Marc Legros

Microsample tensile testing of nanocrystalline metals

Abstract A novel non-contact strain measurement technique has been employed to measure the tensile properties of extremely small ‘microsamples’ of pure high-density ultrafine-grained Al (ufg-Al) nanocrystalline Cu (n-Cu) and nanocrystalline Ni (n-Ni). These microsample tests confirmed the absence of Youngs modulus variations for metals with grain sizes approaching 25 nm. Significant strength enhancements were associated with the nanocrystalline specimens; the tensile stresses achieved in these microsample tests were measured to be an appreciable fraction of the theoretical shear strength for these metals. The ufg-Al samples (diameter, 250 nm) exhibited extensive plasticity while deformation in the n-Ni (diameter, 28 nm) remained almost entirely elastic up to failure at 1500MPa. The n-Cu samples were found to have a multiscale grain structure that produced an attractive balance of strength and ductility. Transmission electron microscopy investigations of deformed n-Ni failed to produce any evidence of dislocation activity. In the absence of dislocation motion, the tensile strength of truly nanocrystalline metals is remarkably high but currently dominated by intrinsic porosity and mesoscale microcrack coalescence.

Observation of Giant Diffusivity Along Dislocation Cores

Diffusion of atoms in a crystalline lattice is a thermally activated process that can be strongly accelerated by defects such as grain boundaries or dislocations. When carried by dislocations, this elemental mechanism is known as “pipe diffusion.” Pipe diffusion has been used to explain abnormal diffusion, Cottrell atmospheres, and dislocation-precipitate interactions during creep, although this rests more on conjecture than on direct demonstration. The motion of dislocations between silicon nanoprecipitates in an aluminum thin film was recently observed and controlled via in situ transmission electron microscopy. We observed the pipe diffusion phenomenon and measured the diffusivity along a single dislocation line. It is found that dislocations accelerate the diffusion of impurities by almost three orders of magnitude as compared with bulk diffusion.

Microstructural evolution in passivated Al films on Si substrates during thermal cycling

In situ and post-mortem transmission electron microscopy (TEM) observations of thermally cycled Al thin films have been made to identify and characterize the deformation mechanisms that govern the thermo-mechanical response of these films. The early stages of thermal cycling were associated with grain growth, untangling and motion of low angle boundaries and the absorption of dislocations into the metal/oxide interfaces. Comparison with wafer curvature experiments indicate that mechanical saturation is related to the generation of large, relatively clean, columnar grains and that the presence of the capping layer slows the rate at which the as-deposited structure is transformed to this state. The formation of large, defect-free grains facilitated the observation of dislocation sources, the motion of threading dislocations across the Al films, and the interaction of these dislocations with local obstacles and grain boundaries. However, the density and velocity of dislocations were too low to account for the thermal strains being imposed during thermal cycling, and no misfit dislocations were observed at the metal/oxide interfaces. These latter findings point to the possible influence of diffusion-related processes.

Prismatic and basal slip in Ti3Al I. Frictional forces on dislocations

Abstract In situ straining experiments have been conducted in polycrystalline Ti3Al, with the aim of analysing the mechanisms controlling dislocation motion and interpreting themechanical properties. This first part is devoted to the glide process of 1/3 〈1120〉 dislocations in prismatic and basal planes. Antiphase boundary (APB) energies have been measured in these planes. Two different APB energies and two different frictional forces have been observed in parallel prismatic planes, corresponding probably to two different cutting planes. The high Peierls-type frictional forces which have been observed in the basal and in one type of prismatic plane are shown to originate from non-planar dislocation cores, in agreement with recent atomistic calculations.

Evidence of grain boundary dislocation step motion associated to shear-coupled grain boundary migration

Abstract The present work reports dynamical observations of the grain boundary (GB)-mediated plasticity during in situ transmission electron microscopy straining experiments at moderate temperature (400 C) both in a 76.4 bicrystalline and a polycrystalline Al sample. We show that the GB migration occurs by the lateral motion of elementary GB dislocation steps. The accumulation of GB dislocation steps eventually form macro-steps. This observation agrees with the idea that GB dislocation steps generally operate in high angle GBs similarly as in twinning or martensitic transformations. The coupling factor, i.e. the strain produced by the motion of the steps was measured using fiducial markers and image correlation. The migration process involves different types of GB dislocation steps, producing different amounts of strain both parallel (coupling factor) and perpendicular to the GB plane.

