Basic Laboratory

Materials Science and Engineering

Scanning Electron Microscopy (SEM)

M109

Stand: 26.10.2017

Aim: Explanation of the basics of scanning electron microscopy using surfaces of fractures and

microstructures as an example.

Comparison and analysis of differing fracture behaviour of metallic and ceramic materials by

means of scanning electron microscopy.

Contents

1. Introduction 2

1.1. Limitations of light-optical microscopy 2

2. Basics 3

2.1. Microscopy by employing electron beams 3 2.2. Interaction between electrons and specimen 4 2.3. Scanning electron microscope (SEM): Design and function 7 2.4. Interrelationship between depth of focus, resolution, and magnification 12

2.5. Fractographic analysis 13 2.6. Transgranular and intercrystalline fracture 13

3. Technological significance 15

3.1. Assessment of damage 15

3.2. Quality assurance and quality control 15 3.3. Medical examination and biological investigation 15

4. Testing 16

5. Evaluation of testing 16

6. Questions 17

7. Bibliography 17

8. Anhang 18

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1. Introduction

Minor defects often result in considerable damage. Small fractures or cracks in materials can have

disastrous effects on the stability of buildings, tools, etc. Once an accident has happened, its causes

have to be found. A microscope examination of the fracture surface shows whether a material defect

or a processing defect has caused the fracture. Light-optical and electron-optical microscopes are

used for this purpose. Electron microscopes are advantageous in that a high degree of magnification

as well as an excellent depth of focus (Fig.1) can be achieved. As a rule, surfaces of fracture are

very rough so that a light-optical microscope often cannot produce a sufficiently clear enlargement

of the relevant image section.

Fig. 1: Photo of blood corpuscles taken by means of a) a light-optical microscope and b) an electron-optical

microscope (same magnification).

1.1. Limitations of light-optical microscopy

The amount of information a micrograph can provide is dependent on resolution. The maximum

resolution that can be achieved using a microscope means the smallest interval distinguishable

between two adjacent points. Any magnification exceeding such maximum would not make sense

since further information cannot be provided.

The maximum resolution mainly depends on the wavelength of the radiation selected for the image.

Beams entering the lens- and aperture system of the microscope produce overlapping diffraction

patterns for each object point. The distance r1 between two diffraction maxima must exceed full

width half maximum (FWHM), otherwise the diffraction maxima cannot be discerned as being

separate (Fig. 2). According to a simple rule found by Rayleigh, distinction is possible when the

maximum of the zero order coincides with the first minimum of the second diffraction pattern. The

distance between the two first minima d1 is inversely proportional to the diameter of the aperture.

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Fig. 2: Minimal distance between two diffraction maxima still projected separately

Diffraction patterns are dependent on the wavelength λ, on the index of refraction of the surrounding

medium µ, and on the angle α formed by the optical axis and the edge beam, which can only just

pass through the aperture. For r1 results:

𝑟1 =𝑑1

2=

0,61𝜆

𝜇 sin 𝛼 (1)

The product µ sin α is referred to as numeric aperture.

Thus, high resolution can be achieved by a short wavelength, a high index of refraction of the

surrounding medium, and a short distance to the sample (hereinafter also referred to as "specimen")

(wide angle α). When normal light-optical microscopes are used, the surrounding medium is air

(µ = 1) and the distance between sample and lens cannot be decreased at discretion. For this reason,

the maximum resolution with regard to wavelengths of visible light (400 - 700 nm) is limited to

about 200 nm, and any degree of magnification beyond 1000 would not make sense.

2. Basics

2.1. Microscopy by employing electron beams

(Hereinafter the term "electron beam is also referred to as "probe"). If electrons are used instead

of optical waves, much smaller wavelengths can be achieved. The wavelength can be varied

depending on the voltage set to accelerate the electrons towards the sample. The velocity v of a

single electron can almost reach the velocity of light c. In that case, relativistic corrections become

necessary. The electron mass changes according to the following equation:

𝑚 =𝑚𝑒

√1−(𝑣

𝑐)

2 (2)

me is the rest mass of the electron.

The deBroglie relation determines the interrelationship between wavelength and momentum.

