This is what I found by google, hope it is helpful.
http://mitghmr.spd.louisville.edu/lutz/resources/sops/sop22.html
Title: Aluminum Etch
Purpose:
To remove the areas of Aluminum unprotected by photoresist and thereby
form the desired metallization pattern.
Precautions:
1. This operation should be started within 30 minutes of the completion of
hard bake.
2. Use only the commercially prepared acid mixture Aluminum Etch by
J.T. Baker.
NOTE: This Aluminum Etch Solution is a mixture of phosphoric,
acetic and nitric acids.
Procedure:
1. Pour Aluminum Etch Solution into the pyrex beaker marked for this
purpose.
2. Keep the pyrex beaker on the hot plate and heat the solution to
about 40° C.
3. Rinse the wafer in running DI water for about 30 secs.
4. Immerse the wafer in the Al Etch Solution. Agitate mildly and
consistently.
NOTE: This etching process proceeds rapidly and typically takes
less than two minutes.
5. The surface of the wafer will start turning darker as the exposed
Aluminum is etched off and the photomasked pattern will begin to
appear. Observe the wafer surface closely to check the etching
process. The pattern should appear clean and uniform over the
wafer surface. There should be no random aluminum spots between
the designed pattern features. At this point etch the wafer for an
extra 20 secs.
6. Remove the wafer. Rinse in running DI water for 5 minutes.
7. Dry the wafers with the N2 gun.
8. Inspect the wafers under the high power optical microscope.
9. If wafer appears unetched or exposed Al is left on the wafer
surface repeat steps 3 to 8 till wafer appears etched. Each
additional etching step should be no longer than 30 secs.
Regards,
Yilei Zhang