--I have found that the following is very good for extremely small
features (I don't know what etch system you have) - the following
proportions of gases:  40:12:5 BCl3:Cl2:CHCL3 (or similar) at the
lowest pressure your RIE will support with approximately 300W RF
power, you can try lower (that was on an older dual-chamber
Plasmatherm RIE system).

PMMA hardening is best dealt with in an oxygen plasma.  The resulting
alumina that forms can be easily removed by a very quick exposure to
that same plasma - in an ideal situation, you would want to switch
your sample under vacuum to an O2 plasma immediately after finishing
the Cl2 etch of the aluminum, and then switch back to the aluminum
etch for about 10 seconds when you were done.

Hi, Eric, your information is also very interesting to me.
I am wondering if you know any metal can be used as etching mask using
the same recipe to etching Al to make submicron features? Because I
can't use PMMA or PR to pattern Al first, I have to deposit a Al layer
then etch it.

Thank you very much!

Maggie