Dielectric properties of 1 MeV electron-irradiated polyimide

Abstract

The dielectric parameters of 50-μm-thick polyimide samples irradiated in air with 1 MeV electrons and in BF3 solution with Co-60 gamma rays were studied. The dielectric constant of polyimide was found to decrease with increasing electron fluence and dose of gamma rays. At an electron fluence of ∼1015 e/cm2, the dielectric constant decreased from its virgin value of 3.15 to 2.4, measured at a frequency of ∼7 MHz. Furthermore, by irradiating polyimide samples in BF3 solution with gamma rays, boron and fluorine diffused into each polyimide sample from both sides. In these polyimide samples, the dielectric constant decreased further to ∼2.1 (7 MHz). In a plot of dielectric loss, ε″, vs log (f ), two peaks were observed over a frequency range 100 Hz–7 MHz, in virgin as well as in the electron- and gamma-ray-irradiated polyimide samples. The dielectric loss increased with increasing electron fluence or dose of gamma rays. The refractive index (632.8 nm) of polyimide decreased from the virgin value of 1.74 to 1.69, at an electron fluence of ∼1015 e/cm2. The observed decrease in the dielectric constant and the refractive index can be attributed to the induced changes in the polarization and density of the irradiated polyimide samples.