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We investigated the formation of CuO or Cu2O nanoparticles in the thick polyimide films by oxidizing Cu nanoparticles at various temperatures during the post heat-treatment. Cu nanoparticles of 4-5 nm in diameter were initially formed in the polyimide film by the reaction between a Cu film and a polyimide precursor, polyamic acid, and a following thermal curing in a reducing atmosphere. After the subsequent post heat-treatments in oxidizing atmospheres, X-ray diffraction patterns revealed that initial metallic Cu nanoparticles were transformed to Cu2O or CuO nanoparticles depending on the temperature during the post heat-treatment. Cu nanoparticles were oxidized to Cu2O during the post heat-treatment at low temperature while Cu nanoparticles were oxidized to CuO during the post heat-treatment at high temperature. Cross-sectional TEM studies showed that about 4.7 nm sized Cu2O nanoparticles or 4.7-5.2 nm sized CuO nanoparticles were fabricated in a thick polyimide film depending on the post heat-treatment condition. In the optical absorption measurements, the absorption peak from surface plasmon resonance of Cu nanoparticles disappeared during the post heat-treatment in an oxidizing atmosphere.

We investigated the formation of nanoparticles in a relatively thick polyimide (PI) film (> 1 microm) in controlled atmospheres and optical properties of these nanoparticles. Polyamic acid of 10 wt% BPDA-PDA was spin-coated on the 25 nm Cu thin film and thermally cured at 350 degrees C in high purity nitrogen or 5% H2 + 95% N2 atmosphere. The fabricated nanoparticles in high purity nitrogen atmosphere had spherical shape and were dispersed in the 1.5 microm thick PI film. Its phase was revealed as Cu and Cuprous Oxide by X-ray diffraction (XRD). Its size and optical absorption depended on deposited metal thin film thickness. After post heat-treatment in 5% H2 + 95% N2 atmosphere, surface plasmon resonance from metallic Cu nanoparticle was enhanced. In the specimens cured in reducing atmosphere, 5% H2 + 95% N2, highly dense and 3.5 nm size nanoparticles were well dispersed in an entire PI film. XRD results and optical data revealed that nanoparticles fabricated in 5% H2 + 95% N2 atmosphere were metallic Cu. Thermal curing in reducing atmosphere produces nanoparticles of high density, and uniform dispersion in a relatively thick PI film.