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𝜆 =ℎ

𝑝=

ℎ

𝑚𝑣 (3)

h is Planck's quantum (constant of action). The energy transmitted to an electron eV can be equated

with the energy of relativistic mass changes:

𝑒𝑉 = (𝑚 − 𝑚𝑒)𝑐2 (4)

By means of these three equations the dependence of wavelength on accelerating voltage can be

derived:

𝜆2 =ℎ2

(1,2𝑒𝑉𝑚𝑒+𝑒2𝑉2

𝑐2 ) (5)

𝜆 = √1,5

(𝑉+10−6𝑉2) nm (6)

An accelerating voltage of e.g. 20 kV results in 8.6E-3 nm = 8.6 pm, whereas at 500 kV only

1.4E-3 nm are reached.

Since electrons would be too strongly scattered in air, a high vacuum is required in an electron

microscope. In addition, the samples to be tested have to be electrically conductive, otherwise they

would be overcharged with electrons during irradiation. For this reason, conductors and insulators

of inferior quality have to be coated with a conductive layer of metal or carbon prior to microscopic

investigation.

2.2. Interaction between electrons and specimen

Electrons in scanning electron microscopes are accelerated at voltages in the range of 2 to 40 kV.

An electron beam < 0.01 µm in diameter is focused on the specimen. These fast primary electrons

(PE) interact in various ways with the surface layers of the specimen. The zone, in which such

interaction occurs, and in which different signals are produced, is called "interaction volume" or

"electron – diffusion cloud". The size of the interaction volume is proportional to the energy of

primary electrons, its shape is determined dependent upon scattering processes by the mean atomic

number. Secondary electrons (SE), back scattered electrons (BSE), and absorbed electrons are

produced, flowing off as specimen current. In addition, X-rays, Auger electrons, and

cathodoluminescence are produced (Fig. 3).

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Fig. 3: Interaction volume

R: The range of primary electrons (PE);

T: Escape level for back scattered electrons (BSE)

Resolution limit of BSE ≈ ½ R

Resolution limit of X-radiation ≈ interaction volume

Resolution limit of secondary fluorescence > interaction volume Secondary electrons (SE)

Although secondary electrons are produced in the entire interaction volume, they can only escape

from surface layers (metals: max. 5 nm, insulators: max. 50 nm, Fig. 4: Escape level t). Secondary

electrons are very slow, their escape energy is ≤ 50 eV. Approximately half of all SE are produced

very near to the point of impact of PE (SE1). Owing to back scattered electrons (BSE) diffusing in

the specimen material, SE are also produced at a distance in the range of 0.1 to some µm to the point

of impact (SE2). Back scattered electrons reacting with the wall of the specimen chamber are the

third source of SE. This reaction process causes background radiation and thus a smaller degree of

contrast, which, however, can partly be increased again electronically. (Fig. 4)

Fig. 4: Production of SE and BSE

The best lateral point resolution can be achieved by means of SE1. The signal can be intensified

when the primary beam hits the samples at an angle of < 90°; this is referred to as inclination

contrast. If radiation can penetrate specimen structures such as tips, fibres, or edges, the images of

these structures will be very bright (edge contrast) owing to a high SE yield.

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The SE signal, comprising all essential information on topography, produces electron-micrographs

of high resolution.

Fig. 5: SE yield δ is dependent on the atomic number Z

2.2.1. Back scattered electrons (BSE)

The electrons escaping from the surface of the sample and having an energy of ≥ 50 eV are referred

to as back scattered electrons (BSE). BSE are produced in the entire interaction volume at a larger

distance to the point of impact of PE (Fig. 4). When atomic numbers are low, the escape

level T is approx. half the range R; at accelerating voltages > 20kV and when atomic numbers are

high, the escape level T is lower. The higher the PE energy and the smaller the atomic number of

the specimen material, the more extends the area of production of BSE and the lower the achievable

resolution. However, the dependence on the atomic number of the sample material is an advantage

in that, apart from the topography contrast, a material contrast can be made visible. Moreover, owing

to higher energy charging occurs less frequently than in case of SE.

Fig. 6: RE yield η is dependent on the atomic number Z

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2.3. Scanning electron microscope (SEM): Design and function

The surface of a specimen is brought into the focus of electron beams. The signals produced control

the brightness of a screen tube such that an image of the surface of the sample appears. Fig. 7

illustrates the basic design of a scanning electron microscope.

