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Written & Edited by Debapratim Pal
What is SEM:
SEM is a surface imaging method by which the incident electron beams scan across the surface of the sample. In this method, the sample interacts with the incident electron beams and generates secondary and backscattered electrons which create the image of the surface. In optical devices, resolution has very important role. Basically, resolution of a microscope denotes the smallest distance between two points on a specimen which can be distinguished as two separate entities. The resolution depends on the several factors but wavelength is one of the most important factors. The resolution is inversely proportional with the applied wavelength. An optical microscope uses a wavelength range of 400-700 nm. On the other hand, the electrons have a very small wavelength (around 1 angstrom). Since the electron has very small wavelength, it has a very high-resolution power. In this way, the SEM has superior resolution than optical microscopes.
The components of SEM:
In SEM, the vacuum condition is maintained. Without vacuum condition, other particles can be present in the column which reduces the resolution and also it deflects the electrons. The vacuum condition protects the electron beam from contamination which also increases the resolution power. For this reason, all the components are kept in a vacuum condition.
The components of SEM are-
Source of electrons
Columns through which electron moves
Condense lens
Electron detector
Sample chamber
Computer and display
How it works:
At first, electrons are produced at the top of the column by electron source. When these electron’s thermal energy overcome the work function of the source material, the electrons start emitting. These electrons are attracted towards anode which is positively charged. To generate a proper electron beam, lenses are needed. Since electrons can’t pass through glass, coils within metal pole pieces are used. A magnetic field is produced when electron beam or current pass through this. The path of the electron is properly controlled by this magnetic field. Two types of lenses are used in SEM. These are - condenser lens and objective lens. The condenser lens helps to define the resolution by adjusting the size of electron beam. The condenser lens converges the electron beam towards the target. The emitted electrons firstly go through condenser lens then goes to objective lens.
The objective lens helps to focus the electron beam on the target sample. Apertures are also used to control the size of the beam. The scanning coils help to raster the electron beam on the sample. The scan coil is placed above of the objective lens which controls the positions of electron beam. The raster scanning process is done on a particular area which is defined by scanning coils. The electron-sample interaction produces many signals which are detected. Basically, the interaction of electron beam and sample generates electrons, photons and even irradiation. In SEM, backscattered electrons, secondary electrons and X rays are produced. The backscattered electrons reflect back after the elastic collision between beam and sample. The backscattered electron beam is a primary electron beam. The backscattered electrons come from deeper region of the sample. For this reason, these electrons are used to determine sensitivity towards the atomic number. The atomic number is proportional with the brightness of the material in the image. On the other hand, the secondary electrons are emitted from the sample which is caused by inelastic interaction between sample and electron beam. The secondary electron beam is produced from surface of the sample so it provides information about the surface of the sample.
X ray is also produced during the incident electron beam and sample interaction. When a high energy electron collides with the electrons which are nearer to the nucleus, the electrons from nearer orbit goes to the vacant place. During this process, X ray is produced. Since every material has unique energy for X ray emission, this phenomenon is used in elemental analysis of the sample. By analyzing the energies of the X ray, different elements of an unknown sample can be determined.
The backscattered electrons and secondary electrons are detected by two different types of detectors. For backscattered electrons, solid state detectors are used. These detectors are placed above the sample. These detectors are used concentrically to the electron beam which increases the backscattered electron collection. The Everhart-Thornley detector is used to detect the secondary detectors. It is made of scintillator and faraday cage. The secondary electrons are attracted by positively charged faraday cage and the scintillator accelerates the secondary electrons. The electrons are converted into light inside the scintillator. Now this light is targeted in photomultiplier to amplify the signal. Since the secondary electron detector is placed at an angle at the side of the electron, it is able to generate 3D image of the sample. This type of orientation increases the efficiency of the detector. The whole process happens in the region of nanoseconds which is almost instantaneous.
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