Question

Do crystalline grains aligned close to the incident ion beam angle give a brighter or darker contrast? What is this effect called and how does it occur?

Answer 1

Finely focused beam FIB interaction with target material will produce the ejection of secondary electrons, secondary atoms and secondary neutrals due to sputtering. Spettering Yeild varies with grain orientation which is associated with the existence of channeling direction i.e crystallographic direction along which ions can penetrate relatively deeply into the crystal. The sputtering yield generally decreases by between two to five times when the ion direction is parallel to channeling direction of a crystal.

FIB allows the use of either secondary electrons or secondary ions for image formation. FIB produce ion channeling contrast in secondary electrons images. The secondary electrons yield varies as a function of crystallographic orientation with the sample which results in ion channeling contrast. Thus, When the crystallographic axis of a crystalline grain is aligned with the incident ion beam the channeling can occur as a result , that grain will appear darker due to decrease in the number of secondary electrons that are emitted.