Dependence of charge buildup in the polyimide on the incident electron energy

Elemental silver was diffused in a number of polyimide (C22H10N2O5, PMDA-ODA, Kapton-H) samples using the developed method of 6 MeV electron irradiation. The virgin and the silver diffused polyimide samples, each of dimension 12x12x25 μm, were irradiated with electrons of different energies, varying from 2 to 20 keV. Each sample was irradiated with single energy electrons for a period of 300 s, at a current density of ∼50 nA/cm2. The variations in the sample current and the backscattered electron current with the irradiation period were studied. The gaseous species evolved from the polyimide during electron irradiation were investigated by a quadrupole mass spectrometer. The negative charge stored and the surface potential developed in the polyimide increased with increasing electron energy, even though the irradiation period and the incident electron current were kept constant. This phenomenon is attributed to the decrease in the number of the backscattered electrons and the increase in the number of charged gaseous species that evolved from the polyimide surface with increasing energy of the incident electrons. For the same electron energy and fluence, the charge stored in the silver diffused polyimide was lower than that developed in the virgin polyimide. This property of the silver diffused polyimide is desirable for the space applications