnification100 000x. Modern TEMs equippedwith a STEM facility can resolve 1 nm at magnifications of up to 1 million times.

Light Beam

Specimen

Light Source(Transmitted Light)

Light Source(Reflected Light)

Electron Source

Electron Beam

Specimen (thin)

Electron Source

Electron Beam

Deflection coil

Specimen (thick)Detector

Vacuum

Monitor

Vacuum

Fluorescent Screen

Fig. 4 Comparison of the light microscope(A) with transmission (B) and scanning(C) electron microscopes7

Fig. 5 A modern scanning electron microscope

the Quanta

A

B

C

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The column is the crucial item. Itcomprises the same elements as the light microscope as can be seenfrom the ray paths of light and elec-trons (Fig. 6). The light source of thelight microscope is replaced by anelectron gun which is built into thecolumn. The glass lenses are replacedby electromagnetic lenses and theeyepiece or ocular is replaced by a fluorescent screen. The entire electronpath from gun to screen has to beunder vacuum (otherwise the elec-trons would collide with air moleculesand be absorbed) so the final imagehas to be viewed through a windowin the projection chamber. Anotherimportant difference is that, unlikeglass lenses, electromagnetic lensesare variable: by varying the currentthrough the lens coil, the focal length(which determines the magnification)can be varied. (In the light micro-scope variation in magnification isobtained by changing the lens or bymechanically moving the lens).

There are four main components to a transmission electron microscope:

an electron optical column, a vacuum system, the necessary electronics

(lens supplies for focusing and deflecting the beam and the high voltage

generator for the electron source), and software. A TEM from the Tecnai

series comprises an operating console surmounted by a vertical column

about 25 cm in diameter and containing the vacuum system, and

control panels conveniently placed for the operator (Fig. 3).

The TransmissionElectron Microscope

The electron gunThe electron gun comprises a fila-ment, a so-called Wehnelt cylinderand an anode. These three togetherform a triode gun which is a very stable source of electrons. The tung-sten filament is hairpin-shaped andheated to about 2700 OC. By apply-ing a very high positive potentialdifference between the filament andthe anode, electrons are extractedfrom the electron cloud round the filament and accelerated towards theanode. The anode has a hole in it sothat an electron beam in which theelectrons are travelling at severalhundred thousand kilometres persecond (see box G) emerges at theother side. The Wehnelt cylinderwhich is at a different potential,bunches the electrons into a finelyfocused point (Fig. 7).Other electron sources exist andthese are discussed briefly under"Additional Techniques" on page 21.

The beam emerging from the gun is condensed into a nearly parallelbeam at the specimen by the con-denser lenses and, after passingthrough the specimen, projected as a magnified image of the specimenonto the fluorescent screen at thebottom of the column.

If the specimen were not thin, theelectrons would simply be stoppedand no image would be formed (see box E "Penetration" on page 6).Specimens for the TEM are usually0.5 micrometres or less thick. Thehigher the speed of the electrons, inother words, the higher the acceler-ating voltage in the gun, the thickerthe specimen that can be studied.

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Filamentsource

High VoltageGenerator

Anode

Electron beam

Filament

Wehneltcylinder

Lamp Electrons

Glass lensCondenser lens

Specimen

Objective lens

First image

Projector lens

Final image

“Illumination”

Glass lens

Glass lens

Electro-magnetic

lens

Electro-magnetic

lens

Electro-magnetic

lens

Ocular Fluorescentscreen

Electron DensityHow many electrons impinge on

the specimen? A typical electronbeam has a current of about 1

picoampere (10 –12 A). One ampere is 1coulomb/sec. The electron has a charge of

1.6 x 10 –19 coulomb. Therefore approxi-mately 6 million electrons per secondimpinge on the specimen.

Fig. 7 Schematic cross-section of the electron gun in an electron microscope.

Fig. 6 Ray paths of light in a light microscope (LM) compared with those of electrons in a transmission electron microscope (TEM).

Electron VelocityThe higher the acceleratingvoltage, the faster the electrons.80 kV electrons have a velocity of150 000 km/second (1.5 x 10 8 m/s)which is half the speed of light. Thisrises to 230 000 km/second for 300 kVelectrons (2.3 x 10 8 m/s – more thanthree-quarters the speed of light).

Box G

Box H

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The electromagnetic lensesFigure 10 shows a cross-section of an electromagnetic lens. When anelectrical current is passed throughthe coils (C), an electromagnetic fieldis created between the pole pieces(P) which form a gap in the magnet-ic circuit. By varying the currentthrough the coils, the magnificationof the lens can be varied. This is theessential difference between themagnetic lens and the glass lens.Otherwise they behave in the sameway and have the same types ofaberration: spherical aberration (themagnification in the centre of thelens differs from that at the edges),chromatic aberration (the magnifica-tion of the lens varies with the wavelength of the electrons in thebeam) and astigmatism (a circle inthe specimen becomes an ellipse in the image).

Spherical aberration is a very important characteristic which islargely determined by the lens design and manufacture. Chromaticaberration is reduced by keeping the accelerating voltage as stable as possible and using very thin speci-mens. Astigmatism can be correctedby using variable electromagneticcompensation coils.

The condenser lens system focusesthe electron beam onto the speci-men under investigation as much as necessary to suit the purpose. The objective lens produces animage of the specimen which is then magnified by the remainingimaging lenses and projected ontothe fluorescent screen.

