Scanning Electron Microscopy

Question 1

What is the resolution of a High-end SEM?

Answer 1

~ 1 nm

Question 2

What can be studied with SEM?

Answer 2

Topography/ morphology
Chemistry
Crystallography
Orientation of grains

Question 3

What is the equation for resolution?

Answer 3

r = 0.61 λ / n sin α

Where n sin α is the numerical aperture

Question 4

What happens when wavelength goes down?

Answer 4

Resolution goes up

Question 5

What is the voltage and subsequent Wavelength of Electrons

Answer 5

200 kV ~ 0.0025nm
r ~ 0.015 Å

Question 6

How does the Aperture angle affect the depth of field?

Answer 6

Larger Aperture angle = Small depth of field

Question 7

Which Depth of field is larger?

Answer 7

SEM

Question 7

What is the equation for depth of field?

Answer 7

D = d/tan α
Where D = depth of field
and d = resolution at magnification

Question 8

What passage do electrons take through an SEM

Answer 8

Electron gun, Condenser Lens, Beam deflector, Objective lens, Specimen, Detector

Question 9

What can the electron gun sources be, and what are they suitable for (from least to most expensive)

Answer 9

Tungsten hairpin ($10), Lanthanum Hexaboride ($1000) - work on Thermionic emission
Field emission ($10000) - Strong electron field

Question 10

What does the Condenser lens do?

Answer 10

Demagnifies the source - produces small beam of electrons

Question 11

What does the beam deflector do?

Answer 11

Moves the beam of electrons to the correct position

Question 12

What does the objective lens do?

Answer 12

Focuses the beam to a small spot, the size of which being controlled by the objective aperture

Question 13

How does the Thermionic Emitter Electron source work?

Answer 13

• A negative cathode is heated to produce electrons (Up to 2700K for Tungsten)
• Electrons accelerated by positive anode potential
• Wehnelt Cap (-500V) repels electrons toward optical axis
• Electrons collect in space between filament tip and Wehnelt cap
• Electrons nearest to anode exit gun through small (<1mm) hole in wehnelt cap

Question 14

What are the properties of an electron beam?

Answer 14

Point source, Similar energies (monochromatic) and parallel to optic axis

Question 15

How does the FEG worl?

Answer 15

Source size = 50 nm
First anode is positively charged by several kV with respect to tip, extracting electrons by enabling them to tunnel out of the tip. Second anode accelerates the electrons to 100 kV or more

Question 16

What are the benefits and drawbacks of using a thermionic source for electrons?

Answer 16

• Cheap, easy to handle, moderate vacuum required, good stability
  -Not long lasting (100 hrs), low Brightness ratio, and bigger attainable resolution

Question 17

What are some benefits and drawbacks of a field emission electron source?

Answer 17

• Large Brightness ratio, much smaller energy spread, longer lifetime
• Significant cost, Requires proper handling, very high vacuum required

Question 18

What are backscattered electrons?

Answer 18

-Elastic interaction with the atom in the material
- Have a high energy as they are close to primary beam
- BSE emission is dependant on the avg atomic number of sample

Question 19

What are secondary electrons?

Answer 19

• Generated from the collision between the incoming electron and loosely bonded outer electron
• Primary electron strikes electron in atomic shell
• this electron becomes a secondary electron (low energy)
• primary Electron continues colliding
• Only SE generated close to surface escape (topographic information is obtained)

Question 20

What has higher energy, Secondary electrons or backscattered electrons?

Answer 20

Back scattered electrons (> 50 ev)

Question 21

What happens to the interaction volume when the accelerating voltage decreases

Answer 21

It decreases, but increases the size of the beam as the wavelength of electrons is increased

Question 22

How is a pixel brightness calculated?

Answer 22

The more a scanner is hit with electrons, the brighter that pixel from that scanner will be (Brighter pixel = more counts)

Question 23

What is the equation for magnification?

Answer 23

magnification = Distance displayed on screen / Scan distance

Question 24

What happens to the number of secondary electrons escaping from an edge as opposed to a flat area?

Answer 24

more

Question 25

What creates a channeling contrast image and why?

Answer 25

BSE, as different atomic numbers will result in altered Backscattered electron energies

Question 26

What is the resolution of Secondary and Backscattered electron imaging, and why?

Answer 26

Secondary = 1 - 50nm, lower energy and smaller specimen volume
BSE = 10 - 300nm, as much greater penetration depth

Question 27

What penetrates more? BSE or SE

Answer 27

BSE (10 - 300 nm)

Question 28

Why is resolution important?

Answer 28

Allows us to distinguish two close by objects instead of seeing just one

Question 29

What info do you get in BSE?

Answer 29

The Z-Contrast