The transmission electron microscope (TEM)

5.4.2 The transmission electron microscope (TEM)

Section 5.2.1 shows that to increase the resolving power of a microscope it is necessary to employ shorter wavelengths. For this reason the electron microscope has been developed to allow the observation of struc- tures which have dimensions down to less than 1 nm. An electron microscope consists of an electron gun and an assembly of lenses all enclosed in an evacuated column. A very basic system for a transmission elec- tron microscope is shown schematically in Figure 5.21. The optical arrangement is similar to that of the glass lenses in a projection-type light microscope, although it is customary to use several stages of magnification in the electron microscope. The lenses are usually of the magnetic type, i.e. current-carrying coils which are completely surrounded by a soft iron shroud except for a narrow gap in the bore, energized by d.c. and, unlike the lenses in a light microscope, which have fixed focal lengths, the focal length can be controlled by regulating the current through the coils of the lens.

Figure 5.21 Schematic arrangement of a basic transmission electron microscope system .

144 Modern Physical Metallurgy and Materials Engineering This facility compensates for the fact that it is difficult

to move the large magnetic lenses in the evacuated column of the electron microscope in an analogous manner to the glass lenses in a light microscope.

The condenser lenses are concerned with collimating the electron beam and illuminating the specimen which is placed in the bore of the objective lens. The function of the objective lens is to form a magnified image of up to about 40ð in the object plane of the intermediate, or first projector lens. A small part of this image then forms the object for the first projector lens, which gives

a second image, again magnified in the object plane of the second projector lens. The second projector lens is capable of enlarging this image further to form a final image on the fluorescent viewing screen. This image, magnified up to 100 000ð may be recorded on a photographic film beneath the viewing screen. A

to the electron gun, according to the approximate relation

⊲ 1.5/V⊳nm

and, since normal operating voltages are between 50 Figure 5.22 Schematic diagram of a basic scanning electron microscope (courtesy of Cambridge Instrument Co.) .

to 0.0035 nm. With a wavelength of 0.005 nm if one comparable to that for optical lenses, i.e. 1.4, it would

the specimen surface. The image observed on the oscil-

be possible to see the orbital electrons. However, mag- loscope screen is similar to the optical image and the netic lenses are more prone to spherical and chromatic

specimen is usually tilted towards the collector at a low aberration than glass lenses and, in consequence, small

angle ⊲<30 ° ⊳ to the horizontal, for general viewing. apertures, which correspond to ˛-values of about 0.002

As initially conceived, the SEM used backscat- radian, must be used. As a result, the resolution of

tered electrons (with E ³ 30 kV which is the inci- the electron microscope is limited to about 0.2 nm. It

dent energy) and secondary electrons (E ³ 100 eV) will be appreciated, of course, that a variable magni-

which are ejected from the specimen. Since the sec- fication is possible in the electron microscope without

ondary electrons are of low energy they can be bent relative movement of the lenses, as in a light micro-

round corners and give rise to the topographic con- scope, because the depth of focus of each image, being

trast. The intensity of backscattered (BS) electrons is inversely proportional to the square of the numerical

proportional to atomic number but contrast from these aperture, is so great.

electrons tends to be swamped because, being of higher energy, they are not so easily collected by the normal collector system used in SEMs. If the secondary elec-

trons are to be collected a positive bias of ³200 V is The surface structure of a metal can be studied in the

5.4.3 The scanning electron microscope

applied to the grid in front of the detector; if only the TEM by the use of thin transparent replicas of the sur-

back-scattered electrons are to be collected the grid is face topography. Three different types of replica are

biased negatively to ³200 V. in use, (1) oxide, (2) plastic, and (3) carbon replicas.

Perhaps the most significant development in recent However, since the development of the scanning elec-

years has been the gathering of information relating to tron microscope (SEM) it is very much easier to study

chemical composition. As discussed in Section 5.4.1, the surface structure directly.

materials bombarded with high-energy electrons can

A diagram of the SEM is shown in Figure 5.22. The give rise to the emissions of X-rays characteristic of electron beam is focused to a spot ³10 nm diameter

the material being bombarded. The X-rays emitted and made to scan the surface in a raster. Electrons from

when the beam is stopped on a particular region of the specimen are focused with an electrostatic elec-

the specimen may be detected either with a solid- trode on to a biased scintillator. The light produced is

state (Li-drifted silicon) detector which produces a transmitted via a Perspex light pipe to a photomulti-

voltage pulse proportional to the energy of the incident plier and the signal generated is used to modulate the

photons (energy-dispersive method) or with an X-ray brightness of an oscilloscope spot which traverses a

spectrometer to measure the wavelength and intensity raster in exact synchronism with the electron beam at

(wavelength-dispersive method). The microanalysis of

The characterization of materials 145 materials is presented in Section 5.4.5. Alternatively,

are difficult to study because they cannot be etched. if the beam is scanned as usual and the intensity of the