In situ deformation of thin films on substrates.

Metallic thin‐film plasticity has been widely studied by using the difference between the coefficients of thermal expansion of the film and the underlying substrate to induce stress. This approach is commonly known as the wafer curvature technique, based on the Stoney equation, which has shown that thinner films have higher yield stresses. The linear increase of the film strength as a function of the reciprocal film thickness, down to a couple hundred nanometers, has been rationalized in terms of threading and interfacial dislocations. Polycrystalline films also show this kind of dependence when the grain size is larger than or comparable to the film thickness. In situ TEM performed on plan‐view or cross‐section specimens faithfully reproduces the stress state and the small strain levels seen by the metallic film during wafer curvature experiments and simultaneously follows the change in its microstructure. Although plan‐view experiments are restricted to thinner films, cross‐sectional samples where the film is reduced to a strip (or nanowire) on its substrate are a more versatile configuration. In situ thermal cycling experiments revealed that the dislocation/interface interaction could be either attractive or repulsive depending on the interfacial structure. Incoherent interfaces clearly act as dislocation sinks, resulting in a dislocation density drop during thermal cycles. In dislocation‐depleted films (initially thin or annealed), grain boundaries can compensate for the absence of dislocations by either shearing the film similarly to threading dislocations or through fast diffusion processes. Conversely, dislocations are confined inside the film by image forces in the cases of epitaxial interfaces on hard substrates. To increase the amount of strain seen by a film, and to decouple the effects of stress and temperature, compliant substrates can also be used as support for the metallic film. The composite can be stretched at a given temperature using heating/cooling straining holders. Other in situ TEM methods that served to reveal scaling effects are also reviewed. Finally, an alternate method, based on a novel bending holder that can stretch metallic films on rigid substrates, is presented. Microsc. Res. Tech., 2009.

Prismatic and basal slip in Ti3Al. II. Dislocation interactions and cross-slip processes

Abstract In the first part, dislocation mechanisms controlling individual dislocation glide in prismatic and basal planes were studied and compared with results of atomistic calculations. In this second part, the plasticity of Ti3Al is studied in situ at an intermediate scale. The critical resolved shear stress (CRSS) of prismatic slip is shown to be controlled by the easy glide process of dislocations in the planes of high APB energy, and the CRSS of basal slip is shown to depend on the collective motion of several dislocation families forming a coplanar network. Other mechanical properties like work-hardening and ductility have been interpreted in terms of dislocation interactions, multiplications and cross-slip processes. The high Peierls-type frictional forces are thus surprisingly of minor importance for explaining the mechanical properties of Ti3Al.

Dynamic observation of Al thin films plastically strained in a TEM

Abstract Aluminum thin films deposited onto oxidized silicon substrates were deformed using the difference in thermal expansion coefficients between the two materials. Stresses in the Al films were measured by the wafer curvature method. The stress-temperature behavior is explained by cross-sectional TEM studies which revealed grain growth and dislocation exhaustion. In order to track down finer dislocation mechanisms, in situ cross-sectional samples were heated from room temperature up to 450°C in a TEM. Dynamic observations demonstrated that the Al/SiO x –Si interface acts as a sink for dislocations and grain boundaries.

Characterization of ageing failures on power MOSFET devices by electron and ion microscopies

Abstract Extreme electro-thermal fatigue tests have been performed to failure on power MOSFET devices that were later observed using electron and ion microcopy. At variance with devices from the former technology generation, fatigue-induced ageing of these components is observed only in the source metallization zone. An increase in drain–source resistance may originate from both a loss of contact between the wire bondings and the Al layer and/or an extensive decohesion between the metal grains. Failure modes include local melting of the Al and creation of eutectic alloys.

Characterization and modelling of ageing failures on power MOSFET devices

A method based on the failure analysis of power MOSFET devices tested under extreme electrothermal fatigue is proposed. Failure modes are associated to several structural changes that have been investigated through acoustic, electron and ion microscopy. The main aging mode is related to the exponential increase in drain resistance due to delamination at the die attach. Earlier failures are observed when very local defects due to electrical over stresses (EOS) occur at the source metallization or at the wire bonding. Aging models were elaborated to account for the die attach delamination, but are still lacking to take in account the structural evolution of the Al metallization. This new methodology, based on accelerated tests and structural observations aims at designing a new generation of power components that will be more reliable.