电子显微分析scanningelectronmicroscopy（sem）课件

电子显微分析 Scanning Electron Microscopy (SEM),6.1 Inelastic Scattering,Electron diffraction and TEM imaging mainly use elastic scattering electrons SEM imaging mainly use inelastic scattering electrons EDS (in TEM and SEM) and EELS (in TEM) also use inelastic scattering electrons,Inelastic scattering electrons generate,Characteristic X-ray Used for EDS in TEM and SEM Secondary electron Slow secondary electron Used for SEM imaging (morphology) Auger Electron Used for surface analysis Others Energy-loss transmitted electron Used for EELS in TEM Backscattered electron Used for SEM imaging (composition),Characteristic X-ray,If more than a critical amount of energy is transferred to an inner shell electron, that electron is ejected. e.g. an K shell electron is ejected from the atom by a high-energy electron, leaving a hole in the K shell. When the hole in the K shell is filled by an electron from L shell, characteristic Ka X-ray emission occurs.,The energy of the characteristic X-ray is characteristic of the difference in energy of the two electron shells involved. If we fill a K-shell hole from the L shell we get a Ka X-ray. If we fill a K-shell hole from the M shell we get a Kb X-ray. In microanalysis we only use the most intense lines, usually the Ka X-ray.,The characteristic X-ray with a specific energy can be used as an unambiguous sign of the presence of an element in the specimen. (Cu Ka 8.02keV, Fe Ka 6.40keV) The characteristic X-ray are used for EDS in TEM and SEM.,Secondary electrons,Secondary electrons (SEs) are electrons in the specimen that are ejected by the incident electron. They can be discussed as two distinct groups: Slow secondary electrons Fast secondary electrons,Slow secondary electrons (SEs),If the electrons are in the conduction or valence bands, it doesnt take much energy to eject them. They are called “slow SEs” energies of SE: 50eV. SEs are not associated with a specific atom and so they contain no specific elemental information.,Because SEs are weak they can only escape if they are near the specimen surface (5-10nm). So we use them in SEMs for forming images of the specimen surface. The number of SEs is dependent on the angle between incident electron beam and surface. If surface is not flat, it will generate different amount of SEs, results in the topographic contrast. SEs is used for morphology observation in SEM.,Fast secondary electrons,If the electrons are strongly bound inner-shell electrons they are less readily ejected, but when they are thrown out of their shells they can have a significant fraction (up to about 50%) of the beam energy, and they are then called “fast secondary electrons,” or FSEs. FSEs can have energies of 50-200 keV, and they can travel significant distance within the specimen. FSEs degrade the spatial resolution of microanalysis in TEM and SEM and they also generate significant numbers of X-rays which can cause problems in quantifying X-ray data. FSEs are useless for imaging and microanalysis.,Backscattered electrons,Come from the regions a few hundred nm below the surface of the specimen. The yield of the BSE increases with increasing the atomic number Z. BSE can be used to show Z-dependent contrast in SEM.,Auger Electrons,If a incident electron has enough energy to eject the inner-shell (K shell) electron in a atom, while the other inner shall in the atom have been filled by electrons, the ejected electron can only escape from the atom (ionization), leaving a hole in the atom. The electron in other shell (say, L1) can fill the hole in the inner shell. The energy released when the L1 electron fills the hole in the K shell is transferred to an electron in the L2,3 shell which is ejected as a KL1L 2,3 Auger electron.,The Auger electron has an energy given by the difference between the original excitation energy and the binding energy of the outer shell from which the electron was ejected. So the Auger electron can give information about the element. Typical Auger electron energies are in the range of a few hundred eV to a few keV.,The Auger electrons come from very close to the surface (1nm). They contain surface chemical information and Auger electron spectrometer (AES) is recognized surface chemistry technique. AES is sensitive to the lighter element. (X-ray is sensitive to the heavier element). It works under UHV. AES is rarely used in TEM/SEM.,Electron-hole pairs and cathodoluminescence (CL),These two signals are closely related. If your specimen is a semiconductor, the incident electron can generate electron-hole pairs. If you dont do anything, the electrons and holes will recombine, giving off light. This process is termed cathodoluminescence (CL),Schematic illustration of CL: Initial state before a beam electron interacts with valence-band electrons. A valence-band electron is excited across the gap into the conduction band, leaving a hole in the valence band. The hole is filled by a conduction-band electron falling back into the valence-band hole. Upon recombination a photon of light is emitted, with a frequency determined by the band gap.,Application of CL,The photon has a frequency equal to the energy of the band gap (EG) divided by Plancks constant (h). If the band gap varies for some reasons (e.g. impurity) there will be a spectrum of light given off, or the light will vary depending on what part of the specimen is being observed. CL spectroscopy has applications in the study of semiconductors and impurity effects in SEM.,Transmitted electrons,The electrons go through the specimen Some transmitted electrons lost their energies and are used for electron energy-loss spectrometry (EELS) Transmitted electrons are used in TEM and STEM,Absorption electrons,The electrons lost all their energies in the specimen Absorption electrons are used in SEM to form absorption electron image,Phonon,When a high-energy electron strikes an atom in the specimen, the lattice shakes