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27 Cards in this Set
- Front
- Back
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Define a primary electron beam? |
electrons from the electron gun, through the lenses |
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Spot Size |
area of the cross-over |
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What is the probe current and how does it effect the spot size? how does the spot size effect resolution? |
the probe current is the amount of electrons in the beam. The higher the probe current, the greater the spot size. The lower the current the smaller the spot size. To maximize resolution you want to have a small spot size, so a lower probe current is better. |
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How can probe current effect resolution? What might be a limiting factor? |
The probe current is directly proportional (or has some correlative mathematical relationship) to the spot size; lower current means smaller spot size however this is definitely a limitation because fewer electrons make detection much harder. |
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What do we call where the beam strikes the surface of the sample? |
Interaction-Volume |
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What are the two things that affect the interaction-volume? |
(1) Atomic number: the higher the atomic number, the more energy electrons have to expend on entering the sample and therefore cannot spread out as far and will have a smaller interaction volume. The opposite is also true. (2) The energy associated with the beam: higher beam = more energy/electrons = larger interaction-volume. |
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What are the two types of interactions electrons in the sample can have with the beams? What types of electrons undergo which type? |
Elastic (back-scatter) and Inelastic (secondary, Auger) |
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Elastic Interactions |
the electron from the beam strikes the electrons in the sample and deflects them at a wide angle and RETAINS all of the energy. |
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Inelastic Interactions |
the electron from the beam strikes the electrons in the sample and deflects them at a small angle and causes the energy transferred to drop considerably. |
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What type of interaction do Secondary Electrons undergo? What is their relative energy level? |
Secondary electrons undergo inelastic reactions which decreases the ejected electron's energy considerably. They are no more than 50eV but are generally at about 2-5eV. These are the most commonly used for E-T detectors. |
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What type of interaction do Back-Scatter Electrons undergo? What is their relative energy level? |
Back-scatter electrons undergo elastic reactions which completely transfer the energy from the striking electron to the sample's electron. This means that back-scatter electrons have roughly the same amount of energy as the probe current. Images made with these lack resolution. |
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What type of interaction do Auger Electrons undergo? What do we use them for? |
Auger electrons undergo inelastic electrons but we only use them on things with atomic numbers less than carbon. |
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SEI |
Produced by the beam hitting the sample and is used to create the image. They make up about 9% of total detected electrons. |
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SEII |
Generated by back-scatter electrons as they are trying to leave the sample and collide with other atoms and create another secondary electron. Secondary electrons created by this are perceived as noise and must be removed from the final image. 28% |
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SEIII |
Generated by back-scatter electrons that escape the sample and strike something in the chamber to create noise. 61% |
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SEIV |
Part of the primary beam interacting with the aperture and causes noise. 2% |
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Briefly explain an Everhart-Thronley detector |
1) Faraday cage - is lightly positively charged (300eV) that acts as an electron sponge. 2) Scintillator - made of CaF2 which is charged at +12,000V and get accelerated drastically and smash into the material (phosphor); when the electron hits it emits a photon that is mirrored by the thin film of Cu into the 3) light pipe (made of quartz) that gets bounced around by internal reflection into the 4) photo multiplier tube, which amplifies the signal. |
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What is a scintillator? What purpose does it serve in the Everhart-Thornley detector? |
The scintillator is made of the phosphor material CaF2 and has a massively positive charge (+12,000v) found behind the Faraday Cage that causes the electrons bumped off the sample to accelerate and strike the CaF2, emitting a photon. These are the signals that are read by the detector. |
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What is a photomultiplier? What purpose does it serve in the Everhart-Thornley detector? |
A chamber located at the end of the light pipe here photons are converted into electrons once more. The incident beam of light strike a photocathode and produce electrons through the photoelectric effect and are focused by an electrode toward electron multipliers that produce more and more electrons through secondary emission. |
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How do we take a back-scatter image using an E-T detector? |
We would need to change the polarity of the charge; instead of +300V we must make it -300V to repel the secondary electrons while the scintillator still accelerates them to strike the target and produce an image. |
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Explain the Robinson BS Detector. How do we increase resolution in this system? |
This is a type of detector that uses a passive scintillator meaning it has no charge; the beam is sent directly through the detector. The best resolution is obtained by reducing the working distance or increase the probe current (more e- to make more BSE). |
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Why do images made with back-scatter electrons have poorer resolution than those made with secondary electrons? |
The back-scatter electrons are generated deeper in the sample than secondary electrons and because of the shape of the interaction-volume this means the apparent spot size is larger, which decreases resolution. To counter this with increasing the amount of electrons hitting the sample would also generate a larger spot size too. |
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What is a Solid State Detector? |
A detector used for back-scatter imaging made of semi conductive material positioned directly over the sample. The 5th quadrant often featured in newer models provides better 3D perception. The computer can manipulate the quadrant activity. |
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What is the cause of a bright area on your image? |
Bright areas on an image indicate negative charging, probably a result of the non conductivity of the sample. |
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What is the cause of a dark area on your image? |
Dark areas on an image indicate positive charging and is probably the result of the non conductivity of the sample. This is because the positively charged area is pulling electrons from the positions around it. |
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What is a catastrophic breakdown? How will it appear on the image? |
A catastrophic breakdown is the sudden release of a groups of electrons from a charged source. This will appear as a bright flash on the monitor. (Back-scatter detectors do not have this issue)
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What is an Electron Mirror? |
An electron mirror is generated on a sample that holds a lot of electrons; after a large build up of high energy electrons congregates on a spot, drastically reduce the probe current - the new low energy electrons are reflected away off the sample and create an image of the chamber. |