If the specimen is crystalline there will be a diffraction pattern at a different point in the lens known as the back focal plane. By varyingthe strength of the lens immediatelybelow the objective lens, it is possible to enlarge the diffractionpattern and project this onto the fluorescent screen. In the Tecnaiseries of TEMs, the objective lens isfollowed by four lenses: a diffractionlens, an intermediate lens and twoprojector lenses. To guarantee a high

What happens in the specimen during the electron bombardment?Contrary to what might be expected,most specimens are not affected bythe electron bombardment as longas it is kept under control. Whenelectrons impinge on the specimen, a number of things can happen:a. Some of the electrons are absorbed

as a function of the thickness andcomposition of the specimen; thesecause what is called amplitude contrast in the image.

b. Other electrons are scattered oversmall angles, depending on thecomposition of the specimen;these cause what is called phasecontrast in the image.

c. In crystalline specimens, the elec-trons are scattered in very distinctdirections which are a function ofthe crystal structure; these causewhat is called diffraction contrastin the image.

d. Some of the impinging electronsare reflected (these are calledbackscattered electrons).

e. The impinging electrons cancause the specimen itself to emit electrons (these are calledsecondary electrons).

f. The impinging electrons causethe specimen to emit X-rayswhose energy and wavelengthare related to the specimen’selemental composition.

g. The impinging electrons causethe specimen to emit photons(or light); this is calledcathodoluminescence.

h. Finally, electrons which have lost an amount of energy because ofinteraction with the specimen can be detected by an Energy Loss Spectrometer which is theequivalent of a prism in light optics.

In a standard TEM the first two phenomena contribute to the formation of the normal TEM imagefor non-crystalline (biological) specimens, while for crystalline specimens (most non-biologicalmaterials), phase contrast and diffraction contrast are the mostimportant factors in image formation.It is necessary to add accessories orperipheral equipment to the basicmicroscope in order to exploit theadditional information which can beobtained by studying the last fiveinteractions listed above.

Fig. 8 Cross-section of the column of a modern transmission electron microscope.

Fig. 9 Perhaps themost efficientkiller world-wideis the influenzavirus.

lens and the diffraction lens can beselected from outside the column asdictated by circumstances.

Observation and recording ofthe imageThe image on the fluorescent screencan be observed through a largewindow in the projection chamber(some models even have two win-dows). In order to examine finedetail or to assist correct focusingof the image, a special fine grainfocusing screen can be inserted intothe beam and observed through ahigh-quality 12x binocular viewer.

As in any other branch of science, a permanent record of what hasbeen observed with the eye isdesired. Antony van Leeuwenhoekpainstakingly drew what he saw inhis microscope. The invention ofphotography enabled microscopiststo photograph what they saw.Electrons have the same influence

on photographic material as light.Therefore it is only necessary toreplace the fluorescent screen with a photographic film in order torecord the image. In practice the fluorescent screen hinges up to allow the image to be projected on the film below. It is also possibleto use a TV camera to record the image digitally, making it suitable for subsequent enhancement and analysis.

TV observation is useful for instruc-tional purposes or other occasionswhen group viewing is desired. It canalso be used for recording dynamicphenomena using a video taperecorder or as the input signal for an image analysis system.

stability and to achieve the highestpossible magnification, the lenses ina modern TEM are water cooled.

On the way from the filament to the fluorescent screen, the electronbeam passes through a series ofapertures with different diameters.These apertures stop those elect-rons which are not required forimage formation (e.g. scattered electrons). Using a special holdercarrying four different apertures, the diameter of the apertures in the condenser lens, the objective

C

C

C

C

p p

p p

Fig. 10 Cross-section of an electromagnetic lens. C is anelectrical coil and P is the softiron pole piece. The electron trajectory is from top to bottom.

Fig. 12 A special electron diffraction technique (HAADF)

produced this spectacular image.

Fig. 11 Just as the umbrella fabric produces a pattern of light spots, a crystal (here asemiconductor) produces a diffraction pattern which can be photographed (see insert).

DifractionWhen a wave passes through a

periodic structure whose periodicity is ofthe same order of magnitude as the wavelength,

the wave emerging is subject to interference whichproduces a pattern beyond the object. The phenomenon

can be observed when ocean waves pass through a regularline of posts or when a street lamp is seen through an

umbrella. The street lamp appears as a rectangular pattern ofspots of light, bright in the centre and then getting fainter. This is

caused by diffraction of light by the weave of the umbrella fabric andthe size and form of the pattern provide information about the struc-ture (closeness of weave and orientation). In exactly the same wayelectrons are diffracted by a crystal and the pattern of spots on thescreen of the microscope gives information about the crystal lattice(shape, orientation and spacing of the lattice planes) – see Fig. 11.

Box I

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VacuumElectrons behave like light only whenthey are manipulated in vacuum.As has already been mentioned, thewhole column from gun to fluores-cent screen and including the camerais evacuated. Various levels of vacuumare necessary: the highest vacuum isaround the specimen and in the gun;a lower vacuum is found in the projection chamber and camerachamber. Different vacuum pumpsare used to obtain and maintainthese levels. The highest vacuumattained is of the order of a ten millionth of a millimetre of mercury(see box J).

To avoid having to evacuate thewhole column every time a specimenor photographic material or afilament is exchanged, a number of airlocks and separation valves are built in. In modern TEMs thevacuum system is completelyautomated and the vacuum level iscontinuously monitored and fullyprotected against faulty operation.

The electronicsTo obtain the very high resolution of which modernTEMs are capable, the accelerating voltage and the

current through the lensesmust be extremely stable. The

power supply cabinet contains anumber of power supplies whose

output voltage or current does not deviate by more than one millionthof the value selected for a particularpurpose. Such stabilities require verysophisticated electronic circuits.

Improved electron optical design has made possible a number ofincreasingly more complicated electron-optical techniques. This inturn has created the need for moreease of operation. Digital electronictechniques in general and micro-processor-based techniques inparticular play an important role in this respect. Modern electronmicroscopes employs a fast, powerfulPC (personal computer) to control,

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monitor and record the operatingconditions of the microscope. Thisresults in a dramatic reduction in the number of control knobs compared with earlier models anda microscope which is very easy touse. Furthermore it allows specialtechniques to be embedded in theinstrument so that the operator cancarry them out using the same con-trols. The PC can be attached to anetwork to allow automatic back-upsto be made and results to be down-loaded to other workstations. Themicroscope can always be broughtup to date by installing new softwareor by replacing the computer withthe latest model.

Fig. 14 Bend contours ina semiconductor.

Fig. 15 Arrangement ofatoms in ceramic.

Fig. 16Chromosome

from a humancancer cell.

Fig. 17Woven fabric(SEM image).