The intensity of back-scattered electrons is also sen- X-ray emission, characteristic of a particular element,

sitive to the orientation of the incident beam relative is used to modulate the CRT, an image showing the

to the crystal. This effect will give rise to ‘orienta- distribution of that element in the sample will result. X-

tion’ contrast from grain to grain in a polycrystalline ray images are usually very ‘noisy’ because the X-ray

specimen as the scan crosses several grains. In addi- production efficiency is low, necessitating exposures a

tion, the effect is also able to provide crystallographic thousand times greater than electron images.

information from bulk specimens by a process known Collection of the back-scattered (BS) electrons with

as electron channelling. As the name implies, the elec-

a specially located detector on the bottom of the lens trons are channelled between crystal planes and the system gives rise to some exciting applications and

amount of channelling per plane depends on its pack- opens up a completely new dimension for SEM from

ing and spacing. If the electron beam impinging on a bulk samples. The BS electrons are very sensitive to

crystal is rocked through a large angle then the amount atomic number Z and hence are particularly impor-

of channelling will vary with angle and hence the BS tant in showing contrast from changes of composi-

image will exhibit contrast in the form of electron tion, as illustrated by the image from a silver alloy

channelling patterns which can be used to provide crys- in Figure 5.23. This atomic number contrast is par-

tallographic information. Figure 5.24 shows the ‘orien- ticularly effective in studying alloys which normally

tation’ or channelling contrast exhibited by a Fe–3%Si

b 2 µ m Figure 5.23 Back-cattered electron image by atomic number contrast from 70Ag–30Cu alloy showing (a) ˛-dendrites C

a 20 µ m

eutectic and (b) eutectic (courtesy of B. W. Hutchinson) .

50 µ m

Figure 5.24 (a) Back-scattered electron image and (b) associated channelling pattern, from secondary recrystallized Fe–3%Si (courtesy of B. W. Hutchinson) .

146 Modern Physical Metallurgy and Materials Engineering specimen during secondary recrystallization (a process

be obtained. In consequence, the thickness of the metal used for transformer lamination production) and the

specimen has to be limited to below a micrometre, channelling pattern can be analysed to show that the

because of the restricted penetration power of the new grain possesses the Goss texture. Electron chan-

electrons. Three methods now in general use for nelling occurs only in relatively perfect crystals and

preparing such thin films are (1) chemical thinning, hence the degradation of electron channelling patterns

(2) electropolishing, and (3) bombarding with a beam may be used to monitor the level of plastic strain, for

of ions at a potential of about 3 kV. Chemical thinning example to map out the plastic zone around a fatigue

has the disadvantage of preferentially attacking either crack as it develops in an alloy.

the matrix or the precipitated phases, and so the The electron beam may also induce electrical effects

electropolishing technique is used extensively to which are of importance particularly in semiconductor

prepare thin metal foils. Ion beam thinning is quite materials. Thus a 30 kV electron beam can generate

slow but is the only way of preparing thin ceramic some thousand excess free electrons and the equiv-

and semiconducting specimens. alent number of ions (‘holes’), the vast majority of

Transmission electron microscopy provides both which recombine. In metals, this recombination pro-

image and diffraction information from the same small cess is very fast (1 ps) but in semiconductors may be a

volume down to 1 µ m in diameter. Ray diagrams for few seconds depending on purity. These excess current

the two modes of operation, imaging and diffrac- carriers will have a large effect on the limited conduc-

tion, are shown in Figure 5.25. Diffraction contrast 1 tivity. Also the carriers generated at one point will

is the most common technique used and, as shown diffuse towards regions of lower carrier concentration

in Figure 5.25a, involves the insertion of an objective and voltages will be established whenever the carriers

aperture in the back focal plane, i.e. in the plane in encounter regions of different chemical composition

which the diffraction pattern is formed, to select either (e.g. impurities around dislocations). The conductiv-

the directly-transmitted beam or a strong diffracted ity effect can be monitored by applying a potential

beam. Images obtained in this way cannot possi- difference across the specimen from an external bat-

bly contain information concerning the periodicity of tery and using the magnitude of the resulting current

to modulate the CRT brightness to give an image of conductivity variation.

The voltage effect arising from different carrier con- centrations or from accumulation of charge on an insu- lator surface or from the application of an external electromotive force can modify the collection of the emitted electrons and hence give rise to voltage con- trast. Similarly, a magnetic field arising from ferromag- netic domains, for example, will affect the collection efficiency of emitted electrons and lead to magnetic field contrast.

The secondary electrons, i.e. lightly-bound electrons ejected from the specimen which give topographical information, are generated by the incident electrons, by the back-scattered electrons and by X-rays. The resolution is typically ³10 nm at 20 kV for medium atomic weight elements and is limited by spreading of electrons as they penetrate into the specimen. The back-scattered electrons are also influenced by beam spreading and for a material of medium atomic weight the resolution is ³100 nm. The specimen current mode is limited both by spreading of the beam and the noise of electronic amplification to a spatial resolution of 500 nm and somewhat greater values ³1 µ m apply to the beam-induced conductivity and X-ray modes.

Figure 5.25 Schematic ray diagrams for (a) imaging and (b) diffraction .