Evaluating focused ion beam induced damage in soft materials

Focused ion beam (FIB) microscopy uses Ga+ ions to remove material from a sample for a variety of imaging and preparation techniques. While considerable work has examined the effects of FIB exposure on a number of materials, optimized FIB conditions for use with softer polymeric materials are yet to be determined. In this report we use phase contrast AFM to measure local changes in the elastic modulus of polycarbonate surfaces parallel to a sectioning FIB at varying beam energies. We show that polycarbonate surfaces exposed to lower FIB energies appear stiffer than the bulk material whereas surfaces exposed to the higher beam energies of up to 25 keV are more representative of the bulk material. Energy dispersive spectroscopy (EDS) indicates that the polymer surfaces become stiffer because of Ga+ implantation from the FIB. Our experimental observations are supported by computer simulations showing an increase in the residual Ga+ concentration near-surface at lower FIB energies. A high energy FIB is therefore shown to be less invasive, producing a surface more representative of the bulk material, than using low energy FIB when sectioning polymers.