Refractive index and dielectric constant of the boron and fluorine diffused polyimide

Abstract

The dielectric constant, ε ′, of polyimide (C 22H 10N 2O 5, PMDA-ODA, Kapton-H) was decreased from 3.15 to 2.57 (1 kHz) by diffusing fluorine and boron in the surface region of the polyimide using the radiation-assisted diffusion technique. Polyimide samples ( 50 μm×12 mm×12 mm) were immersed in a BF 3 solution kept in a thin walled glass bottle, and six such bottles were used. Each bottle was exposed to Co-60 gamma rays and the dose was varied from bottle to bottle over the range of 30–180 kGy. Both the dielectric constant, ε ′, and the refractive index, n, of the boron and fluorine-diffused polyimide decreased with increasing radiation dose, and hence with the concentration of fluorine and boron. For these polyimide samples, the dielectric constant, ε ′, estimated from the measured value of refractive index, n, (632.8 nm) was 2.22, which is close to the dielectric constant, ε ′, (2.26) measured at 7 MHz. The results indicate that the degree of electronic polarization is lowered appreciably as compared to that of atomic and dipole polarization. The decrease in the dielectric constant, ε ′, and the refractive index, n, is attributed to the increase in the free volume of polyimide and to a decrease in the degree of electronic polarization.