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.
Atomic and electronic structure of the TiN/MgO interface from first principles
Abstract The atomic structure, adhesion energy, electronic states, and interfacial bonding nature of the TiN(0 0 1)/MgO(0 0 1) and TiN(1 1 0)/MgO(1 1 0) interfaces are systematically investigated by first principles calculations. Two possible models for TiN(0 0 1)/MgO(0 0 1) interfaces with an orientation relationship of [0 1 0](0 0 1) TiN //[0 1 0](0 0 1) MgO and four models for TiN(1 1 0)/MgO(1 1 0) interfaces with an orientation relationship of [0 0 1](1 1 0) TiN //[0 0 1](1 1 0) MgO were considered. Through the calculations of adhesion energies of all the candidate interfaces, it was found that the interface with cation–anion bonding configurations (namely, Ti–O and Mg–N) is energetically favorable for TiN(0 0 1)/MgO(0 0 1) interfaces and the interface with N(Ti) atoms located above bridge sites between Mg(O) atoms of TiN(1 1 0)/MgO(1 1 0) interfaces shows the largest adhesion energy; in addition, the orientation relationship has a significant impact on the stability of the interface. By applying some analytical methods, we characterized the electronic structure and found that the interfacial bonding is mainly ionic with a small degree of covalence.
First-principles study of the effect of hydrogen on the metal–ceramic interface
We employ density functional theory to investigate the effect of hydrogen on TiN(111) surfaces and Al/TiN(111) interfaces. The results show that hydrogen atoms prefer to sit over different sites when adsorbing on the N- and Ti-terminated TiN(111) surfaces. Subsequent study shows that the work of adhesion of the hydrogen contaminated TiN(111) interface is substantially lower than that of the clean surface. A thorough analysis of the atomic structure and electronic density shows that hydrogen atoms break the original strong interfacial Al–N or Al–Ti bond of the clean surface and the new interfacial Al–H bond is rather weak. To gain insight into the tensile strength of the hydrogen contaminated interface, the tensile process is fully simulated. The different fracture behaviours are revealed for the clean interface and the hydrogen contaminated one: the fracture of the clean interface is ductile inside the Al slab, while that of the hydrogen contaminated interface is at the interface. Our calculated work of adhesion and tensile strength are quantitatively consistent with available theoretical and experimental results.