I have used:

  5% H2O2:5% H2SO4:90% H2O.

to etch 6.66 µm Bi. My notes say that 2.5% may be better, but I have
no data.
Photoresist gap was 5 µm and yielded 12-15 µm etched linewidth.

>
> Does anyone knows about the wet etchant for the Bismuth thin film?