EFFECT OF MOLECULAR-ORIENTATION ON THE DIELECTRIC-PROPERTIES OF SPIN-COATED POLYIMIDE FILMS

The effect of molecular orientation on the dielectric properties of spin-coated polyimide films has been studied in situ for pyromellitic dianhydride with 4,4'-oxydianiline (PMDA/ODA) and biphenyldianhydride with p-phenylenediamine (BPDA/PPD). The degree of molecular orientation is characterized by the optical anisotropy between the in-plane and the through-plane refractive indices. The through-plane dielectric properties are measured by fabricating parallel-plate capacitors directly onto the silicon substrate. Both the birefringence and the dielectric constant of PMDA/ODA polyimide vary with film thickness; however, these properties are independent of film thickness for BPDA/PPD films. To confirm that the measured dielectric constant obtained from the parallel-plate structures is free from a significant edge effect, finite element modeling of the electrostatic potential within the dielectric is performed. Models for both isotropic and anisotropic dielectric properties indicate that the fringing effects are indeed negligible for the film thicknesses examined. Thus, the changes observed in the measured dielectric constant are attributed to the variations in the molecular orientation. The optical anisotropy observed for the polyimides suggests a corresponding dielectric anisotropy in the films. An estimation using the Maxwell relation indicates that the dielectric anisotropy at 10(5) Hz is 0.31 for PMDA/ODA and 0.85 for BPDA/PDD. (C) 1994 John Wiley & Sons, Inc.