Unit 1- SEM microscope

secondary electron detector

An electron detector is placed in the sample chamber. By having a 10 keV positive potential on its face, it attracts the secondary electrons emitted from the sample surface. One advantage of this biased detector is that it can attract secondary electrons emitted from sides of the sample which are physically blocked from the detector face. This greatly reduces shadowing effects in SEM images.

electron beam

a column of high energy electrons

electron gun

a device consisting of the cathode which emits electrons, and a surrounding electrostatic or electromagnetic apparatus, which controls, focuses, and accelerates the stream of electrons

characteristic X-ray production

A hole in an inner shell of the specimen atom is generated by an incident high energy electron that loses the corresponding energy transferred to the ejected electron. The hole in the shell is subsequently filled by an electron from an outer shell. The superfluous energy is emitted as a characteristic X-ray quantum. The energy of the X-ray is characteristic of the specimen atomic number from which it is derived

Backscattered Electrons

If the incident beam collides with a nucleus of a sample atom, it bounces back out of the sample as a backscattered electron. These electrons have high energies and because a sample with a higher density will create more of them, they are used to form backscattered electron images, which generally can discern the difference in sample densities

how an SEM works

Microscopes that take electrons and focuses them onto a specimen; the electrons that scatter off the surface following this interaction can be analyzed with a variety of detectors that provide topographical, morphological and compositional information regarding the surface of a sample.

secondary electron

The incident beam is composed of highly energized electrons. If one of these electrons collides with a sample atom electron, it will knock it out of its shell. This electron is called a secondary electron and is weak in energy (nearly 100 volts). If these secondary electrons are close enough to the sample surface, they can be collected to form an SEM image. The incident beam electron loses little energy in this collosion. In fact, a single electron from the beam will produce a shower of thousands of secondary electrons until it doesn't have the energy to knock these electrons from their shells.

focus

concentration of electrons

vacuum chamber

location in the scanning electron microscope where the sample is placed into very low pressure environment.

sample chamber

location in which the sample is placed

electron beam

stream of electrons

contrast

the darkness of the background relative to the specimen.

brightness

the luminance of a body, apart from its hue or saturation, that an observer uses to determine the comparative luminance of another body. Pure white has the maximum brightness, and pure black the minimum brightness.