Lecture 6

Question 1

What is FIB?

Answer 1

Focused Ion Beam it can be used to image samples however it damages the sample. It can also be used to prepare samples for TEM. It uses a focused beam of Gallium or Agon ions

Question 2

Why must a specimen be polished so well before FIB?

Answer 2

The FIB will erode away any scratches faster than other parts of the sample

Question 3

Why must the specimen be less than 100nm thick in FIB?

Answer 3

Any thicker would lead to scattering therefore we would no longer know how the beam has been affected by the sample

Question 4

How can we create contrast?

Answer 4

Electron microscopes used monochromatic radiation therefore in order to create contrast we must use apertures. We use different lattices to create correlated scattering of the beam. A bright field image focuses on unscattered light. A darkfield image focuses on scattered light.

Question 5

How can lateral coherency be reduced?

Answer 5

Increasing the difference between the source and the detector

Question 6

What is phase contrast?

Answer 6

Introducing a phase difference between parts of the light supplied by a condensor so as to enhance the outlines of the sample or the boundaries between parts of differing optical density. The basic concept involves seperating the sample scattered light from the illuminating background light.

Question 7

How is phase contrast created?

Answer 7

As the light passes through a substance the atoms cause negligible phase shift however this adds up.