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Dielectric constant and surface morphology of the elemental diffused polyimide

Polyimide (C22H10N2O5, PMDA-ODA, Kapton-H) samples were doped with phosphorous or boron and fluorine using the radiation assisted diffusion technique, with Co-60 gamma-rays over the dose range ∼64–384 kGy, at room temperature. The diffusion of phosphorus and fluorine was confirmed by the RBS technique and that of boron by the neutron depth profiling technique. The elemental concentration on the surface was studied by the XPS technique. The relative concentration of phosphorus, fluorine and boron increased with increasing dose of gamma-rays. The dielectric constant, ε′, of the polyimide increased by ∼43% after phosphorus doping but decreased by ∼33% after boron and fluorine doping. The increase in ε′ is attributed to the radiation induced chemical coupling of the phosphorus atoms across the intra-molecular polyimide chains. The down shift in ε′ is attributed to the decrease in the degree of electronic polarization and to the increase in the free volume due to the diffused boron or fluorine atoms. For all the doped samples the dielectric constant, ε′, decreased very slowly with increasing frequency, over the range 100 Hz–7 MHz. AFM results reveal that the surface morphology and the roughness of the doped polyimide are appreciably different than that of virgin polyimide.