A niobium etchant I have used makes use of a two-step reaction:
nitric acid oxidizes the niobium, then HF etches the oxidized niobium.
For this reason, one might guess that HF alone will not etch Nb.
We actually measured the etch rate of Nb in 5:1 BHF. After an hour, zero
etching was measured.
See papers of etch rates online in JMEMS.
    --Kirt Williams

----- Original Message -----
From: "Giuseppe Tettamanzi" 
Subject: [mems-talk] Chemical etching for NbOx and Diffusions coefficients of
Au in Nb and of Nb in A

> Dear Collegues,
> first, I have a very thin ( 100 nm) Nb films and I would like to know what
> is the best way for take off the oxide ( NbOx ? ) layer on my sample, I have
> been suggested to try Hf but in some publication I have found It is stated
> that the Nb will be damaged if I will use Hf...
> Does someone know a better way to "clean" from oxide those samples ? ( so I
> will be able to measure easily the Tc )
>