edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 26 NO. 1 16 specimens often localized within regions of several nm or less. In this way, nm-scale devices, defects, structures, or interfaces can be selected for APT examination. SCOPE OF MATERIAL CHARACTERIZATION APT is an experimental technique that provides sub-nm 3D maps of any element in complex material structures with analytical sensitivity approaching the ppm range. By evaporating single atomic layers of a sample atom-byatom, followed by elemental identification in the mass spectra, the specimen can be virtually “reconstructed” to reveal many microstructural and chemical features relevant to device performance and failure. For example, APT can be used to show evidence of nucleation and clustering, segregation or depletion of species at grain boundaries, and other defects such as dislocations as long as they have an associated chemical marker like a Cottrell atmosphere or segregation of an element to a dislocation core. Additionally, APT can be used to measure chemical composition, oxygen stoichiometry, and spatially resolved dopant concentrations in 3D. It excels at measuring chemical gradients across buried, arbitrarily shaped interfaces, such as those encountered in patterned nanoscale heterostructure devices. Measuring the chemical composition, uniformity, and 3D distributions of dopants and additives is extremely important to the function of nanoscale semiconductor materials, requiring high analytical and spatial resolution of each element in the device. To that end numerous electronic materials and devices have been examined by APT, including various field-effect transistor geometries, magnetic tunnel junctions, photovoltaics, battery materials, ferroelectrics, thermoelectrics, light emitting diodes, and more. Such devices contain materials as varied as metals, oxides, semiconductors, silicides, nitrides, and even 2D materials. Several examples of APT applied to the above-mentioned applications are presented here. COMPOSITION ANALYSIS Composition analysis is important for devices that rely on precise control of the stoichiometry of chemical phases, for example, in InGaN-based light emitting diodes and quantum wells. Understanding the composition and uniformity of each layer is required to verify that the apparatus will perform the required function. As such, in an APT experiment, the composition of individual nanolayers can be determined and compared with other layers in the device. Figure 3 shows a bulk InGaN quantum well structure (Fig. 3a). The inset dashed line indicates the region of interest (ROI) from which the APT tip was extracted. Here, laser-pulsed APT was used to determine the composition of each layer. The colorized 3D reconstruction (Fig. 3b), and mass spectra (Fig. 3c) from the various layers demonstrate the spatial resolution of the technique and how the composition varies in the different layers.[12] As seen in Fig. 3, the quantity and dispersion of indium in each layer can be accurately determined after reconstructing the sample tip, showing the ability of APT to verify the device properties and dispersion of elements in the quantum well. INTERFACE ANALYSIS Within a materials’ microstructure, grain boundaries are critical to the overall bulk mechanical properties. For example, elements within a bulk material may segregate or deplete at grain boundaries during the fabrication process, causing undesirable effects such as embrittlement. APT can be used to provide 1D composition profiles Fig. 3 (a) A TEM image of the InGaN material cross-section with the APT tip area shown and nominal bulk layer composition, (b) the APT reconstruction showing the separate material layers, and (c) the separate mass spectra and composition as measured by APT from each isolated layer. Adapted from Ref 12. (a) (b) (c) (continued on page 18)
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