1. HISTORY ABOUT SEM SEM stands for Scanning Electron Microscopy. The earliest known work describing the concept of a Scanning Electron Microscope was by M. Knoll (1935) who, along with other pioneers in the field of electron optics, was working in Germany. Subsequently M. von Ardenne (1938) followed by Zworykin et al. (1942), working in the RCA Laboratories in the United States also tried for construction of SEM but unfortunately they could not get success, it did suffer from the slight problem. Finally Dennis McMullan and Oatley in 1948 built their first SEM. By 1952 this instrument had achieved a resolution of 50 nm. [1] 2. I TRODUCTIO The simplest of all microscopes is the hand lens or magnifying glass which is a single biconvex lens of glass or plastic. But as all we know ecessity is the Mother of Invention means that without ever having to need anything, nothing would have been invented. The need of certain things, you can help improve your life and the lives of others. The day by day requiring of things need to know about the things, we have to correlate our necessity with existing materials, we have to tailor the things in order to fulfill our requirements. For a particular application the compatibility of any material can be decided by their structure, their properties etc. So how can we determine the structure of any material? This need invented many [2, 3, 4] microscopes, the SEM is one of them. As the name Scanning Electron Microscope sounds that the electrons are used for imaging the material. The next question arises in mind, what happened when electrons incident on the specimen surface. Figure 1 shows the interaction of electron beam with matter. We can 1
realize that how can this electron beam helps us to know about specimen or any matter. I will discuss this in detail one by one in my next section. 3. I TERACTIO OF ELECTRO BEAM WITH MATTER When the primary electron beam strikes the specimen in the scanning microscope a number of different emission and absorption processes occur (figure 1). [2,4] (1) Secondary Electrons: These are low energy (tens of ev) electrons originate from specimen atoms by collision with high energy electrons. (2) Backscattered Electrons: These backscattered electrons are generated from primary beam which have interacted with atoms in the specimen and have been turned back out of the specimen again. They may have energies ranging from the full primary beam energy down to the level of secondary electrons. Figure 1: (a) Interaction of Electron Beam with Matter, (b) Range of Energy of Emitted Electrons 2
(3) Auger Electrons: As higher energy electrons interact with specimen they will knock out inner shell electron. In order to fill this inner shell electron vacancy outer most electron jump from higher energy level to lower one with the emission of X-rays. If these X-rays have sufficient energy then they are able to knock out electron from any shell depending on their energy, these electrons are known as Auger Electrons and has the energies up to 1 to 2 kev. (4) Transmitted Electrons: If specimen the specimen is thin enough then some of the incident electrons penetrate through it. These transmitted electrons may have been deflected from the line of the primary beam and may have lost energy by collisions. (5) Light: Many specimens emit light under electron bombardment, by the process of Cathodoluminescence. (6) Charge on Surface: The net current remaining in the specimen after all the above processes have added to or diminished the primary beam current is conducted to the earth. 4. CO STRUCTIO The essential parts of SEM column are shown schematically in figure, numbered as follows: (1) Electron Gun: This is a conventional triode gun, usually with a tungsten filament. It will deliver a total electron current of up to 250 µa at energies adjustable between 1 and 30 kev. 3
Figure 2: Simplified Cross Section of the Column of a Scanning Electron Microscope (2) Double Condenser Lens: These lenses are magnetic electron lenses, consists by a coil of copper wire carrying a direct current, surrounded by an iron shroud, project a demagnified image of the source onto the surface of the specimen. Interaction with this field causes electrons to be deflected towards the axis, giving properties analogous to those of convex glass lenses used for focusing light. The strength of the lens can be controlled by varying the current in the coil. [3,4] (3) Objective lens (Final condenser lens): This lens projects the diminished image of the electron crossover as a spot focused on the surface of the specimen. (4) Specimen Chamber: The chamber accommodates the specimen holder and mechanisms for manipulating it, as well as detectors for the various emissions (electrons, light, X- radiation) which will form the scanning microscope image. The specimen stage is able to 4
move in three mutually perpendicular directions X, Y, Z, the last being parallel to the axis of the column. The specimen can be tilted and rotated so that every point on its surface can be brought under the electron beam and examined. (5) Vacuum pumps: Electron beam instruments must be evacuated sufficiently well to avoid damage to the electron source and high-voltage breakdown in the gun, as well as allowing electrons to reach the specimen without being scattered. Taking these considerations into account, it is desirable for the operating pressure to be below 10-5 mbar. [2] 5. THE I TERPRETATIO OF SEM MICROGRAPHS (a) Effects of Tilt: In figure 3 we can easily visualize the combined effects of changes in secondary emission and perspective when a specimen is tilted. [5] (a) 5