VacuumNormal atmospheric air pressure is around 760 mm ofmercury. This means that the pressure of the atmos-phere is sufficient to support a column of mercury 760mm high. Modern physicists use the Pascal (Pa).Normal air pressure = 100 000 Pa; residual pressure inthe microscope = 2.5 x 10 –5 Pa. At this pressure, thenumber of gas molecules per litre is about 7x10 12 andthe chance of an electron striking a gas moleculewhile traversing the column is almost zero.

Box J

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Specimen orientation and manip-ulationWith most specimens it is notsufficient to move them only in thehorizontal plane. Although thespecimen is thin, there is neverthelessinformation in the image coming from various depths within the speci-men. This can be seen by tilting thespecimen and taking stereo pho-tographs. In order to define the axis of tilt, it is necessary to be able torotate the specimen. Crystalline specimens need to have a second tiltaxis perpendicular to the first tilt axis in order to be able to orient a part ofthe specimen so as to obtain therequired diffraction pattern. Theserequirements can be fulfilled in adevice called a goniometer.

The goniometer is a specimen stagedesigned to provide, in addition to Xand Y translation of the specimen, tiltabout one or two axes and rotation as well as Z movement (specimen

height) parallel to the beam axis. It is usual also to provide for

heating, cooling and strainingof the specimen for spe-cialised experiments in the microscope. Thegoniometer is mountedvery close to the objec-tive lens; the specimen is

actually located in theobjective lens field between

the pole pieces because it isthere that the lens aberrations

are smallest and the resolution is highest.

The goniometer itself providesmotorised X, Y and Z move-ment and tilt about one axis. The specimen ismounted near the tip of arod-shaped holder whichin turn is introduced intothe goniometer through

an air lock. It is the specimen holderrod which provides the extra tilt axisor the rotation or heating, cooling orstraining, a special holder being need-ed for each purpose.

Specimen preparationA TEM can be used in any branch of science and technology whereit is desired to study the internalstructure of specimens down to theatomic level. It must be possible tomake the specimen stable and smallenough (some 3 millimetres indiameter) to permit its introductioninto the evacuated microscope column and thin enough (less thanabout 0.5 micrometres) to permit the passage of electrons.

Every branch of research has its ownspecific methods of preparing thespecimen for electron microscopy.In biology for example, tissues aresometimes treated as follows: firstthere is a chemical treatment toremove water and preserve the tissueas much as possible in its originalstate; it is then embedded in ahardening resin; after the resin hashardened, slices (sections) with anaverage thickness of 0.5 micrometresare cut with an instrument called anultramicrotome equipped with a glassor diamond knife. The tiny sectionsthus obtained are placed on a speci-men carrier – usually a 3 mm diame-ter copper specimen grid which hasbeen coated with a structureless carbon film 0.1 micrometre thick.

In metallurgy the following method of preparation is sometimes applied: a 3 mm diameter disc of material(thickness say 0.3 mm) is chemicallytreated in such a way that in the centre of the disc the material is fully etched away. Around this holethere will usually be areas that are sufficiently thin (approximately

0.1 micrometre) to permit electronsto pass through. In a semiconductor it is sometimes desired to cut out a section of material perpendicular tothe surface in order to investigate a defect. This is done by ion-beametching. We shall return to this in a later section.

A technique which is gaining impor-tance in the field of structural biologyis that of freezing the specimen andobserving it in the frozen state.

It is beyond the scope of this bookletto describe the many specimenpreparation techniques that are usednowadays. What can be said is thatthe preparation of a tiny specimen forTEM is often relatively complicatedand certainly more complicated thanthe operation of present-day TEMs.

Scanning Transmission ElectronMicroscopy (STEM)The technique can be applied in aSEM but there is much more interestin applying it in TEM because of thelenses beneath the specimen whichgreatly expand the number of possi-bilities for gathering information. Thistechnique was first demonstrated byFEI in 1969. Today most TEMs can beequipped with this facility and whatwas once an attachment is now anintegral part of the instrument.

The latest TEM from FEI offers a resolution of 1 nm at a magnificationof more than 1 million times.

Once a scanning facility has beenintroduced into the TEM column, it is possible to take advantage of thebackscattered electrons as well as ofthe secondary electrons which areemitted during specimen bombardment.

Fig. 13 A site-specific specimen from a 200 mm semiconductor wafer is thinned to about 300 nm. Cuts are made to partially detach the section. The wafer is tilted to 60° and FIB cuts made at the

base and sides. From here, milling to final thickness will be done at low beam currents and final"release" cuts will be made at 0° tilt.

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All the components of a SEM are usually housed in one unit (Fig. 5page 7). On the right is the electron-optical column mounted on top ofthe specimen chamber. In the cabinetbelow this is the vacuum system.On the left is the display monitor, the keyboard and a "mouse" for controlling the microscope and thecamera. All the rest is below the desktop which gives the whole instrumentits clean appearance.

The electron gun at the top of thecolumn produces and electron beamwhich is focused into a fine spot lessthan 4 nm in diameter on the speci-men. This beam is scanned in arectangular raster over the specimen.Apart from other interactions at thespecimen, secondary electrons areproduced and these are detected bya suitable detector. The amplitude ofthe secondary electron signal varieswith time according to the topogra-phy of the specimen surface. The sig-nal is amplified and used to causethe brightness of the electron beamin a cathode ray tube (CRT) to vary

in sympathy. Both the beam in themicroscope and the one in the CRTare scanned at the same rate andthere is a one to one relationshipbetween each point on the CRTscreen and a corresponding point onthe specimen. Thus a picture is builtup (see Fig. 18). The ratio of the sizeof the screen of the viewing monitor(CRT) to the size of the area scannedon the specimen is the magnification(see box K). Increasing the magnifi-cation is achieved by reducing thesize of the area scanned on the specimen. Recording is done by photographing the monitor screen(or, more usually, a separate high resolution screen), making videoprintsor storing a digital image.

The electron gunThe electron gun consists of a fila-ment and Wehnelt cylinder and isthe same as that in a TEM. Also theillumination system, consisting ofelectron gun, anode and condenserlenses is not much different. Thefinal lens focuses the beam onto thesurface of the specimen to be studied.

