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/SEM Technology Overview
SEM Technology Overview 2017-06-06T13:52:47+00:00

SEM Technology Overview – Scanning Electron Microscopy 

The information below is provided only as a general overview of Scanning Electron Microscopes and the imaging and analysis they can provide.  The interested reader is encouraged to explore some of the much more thorough reviews found in books and around the web.  Here are a few that we recommend:

See more references at our LINKS page

Interaction between electron and solid

SEM Overview

Optical Microscope
Low resolving power, Low depth of focus

Scanning Electron Microscope
High resolving power, High depth of focus

Overview of Scanning Electron Microscope Components

SE (Secondary Electron)

Electrons having energy less than 50eV generated in the specimen by inelastic collision of electronic beam and the specimen when incident electron beam collided with the specimen

BSE (Backscattered Electron)

Electron emitted backwards elastically when incident electron beam collided with the specimen

SEM Overview

Schematic of Scanning Electron Microscope internal components

Electron Beam Manipulation

1) How small the size of electron beam shot into specimen can get is the key of high resolution of SEM
2) ELECRTON LENS
–Condenser Lens

  • Reduces crossover made up in Electron gun
  • Controls Spot size

–Objective Lens

  • Changes location of Prove crossover depending on optic axis
  • Focuses image
  • Generating electron ray
  • Lens system controlling electron ray
  • Scan Coil deciding magnification by deciding scanning area on the specimen
  • Stigmator eliminating astigmatism
  • Detector collecting Signal gained from specimen
  • CRT showing information gained from Detector

Principle of Magnification of SEM

Interaction volume and signal

SE / BSE Detector Images

pic_01-300x231

SE(Secondary Electron)

Secondary Electrons form images with information with topographic contrast and are generally used to compose the overall image by observing the surface shape.

BSE (Backscattered Electron)

BSE 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

SE (Secondary Electron) Detector

Secondary Electrons generated on the surface of a specimen by the energy gained from inelastic collision with beam electrons. Only a small amount of kinetic energy is transferred to secondary electrons because the amount of energy in beam electron is very small compared to the electrons of specimen.

Most SEMs utilize the Everhart-Thornley (E-T) Detector design. Photons are generated when activated electrons collide with a Scintillator.  They then move into a photomultiplier tube by total reflection in an optical waveguide. Photons can go through vacuum and quartz windows because they are in the form of light. Photons attract current from both poles, collect and go back to current at detectable points.

Electrons directed at the sample and are scattered in various directions. These are divided into elastic or inelastic scattering of electron.

Elastic scattering of primary electron – Scattered electrons change momentum but energy changes less that 1eV. Because P=mv and m doesn’t change, only the direction of speed vector changes. Scattering angle is 0-180° and typically 5°. Elastic scattering is generated between negative electron and positive atomic nucleus. Some electrons have large scattering angle to be directed outside the sample and these are backscattered electrons.

Brighter portions of the BSE image above is copper and darker portions are aluminium. Energy during inelastic scattering is transferred to electrons around the atom and kinetic energy in activated electrons is reduced.

Image change depending on Accelerating Voltage

Image distortion depending on Astigmatism

Observation by SEM

Observation of surface image

Observation of cross-section structure

Thickness measurement

Conclusion. Principle and feature of electron beam analyzing technique

References

1. Principle and Application of electron microscope
2. Analysis of SEM and X-ray microanalysis