SEM intro, signals and detection

Question 1

SEM general structure

Answer 1

source -> set of lenses and appertures -> detector -> sample

Question 2

Types of lenses

Answer 2

condenser and objective lenses

Question 2

Limitations on probe size/ reolution

Answer 2

1. diffraction: lambda/(2*a)
2. spherical aberration: ds = Cs*a^3
3. chromatic aberration: dc = CcdE/Ea
4. source size: d_imaging = M_t*d_source

Question 3

Types of electron sources

Answer 3

1) heated W filament (the og) (thermionic)
2) LaB6 (thermionic)
3) FEG (Field Emmision Gun)(again from W but also ZrOx reservoir)

Question 4

What are different approaches to generate a beam of electrons / different general principles important for the design of an electron source?

Answer 4

1. High Electric Field which will pull electrons from a material
   1.5 Enhanced electric field through sharp tip of material (e.g. FEG)
2. Heat
3. Photon emission (rarely used)

Question 5

Disadvantage of heating as method to generate electrons

Answer 5

dE/E0 is significantly affected => chromatic aberration

dE: beam energy spread
E0: beam energy

Question 6

Disadvantage of FEG

Answer 6

sharp tip also means lower source diameter => lower imaging diameter => lower resolution

Question 7

Types of Apertures in SEM

Answer 7

Condenser Apertures: Located in the electron optics system before the sample, they help control the beam size and intensity reaching the sample.

Objective Apertures: Placed very close to the sample, these apertures help enhance resolution by limiting the acceptance angle of electrons that contribute to the image, thereby improving depth of field and sharpness.

Annular Apertures: Used in backscattered electron detectors, these allow for selective detection of backscattered electrons from specific angles, enhancing compositional contrast in images.

Question 8

Why does SEM provide topography contrast?

Answer 8

Backscattering electrons with lower angle can pass through thin edges away from the electron scattering volume, if they are at a locally higher region and thus be detected. This cannot happen on a flat surface. See figure from lecture if still confused.

Question 9

What does a lens do? Why is it challenging to make an electron lens?

Answer 9

Deflects rays with different intensity depending on how close they are to the central axis on which the rays are being focused on.
This is challenging to implement with electrons since you need to apply a heterogeneous electric field on them.

Question 10

Electric field lenses

Answer 10

an electric field pointing parallel to the electron beam would be curved in the begining where the beam enters. This will result in unequal bending of the outer electrons of the beam. While the opposite would be expected at the other end, the electrons are additionally being accelerated and being already focused on the axis so this effect is negligible. However, compared to magnetic lenses, electric ones suffer more from abberations also because curvature of the field which this is based on could not be perfectly spherical or contaminated or if the beam is not centered on same axis as opening

Question 11

Magnetic lenses

Answer 11

The principle is the same but with a magnetic field. An electric coil is put around the vacuum chamber and it creates a magnetic field. Similar to the electric field, at the edge of the lens electrons on the outside are affected less by the Lorentz force. However, due to the different nature of this force, which is influenced by the direction of the electron movement, they will begin to rotate and thus spiral towards the axis.