All You Wanted to Know About Electron Microscopy

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All you wanted to know about Electron Microscopy...

...but didn’t dare to ask! 4

20 Introduction What is Electron This booklet is written for those who know little or nothing about electron Microscopy? microscopy and would like to know how an electron microscope works, why it is used and what useful results it can produce.

"With a microscope you see the surface of things. It magnifies them but does not show you reality. It makes things seem higher and wider. But do not suppose you are seeing things in themselves." Feng-shen Yin-Te (1771 – 1810) In The Microscope 1798

A publication of FEI Electron Optics FEI Company, one of the world’s leading suppliers of transmission and The Transmission scanning electron microscopes.

Electron Our commitment to electron microscopy dates back to the mid- Microscope 1930s, when we collaborated in EM research programmes with universi- ties in the UK and the Netherlands. In 1949, the company introduced its first EM production unit, the EM100 transmission electron microscope.

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

Microscope ISBN nummer 90-9007755-3

Additional Techniques c o n t e n t s 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 What is Electron valuable insight into the origins of our planet and its valuable Microscopy? mineral resources.

Nobody knows for certain who Why use electrons instead the objective lens in a medium with invented the microscope. The light of light? a high refractive index (oil) gave microscope probably developed A modern light microscope (often another small improvement but from the Galilean telescope during abbreviated to LM) has a magnifica- these measures together only the 17th century. One of the earliest tion of about 1000x and enables brought the resolving power instruments for seeing very small the eye to resolve objects separated of the microscope to objects was made by the Dutchman by 0.0002 mm (see box A). In the just under 100 nm. Antony van Leeuwenhoek (1632- continuous struggle for better 1723) and consisted of a powerful resolution, it was found that the convex lens and an adjustable holder resolving power of the microscope for the object being studied (speci- was not only limited by the number men). With this remarkably simple and quality of the lenses but also by microscope (Fig. 1), Van Leeuwenhoek the wavelength of the light used for may well have been able to magnify illumination. It was impossible to objects up to 400x and with it he resolve points in the object which discovered protozoa, spermatozoa were closer together few hundred and bacteria and was able to classify nanometres – see box B). Using light red blood cells by shape. with a short wavelength (blue or ultraviolet) gave a small improve- The limiting factor in Van ment; immersing the Leeuwenhoek’s microscope was the specimen and quality of the convex lens. The prob- the front of lem can be solved by the addition of Resolution and another lens to magnify the image Magnification (1) produced by the first lens. This com- Box A Given sufficient light, the unaided pound microscope – consisting of human eye can distinguish two points 0.2 an objective lens and an eyepiece mm apart. If the points are closer together, only one point will be seen. This distance is together with a means of focusing, called the resolving power or resolution of the a mirror or a source of light and a eye. A lens or an assembly of lenses (a micro- specimen table for holding and scope) can be used to magnify this distance and positioning the specimen – is the enable the eye to see points even closer together basis of light microscopes today. than 0.2 mm. Try looking at a newspaper photograph or one in a magazine through a magnifying glass for example.

4 In the 1920s it was discovered that accelerated electrons (parts of the atom – see box C) behave in vacuum just like light. They travel in straight lines and have a wavelength which is about 100 000 times smaller than that of light. Furthermore, it was found that electric and magnetic fields have the same effect on electrons as glass lenses and mirrors have on visible light. Dr. Ernst Ruska at the University of Berlin combined these characteristics and built the first transmission electron microscope (often abbreviated to TEM) in 1931. Box D The Nanometre For this and subsequent work on the As distances become shorter, the subject, he was awarded the Nobel number of zeros after the decimal Prize for Physics in 1986. The first point becomes larger, so microscopists electron microscope used two mag- use the nanometre (abbreviated to nm) netic lenses and three years later he as a unit of length. One nanometre is a added a third lens and demonstrated millionth of a millimetre (10 –9 metre). An a resolution of 100 nm (see box D), intermediate unit is the micrometre (abbreviated twice as good as that of the light to µm) which is a thousandth of a millimetre or 1000 nm. microscope. Today, using five magnetic lenses in the imaging Some literature refers to the Ångström unit (abbre- system, a resolving power of 0.1 nm viated to Å) which is 0.1 nm and the micron for at magnifications of over 1 million micrometre. times can be achieved.

Fig. 1 Replica of one of the 550 light microscopes The Electron made by Antony van Leeuwenhoek. One way of looking at an atom is to visualise it as a minute "solar system" in which electrons orbit like planets round a central nucleus. The nucleus consists of neutral and positively charged particles and the

Box C electrons have a compensating negative charge so that the atom is neutral. The electrons which are about 1800x lighter than the nuclear particles occupy distinct orbits each of which can accommodate a fixed maximum number of electrons. Box B Resolution and Magnification (2) When electrons are liberated from the atom, however, The resolving power of a microscope they behave in a manner analogous to light. It is this determines its maximum magnification. behaviour which is made use of in the electron It is only necessary to magnify the resolving microscope, although we should not lose sight of power to 0.2 mm, the resolving power of the the electron’s role in the atom, which will be unaided human eye, for all the fine detail of an made use of later. object to be revealed. Higher magnification will not reveal any more information and is unnecessary.

The resolving power of a microscope is one of its most important parameters. The reason that magnifications are often quoted is that it gives an idea of how much an image has been enlarged.

5 Transmission Electron Not all specimens can be made thin Microscope (TEM) enough for the TEM. Additionally, The transmission electron microscope there is considerable interest in can be compared with a slide projec- observing surfaces in more detail. tor (Fig. 2). In the latter, light from a Early attempts at producing images light source is made into a parallel from the surface of a specimen beam by the condenser lens; this involved mounting the specimen passes through the slide (object) and nearly parallel to the electron beam is then focused as an enlarged image which then strikes the surface at a onto the screen by the objective lens. very small angle. Only a very narrow

In the electron microscope, the light region of the specimen appears in Box F source is replaced by an electron focus in the image and there is con- source (a tungsten filament heated in siderable distortion. The technique Scanning Microscopy vacuum), the glass lenses are has not found wide application in Imagine yourself alone in an replaced by magnetic lenses and the the study of surfaces. unknown darkened room with only a projection screen is replaced by a fine beam torch. You might start exploring fluorescent screen which emits light the room by scanning the torch beam system- when struck by electrons. The whole atically from side to side gradually moving Box E trajectory from source to screen is Penetration down so that you could build up a picture of the under vacuum and the specimen Electrons are easily objects in the room in your memory. (object) has to be very thin to allow stopped or deflected by A scanning microscope uses an electron beam the electrons to penetrate it. matter (an electron is nearly 2000x smaller and lighter instead of a torch, an electron detector instead than the smallest atom). That of eyes and a fluorescent screen and camera is why the microscope has to be as memory. evacuated and why specimens – for the transmission microscope – have to be very thin in order to be imaged with electrons. Typically, the specimen must be no thicker than a few hundred nanometres.

