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

The effect of molecular orientation on the dielectric properties of sp in-coated polyimide films has been studied in situ for pyromellitic di anhydride with 4,4'-oxydianiline (PMDA/ODA) and biphenyldianhydride wi th p-phenylenediamine (BPDA/PPD). The degree of molecular orientation is characterized by the optical anisotropy between the in-plane and th e through-plane refractive indices. The through-plane dielectric prope rties are measured by fabricating parallel-plate capacitors directly o nto the silicon substrate. Both the birefringence and the dielectric c onstant of PMDA/ODA polyimide vary with film thickness; however, these properties are independent of film thickness for BPDA/PPD films. To c onfirm that the measured dielectric constant obtained from the paralle l-plate structures is free from a significant edge effect, finite elem ent modeling of the electrostatic potential within the dielectric is p erformed. Models for both isotropic and anisotropic dielectric propert ies indicate that the fringing effects are indeed negligible for the f ilm thicknesses examined. Thus, the changes observed in the measured d ielectric constant are attributed to the variations in the molecular o rientation. The optical anisotropy observed for the polyimides suggest s a corresponding dielectric anisotropy in the films. An estimation us ing the Maxwell relation indicates that the dielectric anisotropy at 1 0(5) Hz is 0.31 for PMDA/ODA and 0.85 for BPDA/PDD. (C) 1994 John Wile y & Sons, Inc.