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Abstract: Local approach to brittle fracture for low-alloyed steels is discussed in this paper. A bibliographical introduction intends to highlight general trends and consensual points of the topic and evokes debatable aspects. French RPV steel 16MND5 (equ. ASTM A508 Cl.3), is then used as a model material to study the influence of temperature on brittle fracture. A micromechanical modelling of brittle fracture at the elementary volume scale already used in previous work is then recalled. It involves a multiscale modelling of microstructural plasticity which has been tuned on experimental inter-phase and inter-granular stresses heterogeneities measurements. Fracture probability of the elementary volume can then be computed using a randomly attributed defect size distribution based on realistic carbides repartition. This defect distribution is then deterministically correlated to stress heterogeneities simulated within the microstructure using a weakest-link hypothesis on the elementary volume, which results in a deterministic stress to fracture. Repeating the process allows to compute Weibull parameters on the elementary volume. This tool is then used to investigate the physical mechanisms that could explain the already experimentally observed temperature dependence of Beremin’s parameter for 16MND5 steel. It is showed that, assuming that the hypothesis made in this work about cleavage micro-mechanisms are correct, effective equivalent surface energy (i.e. surface energy plus plastically dissipated energy when blunting the crack tip) for propagating a crack has to be temperature dependent to explain Beremin’s parameters temperature evolution. [ABSTRACT FROM AUTHOR]

Abstract: An original approach is proposed to predict the behavior of the 16MND5 bainitic steel (similar to U.S. A508 cl.3) in the lower range of the ductile-to-brittle transition region and at lower temperatures [−196°C; 20°C], by developing a new polycrystalline modeling concurrently with X-ray diffraction (XRD) analysis. A two-level homogenization is used to take into account each kind of heterogeneity as well as the phase and grain interactions. A Mori–Tanaka formulation first enables to describe the elastoplastic behavior of a bainitic single crystal (modeled as a single crystal ferritic matrix reinforced by cementite inclusions), while the transition to polycrystal is achieved by a self-consistent approach. This model can simulate in particular the effects of temperature. It reproduces qualitatively the stress distribution in the material (stress states are lower in ferrite than in the bulk material due to cementite particles, the difference never exceeding 150MPa), the intergranular strain heterogeneity (ripples observed on the ɛ ϕψ = f(sin2 ψ) curve) and the pole figures determined by XRD on different scales. The proposed approach is validated here on the macroscopic, phase and intraphase scale. [Copyright &y& Elsevier]

Abstract: An inverse approach to dislocation microstructure analysis is presented, able to provide dislocation density, character of the activated dislocations and range of the corresponding distortion field. It is based on the coupling between the micromechanical dislocation theory (providing the distortion field within a given crystal) with X-ray line profile analysis (relating a distortion field with the broadening of diffraction line profiles). In particular, lattice distortion is calculated by using both the dislocation density tensor (expressing the incompatibility of the microstructure) and the general Green''s function formalism (characterizing the interaction between lattice spatial positions) whereas a Fourier treatment is used to evaluate the X-ray diffraction line broadening. Within the proposed formalism, the case of a periodic arrangement of dislocations is analysed, and numerical simulations, compared with high resolution X-ray diffraction, are shown to provide results quantitatively compatible with literature data both on dislocation density, arrangement and mean distance. [Copyright &y& Elsevier]

This paper aims at determining thermomechanical stress variations induced by annealed copper filled through-silicon via (TSV) in single crystalline silicon using metal–oxide–semiconductor (MOS) rosette sensors. These eight branches sensors were specifically designed and embedded in a 65-nm CMOS technology test vehicle. An in-house four-point bending tool was employed to calibrate and to extract the six independent piezoresistive coefficients. Through the piezoresistive relations, the stress tensor was evaluated by carrying out electrical measurements on wafer splits. A finite-element approach was also adopted to evaluate numerically the stresses and the expected mobility variations induced by TSV. According to this paper, a large variation of stresses (up to 100 MPa) in the sensor area was estimated, suggesting possible sensor design improvements to better accuracy. A good agreement was obtained between numerical and experimental results, except for the orthoradial component, which was found slightly compressive experimentally. Based on a critical analysis of the experimental–numerical methodology and results detailed in this paper, guidelines are drawn to get better accuracy through the improvement of MOS size and positions as well as recommendations regarding test strategy to overcome process variability. In the longer term, such improvements should lead to the definition of a comprehensive strategy for mechanical stress probing with in situ structures in advanced semiconductor products. [ABSTRACT FROM AUTHOR]

RF MEMS (Radio Frequency Micro Electro Mechanical System) switches are promising devices but their gold-on-gold contacts, assimilated for this study to a sphere/plane contact, represent a major reliability issue. A first step toward understanding failure mechanisms is to investigate the contact metal microstructure evolution under static and cyclic loading. After static and cyclic loading of sputtered gold thin films under spherical indentation, high-resolution Electron Back Scatter Diffraction (EBSD) is used to investigate the contact area. Grain rotation against {111} fiber texture of 1-μm-thick sputtered gold thin film is a signature of plastic deformation. Grain rotation is observed above 1.6 mN under static loading using a spherical diamond indenter with 50-μm tip radius. The heterogeneity in grain rotation observed corresponds to a greater plastic deformation in the middle of the indent than at the edge. A 30° grain rotation due to cyclic work hardening is observed for a half-million mechanical cycles under 300 μN load using a spherical gold tip (20 μm radius). The same test in hot switching mode induces a grain growth in the contact area. Therefore, thermal effects occurring during hot switching are underlined. [ABSTRACT FROM AUTHOR]

An accurate knowledge of the phenomenon is required to develop a predictive modeling of the electromigration failure. Thus, a hitherto unseen SEM in operando observation method is devised. The test structure with “high density” through silicon vias (TSV) is tested at 623 K with an injected current density of 1 MA/cm 2 . Regular shots of micrographs inform about the voids nucleation, forced in copper lines above the TSV, and about the scenario of their evolution. A clear relation is established between voids evolution and the one of the electrical resistance. The lack of impact of test conditions on the failure mechanism is demonstrated. Finally, the impact of microstructure on the depletion mechanism is discussed. Grain boundaries are preferential voids nucleation sites and influence the voids evolution. A probable effect of grain size and crystallographic orientation is revealed. [ABSTRACT FROM AUTHOR]

The effect of thermal profile on microstructure is studied in the frame of thin films deposited by inkjet-printing technology. The role of sintering temperature and thermal ramp is particularly investigated. Fast heating ramps exhibit coarse grains and pores, especially when a hybrid microwave curing is performed. This enhanced growth is attributed to the quick activation of densifying sintering regimes without undergoing thermal energy loss at low temperature. Microstructural evolution of various sintered inkjet-printed films is correlated with electrical resistivity and with the Young's modulus determined by nanoindentation. A strong link between those three parameters is highlighted during experiments giving credit to either a surface or a fully volumetric sintering, according to the process. Sintering is then mainly triggered by surface mass transfer or by grain boundary diffusion. Silver thin films with an electrical resistivity 4-5 times higher than the bulk, has been reached in a few minutes with a Young's modulus of 38 GPa. [ABSTRACT FROM AUTHOR]