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.