A Concept for Three-Dimensional Particle Metrology Based on Scanning Electron Microscopy and Structure-from-Motion Photogrammetry.

Scanning electron microscopy (SEM) has been frequently used for size and shape measurements of particles. SEM images offer two-dimensional (2D) information about a particle's lateral dimensions. Unfortunately, information about the particle's three-dimensional (3D) size and shape remains unavailable. To resolve this issue, I propose a new concept in SEM: 3D particle metrology obtained by applying structure-from-motion (SfM) algorithms to multiple rotational SEM images of particles deposited onto a cylindrical substrate to generate a 3D model from which size and shape information can be extracted. Particles can have any size that is suitable for SEM imaging. SEM images of the sample can be acquired from 0° to 360° using a rotational-tip SEM substage. Here, I will discuss the concept and, for clarity, illustrate it with aquarium gravel particles that are glued onto a craft roll and imaged optically before generating the 3D model of that handmade craft. Future work will include the experimental SEM realization, as well as further development of the SfM algorithms. In my view, this proposed concept may become an integral part of SEM-based particle metrology.

Three-Dimensional (3D) Nanometrology Based on Scanning Electron Microscope (SEM) Stereophotogrammetry.

Three-dimensional (3D) reconstruction of a sample surface from scanning electron microscope (SEM) images taken at two perspectives has been known for decades. Nowadays, there exist several commercially available stereophotogrammetry software packages. For testing these software packages, in this study we used Monte Carlo simulated SEM images of virtual samples. A virtual sample is a model in a computer, and its true dimensions are known exactly, which is impossible for real SEM samples due to measurement uncertainty. The simulated SEM images can be used for algorithm testing, development, and validation. We tested two stereophotogrammetry software packages and compared their reconstructed 3D models with the known geometry of the virtual samples used to create the simulated SEM images. Both packages performed relatively well with simulated SEM images of a sample with a rough surface. However, in a sample containing nearly uniform and therefore low-contrast zones, the height reconstruction error was ≈46%. The present stereophotogrammetry software packages need further improvement before they can be used reliably with SEM images with uniform zones.

An electron microscopy investigation of the structure of porous silicon by oxide replication.

The morphology of porous silicon is studied by scanning electron microscopy (SEM) by making an oxide replica of the pore structure. Highly branched n-type porous silicon samples were prepared and a replica was formed by oxidation of the pores followed by selective removal of the silicon substrate to expose the oxide pores. Scanning and transmission electron microscopy images confirmed many previously held assumptions about porous silicon formation, including the fractal structure and crystallographic propagation; they also provided a clearer understanding of the details of pore formation. The replica procedure also provides a platform for a more facile and comprehensive analysis of the porous silicon morphology.