Quantitative nanoscale imaging using transmission He ion channelling contrast: Proof-of-concept and application to study isolated crystalline defects

Abstract

A newly developed microscope prototype, namely npSCOPE, consisting of a Gas Field Ion Source (GFIS) column and a position sensitive Delay-line Detector (DLD) was used to perform Scanning Transmission Ion Microscopy (STIM) using keV He+ ions. One experiment used 25 keV ions and a second experiment used 30 keV ions. STIM imaging of a 50 nm thick free-standing gold membrane exhibited excellent contrast due to ion channelling and revealed rich microstructural features including isolated nanoscale twin bands which matched well with the contrast in the conventional ion-induced Secondary Electron (SE) imaging mode. Transmission Kikuchi Diffraction (TKD) and Backscattered Electron (BSE) imaging were performed on the same areas to correlate and confirm the microstructural features observed in STIM. Monte Carlo simulations of the ion and electron trajectories were performed with parameters similar to the experimental conditions to derive insights related to beam broadening and its effect in the degradation of transmission image resolution. For the experimental conditions used, STIM imaging showed a lateral resolution close to30 nm. Dark twin bands in bright grains as well as bright twin bands in dark grains were observed in STIM. Some of the twin bands were invisible in STIM. For the specific experimental conditions used, the ion transmission efficiency across a particular twin band was found to decrease by a factor of 2.8. Surprisingly, some grains showed contrast reversal when the Field of View (FOV) was changed indicating the sensitivity of the channelling contrast to even small changes in illumination conditions. These observations are discussed using ion channelling conditions and crystallographic orientations of the grains and twin bands. This study demonstrates for the first time the potential of STIM imaging using keV He+ ions to quantitatively investigate channelling in nanoscale structures including isolated crystalline defects.