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

Projector Screen Objective Lens Condenser Lens Slide Light Source

Fluorescent Screen Specimen Aperture (thin) Electron Beam

Electron Source

Objective Condenser Lens Lens Fig. 3 A modern transmission electron microscope the Tecnai 12. TEM 6 Light Source (Reflected Light)

Light Beam

Specimen

Light Source (Transmitted Light)

Electron Source

Fig. 5 A modern Electron Beam scanning electron microscope the Quanta

Specimen (thin)

Scanning Electron Microscope Figure 4 compares light microscopy (SEM) (using transmitted or reflected light) It is not completely clear who first with TEM and SEM. A combination Vacuum proposed the principle of scanning of the principles used in both TEM the surface of a specimen with a and SEM, usually referred to as Fluorescent Screen Slide projector finely focused electron beam to scanning transmission electron produce an image of the surface. microscopy (STEM), was first B The first published description described in 1938 by Dr. Manfred appeared in 1935 in a paper by the von Ardenne. It is not known what German physicist Dr. Max Knoll. the resolving power of this instru- Although another German physicist ment was. The first commercial Dr. Manfred von Ardenne performed instrument in which the techniques some experiments with what could were combined was a Philips EM200 Electron Source be called a scanning electron micro- equipped with a STEM unit devel- scope (usually abbreviated to SEM) oped by Dr. Ong of Philips Electronic Electron Beam in 1937, it was not until 1942 that Instruments in the U.S.A. (1969). three Americans, D. Zworykin, Dr. At that time, the resolving power Hillier and Dr. Snijder first described was 25 nm and the magnification a true SEM with a resolving power of 100 000x. Modern TEMs equipped Deflection coil 50 nm and a magnification of 8000x. with a STEM facility can resolve Nowadays SEMs can have a resolving 1 nm at magnifications of up to power of 1 nm and can magnify 1 million times.

over 400 000x. Specimen (thick) Detector Monitor Electron Source Vacuum

Fig. 4 Comparison of the light microscope TEM (A) with transmission (B) and scanning 7 (C) electron microscopes Electron Velocity The higher the accelerating voltage, the faster the electrons. There are four main components to a transmission electron microscope: 80 kV electrons have a velocity of 150 000 km/second (1.5 x 10 8 m/s) which is half the speed of light. This an electron optical column, a vacuum system, the necessary electronics rises to 230 000 km/second for 300 kV electrons (2.3 x 10 8 m/s – more than (lens supplies for focusing and deflecting the beam and the high voltage three-quarters the speed of light).

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 Transmission Electron Microscope

The column is the crucial item. It The electron gun The beam emerging from the gun comprises the same elements as The electron gun comprises a fila- is condensed into a nearly parallel the light microscope as can be seen ment, a so-called Wehnelt cylinder beam at the specimen by the con- from the ray paths of light and elec- and an anode. These three together denser lenses and, after passing trons (Fig. 6). The light source of the form a triode gun which is a very through the specimen, projected as light microscope is replaced by an stable source of electrons. The tung- a magnified image of the specimen electron gun which is built into the sten filament is hairpin-shaped and onto the fluorescent screen at the column. The glass lenses are replaced heated to about 2700 OC. By apply- bottom of the column. by electromagnetic lenses and the ing a very high positive potential eyepiece or ocular is replaced by a difference between the filament and If the specimen were not thin, the fluorescent screen. The entire electron the anode, electrons are extracted electrons would simply be stopped path from gun to screen has to be from the electron cloud round the and no image would be formed under vacuum (otherwise the elec- filament and accelerated towards (see box E "Penetration" on page 6). trons would collide with air molecules the anode. The anode has a hole in Specimens for the TEM are usually and be absorbed) so the final image it so that an electron beam in which 0.5 micrometres or less thick. The has to be viewed through a window the electrons are travelling at several higher the speed of the electrons, in in the projection chamber. Another hundred thousand kilometres per other words, the higher the acceler- important difference is that, unlike second (see box G) emerges at the ating voltage in the gun, the thicker glass lenses, electromagnetic lenses other side. The Wehnelt cylinder the specimen that can be studied. are variable: by varying the current which is at a different potential, through the lens coil, the focal length bunches the electrons into a finely (which determines the magnification) focused point (Fig. 7). can be varied. (In the light micro- Other electron sources exist and scope variation in magnification is these are discussed briefly under obtained by changing the lens or by "Additional Techniques" on page 21. mechanically moving the lens).

8 Electron Velocity The higher the accelerating Box G voltage, the faster the electrons. 80 kV electrons have a velocity of Filament 8 150 000 km/second (1.5 x 10 m/s) source which is half the speed of light. This rises to 230 000 km/second for 300 kV electrons (2.3 x 10 8 m/s – more than Filament three-quarters the speed of light).

Wehnelt cylinder

Electron beam

High Voltage Generator Anode

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).

Lamp Electrons “Illumination”

Electro- magnetic Glass lens lens Condenser lens

Box H Electron Density Specimen How many electrons impinge on Electro- magnetic the specimen? A typical electron Glass lens lens beam has a current of about 1 Objective lens picoampere (10 –12 A). One ampere is 1 coulomb/sec. The electron has a charge of 1.6 x 10 –19 coulomb. Therefore approxi- First image mately 6 million electrons per second impinge on the specimen.

