Hi,

I'm trying to make 70nm wide (50nm deep) Nb trenches separated by about 100 nm
(edge-to-edge).  With Al2O3 as an etch mask, I've been using SF6/O2 RIE to try
to etch away the Nb.
(20sccm SF6 and 0.6  sccm O2, 60 mtorr, 60W)

I think my Nb is being completely undercut (all Nb under the 70nm wide Al2O3
etch mask is being etched away).

How can I rid of the undercut?  Is it possible?  Is my pattern too small to use
RIE? I know I should reduce the pressure to less than 10mtorr, but what else?
Should I try CHF3/CF4?

Thanks,
Serena