Wavelength Dependence of Refractive Indices of Polyimides in Visible and Near-IR Regions

Abstract

The refractive indices of eleven kinds of polyimide (PI) films formed on silicon substrates have been measured at three wavelengths (0.633, 1.320, and 1.523 µm) in the transverse electric (TE) and transverse magnetic (TM) polarization modes using the prism-coupling method. The wavelength dependence of average refractive indices was fitted using the simplified Cauchy's formula, and the estimated refractive indices at infinite wavelength (ninf) and the coefficients of wavelength dispersion (D) were determined. The PIs having higher ninf exhibit larger D, and the values of D are linearly proportional to ninf for aromatic PIs, while those of wholly alicyclic PIs are very small and deviate from the linearity. In a similar manner, the in-plane/out-of-plane birefringence (Δn) values of the aromatic PIs having higher refractive indices are large and show significant wavelength dependence, while the values of Δn of alicyclic PIs are negligible. Approximate equations are presented for aromatic PIs to estimate ninf and refractive indices at 1.320 and 1.523 µm from a refractive index at 0.633 µm.