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

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

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

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

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

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

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

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