Optical properties of polyimide layers prepared by vacuum deposition in the presence of gas

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Thin polyimide (PI) layers were prepared by simultaneous vacuum evaporation of the polyimide precursors pyromellitic dianhydride (PMDA) and 4,4'-oxydianiline (ODA). After deposition the layers were thermally treated 1 hour at 170°C followed by 1 hour at 250°C. Precursors were deposited in a residual atmosphere of different gases - argon, nitrogen, helium and air at a pressure 10−2 Pa. Conditions for fabrication of PI layers with reproducible composition, uniform and smooth surface, without mechanical and chemical defects have been established. It was found that the preparation conditions do not significantly affect the optical properties of the layers. The PI layers before and after thermal treatments are transparent over 85% in the studied spectral range (from 350 to 900 nm). The thermal treatment leads to a bathochromic shift, in a consequence of the transmission edge shift to the longer wavelengths, for all the samples, independently of the deposition conditions in the vacuum chamber. The average refractive indices at 633 nm varied from 1.67 towards 1.72 depending on the deposition conditions and thermal treatment.