We're using a foundry service (through MOSIS) that gives us a standard 5
metal layer CMOS chip back.  We define the top level interconnect metal
for use as a post processing etch mask.  So, the aluminum mask is a
must, and can't modify it to any great extent.  The depth of the etch is
such that we are beginning to see ARDE effects, and need the lower
pressure/higher bias to clear out the SiO2 from the bottom of the
trenches.  So, mainly looking at options for modifying the aluminum
(i.e., oxidizing or something) to increase its sputtering resistance.
We would like the aluminum layer to still be mostly aluminum when we're
done, and it's only 1micron thick to start with.  So, we need something
that will controllably oxidize 100-300nm.  Most anodization schemes I've
looked at are likely to turn the whole layer into Al2O3 in a very short
time.  maybe I'm wrong. I started looking a bit at plasma oxidation, and
it would be very nice if we could do it in the RIE, as then we wouldn't
have to break vacuum.  We could even oxidize a bit, etch, oxidize some
more, etc.  But I have no idea where to start as far as plasma
parameters, and oxidation times are concerned.  If they are
self-limiting that would be great, because then we could set up the
conditions, and the thickness wouldn't be very sensitive to exposure
time.  Another reason I'm shying away from the wet processes for
oxidation is that the majority of the milling occurs at the end of the
etch when we have to drop pressure and turn up the bias to clear out the
last of the SiO2.  At that point I wouldn't want to put the chip in a
wet environment if I didn't have to.  We could possibly even wait to do
the oxidation until this point, but all of that means I need to find
some rate temp dependence data, and so far I've come up short :)

nick j