GaBiLi - A Novel Focused Ion Beam (FIB) Source for Ion Microscopy and Related Workflows for 3D Tomography with a Top-Down FIB From Liquid Metal Alloy Ion Sources (LMAIS)

Liquid Metal Alloy Ion Source (LMAIS) is a Focused Ion Beam (FIB) source technology that has been established for nanofabrication over the last years [1]. Most recently, ion imaging with Lithium ions from GaBiLi eutectic alloy has become feasible [2]. Multiple ion species such as Lithium and Bismuth are emitted simultaneously from a single source. Related light or heavy ions in different charge states have different kinetic ion energies at a given acceleration voltage and are separated in a downstream Wien filter. Appropriate ions can be selected quickly without any mechanical sample or column adjustment [3]. Switching between Lithium and Bismuth can be accomplished within a few seconds and enables access to different ion beams from the same FIB column while maintaining the top-down FIB geometry. Just electrical field settings of the Wien filter with perpendicular electrical and magnetic fields need to be adjusted to access different ion species from the GaBiLi source. In this way different ions are accessible without any beam offset. Lithium is the lightest ion available from LMAIS. It features the smallest virtual source diameter and lowest energy spread [4]. Hence Lithium offers the smallest probe size of all ions emitted from eutectic alloys, resulting in sub 2 nm image resolution at high depth of focus (Figure 1). Images acquired with a FIB microscope using Lithium not only have a very high resolution, but also allow nearly damage free characterization and surface sensitive imaging of the sample (Figure 2). Bismuth ions and in particular Bismuth clusters have an excellent depth resolution for sample delayering [5]. The selection of various beam paths or looping strategies as known from many applications in nanofabrication circumvent selective sputtering and redeposition while digging into the sample. Intermittent imaging with light Lithium ions prevents further sputtering and allows 2D sample imaging for mapping the region of interest layer by layer. Obtained Lithium images are then stacked for sample reconstruction (Figure 3). This unique workflow for 3D tomography takes advantage of a lithography platform in combination with the reliable availability of light and heavy ions from a single source. With the FIB system used for this work ion beams are always perpendicular to the sample and no sample tilt is needed, resulting in a stable and reliable process for tomography and 3D sample reconstruction. Workflows have been developed on a Raith VELION FIB-SEM system. Achievements from a lithography architecture have been utilized for advanced 3D ion microscopy. VELION’s smart patterning strategies guarantee a homogenous milling of the sample regardless of the requirements of the sample for 3D analysis. The precision of the laser interferometer stage enables taking single images at well-defined absolute positions. It is therefore possible to stack images accurately to create accurate 3D models of the sample and to convert a nanofabrication tool into a powerful ion microscope [6]. Alternating Bismuth ion milling and Lithium ion imaging allow stable 3D imaging of samples for reconstruction. 80 Images are stacked precisely on top of each other and are combined for a 3D nano-reconstruction of the sample. Lithium ion images of Bi2-Ca2-Co revealing the high depth of focus (left) and high image resolution on Sn/C (right). Comparison of images from single Carbon Nano Membranes (CNM) on perforated carbon foil. Left: Lithium ion image. Middle: Bismuth ion image, Right: corresponding SEM image. Both ion images resolve more details than the SEM Image and provide much higher image contrast between the different areas of the sample. Even the fold of the single CNM in the center is clearly visible in both ion images. At first glance the low damage Lithium ion beam delivers best results. The image appears contrasty, and the thin membrane remains well preserved. Because of the high surface sensitivity Lithium-ion beam is well suited to image ultrathin CNM with a high signal to noise ratio. Left: 5µm x 5µm region of interest after mill and image sample analysis. Middle: high-resolution, damage free 2D images with lithium are obtained after delayering. Right: Reconstruction of the analyzed sample area (5µm x 5µm x 800nm).