(b) (c) Figure 3: SEM micrograph (Scale bar 30µm) of a deposited egg of Acrosternum marginatum with, (a) Tilt angle 0 o, (b) 30 o, (c) 60 o Reference: Klaus W. Wolf et al, Micron, 34, 57, (2003) In figure 3(a, b, c) the same sample is seen at 0 o, tilted 30 o and 60 o from the respectively. In figure 3b micrograph at 30 o the region around pores are brighter than micrograph at 0 o because high secondary electron emission is combined with a high efficiency of collection. Further increment in the tilt angle (at 60 o ) the secondary emission would be expected to be high because of the surface inclination to the scanning beam, but the final effect is of low brightness, because the emitted and backscattered electrons will be immediately reabsorbed by the surrounding surfaces. (b) Effects of Magnification: In SEM, magnification is increased by the reduction in scanning area. From figure 4, it is clear that at lower magnification the product is bundlelike structure with lengths of ~500 nm but as the magnification is increased from 20K X to 120K X the micrograph becomes clearer and we conclude that these structures are composed from small rods of 500 nm length and ~50 nm in diameter. [6,7] 6
Figure 4: (a) Low- and (b) high-magnification SEM images of the as-synthesized bundle-like Te nanostructures Reference: Jun Li et al, Solid State Sciences, 10, 1549, (2008) (c) Effects of bias voltage: SEM image intensity is a result of back scattered electrons or secondary electrons, or both, we applied a bias (50 ev) to the sample by modifying a Faraday-cup sample stage figure 5c. Secondary electrons (defined as <50 ev energy) are considered to be produced mainly through interactions between energetic beam electrons and weakly bound conduction-band electrons in metals, or outer-shell valence electrons in semiconductors and insulators. Most secondary electrons are emitted with less than 10 ev energy; thus, a positive bias on the specimen will suppress the emission to be collected by the detector. In contrast, the BSEs are those incident electrons that underwent elastic scattering from the sample and changed their direction while losing little energy; therefore, the bias will not affect the image intensity. [8] 7
(a) (b) (c) Figure 5: SEM images of a thick holey carbon film (~20-nm-thick) on top of a Ti grid and covered with a thin (~2-nm-thick) carbon film with, (a) 0 ev, (b) +12.7 ev bias, (c) effect of accelerating voltage (E o ) and atomic number (Z) on the droplet-shaped volume inside the specimen Reference: Y. Zhu et al, ature Materials, 8, 808, (2009) When a small bias is applied, the contrast of the thin carbon film generated by secondary electrons disappears almost completely, whereas the contrast of the thick grid stays the same. 6. OPERATI G MODES OF THE SEM 6.1 Bright Field and Dark Field Image The bright field (BF) image is formed from transmitted electrons (Appendix 1) and dark field (DF) image is formed from scattered electrons (Appendix 2). Micrographs are shown in 8
figure 6b it is clear that the BF image has more information about crystallographic orientations of molecules than Secondary electron image (figure 6c). In multiphase materials with varying electron transmission the BF images can be difficult to acquire with good contrast in all phases then the DF mode is often more useful. [9] Pt (a) (b) Al 2 O 3 Mo 5 (Al, Si) 3 Mo(Al,Si) (c) Figure 6: Images of the molybdenum disilicide composite thin foil sample. (a) SE SEM image with labels (b) BF SEM image (c) DF SEM image Reference: M. Halvarsson et al, Journal of Physics, Conference Series, 126, 012075, (2008) 9
6.2 Transmission Electron Mode Thick substrates commonly used in BSE detection will generate BSE signals that exceed the BSE signals from nanoparticles many times resulting in a poor image contrast. The unwanted signal contributions of the substrate can, in principle, be reduced by using thin film substrates; however, this does not help to enhance the small portion of electrons that are scattered back from the nanoparticles. Since more electrons are transmitted and scattered in the forward direction, a measurement of transmitted electrons results in higher signals and thus in better signal-to-noise ratios. [10] Figure 7: SEM images of silica particles deposited on holey carbon film, (a) Dark field image (b) Bright field image Reference: E Buhr et al, Meas. Sci. Technol, 20, 084025, (2009) 10
