Mechanism and application of refractive index increase induced in fluorinated polyimide by ion irradiation

This paper reports a result carried out in order to clarify the applicability of ion irradiation effects to polymer materials used for optical waveguides. We irradiated 950-keV He⁺ ions or 1.0-MeV H⁺ ions to a fluorinated polyimide film to a fluence between 1 × 10¹⁴ and 7 × 10¹⁶ cm^-2, and the film surface was scanned by a profilometer. The depth of a dent induced by the irradiation increases with an increase in the fluence. From the depth of the dent, the projected range of the He⁺ ions, and the Lorentz-Lorenz equation, the refractive index of the ion-irradiated region was found to increase by 2.9 %. This value agrees with the increment in refractive index measured by spectroscopic ellipsometry, which also increases as the fluence increases. Furthermore, the increment in refractive index, 0.21 %, induced by the irradiation of H⁺ ions to the fluence of 1 × 10¹⁵ cm^-2 is comparable to the value, 0.35 %, observed when H⁺ ions of a similar fluence were irradiated to SiO₂ glass. Therefore, it is natural to assume that the ion irradiation to the polymer can be a good tool to fabricate a high-performance polymer-based optical waveguide.