Characterization of thin photosensitive polyimide films for future metallization schemes

Photosensitive polyimide (PSPI) is being integrated as potential low dielec tric constant layers in next generation advanced silver metallization. Thes e dielectrics have excellent thermal stability as represented by a thermal decomposition temperature of 450 degreesC and an average thermal expansion coefficient of 20 x 10(-6)/degreesC. This results in a low stress (< 26 MPa ) in the polyimide film as measured during in situ curing and cooling cycle s. The morphology and structure of two types of polyimides, 3,3 ' ,4,4 ' -b iphenyl tetracarboxylic acid dianhydride-p-phenylenediamine (BPDA/PPD) and the PSPI based on BPDA/PPD, have been investigated in this work. Both refra ctive indices measurements and x-ray scans indicated that the 85 degreesC d ried BPDA/PPD is slightly anisotropic, but the dried PSPI films are isotrop ic. Both polyimide films cured at 300 degreesC exhibit optical anisotropy w ith the average in-plane refractive index of 1.74 and the out-of-plane inde x 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 st eps associated with PSPI and Ag etch over conventional Cu chemical mechanic al polishing, shows great potential in terms of future process integration. (C) 2001 American Vacuum Society.