Fig. 7: Basic design of an SEM

In a scanning electron microscope the signal-producing system and the signal-processing system

operate independently.

2.3.1. Signal-producing system

The signal-producing system (see Fig. 7 to the left and Fig. 8) is to generate a probe of the smallest

diameter possible and of maximum brightness when hitting the surface of the specimen. It consists

of an electron gun, (cathode – Wehnelt cylinder – anode), lens system (lenses, apertures, beam

deflection coils and stigmator coils) and the specimen chamber.

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Fig. 8: Course of the probe in the signal-producing system

At least two pumps are required to reduce pressure to a vacuum. A vane-type rotary pump produces

a pre-vacuum of approx. 10-3 mbar. Either a turbomolecular pump or an oil diffusion pump maintain

the operation vacuum of at least 10-6 mbar in the column and in the chamber. Dependent upon the

type of cathode used, a third pump, the ion getter pump, may be operated.

For further information please refer to technical literature!

2.3.1.1. Generation of the probe

In the field of electron microscopy free electrons are produced either by thermal emission or by

means of field emission (> 109 V/m). As such tungsten filaments or LaB6-crystals serve as cathode

for thermal emission. However most modern SEM systems use field emission guns (FEG) since

those exhibit a much better coherency and therefore smaller beam diameter than traditional sources.

The electron emitter consists of a three-electrode arrangement (Fig. 9).

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Fig. 9: Basic design of an electron gun.

In case of thermal emission an electric heating current heats up the filament on the negative potential

(cathode) opposite the anode. The relevant accelerating voltage accelerates the emitted electrons

towards the anode where they pass through a gap to enter the microscope column. The filament is

situated in a Wehnelt cylinder so that the electrons can be focused. The potential of the Wehnelt

cylinder is slightly more negative than that of the filament. The Wehnelt cylinder focuses the

electrons by emitting them from one point. This point, also referred to as virtual electron emitter or

as cross-over, can be shifted by a variable bias resistance. The Wehnelt cylinder does not only adjust

the diameter of the cross-over but the number of electrons leaving the cathode (emission current).

2.3.1.2. Lens system

Magnetic lenses and various apertures focus the electron beam. When an electron with the

charge e and the velocity v reaches a magnetizing field of the intensity B, force F acts on the electron

such that the force vector F is perpendicular to the velocity vector v and the magnetizing field vector

B.

𝐹 = 𝑒(𝐵 ∧ 𝑣) (7)

Fig.10: Force vectors of a charge moving in the magnetizing field

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The magnetizing field of an electromagnetic lens can be divided into an axial and a radial part. The

axial part, running in parallel to the direction of movement of the electron, does not influence the

electron. The radial part, however, forces the electron to take a helix-path by the force

(Brad e v). Thus, due to such circular component the velocity vector is influenced by the axial

magnetizing field (Bax e vzirk). As a result the radius of the helix-path is becoming ever smaller.

The electromagnetic lenses of an SEM produce an image reduced in diameter of the cross-over in

the gun on the surface of the specimen. Two condenser lenses (Fig. 8) reduce the diameter of the

electron beam (the diameter of the electron beam is also referred to as "probe size") from d0 to d2.

The higher the lens current, the smaller the diameter (Fig. 11).

Fig. 11: Schematic illustration of the probe a) low, b) high lens current

The smaller the probe size, the smaller the portion of electrons reaching the specimen since not all

electrons leaving lens 1 can pass through lens 2. (Fig. 11): 12 αα < . Increasing noise results, limiting

the resolving power of the SEM.

The third lens, i.e. the objective lens, focuses the probe towards the specimen.

Lens holes which are not absolutely symmetrical mechanically, whose magnetizing fields are

inhomogeneous, and whose pole piece holes are contaminated, and contaminated apertures in

particular, will result in an elliptical probe producing "axial astigmatism". The surface of the

specimen cannot be brought into focus accurately since an elliptical probe will produce a distorted

image of specimen structures during the focusing process. A corrective magnetic field, required to

recover the rotational symmetry of the probe, is to be produced by a stigmator. A stigmator consists

of 2 times 4 coils arranged centrically towards the optical axis.

