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What is scanning electron microscope?  Scanning Electron Microscopy (SEM) is one of the most employed techniques for its ability to imaging the material with a resolution of only a few nanometers.
Scanning electron microscopy is a highly versatile technique used to obtain high-resolution images and detailed surface information of samples. It is a type of electron microscopy that uses a focused beam of electrons to scan the surface of a specimen and generate images at a much greater resolution compared to optical microscopy. The resolution of SEM instruments can range from < 1 nanometer up to several nanometers.
A scanning electron microscope (SEM) projects and scans a focused stream of electrons over the surface of a sample and collects the different signals produced using specialized detectors. The electrons in the beam interact with the atoms within the sample, thereby producing various signals that can be used to obtain information about the surface’s topography and composition. Images are viewed in real-time on an external monitor using software that correlates the beam’s position with the intensity of electrons acquired by the detector(s). A secondary electron detector (SED) and a backscattered electron detector (BSD) are the two most common types of detectors used for high-resolution imaging in an SEM. Microanalysis of the surface composition can be accessed using energy dispersive X-ray spectroscopy (EDS) detectors. The unique configuration of the SEM will ultimately determine its resolution and available imaging modes.
A wide range of solid materials can be directly analyzed using SEM. Biological materials and insulating samples often require specialized imaging, such as low-vacuum or low-kV imaging, to reduce surface charging and beam damage. Insulating samples can be sputter coated with a thin layer of gold or platinum to minimize charging.
WHY USE ELECTRONS INSTEAD OF LIGHT?The electron microscope is developed when the wavelength became the limiting factor in light microscopes.
 Electrons have much shorter wavelengths, enabling better resolution. The incident electron’s wavelength is dependent upon the accelerating voltage of the electron microscope
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Optical microscope VS. A scanning electron microscope image at 4000x magnification of the same nanofibers
HOW SEM WORKSThe SEM is an instrument that produces a largely magnified image by using electrons instead of light to form an image. A beam of electrons is produced at the top of the microscope by an electron gun. The electron beam follows a vertical path through the microscope, which is held within a vacuum. The beam travels through electromagnetic fields and lenses, which focus the beam down toward the sample. Once the beam hits the sample, electrons and X-rays are ejected from the sample.
 Detectors collect these X-rays, backscattered electrons, and secondary electrons and convert them into a signal that is sent to a screen similar to a television screen. This produces the final image.
 HOW IS A SAMPLE PREPARED?Because the SEM utilizes vacuum conditions and uses electrons to form an image, special preparations must be done to the sample. All water must be removed from the samples because the water would vaporize in the vacuum. All metals are conductive and require no preparation before being used. All non-metals need to be made conductive by covering the sample with a thin layer of conductive material. This is done by using a device called a "sputter coater."
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The sputter coater uses an electric field and argon gas. The sample is placed in a small chamber that is at a vacuum. Argon gas and an electric field cause an electron to be removed from the argon, making the atoms positively charged. The argon ions then become attracted to a negatively charged gold foil. The argon ions knock gold atoms from the surface of the gold foil. These gold atoms fall and settle onto the surface of the sample producing a thin gold coating.
 A spider coated in gold to prepare it as a specimen for Scanning electron microscopy. This item was on display at the Australian Museum in Sydney, New South Wales, Australia.
HOW DO ELECTRON SOURCES WORKSThe electron source is one of the most important components of a scanning electron microscope (SEM) and is a major factor in determining its maximum analytical performance. There are three common types of electron sources found in SEMs: tungsten filaments, solid state hexaboride crystals, and field emission guns.
 1.Tungsten Filament This consists of an inverted V-shaped wire of tungsten, about 100 µm long, which is heated resistively to produce electrons. This is the most basic type of electron source and was developed for the first generation of electron microscopes. It remains the most common type of electron source to this day because it is low-cost. The emission area of a tungsten hairpin is relatively large, meaning that the resolution is supplies is also the lowest. Since they operate at extremely high temperatures (~2800 K), the tungsten gradually evaporates over time leading to contamination of the column. This also renders tungsten filaments susceptible to abrupt burnout after about 100 hours of beam-on operation. SEM instruments containing tungsten sources also require frequent adjustment of beam stigmation and alignment because they are sensitive to thermal fluctuations.
2.Lanthanum Hexaboride (LaB6) or Cerium Hexaboride (CeB6) |
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These sources use a precision polished crystal of LaB6 or CeB6 with a sharp tip for thermionic emission. Hexaboride crystals can produce a brighter emission because they have a lower work function than tungsten, allowing them to emit ten times the number of electrons at any accelerating voltage. The high-brightness characteristic of these sources makes them a popular choice for high-performance SEM instruments. They provide two notable advantages: higher signal-to-noise and higher resolution. Compared to tungsten, they are significantly more stable allowing them to require infrequent stigmation adjustment and a working lifetime that is 10-15 times longer. These sources also slowly degrade over time instead of burning out, allowing for planned source changes to take place. Compared to LaB6, CeB6 sources offer better resistance to evaporation and consequently, extended lifetime.
