Scanning Electron Microscopy

Scanning electron microscope (SEM)

• A type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons.
• The electrons interact with atoms in the sample, producing various signals that contain information about the surface topography and composition of the sample.
• The electron beam is scanned in a raster scan pattern, and the position of the beam is combined with the intensity of the detected signal to produce an image.
• SEM can achieve resolution better than 1 nm.
• Specimens are observed in high vacuum in conventional SEM, or in low vacuum or wet conditions in variable pressure or environmental SEM, and at a wide range of cryogenic or elevated temperatures with specialized instruments.

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Image of pollen grains taken on an SEM

History of SEM

• Max Knoll produced a photo with a 50 mm object-field-width showing channelling contrast by the use of an electron beam scanner.
• Manfred von Ardenne in 1937 invented a microscope with high resolution by scanning a very small raster with a demagnified and finely focused electron beam. 
• Ardenne applied scanning of the electron beam in an attempt to surpass the resolution of the transmission electron microscope (TEM), as well as to mitigate substantial problems with chromatic aberration inherent to real imaging in the TEM. 

M. von Ardenne's first SEM

How SEM works?

• The electron source (or gun) is usually of the tungsten filament thermionic emission type, although field emission gun (FEG) sources are increasingly being used for higher resolution.
• The electrons are accelerated to an energy which is usually between 1 keV and 30 keV which is considerably lower than the energies typical of the TEM (100 300 keV).
• Two or three condenser lenses then demagnify the electron beam until, as it hits the specimen, it may have a diameter of only 2-10 nm.  

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How scanning mode works?

• For example, if the electron beam is made to scan a raster 10 μm x 10 μm on the specimen, and the image is displayed on a CRT/CCD screen 100 mm x 100 mm, the linear magnification will be 10000x.

Signal generation in SEM: Inelastic scattering  

Phonon scattering

• Phonons are the quanta of elastic waves, that is of atomic vibrations in a solid.  
• A primary electron can lose energy by exciting a phonon and effectively heating the solid.
• The amount of energy lost is rather small, generally <1 eV, and the mean free path for high energy electrons is quite large, of the order of microns.
• Phonon scattering is important for two main reasons:
• All electrons which remain in the solid are likely to excite phonons eventually, and this is how the solid is heated by the electron beam.
• Also, when phonon scattering occurs, the scattered electron is generally deflected through quite a large angle, typically ~10^o

Plasmon scattering

• A plasmon is a wave in the ‘sea’ of electrons in the conduction band of a metal; there are similar effects among the bonding electrons of non-metals.
• In exciting a plasmon, the primary beam loses 5-30 eV and the mean free path for this event is around few hundred nm in most materials.
• Consequently, plasmon scattering is a frequent occurrence in all electron-solid interactions.  
• Energy losses by plasmon scattering dominate the ‘energy loss spectrum“ but are not very useful for analysis because the energy loss is not particularly characteristic of the scattering element.  

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• It is possible (but less likely) that a primary electron will transfer some energy to a single valence electron rather than to the ‘sea’ collectively.
• The mean free path for this process is quite large (μm), the energy loss is small (~ 1 eV) and the typical scattering angle is also small so the process is not exploited in electron microscopy.  

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Single valence electron excitation  

Inner shell excitation  

Secondary effects  

• A secondary effect can be loosely defined as an effect caused by the primary electron beam which can be detected outside the specimen.
• The secondary effects with which we will be mainly concerned are either electrons or electromagnetic radiation.

Relaxation of excited atoms  

• If a localized electron has been knocked out of an atom, the atom is in an excited, high energy, state.
• At some later time, the empty electron state will be filled and the atom will relax giving off the excess energy as a secondary effect.  
• There are essentially three ways in which this relaxation can happen.
• If the vacant electron state is an outer state then the energy to be given off will be small and is commonly emitted in the form of a photon which may be in the visible range.
• This effect is known as cathodoluminescence.  

• If, on the other hand, the vacant state is an inner state, the amount of energy to be released is larger and there are two main possibilities: a characteristic X-ray or a characteristic (Auger) electron may be emitted.

• An alternative to X-ray emission is the ejection of an outer electron carrying excess energy as kinetic energy.
• This process is known as Auger emission.
• Three electrons are now involved
• Original vacancy
• Outer electron which jumps into it
• Other outer electron which leaves carrying the surplus energy.
• Measurement of the energy of the characteristic Auger electrons forms the basis of Auger electron spectroscopy.  

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• The relative abundance of the three types of electron emission (secondary, backscattered and Auger) shows large number of secondary and backscattered electrons but relatively few of the (analytically most useful) Auger electrons.  