Electro- magnetic Glass lens lens Projector lens

Final image Ocular Fluorescent screen

9 What happens in e. The impinging electrons can The electromagnetic lenses the specimen during the cause the specimen itself to Figure 10 shows a cross-section of electron bombardment? emit electrons (these are called an electromagnetic lens. When an Contrary to what might be expected, secondary electrons). electrical current is passed through most specimens are not affected by f. The impinging electrons cause the coils (C), an electromagnetic field the electron bombardment as long as the specimen to emit X-rays is created between the pole pieces it is kept under control. When whose energy and wavelength (P) which form a gap in the mag- electrons impinge on the specimen, are related to the specimen’s netic circuit. By varying the current a number of things can happen: elemental composition. through the coils, the magnification a. Some of the electrons are absorbed g. The impinging electrons cause of the lens can be varied. This is as a function of the thickness and the specimen to emit pho- the essential difference between the composition of the specimen; these tons (or light); this is called magnetic lens and the glass lens. cause what is called amplitude cathodoluminescence. Otherwise they behave in the same contrast in the image. h. Finally, electrons which have lost way and have the same types of b. Other electrons are scattered over an amount of energy because of aberration: spherical aberration (the small angles, depending on the interaction with the specimen magnification in the centre of the composition of the specimen; can be detected by an Energy lens differs from that at the edges), these cause what is called phase Loss Spectrometer which is the chromatic aberration (the magnifica- contrast in the image. equivalent of a prism in light optics. tion of the lens varies with the c. In crystalline specimens, the elec- wavelength of the electrons in the trons are scattered in very distinct In a standard TEM the first two beam) and astigmatism (a circle in directions which are a function of phenomena contribute to the the specimen becomes an ellipse the crystal structure; these cause formation of the normal TEM image in the image). what is called diffraction contrast for non-crystalline (biological) in the image. specimens, while for crystalline Spherical aberration is a very d. Some of the impinging electrons specimens (most non-biological important characteristic which is are reflected (these are called materials), phase contrast and largely determined by the lens backscattered electrons). diffraction contrast are the most design and manufacture. Chromatic important factors in image formation. aberration is reduced by keeping It is necessary to add accessories or the accelerating voltage as stable as peripheral equipment to the basic possible and using very thin speci- microscope in order to exploit the mens. Astigmatism can be corrected additional information which can be by using variable electromagnetic obtained by studying the last five compensation coils. interactions listed above. The condenser lens system focuses the electron beam onto the specimen under investigation as much as necessary to suit the purpose. The objective lens produces an image of the specimen which is then magnified by the remaining imaging lenses and projected onto the 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 varying the strength of the lens immediately Fig. 9 Perhaps below the objective lens, it is the most efficient possible to enlarge the diffraction killer world-wide pattern and project this onto the is the influenza fluorescent screen. In the Tecnai virus. series of TEMs, the objective lens is followed by four lenses: a diffraction lens, an intermediate lens and two projector lenses. To guarantee a high

10 Fig. 8 Cross-section of the column of a modern transmission electron microscope. stability and to achieve the highest lens and the diffraction lens can be on photographic material as light. possible magnification, the lenses in selected from outside the column as Therefore it is only necessary to a modern TEM are water cooled. dictated by circumstances. replace the fluorescent screen with a photographic film in order to On the way from the filament to Observation and recording of record the image. In practice the the fluorescent screen, the electron the image fluorescent screen hinges up to beam passes through a series of The image on the fluorescent screen allow the image to be projected apertures with different diameters. can be observed through a large on the film below. It is also possible These apertures stop those elect- window in the projection chamber to use a TV camera to record the rons which are not required for (some models even have two win- image digitally, making it suitable image formation (e.g. scattered dows). In order to examine fine detail for subsequent enhancement electrons). Using a special holder or to assist correct focusing and analysis. carrying four different apertures, of the image, a special fine grain the diameter of the apertures in focusing screen can be inserted into TV observation is useful for instruc- the condenser lens, the objective the beam and observed through a tional purposes or other occasions high-quality 12x binocular viewer. when group viewing is desired. It can Fig. 10 Cross-section of an also be used for recording dynamic electromagnetic lens. C is an As in any other branch of science, phenomena using a video tape electrical coil and P is the soft a permanent record of what has recorder or as the input signal for iron pole piece. The electron been observed with the eye is an image analysis system. trajectory is from top to bottom. desired. Antony van Leeuwenhoek painstakingly drew what he saw in Fig. 11 Just as the umbrella fabric produces his microscope. The invention of a pattern of light spots, a crystal (here a photography enabled microscopists semiconductor) produces a diffraction pattern to photograph what they saw. which can be photographed (see insert). C C Electrons have the same influence

p p

C C

‑

Box I Difraction When a wave passes through a periodic structure whose periodicity is of the same order of magnitude as the wavelength, the wave emerging is subject to interference which produces a pattern beyond the object. The phenomenon can be observed when ocean waves pass through a regular line of posts or when a street lamp is seen through an umbrella. The street lamp appears as a rectangular pattern of spots of light, bright in the centre and then getting fainter. This is Fig. 12 A special electron caused by diffraction of light by the weave of the umbrella fabric and diffraction technique (HAADF) the size and form of the pattern provide information about the struc- produced this spectacular image. ture (closeness of weave and orientation). In exactly the same way electrons are diffracted by a crystal and the pattern of spots on the screen of the microscope gives information about the crystal lattice (shape, orientation and spacing of the lattice planes) – see Fig. 11.

11 Vacuum Normal atmospheric air pressure is around 760 mm of mercury. This means that the pressure of the atmosphere is sufficient to support a column of mercury The electronics monitor and record the operating 760 mm high. Modern physicists use the Pascal (Pa). To obtain the very high conditions of the microscope. This Normal air pressure = 100 000 Pa; residual pressure resolution of which modern results in a dramatic reduction in in the microscope = 2.5 x 10 –5 Pa. At this pressure, TEMs are capable, the the number of control knobs the number of gas molecules per litre is about 7x10 12 accelerating voltage and the compared with earlier models and and the chance of an electron striking a gas molecule current through the lenses a microscope which is very easy to while traversing the column is almost zero. must be extremely stable. The use. Furthermore it allows special power supply cabinet contains a techniques to be embedded in the number of power supplies whose instrument so that the operator can output voltage or current does not carry them out using the same con- deviate by more than one millionth trols. The PC can be attached to a of the value selected for a particular network to allow automatic back-ups Box J purpose. Such stabilities require very to be made and results to be down- sophisticated electronic circuits. loaded to other workstations. The microscope can always be brought Improved electron optical design up to date by installing new software has made possible a number of or by replacing the computer with Vacuum increasingly more complicated the latest model. Electrons behave like light only when electron-optical techniques. This in they are manipulated in vacuum. turn has created the need for more As has already been mentioned, the ease of operation. Digital electronic whole column from gun to fluores- techniques in general and micro- Fig. 16 cent screen and including the camera processor-based techniques in Chromosome Fig. 17 is evacuated. Various levels of vacuum particular play an important role from a human Woven fabric are necessary: the highest vacuum is in this respect. Modern electron cancer cell. (SEM image). around the specimen and in the gun; microscopes employs a fast, powerful a lower vacuum is found in the PC (personal computer) to control, projection chamber and camera chamber. Different vacuum pumps are used to obtain and maintain these levels. The highest vacuum Fig. 15 Arrangement of attained is of the order of a ten atoms in ceramic. millionth of a millimetre of mercury (see box J).

To avoid having to evacuate the whole column every time a specimen or photographic material or a filament is exchanged, a number of airlocks and separation valves are built in. In modern TEMs the vacuum system is completely automated and the vacuum level is continuously monitored and fully protected against faulty operation.

