Stability of the hydrophilic behavior of oxygen plasma activated SU-8

The effect of O₂ plasma treatment on surface energy, topography and surface chemistry of the negative photoresist epoxy novolak SU-8 was investigated by contact angle goniometry, atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS). Directly after plasma treatment, the surfaces were completely wetted by water with a contact angle between water and the SU-8 surface below 5°. The surface free energy can be increased significantly depending on the plasma dose. The surfaces remained hydrophilic for several months showing a moderate hydrophobic recovery. The surface topography of the plasma treated SU-8 showed a formation of nanoscale aggregates. The rms-roughness of the topography was correlated with the plasma dose. An increased plasma dose induced aggregates of up to 200 nm in size. XPS measurements revealed changes in surface chemistry due to the oxygen plasma process and an increased antimony concentration on the surface.