Removal of SU-8 photoresist for thick film applications

Abstract

Abstract SU-8 photoresist has consistently shown excellent resolution in thick film applications, has been utilized as an electroplating mold, and is sensitive to inexpensive UV sources. However, the highly crosslinked epoxy remaining after development is difficult to remove reliably from high aspect ratio structures without damage or alteration to the electroplated metal. A review of physical and chemical removal options is discussed with data on the most promising options shown. Several standard solvent mixtures have proven particularly useful in our laboratory. The solvent systems remove the resist through crazing and peeling rather than dissolution. They are inexpensive, and can be utilized on very low aspect ratio features, or on parts with no included SU-8. Alternatively, a very promising option for reliable removal is downstream chemical etching (DCE; Matrix Integrated Systems, Richmond, CA) which achieved removal rates of approximately 7–10 μm/min at 225 °C for several hundred micron thick molds. At higher temperatures, an inexpensive molten salt bath has shown to be reliable. The K10 (Kolene Corp., Detroit, MI) process salt bath operated at 350 °C is efficient at completely oxidizing the highly crosslinked epoxy. Sputter Auger depth profiling of Ni parts after removal by DCE and molten salt bath indicated only superficial elemental damage to the metal, though deposits of antimony from the photocatalyst are left after DCE. Initial mechanical properties of electroplated Ni tensile specimens subsequent to the salt bath and DCE processing are presented.