Copper diffusion into single-crystalline TiN studied by transmission electron microscopy and atom probe tomography
Abstract TiN/Cu bilayers were grown by unbalanced DC magnetron sputter deposition on (001)-oriented MgO substrates. Pole figures and electron back-scatter diffraction orientation maps indicate that both layers in the as-deposited state are single-crystalline with a cube-on-cube epitaxial relationship with the substrate. This is confirmed by selected area electron diffraction patterns. To study the efficiency of the TiN barrier layer against in-diffusion of Cu, we annealed samples at 900 °C for 1 h in vacuum and at 1000 °C for 12 h in Ar atmosphere. The single-crystalline structure of the TiN layer is stable up to annealing temperatures of 1000 °C as shown by high resolution transmission electron microscopy. While no Cu diffusion was evident after annealing at 900 °C, scanning transmission electron microscopy images and energy-dispersive X-ray spectrometry maps show a uniform diffusion layer of about 12 nm after annealing at 1000 °C for 12 h. Concentration depth profiles obtained from 3D atom probe tomography reconstructions confirm these findings and reveal that the TiN film is slightly substoichiometric with a N/Ti ratio of 0.92. Considering this composition, we propose a lattice diffusion mechanism of Cu in TiN via the formation of Cu–N vacancy complexes. The excellent diffusion barrier properties of single-crystalline TiN are further attributed to the lack of fast diffusion paths such as grain boundaries.
Cu diffusion in single-crystal and polycrystalline TiN barrier layers: A high-resolution experimental study supported by first-principles calculations
Dense single-crystal and polycrystalline TiN/Cu stacks were prepared by unbalanced DC magnetron sputter deposition at a substrate temperature of 700 °C and a pulsed bias potential of −100 V. The microstructural variation was achieved by using two different substrate materials, MgO(001) and thermally oxidized Si(001), respectively. Subsequently, the stacks were subjected to isothermal annealing treatments at 900 °C for 1 h in high vacuum to induce the diffusion of Cu into the TiN. The performance of the TiN diffusion barrier layers was evaluated by cross-sectional transmission electron microscopy in combination with energy-dispersive X-ray spectrometry mapping and atom probe tomography. No Cu penetration was evident in the single-crystal stack up to annealing temperatures of 900 °C, due to the low density of line and planar defects in single-crystal TiN. However, at higher annealing temperatures when diffusion becomes more prominent, density-functional theory calculations predict a stoichiometry-dependent ...
Electronic, spectroscopic and elastic properties of early transition metal compounds
Abstract Photoemission experiments on early 3d transition metal compounds (TMC), involving both valence bands and core levels of the 3d elements, are reviewed. Extensive use is made of ab initio schemes as well as simple models and the emphasis is put on understanding the results of experiments more than on the experimental details and methods. Compounds of the TMs are first analyzed in terms of ab initio band structure calculations, which are shown to be usually sufficient as far as the interpretation of valence photoemission spectra is concerned. A discussion of ultraviolet valence band photoemission (UPS) and bremsstrahlung isochromat spectroscopy (BIS) is also made. Other theories involving configuration interaction (CI) in the modelization are then shown to be necessary for an understanding of the core-level photoemission spectra and the observed satellite features. The electronic structure of a wide range of early TMCs, from Sc to Cr, is discussed by means of the CI cluster model analysis of the metal 2s-, 2p-, 3s- and 3p-level X-ray photoemission spectra (XPS). Early TMCs, like Ti, V, Cr oxides and halides (e.g. CrF 3 , CrCl 3 ) have been originally regarded as typical Mott–Hubbard (MH) systems. The MH model results from the electron correlations which dominate the inter-atomic overlaps that lead to bands. The concept of 3d-ligand orbital hybridization leads to the Zaanen–Sawatsky–Allen (ZSA) theory and to the charge transfer (CT) systems. Moreover, we discuss how the analysis of 3s XPS spectra can predict or not the formation of localized magnetic moments. The values of the charge transfer energy Δ and d–d Coulomb repulsion energy U point to systematic trends for the early TM compounds as found in the case of late TM compounds. Simple and competing mechanisms for the excitation of photoemission satellites are presented and the systematic trends for the compounds of the early TM series are discussed. Finally, in addition to the study of the above stoichiometric compounds, we review recent results on the electronic properties of substoichiometric binary alloy (TiN x , TiC x ) by means of core and valence XPS spectra. Self-consistent ab initio calculations with empty spheres at the empty lattice ligand sites performed on these alloys provide the total densities of the occupied states to be compared with the observed valence XPS spectra. An extension of calculations to full potential methods is necessary for interpreting the elastic properties, e.g. the bulk modulus.