What is the function of wehnelt cylinder in an electron gun?

A Wehnelt cylinder (also known as Wehnelt cap, grid cap or simply Wehnelt) is an electrode in the electron gun assembly of some thermionic devices, used for focusing and control of the electron beam. It is named after Arthur Rudolph Berthold Wehnelt, a German physicist, who invented it during the years 1902 and 1903.

How do you increase magnification in SEM?

Magnification of SEM Since the size of the monitor screen is unchanged, decreasing the scan width increases the magnification, whereas increasing the scan width decreases the magnification.

Why do we need vacuum in SEM?

Besides protecting the electron source from being contaminated, vacuum also allows the user to acquire a high-resolution image. In the absence of vacuum, other atoms and molecules can be present in the column. Their interaction with electrons causes the electron beam to deflect and reduces the image quality.

What is the function of anode in electron microscope?

Anode A metal plate which has a positive charge. It attracts the electrons to form a beam.

How does a field emission gun work?

A field emission gun (FEG) is a type of electron gun in which a sharply pointed Müller-type emitter is held at several kilovolts negative potential relative to a nearby electrode, so that there is sufficient potential gradient at the emitter surface to cause field electron emission.

What is an advantage of field emission electron gun over thermionic gun?

15.13. Thermionic sources (W and LaB6) are cheaper, can operate under lower vacuum conditions, and provide greater brightness for large-area illumination than field emission sources. Field emission sources provide greater brightness, narrower energy spread, and more coherency than thermionic sources.

What is the magnification of SEM?

Areas ranging from approximately 1 cm to 5 microns in width can be imaged in a scanning mode using conventional SEM techniques (magnification ranging from 20X to approximately 30,000X, spatial resolution of 50 to 100 nm).

How does an SEM microscope work?

The 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. Once the beam hits the sample, electrons and X-rays are ejected from the sample.

Why are electrons used in SEM?

A scanning electron microscope (SEM) scans a focused electron beam over a surface to create an image. The electrons in the beam interact with the sample, producing various signals that can be used to obtain information about the surface topography and composition.

Does SEM have objective lens?

The SEM ‘objective lens’ lies above the specimen, while that in the TEM is below the specimen. Thus, the SEM objective lens is the last lens in the series of lenses that create the electron beam that scans the specimen. This lens focuses the scanning beam to a very small spot.

What is the role of objective lens in SEM and TEM?

Thus, the SEM objective lens is the last lens in the series of lenses that create the electron beam that scans the specimen. This lens focuses the scanning beam to a very small spot. In the TEM, the objective lens focuses the image, while in the SEM the objective lens focuses the scanning probe.