resulting in generation of phonon. Any shaking of the atoms is equivalent to heating up the specimen and the net result of all phonons is that the specimen gets warmer.Typically, a phonon vibration causes a very small energy loss of 0.1eV. These phonon scattered electrons account for the diffuse background intensity present between the Bragg spots in diffraction patterns. They carry no useful micro-chemical information. The phonon scatter is increased as the temperature rises. If you want to obtain good clear diffraction patterns, cool the specimens.,Most commonly used signals in SEM,Secondary electrons for topographic imaging Backscattered electrons for compositional imaging Characteristic X-ray for microanalysis (EDS),6.2 SEM structure and how it works,Knoll proposed the principle of the SEM (1935). Hill made the 1st SEM (1942). The modern SEM came from the research results of Oatley group of Cambridge University (1948-1965). The 1st commercial SEM was made in Cambridge (1965). SEM is mainly used for topographic observation and microanalysis in bulk specimen. The best SEM has a resolution of 0.4nm. Main SEM manufacturers: HITACHI, JOEL, FEI, ZEISS,The SEM is similar to the TEM in that they both employ a beam of electrons directed at the specimen. This means that certain features, such as the electron gun, condenser lenses and vacuum system, are similar in both instruments.,However, the ways in which the images are produced and magnified are entirely different TEM provides information about the internal structure of thin specimens. TEM uses the transmitted electrons SEM is primarily used to study the surface morphology, or near surface structure of bulk specimens. SEM uses the “ejected” electrons.,TEM,SEM,TEM image of columnar grains in Al/Ti multilayer system,SEM image of a fracture surface of the Al/Ti multilayer system,How SEM works,An electron gun produces electrons, and accelerates them to an energy between 1 keV to 30 keV. Two or three condenser lenses then demagnify the electron beam until, as it hits the specimen, it may have a diameter of only 2-10nm.,The fine beam of electron is scanned across the specimen by the scan coils, while a detector counts the number of low energy secondary electrons, or other radiation given off from each point on the surface. At the same time, the spot of a cathode ray tube (CRT) is scanned across the screen, while the brightness of the spot is modulated by the amplified current from the detector. The electron beam and the CRT spot are both scanned in a similar way to a television receiver, that is, in a rectangular set of straight lines known as a raster.,The mechanism by which the image is magnified is extremely simple and involves no lenses at all. The raster scanned by the electron beam on the specimen is made smaller than the raster displayed on the CRT. The linear magnification M is then the side length of the CRT (l) divided by the side length (L) of the raster on the specimen (M= l /L). For example, if the electron beam is made to scan a raster 10m 10m on the specimen, and the image is displayed on a CRT screen 100mm 100mm, the linear magnification will be 10,000.,How SEM works,An electron gun produces electrons, and accelerates them to an energy between 1 keV to 30 keV. Two or three condenser lenses then demagnify the electron beam until, as it hits the specimen, it may have a diameter of only 2-10nm.,The fine beam of electron is scanned across the specimen by the scan coils, while a detector counts the number of low energy secondary electrons, or other radiation given off from each point on the surface. At the same time, the spot of a cathode ray tube (CRT) is scanned across the screen, while the brightness of the spot is modulated by the amplified current from the detector. The electron beam and the CRT spot are both scanned in a similar way to a television receiver, that is, in a rectangular set of straight lines known as a raster.,Structure of SEM (JSM 6301F SEM),Secondary electron detector,Schematic sectional view of a SEM,SEM structure,Many parts of SEM is quite similar to that of TEM. Here only the differences between TEM and SEM are mentioned.,SEM structure,Electron Gun Magnification of SEM is depending on the length of the scan area of specimen. So very fine electron beam is needed for SEM. SEM require a high intensity fine electron beam crossover size probe size W hairpin 50m 10nm LaB6 5-50 m FEG 5-25nm 1-2nm Demagnification of electron beam is achieved by use of 2-3 condense lenses. Using 2 or 3 lenses to demagnify, one can reduce the probe size for a typical tungsten hairping SEM to about 10nm (5000x) The working voltage of SEM is 1-30kV.,SEM structure,Lenses Demagnification of electron beam is achieved by use of 2-3 condense lenses. The last lens is called objective lens. But it does not play a role of imaging (it is used for focusing and additional demagnification). The objective lens should have enough space to place the scan coils and stigmators The up and low polepieces are not symmetric and with different diameters. This is for easy collection of the secondary electrons.,Scan coils Scan coils scan the electron beam on the surface of the specimen, synchronized with the scan coils of CRT. The scan coils are located between the last two lenses. The magnification of SEM (M) is M= l /L where l is the side length of the CRT (l is fixed) , and L is the side length of the raster on the specimen (L is variable). The magnification M can be adjusted by using a different scan current i. If i is small, then electron beam has a small deflection, resulting in a small L (large M). Vice versa. The magnification of SEM is 10-500,000 (M can be adjusted continuously).,Specimen chamber,Specimen chamber As all the signal detectors are located inside specimen chamber, the design of the specimen chamber should consider how to collect various signals efficiently. The most important part in the specimen chamber is specimen