The most important differencesbetween TEM and SEM are:a. the beam is not static as in the

TEM: with the aid of an electro-magnetic field, produced by the scanning coils, the beam is scanned line by line over an extremely small area of the specimen’s surface (see box K);

b. the accelerating voltages are much lower than in TEM becauseit is no longer necessary topenetrate the specimen; in a SEM they range from 200 to 30 000 volts.

c. specimens need no complex preparations.

A scanning electron microscope, like the TEM, consists of an electron

optical column, a vacuum system and electronics. The column is consid-

erably shorter because there are only three lenses to focus the electrons

into a fine spot onto the specimen; in addition there are no lenses

below the specimen. The specimen chamber, on the other hand, is

larger because the SEM technique does not impose any restriction on

specimen size other than that set by the size of the specimen chamber.

The electronics unit is more

compact: although it now

contains scanning and display

electronics which the basic TEM

did not, the lens supplies and

the high voltage supplies are

considerably more compact.

The ScanningElectronMicroscope

Box L

ScanningElectron MicroscopeIn the introduction, the functioningof a SEM was compared, with somerestrictions, with a reflected light micro-scope (Fig. 3). Another valid comparison isillustrated in Fig. 18. The electron beam trajec-tories in a SEM are very similar to those in aTV monitor. Both devices have an electron gun, both areevacuated, both have deflection coils.

The electrons produced when the primary beam strikes the specimen ina SEM are detected and turned into an electrical signal; in the monitor,the primary electrons are turned into light to produce an image on thefluorescent screen.

Filament

Wehnelt aperture

Wehnelt aperture

Electromagnetic lens

Electromagnetic lens

Specimen

Vacuum

Deflection coil

Anode

SignalAmplifier

Image onfluorescent screen

of TV monitor

Deflectioncoil

Vacuum

FilamentWehnelt aperture

Anode

Vacuum

Electron Beam

ScanGenerator

A

BC

SEM

High VoltageGenerator

Filament source

Fig. 18 Schematic

diagram of a SEM showing

the column andhow the image

is formed on the monitor

SEM Magnification

length of one line "written" by the electron beam on the monitor

SEM magnification =length of one "track" of theelectron beam on the specimen

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Box K

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What happens in the specimenduring electron bombardment?When discussing the TEM, it wasseen that when electrons strike the specimen several phenomenaoccur. In general, five of these phenomena are used in an ordinarySEM (see box M).

a. The specimen itself emitssecondary electrons.

b. Some of the primary electrons are reflected (backscatteredelectrons).

c. Electrons are absorbed by the specimen.

d. The specimen emits X-rays.e. The specimen sometimes emits

photons (= light).

All these phenomena are interrelatedand all of them depend to someextent on the topography, the atomicnumber (see box M) and the chemicalstate of the specimen. The numberof backscattered electrons, secondaryelectrons and absorbed electrons ateach point of the specimen dependson the specimen’s topography to amuch greater extent than the otherproperties mentioned. It is for thisreason that these three phenomenaare exploited primarily to image thespecimen’s surface.

Electron detectionDetectors for backscattered elec-trons and secondary electrons areusually either a scintillation detectoror a solid state detector. In the former case, electrons strike a fluorescent screen which thereuponemits light which is amplified andconverted into an electrical signal bya photomultiplier tube. The latterdetector works by amplifying theminute signal produced by theincoming electrons in a semi-conductor device. When the specimen is not directly connected to earth but via a resistor, the electrons that are not reflected generate a potential difference across the resistor. This changingpotential difference can be amplifiedand the resulting signal used to produce a third sort of image on the monitor. This facility also permits

the study of (dynamic) electrical phenomena in electronic devicessuch as integrated circuits.

Magnification and resolutionIn the SEM, the magnification isentirely determined by the electroniccircuitry that scans the beam overthe specimen (and simultaneouslyover the fluorescent screen of themonitor where the image appears).

Magnification can be as high as 300 000x which is usually more thansufficient. In principle the resolutionof a SEM is determined by the beamdiameter on the surface of the speci-men. The practical resolution how-ever depends on the properties ofthe specimen and the specimen pre-paration technique and on manyinstrumental parameters such asbeam intensity, accelerating voltage,scanning speed, distance from thelast lens to the specimen (usuallyreferred to as the working distance)and the angle of the specimensurface with respect to the detector.Under optimum conditions a resolu-tion of 1 nm can be attained (see box N).

Observation and recording ofthe imageA SEM is usually equipped with twoimage monitors, one for observationby the operator and the other, a high resolution monitor, is equippedwith an ordinary photo camera(which can be 35 mm or Polaroid).

To facilitate the observation and correct choice of the parametersmentioned above, FEI SEMs have an image store in which the image isbuilt up scan by scan and displayedat TV speed so that there is a steady,flicker-free image on the viewingmonitor. Images are digital and canbe stored electronically for subse-quent enhancement and analysis.

Image treatmentBecause the image in a SEM is com-pletely electronically produced, it can be subject to all kinds of treatmentusing modern electronics. Thisincludes contrast enhancement,

Electron Interactionswith MatterIn the modern view of matter,an atom consists of a heavycharged nucleus surrounded by anumber of orbiting electrons (see boxC, page 5). The number of electrons isequal to the number of protons in thenucleus, this is known as the atomicnumber of the atom. The incomingelectron can interact with the nucleusand be backscattered with virtuallyundiminished energy (just as a space probeis deviated by the gravity of a planet duringfly-past). Or it can interact with the orbit-ing electrons: an electron may be ejectedfrom the atom, (this is a secondary elec-tron), and the atom with one electronshort restores the status quo by emittingits excess energy in the form of anX-ray quantum or a light photon.(See box P, page 20)

Box M

inversion (black becomes white etc.),mixing of images from various detectors, subtraction of the imagefrom one detector from that pro-duced by a different detector, colourcoding and image analysis. All thesetechniques may be applied if it suitsthe primary aim of extracting thebest possible information from thespecimen.

