26 Electron Beam Interaction with Materials
Vinay Gupta
Introduction:
In electron microscopy, electrons emitted from a source are accelerated by an applied voltage which can vary from a few kV (SEM) to a few hundred or even thousand kV (TEM). The high energy electrons are focused to a fine probe on a sample (bulk or specially prepared thin). The interaction between high energy electrons and the atoms of the sample gives rise to a variety of signals. Detection and quantitative characterization of the signals form the basis of SEM and TEM. The dual nature of electrons offers unique opportunities for the study of samples; the wave behavior is utilized in the formation of images and diffraction patterns. The charged particle behavior results in strong interaction with the atoms of the sample giving rise to information on the chemical composition of the sample. Thus it is possible to concentrate on a small region of the sample (typically from µm level downwards) and obtain information on microstructure, crystallographic details and chemical composition. The information available may broadly be summarized as
- Morphology – size, shape and distribution of phases and their relationship to one another, high resolution, µm to nm (or fraction of it) level
- Crystallography – arrangement of atoms and their degree of order, detection of equilibrium and metastable precipitates, orientation relationship
- Composition – elements present in the sample and their local variation – high resolution
Fig.1 is an illustration of the various possibilities of electron optical techniques. Microstructural information can be obtained from the SEM, TEM (including STEM and HREM), crystallographic details from the diffraction patterns and chemical composition from analytical microscopy.
Fig. 1: Illustration of information provided by the electron microscope
In this module, we shall focus our attention on the signals arising from the interaction between the atoms of the specimen and the high energy electrons, their characteristics, information available and the methods of detecting them. It will be emphasized that more than one signal may be used to obtain a specific information, say chemical composition.
Specimen-Electron Beam Interaction:
The signals arising from the interaction of high electrons with the sample are schematically illustrated in Fig. 2.
Fig. 2: Signals emerging from the interaction of high energy electrons with the sample.
Several signals, secondary, backscattered and Auger electrons, X-rays, cathodoluminiscence (CL) are emitted on the incident side of the electron beam (the top surface of the sample) while some are emitted on the exit side (the bottom surface of the specimen). As electrons are absorbed easily in thick samples (thickness exceeding 1µm), the transmitted electrons are observed only in thin specimens. A study of the signals emitted on the top surface forms the basis of Scanning Electron Microscopy (SEM) while transmitted electrons are used in Transmission Electron Microscopy (TEM). It may also be seen from this figure that a considerable amount of heat is generated which is to be transported away from the sample for prolonged observation. Specimen current is of interest in semi-conducting materials.
Typical depths in the sample from which these signals emerge are shown in Fig. 3. The penetration of electrons in the specimen is a function of the electron energy as well as the atomic number of the sample. The higher the energy of the electrons and the atomic number of the atoms in the sample the greater is the penetration. (Fig. 4).
Fig. 3: Illustration of the depths from which various signals arise.
Fig. 4: Dependence of electron penetration on primary energy and atomic number of atoms in the sample.
It is clear from Fig 3 that Auger electrons are emitted from regions very close to the surface (0.5-5.0 nm), secondary electrons from a slightly deeper region (~10nm) while back scattered electrons emerge from a region of 1- 2 µ m. Characteristic and continuous X- rays emerge from a nearly pear shaped (or tear drop shaped) volume with dimensions of a few µm.
Emission mechanism and characteristics of signals:
The mechanism of emission of secondary, backscattered and Auger electrons and X-rays (K and L) is shown in Fig. 5.
Fig: 5: Excitation of (a) back scattered and (b) secondary electrons
Fig. 5 (contd.) Emission of (c) Auger electrons, (d) X-rays (L lines) and (e) X-rays K lines.
Secondary electrons (SE) are loosely bound electrons near the surface of the sample ejected by the incident high energy electrons. They have energy in the range 10 eV – 50 eV. Secondary electron yield increases with increasing tilt of the sample with respect to the electron beam direction and increases with decreasing energy.
