Basic Laboratory Materials Science and Engineering Scanning Electron Microscopy (SEM)
Aim: Explanation of the basics of scanning electron microscopy using surfaces of fractures as an example. Comparison and analysis of differing fracture behaviour of metallic and polymeric materials by means of scanning electron microscopy.
Contents 1. Introduction
1.1. Limitations of light-optical microscopy
2.1. 2.2. 2.3. 2.4. 2.5. 2.6.
Microscopy by employing electron beams Interaction between electrons and specimen Scanning electron microscope (SEM): Design and function Interrelationship between depth of focus, resolution, and magnification Fractographic analysis Transgranular and intercrystalline fracture
3 4 7 12 13 13
3. Technological significance
3.1. Assessment of damage 3.2. Quality assurance and quality control 3.3. Medical examination and biological investigation
15 15 15
5. Evaluation of testing
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.
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 0,61λ = 2 μ sin α
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 v 2 1 − c
me is the rest mass of the electron. The deBroglie relation determines the interrelationship between wavelength and momentum.
h h = , p mv
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 )c 2
By means of these three equations the dependence of wavelength on accelerating voltage can be derived:
h2 2eVme + e 2V 2 c2
2 1,5 λ= nm −6 2 V + 10 V
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 < 0.01μm 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).
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 volumeSecondary 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
M109: SEM 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. 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
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.
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 10-5 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! 18.104.22.168. Generation of the probe In the field of electron microscopy free electrons are usually produced by thermal emission. Other microscopes operate by means of field emission (> 109 V/m). Mostly, tungsten filaments or - as described here - LaB6-crystals serve as cathode. The electron emitter consists of a three-electrode arrangement (Fig. 9).
Fig. 9: Basic design of an electron gun.
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 crossover, 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). 22.214.171.124. 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
M109: SEM 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): α 2 < α 1 . 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. 126.96.36.199. 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. 10
M109: SEM 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 1mm x 1mm 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.
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.
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).
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.
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.
4. Testing In the course of this test you are to analyse and compare the differing fracture behaviour of metallic and polymeric materials in order to give an example of scanning electron microscopy as frequently used in practice. Use the surfaces of fracture obtained as a result of other tests conducted in this laboratory course, e.g. the tensile test or the notched-bar impact test. As far as metals are concerned use the samples cut to adequate size (no further preparation necessary). In case of polymeric materials, insulators are usually used. Prior to testing, coat the specimen surfaces with a thin film of precious metal to prevent charging. For this purpose a low-vacuum cathode sputtering unit is available Do not operate the electron microscope and the cathode sputtering unit 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.
Testing equipment Scanning electron microscope SEM XL30 (Philips) Cathode sputtering unit SCD 050 (Baltec)
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 individual fracture behaviour of each specimen. Determine - to the extent possible the average dimensions of characteristic features such as size of crystallite, size of dimple, fatigue striation. Briefly describe the differing material properties or conditions of fracture, respectively, that have caused the surfaces of fracture you observed.
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 of this method?
7. Bibliography •
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
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
8. Anhang Abbildungen des Versuchs M109 Abb. 3 Primärelektronenstrahl 1 mm Auger Elektronen Rückstreuelektronen Charakterische Röntgenstrahlung Röntgenstrahlung: Kontinuum Sekundäre Fluoreszenz (Kontinuum und charakteristische Röntgenstrahlung) RE-Auflösung Röntgenstrahlung, Auflösung Abb. 4 Wandung zum Detektor Probenoberfläche Austrittstiefe Reichweite Elektronendiffusionswolke Abb. 5 Ordnungszahl Abb. 7 Elektronenstrahl Kondensorlinsen Ablenkspulen Objektivlinse Probe Verstärker Rastereinheit Signaldetektor Sichtbildschirm Abb. 8 Kathode Wehneltzylinder Anode Sprayblende Kondensorlinse Stigmator Objektiv Bildfeinverschiebung Aperturblende Probe Abb. 9 Elektronenkanone Hochspannungskabel
primary electron beam Auger electrons, 1 mm back scattered electrons characteristic X-radiation X-radiation: continuum Secondary fluorescence (continuum and characteristic X-radiation) BSE resolution X-radiation, resolution wall towards detector specimen surface escape level range electron diffusion cloud atomic number electron beam condenser lenses beam deflection coils objective lens specimen amplifier scanning unit signal detector screen cathode Wehnelt cylinder anode dispersion aperture condenser lens stigmator objective fine-adjustment of micrograph aperture diaphragm specimen
electron gun high-voltage cable 18
M109: SEM Keramikisolator Wolframdrahtfaden Vakuumdichtung Abschirmung oder Wehneltzylinder Anode Abb. 11 Linse Abb. 12 Elektronen-optische Parameter Vorverstärker Videoverstärker Kontrast Helligkeit Differenzierung Inversion Oszillograph weiß schwarz Photobildschirm Beobachtungsbildschirm Abb. 14 Punktauflösung Schärfentiefe REM Förderliche Vergrößerung
ceramic insulator tungsten filament vacuum seal shielding or Wehnelt cylinder anode lens electron-optical parameters preamplifier video amplification contrast brightness differentiation inversion oscillograph white black screen / micrographs screen point resolution depth of focus SEM useful magnification