VacuumIn general a sufficiently low vacuumfor a SEM is produced by either anoil diffusion pump or a turbo-molecular pump in each case backedby a rotary pre-vacuum pump. These combinations also provide reasonable exchange times for specimen, filament and aperture (less than 2 minutes) without theneed to use vacuum airlocks. The SEMvacuum system is fully automaticallycontrolled and protected againstoperating failures.

ElectronicsIt goes without saying that in a SEMthe voltages and currents requiredfor the electron gun and condenserlenses should be sufficiently stable to attain the best resolution. Similarly the stability of the elec-tronic circuitry associated with thedetectors should be extremely wellcontrolled. Stabilities of 1 part permillion are no exception.

All the electronic units are housed in the microscope console and arecontrolled by a personal computer(PC) using a keyboard and mouse.

Application and specimenpreparationA SEM can be used whenever infor-mation is required about the surfaceof a specimen. This applies in manybranches of science and technologyas well as life sciences. The onlyrequirement is that the specimen can withstand the vacuum of thechamber and the electron bombardment.

Many specimens can be brought intothe chamber without preparation ofany kind. If the specimen containsany volatile components such aswater, this will need to be removedby using a drying process (or in some circumstances it can be frozensolid). Non-conducting specimenswill charge up under electron bom-bardment and need to be coatedwith a conducting layer. Because aheavy element like gold also gives agood yield of secondary electronsand thereby a good quality image,this is the favourite element for coating. In addition it gives a finegrain coating and is easily applied ina sputter coater. The layer requiredto ensure a conducting layer is quitethin (about 10 nm). All in all, thepreparation of specimens to be investigated by SEM is not as complicated as the preparation ofspecimens for TEM.

Sometimes it is impossible orundesirable to prepare a specimenfor SEM: many specimens in forensic

science, for example, silicon wafers used in IC manufacture, and integrat-ed circuits themselves which need tobe studied whilst in operation. Insuch cases special techniques need to be employed in order to obtainsatisfactory images. Low voltage SEMis one of these techniques. The mod-ern SEM can be adapted to all thesespecial techniques, sometimes withthe addition of a suitable accessory.

17

Fig.19 Forensic investigation of a tungsten car-lamp filamentfollowing an accident. Globules of melted glass on the wire

show that at the time of impact, the lamp was alight.Photo courtesy of Dr Rabbit, Military School, Brussels

Resolution in SEMThe resolution (determined by the size

of the scanning spot) means that mag-nifications of the order of 300 000x canbe realised. If the viewing screen is, say,300 mm wide, then the full width of thescanned area is only 1 micrometre.

Box N

Specimen orientation andmanipulationAs has been mentioned, the qualityof the image in a SEM depends onthe orientation and the distance ofthe specimen from the detectors andthe final lens. The specimen stageallows the specimen to be moved ina horizontal plane (X and Y direction)and up and down (Z direction) androtated and tilted as required. Thesemovements are often motorised andcontrolled by the PC.

The various SEM models in a rangediffer in the size of their specimenchambers allowing various sizes ofspecimens to be introduced andmanipulated. The maximum speci-men size also determines the pricebecause the larger the specimenchamber, the larger the goniometermovement needed and the larger thepumping system needed to obtainand maintain a good vacuum.

The simplest model accepts specimens of a few centimetres indiameter and can move them 50 mmin the X and Y directions. The largest chamber accepts samples up to 200 mm in diameter and canmove them 150 mm in each direc-tion. All models allow samples to betilted up to high angles and rotatedthrough 360 degrees.

There are special stages for heating,cooling and straining specimens butbecause of the wide variety of pos-sible sample sizes, these stages areproduced by specialist firms.

ESEMAs mentioned above, samples forconventional SEM generally have tobe clean, dry, vacuum compatibleand electrically conductive. In recentyears the Environmental ScanningElectron Microscope (ESEM) has been developed to provide a uniquesolution for problematic samples.Examples of specimens which poseproblems are wool or cotton tissue,cosmetics, fats and emulsions (e.g. margarine).

Attempts to view a specimen containing volatile components by placing it in an environmental chamber (see box O) isolated fromthe main column by one or more differential pumping apertures usedto be hampered by the lack of a suitable electron detector which can work in the atmosphere of the chamber. The Gaseous SecondaryElectron Detector makes use of cascade amplification (see Fig. 20)not only to enhance the secondaryelectron signal but also to producepositive ions (see page 22) whichare attracted by negative charge on the insulated specimen surfaceand effectively suppresscharging artefacts.

18

Environmental ChamberThe pressure-temperature phase

diagram for H2O indicates that true "wet"conditions only exist at pressures of at least

600 Pa at 0°C (environmental microscopistsusually refer to 4.6 Torr = 4.6 mm of mercury –see box J). In the range 650 to 1300 Pa (5 –10 Torr)therefore the specimen may be observed whilst atequilibrium with water.

Fig. 21 Ice cream in theCryo-SEM: the amount of emulsionin relation to air and water (ice) is an

important parameter to study.

Box O

19

Signal Amplification in the ESEM

Primary ElectronBeam

Detector

Gaseousenvironment

CascadeAmplification

Secondary Electrons

Fig. 20 Gas ionisation amplifies thesecondary electron signal and, ina non-conductive specimen,positive ions are attractedto the specimen surfaceas charge accumu-lates there.

Fig. 24 The cocchlea is the hearing organ inside the ear. It is the vibration of

the fibres which transmitssounds to the brain.

Fig. 22 House mites havebeen filmed live and movingin the ESEM.

Fig. 23Biomineralisation

by a number oforganisms illustrates

an astounding range ofskeletal ulstrastructure.

Photos courtesy of Natural History Museum, London.

20

More information from the specimenAs already mentioned, the electronsbombarding the specimen cause it to emit X-rays whose energy is determined by the elemental com-position. When a suitable detector is placed near the specimen (thegoniometer allows for this), it is pos-sible to detect very tiny quantities of elements. Quantities down to onethousandth of a picogram (10 –12 g)or less can thus be detected (see box P). The detector in question iscalled an Energy Dispersive X-rayDetector (EDX). With this detector,spectra are acquired showing distinctive peaks for the elements present with the peak height indicat-ing the element concentration. Thisapplies to both SEM and TEM.

