View article

Thin films ranging from 400 nm to 4 μm thickness of poly( p‐phenylene biphenyltetracarboximide) (BPDA‐PDA), prepared by thermal imidization of the precursor poly(amic acid) on substrates, have been investigated by the optical waveguide spectroscopy. These polyimide films, most prominent for potential applications as the interlevel dielectrics in multilevel interconnect technologies owing to their low coefficients of thermal expansion and excellent thermal/mechanical properties, are found to exhibit an extraordinarily large anisotropy in the refractive index, with the measured in‐plane refractive index n_∥≂1.852 and the out‐of‐plane value n_⊥≂1.612 at 632.8 nm wavelength, nearly independent of the film thickness. This large optical anisotropy indicates a very strong preference of polymer chains to orient along the film surface, and suggests a considerably larger (by ca. 27%) dielectric constant in the film plane …