Fig. 14 Bend contours in a semiconductor.

12 Specimen orientation and manip- an air lock. It is the specimen holder 0.1 micrometre) to permit electrons ulation rod which provides the extra tilt axis to pass through. In a semiconductor With most specimens it is not or the rotation or heating, cooling it is sometimes desired to cut out a sufficient to move them only in the or straining, a special holder being section of material perpendicular to horizontal plane. Although the needed for each purpose. the surface in order to investigate specimen is thin, there is nevertheless a defect. This is done by ion-beam information in the image coming Specimen preparation etching. We shall return to this in from various depths within the speci- A TEM can be used in any branch a later section. men. This can be seen by tilting the of science and technology where specimen and taking stereo photo- it is desired to study the internal A technique which is gaining impor- graphs. In order to define the axis structure of specimens down to the tance in the field of structural biology of tilt, it is necessary to be able to atomic level. It must be possible is that of freezing the specimen and rotate the specimen. Crystalline to make the specimen stable and observing it in the frozen state. specimens need to have a second tilt small enough (some 3 millimetres in axis perpendicular to the first tilt axis diameter) to permit its introduction It is beyond the scope of this booklet in order to be able to orient a part into the evacuated microscope to describe the many specimen of the specimen so as to obtain the column and thin enough (less than preparation techniques that are used required diffraction pattern. These about 0.5 micrometres) to permit nowadays. What can be said is that requirements can be fulfilled in a the passage of electrons. the preparation of a tiny specimen device called a goniometer. for TEM is often relatively complicated Every branch of research has its own and certainly more complicated than The goniometer is a specimen stage specific methods of preparing the the operation of present-day TEMs. designed to provide, in addition to X specimen for electron microscopy. and Y translation of the specimen, tilt In biology for example, tissues are Scanning Transmission Electron about one or two axes and rotation sometimes treated as follows: first Microscopy (STEM) as well as Z movement (specimen there is a chemical treatment to The technique can be applied in a height) parallel to the beam axis. remove water and preserve the tissue SEM but there is much more interest It is usual also to provide for as much as possible in its original in applying it in TEM because of the heating, cooling and straining state; it is then embedded in a lenses beneath the specimen which of the specimen for spe- hardening resin; after the resin has greatly expand the number of possi- cialised experiments in hardened, slices (sections) with an bilities for gathering information. This the microscope. The average thickness of 0.5 micrometres technique was first demonstrated by goniometer is mounted are cut with an instrument called an FEI in 1969. Today most TEMs can be very close to the objec- ultramicrotome equipped with a glass equipped with this facility and what tive lens; the specimen or diamond knife. The tiny sections was once an attachment is now an is actually located in the thus obtained are placed on a speci- integral part of the instrument. objective lens field between men carrier – usually a 3 mm diame- the pole pieces because it is ter copper specimen grid which has The latest TEM from FEI offers a there that the lens aberrations been coated with a structureless resolution of 1 nm at a magnification are smallest and the resolution carbon film 0.1 micrometre thick. of more than 1 million times. is highest. In metallurgy the following method Once a scanning facility has been The goniometer itself provides of preparation is sometimes applied: introduced into the TEM column, it motorised X, Y and Z move- a 3 mm diameter disc of material is possible to take advantage of the ment and tilt about one (thickness say 0.3 mm) is chemically backscattered electrons as well as of axis. The specimen is treated in such a way that in the the secondary electrons which are mounted near the tip of a centre of the disc the material is emitted during specimen bombardment. rod-shaped holder which fully etched away. Around this hole in turn is introduced into there will usually be areas that are the goniometer through sufficiently thin (approximately

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.

13 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 The Scanning compact: although it now contains scanning and display Electron electronics which the basic TEM did not, the lens supplies and Microscope the high voltage supplies are considerably more compact.

All the components of a SEM are vary in sympathy. Both the beam in The most important differences usually housed in one unit (Fig. 5 the microscope and the one in the between TEM and SEM are: page 7). On the right is the electron- CRT are scanned at the same rate a. the beam is not static as in the optical column mounted on top of and there is a one to one relation- TEM: with the aid of an electro- the specimen chamber. In the cabinet ship between each point on the CRT magnetic field, produced by the below this is the vacuum system. screen and a corresponding point on scanning coils, the beam is On the left is the display monitor, the specimen. Thus a picture is built scanned line by line over an the keyboard and a "mouse" for up (see Fig. 18). The ratio of the size extremely small area of the spec - controlling the microscope and the of the screen of the viewing monitor imen’s surface (see box K); camera. All the rest is below the desk (CRT) to the size of the area scanned b. the accelerating voltages are top which gives the whole instrument on the specimen is the magnification much lower than in TEM because its clean appearance. (see box K). Increasing the magni- it is no longer necessary to fication is achieved by reducing the penetrate the specimen; in a Scanning The electron gun at the top of the size of the area scanned on the SEM they range from 200 to Electron Microscope column produces and electron beam specimen. Recording is done by 30.000 volts. In the introduction, the functioning which is focused into a fine spot less photographing the monitor screen c. specimens need no complex of a SEM was compared, with some than 4 nm in diameter on the speci- (or, more usually, a separate high preparations. restrictions, with a reflected light micro- men. This beam is scanned in a resolution screen), making videoprints Box L scope (Fig. 3). Another valid comparison is rectangular raster over the specimen. or storing a digital image. illustrated in Fig. 18. The electron beam trajec- Apart from other interactions at the tories in a SEM are very similar to those in a specimen, secondary electrons are The electron gun TV monitor. Both devices have an electron gun, both are produced and these are detected by The electron gun consists of a fila- evacuated, both have deflection coils. a suitable detector. The amplitude ment and Wehnelt cylinder and is of the secondary electron signal the same as that in a TEM. Also the The electrons produced when the primary beam strikes the specimen varies with time according to the illumination system, consisting of in a SEM are detected and turned into an electrical signal; in the topography of the specimen surface. electron gun, anode and condenser monitor, the primary electrons are turned into light to produce an The signal is amplified and used to lenses is not much different. The final image on the fluorescent screen. cause the brightness of the electron lens focuses the beam onto the beam in a cathode ray tube (CRT) to surface of the specimen to be studied.

