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 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.

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 20-30 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).