Hi colleagues,
Sorry for the belated summary because my hands were full of loads the past 2
weeks.  Here is the summary:

1.      submerging or smearing your samples (Al layer side!) with
o-phosphoric acid with some mild temperature elevation of the o-phosphoric
acid will increase the rate of etching (contributed by Karl Cazzini);
2.      NaOH (contributed by Marisa Ahmad); KOH or any other alkaline
solutions surely will do the job, however, these etchant most probably will
also attack Si which is the structural material.  I didn't specify
non-alkaline etchants in my post, my apologies here;
3.      This is an ideal application for Xenon Difluoride etching.  It may
not be as fast as you like but it should work.  (contributed by Greg Ortiz)
Greg, I think maybe XeF2 is meant for plasma etching.  Maybe I did not
specify wet etching, my apologies here again;
4.      Dilute HCl (18.5% by vol., which is conc. HCl:H2O 1:1) at 50°C (you
can try higher temperature if you want faster rates) which undercuts 0.5 µm
Al with a rate of 10 µm/min.  Be careful not to have Cr/Au/Ti structure
(Cr/Au/Ti in direct electrical contact) which presents electrochemical
effects that undercuts Cr severely.  The etchant does not attack Au, etches
Cr at a minimal rate of 30 Å/min and Ti at 50 Å/min.  Stand-alone Au/Cr and
Au/Ti structures do not show visible undercuts.  This is our in-house
developed recipe, simple and works beautifully without any nasty stuff left
after releasing.  Give it a try colleague if you have similar needs.

Well, that's it for the summary.  Thank you all for your inputs and this
mailing list is surely informative.
Keep sending your emails.

Best,

G. Chen