Draw the schematic diagram of Scanning Electron Microscope and explain its construction, working, advantages, disadvantages and applications.
SEM is a type of electron microscope that uses electron beam which is focused on the specimen and by detecting the signals from the interaction of incident electron with sample surface.
The sample is placed inside the microscope’s vacuum column through an air tight door. The electron gun assembly produces a high energy electron beam which travels through a series of magnetic lens which helps in focusing the electron beam as a small spot on the specimen surface. It scans the specimens in the raster pattern.
When the high energy electron strikes the surface, some electrons are scattered due to electron scattering. Some are knocked off from the surface, some electrons penetrate deep into inner shells to knock off inner shells electron due to which x rays are produced.
These electrons detected using detector. the signals are amplified and displayed on TV monitor. The final image is built up from the number of electron emitted from each spot of the sample.
If the scattering is high at particular spot, the corresponding point on the screen is bright and vice versa. This creates an image of the objects.
Advantages:-
- High magnification range
- Good image resolution
Limitations:-
- Vacuum needed
- gives only surface image of specimen
- the specimen must be conductive
Application: – useful in material science, forensic science, solid state physics, medical science