Molecular depth-profiling of polycarbonate with low-energy Cs⁺ ions

In this work, we explored the possibility of performing molecular depth-profiling by using very low-energy (about 200 eV) monoatomic Cs ions. We show, for the first time, that this simple approach is successful on polymer layers of polycarbonate (PC). Under 200 eV Cs irradiation of PC, a fast decrease of all characteristic negatively charged molecular ion signals is first observed but, rather surprisingly, these signals reach a minimum before rising again. A steady state is reached at which time most specific PC fragments are detected, some with even higher signal intensity (e.g. CHO) than before irradiation. It is believed that the implanted Cs plays a major role in enhancing the negative ionisation of molecular fragments, leading to their easy detection for all the profile, although some material degradation obviously occurs. In the positive ion mode, all molecular fragments of the polymer disappear very rapidly, but clusters combining two Cs atoms and one molecular fragment (e.g. Cs CHO) are detected during the profile, proving that some molecular identification remains possible. In conclusion, this work presents a simple approach to molecular depth-profiling, complementary to cluster ion beam sputtering.