stage, which should be large enough to hold a large specimen (100mm). The specimen stage can translate in three directions, tilt, and rotate with a high accuracy and vibration free. The movement of the specimen stage is controlled by computer with a accuracy of 1 m.,Signal detectors and display system,Secondary electron detector Backscattered electron detector Absorption electron detector X-ray detector Energy-dispersive spectrometer (EDS) Wavelength-dispersive spectrometer (WDS) Display system Computer screen Camera or CCD camera,6.3 Information obtained from SEM,Secondary electron image Backscattered electron image Absorption electron image Electron Backscatter Diffraction (EBSD),6.3.1 Secondary electron image,Secondary electron yield is the number of secondary electron generated by each incident electron. The relationship between secondary electron yield and energy of incident electron is shown in the following figure.,Angular distribution of secondary electron emission follows cosine rule (Ip cos, where is the angle between the normal of the specimen surface and the electron emission). If placed in the position of =0, the detector can collect the largest signal.,The angular distribution of secondary electron emission is neither relevant to the crystal structure of the specimen nor relevant to the incident angle of the incident electron.,Secondary electron yield d 1/cosa, where a is the angle between the normal of the specimen surface and the incident beam. The larger the a, the more the secondary electrons (larger d) To get stronger SE signal, you can title the specimen ( a45) to get more SE emission.,Topographic contrast As we know, the larger the incident angle a, the more the secondary electrons. So the area A in the right figure will generate more SEs than the area B, leading to the difference in contrast. This contrast is called topographic contrast SEs can reach the detector along the curved trajectory, i.e. even the electron generated by the surface not facing the detector can reach the detector. So SE image has no sharp shadow and shows a soft stereoscopic contrast.,Composional contrast When Z20, SE yield is almost independent on the Z,Secondary electrons (SE) From the surface layers of 5-10nm. sensitive to the topography of sample surface (main image mode of SEM) Almost not sensitive to the atomic number Resolution of SE image is 0.4-5nm,SE images,Voltage contrast,In conductor The area with positive potential generates less secondary electrons. Its SEM image looks darker. The area with negative potential generates more secondary electrons. Its SEM image looks brighter. This causes the voltage contrast. Voltage contrast is used for observation of IC.,Voltage Contrast Detection of voltages on the surface of a biased device through SE emission, yielding information on distribution of electrical fields on the specimen surface.,Unbiased sample,biased sample,Image of the “difference”,6.3.2 Backscattered electron image,Backscattered electron coefficient is a probability of generation of a BSE with its energy larger than 50eV by an incident electron. increases with increasing atomic number Z. The energy of incident electron has little effect on . Number of BSE increases with the increase of angle between the incident electron and the normal of the surface of specimen,The contrast of BSE image is mainly depend on the composition and topography of the specimen surface. BSE image is Z-dependent imaging,BSE image of Ag-Cu alloy. White area is Ag (Z=47) and black area is Cu (Z=29),The backscattered electron images show light Ti3Al5 phase within the darker TiAl2 matrix alloy in Ti35Al65 alloys annealed at (a) 1000C and (b) 700C.,As the BSE trajectory is straight line, so BSE image has obvious shadow. The details in the shadow region is too dark to see.,Number of BSE increases with the increase of angle between the incident electron and the normal of the surface of specimen,Channelling contrast,The backscattered electron coefficient is dependent on the orientation of a crystal with respect to the incident beam. This effect, known as electron channelling. can be used for the study of the grain orientation,Backscattered electrons (BSE) From the depth of 0.1-1m below surface. sensitive to the atomic number Z, suitable for observation of spatial distribution of composition Sensitive to the orientation of a crystal with respect to the incident beam, can be used for the study of the grain orientation Resolution of BSE image is 6-200nm,6.3.3 Absorption electron image,II = ISE + IBSE + IA + IT,Intensity of incident electron,Intensity of SE,Intensity of BSE,Intensity of absorption electron,Intensity of transmitted electron,For SEM, the specimen is so thick that IT=0. In a certain experimental condition, II is a constant. So II = ISE + IBSE + IA =constant, i.e. IA = constant - (ISE + IBSE ) The contrast of the absorption electron image is complement with BSE contrast and SE contrast. The resolution of the absorption electron image is about 0.1-1m.,SEM images of AgCu alloy，x1200 (a) BSE image, white area is Ag，black area is Cu； (b) Absorption electron image， white area is Cu， black area is Ag；,a,b,The contrast of the absorption electron image is complement with BSE contrast and SE contrast,6.3.4 Electron Backscatter Diffraction (EBSD),EBSD is a diffraction technique for obtaining local crystallographic information from the surface of bulk samples, such as texture and grain boundary misorientation in multiphase materials, crystal deformation, etc.,EBSD will be discussed later.,6.4 Resolution of SEM,The resolution of SEM is limited by: The real diameter of electron beam Electron scattering in specimen Ratio of signal to noise,Effect of electron diameter,For SEM, the spherical aberration is larger than the chromatic aberration and diffraction effect. The real diameter of electron beam d is mainly composed of the diameter of electron beam generated by electron gun (d0) and the diame