Explain scanning process of scanning electron microscope, Physics

Assignment Help:

Explain about the scanning process and image formation of scanning electron microscope.

Scanning process and image formation of scanning electron microscope

In a typical scanning electron microscope, an electron beam is thermionically emitted through an electron gun fitted along with a tungsten filament cathode. Tungsten is usually used in thermionic electron guns since this has the highest melting point and lowest vapour pressure of each metal, thereby allowing this to be heated for electron emission, and due to its low cost. Many types of electron emitters comprise lanthanum hexaboride (LaB6) cathodes that can be used into a standard tungsten filament SEM when the vacuum system is upgraded and field emission guns (FEG) that may be of the cold-cathode type by using tungsten single crystal emitters or the thermally-assisted Schottky kind, by using emitters of zirconium oxide.

The electron beam, that typically has an energy ranging from small hundred eV to 40 keV, is focused through one or two condenser lenses to a spot around 0.4 nm to 5 nm into diameter. The beam passes by pairs of scanning coils or pairs of deflector plates into the electron column, classically in the last lens that deflect the beam into the x and y axes so that this scans in a raster fashion over a rectangular region of the sample surface.

While the primary electron beam interacts along with the sample, the electrons lose energy by repeated random scattering and absorption into a teardrop-shaped volume of the specimen termed as the interaction volume that extends from less than 100 nm to about 5 µm in the surface. There size of the interaction volume depends onto the specimen's density, the electron's landing energy and the atomic number of the specimen. The energy exchange among the electron beam and the sample results into the reflection of high-energy electrons through elastic scattering, emission of secondary electrons by inelastic scattering and the emission of electromagnetic radiation, all of which can be detected through specialized detectors. The beam current absorbed through the specimen can also be detected and used to make images of the distribution of specimen current. Electronic amplifiers of different types are used amplify the signals that are displayed as variations into brightness onto a cathode ray tube. The raster scanning of the cathode ray tube display is synchronised with which of the beam on the specimen in the microscope, and the resulting image is thus a distribution map of the intensity of the signal being emitted through the scanned area of the specimen. The representation may be captured through photography by a high resolution cathode ray tube, but into modern machines is digitally captured and displayed onto a computer monitor and saved to a computer's hard disk.


Related Discussions:- Explain scanning process of scanning electron microscope

Verify the rms potential drops across the three elements, The given circuit...

The given circuit diagram shows a series LCR circuit linked to a variable frequency 230 V source: (a) Verify the source frequency which drives the circuit in resonance. (b) G

What is the dimentional formula of plank''s constant?, The dimentional for...

The dimentional formula of  plank;s constant ML 2 T -1

Projectile motion, formula for height in a horizontal projection on an incl...

formula for height in a horizontal projection on an inclined plane?

Heat energy, A body of mass 500g IS heated such that its température roses ...

A body of mass 500g IS heated such that its température roses from -4*c to 100*c calculate the total heat energy consumed

., what is radioactivity?

what is radioactivity?

What are the applications of laser transitions, What are the applications o...

What are the applications of laser transitions? Applications of laser transitions: a. High coherence of light is helpful for measuring length precisely by interferometry or

Magnetic domains, Ferromagnetic materials get magnetic properties not only ...

Ferromagnetic materials get magnetic properties not only because their atoms owns a magnetic moment but also because the material consists of small regions called as magnetic domai

Magnetism, Differentiate between teporary and permanent magnet

Differentiate between teporary and permanent magnet

Free body force diagram, A person attempts to calculate the gravitational c...

A person attempts to calculate the gravitational constant G by suspending two 100.0kg spherical masses from a high ceiling in a shopping mall and calculating the deflection of the

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd