Vivek,

Each photoresist have a spin-speed curve in the application sheet. So
in order to get  a thicker photoresist you can spin coat it at less
rpm. But the best is to check out the application data sheet from the
industry you get the photoresist. Try using Phosphoric acid, along
with acetic acid and nitric acid. You can get a direct Al etch
solution too from companies. Also heating you the solution will lessen
the etching time.


Let me know if you need to know anything more about the process.

- Aasutosh Dave