Elsevier

Ultramicroscopy

Volume 109, Issue 4, March 2009, Pages 326-337
Ultramicroscopy

Four-dimensional STEM-EELS: Enabling nano-scale chemical tomography

https://doi.org/10.1016/j.ultramic.2008.12.012Get rights and content

Abstract

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° 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, θ, ΔE), 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 99 eV 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.

Introduction

The power and flexibility of modern microscopes, coupled with the high throughput of modern detectors, allows new modalities in multidimensional data acquisition to be realized. The technique of STEM-EELS spectrum imaging (SI) has been used to probe elemental, physical and chemical state information at the nano-scale [1], [2], [3], [4], [5], [6]. More recently, tilt- or rotation-series tomography has matured into a powerful tool for imaging and spectroscopic tomography of nanostructures in three dimensions [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. In this work, these techniques are combined in a complementary manner to perform 4D STEM-EELS tomography, allowing chemical characterization over a full 3D sample volume.

Tomographic reconstructions based on tilt or rotation series can provide detailed information about the structure of a material in three dimensions. STEM high-angle annular dark-field (HAADF) imaging is commonly used to probe nanostructures in 3D due to its insensitivity to diffraction effects. The HAADF signal is further suited to tomographic studies since it satisfies, to a good approximation, the “linear projection requirement” necessary to perform tomographic reconstruction [13], [19], [20], [21]. The HAADF tilt-series images can be readily combined to provide structural information in 3D, and in some special cases composition can be inferred from the Z-contrast of HAADF images [9], [10], [14]. Image acquisition rates for individual HAADF images are usually measured in tens of seconds. Similarly, energy-filtered TEM (EF-TEM) images can also be acquired in tens of seconds allowing for the tilt-series tomographic reconstruction of a collection of individual EELS intensities [10], [13], [19], [21], [22], [23]. Gass et al. [19] have shown that it is possible to extract voxel-specific EELS spectra from EF-TEM tomograms acquired in this way. Spectroscopic imaging techniques, such as EELS and EDS SI, typically required considerably longer acquisition times, at the order of tens of minutes, which until now has made acquiring spectrum images as a tilt series impractical.

The 4D STEM-EELS technique reported here was enabled by matching a high-brightness FE-STEM, equipped with a rotation holder [24], with a high-acquisition-rate EELS spectrometer. The utilization of pillar-shaped samples and the 360° rotation holder offered two advantages: a full rotation to eliminate missing wedge artifacts, and a sample with a constant projection thickness at all rotation angles (desirable for EELS studies). With acquisition rates of over 100 spectra/s, it was possible to obtain relatively large (>200×200) pixel count spectrum images in a matter of minutes. It was therefore practical to record spectrum images at regular angular intervals with the total 4D data acquisition requiring only a few hours per sample. The SI rotation series was then merged and aligned to create a single 4D STEM-EELS data set (containing dimensions of x and y spatial information, θ the rotation angle, and ΔE the electron energy-loss spectra). The spectral properties of interest were extracted from these 4D data sets using standard EELS-SI extraction techniques. In cases where the chosen EELS signal(s) met or approximated the linear projection requirement [13], [19], [20], [21], tomographic reconstructions were then performed to yield “volumetric” maps of the properties of interest. When intrinsic material anisotropies caused variations in the EELS signals as a function of orientation, the projection requirements for 3D tomographic reconstruction are not met and the data are reported without reconstruction. The 4D STEM-EELS technique allows for direct probing, and in many cases reconstruction and rendering of the elemental, physical and chemical state information of a material in three dimensions. Two examples are reported here to illustrate how 4D STEM-EELS can be used to probe 3D electronic structure on the nano-scale. In the first example, the extrinsic properties (e.g., local variations in composition and bonding) of a W-to-Si contact from a semiconductor device are mapped in three dimensions. The high-Z/low-Z interfaces surrounding a W contact make analysis from a single two-dimensional (2D) projection difficult. For example, the thickness or uniformity of the low-Z barrier layer which surrounds a W contact cannot be accurately judged from a single 2D projection. While tilt- or rotation-series imaging techniques are widely used to image these structures in 3D, it is only possible to infer composition or chemical information if contrast mechanisms can be reliably modeled. The W-to-Si contact example reported here illustrates how 4D STEM-EELS can be used to extract chemical signals that directly reflect the different phases, composition and bonding of a nanostructured material in 3D.