Backscattered electrons (BSE) are those deflected by the nuclei of the atoms; they have energy in the range100 eV to Ep , the energy of the primary electrons. The lower energy electrons have obviously reacted with the sample before being ejected out of the sample and in the process lost a part of their original energy. They are referred to high loss BSEs. The term back scatter coefficient (η) refers to the ratio of the number of BSEs ejected to the total number of primary electrons. η increases with increase in the atomic number of the elements in the sample and with increasing tilt and is marginally affected by the primary beam energy.
The high energy primary electrons can cause ionisation of the atom by ejecting one of the inner shell electrons (K, L or M). The ionised atom returns to the ground state when an electron from a higher level fills the hole in the inner shell caused by the ejected electron. The extra energy available is utilised for the release of an X-ray photon (K , L or M characteristic X-ray) or to knock out an electron from an outer shell (Auger electron). Energy is specific to the element emitting X-rays; e.g. Al K 1.48 keV. Energy generally lies in the range 1 keV to 10 keV for K radiation of common elements and L and M radiation for heavier elements. The energy E or wavelength λ is characteristic of the element and the intensity is a measure of the concentration. This forms the basis for electron probe microanalysis (EPMA) and the analytical electron microscopy (AEM); also the variation in the concentration of an element along a line or area can be measured (line or area mapping).
Fluorescent yield (ω) is the ratio of the number of X-ray photons emitted to the total number of ionisation caused. Suppose there are 1000 vacancies caused in the K-shell and 300 K X-ray photons are emitted. The fluorescent yield for the K-X rays is 0.3.The variation of ω with atomic number is shown in Fig. 7. It is seen that for elements with atomic number leass than 10, the fluorescent yield for K, L and M x-rays is negligible. Since X -ray and Auger electron emission are complementary processes, Auger electron yield is high for the low atomic number elements. This explains the common use of Auger electron spectroscopy for the study of light elements such as C, O and N.
Fig. 7: Variation of fluorescent yield with atomic number for K, L and M X-rays
The energy of the Auger electrons: ~10 eV to 2000 eV. Since these electrons are easily absorbed by the environment in the microscope; high vacuum conditions are necessary for characterization, a condition difficult to obtain in a conventional SEM. Since the Auger signal originates from the first few atom layers, in-depth profile of elemental distribution possible by ion beam sputtering.
Additional signals in TEM:
As pointed out earlier, thin samples are required for study in the TEM. The high energy electrons will penetrate the thin sample and signals emerging from the exit side can be used for characterizing the sample. It may be noted that even in thin foils all the signals mentioned so far (with respect to the bulk sample) will be present but with reduced intensity. Elastically scattered and inelastically scattered electrons arising from thin foils are used to provide information on microstructure, crystallography and chemical composition.
Elastically scattered electrons have energy equal to that of the primary electrons. They will be prominent when the specimens are very thin. They can be used for the formation of bright field (BF) and dark field (DF) images and diffraction patterns (SADP). Diffraction patterns provide useful information on crystallographic relationship, sample thickness and chemical composition.
In elastically scattered electrons are those which have interacted with the sample and in the process suffered energy loss. The analysis of plasmon peaks and ionization edge, forming the basis of electron energy loss spectroscopy (EELS), provides information on chemical composition and the fine structure before the ionization edges (EXELFS, extended electron absorption fine structure) on the nature of bonding.
Detectors:
The signals arising from the interaction of high energy electrons with the atoms of the sample are studied with a number of detectors (Table 2). The more commonly used ones are the scintillation-photomultiplier or Everhart-Thornley (ET) detector for SEs and BSEs, solid state detectors for BSEs and the energy dispersive and wavelength dispersive systems for X-rays.
Table 2: Detectors for various signals
Signal Detectors
The E-T detector is schematically illustrated in Fig. 7. It is basically a scintillator-photomultiplier system. The secondary electrons are first collected by attracting them towards an electrically-biased grid at + 400 V to +500 V, and then further accelerated towards a phosphor or scintillator at about + 2000V. The accelerated secondary electrons are now sufficiently energetic to cause the scintillator to emit flashes of light (CL) which are conducted to a photomultiplier outside the SEM column via a light pipe and a window in the wall of the specimen chamber. The amplified electrical output by the photomultiplier is displayed as a two dimensional intensity distribution that can be viewed and photographed on an analogue video display or subjected to analog-to-digital and displayed and saved as a digital image. The brightness of the signal depends on the number of secondary electrons reaching the detector. If the beam enters the sample perpendicular to the surface, then the activated region is uniform about the axis of the beam and a certain number of electrons “escape” from within the sample. As the angle of incidence increases, the “escape” distance of one side of the beam will decrease, and more secondary electrons will be emitted. Thus steep surfaces and edges tend to be brighter than flat surfaces, which results in images with a well-defined, three dimensional appearance. It is possible to obtain a resolution of < 1 nm.
