SEM Basics
A Scanning Electron Microscope (SEM) uses a focused beam of electrons to create a magnified image of a sample. The electron beam is scanned in a regular pattern across the surface of the sample and the electrons that come out of the sample are used to create the image.
 
Essentially, the way the scanning electron microscope “looks” at the surface of a sample can be compared to a person alone in a dark room using a fine beamed torch to scan for objects on a wall. By scanning the torch systematically side-to-side and gradually moving down the wall, the person can build up an image of the objects in their memory. The SEM uses an electron beam instead of a torch, an electron detector instead of eyes, and a viewing screen and camera as memory.

The SEM is a tool for creating images of the otherwise invisible worlds of microspace 1um and nanospace 1nm. SEMs can magnify an object from about 10 times up to 300,000 times. A scale bar is usually provided on an SEM image. The scale bar is used to calculate the sizes of features in the image. SEM images have no colour (but may be artificially coloured), they may look quite three dimensional (due to depth of field) and they show only the surface of the sample (due to minimal penetration of the electron beam into the sample).
 
Overview of Scanning Electron Microscope Components

 
SE (Secondary Electron)
When incident electrons travel towards a specimen, these electrons lose their energy while repeating collision with constituent atoms in the specimen (inelastic scattering). In this process, outer-shell electrons of the constituent atoms are ejected, and then part of them overcome the binding energy and are emitted from the specimen surface. These emitted electrons are called “secondary electrons.” Since the energy of secondary electrons is small (normally, several 10 eV), only those generated near the top surface of the specimen (depth: 10 nm or less) are emitted from the specimen. The secondary electron yield becomes larger as the incidence angle of the electron beam with the specimen is smaller (at a grazing incidence). The difference of the secondary electron yields in a secondary electron image reveals surface morphology of the specimen. Secondary Electrons form images with information with topographic contrast and are generally used to compose the overall image by observing the surface shape.

BSE (Back Scattered Electron)
When incident electrons travel towards a specimen, a part of electrons is reflected (scattered) backward and emitted from the specimen surface. These emitted electrons are called “backscattered electrons.” The intensity of the backscattered electrons is larger as the atomic number of the constituent atoms in the specimen is larger. The energy of the electrons is close to that of incident electrons, indicating that backscattered electrons possess higher energy than secondary electrons. Thus, the backscattered electrons are emitted from a deep region from the top surface (depth: 100 nm or less) compared to secondary electrons. A backscattered electron image provides the difference of the specimen composition and topographic shape. If the specimen is a crystal, the backscattered electron intensity largely depends on the orientation of the incident electron beam due to electron channeling. Thus, a backscattered electron image obtained from a crystalline specimen shows the difference of the crystal orientation in the specimen. backscattered electron images have atomic weight contrast and show how the specimen is composed of various elements. The brightness increases in proportion to the increase in atomic weight or number.

Schematic Diagram of Scanning Electron Microscope

Image change depending on Accelerating Voltage