Scanning Electron Microscope (SEM)

Definition

Scanning electron microscope is a classification of electron microscope that uses raster scanning to produce the images of a specimen by scanning using a focused electron beam on the surface of the specimen.

  • An SEM creates magnified images of the specimen by probing along a rectangular area of the specimen with a focused electron beam. This process is called the raster scanning.
  • It is called a scanning electron microscope because the image is formed by scanning the surface of the specimen in a raster pattern using a focused electron beam.
  • SEM relies on the secondary emission of electrons from the surface of the specimen to achieve magnified image to be viewed
  • The major advantage of a SEM over TEM is that it can produce detailed image of the whole organisms and surfaces of the cells.

Scanning Electron Microscope (SEM) photo image

Working principle of SEM

working principle of Scanning Electron Microscope (SEM) diagram and image representation

A scanning electron microscope works on the principle of targeting a focused beam of electrons moving with high kinetic energy on a specimen.

When these electrons scan the surface of the specimen, electrons are scattered and these secondary electrons that are slowed down on the impact of hitting the surface of the specimen are collected by a detector and these secondary electrons create the magnified image of the specimen thereby illustrating the morphology and topology of the specimen.

Similarly, the backscattered electrons from the surface of the specimen help us in illustrating the difference in composition of the specimen thereby making this type of microscope a useful instrument.

Difference between Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM)

Difference between Scanning Electron Microscope (SEM) and Transmission Electron Microscope

The main difference between a transmission electron microscope (TEM) and scanning electron microscope is the fact that TEM operates by penetrating electrons through the specimen and producing image by capturing the emitted electrons from the specimen.

On the other hand, SEM employs the technique of raster scanning the surface and capturing the secondary and back scattered electrons to create an image that illustrates the topology and morphology of the specimen and also plot the changes in composition using the backscattered electrons.

Components (Parts)

The scanning electron microscope consists of the following major parts:

Schematic drawing of the electron and x-ray optics of a combined SEM EPMA and parts description
  • Electron Gun
  • Condenser Lens
  • Objective Lens
  • Sample stage
  • Scanning Coil
  • Detector 
  • Other infrastructure elements
    • Power Supply
    • Vacuum system
    • Cooling system
    • Area devoid of electric and magnetic fields
    • Vibration-free floor

Electron Gun

  • This is the basic part where the electrons are produced and directed towards the specimen for raster scanning.
  • It contains a cathode and an anode and electrons are produced under thermal heat.
  • The three types of electron sources that are used to generate a focused beam of electrons include : 1. Field emission gun (FEG), 2. Lanthanum hexaboride or a 3. Tungsten filament 

Condenser Lens

  • The condenser lens is used to adjust the amount of electrons that travel through the column to be focused on the specimen.
  • The condenser can be used to increase or decrease the amount of electrons that are required to scan the surface of the specimen

Objective Lens

This lens is used to focus the beam of electrons onto the surface of the specimen

Sample Stage

  • This is the area on which the sample is mounted.
  • The sample stage can be moved up and down as per the requirement.

Scanning Coil

Also called as the deflection coil, the scanning coil is used to deflect the incoming beam of electrons onto the surface of the specimen.

Detector

  • An SEM has several detectors that differentiate between the secondary electrons, back scattered electrons and diffracted electrons among others.
  • These detectors help in creating the magnified image of the specimen

Applications

Scanning electron microscopes find great use in the fields of nanoscience studies, microbiology, and biomedical studies

  • SEMs are used to analyze and study the very tiny filament structures of microorganisms
  • They are used in analyzing the nanoscale ingredients that are used in cosmetic industry
  • They are used to study the morphology and topology of  elements used in various industries
  • SEMs are used in spot analysis of chemical compositions in energy-Dispersive X-ray Spectroscopy.
collage images of the organelles under the scanning electron microscope - magnification
Images : Left column (top: pollen of Tradescantia; middle: hederelloid from the Devonian of Michigan; botton: kidney stone surface (crystals of calcium oxalate dihydrate) Right column (top: Cyst of nematode and its egg; middle: photoresist layer used in the semiconductors; botton: stomata on the leaf lower surface). Pictures reference – Wikipedia

Magnification Power of Scanning Electron Microscope

  • The magnification of an SEM ranges from 10X to 3,000,000X
  • The image magnification on SEM is not dependent on the power of the objective lens, as is the case with transmission electron and optical microscopes.
  • Areas as tiny as 5 microns to approximately 1cm in width can be viewed using the scanning mode on an typical SEM

Advantages

  • SEMs are robust, reliable and technologically advanced instruments
  • They are user-friendly interfaces and trained users find it easy to operate
  • Modern-day SEMs help in creating digital data of the specimen thereby making it easier to analyze it
  • The SEMs are quick and the results are delivered fairly quickly, therefore aiding faster analysis of samples.