Fig. 7: The Everhart-Thornley detector
Preferential collection of the SEs and BSEs can be achieved by varying the collector (electrically-biased grid) voltage. When the voltage is +400 V to + 500 V, SEs are preferentially attracted. If it is maintained at a negative value (-100 V), SEs are repelled and BSEs are collected (Fig. 8).
The principle of the solid detectors is illustrated in Fig. 9. The high energy BSEs are incident on a semiconductor detector where they cause the formation of a number of electron-hole pairs depending on their energy. If electrodes are placed on opposite faces of the detector, and a potential applied with an external circuit, the free electrons and holes flow in opposite directions, producing a current flow in the external circuit. This current can be amplified and used as a video signal. Background or dark current in the circuit (the flow of current even when no electrons strike the detector) can be minimized by the use of a p-n junction.
Fig. 8: Preferential detection of SE and BSE by varying collector voltage
Fig. 9: Solid state back scattered electron detector
Common Modes of operation of the TEM:
ρ Bright Field (BF) microscopy
ρ Dark Field (DF)
ρ Selected Area Diffraction
Electrons passing through a thin foil are diffracted from various planes and the diffracted beams from a family of planes (h1 kil1 and h2k2l2 in Fig 10) are focused in the back focal plane of the objective lens. Thus a set of diffracted spots are formed on this plane. The formation of diffraction pattern and intermediate image is shown in Fig. 11. A suitable aperture placed in the back focal plane of the objective lens can be used for the formation of bright and dark field images. If the aperture blocks the diffracted electrons but allows the directly transmitted electrons to form the final image, a bright field image is obtained (Fig. 12). Dark field images can be formed either by moving the aperture so as to allow diffracted electrons at a particular angle to pass through the aperture or by tilting the electron beam so that the diffracted electrons at a particular angle go through the optic axis, i.e. one diffracted beam moves to the centre (Fig. 12).
Fig. 10: Formation of diffraction spots in the back focal plane of the objective lens
The basic diffraction pattern is formed in the back focal plane of the objective lens. This pattern can be observed by projecting the back focal plane of the objective lens on to the viewing screen. In this case the intermediate lens is used as a long focal length lens by running it reduced excitation. The use of the selector area or diffraction aperture at the intermediate image plane allows the selection of the area on the specimen from which the diffraction pattern is obtained. If the size of the intermediate aperture is D and the objective lens magnification is M, then D/M is the size of the selected area. It is necessary to ensure that the plane of the intermediate aperture coincides with that of the intermediate image.
Fig. 11: Image and SAD formation in the electron microscope
Fig. 12: Imaging in (a) BF mode, (b) DF mode with shift of objective aperture and (c) DF mode with beam tilting
Questionnaire
- What kind of information can be obtained from electron microscopes?
- What are the different kinds of signals which arise due to the interaction of specimen with electron beam?
- Which parameters define the depth of penetration of electron beam in a specimen?
- What are the different signals which can be observed in TEM?
- What are the common modes of operation in TEM?
- Describe the detectors for various signals.
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References
- Electron Microscopy: Principles and Fundamentals by S. Amelinckx (Editor), Dirk van Dyck (Editor), J. van Landuyt (Editor), Gustaaf van Tendeloo (Editor).
- Scanning Electron Microscopy and X-ray Microanalysis: Third Edition by Dale E. Newbury, David C. Joy, and Joseph I. Goldstein.
- Electron microscopy by John J. Bozzola.
- Transmission Electron Microscopy: A Textbook for Materials Science, Authors: Williams, David B., Carter, C. Barry