Question

name three usual defects of the electromagnetic lens and how do they differ from each other

what are the main parts of the scanning electron microscope? explain its functions

Answer 1

The three usual defects of electromagnetic lens are,

1. Chromatic aberration: The lens is unable to focus lights of different wavelengths to the same focal point, due to different colors of light having different refractive indices.

2. Spherical aberration: Spherical surfaces are unable to focus the rays parallel to the optic axis at the same point.

3. Diffraction: At the edges of the aperture the waves tend to diffract and spread out the focus, thereby decreasing the resolution.

The main parts of Scanning electron microscope and their functions are:

1. Electron gun: The electrons are produced either by thermionic emissions (by heating a tungsten filament) or by field emissions (electrons produced using high electric field). This electrons are then accelerated to 1-40 kV and are condensed to narrow beams for analysing and imaging.

2. Lenses: A series of condenser lens are required to focus the electron beam coming from the source, down the column. The narrower the beam the smaller the spot it creates on contact with the sample.

3. Scanning coil: It deflects the beam in x and y directions to analyse the surface of the sample.

4. Sample chamber: It is an evacuated chamber in which the sample is mounted.

5. Detector: There are different detectors in general, to detect the different electrons (secondary electrons, backscattered electrons, or characteristic X-rays) generated by the beam of electrons in a SEM. The signals comes from different penetration depths and depends on the accelerating voltage and sample density.