It has also been mentioned thatsome of the electrons lose energywhen they travel through the speci-men. This loss of energy can be measured with an Electron EnergyLoss Spectrometer (EELS). This detector is mounted under the projection chamber of the TEM.

Scanning Transmission Electron MicroscopyIf the specimen in a SEM is sufficientlytransparent for electrons to be transmitted through the specimen,these can be collected with a suitablyplaced detector. This STEM technique,sometimes applied in a SEM is moreoften encountered in a TEM becauseuse can also be made of the lensesbelow the specimen. It is discussedon page 13.

AdditionalTechniques

X-ray AnalysisTo return for a moment to the analogy of a person in an

unknown darkened room with a fine beam torch (box F). Whilescanning the room to determine its topography, the viewer would also be

aware of the colour of objects. Likewise by looking at the wavelengths of X-raysemitted (their "colour") we can determine which chemical elements are present at

each spot and the intensity of the X-rays is a measure of the element concentration.

The impinging electron in the primary beam may cause an electron in an atom of thespecimen to be removed from its orbit (if the atom is near the surface, it can become a

secondary electron). The atom is left in an excited state (it has too much energy) and itreturns to its stable state by transferring an electron from an outer orbit to replace the one

removed (see box C). Finally the outermost missing electron is replaced by one from thefree electrons always present in a material. Each of these transfers results in the release ofsurplus energy as an X-ray quantum of characteristic wavelength (in the case of outer toinner orbital transitions) or a light quantum (in the case of free electron to outer orbittransfers). We can see that different materials produce different colours by examining acolour television screen with a magnifying glass. Different phosphors produce differentcolours of light under the impact of the scanning electron beam.

Box P

The other phenomena that occurwhen the electron beam goesthrough the specimen (backscatteringof the incident electrons and theemission of secondary electrons) are exploited by using the STEMtechnique which was discussed on page 13.

Other electron sourcesThe same type of hairpin-shapedtungsten filaments are used in bothTEM and SEM. The higher thetemperature of the filament, themore electrons are generated by theelectron gun and the brighter theimage. The lifetime of the filamenthowever is reduced and a practicalcompromise sets a limit on thebrightness of the tungsten gun. Twobrighter sources of electrons havebecome popular in recent years: thelanthanum hexaboride (LaB 6) gunand the field emission gun (FEG). Inthe former, a lanthanum hexaboridecrystal when heated gives off up to10x more electrons than tungstenheated to the same temperature. Inthe latter, electrons are pulled out ofa very fine pointed tungsten tip bya very high electric field. Electrondensities up to 1000 times thosefrom tungsten emitters can beobtained with the FEG.

The larger number of electrons or the greater electron density providedby these sources permits reductionsin beam diameter which results inbetter resolution in both SEM andSTEM. It also permits more accurateX-ray analysis in both instruments tobe obtained. The gains are not how-ever proportional to the increase inbrightness: other factors such as lensaberrations also play a role.

Field emission TEMs and SEMs costmore than regular instruments butprovide much higher performance.

21

Fig. 25 The quality of a soldered joint depends on getting the right structure and mixture on cooling. The coloured X-ray maps show the locations of

lead (red) and tin (green) corresponding to the secondary electron image.

Ion Beam technologySo far this booklet has been aboutelectron microscopy and what usefulinformation can be obtained using aprimary electron beam. However,electrons are not the only chargedparticles that can be accelerated andfocused using electrical and magneticfields. It was explained earlier (seebox C) that the atom consists of apositively charged nucleus surroundedby electrons in orbits. Normally theatom is neutral because there areequal numbers of protons and elec-trons. An atom that has lost oneor more of its outermost electronshas a positive charge and can beaccelerated, deflected and focussedin a similar way to its negativelycharged cousin the electron.

The difference lies in the mass(analogous to weight – see box Q).The electron is almost 2000x lighterthan the lightest ion and ions canbe up to 250x heavier still. The rela-tively low-mass subatomic electronsinteract with a sample and releasesecondary electrons which, when collected, provide high quality imagesat near atomic-level resolution.The greater-mass ions dislodgesurface particles, also resulting in displacement of secondary ions andelectrons. The ion beam directlymodifies or mills the surface withsubmicron precision. Secondary elec-trons and ions are used for imagingand compositional analysis.By carefully controlling the energyand intensity of the ion beam, it ispossible to carry out very precisemicro-machining to produce minutecomponents or to remove unwanted

material. Examples of applications are the production of thin-film headsfor reading computer storage mediaand the removal of material round adefect in a semiconductor device toallow the defect to be studied in atransmission electron microscope(see Fig. 13, page 13).

How is such a fine beam controlledand how does the operator knowwhen the micro-machining is finished?Quite simply by having a detector toobserve the secondary electrons produced by the impact of the ions.By having a scanning electron micro-scope as an integral part of the sameinstrument, the point of impact of the ion beam can be observed non-destructively and the operatorcan control the ion beam process precisely. These instruments are called dual-beam instruments.

FIBs in IC Manufacturing By precisely focusing a high currentdensity ion beam, a FIB workstationcan not only remove material andexpose defects, but also deposit newconducting paths or insulating layers,analyse the chemical composition ofa sample and view the area beingmodified, all within submicron toler-ances. IC manufacturers find it acost-effective tool with a versatile range of applications that can reducetime-to-market and improve yields.During IC design and testing, theseproducts speed the fabrication anddebugging of prototypes. Focusedion beam technology gives rapidaccess to design problems, then gas injection techniques allow fast cut-and-splice modifications to the

circuit. Used in defect review, a FIBunit can improve manufacturingyields by analysing surface and sub-surface defects on wafers from themanufacturing (fab) line. It can alsocross-section critical structures toprovide three-dimensional monitoringof process performance. As a failureanalysis tool, it speeds the diagnosisof IC malfunction and the locationof subsurface defects. As micro-electronic structures get smaller, contamination control becomesmore important. Some of the newFIB models feature robotic wafer handling and automatic wafer align-ment to comply with the cleanroomrequirements of the fab line.

