The remainder might be Ti-based.  Can you try a Ti etch like 1::3 HNO3 :: H2O2?

ahajjiah  wrote:

I have a Ti/Al metalization on a GaN wafer. I have tried to strip off the
metals by using Aqua Regia (3:1 HCL:HNO3). I left the sample in the acid for 6
hrs. It seems that most of the metals got stripped off, but there are still
some metals on the sample.

Any suggestions on how to remove the metals completely?