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 (about 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. Positive charge layers extending from both surfaces to depths (order 1 μm) depending on the depth of the negative charges were also observed. This behavior provides new insights in the charging of dielectrics using electron beams