3.Field Emission Gun (FEG) The FEG source is a wire of tungsten with a very sharp tip, less than 100 nm, that uses field electron emission to produce the electron beam. The process works through electron tunneling wherein an electric field is applied to the tip to extract the electrons, and a second field is used to accelerate them down the column. Schottky FEGs are typically used in SEM. These sources function as field-assisted thermionic emitters. FEG sources produce electron beams with the highest brightness and coherence, allowing the highest resolutions to be obtained.
DETECTORSWhen the electron beam interacts with a sample in a scanning electron microscope (SEM), multiple events happen. In general, different detectors are needed to distinguish secondary electrons, backscattered electrons, or chcharacteristic X-rays.aracteristic X-rays. Depending upon the accelerating voltage and sample density, the signals come from different penetration depths.
 Schematic diagram of Backscattered electrons vs Secondary electrons generation.
1.Backscattered Electron Detector (BSD)A backscattered electron detector (BSD) detects elastically scattered electrons. These electrons are higher in energy and originate from below the surface of the sample. Therefore, the resolution of a BSD image will be worse than compared to an image acquired with an SED. Using a BSD allows for lower vacuum levels, reducing sample preparation requirements and minimizing beam damage.
Backscattered electrons vary in their amount and direction due to the composition and topography of the specimen. The contrast of the backscattered electron image depends on multiple factors, including the atomic number (Z) of the sample material, the acceleration voltage of the primary beam, and the specimen angle (tilt) in relation to the primary beam. Materials with elements composed of higher Z elements yield more backscattered electrons than lower Z elements. BSD images can be used to quickly identify phases with different compositions.
The most common type of BSD is composed of a solid-state sensor. Incident electrons form electron-hole pairs in the semiconductor sensor material, generating a current that is proportional to the backscatter electron yield. A four-quadrant BSD is configured in an annulus around the optic axis above the sample. This design allows a large number of the backscatter electrons to be captured as well as providing individual signals in each quadrant, allowing for elemental contrast (composition) imaging and topographic imaging modes.
By adding the signal from all quadrants, the conventional BSD images can be generated that exhibit compositional contrast. Operating the BSD quadrants in pairs and then subtracting the signals yields topographic images. Compositional and topographic images can be acquired in the same location to provide insight to correlate material properties to topography, grain size, or morphology.
 Silver Paste on Sandpaper
 BSD-acropora-cf-cerealis
2.Secondary Electron Detector (SED)A secondary electron detector (SED) for scanning electron microscopy offers images with resolution independent of the material. SED images allow for a visualization of the inelastically scattered electrons generated close to the sample surface and provide topographical information with the best possible resolution. No material composition information is available from SED images.
Secondary electrons are detected with an Everhart-Thornley (E-T) detector. The ET detector is composed of a scintillator inside a Faraday cage and is placed above the sample to one side. A positive bias is applied to the Faraday cage to attract the low-energy electrons where a scintillator converts the electrons into light (phontons). The photon signals is then amplified using a photomultiplier tube to yield the final signal.
 SED-velvet-mite
 SED-pollen
3.Energy Dispersive Spectroscopy (EDS)EDS makes use of the characteristic X-rays that are emitted from the sample during SEM imaging. It is a fast, accurate, and non-destructive method for identifying elemental composition on the micron scale. Characteristic X-rays are emitted when the electron beam displaces an inner shell electron (that belongs to an atom in the sample) that is replaced by an outer shell electron. Because each element has a unique energy difference between outer and inner electron shells, the X-rays that are detected are at specific energies that can be correlated with an elemental identification. EDS data can be obtained at a point, along with a line, or mapped over an area.
A silicon drift detector (SDD) is the most common type of EDS detector used in a SEM instrument. The SDD is composed of a solid-state sensor that generates electron-hole pairs by absorbing X-rays. By measuring the charge generated in the detector over a signal collection period, a spectrum of signal intensity vs. X-ray energy can be acquired allowing for the identification of almost all elements. Most SDDs contain a thin window, which provides a physical barrier for the detector to maintain appropriate vacuum levels while samples are being exchanged. The window allows all but low-energy X-rays to pass through, depending on the specific material and thickness. Beryllium windows are robust but do absorb X-rays from elements up to Be, meaning that elements from B and of higher atomic numbers can still be detected.
 EDS-semicon-full
 EDS-Semicon-Sn
4.Electron Backscatter Diffraction (EBSD)Electron backscatter diffraction (EBSD) is a scanning electron microscope–based microstructural-crystallographic characterization technique commonly used in the study of crystalline or polycrystalline materials. The technique can provide information about the structure, crystal orientation, phase, or strain in the material. Traditionally these types of studies have been carried out using X-ray diffraction (XRD), neutron diffraction and/or electron diffracti。
  TYPICAL SEM IMAGE  Melamine Foam 690x
  Lily Pollen 2000x
  Charcoal 8600x
  Gold 100,000x
  Chromosomes 100,000x
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Huang Xin