Hi Michael,

Thank you very much for your informative reply. Indeed, I find there's
noticeable aluminum redeposistion-like phenomenon happend after CF4 RIE
etching. The trenches got contaminated and became very rough after etching.
As you mentioned, I did get a few better results from the RIE equipment with
ICP. However, the surface was very rough with certain particles sitting
around trench edges. Still, the trench patterns were not transfer to silicon
as well as expected, size shrinking and not straight edges.

Actually, I tried photoresist (PMMA) rather than aluminum as mask. But I
found the selectivity is not as good to protect PMMA, though the selectivity
heavily depends on the recipe.

Thanks,
Li