My structure consists of 250nm PMMA / 700nm of Al / 1um SiO2.

I have done e-beam lith on the PMMA layer and developed.  I now want to etch the
Al down to the SiO2.

Can anyone recommend an Al etchant that should be used in these circumstances?
The selectivity of the etch between the Al and the PMMA should be as high as
possible.