Alumina mask - NanoLabStaff/nanolab GitHub Wiki
by Kai Sandvold Beckwith ([email protected]).
This protocol describes the procedure for depositing an alumina (Al2O3)-based hard-mask for cryogenic etching using the AJA sputter coater. See separate Si cryogenic etch protocol for etching recipe. Due to high selectivity, this allows the use of extremely thin masks for very deep etching (selectivity of at least 1:1000). Although alumina is the preferred etch mask, it is also very hard to remove, so an optional aluminum (Al) layer can first be deposited to aid removal of entire mask after process completion.
- Piranha clean or oxygen plasma clean
- Use standard processes for photoresist, such as SPR700-1um or Image-reversal.
- Ensure full resist development by visual inspection or profilometer.
- A mild oxygen plasma clean can be used here.
- Using the aluminum DC-target, sputter 5-15 nm at 300W and 3mtorr. Deposition rate about 1.4Å/s.
- Close the shutter, but leave the target on. Let 6 sccm O2 into the chamber, and then reduce the argon to 60 sccm. Leave for 1 minute to stabilize.
- Open the shutter, now depositing 5-15 nm alumina (Al2O3). Deposition rate is roughly same, about 1.4Å/s.
- Check the resist protocols for lift-off procedures. Typically, remove the resist by high energy sonication in Remover PG and/or Remover PG heated to 80°C. The adhesion of Al/Al2O3 to Si is good, so detachment is typically not an issue.
- Proceed with etching process, such as Si cryogenic etch.
- Remove mask by incubation MF-26 developer. The alkaline developer etches aluminum readily, lifting off the alumina deposited on top. 1-5 minutes is typically sufficient. Heat or sonication can be applied to speed up the removal, but this is not usually necessary.