Time-of-flight scanning transmission ion microscopy with high energy resolution in the low keV range

A prototype scanning transmission ion microscope (STIM) with time-of-flight (ToF) capabilities has been developed at the Luxembourg Institute of Science and Technology. It simultaneously collects 2D sample plane, 2D detector plane, and time-of-flight data to generate rich 5D datasets, with a sample spatial resolution of around 2 μm, a detector angular resolution of 0.01°, and a ToF resolution of under 1 ns. Pulsed beam chopping was implemented to achieve the sub-ns time resolution. At 25 keV, an energy resolution of ∼100 eV in ion energy loss spectroscopy is achieved for the flight path of 470 mm. The system is demonstrated for an application of sample thickness measurements for partially eroded metallic thin films of Pd, Pt, and Au that are nominally 2-3 nm thick. The experimental results are compared to theoretical results from SRIM for the ion energy-loss per unit thickness, which indicate that the energy resolution of 100 eV gives a theoretical thickness sensitivity of ∼0.5 nm for a thin Pd film.