14 SEM Magnification length of one line "written" by the electron beam on the monitor SEM magnification = length of one "track" of the electron beam on the specimen

Box K

Filament source Filament Wehnelt aperture Filament

High Voltage Anode Wehnelt aperture Generator

Anode Vacuum Wehnelt aperture Electron Beam Electromagnetic lens

Fig. 18 Schematic Vacuum diagram of a Scan SEM showing Generator the column and Deflection coil how the image is formed on the monitor Electromagnetic lens Deflection coil Vacuum

Signal Specimen Amplifier

Image on A fluorescent screen of TV monitor B C Scanning Electron Microscope SEM In the introduction, the functioning of a SEM was compared, with some restrictions, with a reflected light microscope (Fig. 3). Another valid comparison is illustrated in Fig. 18. The electron beam trajec- tories in a SEM are very similar to those in a TV monitor. Both devices have an electron gun, both are evacuated, both have deflection coils.

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

15 What happens in the specimen the study of (dynamic) electrical during electron bombardment? phenomena in electronic devices When discussing the TEM, it was such as integrated circuits. seen that when electrons strike the specimen several phenomena Magnification and resolution Electron Interactions occur. In general, five of these In the SEM, the magnification is with Matter Box M phenomena are used in an ordinary entirely determined by the electronic In the modern view of matter, SEM (see box M). circuitry that scans the beam over an atom consists of a heavy the specimen (and simultaneously charged nucleus surrounded by a a. The specimen itself emits over the fluorescent screen of the number of orbiting electrons (see box C, page 5). The number of electrons is secondary electrons. monitor where the image appears). equal to the number of protons in the b. Some of the primary electrons nucleus, this is known as the atomic are reflected (backscattered Magnification can be as high as number of the atom. The incoming elec- electrons). 300 000x which is usually more than tron can interact with the nucleus and be c. Electrons are absorbed by the sufficient. In principle the resolution backscattered with virtually undiminished specimen. of a SEM is determined by the beam energy (just as a space probe is deviated by d. The specimen emits X-rays. diameter on the surface of the the gravity of a planet during fly-past). Or e. The specimen sometimes emits specimen. The practical resolution it can interact with the orbiting electrons: photons (= light). how-ever depends on the properties an electron may be ejected from the atom, (this is a secondary electron), and the of the specimen and the specimen atom with one electron short restores the All these phenomena are interrelated pre-paration technique and on many status quo by emitting its excess energy and all of them depend to some instrumental parameters such as in the form of an X-ray quantum or a extent on the topography, the atomic beam intensity, accelerating voltage, light photon. (See box P, page 20) number (see box M) and the chemical scanning speed, distance from the state of the specimen. The number last lens to the specimen (usually of backscattered electrons, secondary referred to as the working distance) electrons and absorbed electrons at and the angle of the specimen each point of the specimen depends surface with respect to the detector. on the specimen’s topography to a Under optimum conditions a resolu- much greater extent than the other tion of 1 nm can be attained properties mentioned. It is for this (see box N). reason that these three phenomena are exploited primarily to image the Observation and recording of specimen’s surface. the image A SEM is usually equipped with two Electron detection image monitors, one for observation Detectors for backscattered elec- by the operator and the other, a trons and secondary electrons are high resolution monitor, is equipped usually either a scintillation detector with an ordinary photo camera or a solid state detector. In the (which can be 35 mm or Polaroid). former case, electrons strike a fluorescent screen which thereupon To facilitate the observation and emits light which is amplified and correct choice of the parameters converted into an electrical signal by mentioned above, FEI SEMs have a photomultiplier tube. The latter an image store in which the image is detector works by amplifying the built up scan by scan and displayed minute signal produced by the at TV speed so that there is a steady, incoming electrons in a semi- flicker-free image on the viewing conductor device. When the monitor. Images are digital and can specimen is not directly connected be stored electronically for subse- to earth but via a resistor, the quent enhancement and analysis. electrons that are not reflected generate a potential difference Image treatment across the resistor. This changing Because the image in a SEM is com- potential difference can be amplified pletely electronically produced, it can and the resulting signal used to be subject to all kinds of treatment produce a third sort of image on using modern electronics. This the monitor. This facility also permits includes contrast enhancement,

16 Fig.19 Forensic investigation of a tungsten car-lamp filament following 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

inversion (black becomes white etc.), mixing of images from various detectors, subtraction of the image from one detector from that produced by a different detector, colour coding and image analysis. All these techniques may be applied if it suits the primary aim of extracting the best possible information from the specimen.

Vacuum In general a sufficiently low vacuum for a SEM is produced by either an oil diffusion pump or a turbomolecular pump in each case backed by a rotary pre-vacuum pump. These combinations also provide reasonable exchange times for specimen, filament and aperture (less than 2 minutes) without the need to use vacuum airlocks. The SEM vacuum system is fully auto- matically controlled and protected against operating failures.

Electronics Many specimens can be brought into It goes without saying that in a SEM the chamber without preparation of the voltages and currents required any kind. If the specimen contains for the electron gun and condenser any volatile components such as Resolution in SEM lenses should be sufficiently stable water, this will need to be removed The resolution (determined by the size to attain the best resolution. by using a drying process (or in of the scanning spot) means that mag- Similarly the stability of the elec- some circumstances it can be frozen nifications of the order of 300 000x can tronic circuitry associated with the solid). Non-conducting specimens be realised. If the viewing screen is, say, detectors should be extremely well will charge up under electron bom- 300 mm wide, then the full width of the controlled. Stabilities of 1 part per bardment and need to be coated scanned area is only 1 micrometre. million are no exception. with a conducting layer. Because a Box N heavy element like gold also gives a All the electronic units are housed good yield of secondary electrons in the microscope console and are and thereby a good quality image, controlled by a personal computer this is the favourite element for (PC) using a keyboard and mouse. coating. In addition it gives a fine grain coating and is easily applied in Application and specimen a sputter coater. The layer required science, for example, silicon wafers preparation to ensure a conducting layer is quite used in IC manufacture, and integrat- A SEM can be used whenever infor- thin (about 10 nm). All in all, the ed circuits themselves which need mation is required about the surface preparation of specimens to be to be studied whilst in operation. In of a specimen. This applies in many investigated by SEM is not as such cases special techniques need branches of science and technology complicated as the preparation of to be employed in order to obtain as well as life sciences. The only specimens for TEM. satisfactory images. Low voltage SEM requirement is that the specimen is one of these techniques. The mod- can withstand the vacuum of the Sometimes it is impossible or ern SEM can be adapted to all these chamber and the electron undesirable to prepare a specimen special techniques, sometimes with bombardment. for SEM: many specimens in forensic the addition of a suitable accessory.

17 Specimen orientation and ESEM manipulation As mentioned above, samples for As has been mentioned, the quality conventional SEM generally have to of the image in a SEM depends on be clean, dry, vacuum compatible the orientation and the distance of and electrically conductive. In recent the specimen from the detectors and years the Environmental Scanning the final lens. The specimen stage Electron Microscope (ESEM) has allows the specimen to be moved in been developed to provide a unique a horizontal plane (X and Y direction) solution for problematic samples. and up and down (Z direction) and Examples of specimens which pose rotated and tilted as required. These problems are wool or cotton tissue, movements are often motorised and cosmetics, fats and emulsions controlled by the PC. (e.g. margarine).

