Hello Daxiang,
There are several options to address the issue.
You can try copper or gold. These metals stay in KOH.
Nickel is also a good mask in KOH.
However, masking layer should be thicker than what you are using.
It is important to have a masking sub-layer as well. Otherwise, KOH will
penetrate through metal layer via pinholes or places with poor adhesion
and form cavities under the mask.
With respect
Nickolai Belov
Nanochip, Inc.