1. 08/03/09
SEM signal generation
Water droplet analogy – the electron beam consists of individual electrons that strike the surface of the specimen. These electrons act much like droplets of water falling into a bucket of water
XL Course - Module 1

2. Water droplet analogy Potential energy (~H) primary droplet
08/03/09
Water droplet analogy
primary droplet
“primary” splash – backscattered water molecules from primary droplet
Potential energy (~H)
“secondary” splash – water molecules from water in the “bucket”
Energy dissipation (heat, waves, etc)
XL Course - Module 1

3. 08/03/09
Energy of an electron
In an electron gun, electrons are accelerated by a voltage applied at the anode plate. In electron microscopy this voltage is usually referred to in thousands of volts, or kV.
The energy of an electron that has been accelerated by a 1 kV accelerating potential is 1 keV.
The Phenom accelerates electrons to an energy of 5 keV
XL Course - Module 1

4. Beam Interaction volume
08/03/09
Beam Interaction volume
Electrons undergo many random scattering events after entering a solid specimen. The computer simulations below show the paths of 250 electrons in different materials. Fe
SiO2
Organic
XL Course - Module 1

5. Backscattered electrons
08/03/09
In backscattering, a primary electron (an electron from the incoming beam) is deflected by the electrostatic field of the positive nucleus
Elastically scattered electrons (those that don’t lose energy) tend to be scattered at high angles
This process happens many times as a primary electron travels through the solid
e-1
e-1 + + +
XL Course - Module 1

6. Backscattered electrons
08/03/09
The red lines in this simulation depict electrons that were scattered back to the surface of the specimen and then escaped
XL Course - Module 1

7. Backscattered electron (BSE) information
08/03/09
The backscatter coefficient is proportional to Z, the atomic number
nuclei with more protons scatter more electrons by larger angles
In a compound or alloy, the determining factor is the average atomic number
Thus BSE’s are used for compositional imaging
Materials with higher average Z will deflect more electrons back towards the detector
BSE’s can be also used to measure surface topography
Surfaces tilted away from a detector will be less bright
XL Course - Module 1

8. 08/03/09
Other SEM signals
When irradiated by an electron beam, electrons penetrating the sample cause many different electronic transitions. Each of these transitions cause the sample to emits a signal. Each conveys information about the electronic transition, and therefore about the specimen itself. Each signal can be intercepted with the appropriate detector.
Backscattered electrons (composition, topography)
Secondary electrons (high resolution topography)
X-rays (elemental composition)
Visible and UV light (elemental composition)
And others
XL Course - Module 1