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