Simple lift-off for > 20 nm resolution¶
The lift-off process for > 20 nm resolution uses a two layer polymer stack (see Figure 104). The base layer is polymethylglutarimide (PMGI, SFG 2S™, also known as LOR™) and the top layer is polyphthalaldehyde (PPA). The thicknesses can be, for example, 50 nm PMGI and 30 nm PPA. The advantage of this recipe is that no advanced tools are required for preparing the stack, only a spin-coater and a hot plate are necessary. During the patterning process, the PMGI underneath the PPA is exposed and a short wet etch of the PMGI exposes the substrate. Now, a thin film can be deposited on the sample. Finally, the lift-off is carried out in a hot solvent. A disadvantage of the technique is the isotropic underetching of the PMGI which might lift small freestanding structures or result in collapse of the underetched structures due to capillary forces. An example of structures patterned using the simple lift-off process is shown in Figure 106).

Figure 105 SEM cross sections images of the simple lift-off stacks after metal evaporation. 250 nm PMGI, 70 nm PPA, 30 nm metal on the left. 50 nm PMGI, 30 nm PPA and 15 nm metal on the right.

Figure 106 An example of structures fabricated via the simple lift-off process: an array of lines with a half-pitch of 70 nm.
- Prepare PPA solution of 1.3 w-%
- Solvent: Anisole (CH3OC6H5, 0.995 g/mL, bp: 154 °C) or Cyclohexanone (C6H10O, 0.9478 g/mL, bp: 155.6 °C).
- For example: use 65 mg of PPA and 4.935 g of solvent for approximately 5 ml of PPA solution.
- Shake the solution well to fully dissolve the PPA and allow to stabilize for a couple of hours.
- Sample preparation
- Clean the sample using either mild oxygen plasma or a hydrofluoric acid (HF) dip.
- Deposit PMGI (SFG 2S) by spin coating for 35 seconds at 3,000 rpm for a 50 nm thick layer. Bake the sample for 1 minute at 200 °C.
- Deposit 30 nm of PPA by spin-coating for 35 seconds at 2,000 rpm. Bake at 90 °C for 3 min.
- Use the sample within a few days, or store it in a nitrogen atmosphere.
- Pattern the PPA using the NanoFrazor
- The standard settings for the process are:
- Pixel size: 10 nm.
- Set the target depth to about 10 % less than the thickness of the PPA.
- Etch the PGMI
- Use 0.17 mol/L TMAH developer (e.g. Diluted AR 300-47™) which gives an etching rate of approximately 1 nm/s. Do not stir the sample during the etching. If necessary, extend the etching time to get a bigger undercut.
- Rinse the sample in deionized water (DIW) to stop the development followed by a rinse in isopropyl alcohol (IPA). IPA has a lower surface tension than DIW, therefore reducing the risk of collapse of the undercut layers.
- Blow the sample dry with nitrogen.
- Descumming (optional)
- For a cleaner outcome, remove 5 nm of the PPA using a barrel etcher (e.g. Tepla BA; oxygen plasma for 5 seconds at 200W).
- Metal deposition
- Evaporate up to 25 nm metal, eg. 3 nm titanium and 22 nm platinum.
- As a general rule, the evaporated layer thickness should not exceed 50 % of the thickness of the PMGI.
- Lift-off the metal
- Immerse the sample in hot solvent (for example, N-Methyl-2-pyrrolidone, NMP). PMGI should dissolve very easily in the NMP. Ask the responsible person in your facility for the maximum allowed temperature for the solvent you are using. Be careful, NMP is a hazardous chemical!
- For a faster lift-off press a piece of scotch tape gently on the sample and rip it off immediately. Only the metal on the sample should stick on the tape, not the polymers. Then place the sample in a hot solvent, as described above.
- If the metal does not lift-off, try placing the sample (still immersed in the hot solvent in suitable container) in an ultrasonic bath.