Specific Process Knowledge/Lithography/nLOF
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AZ nLOF 2020 is a negative UV photoresist, suitable for loft-off processes.
Spin coating
Post-exposure bake
The recommended PEB for nLOF is 60s at 110°C, regardless of resist film thickness.
While 1min@110°C is adequate for Si substrates, less thermally conductive substrates (glass, III-V materials, chips bonded to carrier), have shoved problems using the standard PEB. These problems were largely solved by increasing the PEB time to 2min. Tests (on Aligner: Maskless 02) have shown that the lithographic performance of nLOF on Si is improved by using 2min@110°C PEB (less stitching, less bias, more negative profile).
Development
A 2µm nLOF resist film is fully developed in 20-30s using TMAH (AZ 726). However, the development can be continued to 60s in order to get a nore negative resist profile (increase the under-cut).