Sunday, June 26, 2022

Scanning electron microscope Sem, definition, Principle, Working, Diagram, images, use

Scanning electron microscope definition.

A (Sem) scanning electron microscope is simply a type of electron microscope.

A scanning electron microscope uses a high-energy electron beam to produce an image of an ultra-thin sample after scanning it.

The signals which are produced after interacting electron beam with the atoms of a sample give information about the sample’s surface topography,

material orientation, chemical composition, and many other material properties.

Scanning electron microscope principle:

Scanning electron microscope (sem) works on the principle of interaction of electron beam (accelerated electrons),

with the atoms of a sample at various depths and reveal the information in the form of signals. 

How does a scanning electron microscope work?

A Sem uses an electron beam instead of a light beam. The scanning of the sample is done in a raster scan pattern.

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Here, after the interaction, various types of signals are produced.

But in Sem, we considered mainly two types of electrons.

The two types of electrons are secondary electrons (SE), and the reflected one, commonly called backscattered electrons (BSE).

After balancing, scanning, or combining the position of the beam with the intensity of the detected signal an image of the sample is achieved.

Question and answer session.

The question to ask friends:

Why the electron beam is used in sem and why not light?

Answer:

Because the wavelength of the electron is smaller than light, which provides a thousand times better resolution than light.

Scanning electron microscope construction and working.

sem scanning electron microscope
The image is modified and credit goes to Wikimedia.

Component or instrument used in scanning electron microscope.

Electronic Gun – For the source of the electron.

Vacuum chamber.

Electromagnetic lenses (Condenser lens, Objective lens).

Scanning coils.

Sample.

Backscattered electron detector (BSED).

Secondary electron detector (SED).

X-rays or light detector.

Computer or Tv Scanner.

The electronic gun provides the required electron beam by thermionic emission.

Thermionic emission: 

It is the thermally induced flow of charge carriers (electrons, ions) from a surface or over a potential energy barrier.

In electronic guns, we use a tungsten wire and kept at a negative potential around and heated at a high temp range by resistive heating.

In this process, the free-electron emits from the surface of the tungsten wire due to thermionic emission.

Question and answer session.

The question to ask friends:

Why tungsten is used for this purpose?

Answer:

In all metals, Tungsten has the highest melting point and lowest vapor pressure. Also, tungsten has a low price.

Scanning electron microscope Lens.

The electron beam is focused on the two electromagnetic lenses, condenser, and objective.

The main function of these two lenses is to control the path of electrons inside the column.

Also, the condenser lens defines the size of the electron beam in a few nanometers and converges it, to focus on the sample.

The Objective lens also converged the beam and deflect it in the x-axis and y-axis, before striking onto the sample.

Here the deflection is in the x-axis and y-axis so that the scanning coils scan into a raster scan pattern over a rectangular area onto the sample.

Scanning electron microscope types of electrons.

As you know there are two types of electrons in Sem for imaging the sample. 

BSE, Backscattered electrons:

BSE are the electrons that are reflected back by elastic scattering, after interaction between the electron beam and atoms of the sample.

In BSE imaging higher the atomic number of sample,

brighter the substance appears in the image,

as the intensity of the BSE are strongly related to the atomic number of the sample used.

The backscattered electrons come from the deeper region of the sample surface.

The BSE imaging reveals the information about the chemical composition in the sample and determines the crystallographic structure.

These electrons are collected by the Backscattered electron detector, which is used in the Sem.

SE, Secondary electrons:

Secondary electrons are the electrons that originate from the surface of the sample by inelastic scattering,

after interaction between the electron beam and atoms of the sample.

As the Secondary electrons come from the surface region of the sample, the Secondary electron imaging reveals information about the sample’s surface topography, etc.

X-rays: X-rays are produced during the interaction between the electron beam and sample at various depths.

After analyzing or detecting the energy of x-rays signals, which is come out from the sample, we can map the distribution and quantity of elements in the sample.

So that we use an x-rays detector for this purpose.

The computer screen or CRT monitors:

These are used to form images of the sample after collecting signals by various detectors.

♦ Colors in Sem:

The images produced by the Sem are not colored.

Various technologies are used to obtain colored images like photo-editing software and by using multiple detectors.

After this, we can see the images in a more detailed view.

Application of scanning electron microscope and scanning electron microscope uses.

Scanning electron microscope Sem
Photo by Rodion Kutsaev on Unsplash

Sem is a very important tool in many fields such as Biology, Gemology, Metallurgy, Forensic science, etc.

Any substance which is to be examined at nanoscale and Sem is very important to reveal info about the chemical composition, surface topography, examining fracture surfaces of metals, dimensional measurements, special for characterization of materials, corrosion measurements, etc. 

Sem has many different applications in industrial work and in electronic technologies like in Miniscule products, assembly of computer integrated circuits, etc.

Sem is used in semiconductor inspection (Where detecting any particles or defects in the wafer in the semiconductor wafer manufacturing process).

Scanning electron microscope advantages.

It is seen that many applications require minimal sample preparation and rapid results also.

Sem gives two-dimensional detailed surface topography imaging as well as three-dimensional imaging by using different detectors and techniques and software, etc.

The Sem is easy to use if someone has the proper training (knowledge of computer software is required for the study of the images at different scales).

In Sem, we can use CRT monitors as well as new technology’s high-quality monitors, etc.

Scanning electron microscope magnification.

Magnification in Sem is around 1nm to 10nm and in some Modern Sem magnification is more than 50nm.

In Sem magnification is controlled by the voltage supplied to scanning coils.

The scanning electron microscope resolution is very high and makes this instrument a great tool for detailed surface imaging.

Scanning electron microscope sem Conclusion:

According to us this instrument is a great tool in material characterization for researchers in physics and saves humans valuable time.

Some info is taken from Wikipedia.

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