The various SEM models in a range Attempts to view a specimen differ in the size of their specimen containing volatile components chambers allowing various sizes of by placing it in an environmental specimens to be introduced and chamber (see box O) isolated from manipulated. The maximum speci- the main column by one or more men size also determines the price differential pumping apertures used because the larger the specimen to be hampered by the lack of a chamber, the larger the goniometer suitable electron detector which movement needed and the larger the can work in the atmosphere of the pumping system needed to obtain chamber. The Gaseous Secondary and maintain a good vacuum. Electron Detector makes use of cascade amplification (see Fig. 20) The simplest model accepts not only to enhance the secondary specimens of a few centimetres in electron signal but also to produce diameter and can move them 50 mm positive ions (see page 22) which in the X and Y directions. The are attracted by negative charge largest chamber accepts samples on the insulated specimen surface up to 200 mm in diameter and can and effectively suppress move them 150 mm in each direc- charging artefacts. tion. All models allow samples to be tilted up to high angles and rotated Fig. 21 Ice cream in the through 360 degrees. Cryo-SEM: the amount of emulsion in relation to air and water (ice) is an There are special stages for heating, important parameter to study. cooling and straining specimens but because of the wide variety of possible sample sizes, these stages are produced by specialist firms.

Box O Environmental Chamber The pressure-temperature phase

diagram for H2O indicates that true "wet" conditions only exist at pressures of at least 600 Pa at 0°C (environmental microscopists usually 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 at equilibrium with water.

18 Signal Amplification in the ESEM

Detector Primary Electron Beam

Gaseous Cascade environment Amplification

Secondary Electrons Fig. 24 The cocchlea is the hearing organ inside the ear. It is the vibration of Fig. 20 Gas ionisation amplifies the the fibres which transmits secondary electron signal and, in sounds to the brain. a non-conductive specimen, positive ions are attracted to the specimen surface as charge accumu- lates there.

Fig. 22 House mites have been filmed live and moving Fig. 23 in the ESEM. Biomineralisation by a number of organisms illustrates an astounding range of skeletal ulstrastructure. 19 Photos courtesy of Natural History Museum, London. Scanning Transmission More information from Electron Microscopy the specimen If the specimen in a SEM is sufficiently As already mentioned, the electrons transparent for electrons to be bombarding the specimen cause it transmitted through the specimen, to emit X-rays whose energy is these can be collected with a suitably determined by the elemental com- placed detector. This STEM technique, position. When a suitable detector sometimes applied in a SEM is more is placed near the specimen (the often encountered in a TEM because goniometer allows for this), it is pos- use can also be made of the lenses sible to detect very tiny quantities below the specimen. It is discussed of elements. Quantities down to one on page 13. thousandth of a picogram (10 –12 g) or less can thus be detected (see box P). The detector in question is called an Energy Dispersive X-ray Detector (EDX). With this detector, spectra are acquired showing Additional distinctive peaks for the elements present with the peak height indicat- ing the element concentration. This Techniques applies to both SEM and TEM. It has also been mentioned that some of the electrons lose energy when they travel through the specimen. This loss of energy can be measured with an Electron Energy Loss Spectrometer (EELS). This detector is mounted under the projection chamber of the TEM.

X-ray Analysis To return for a moment to the analogy of a person in an unknown darkened room with a fine beam torch (box F). While scanning 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-rays

Box P emitted (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 the specimen 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 it returns 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 the free electrons always present in a material. Each of these transfers results in the release of surplus energy as an X-ray quantum of characteristic wavelength (in the case of outer to inner orbital transitions) or a light quantum (in the case of free electron to outer orbit transfers). We can see that different materials produce different colours by examining a colour television screen with a magnifying glass. Different phosphors produce different colours of light under the impact of the scanning electron beam.

20 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.

The other phenomena that occur when the electron beam goes through the specimen (backscattering of the incident electrons and the emission of secondary electrons) are exploited by using the STEM technique which was discussed on page 13.

Other electron sources The same type of hairpin-shaped tungsten filaments are used in both TEM and SEM. The higher the temperature of the filament, the more electrons are generated by the electron gun and the brighter the image. The lifetime of the filament however is reduced and a practical compromise sets a limit on the brightness of the tungsten gun. Two brighter sources of electrons have become popular in recent years: the

lanthanum hexaboride (LaB 6) gun and the field emission gun (FEG). In the former, a lanthanum hexaboride crystal when heated gives off up to 10x more electrons than tungsten heated to the same temperature. In the latter, electrons are pulled out of X-ray Analysis a very fine pointed tungsten tip by To return for a moment to the analogy of a person in an a very high electric field. Electron unknown darkened room with a fine beam torch (box F). While densities up to 1000 times those scanning the room to determine its topography, the viewer would also be from tungsten emitters can be aware of the colour of objects. Likewise by looking at the wavelengths of X-rays obtained with the FEG. emitted (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 larger number of electrons or the greater electron density provided by these sources permits reductions The impinging electron in the primary beam may cause an electron in an atom of the in beam diameter which results in specimen to be removed from its orbit (if the atom is near the surface, it can become a better resolution in both SEM and secondary electron). The atom is left in an excited state (it has too much energy) and it STEM. It also permits more accurate returns to its stable state by transferring an electron from an outer orbit to replace the one X-ray analysis in both instruments to removed (see box C). Finally the outermost missing electron is replaced by one from the be obtained. The gains are not how- free electrons always present in a material. Each of these transfers results in the release of ever proportional to the increase in surplus energy as an X-ray quantum of characteristic wavelength (in the case of outer to brightness: other factors such as lens inner orbital transitions) or a light quantum (in the case of free electron to outer orbit aberrations also play a role. transfers). We can see that different materials produce different colours by examining a colour television screen with a magnifying glass. Different phosphors produce different Field emission TEMs and SEMs cost colours of light under the impact of the scanning electron beam. more than regular instruments but provide much higher performance.

