My devices consist of a “sandwich” structure with two 1µm SU-8 layers and an intermediate Ti/Au layer. During the second SU-8 lithography step, however, the unexposed SU-8 appears to interact with the gold layer, leading to unwanted SU-8 residue that persists after development.

Steps I have tested without success include:

• Using different types of SU-8 (such as TF 6002 and TF 3050), as well as fresh SU-8 bottles
• Employing alternative metals; the same crosslinking issue occurs with aluminum and platinum
• Applying different oxygen plasma treatments before the second SU-8 lithography step to rule out a contamination from the plasma chamber
• Slowing the temperature ramp for all baking steps to prevent potential overheating of SU-8 on the metal
• Adjusting exposure and development times

I have attached SEM images to illustrate the issue. Note that this problem also affects larger patterns, such as 500µm-wide pads.

Finally, it is important to point out that these devices are used in contact with living cells, therefore biocompatibility should be ensured.

I would appreciate any suggestions or insights to help resolve this!