Scanning Electron Microscopy Instrument Imaging Chemical Analysis (EDX)
Structural and Chemical Analysis of Materials J.P. Eberhart John Wiley & Sons, Chichester, England, 1991.
Scanning Electron Microscopy and X-Ray Microanalysis J. Goldstein, D. Newbury, D. Joy, C. Lyman, P. Echlin, E. Lifshin, L. Sawyer, J. Michael
Kluwer Academic/Plenum Publishers, New York, 2003.
1. A column which generates a beam of electrons. 2. A specimen chamber where the electron beam interacts with the sample. 3. Detectors to monitor the different signals that result from the electron beam/sample interaction. 4. A viewing system that builds an image from the detector signal.
Reduced image of crossover
Project the crossover image onto the specimen
X-Y translation + rotation
SEM Image not formed by focusing of lenses X-ray maps can be displayed. Resolution not limited by lens aberrations (in the usual sense of image forming lenses is limited by the objective lens aberrations which determines the minimum probe size).
Imaging involves digital processing online image enhancement and offline image processing. Resolution limited by probe size and beam spreading on interaction with specimen. Hence, resolution depends on the signal being used for the formation of the image.
A fine electron probe is scanned over the specimen. Various detectors (Secondary Electron (SE), Back Scattered Electron (BSE), XRay, Auger Electron (AE) etc.) pick up the signals. The amplified output of a detector controls the intensity of the electron beam of a CRT (synchronized scanning) of the pixel of display
Scanning Electron Beam
Various Detectors (SE, BSE, EDX, AE)
Display on CRT Note that the resolution depends on the type of signal being used
~ 40 Å (SE); ~ (100-500) Å (BSE)
10 – 105
Depth of field
High (~ m)
Size of specimen
1 – 5 cm (usual range)
Importance of SEM
Many signals are generated by the interaction of the electron beam with the specimen. Each of these signals is sensitive to a different aspect of the specimen and give a variety of information about the specimen.
Backscattered Electrons (BSE)
Incident High-kV Beam
Secondary Electrons (SE) Characteristic X-rays
Auger Electrons Bremsstrahlung X- rays Visible Light Absorbed Electrons
In a SEM these signals are absent Elastically Scattered Electrons
Inelastically Scattered Electrons
Signals An important point to note is the fact that the different signals are generated ‘essentially*’ from different regions in the specimen. This determines: as to what the signal is sensitive to the intensity of the signal.
Not to scale
• The X-rays generated by the electrons are the “Primary X-rays” • The primary X-rays can further lead to electronic transitions which give rise to the “Secondary X-rays” (Fluorescent X-rays)
Interaction volume volume which the electrons interact with Sampling volume volume from which a particular signal (e.g. X-rays) originates * Monte Carlo simulations are used to find the trajectory of electrons in the specimen and determine the probability of various processes
X-ray fluorescence and Auger electrons 1 3-10 keV e−
Electron from beam knocks out a core electron
Transition from higher energy level to fill core level
3 Generation of x-rays accompanying the transition
Further the x-ray could knock out an electron from an outer level → this electron is called the Auger electron
Photoluminescence Photon induced light emission
Electron-hole pairs and cathodoluminescence e Incident electron excites an electron from the valence band to the conduction band → creating an electron hole pair
Cathodoluminescence Electron induced light emission
Band gap hole
Cathodoluminescence (CL) Spectroscopy h
Electron beam induced current ( EBIC) Charge collection microscopy
Secondary Electrons (SE) Produced by inelastic interactions of high energy electrons with valence electrons of atoms in the specimen which cause the ejection of the electrons from the atoms. After undergoing additional scattering events while traveling through the specimen, some of these ejected electrons emerge from the surface of the specimen. Arbitrarily, such emergent electrons with energies less than 50 eV are called secondary electrons; 90% of secondary electrons have energies less than 10 eV; most, from 2 to 5 eV. Being low in energy they can be bent by the bias from the detector and hence even those secondary electrons which are not in the ‘line of sight’ of the detector can be captured.
Some Z contrast! SE are generated by 3 different mechanisms: SE(I) are produced by interactions of electrons from the incident beam with specimen atoms SE(II) are produced by interactions of high energy BSE with specimen atoms SE(III) are produced by high energy BSE which strike pole pieces and other solid objects near the specimen. http://www.emal.engin.umich.edu/courses/semlectures/se1.html
Back Scattered Electrons (BSE)
Produced by elastic interactions of beam electrons with nuclei of atoms in the specimen Energy loss less than 1 eV Scattering angles range up to 180°, but average about 5° Many incident electrons undergo a series of such elastic event that cause them to be scattered back out of the specimen The fraction of beam electrons backscattered in this way varies strongly with the atomic number Z of the scattering atoms, but does not change much with changes in E0.
Dependence on atomic number BSE images show atomic number contrast (features of high average Z appear brighter than those of low average Z) http://www.emal.engin.umich.edu/courses/semlectures/se1.html
Note that SE not in traveling in the line of sight can also be captured by the detector
Magnification The magnification in an SEM is of ‘Geometrical origin’ (this is unlike a TEM or a optical microscope) Probe scans a small region of the sample, which is projected to a large area (giving rise to the magnification).
Area scanned on specimen Area projected onto display
Depth of field
Dependent on the angle of convergence of the beam Depth of field is the same order of magnitude as the scan length Magnification 10,000 Scan length 10 m Depth of Field 8 m
What determines the resolution in an SEM? Probe size (probe size is dependent on many factors) Signal being used for imaging This is because the actual interaction volume/cross section is different from the probe diameter. Additionally, each signal is sensitive to a different aspect of the specimen. In terms of parameters: Accelerating voltage Beam current Beam diameter Convergence angle of beam
Topographic Contrast in SEM Inclination Effect Shadowing Contrast Edge/Spike Contrast
Line of sight with the detector
Operating parameters affecting signal quality Accelerating Voltage Probe Current Working Distance Specimen Tilt Aperture Size Edge effect Contamination Charging
LaB6 Field Emission Vacuum
Probe current Probe diameter Resolution This leads to decrease in image intensity we have to use a brighter source (W filament < LaB6 < Field Emission gun)
Comparison of Electron Sources at 20kV Units