Weng:
I am not sure how much oxidation has occured on the surface of the wafer but
I would guess that no more than 500-1000 angstroms. That material is now
Al203 and very hard to etch. A significant amount of ion energy will be
required to etch the material in a plasma a high bias with BCL3 would do the
trick. Also most phosphoric acid based etchants will also etch the AL203 with
a little surfactant in the etchant for uniform breakthrough of the oxide. Bob
Henderson