2.3.1.3. Scanning system / magnification

Beam deflection coils in a scanning generator (Fig. 7) scan the specimen surface by means of the

primary electron beam for a certain period of time; beam deflection coils are installed in the pole

piece duct of the objective lens. Simultaneously a cathode ray scans the screen of a monitor.

Due to the principle of scanning, an SEM lineagraph consists of many spots. The beam deflection

coil can be used to produce horizontal and vertical deflections by means of the electron beam.

Horizontal deflection generates a line whose position is determined by vertical deflection.

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Scanning speed depends on the time set for the scanning of one line and on the number of lines per

scanning process ("frame").

In order to increase magnification the current in the beam deflection coils must be increased. This

involves a reduction of the scanning pattern produced on the specimen, whereas the size of the

image displayed remains unchanged. Thus, magnification results from the ratio between the edge

length of the screen and the edge length of the zone scanned on the specimen (Fig. 7). If, e.g., a zone

of 1 mm x 1 mm is scanned, while the edge length of the screen is 30 cm, the degree of magnification

will be 300-fold.

2.3.2. Signal-processing system

Fig.12: System of signal processing

Due to the principle of scanning, signals - e.g. secondary electrons - are successively produced by

each object point. After registration by means of a detector an electrical signal, the video signal, is

generated and amplified by a preamplifier and by video amplification. The video signal, such

amplified, modulates the cathode ray deflected simultaneously to the primary electron beam such

that an image appears on the monitor. In this way, there is a spot-by-spot-correlation between the

signal level of an object point and the brightness of the corresponding display spot.

The amplitudes of the signal can be displayed as Y-modulation.

The modulation of object signals to successive electrical signals is advantageous in that the latter

can be modified in order to optimise image information (brightness, contrast etc.).

2.3.3. Detectors

Detectors connect the signal-producing and the signal-processing system of an SEM. They convert

the signals produced (electrons) into electrical signals. As a rule, each signal (secondary electrons,

back scattered electrons, X-rays) requires a special detector.

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Fig. 13: Everhart - Thornley – Detector

K: Collector, S: Scintillator, LL: Optical fibre, V: Preamplifier, PM: Photo multiplier

The most widely used detector of secondary electrons is the Everhart-Thornley-Detector (Fig. 13).

A driving potential of e.g. +300 to 400 V is applied between the specimen and the collector for the

intake of secondary electrons of low energy. Between collector and scintillator, high voltage of

10 kV is applied, accelerating the SE to come forcibly into contact with the scintillator. The

scintillator consists either of a glass plate coated with luminescent powder (phosphor compound) or

of a YAG- or YAP- monocrystal. The photons produced pass via the optical fibre to the photo

multiplier. The photons release electrons at the photocathode of the multiplier. The multiplier

voltage accelerates these electrons towards the dynodes where they produce cumulatively a multiple

of electrons.

BSE are also detected. If an image is to be produced by BSE only, no SE must be present; the

collector must be switched off or a negative voltage must be applied to repulse the SE.

Scintillator detectors (Robinson detector) or semiconductor detectors are especially in use to detect

BSE.

2.4. Interrelationship between depth of focus, resolution, and magnification

Great depth of focus is required for an analysis of fracture surfaces. The term "depth of focus"

describes that zone of object positions, in which a change in focus cannot be perceived through the

sight. Fig. 14 shows the interrelationship between the depth of focus and the point resolution X or

magnification.

At a 1000-fold magnification, the light-optical microscope can only project a depth of approx.

0.2 µm, whereas 100 µm can be achieved by means of an electron microscope.

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Fig. 14: Interrelationship between depth of focus, point resolution, and magnification: Light-optical microscope

and scanning electron microscope.

2.5. Fractographic analysis

Any fracture of a body starts with the formation and propagation of cracks in submicroscopic,

microscopic, and eventually macroscopic dimensions. The structure of the fracture surface varies

depending on the composition and microstructure of the material in question as well as on other

conditions given during the process of breaking, such as temperature and stress state. Thus an

analysis of the fracture surface can provide essential information on the cause of fracture.

2.6. Transgranular and intercrystalline fracture

Metals are composed of a multitude of small crystallites formed when the melt is cooling down.

