In-situ deformation monitoring of thin electrochemically deposited copper lines during thermo-mechanical pulsing

In semiconductor industry, the development of the last years led to smaller and smaller devices in order to maximize efficiency and minimize costs. As a result, a miniaturization of the test structures is required as well as a proper method to monitor gradual deformation processes during repetitive thermal cycling. Thin metal films, e.g., Cu are commonly used in power semiconductor devices. Rapid temperature changes combined with a mismatch in thermal expansion coefficients of the different materials in the layer stack lead to thermo-mechanical stresses and as a result to deformation of the metallization. In order to realize high heating rates (up to 106 K/s) and to be able to observe deformation on the metallization surface, polyheater structures are used. There, a polysilicon layer works as a heating plate (Joule heating) for the Cu layer above, allowing repetitive heating and cooling on short timescales. The temperature of the system is measured using an integrated sensor. Since the deformation features, e.g. slip bands and extrusions, are on the sub-micron length scale, a scanning electron microscope (SEM) is necessary for in-situ deformation monitoring. This novel approach provides the possibility to observe the gradual deformation of metallizations under variable test parameters at high magnification and in vacuum. As test structures, 20x20x300 µm³ Cu lines with different types of copper on top of the polysilicon were chosen to be able to observe the surface as well as the side walls of a metallization structure. It is revealed, that different Cu grain microstructures lead to differences in deformation behavior during thermo-mechanical cycling. Videos of the deformation process and EBSD images are presented to demonstrate the method

Structure and Migration Mechanisms of Small Vacancy Clusters in Cu: A Combined EAM and DFT Study

Voids in face-centered cubic (fcc) metals are commonly assumed to form via the aggregation of vacancies; however, the mechanisms of vacancy clustering and diffusion are not fully understood. In this study, we use computational modeling to provide a detailed insight into the structures and formation energies of primary vacancy clusters, mechanisms and barriers for their migration in bulk copper, and how these properties are affected at simple grain boundaries. The calculations were carried out using embedded atom method (EAM) potentials and density functional theory (DFT) and employed the site-occupation disorder code (SOD), the activation relaxation technique nouveau (ARTn) and the knowledge led master code (KLMC). We investigate stable structures and migration paths and barriers for clusters of up to six vacancies. The migration of vacancy clusters occurs via hops of individual constituent vacancies with di-vacancies having a significantly smaller migration barrier than mono-vacancies and other clusters. This barrier is further reduced when di-vacancies interact with grain boundaries. This interaction leads to the formation of self-interstitial atoms and introduces significant changes into the boundary structure. Tetra-, penta-, and hexa-vacancy clusters exhibit increasingly complex migration paths and higher barriers than smaller clusters. Finally, a direct comparison with the DFT results shows that EAM can accurately describe the vacancy-induced relaxation effects in the Cu bulk and in grain boundaries. Significant discrepancies between the two methods were found in structures with a higher number of low-coordinated atoms, such as penta-vacancies and di-vacancy absortion by grain boundary. These results will be useful for modeling the mechanisms of diffusion of complex defect structures and provide further insights into the structural evolution of metal films under thermal and mechanical stress

Surface energies of AlN allotropes from first principles

In this Letter we present first principle calculation of surface energies of rock-salt (B1), zinc-blende (B3), and wurtzite (B4) AlN allotropes. Out of several low-index facets, the highest energies are obtained for mono-atomic surfaces (i.e. only by either Al or N atoms): \gamma_{\{111\}}^{\rm B1}=410\uu{meV/\AA}^2, \gamma_{\{100\}}^{\rm B3}=346\uu{meV/\AA}^2, \gamma_{\{111\}}^{\rm B3}=360\uu{meV/\AA}^2, and \gamma_{\{0001\}}^{\rm B4}=365\uu{meV/\AA}^2. The difference between Al- and N-terminated surfaces in these cases is less then 20\uu{meV/\AA}^2. The stoichiometric facets have energies lower by 100\uu{meV/\AA}^2 or more. The obtained trends could be rationalised by a simple nearest-neighbour broken-bond model.Comment: 7 pages, 2 figure

Modelling the nucleation, growth and coarsening kinetics of γ^″ (D0<inf>22</inf>) precipitates in the Ni-base Alloy 625

Alloy 625 is susceptible to significant precipitation hardening through the formation of γ″ (D022-NbNi3) particles. These precipitates can form both during manufacture and in high temperature service and, consequently, the accurate prediction of their behaviour is crucial. To this end, a model is presented here which describes γ″ precipitation in Alloy 625, encompassing the concurrent nucleation, growth and coarsening of different particles and allowing for the particles to be shape changing. This model is calibrated with respect to the experimentally measured aspect ratio evolution observed at 650 °C. The resultant outputs for interfacial energy, particle size distribution and number density are in agreement with experimental data for a simulation of 1000 h at 650 °C

Atomic scale investigation of Cr precipitation in copper

The early stage of the chromium precipitation in copper was analyzed at the atomic scale by Atom Probe Tomography (APT). Quantitative data about the precipitate size, 3D shape, density, composition and volume fraction were obtained in a Cu-1Cr-0.1Zr (wt.%) commercial alloy aged at 713K. Surprisingly, nanoscaled precipitates exhibit various shapes (spherical, plates and ellipsoid) and contain a large amount of Cu (up to 50%), in contradiction with the equilibrium Cu-Cr phase diagram. APT data also show that some impurities (Fe) may segregate along Cu/Cr interfaces. The concomitant evolution of the precipitate shape and composition as a function of the aging time is discussed. A special emphasis is given on the competition between interfacial and elastic energy and on the role of Fe segregation

Predicting microstructure and strength of maraging steels: Elemental optimisation

A physics–based modelling framework to describe microstructure and mechanical properties in maraging steels is presented. It is based on prescribing the hierarchical structure of the martensitic matrix, including dislocation density, and lath and high–angle grain boundary spacing. The evolution of lath–shaped reverted austenite is described using grain–boundary diffusion laws within a lath unit. The dislocation density provides the preferential nucleation sites for precipitation, whereas descriptions for particle nucleation, growth and coarsening evolution are identified for Ni 3 Ti, NiAl and its variants, and BCC–Cu clusters. These results are combined to describe the hardness at different ageing temperatures in several [Formula presented], [Formula presented] and [Formula presented] steels. A critical assessment on individual contributions of typical alloying elements is performed. Ni and Mn control the kinetics of austenite formation, where the latter shows stronger influence on the growth kinetics. Ti additions induce higher hardness by precipitating stronger Ni 3 Ti, whereas Cu clusters induce low strength. A relationship between the reverted austenite and the total elongation in overaging conditions is also found. This result allows to identify optimal process and alloy design scenarios to improve the ductility whilst preserving high hardness in commercial maraging steels