Three-dimensional Characterization of Gd Nanoparticles using STEM-in-SEM Tomography in a DualBeam FIB-SEM

Serial sectioning using the FIB and subsequent imaging of the same FIB-exposed surface by both FIB microscopy and scanning electron microscopy (SEM) in a DualBeam has proven especially useful to study the three-dimensional (3D) morphology of complex engineered materials systems. The technique was first introduced as an automated process in 2004 and since then has established itself as one of the primary applications for FIB and DualBeams. While state-of-the-art systems can produce datasets with a z-axis slice thickness of 3-5 nm, FIB nanotomography remains a destructive technique and is limited in resolution by the z-axis slice thickness. Electron tomography is another technique used to visualize 3D structures within a transmission electron microscope used in TEM or STEM mode. Using a thin sample focused on a region of interest, the electron beam passes through the specimen incrementally tilting around the center of the region of interest as images are acquired sequentially on a camera (TEM) or a Detector (STEM). The resulting images are reconstructed into a 3D volume using a variety of algorithms including Weighted Back Projection (WBP), or Serial Iterative Reconstruction Technique (SIRT). Low energy STEM in SEM is a routine analysis in SEMs and DualBeam FIB-SEM instrumentation for morphological characterization and ultra high-resolution imaging. With a DualBeam or SEM configured with a solid state silicon diode STEM detector and a stage with adequate tilt freedom, it is possible to acquire a sufficient number of images for 3D reconstruction using STEM tomography in SEMs and DualBeam instruments. A thin section sample of gadolinium nanoparticles ranging in size up to 50 nm mounted on an aluminum substrate was prepared using in-situ lift-out (INLO) by FIB. The sample was thinned using 30 kV Ga+ FIB to approximately 125 nm. Using an in-situ stage with 360 degree continuous tilt, the thin section was imaged every 1 degree with 30 keV SEM and the STEM detector through approximately 125 degrees of tilt. The data set was then reconstructed into a 3D rendering using FEI's tomography reconstruction software, Inspect3D Express, and visualized using FEI's Avizo image and data analysis software. The technique and results compared with conventional TEM and STEM tomography using a 200 keV FEI Talos TEM will be discussed.