1. Pattern transfer by etching or lift-off processes

2. Basic pattern transfer steps
Positive resist (~1mm thick)
Film
substrate
Hard mask
UV light
Glass
opaque pattern
Film
etching
Exposure
development
Resist stripping
- Exposed resist removed by developer
- The resist pattern is the positive image of the pattern on the mask
Unexposed resist removed by resist striper
Resist protects the film areas

3. Pattern transfer by “lift-off “
etching
- Allows use of hard-to-etch materials
- Not for sub-micron features
rounded top
Wing tips

5. Requirements and constraints on lift-off
Must avoid lateral step coverage
good edge definition depends on the lateral walls
avoid tearing or peeling of the remaining metal
easier to liftoff (solvent can reach the resist)
Resist with slope or undercut profile:
Create a descontinuity of the deposited film
provides an area through which a solvent can pass to remove the resist
resist
resist
substrate
substrate
a) bake resist at 110ºC before exposure
(top surface becomes harder)
b) Double resist: resist I/resist II

6. Resist thickness > metal thickness
For thick metal leyers:
a) Increase the PR thickness
b) Multiple layers: Resist/oxide
Deposited film descontinuous
resist
resist
substrate
substrate
Metal deposition temperature < ~250ºC (resist degradation)
Some materials are deposited at high temperatures:
- sputtered AlSiCu @400ºC
“Burn” resist: ~100ºC
(difficult to clean resist residues)
Metal ductility determines success of liftoff
Ductile materials (e.g. Au):
poor edge definition or
impossible to liftoff
Britle materials:
tend to fracture at the edge of pattern steps (film discontinuity)

7. Problems: fabrication of surface sensitivity devices
Cannot deposit directly the device layers:
The magnetic structure at the edges can be affected:
The wafer surface was exposed to chemicals (vapor prime HMDS, resist, developer)
resist
1.5 mm
(15 000Å)
600 Å Ta
30 Å
MnIr Å
90 Å
NiFe
70 Å
TiWN2
150 Å
Magnetic tunnel junction

8. Pattern transfer by etching
Isotropic or anisotropic
selectivity
overetch
The resist and the substrate are also etched
If the mask is etched too quickly: no control of the feature size
To cope with film thickness and etch rate non-uniformities

9. Pattern transfer by etching
Dry etching
Wet etching

10. Wet etching SiO2 etching: SiO2 + 4 HF  SiF4 + 2 H2O Al2O3 etching:
Si etching:
Anisotropic etching of Si: atomically smooth planes and atomically sharp edges, at low cost

11. Defect-selective wet etching
Al2O3 etching
Oxide “pin-hole” detection
Nucleation sites after wet etching
Enhancement of local defects – can be counted in an optical microscope
Si/NiFe 500Å/AlN 500Å

12. Dry etching
plasma
Plasma +
acceleration

13. Mechanisms in a plasma etch process

14. Dry etching profiles

15. Avoid redeposition – sample tilting