:: IPCI ::

Definition: removing part of the layer
Purpose: Second step of transferring an image to a layer (making a structure)
How: Photoresist mask protects a part, the other is removed by etchant (see picture below)

Figures of merit for etching
- Etch rate
  - defined as the ratio: thicknessof the removed layer/etching time
  - if it is too high, then the etching becomes difficult to control
  - if it is too low, it is inefficient, requiring too much time = money

Etch rate uniformity
- The etched thickness should be the same on the whole wafer, but also from wafer to wafer to insure reproducible results

Selectivity
- it is desirable to remove the top layer without attacking the underlying material, to insure the fact that etching stops at the substrate
- thus the substrate material should not be attacked by the etchant (or the etching rate should be much lower)
- in the picture below it is observed how different materials have different etch rates, and what is the effect
- selectivity is defined as the ratio between the the etched thicknesses of the two films:

Undercut
- In wet etching, a part of the region protected by the resist is also etched (as can be seen in the picture), due to the anisotropy characteristcs
- the magnitude of the undercut is defined by:
  - A = 1 for anisotropic etching (the etching takes place in all directions)
  - A = 0 for isotropic etching (etching occurs just in one direction, or occurs with different etch rates in different directions)

Damage
- the underlying film may get damaged or attacked in the etching process by the etchant - which is undesirable
Safety
- some etchants may be dangerous to ambient, human