Inelastic scattering and absorption  

• Even for the thinnest specimen, more than one of the inelastic scattering processes can take place.
• In a ‘solid’ specimen many such events will occur until the electron is stopped or leaves by the surface it entered.
• Majority of electrons are brought to a halt within the solid but a few are backscattered and leave the specimen.
• The volume within which 95% or so of the primary electrons are brought to rest is generally referred to as the interaction volume.

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Secondary electrons

• Secondary electrons: Electrons which escape from the specimen with energies below about 50 eV.
• They could conceivably be the primary electrons which at the very end of their trajectory reach the surface with a few eV remaining.
• They are more likely to be electrons to which a large amount of energy has been transferred within a short distance of the surface.
• The yield of secondary electrons, that is the number of SE emitted per primary electron can be as high as, or higher than 1.
• Secondary electrons are abundant and are the most commonly used as imaging signal in scanning electron microscopy.  

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Types of SEs

• SE1 electrons are generated and leaves the surface directly at the spot where the PE beam impacts on the specimen surface.
• SE2 electrons are generated after multiple scattering inside the interaction volume, and leave the sample at a grater distance from the primary beam’s impact point.
• SE3 electrons are generated by BSE colliding with chamber walls or the lens system

Secondary electron energy distributions for PE of 75 eV through 500 eV impacting on graphite.

Backscattered electrons

• Backscattered electrons are electrons resulting from the interaction of the electrons of the primary beam with the atomic nuclei of the sample that have reacted in a quasi-elastic manner with the atoms of the sample.
• Some primary electrons may leave the surface before giving up all their energy.
• They are most likely to do this while they still have a large fraction of their incident energy.  
• Backscattered electrons are not usually as numerous as secondary electrons but most of them carry high energies.
• They are used for imaging, diffraction and analysis in the SEM.

• Backscattered electrons have a broad energy spread.
• Those of the highest energy are electrons which have been scattered only a few times.
• These originate near the incident beam (A in Figure) are capable of giving information at high spatial resolution as well as yielding crystallographic information.
• Electrons which have undergone multiple scattering (B in Figure) lose more energy, come from a larger area and therefore yield information at worse spatial resolution.  

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Obtaining a signal in the SEM

• Due to the interaction of electrons with a thick specimen, the energy of the incident electrons is dissipated resulting in
• Various secondary emissions from the specimen
• Emission of electron as a result of inelastic scattering (secondary electrons)
• Emission of elastically scattered electrons i.e. backscattered electrons out of the specimen.  
• Each signal is the result of some particular interaction between the incident electrons and the specimen and may provide different information about the specimen.
• In SEM, any radiation from the specimen or any measurable change in the specimen may be used to provide the signal to modulate the CRT or digital display and thus provide contrast in the image.

• All scanning electron microscopes normally have facilities for detecting secondary electrons and backscattered electrons.
• X-rays are used primarily for chemical analysis rather than imaging.
• Auger electrons (and cathodoluminescence) are of such low energy, and are so easily absorbed that they require an ultra high vacuum system and specialized equipment for their efficient use.
• The region into which the electrons penetrate the specimen is known as the interaction volume and throughout it, various radiations are generated as a result of inelastic scattering
• As the primary electrons lose energy the amount and type of the secondary radiations will alter.  

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• Even though radiation is generated within this volume, it will not be detected unless it escapes from the specimen, and this will depend on the radiation and the specimen.
• X-rays are not easily absorbed, and most will escape from the specimen.
• The volume of material contributing to the X-ray signal, or sampling volume, is of the same order as the interaction volume, which may be several micron in diameter.
• Electrons will not be backscattered out of the specimen if they have penetrated more than a fraction of a micrometer and the backscattered signal therefore originates from a much smaller region.

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• Although secondary electrons are generated both by the primary electrons entering the specimen and by the escaping backscattered electrons, the former are more numerous.
• Detected secondary electron signal originates mainly from a region which is little larger than the diameter of the incident beam.
• The resolution of the SEM is closely related to the sampling volume of the signal used.
• Secondary electrons having the smallest sampling volume, are therefore capable of giving a better spatial resolution than the other signals.  

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SEM images of Fe particles in carbon obtained with secondary electrons (left) and back-scattered electrons (right). The BSE image shows the Fe particles with bright contrast.

The left image is made by secondary electrons (ETD), the right one is a topographic view made with backscattered electrons (BSE). 

•  If the electron yield is not equal to unity, then unless the sample is a conductor, it will tend to become charged during examination.
• Careful control of the accelerating voltage can produce an electron yield of unity which will overcome this effect  

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Li, SS and W SE yields vs. mean electron energy for He and Ar Dc-GD

Backscattered electron coefficient of carbon, stainless steel, and copper with normal incidence electrons

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