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 a Gaussian-like spot probe ion beam, we characterize the use of the stigmator as quasi-cylindrical lens to create a highly astigmatic beam that we term ion knife. Using the ion knife allows for material ablation by spreading the current over a larger area and changes the dimension of the probe with an anisotropic change in apparent beam resolution, as observed in spot burn cross-sections. We demonstrate a method to approximate beam shapes by imaging that allows for convenient alignment of parameters for beam shaping. 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.