Exploring shaped focused ion beams for lamella preparation

Focused ion beams (FIB) are widely used instruments in transmission electron microscopy (TEM) sample preparation across scientific disciplines. Generally, site-specific ablation of material is achieved by scanning a highly focused probe across a selected area, leading to the removal of material. However, the geometries of TEM lamellae milled with the FIB are usually highly non-isometric, with their thickness generally being orders of magnitude smaller than their width and length. Here, we explore a changed probe shape for milling. Instead of using an ion beam with the standard, Gaussian-like probe, we characterize the use of the stigmator as quasi-cylindrical lens to create a highly astigmatic probe that we term 'ion knife'. Using the ion knife allows for material removal by spreading the current over a larger area and changes the dimension of the probe as observed in spot burn cross-sections. To allow for rapid alignment of parameters in beam shaping, we demonstrate a method to approximate the shapes of our probes by imaging. Finally, exploring shaped probes in cryogenic lamella preparation, we demonstrate the feasibility of cellular lamella milling and sectioning of cryo-lift-out volumes with the ion knife.