Enhanced adhesive strength between SU-8 photoresist and titanium substrate by an improved anodic oxidation method for high aspect-ratio microstructures

Abstract

This paper presents an anodic oxidation treatment method for improving the adhesive strength between SU-8 photoresist and titanium substrate. The adhesion strengths were measured for the treated titanium substrates by different anodic oxidation techniques, which increased with the enhancement of the surface roughness and thorny structures on the treated substrate. It was found that the adhesion was increased by approximately 86% through anodic oxidation treatment in 3.0 mol l−1 NaOH solution compared to Si wafer substrate. Additionally, the influence of UV dosage and hard bake temperature on the adhesion strength was examined for the anodic oxidation treated substrates. The UV dosage effect, with a range from 0.8 to 2.08 J cm−2, was measured and analyzed for a 300 µm thickness SU-8 cylinder, and the adhesion strength corresponding to the optimal dosage (1.52 J cm−2) was 45.82 ± 3.08 MPa. The adhesion was further strengthened by a hard bake at 110 °C for 60 min and its value was up to 71.64 MPa, which almost approaches the breaking strength of the SU-8 polymer. Using the optimized anodic oxidation and UV lithography conditions, high aspect ratio (24) SU-8 micropillar arrays with 600 µm height were fabricated without any distortion, collapse or delamination. Finally, to evaluate the feasibility of the anodic oxidation treated titanium substrate to serve as a seed layer, the electrochemical properties were measured by polarization curves.