Specific Process Knowledge/Lithography/nLOF

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Resist description

AZ nLOF 2020 is a negative UV photoresist, suitable for lift-off processes.

Spin coating

Spin curves for AZ nLOF 2020 using a 30s spin-off, and a 60s@110°C softbake

Typical spin parameters:

• Spin off: 30-60 s
• Soft bake: 60 s @ 110°C

Post-exposure bake

Typical PEB parameters:

• PEB temperature: 110°C
• PEB time: 60 s

The manufacturer recommended PEB for nLOF is 60 seconds at 110°C, regardless of resist film thickness.

PEB baking time investigation

While 60 s @ 110°C is adequate for Si substrates, less thermally conductive substrates (glass, III-V materials, chips bonded to carrier), have shown problems using the standard PEB recipe.
These problems were largely solved by increasing the PEB time to 120 s. Tests (on Aligner: Maskless 02) have shown that the lithographic performance of nLOF on Si is improved when using 120 s @ 110°C PEB (less stitching, less bias, more negative profile).

A small report on the tests can be found here: nLOF PEB time 2019.

Development

Development speed:

• Puddle development in 2.38% TMAH (AZ 726 MIF): ~4 µm/min

A 2 µm nLOF resist film is fully developed in ~30 s in 2.38% TMAH (AZ 726 MIF). However, the development can be continued to 60 s in order to get a more negative resist profile (due to increased under-cut).

Solvent development

Cross-linking negative resists have the potential to be developed using organic solvents instead of the normally used alkaline aqueous solutions. A quick test of this was carried out by Thomas Anhøj @ DTU Nanolab, showing that nLOF 2020 could potentially be used for water-free lithography.

A small report on the test can be found here: nLOF solvent development 2024.