| Due to its processing ease, chemical resistance, thermal stability, and planarizability, polyimide is considered a material of choice as inter-level dielectric for microelectronic devices. Thin polyimide films (2-10 $mu$m) spin-coated from polyamic acid solution are known to develop an in-plane orientation. In order to ensure the proper selection and utilization of polyimide for microelectronic applications, it is important to understand the origin of the molecular orientation and the its effect on the critical performance properties.;In this work, processing variations have been employed to determine the origin of the molecular orientation, characterized by measuring the in-plane and the through-plane refractive indices of the films. The optical anisotropy in these films is influenced by the substrate coefficient of thermal expansion, substrate solubility, final bake temperature, the film thickness, and the back bone rigidity of the polymer chains. The effects of the molecular orientation variation with the film thickness on the biaxial stress and the through-plane dielectric constant of the films have been investigated. Although the biaxial stress does not vary with film thickness for these films, the through-plane dielectric constant is correlated with the molecular orientation. Therefore, a dielectric anisotropy is expected to exist in these thin films.;As a result of the in-plane orientation, the in-plane dielectric constant should be greater than the through-plane dielectric constant. The through-plane dielectric constant can be obtained from parallel plate capacitor structures, but the in-plane dielectric constant cannot be directly measured on these thin-films. Finite element modeling of the electrostatic potential distribution has been performed to identify electrode designs suitable for the determination of the in-plane dielectric constant. The dielectric anisotropy experimentally obtained from an inter-digitated electrode design agrees with the theoretical dielectric anisotropy predicted from the Maxwell relation. |
