Measuring crystal orientation from etched surfaces via directional reflectance microscopy

Abstract

Mapping crystal orientation has always been the domain of diffraction-based techniques. However, these measurements have limited throughput and require specialized equipment. In this work, we demonstrate crystal orientation mapping on chemically etched aluminum samples using a simple and inexpensive optical technique called directional reflectance microscopy (DRM). DRM quantifies surface reflectance as a function of illumination angle. We identify directional reflectance characteristics of grains with (111) out-of-plane orientation and infer their surface topography to calculate their underlying crystal orientation. We confirm the surface topography using atomic force microscopy and validate DRM orientation measurements with electron backscatter diffraction.

A machine learning approach to map crystal orientation by optical microscopy

“…In practice, however, developing such a model is challenging, time-consuming, and relies heavily on the input of human experts. It requires sophisticated digital signal analysis to accurately identify the key directional reflectance features in the optical signals, detailed characterization of the morphology, distribution, and crystallography of the microstructural constituents responsible for directional reflectance, and an in-depth understanding of how these constituents interact with visible light to produce the directional reflectance signal 10 . The DRM apparatus (here, represented schematically) consists of a stereomicroscope focused on a fixed specimen and a moving white light source.…”

“…Since the atomic lattice cannot be resolved directly under visible light, however, optical orientation mapping may only be achieved indirectly, by analyzing optical signals that encode the underlying crystallographic orientation 6,7 . Based on this principle, techniques have been developed to quantify orientation-dependent changes in light intensity and polarization upon reflection from optically active materials 8,9 , or to reconstruct the topography of etch-pits 10 , which inherit their geometry and orientation from the underlying atomic lattice. Directional reflectance microscopy (DRM) falls in this second category 10,11 .…”

“…Based on this principle, techniques have been developed to quantify orientation-dependent changes in light intensity and polarization upon reflection from optically active materials 8,9 , or to reconstruct the topography of etch-pits 10 , which inherit their geometry and orientation from the underlying atomic lattice. Directional reflectance microscopy (DRM) falls in this second category 10,11 . The working principle of DRM is based on measuring and analyzing the reflection of light from the surface of the material as a function of the illumination direction [10][11][12] .…”

“…Directional reflectance microscopy (DRM) falls in this second category 10,11 . The working principle of DRM is based on measuring and analyzing the reflection of light from the surface of the material as a function of the illumination direction [10][11][12] . When the material is chemically etched, the preferential dissolution of specific crystallographic planes [13][14][15] or phases 16 can generate topographical surface features which are linked to crystallographic orientation.…”

“…These features reflect visible light preferentially at certain angles, producing a directional (i.e., anisotropic) reflectance effect. Once captured by DRM, directional reflectance data is analyzed by computational methods to enable spatial mapping of grain orientation 10,11 . Until now, however, orientation imaging by optical means has only been attained on pure crystalline solids, for which the measured optical signals could be indexed using material-specific, physics-based models.…”