Focused Ion Beam - Scanning Electron Microscopy Applied to Electrically Insulating Materials

Abstract

The Focused Ion Beam – Scanning Electron Microscope (FIB-SEM) is a versatile instrument originating from the semiconductor industry. The FIB is used to produce cross sections of pre-defined locations of interest, which are imaged and analyzed with the SEM. Repeated FIB cross sectioning and subsequent SEM imaging –a process called FIB-SEM tomography– results in a full three-dimensional analysis of the structure of interest, allowing for the 3D reconstruction of cellular materials or porous media. In recent years, the capabilities of the FIB-SEM have attracted a considerable amount of attention from various scientific disciplines. This PhD Thesis presents a survey across samples from Life Sciences, Earth Sciences and Material Sciences, exploring and validating the application of the FIB-SEM instrument to scientific questions from the three disciplines. The common denominator of the various samples under investigation is their electrically insulating nature, requiring additional care in dealing with the samples in the microscope. Life Sciences Electron Microscopy (EM) in Life Sciences can be split in room temperature (cells are dehydrated, optionally embedded) and cryo applications. A contribution is made in both fields. Room temperature FIB-SEM tomography is demonstrated on cells embedded in a resin. The cellular membranes are stained with heavy metals, providing sufficient contrast in Back Scattered Electron (BSE) mode. It is shown that the distribution of endoplasmic reticulum and the mitochondria can be reconstructed in three dimensions. As for cryo-EM, a major quest in Life Sciences is the preparation of artifact-free thick cryo-sections or lamellas (>200 nm) for cryo-Transmission Electron Microscopy (TEM) tomography. A dedicated workflow has been developed to produce such thick cryo-sections with the FIB, while is SEM is used to check the quality of the sections in-situ. Earth Sciences The importance of 3D reconstructions over 2D imaging is demonstrated with an olivine sample. The permeability of olivine is linked to the wetting angle –the dihedral angle– of the melt and the rock. FIB-SEM tomography data of melt pockets indicates that measuring the dihedral angle based on 2D data may not be valid because of the presence of facets in the third dimension. Material Sciences Two different Material Sciences samples are investigated: 1) Large steamed zeolite ZSM-5 crystals and 2) Fluid Catalytic Cracking (FCC) particles. The steaming process of large ZSM-5 crystals creates mesoporosity, which is characterized by FIB-SEM tomography. To prevent charging artifacts, low-kV BSE imaging is used successfully. FIB-SEM tomography through the FCC particles revealed two major challenges considering the post-processing of the 3D data. The first challenge considers automated segmentation of the pore space from FIB-SEM tomography data. A combination of several techniques is found to be sufficiently accurate. The second challenge involves upscaling of transport properties from the dimensions analyzed by the FIB-SEM. A workflow is presented which employs experimentally based statistical means to extrapolate high resolution data to much larger length scales.