Thermal and acoustic pulse studies of space-charge profiles in electron-irradiated fluoroethylene propylene

We report complementary studies of space-charge distributions in electron-beam irradiated fluoroethylene propylene (FEP) (thicknesses of 13 and 26 µm) using the thermal pulse (TP) method and the laser-induced pressure-pulse (LIPP) method. The distributions obtained by the two methods are compared to show the strengths of both methods, the high near-surface resolution of the TP method (better than 100 nm) and the nearly constant resolution versus depth of the LIPP method (1-2 µm). In addition to the expected implanted negative charge at a depth governed by the electron energy, a negative near-surface charge layer was found close to the metal/FEP interface if the electron beam entered the dielectric through that interface. This finding provides new insights into the charging of dielectrics using electron beams.