21 Ion Beam technology material. Examples of applications circuit. Used in defect review, a FIB So far this booklet has been about are the production of thin-film heads unit can improve manufacturing electron microscopy and what useful for reading computer storage media yields by analysing surface and sub- information can be obtained using and the removal of material round surface defects on wafers from the a primary electron beam. However, a defect in a semiconductor device manufacturing (fab) line. It can also electrons are not the only charged to allow the defect to be studied in cross-section critical structures to particles that can be accelerated and a transmission electron microscope provide three-dimensional monitoring focused using electrical and magnetic (see Fig. 13, page 13). of process performance. As a failure fields. It was explained earlier (see analysis tool, it speeds the diagnosis box C) that the atom consists of a How is such a fine beam controlled of IC malfunction and the location positively charged nucleus surrounded and how does the operator know of subsurface defects. As micro- by electrons in orbits. Normally the when the micro-machining is finished? electronic structures get smaller, atom is neutral because there are Quite simply by having a detector to contamination control becomes more equal numbers of protons and elec- observe the secondary electrons important. Some of the new FIB trons. An atom that has lost one produced by the impact of the ions. models feature robotic wafer or more of its outermost electrons By having a scanning electron micro- handling and automatic wafer align- has a positive charge and can be scope as an integral part of the same ment to comply with the cleanroom accelerated, deflected and focussed instrument, the point of impact of requirements of the fab line. in a similar way to its negatively the ion beam can be observed charged cousin the electron. non-destructively and the operator FIBs in Materials Science can control the ion beam process For materials science applications, The difference lies in the mass precisely. These instruments are a FIB unit can cross-section complex (analogous to weight – see box Q). called dual-beam instruments. alloys and layered materials and pre- The electron is almost 2000x lighter pare extremely thin TEM specimens. than the lightest ion and ions can FIBs in IC Manufacturing An ion beam’s submicron milling be up to 250x heavier still. The rela- By precisely focusing a high current has uses in the manufacture and test tively low-mass subatomic electrons density ion beam, a FIB workstation of microelectromechanical systems, interact with a sample and release can not only remove material and such as acceleration detectors. secondary electrons which, when expose defects, but also deposit new Other hard surface applications collected, provide high quality images conducting paths or insulating lay- include micromachining fresnel at near atomic-level resolution. ers, analyse the chemical composi- lenses and sharpening scanning The greater-mass ions dislodge tion of a sample and view the area tunnelling microscope tips. surface particles, also resulting in being modified, all within submicron displacement of secondary ions and tolerances. IC manufacturers find it a FIBs in Biological Science electrons. The ion beam directly cost-effective tool with a versatile Used in the biological sciences, an modifies or mills the surface with range of applications that can reduce ion beam’s precise cut can remove submicron precision. Secondary elec- time-to-market and improve yields. the surface of a cell or prepare a trons and ions are used for imaging During IC design and testing, these cross-section of a virus to allow for and compositional analysis. products speed the fabrication and detailed examination under an By carefully controlling the energy debugging of prototypes. Focused electron microscope. It is opening and intensity of the ion beam, it is ion beam technology gives rapid up new possibilities in the field of possible to carry out very precise access to design problems, then structural biology. micro-machining to produce minute gas injection techniques allow fast components or to remove unwanted cut-and-splice modifications to the

Box Q Weight and Mass Everything has mass. Even a weightless astronaut has the same mass he had before he took off. Weight is the force of gravity acting on a mass. For more information about the latest technology If the gravity is lower, as on the moon, we and innovations in microscopy, feel free to surf to weigh less, but our mass is still the same. That’s why scientists refer to mass rather www.fei.com than weight. Mass is independent of the frame of measurement.

