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

Abstract

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 cm−2. 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.

Laser ablation of polymers: a review

“…PI is coated over a silicon wafer substrate where laser ablation is then utilized to create microfeatures with high aspect ratios on the polymer material. Once the features have been formed on the polymer, electroplating is done to coat the polymer surface with metals . Subsequent to the metal deposition, the polymer material can be dissolved resulting in a shell created by the metal coating.…”

Optical modeling of laser ablated microstructures

“…For polyimide irradiated with KrF laser pulses, the photochemical model works well for fluences in the range E Շ 0.6 J/cm 2 /pulse or E 0 Շ 30. 13,26 Many excimer laser ablation studies have been reported in the literature for this material. For our calculations at 248 nm, we use values for the refractive index n ¼ 1.79; the absorption coefficient a ¼ 2.8 Â 10 5 cm À1 ; the photon absorption depth L ¼ 36 nm and threshold fluence for ablation E T ¼ 19 mJ/cm 2 /pulse.…”

“…Validation of our ablation model was carried out by comparing computations of the ablation rate and taper angle in polyimide with measurements made by ourselves and other workers. [9][10][11][12][13][14][15] Polyimide is important industrially as an electronic material, and for fabricating ink jet printer heads whereby excimer laser drilling of nozzle plates is a key process. 16 Our measurements used an Exitech M8000 mask projection micromachining system incorporating a Lambda Physik LPX210 KrF laser with a beam delivery system consisting of shaping lenses, variable attenuator, fly's eye homogenizer arrays, field lens and a 0.13 NA, Â5 demagnification diffraction-limited imaging objective.…”

Experimental studies and simulation of laser ablation of high-density polyethylene films

“…This process is typically used to create a pattern that is then used for mass fabrication. The most commonly used polymer in the LIGA process is PI [113].…”