FIBs in Materials ScienceFor materials science applications, a FIB unit can cross-section complexalloys and layered materials and pre-pare extremely thin TEM specimens.An ion beam’s submicron milling hasuses in the manufacture and test ofmicroelectromechanical systems,such as acceleration detectors. Other hard surface applicationsinclude micromachining fresnellenses and sharpening scanningtunnelling microscope tips.

FIBs in Biological ScienceUsed in the biological sciences, anion beam’s precise cut can removethe surface of a cell or prepare across-section of a virus to allow fordetailed examination under anelectron microscope. It is openingup new possibilities in the field ofstructural biology.

Weight and MassEverything has mass. Even aweightless astronaut has the samemass he had before he took off. Weightis the force of gravity acting on a mass.If the gravity is lower, as on the moon, weweigh less, but our mass is still the same.That’s why scientists refer to mass ratherthan weight. Mass is independent of theframe of measurement.

22

Box Q

For more information about the latest technology and innovations in microscopy, feel free to surf to

www.feicompany.com

23

g l o s s a r yAberrationThe deviation from perfect imaging in an optical system which is caused by imperfec-tions in the lens or by non-uniformity of theelectron beam.

Accelerating voltageThe potential difference in an electron gunbetween cathode and anode over which elec-trons are accelerated. The higher the voltage,the faster the electrons and the more pene-trating power they have. Voltages usuallyrange from a few thousand volts up to severalhundred thousand.

AirlockA chamber within the electron microscopethat can be isolated from the rest to allow thespecimen to be inserted. The airlock is thenpumped out and the specimen moved intothe column vacuum. This reduces the amountof air and other contaminants brought intothe column. Airlocks also facilitate exchangeof photographic material and gun emitter.

AmplitudeThe maximum value which a periodicallychanging physical magnitude can reach. For electromagnetic waves the intensity of the disturbance is proportional to the square of their amplitude.

Amplitude contrastImage contrast caused by the removal of elec-trons (or light) from the beam by absorptionin the specimen.

ÅngströmUnit of length used in the early days ofmicroscopy. 1 Å = 0.1 nm (see box D, page 5).

AnodeIn an electron gun (see page 8), the negative-ly charged electrons are accelerated towardsthe anode which has a positive charge relativeto the filament (cathode) from which theyemerge. In practice (for ease of construction),the filament has a high negative charge andthe anode is at earth (ground) potential.

ApertureA disc of metal with a small hole designed tostop those electrons which are not requiredfor image formation (e.g. scattered electrons).

AstigmatismA lens aberration (q.v.) A circle in the speci-men becomes an ellipse in the image.

AtomThere are many ways of looking at the atom.The most useful one for electron microscopistsis to think of it as consisting of a positivelycharged nucleus (containing positivelycharged protons and uncharged neutrons)surrounded by negatively charged electrons in discrete orbits.

Atomic numberThe number of protons in the atomic nucleus.This number determines the chemical natureof the atom. An atom of iron for example has29 protons, an atom of oxygen 8 and so on.

Backscattered electronsPrimary electrons which have been deflected by the specimen through an angle generallygreater than 180

owith little or no loss of energy.

Binocular viewerA light microscope built into a TEM for viewinga fine-grain fluorescent screen for criticalfocussing and astigmatism correction.

CathodoluminescenceThe emission of light photons by a materialunder electron bombardment (see page 11).

Chromatic aberrationSee aberration. The magnification of the lensvaries with the wavelength of the electrons in the beam.

CleanroomRoom containing extremely low numbers ofdust particles used for the manufacture ofsemiconductors.

ColumnThe evacuated electron beam path, the elec-tromagnetic lenses and the specimen and aperture mechanisms are accommodated in a vertical column (see page 8).

Condenser lensPart of the illumination system between thegun and the specimen designed to concen-trate the electron beam into a parallel or intoa finely focussed spot on the specimen (or, inthe case of the scanning electron microscope,into the objective lens).

CRTCathode ray tube. The screen of a conventionalTV set or a PC monitor is the front end of acathode ray tube. Behind is an electron gunproducing an electron beam, lenses to focus itand a scanning system to make the beam scan araster (q.v.) on the screen to produce an image.

CrystalA material in which the atoms are ordered intorows and columns (a lattice) and because ofthis periodicity, electrons, whose wavelength(q.v.) is about the same size as the spacingbetween atoms, undergoes diffraction (q.v.)

DetectorA device for detecting particular electrons orphotons in the electron microscope.

DiffractionDeviation of the direction of light or otherwave motion when the wavefront passes theedge of an obstacle.

Diffraction contrastImage contrast caused by the removal of elec-trons (or light) from the beam by scattering by a periodic (e.g. crystalline) structure in thespecimen.

EDXEnergy Dispersive X-ray Analysis orSpectrometry. An EDX spectrometer makes aspectrum (q.v.) of X-rays emitted by the spec-imen on the basis of their energy. An alterna-tive method of making the spectrum,Wavelength Dispersive Spectrometry (WDS),exists but is not referred to in this book.

EELSElectron Energy Loss Spectroscopy (or Spectrometry) is defined on page 20.

ElectronFundamental atomic particle rotating in anorbit around the nucleus of the atom (see box C). Free electrons can easily flow in aconductor and can be extracted into a vacuum by heat and or an electrical field.

Electron microscopeA microscope (q.v.) in which a beam of electrons is used to form a magnified imageof the specimen.

ESEMEnvironmental Scanning Electron Microscopyis defined on page 18.

ExcitationThe input of energy to matter leading to theemission of radiation.

Excited atomAn atom which has lost one of its inner elec-trons (see also ion) has a higher energy (theextra energy is that needed to remove theelectron). It seeks to return to its ground stateby rearranging its electrons and emitting theexcess energy as an X-ray quantum.

Fab lineThe semiconductor production line which islocated in a clean room (q.v.).