22 g l o s s a r y

Aberration Chromatic aberration Excited atom Nanometre Scintillation detector The deviation from perfect imaging in an See aberration. The magnification of the lens An atom which has lost one of its inner elec- Unit of length (distance). One nanometre Electron detector used in SEM or STEM in optical system which is caused by imperfec- varies with the wavelength of the electrons trons (see also ion) has a higher energy (the (abbreviated to nm) is a millionth of a millime- which electrons to be detected are accelerated tions in the lens or by non-uniformity of the in the beam. extra energy is that needed to remove the tre (10-9 metre) (see box D, page 5). towards a phosphor which fluoresces to pro- electron beam. electron). It seeks to return to its ground state duce light which in turn is amplified by means Cleanroom by rearranging its electrons and emitting the Objective lens of a photomultiplier (q.v.) to produce an Accelerating voltage Room containing extremely low numbers of excess energy as an X-ray quantum. In a TEM, this is the first lens after the specimen electrical signal. The potential difference in an electron gun dust particles used for the manufacture of whose quality determines the performance between cathode and anode over which semiconductors. Fab line of the microscope. In a SEM it is the last Secondary electrons electrons are accelerated. The higher the The semiconductor production line which is lens before the specimen and produces the Electrons originating in the specimen which voltage, the faster the electrons and the more Column located in a clean room (q.v.). extremely fine electron spot with which the are emitted under influence of the primary penetrating power they have. Voltages usually The evacuated electron beam path, the elec- specimen is scanned. beam. range from a few thousand volts up to several tromagnetic lenses and the specimen and FEG hundred thousand. aperture mechanisms are accommodated in Field Emission Gun is defined on page 21. Oil diffusion pump SEM a vertical column (see page 8). Vacuum pump where the pumping action is Scanning Electron Microscopy is defined Airlock FIB produced by the dragging action of a stream on page 7. A chamber within the electron microscope Condenser lens Field ion beam instrument. A beam of ions of oil vapour though an orifice. that can be isolated from the rest to allow the Part of the illumination system between the replaces the beam of electrons. Dual beam Semiconductor detector specimen to be inserted. The airlock is then gun and the specimen designed to concen- instruments also have an electron column. Phase Electron detector used in SEM or STEM in pumped out and the specimen moved into trate the electron beam into a parallel or into Relative position in a cyclical or wave motion. which the electron to be detected causes a the column vacuum. This reduces the amount a finely focussed spot on the specimen (or, in Filament It is expressed as an angle, one cycle or one small change in the semiconductor material of air and other contaminants brought into the case of the scanning electron microscope, Metal wire, usually in the form of a hairpin, wavelength (q.v.) corresponding to 180o. which is amplified by making the semi- the column. Airlocks also facilitate exchange into the objective lens). which, when heated in vacuum, releases conductor part of an electrical circuit. of photographic material and gun emitter. free electrons and so provides the source Phase contrast CRT of electrons in the electron microscope. Image contrast caused by the conversion of Spectrometer Amplitude Cathode ray tube. The screen of a conventional phase differences in light leaving the object Instrument for obtaining a spectrum (q.v.). The maximum value which a periodically TV set or a PC monitor is the front end of a Fluorescent screen into amplitude differences in the image. changing physical magnitude can reach. cathode ray tube. Behind is an electron gun Large plate coated with a material (phosphor) Spectrum For electromagnetic waves the intensity of producing an electron beam, lenses to focus it which gives off light (fluoresces) when Phase diagram A display produced by the separation of a the disturbance is proportional to the square and a scanning system to make the beam scan a bombarded by electrons). In the TEM, the Graph of temperature and pressure showing complex radiation into its component of their amplitude. raster (q.v.) on the screen to produce an image. image is formed on the fluorescent screen the range of each under which agiven mate- wavelengths or energies. (see page 11). rial can exist in the solid, liquid or vapour Amplitude contrast Crystal phase. Spherical aberration Image contrast caused by the removal of A material in which the atoms are ordered Focal length of a lens See aberration. The magnification in the centre electrons (or light) from the beam by absorp- into rows and columns (a lattice) and The distance (measured from the centre of Photomultiplier of the lens differs from that at the edges. tion in the specimen. because of this periodicity, electrons, whose the lens) at which a parallel incident beam is Electronic tube in which light is amplified wavelength (q.v.) is about the same size as brought to a focus. without interference (noise) to produce an Sputter coater Ångström the spacing between atoms, undergoes dif- electrical signal. Instrument for coating a non-conducting Unit of length used in the early days of micros- fraction (q.v.) Focusing specimen with a very thin uniform layer of a copy. 1 Å = 0.1 nm (see box D, page 5). The act of making the image as sharp as pos- Photons conducting element such as gold to make it Detector sible by adjusting the objective lens (q.v.). Discrete packets of electromagnetic radiation. A suitable for examination in a scanning electron Anode A device for detecting particular electrons or light beam is made up of a stream of photons. microscope without danger of it charging up. In an electron gun (see page 8), the negative- photons in the electron microscope. Goniometer ly charged electrons are accelerated towards Specimen stage allowing linear movement of Primary electrons STEM the anode which has a positive charge relative Diffraction the specimen in two or more directions and Electrons in the beam as it leaves the gun and Scanning Transmission Electron Microscopy is to the filament (cathode) from which they Deviation of the direction of light or other rotation of the specimen in its own plane and impinges on the specimen. defined on page 7. emerge. In practice (for ease of construction), wave motion when the wavefront passes the tilting about one or more axes (see page 13). the filament has a high negative charge and edge of an obstacle. Quantum TEM the anode is at earth (ground) potential. Ground state A discrete packet of X-radiation. Transmission Electron Microscopy is defined Diffraction contrast The lowest energy state of an atom. All elec- on page 6. Aperture Image contrast caused by the removal of electrons in their proper places and no charge. Raster A disc of metal with a small hole designed to trons (or light) from the beam by scattering The track of the beam in a SEM or STEM. It is Turbomolecular pump stop those electrons which are not required by a periodic (e.g. crystalline) structure in Ion analogous to eye movements when reading Vacuum pump in which the centrifugal force for image formation (e.g. scattered electrons). the specimen. An atom which has lost one or more of its a book: left to right word by word and down of fast rotating discs forces molecules from the outer electrons. It has a positive charge. the page line by line. axis to the periphery. Astigmatism EDX A lens aberration (q.v.) A circle in the speci- Energy Dispersive X-ray Analysis or Ion getter pump Refraction Vacuum men becomes an ellipse in the image. Spectrometry. An EDX spectrometer makes Vacuum pump in which the movement of Changes in speed (and sometimes direction) Space from which (most) gases and vapours a spectrum (q.v.) of X-rays emitted by the ions in a strong magnetic field drag molecules when light or electrons pass through a have been removed (see box J, page 12). Atom specimen on the basis of their energy. An of gas and embed them in the cathode of specimen. There are many ways of looking at the atom. alternative method of making the spectrum, the pump. Wavelength The most useful one for electron microscopists Wavelength Dispersive Spectrometry (WDS), Refractive index The distance on a periodic wave between is to think of it as consisting of a posi- exists but is not referred to in this book. Lattice The ratio of the speed of light in a vacuum two successive points at which the phase is tively charged nucleus (containing positively Regular array of atoms in a crystal (q.v.). to that in a given medium such as glass, water the same. charged protons and uncharged neutrons) EELS or oil. surrounded by negatively charged electrons Electron Energy Loss Spectroscopy Lens Wehnelt cylinder in discrete orbits. (or Spectrometry) is defined on page 20. In a light microscope, a piece of transparent Resolving power An electrode between the cathode (filament) material with one or more curved surfaces, The ability to make points or lines which are and the anode (earth) in an electron gun Atomic number Electron which is used to alter systematically the direc- closely adjacent in an object distinguishable in which focuses and controls the electron The number of protons in the atomic nucleus. Fundamental atomic particle rotating in an tion of rays of light. In an electron microscope, an image. (see box B, page 5) current in the beam and keeps it constant This number determines the chemical nature orbit around the nucleus of the atom (see a similar effect is achieved on a beam of elec- (see Fig. 7, page 9). of the atom. An atom of iron for example has box C). Free electrons can easily flow in a trons by using a magnetic (or electrostatic) Resolution 29 protons, an atom of oxygen 8 and so on. conductor and can be extracted into a field (see pages 5 & 10). The act or result of displaying fine detail in an Working distance vacuum by heat and or an electrical field. image. (see box A, page 4) The physical distance between the external Backscattered electrons Micrometre metal parts of the objective lens and the speci- Primary electrons which have been deflected Electron microscope Unit of length (distance). One micrometre Rotary pump men surface. This is the space available for by the specimen through an angle generally A microscope (q.v.) in which a beam of (abbreviated to µm) is a thousandth of a mil- Vacuum pump in which the pumping action placing certain electron, X-ray and cathodolu- greater than 180o with little or no loss of energy. electrons is used to form a magnified image limetre or 1000 nm (see box D, page 5). is produced by moving packets of air from minescence detectors. For highest resolution, of the specimen. one side of a rotating cylinder to another by the working distance has to be made as small Binocular viewer Microscope means of an eccentric drum. as possible which leads to compromises. A light microscope built into a TEM for viewing ESEM An instrument designed to extend man's a fine-grain fluorescent screen for critical focuss- Environmental Scanning Electron Microscopy visual capability, i.e. to make visible minute Scanning X-rays ing and astigmatism correction. is defined on page 18. detail that is not seen with the naked eye. Process of investigating a specimen by moving Extremely short wavelength radiation. In the a finely focussed electron beam systematically electron microscope, an X-ray photon (q.v.) Cathodoluminescence Excitation Microtome in a raster over the surface. The same tech- is emitted when an excited atom releases its The emission of light photons by a material The input of energy to matter leading to the Instrument for cutting extremely thin sections nique in the display monitor leads to an image excess energy and returns to its ground state. under electron bombardment (see page 11). emission of radiation. from a specimen prior to examination in the in which every point corresponds to a point microscope. In electron microscopy this is on the specimen. usually referred to as an ultramicrotome.

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