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15 Cards in this Set
- Front
- Back
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What do you use to get a 'light' vacuum? |
We could use a rotary pump to get a light vacuum but not good enough to take the sample |
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What type of rotor does a rotary pump have? How does that affect the removal of air? |
It is an eccentrically positioned rotor - it is always touching the side but is off center and pushes air out by opening and closing areas in the pump with its rotation |
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What do we call when the oil feeds back in slightly? |
Backstreaming - oil that the rotors are submerged in sometimes leak back into the chamber if you leave the chamber door slightly ajar |
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What do we use to generate high vacuums? |
We use turbo molecular pumps to create as perfect a vacuum as possible. They remove the air using alternately tilted rotors and stators moving (or staying put) air down a column and out, the rotor's movements always pushing the air out. We often use a rotary pump to pull the air through faster.
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What makes up 99% of the molecules that get left in the chamber by the Turbo Molecular Pumps? |
Only very small molecules will remain after using a Turbo Molecular Pump - mostly HYDROGEN |
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What makes catastrophic failure of turbo molecular pumps do fear inducing? |
The ball bearings - should something fail heavy metal balls ricochet around a very delicate machine |
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Why would we want to vary the pressure of the chamber? |
ionizes the air molecules - can carry electrons and reduce CHARGING, also good for damp or uncoated samples. |
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When we let air in, what causes the reduction of resolution in the image? |
Skirting is the end of the beam interacting with air molecules and causes beam scattering which increases the apparent spot size. Also this reduces the signal because the electrons have a greater chance of getting deflected by air molecules |
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Why can we not use an E-T detector when we introduce air into the chamber? What should we use instead? |
This can cause arcing between the Faraday Cage and the sample which can be very damaging! We use a BSE detector OR a Gaseous Secondary Electron Detector (GSED) |
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How is an ESEM different from your everyday SEM? |
Environmental Scanning Electron Microscope - environmental part means controlling temperature and humidity by adding water vapor to the chamber. These can be used to image a sample that is a little hydrated. |
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How does an ESEM keep the sample's surface moist? |
A small amount of water vapor is allowed to leak into the chamber. The peltier stage is then set to a slightly lower temperature causing the water to condense on the surface of the sample. |
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What are some issues with ESEM? What type of detector is optimal? |
Because the sample needs contact with the water vapor, they cannot be coated and so may have serious charging issues. To reduce the charging problems we use a GSED. |
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How is a Peltier Stage different the the Cryogenics stage? |
Peltier stages are meant to alter the temperature in small amounts, cryo stages are meant to be taken down as cold as they can to maintain the frozen sample. |
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Explain the Gaseous Secondary Electron Detector |
The beam is focused through the final aperture and an extremely thin disk that has a slight positive charge and hits the sample. The beam causes the water to ionize into OH- and H+, the secondary electrons ejected from the sample 'bump' into the OH- which carries the charge until it bumps into another OH ion and so on until the signal reaches the detector - called gaseous amplification. It is rarely the secondary electron that strikes the detector. The beam does interact with the detector slightly and the ions present in the chamber cause skirting, which reduces resolution. |
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How is TSEM different from SEM? |
A transmission electron microscopes send the beam THROUGH the sample and require up to 8 days of preparation to look at interior structures/features. |
