Precise identification of subnanometer chemical defects within a fin field-effect transistor

Three-dimensional electron energy-loss spectroscopy (EELS) has long been recognized as a promising technique for the chemical mapping of nanoscale devices; however, its spatial resolution has historically been constrained. In this study, we demonstrate subnanometer resolution in EELS tomography, enabling the precise identification of chemical defects within a fin field-effect transistor (finFET) structure. The finFET specimens were fabricated into <50 nm diameter needle-shaped geometries using focused ion beam milling to facilitate a full 180° tilt series of scanning transmission electron microscopy EELS spectrum images. To overcome severe carbon contamination and low signal-to-noise ratios, we employed a structurally validated background subtraction method paired with multivariate statistical analysis for robust denoising. Three-dimensional chemical maps were subsequently computed using the generalized Fourier iterative reconstruction algorithm. The resulting tomograms successfully resolve a localized ∼0.79 nm oxide defect resulting from oxygen incorporation within the TaN metal gate. Furthermore, by evaluating the reconstructions via Fourier shell correlation using the 1/2-bit information criterion, we demonstrate that a spatial resolution of 0.87 nm is achievable even under a 50% reduced electron dose. These findings establish a reliable EELS tomographic workflow for investigating buried, subnanometer chemical inhomogeneities in advanced semiconductor architectures.