FEGField Emission Gun is defined on page 21.

FIBField ion beam instrument. A beam of ionsreplaces the beam of electrons. Dual beaminstruments also have an electron column.

FilamentMetal wire, usually in the form of a hairpin,which, when heated in vacuum, releases free electrons and so provides the source of electrons in the electron microscope.

Fluorescent screenLarge plate coated with a material (phosphor)which gives off light (fluoresces) when bom-barded by electrons). In the TEM, the image isformed on the fluorescent screen (see page 11).

Focal length of a lensThe distance (measured from the centre ofthe lens) at which a parallel incident beam isbrought to a focus.

FocusingThe act of making the image as sharp as possi-ble by adjusting the objective lens (q.v.).

GoniometerSpecimen stage allowing linear movement ofthe specimen in two or more directions androtation of the specimen in its own plane andtilting about one or more axes (see page 13).

Ground stateThe lowest energy state of an atom. All elec-trons in their proper places and no charge.

IonAn atom which has lost one or more of itsouter electrons. It has a positive charge.

Ion getter pumpVacuum pump in which the movement of ionsin a strong magnetic field drag molecules of gasand embed them in the cathode of the pump.

LatticeRegular array of atoms in a crystal (q.v.).

LensIn a light microscope, a piece of transparentmaterial with one or more curved surfaces,which is used to alter systematically the direc-tion of rays of light. In an electron microscope,a similar effect is achieved on a beam of elec-trons by using a magnetic (or electrostatic)field (see pages 5 & 10).

MicrometreUnit of length (distance). One micrometre(abbreviated to µm) is a thousandth of a mil-limetre or 1000 nm (see box D, page 5).

MicroscopeAn instrument designed to extend man'svisual capability, i.e. to make visible minutedetail that is not seen with the naked eye.

MicrotomeInstrument for cutting extremely thin sectionsfrom a specimen prior to examination in themicroscope. In electron microscopy this isusually referred to as an ultramicrotome.

NanometreUnit of length (distance). One nanometre(abbreviated to nm) is a millionth of a millime-tre (10-9 metre) (see box D, page 5).

Objective lensIn a TEM, this is the first lens after the specimenwhose quality determines the performance ofthe microscope. In a SEM it is the last lensbefore the specimen and produces theextremely fine electron spot with which thespecimen is scanned.

Oil diffusion pumpVacuum pump where the pumping action isproduced by the dragging action of a streamof oil vapour though an orifice.

PhaseRelative position in a cyclical or wave motion.It is expressed as an angle, one cycle or onewavelength (q.v.) corresponding to 180

o.

Phase contrastImage contrast caused by the conversion ofphase differences in light leaving the objectinto amplitude differences in the image.

Phase diagramGraph of temperature and pressure showingthe range of each under which agiven materialcan exist in the solid, liquid or vapour phase.

PhotomultiplierElectronic tube in which light is amplifiedwithout interference (noise) to produce anelectrical signal.

PhotonsDiscrete packets of electromagnetic radiation. Alight beam is made up of a stream of photons.

Primary electronsElectrons in the beam as it leaves the gun andimpinges on the specimen.

QuantumA discrete packet of X-radiation.

RasterThe track of the beam in a SEM or STEM. It isanalogous to eye movements when reading abook: left to right word by word and downthe page line by line.

RefractionChanges in speed (and sometimes direction)when light or electrons pass through a specimen.

Refractive indexThe ratio of the speed of light in a vacuum to that in a given medium such as glass, water or oil.

Resolving powerThe ability to make points or lines which areclosely adjacent in an object distinguishable inan image. (see box B, page 5)

ResolutionThe act or result of displaying fine detail in animage. (see box A, page 4)

Rotary pumpVacuum pump in which the pumping action isproduced by moving packets of air from oneside of a rotating cylinder to another bymeans of an eccentric drum.

ScanningProcess of investigating a specimen by moving a finely focussed electron beam systematically in a raster over the surface. The same tech-nique in the display monitor leads to an imagein which every point corresponds to a point on the specimen.

Scintillation detectorElectron detector used in SEM or STEM inwhich electrons to be detected are acceleratedtowards a phosphor which fluoresces to pro-duce light which in turn is amplified by meansof a photomultiplier (q.v.) to produce anelectrical signal.

Secondary electronsElectrons originating in the specimen whichare emitted under influence of the primarybeam.

SEMScanning Electron Microscopy is defined onpage 7.

Semiconductor detectorElectron detector used in SEM or STEM inwhich the electron to be detected causes asmall change in the semiconductor materialwhich is amplified by making the semi-conductor part of an electrical circuit.

SpectrometerInstrument for obtaining a spectrum (q.v.).

SpectrumA display produced by the separation of acomplex radiation into its component wavelengths or energies.

Spherical aberrationSee aberration. The magnification in the centreof the lens differs from that at the edges.

Sputter coaterInstrument for coating a non-conductingspecimen with a very thin uniform layer of aconducting element such as gold to make itsuitable for examination in a scanning electronmicroscope without danger of it charging up.

STEMScanning Transmission Electron Microscopy isdefined on page 7.

TEMTransmission Electron Microscopy is definedon page 6.

Turbomolecular pumpVacuum pump in which the centrifugal forceof fast rotating discs forces molecules from theaxis to the periphery.

VacuumSpace from which (most) gases and vapourshave been removed (see box J, page 12).

WavelengthThe distance on a periodic wave between two successive points at which the phase is the same.

Wehnelt cylinderAn electrode between the cathode (filament)and the anode (earth) in an electron gunwhich focuses and controls the electron current in the beam and keeps it constant (see Fig. 7, page 9).

Working distanceThe physical distance between the externalmetal parts of the objective lens and the speci-men surface. This is the space available forplacing certain electron, X-ray and cathodolu-minescence detectors. For highest resolution,the working distance has to be made as smallas possible which leads to compromises.

X-raysExtremely short wavelength radiation. In theelectron microscope, an X-ray photon (q.v.) isemitted when an excited atom releases itsexcess energy and returns to its ground state.

©2004. Data subject to change without notice.

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