Hi Vivek,

" Cl2/BCl3 should be the common gas mixture for Al RIE. At the end
of the process, you may remove BCl3, then it will stop at your Al2O3
layer."

In response to the quote above.  I do not think you should count on Cl2 or BCl3
to stop at the Al2O3 layer.  I currently use Cl2 in an ICP to etch Al2O3
(sapphire).  You may be able to adjust the RIE parameters so that you get a good
enough selectivity...  But Cl2 and BCl3 are common gasses for etching Al2O3 as
well.

-Marie