7. DISCUSSIO As I have discussed above, in SEM a beam of electrons is generated in the electron gun, this beam is attracted through the anode, condensed by a condenser lens, and focused as a very fine point on the sample by the objective lens. The scan coils are energized (by varying the voltage produced by the scan generator) and create a magnetic field which deflects the beam back and forth in a controlled pattern. The varying voltage is also applied to the coils around the neck of the Cathode-ray tube (CRT) which produces a pattern of light deflected back and forth on the surface of the CRT. The pattern of deflection of the electron beam is the same as the pattern of deflection of the spot of light on the CRT. Actually what happened, when an electron beam incident on the specimen, it will interact with electron cloud of atoms as well as positively charged nucleus. Because of the positive charge of nucleus they will attract towards nucleus whereas because of negatively charged electron cloud they will repel from these electrons. Depending on certain phenomenon these electron beam will form the image of specimen. Electrons impinging on solid materials are slowed down principally through inelastic interactions with outer atomic electrons, while elastic deflections by atomic nuclei determine their spatial distribution. Some leave the target again, having been deflected through an angle. Both these backscattered electrons and secondary electrons dislodged from the surface of the sample are used for image formation. In addition, interactions between bombarding electrons and atomic nuclei give rise to the emission of X-ray photons with any energy up to 1-10 µ, the energy of the incident electrons, resulting in a continuous X-ray spectrum (or continuum ). Characteristic X-rays (used for chemical analysis) are produced by electron transitions between 11
inner atomic energy levels, following the creation of a vacancy by the ejection of an inner-shell electron. If we use thin specimen then transmitted electrons can form image. Because for thin sample the number of effective scattering centers is lesser since the number of atomic layers is less. 8. CO CLUSIO The wave nature of moving electrons is the basis of the electron microscope. The resolving power (appendix 3) of any optical instrument is proportional to the wavelength of whatever is used to illuminate the specimen. In the case of a good microscope that uses visible light, the maximum useful magnification is about 500X; higher magnifications give larger images but do not reveal any more detail. Fast electrons, however, have wavelengths very much shorter than those of visible light and are easily controlled by electric and magnetic fields because of their charge. In a scanning electron microscope (SEM), current-carrying coils produce magnetic fields that act as lenses to focus an electron beam on a specimen and then produce an enlarged image on a fluorescent screen or photographic plate. To prevent the beam from being scattered and thereby blurring the image, a thin specimen is used and the entire system is evacuated. 12
APPE DIX 1. Bright Field Image: The image formed by using the transmitted beam only referred to as the bright image. To get this the objective aperture is inserted in the back focal plane of the objective lens around the transmitted beam thus blocking all diffracted beams. 2. Dark Field Image: Dark field image is formed the diffracted beams only and is obtained by inserting the objective aperture around the particular beam. Figure: showing dark field patch 3. Resolving Power: The resolving power of a microscope is given by 0.61λ / µ sin α Where µ is the refractive index of the medium, µ sin α, is the numerical aperture. 13
The resolving power of electron microscope ~2 Å as compared to ~1700 Å for optical microscopy. 4. Depth of Field: Depth of field denotes the vertical distance in the object space that can be focused on the plane of the final image without causing any aberration in the image. This can be derived to give a value of 2d/α where d is the resolving power ~2Å and α is the semi-apex angle of the cone of rays (0.01 0.001). The depth of field is therefore about 2 µm while the specimen thickness is less than 1500Å. 5. Depth of Focus: Depth of focus determines the height in the image space over which the image can continue in sharp focus. This value comes to 2dM 2 /α, which will be about 2 metres. 14
References: 1. The scanning electron microscope and its fields of application. Smith KCA and Oatley CW, Br. J. Appl. Phys. 6, 391, (1955) 2. The principles and practice of electron microscopy. Ian M. Watt 3. Electron Microscope Analysis and Scanning Electron Microscopy in Geology S. J. B. Reed, Cambridge 4. Electron Microscopy, Principle and Fundamentals S. Amelinckx, D. van Dyck, J. van Landuyt, G. van Tendeloo 5. Optical illusions in scanning electron micrographs: the case of the eggshell of Acrosternum (Chinavia ) marginatum (Hemiptera: Pentatomidae). Klaus W. Wolf, Walton Reid et al, Micron, 34, 57, (2003) 6. Self-assembly of inorganic magnetic nanocrystals: a new physics emerges M.P. Pileni, J. Phys. D: Appl. Phys. 41, 134002, (2008) 7. Surfactant-assisted synthesis of bundle-like nanostructures with well-aligned Te nanorods Jun Li et al, Solid State Sciences, 10, 1549, (2008) 8. Imaging single atoms using secondary electrons with an aberration-corrected electron microscope Y. Zhu et al, ature Materials, 8, 808, (2009) 9. Thin foil analysis in the SEM. M. Halvarsson et al, Journal of Physics, Conference Series, 126, 012075, (2008) 10. Characterization of nanoparticles by scanning electron microscopy in transmission mode 15
E Buhr et al, Meas. Sci. Technol, 20, 084025, (2009) 11. www.vcbio.science.ru.nl/images/tem-sem 12. www.materialscience.uoregon.edu/ttsem/sem_mos.gif 13. http://mse.iastate.edu/microscopy/home.html 14. www4.nau.edu/microanalysis/microprobe-sem/ 16