Atoms are very regularly arranged in the crystallites. At the boundary between two crystallites the

order of the crystal lattice is disarranged. These crystal boundaries show two-dimensional lattice

defects. As the atoms at the crystal boundaries are not in an equilibrium state, the crystal boundaries

in engineering materials are in general of higher strength than those of regular crystallites. They

form a barrier to the propagation of small cracks so that - at room temperature and at lower

temperatures - cracks normally run through the grains. This process is referred to as transgranular

fracture (Figs. 15 a and c).

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Fig. 15: a) Transgranular cleavage fracture, b) intercrystalline cleavage fracture c) dimple fracture

(transgranular), d) fatigue fracture

Various types of separation occur in brittle and tough material. In the case of transgranular brittle

fracture, crystallites are split without deformation (Fig. 15a). If the material is tough, sliding

processes occur in crystallographically preferred planes; microvoids and cavities form themselves.

The cavities widen, any metal remaining in between propagates and narrow edges are formed. The

resulting microstructure is called dimple fracture, see Fig.15c).

Cyclic straining (cf. Test M512) leads to transgranular cracks showing fracture paths and fatigue

striation (Fig. 15d).

At higher temperatures atoms move more easily, and the strength of crystal boundaries is reduced.

The path of fractures that have occurred after a long time of load at high temperatures runs along

crystal boundaries. Such fractures are referred to as intercrystalline fractures (Fig. 15b); they do not

occur at room temperature unless crystal boundaries have been weakened or embrittled due to

precipitation or impurities. In particular the influence of hydrogen can also lead to intercrystalline

fractures.

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3. Technological significance

3.1. Assessment of damage

As has been mentioned in the introduction, scanning electron microscopy is essential to an

assessment of causes of damage due to fracture. Microscopic analysis has made it possible to

distinguish between material defects and processing defects. Thus, considerable legal consequences

may result with regard to liability for damage.

Slag inclusion in welding seams, e.g., cannot be clearly identified using a light-optical microscope

whereas, owing to the fact that the conductivity of metal differs considerably from that of slag,

contrasts become clearly visible when an electron microscope is used. In connection with X-ray

analysis, such slag inclusion can be clearly identified.

Heavy expenses occur to insurance companies, both in the commercial and in the private field, for

the evaluation of damage due to corrosion of water pipes etc. The cause of corrosion can be

determined by electron-microscopic investigation such that e.g. defective connections between

different metals can be located.

3.2. Quality assurance and quality control

Electron microscopes are well suitable for controlling and ensuring e.g. a constant surface quality

or a defined roughness of workpieces. However, some disadvantages must be mentioned here, too.

In practice, electron microscopes cannot be integrated directly in a production line as they require

high-vacuum for operation so that usually investigation can only be made by taking samples.

Apart from vacuum resistance, the electric conductivity of the specimen surface is of utmost

importance. Although electric conductivity is easy to achieve by coating even relatively sensitive

organic material with metal or carbon, there are high expenses involved so that a wide application

of this method would be disadvantageous. Meanwhile the development of atomic force microscopy

has become a competitive alternative as far as topographic investigation is concerned: The forces of

attraction acting between the specimen surface and the measuring prod are determined in atomic

dimensions.

3.3. Medical examination and biological investigation

Particularly in the field of medical and biologic research, electron microscopy has enormously

contributed to improve examination and investigation. Here, low-vacuum units have been

developed, enabling the investigation of non-conducting, hydrous, organic preparations. The great

depth of focus is not as important as the fact that the 1000-fold magnification achieved by an optical

microscope can be exceeded.

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4. Testing

In this lab experiment you are to analyse the fracture behaviour of a metallic NiTi alloy in order to

give an example of scanning electron microscopy as frequently used in practice. Furthermore a

ceramic sample is to be investigated and subsequently characterized. Finally a sample containing

ZnO tetrapods is to be analysed and discussed. Remember the difficulties that might arise when

using SEM for observations on insulating materials.

Do not operate the electron microscope unless the adviser is present. Follow the instructions to the

letter!

For purposes of documentation and later evaluation store and print typical images of the specimen

you have tested.

Carefully note down in writing information obtained and experience gained during the laboratory

course!

Specimen The adviser will hand over the specimen to you.

5. Evaluation of testing

Prior to giving your results, describe in detail the theory of the electron microscope. Describe

experience gained and information obtained from the specimen, referring to theory. If necessary,

consult technical literature.