In the second example, 4D STEM-EELS was used to map the intrinsic anisotropy of a ZnO thin film in three dimensions. With a number of new applications (and more in development), there is a renewed interest in ZnO [25], [26]. In its most common phase ZnO has an anisotropic wurtzite structure and many studies of its material's properties have been performed [27], [28], [29], [30], [31]. More specifically, EELS has been used to characterize bulk ZnO and nanowire ZnO to document local electronic structure as well as the band gap energy [32], [33], [34], [35]. Most recently Wang et al. [36] used EELS spectroscopy to explore the relationship between crystal and surface orientation and the material's electronic structure. They reported subtle but systematic differences in low-loss and core-loss structure when comparing spectra obtained with the electron beam parallel and perpendicular to the c-axis of ZnO nanoribbons. Their studies also show that these differences vary off-axis but they do not explore this variation systematically. In the work reported here, 4D STEM-EELS was used to probe the 3D low-loss structure of a 200-nm-thick ZnO film with embedded nano-dots [37], [38]. The ZnO thin film was specially prepared to allow systematic sampling of the electronic properties over a range of crystallographic orientations using a single axis of rotation. The film was rotated from e-beam parallel to the c- or growth-axis, to e-beam perpendicular to the c-axis, and then back again while recording EELS-SIs at regular intervals. Subtle, but systematic changes in low-loss structure were observed as a function of sampling angle, similar to Wang et al. [39]. These anisotropies can cause artifacts in 3D reconstruction algorithms, which depend on a linear projection relationship, and are therefore reported directly. The ZnO example illustrates the sensitivity and repeatability of 4D STEM-EELS and how it can be used to directly probe the effects of crystal anisotropy found in some nanostructured materials.

Section snippets

The experiment

The acquisition of 4D STEM-EELS data was made possible by matching a high-brightness FE-STEM, equipped with a rotation holder, with a high acquisition rate EELS spectrometer. Acquisition rates of over 100 spectra/s were obtained by optimizing EELS-SI conditions on a Hitachi HD-2300A equipped with a Gatan Enfina® dedicated EELS spectrometer. The EELS-SI rotation series were aligned and merged to form 4D STEM-EELS data sets I(x, y, θ, ΔE). Standard EELS analysis techniques were then used to

The results

The 4D STEM-EELS technique reported here was used to probe the 3D electronic properties of an individual W-to-Si contact and a 200-nm-thick ZnO thin film with embedded Au nano-dots. EELS-SI rotation series were acquired and then combined to form 4D STEM-EELS data sets I(x, y, θ, ΔE), containing spatial, rotation angle and energy-loss information. Acquisition artifacts such as energy drift and spatial drift were then removed using the procedure outlined above. Standard EELS analysis techniques

Discussion

The 4D STEM-EELS technique reported here provides a new microscopy tool for probing the 3D chemical structure of materials on the nanometer scale. The benefits of conventional EELS-SI acquisition and analysis have been combined with the tilt-series approach to resolve the third spatial dimension. The 4D STEM-EELS data sets offer a wealth of information about the composition, bonding, orientation and phase of a sample which makes it possible and practical to do 3D chemical tomography. This

Conclusions

A new technique “4D STEM-EELS” was reported for probing the 3D electronic structure of materials and then used to characterize a W-to-Si contact and a ZnO thin film. This technique combines the advantages of 2D STEM-EELS spectrum imaging with the tilt-series approach to resolve the third dimension. Improvements in instrumentation were used to obtain acquisition rates of over 100 spectra/s and make the acquisition of STEM-EELS tilt or rotation series practical. Pillar-shaped samples and 360°

Acknowledgements

The authors would like to thank Drs. Shivaraman Ramachandran and Jay Narayan of North Carolina State University's Department of Materials Science & Engineering for providing the ZnO/Au nano-dot thin film.

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