Midterm Study Guide Flashcards
(18 cards)
Draw a schematic of the Tecnai labeling the following: electron source, CCD camera, sample position, and the three apertures
COD: Condenser (above the sample), objective (below the sample), and diffraction (selected area)
Describe one method for setting the sample to the eucentric position in the TEM
- At eucentric position: the image doesn’t move when tilted
- When we find a region where we’re interested, we find an SA range mag
- Start alpha wobbler
- Eucentric focus (before you EVER touch the z button, always press the eucentric focus)
- Minimize movement
- At the eucentric position, minimum contrast = in focus (ie: it looks like it should be fading out)
What stigmator is used to correct for astigmatism in a TEM?
Condensor stigmator: for beam shape
Objective stigmator: for the image (objective aperture forms the image, stimgator corrects for astigmatism in the image)
Diffraction stigmator: corrects for astigmatism in the diffraction pattern
How is the mean free path of an electron related to the accelerating voltage? Does the sample have an effect? If so, what is it?
The greater the interaction cross section, the smaller the MFP of the electron (the MFP is a better descriptor of the electron beam; the interaction cross section is more related to the material type of the sample, but it does have an effect and they are inversely related)
Which surface of the sample do the electrons we detect in the SEM exit? What about the TEM?
- “Where is the detector position relative to the surface of the sample?”
- You collect electrons from the top of the surface in SEM since samples are bulk and can’t penetrate through; you collect from the bottom in TEM since it’s like shadow puppets, the electrons are passing through the sample
Write out the procedure for the following TEM alignments: Gun Tilt, Gun Shift, pivot points X&Y, rotation center.
• Gun tilt: you want the electron beam to go straight down instead of hitting the sides of the wall (solution: maximize your brightness)
• Gun shift: beam shift to the center (Spot #9 and beam shift to the center as a reference; then change to spot #3, and if the beam moves, you gun shift back to the center. Keep iterating this process it’s helping to align the lenses so that the beam goes straight through the center); spot 9 is always related to beam shift; higher magnitudes can help, but you lose benefits if it’s past 60 KeV, diminishing returns essentially its coils shifting the lens so that the beam can get into the right position
• Pivot points X&Y/Beam tilt: before rotating the beam into the center, you need to make sure that the beam is still parallel to the optical axis; ie: you want the beam to hit the sample in the right position regardless of how you tilt the sample
o If it’s aligned, you see one spot; if it’s not aligned, it’s two spots (the instrument is rapidly moving back and forth)
• Rotation center: essentially the same as gun shift, but on a different lens in the sample; we spread the beam out so that the image pulses minimizes the image; pro tip: use small plate; we want the beam to go through the center of the lens so we can limit spherical aberration
How to focus bright field images?
Look for Fresnel fringes – determines if the sample is above or below the image plane (determined by the eucentric position of the microscope which is inherent to the manufacturing process)
Overfocused –> if the image falls ABOVE the image plane
Underfocused –> if it is underfocused
overfocused –> dark rings
underfocused –> white rings
Equation to calculate beam penetration depth into a sample; how does changing voltage affects image interpretation?
Rk-o (μm) = 0.0276(A/(Z^0.89*p))E0^1.67
How does current affect resolution in the SEM?
Increasing current reduces resolution since the spot size gets much larger, but you need current to flow
• Introduces aberrations and may create spots, but more current = more signal
How does increasing the voltage affect resolution in the SEM?
Depends on the sample. Increasing the voltage increases penetration depth, which may be valuable if determination of properties and their concentrations within a sample is desired. However, high voltage will cause greater charging in low atomic samples, and you lose surface topography.
What strategies can you use for imaging and focusing with low current?
- Decrease scan rate
* Reduced area mode – if you make the whole window slow, then it’s going to be hard to bring the image back into focus
Why do longer working distances in an SEM allow for better depth of field?
- Depth of field represents the furthest you can resolve two different things in an image
- Long working distance represents the distance between the objective lens and the sample; the longer the working distance, the better the DOF but less topography since the topography for each part of the sample is at different planes
How do you maximize the solid angle (sr) to a detector? Why is this important?
- When the beam hits the sample, a lot of the sample reflects back signal, but most of the signal doesn’t hit the detector since they scatter at wide angles (you’re missing a lot of incoherent scattering, in specific)
- Therefore, if you decrease the working distance, you would increase the amount of signal that you’re getting since the detector is much closer to the signal
Understand the basic pump down sequence of an SEM
V2 is open, rough pump depressurizes both its chamber and the turbo pump; V2 closes, and the V3 pumps down whole chamber; V3 chamber closes, and the V1 opens, turbo pump depressurizes the whole chamber
Understand what changing magnification does in an SEM
Magnification is the distance your beam is rastering; if you have high magnification, it’s rastering fast at a smaller distance; low mag means slow fastering at long distances; this is why if you resolve at high mag, and then pull back to see a greater amount of the image, then you are going to be better off
What does changing the bias on an ETD do to the image
Everhart-Thornly Detector; the most common detector, the “OG” detector, generic detector that sits in the detector off-axis from the beam; it’s positively charged detector since the electrons are negative, applying a bias means that they get pulled into the electrons; if it’s not positively biased, then other electrons that aren’t the secondary electrons (which are the ones being detected by ETD since they need the positive charge) are going to get into the ETD; it’s actually a good way to determine contrast and get rid of surface artifact if you want to get more information on the properties of the material if you wanted a ETD without the bias on
Know the average atomic number influences the backscatter coefficient in phases containing more then one element, and that the backscatter coefficient changes with incident angle.
The backscattered electrons tells you the atomic number, and the EDS sample can tell you about the wave nature of the electrons coming off at what angle, so you can increase your detection and understand subtle differences in what kinds of materials are in what part of your sample, at specific values too
Understand the use of coatings for sample prep
Samples often get charged since the beam is destructive by nature; anything really conductive can be a good coating (ie: metals like gold and copper; can use paint too; sputter coatings and thermal deposition coatings
