Dear  Hongtao,

i have faced similar problem in past but it got rid from this.
you may increase the Au thickness (if Possible for your requirement) upto
500nm. Rotation of the wafer during deposition may serve good for almost
pinhole free film.

you may also try annealing in nitrogen ambient at 250 degree Celsius for
60 min.

Hope that you may get rid of it.
Good Luck,

Manish Hooda