About cookies on this site Our websites require some cookies to function properly (required). In addition, other cookies may be used with your consent to analyze site usage, improve the user experience and for advertising. For more information, please review your options. By visiting our website, you agree to our processing of information as described in IBM’sprivacy statement. To provide a smooth navigation, your cookie preferences will be shared across the IBM web domains listed here.
Publication
Applied Physics Letters
Paper
Large anisotropy in optical properties of thin polyimide films of poly (p-phenylene biphenyltetracarboximide)
Abstract
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 than that along the film thickness. Moreover, there is some evidence for the existence of a very thin polyimide layer of slightly lower density and higher anisotropy adjacent to the substrate.