Photoablation characteristics of novel polyimides synthesized for high-aspect-ratio excimer laser LIGA process

The photoablation properties of two soluble polyimides DMDB/6FDA and OT/6FDA with thicknesses of over 300 μm, synthesized by the polycondensation of a hexafluoropropyl group contained in a dianhydride with two kinds of diamines, are investigated using a 248 nm krypton fluoride (KrF) laser. The incorporation of the hexafluoropropyl group into the chemical structure gives these two polyimides higher etching rates than Kapton (a commercial polyimide film which is difficult to dissolve). The etching rates of synthesized polyimides are about 0.1-0.5 μm/pulse over a fluence range of 0.25-2.25 J cm^-2. The photothermal mechanism for DMDB/6FDA contributes about 19% of etching depth at a laser fluence of 0.82 J cm². Moreover, the number of laser pulses seriously affects the taper angle of microstructures, especially at low fluence. Near-vertical side-wall structures can be built at high fluence (∼2 J cm^-2). Fresnel patterns with a thickness of 300 μm and a linewidth of 10 μm were fabricated, with an attainable aspect ratio of around 30. After photoablation, the complementary metallic microstructures were also fabricated by a sequential electroplating procedure. Then, those two new polyimides could be dissolved easily in most common solvents (such as THF, DMSO, NMP and DMF). These results indicate that these two soluble polyimides are highly suitable for use in the KrF laser LIGA process.