I am trying to selectively etch ~75 nm of Ta. I am currently not able o use dry etch (instrument down). I have seen in the literature that HF:HNO3:H2O mixture can remove Ta by forming tantalic acid. I am struggling with deciding on a mask for etching this Ta. I was initially thinking SU-8 2005, but that will be extremely difficult to remove after crosslink (exposure). As for using a sacrificial layer (OmniCoat/LOR), I think it will be dissolved by the strong acid. I am considering Au as a mask as well, but assume Au etchant might damage the structure underneath when I try to remove it. In the sample surface I have Ta and SiO2. What is the best approach?