Large volume tomography using plasma FIB-SEM: A comprehensive case study on black silicon.

The xenon plasma focused ion beam and scanning electron microscopy (PFIB-SEM) system is a promising tool for 3D tomography of nano-scale materials, including nanotextured black silicon (BSi), whose topography is difficult to measure with conventional microscopy techniques. Advantages of PFIB-SEM include high material removal rates, precise control of milling parameters and automated slice-and-view procedures. However, there is no universal sample preparation procedure nor is there an established ideal workflow for the PFIB-SEM slice-and-view process. This work demonstrates that specimen preparation, including the orientation of the volume of interest, is critical for the quality of the final reconstructed 3D model. It thoroughly explores three unique configurations incrementally optimized for higher total throughput. All three sampling configurations are applied to a resin-embedded BSi sample to determine the most favourable workflow and highlight each approach's advantages and disadvantages. The reconstructed 3D models of the BSi surface obtained are shown to be qualitatively closer to the topography measured directly by SEM. The height distribution data extracted from the rendered 3D models reveal a higher structure depth compared to that obtained from an atomic force microscopy measurement. Furthermore, the work demonstrates how samples with different rigidity react to long-term ion-beam interaction, as both amorphous (resin) and crystalline (Si) material is present in the tested specimen. This study improves the understanding of sample-beam interaction and broadens the utility of the 3D PFIB-SEM for more complicated sample structures.

3D characterisation using plasma FIB-SEM: A large-area tomography technique for complex surfaces like black silicon.

This paper demonstrates an improved method to accurately extract the surface morphology of black silicon (BSi). The method is based on an automated Xe+ plasma focused ion beam (PFIB) and scanning electron microscope (SEM) tomography technique. A comprehensive new sample preparation method is described and shown to minimize the PFIB artifacts induced by both the top surface sample-PFIB interaction and the non-uniform material density. An optimized post-image processing procedure is also described that ensures the accuracy of the reconstructed 3D surface model. The application of these new methods is demonstrated by applying them to extract the surface topography of BSi formed by reactive ion etching (RIE) consisting of 2 µm tall needles. An area of 320 µm2 is investigated with a controlled slice thickness of 10 nm. The reconstructed 3D model allows the extraction of critical roughness characteristics, such as height distribution, correlation length, and surface enhancement ratio. Furthermore, it is demonstrated that the particular surface studied contains regions in which under-etching has resulted in overhanging structures, which would not have been identified with other surface topography techniques. Such overhanging structures can be present in a broad range of BSi surfaces, including BSi surfaces formed by RIE and metal catalyst chemical etching (MCCE). Without proper measurement, the un-detected overhangs would result in the underestimation of many critical surface characteristics, such as absolute surface area, electrochemical reactivity and light-trapping.

Application and validity of the effective medium approximation to the optical properties of nano-textured silicon coated with a dielectric layer.

The emergence of nanotextures in photovoltaics has resulted in challenges associated with optical modelling. Whilst rigorous methods exist to accurately solve these textures, the computational effort required limits the scope of modeling applications. The effective medium approximation (EMA) is a potential alternative to provide rapid modeling results which can be easily integrated with ray tracing of large complex structures. However, the validity of this technique is strongly dependent on the size of features relative to the wavelength of interest, making the application of EMA ambiguous for many situations. This paper aims to address this issue by comparing the simulated results between EMA and finite element methods for three randomly distributed silicon textures with and without a dielectric layer. Criteria for which the EMA approach is valid are proposed and generalized using ratios between root-mean-square roughness, correlation length and incident wavelength, making these limits broadly applicable, beyond that of just the nanotexture under specific solar spectrum regimes. The results in this work apply to random, isotropic textures under normally incident light. Based on the proposed criteria, the validity of different optical simulation techniques for a set of industrial photovoltaic textures is discussed. This analysis reveals a region within which neither geometric optics nor EMA are adequate for calculating the reflectivity of a textured surface, and hence FDTD or other new approaches are required.