The effect of solvent absorption on the electrical and dielectric properties of polyimide films

Abstract

A distinct difference in the solvent absorption behavior was observed in three types of polyimide films, the pyromellitic dianhydride—oxyclianiline (PMDA-ODA), the SjS-biphenyl tetracarboxyiic diahydride-p-cliaminophenyl (BPBA-PDA) and the 3, 35454-benzophenone tetracarboxylic dianhvdride-l, 3-bis(3-aminophenoxy)benzene (BTDA-APB), which is an isoimide film. All films were spin cast and imidized on silicon substrates to a peak temperature of 375 °C in nitrogen. In the BPDA-PDA films, no solvent absorption was detected after immersion in ft-methyl-2-pyrrolidone (NMP) at temperatures up to 96 °C for 18 h. The PMDA-ODA films exhibit a somewhat anisotropic solvent absorption mode, in which fast solvent diffusion and film swelling were observed in the direction normal to the film surface, while diffusion of solvent in the film plane (lateral direction) was much slower, especially at lower temperatures. However, as the temperature was raised higher (above — 65 °C), a slow but measurable in-plane solvent diffusion could then be detected, which gives a diffusion coefficient ofD — 1.4 × 10 9cm2/s at 96 °C, and it follows Pick's law of diffusion. With the isoimide film, we observed fast solvent diffusion in both directions, i.e., perpendicular and parallel to the film plane, while swelling only occurred in the perpendicular direction due to the containment of the polyimide film by the supporting silicon substrate. By monitoring the solvent advancement as functions of temperature (51–96 °C) and time, the diffusion equation of solvent in the lateral direction (parallel to the film plane), was determined experimentally: D = — 0.0553Xexp (— 9273 cal/RT) cm2/sec. which again follows Pick's law of diffusion. The profiles of the diffusion front appear to have the shape of an error function, with the angles measured at 1.2°-4.3° from the horizontal plane. In addition, with the solvent penetrating and swelling both the isoimide and PMDA-ODA films, a significant increase in the dielectric permittivity (ϵ') and loss factor (ϵ”), and factors of 105 reduction in the insulation resistance were measured at the frequency range of 1-1G6 Hz. A dipole relaxation time of 1.6 × ICT3s was measured on the fully swollen isoimide film according to the Debye dispersion equation. The relaxation time decreases with increasing swelling time in NMP. The reversibility of the solvent sor ption-desorption processes was also evaluated. © 1990, American Vacuum Society. All rights reserved.