Describe the fracture behaviour of the NiTi as well as the characteristics of the ceramic and ZnO

specimen. Determine - to the extent possible - the average dimensions of characteristic features such

as size of crystallite, size of dimple, size of grains, dimensions of tetrapods. Briefly describe the

differing material properties or conditions of fracture, in the specimen you observed.

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6. Questions

How can the range of usage of a light-optical microscope be extended?

Which are the advantages/ disadvantages of transmission electron microscopy (TEM) in

comparison to scanning electron microscopy (SEM)?

Which are the scattering types that can occur at atoms in case of accelerated electrons? Give

some examples!

Field emission SEM: Describe the principle! Which are the advantages/disadvantages of this

method?

7. Bibliography

Macherauch, Eckard: Praktikum in Werkstoffkunde (Laboratory course in

metallography); Publishing house: F. Vieweg & Sohn,

Braunschweig / Wiesbaden 1992

Flegler, Heckman, Klomparens: Elektronenmikroskopie (Electron microscopy);

Publishing house: Spektrum Akademischer Verlag,

Heidelberg 1995

L. Reimer, G. Pfefferkorn: Rasterelektronenmikroskopie (Scanning electron

microscopy); Publishing house: Springer-Verlag,

Berlin 1977

L. Engel, H. Klingele: Rasterelektronenmikroskopische Untersuchungen von

Metallschäden (Scanning electron microscopy used for

the inspection of damage to metal), Publishing house:

Gerling, Köln 1982

W. Schatt.: Einführung in die Werkstoffwissenschaft (Introduction to

materials science), Publishing house: Deutscher Verlag

für Grundstoffindustrie, Leipzig 1972

E. Hornbogen, B. Skrotzki Werkstoffmikroskopie (Materials microscopy),

Publishing house: Springer Verlag, Berlin 1993

M109: SEM

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8. Anhang

Abbildungen des Versuchs M109 Abb. 3

Primärelektronenstrahl primary electron beam

1 mm Auger Elektronen Auger electrons, 1 mm

Rückstreuelektronen back scattered electrons

Charakterische Röntgenstrahlung characteristic X-radiation

Röntgenstrahlung: Kontinuum X-radiation: continuum

Sekundäre Fluoreszenz (Kontinuum und

charakteristische Röntgenstrahlung)

Secondary fluorescence (continuum and

characteristic X-radiation)

RE-Auflösung BSE resolution

Röntgenstrahlung, Auflösung X-radiation, resolution

Abb. 4

Wandung wall

zum Detektor towards detector

Probenoberfläche specimen surface

Austrittstiefe escape level

Reichweite range

Elektronendiffusionswolke electron diffusion cloud

Abb. 5

Ordnungszahl atomic number

Abb. 7

Elektronenstrahl electron beam

Kondensorlinsen condenser lenses

Ablenkspulen beam deflection coils

Objektivlinse objective lens

Probe specimen

Verstärker amplifier

Rastereinheit scanning unit

Signaldetektor signal detector

Sichtbildschirm screen

Abb. 8

Kathode cathode

Wehneltzylinder Wehnelt cylinder

Anode anode

Sprayblende dispersion aperture

Kondensorlinse condenser lens

Stigmator stigmator

Objektiv objective

Bildfeinverschiebung fine-adjustment of micrograph

Aperturblende aperture diaphragm

Probe specimen

Abb. 9

Elektronenkanone electron gun

Hochspannungskabel high-voltage cable

Keramikisolator ceramic insulator

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Wolframdrahtfaden tungsten filament

Vakuumdichtung vacuum seal

Abschirmung oder Wehneltzylinder shielding or Wehnelt cylinder

Anode anode

Abb. 11

Linse lens

Abb. 12

Elektronen-optische Parameter electron-optical parameters

Vorverstärker preamplifier

Videoverstärker video amplification

Kontrast contrast

Helligkeit brightness

Differenzierung differentiation

Inversion inversion

Oszillograph oscillograph

weiß white

schwarz black

Photobildschirm screen / micrographs

Beobachtungsbildschirm screen

Abb. 14

Punktauflösung point resolution

Schärfentiefe depth of focus

REM SEM

Förderliche Vergrößerung useful magnification