Characterization of thin photosensitive polyimide films for future metallization schemes

Photosensitive polyimide (PSPI) is being integrated as potential low dielectric constant layers in next generation advanced silver metallization. These dielectrics have excellent thermal stability as represented by a thermal decomposition temperature of 450 °C and an average thermal expansion coefficient of $20×10−6/°C.$ This results in a low stress (<26 MPa) in the polyimide film as measured during in situ curing and cooling cycles. The morphology and structure of two types of polyimides, 3,3^′,4,4^′-biphenyl tetracarboxylic acid dianhydride-p-phenylenediamine (BPDA/PPD) and the PSPI based on BPDA/PPD, have been investigated in this work. Both refractive indices measurements and x-ray scans indicated that the 85 °C dried BPDA/PPD is slightly anisotropic, but the dried PSPI films are isotropic. Both polyimide films cured at 300 °C exhibit optical anisotropy with the average in-plane refractive index of 1.74 and the out-of-plane index of 1.62, indicating a strong preference of polymer chains to orient along the film plane. The dielectric constants were 3.0 and 2.6 in the in-plane and out-of-plane directions, respectively. The reduced number of process steps associated with PSPI and Ag etch over conventional Cu chemical mechanical polishing, shows great potential in terms of future process integration.