Process and challenges of ultra-thick spin-on photoresist

In this paper, spin-on photoresists were evaluated to form ultra-thick photoresist mold for electroforming of 200 μm height Cu pillar structures with a diameter of 150 μm. Two photoresists materials of different exposure tone (positive & negative) were assessed. A double spin-coating process was required to achieve the required resist film thickness. The soft-bake, exposure and developing parameters were optimized to obtain suitable photoresist profile. Substrates with developed photoresists were subjected to electroplating process to test their chemical resistance A stepped EBR was implemented to overcome poor electrical contact with plating jig due to the thick edge bead. After Cu electroplating, the different materials were subjected to stripping solvent to assess their stripping performance.