Disadvantages

  • SEMs are very expensive and contains several sensitive components
  • They are bulky and require several ancillary requirements to ensure proper function.
  • They require special training to be handled 
  • They need special cooling systems, hence rendering their maintenance to be a hassle.

Price of SEM

The type of electron microscope also decides the price of the microscope because of the various uses the microscope has and also on the components used in them.

The common types of electron microscopes that find widespread usage are Scanning Electron Microscopes (SEM), Focussed ion beam electron microscope (FIB) and Transmission Electron Microscope (TEM).

The table below gives you a fair bit of idea of prices about used as well as new electron microscopes.

ModelTypeUsed PriceEstimated Price (New)
FEI Tecnai F20TEM$395,000$592,500
FEI Tecnai G2 F30 TwinTEM$395,000$592,500
FEI Altura 810SEM$350,000$525,000
Philips CM 200F TWINTEM$300,000$450,000
FEI Tecnai 20 TWINTEM$300,000$450,000
Hitachi SU-70SEM$275,000$412,500
Hitachi 5500SEM$250,000$375,000
FEI Strata 210SEM$250,000$375,000
JEOL 1200ex IITEM$250,000$375,000
FEI FIB 800-MFIB$250,000$375,000
Hitachi 4700-IISEM$225,000$337,500
FEI Tecnai 12TEM$225,000$337,500
FEI Quanta 200SEM$200,000$300,000
FEI FIB 200-SFIB$200,000$300,000
Hitachi 5200SEM$175,000$262,500
Hitachi 4700-ISEM$175,000$262,500
FEI CM200TEM$175,000$262,500
Philips CM 120 BTTEM$175,000$262,500
FEI FIB 200-MFIB$175,000$262,500
Philips CM12TEM$161,000$241,500
FEI XL40SEM$155,000$232,500
FEI XL30SEM$150,000$225,000
ZEISS EVO 50SEM$150,000$225,000
JEOL 1011TEM$150,000$225,000
Philips CM 120TEM$150,000$225,000
JEOL 1200ex ITEM$150,000$225,000
JEOL JEM-2011TEM$150,000$225,000
Philips CM 100TEM$150,000$225,000
FEI FIB 200-PFIB$150,000$225,000
FEI MorgagniTEM$135,000$202,500
FEI Quanta 400SEM$130,000$195,000
Philips CM10TEM$125,000$187,500
Philips EM 208TEM$125,000$187,500
Hitachi 7000TEM$105,000$157,500
Zeiss EVO 40SEM$95,000$142,500
Hitachi 3000NSEM$85,000$127,500

Frequently Asked Questions

Q1. How many times can a scanning electron microscope magnify?

An SEM can magnify up to a million times (1,000,000x)

Q2. What can you see with a scanning electron microscope?

The SEM is typically used to create high-resolution images of shapes of objects and to show spatial variations in chemical compositions. They are used to understand the morphology and topology of the compositions of specimens

Q3. What is a Field emission scanning electron microscope?

Field emission scanning electron microscope (FE-SEM) is an advanced technology instrument that is used to capture the microstructure image of the materials.

Q4. Who discovered the scanning electron microscope?

Manfred von Ardenne developed the first version of the SEM in 1937

Q5. What is the cost of a scanning electron microscope?

The price of a new electron microscope ranges between $80,000 to $10,000,000 and above depending on the customizations, configurations, resolution, components, and brand value

References
  1. https://www.sciencedirect.com/topics/neuroscience/scanning-electron-microscope
  2. https://en.wikipedia.org/wiki/Scanning_electron_microscope
  3. https://www.britannica.com/technology/scanning-electron-microscope
  4. https://www.biosciencenotes.com/scanning-electron-microscope-sem/
  5. https://serc.carleton.edu/research_education/geochemsheets/techniques/SEM.html
  6. https://tssmicroscopy.com/instruments/
  7. https://microscopeclarity.com/electron-microscope-cost/

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