Metal Lift-Off
Patterned metal from a resist stencil, lifted off in solvent.
- Wafer size
- Up to 4"
- Optical features
- 1 – 2 µm
- E-beam features
- 50 – 100 nm
- Overlay
- 1 – 2 µm
- PMGI / LOR bilayer
- Image reversal
- Negative resist
- MMA / PMMA bilayer
- Metal under ⅓ of the resist height
- Evaporated metal only: sputtered sidewalls bridge
- Strip solvents: list exposed metals and polymers
Resist stacks
| Stack | Written by | Feature | Bake |
|---|---|---|---|
| PMGI / LOR bilayer | Aligner | 1 – 2 µm | ≤ 200 °C |
| Image reversal | Aligner | 1 – 2 µm | 90 – 130 °C |
| Negative resist | Aligner | 1 – 2 µm | 90 – 130 °C |
| MMA / PMMA bilayer | E-beam | 50 – 100 nm | 180 °C |
Full specifications
- Substrates
- Flat Si, glass, sapphire and SiC; existing films must tolerate resist coating, baking and stripping.
- Sample size
- Pieces to 100 mm wafers.
- Resist stacks
- PMGI or LOR underlayer under an imaging photoresist; image-reversal or strippable negative resist; MMA copolymer under PMMA for electron-beam writing.
- Minimum feature
- 2 µm with the mask aligner; 1 µm with the maskless aligner; 50–100 nm by electron-beam lithography.
- Alignment
- Optical overlay 1–2 µm with suitable marks; electron-beam overlay depends on marks and writefield.
- Process temperature
- Resist bakes 90–200 °C, by stack; stripping from room temperature to below 80 °C.
- Metal thickness
- Below about one-third of the total resist height, and thinner than the underlayer.
- Deposition
- Directional e-beam or thermal evaporation; sputtered films coat the resist sidewalls and bridge the edge.
- Stripping solvents can attack exposed metals, ITO, polymers and III–V materials.
- Sidewall coating leaves metal fences or keeps the metal from separating cleanly.
- Overheated resist rounds its profile and is harder to strip.
- Insulating samples need a removable charge-control coating for electron-beam writing.
- Fragile features need a gentle strip; strong agitation can lift wanted metal.