Four-dimensional STEM-EELS: enabling nano-scale chemical tomography.

Advances in electron-based instrumentation have enabled the acquisition of multidimensional data sets for exploring the unique structure-property relationship of nanomaterials. In this manuscript, we report a technique for directly probing and analyzing the three-dimensional (3D) electronic structure of a material at the nano-scale. This technique, referred to here as 4D STEM-EELS, utilizes a rotation holder and pillar-shaped samples to allow STEM mode high-angle annular dark-field (HAADF) and EELS spectrum images to be recorded over a complete 180 degrees rotation to minimize artifacts. The end result is a four-dimensional data set, containing two spatial dimensions, rotation angle and energy-loss information I(x, y, theta, DeltaE), which can then be processed to extract any EELS signal as a rotation or "tilt-series" map. If the extracted properties satisfy the linear projection criteria, these maps can then be used for tomographic reconstruction to yield volumetric maps of the corresponding properties. Hence by combining STEM HAADF and energy-loss information from such a series of spectrum images, it is possible to map not only the microstructure, but also the elemental, physical and chemical state information of a material in three dimensions. Two examples are reported here to demonstrate the potential of this technique. To illustrate chemical tomography, 4D STEM-EELS was used to directly probe the 3D electronic structure of a W-to-Si contact from a semiconductor device. Core-loss data were used to reconstruct and render the composition of the W-to-Si contact in three dimensions. The fine structure of the 99eV Si edge was analyzed with MLLS fitting to map the variations in Si bonding in 3D. To illustrate the direct probing of intrinsic material anisotropy, 4D STEM-EELS was used to probe a ZnO thin film. Subtle but systematic changes in low-loss structure were observed as a function of electron-beam orientation with respect to the ZnO crystallographic axes. Together these examples illustrate how the 4D STEM-EELS technique reported here can be used to probe the elemental, physical and chemical state information of a material in three dimensions and extend our knowledge of nano-scale structures.