Towards new approaches for APT specimen preparation with fs-laser ablation

Atom probe tomography (APT) enables three-dimensional compositional imaging at the atomic scale and is based on the principle of field evaporation of a sharp needle shaped specimen. Hereby, a radius at the specimen apex of 10-150 nm is essential. The main aim of this work was to integrate a femtosecond (fs)-laser ablation system into as many stages of the APT specimen preparation process as possible.
Starting from cutting blanks (all pure Fe), the fs-laser ablated dimension was gradually decreased resulting in radii at the specimen apex of the low μm regime. Those specimens were subsequently finished with either electropolishing, broad ion beam (BIB) milling or focused ion beam annular milling. Between and after each step the specimens were investigated using scanning electron microscopy.
Another focus of this work was the parameter study for BIB milling. After varying the acceleration voltage, preparation time and the angle between ion gun and specimen, suitable parameters were found to achieve an adequate size and shape of the specimen apex. The fs-laser ablated and BIB milling finished specimen performed best in the APT proof of concept measurements, reaching 29 million detected ions in voltage mode. The specimens finished with electropolishing suffered from early fracture events and needed several attempts for a successful proof of concept. In summary, fs-laser ablation has proven to be a versatile technique that can be used at many stages of the preparation process. Further studies investigating BIB milling of different materials and experimental setups for finishing APT specimens are recommended as scope for future work.