J. Lépinoux

In situ TEM observations of the cyclic dislocation behaviour in persistent slip bands of copper single crystals

Abstract The dislocation processes occurring during cyclic deformation in persistent slip bands (PSBs) have been investigated in copper single crystals by in situ electron microscopy. The shapes of screw dislocations and of dislocation loops expanding in the channels of PSBs of different widths have been examined under stress. The local stress values and the shape of the stress profiles across PSBs have been determined, allowing for a comparison between experimental and computed dislocation shapes in such non-uniform stress fields. Several microstructural parameters involved in the model elaborated by Mughrabi and Essmann for the saturation stage of cyclic deformation are discussed quantitatively. The present results confirm the basic mechanisms proposed by these authors. In addition, it is found that because of many interactions with obstacles, the screw dislocations move as individuals rather than in groups, and the relative contribution of edge dislocations to the deformation of PSBs cannot be neglecte...

Mechanisms and Kinetics of Recrystallisation: A Two Dimensional Vertex Dynamics Simulation

The recrystallisation process in single phase materials is investigated using a vertex dynamics simulation. The situation is idealised (two dimensional and isotropic) to better understand the role of physical parameters (energy and mobility of subgrain and grain boundaries, shape and relative size of grains) on the competition between recrystallisation and recovery. Simulations show that subgrain growth can be very heterogeneous in the vicinity of grain boundaries, i.e. that recrystallisation develops following the bulging mechanism. Bulging is enhanced for low mobilities of subgrain boundaries, high relative desorientation and small relative sizes. The recrystallisation kinetics is well described by an Avrami law of low exponent.

Contribution of matrix frustration to the free energy of cluster distributions in binary alloys

A generalized version of Frenkels model of a cluster gas is proposed to provide a rigorous description of the contribution of the configuration entropy to the total free energy of cluster distributions in a binary alloy. It is shown that the predicted cluster distributions are in excellent agreement with those obtained in kinetic Monte Carlo simulations. The emission and absorption coefficients to be used in cluster dynamics are fully defined: they depend not only on the free energy of clusters but also on the whole cluster distribution. Alternatively, this model can provide accurate values of the nucleation driving force used in classical nucleation theory.

Microstructural defects induced by implantation of hydrogen in (111) silicon

Abstract Silicon crystals have been implanted with hydrogen at high energy. In order to obtain a constant distribution of hydrogen through a wide zone located from 20 to 50 pm from the surface, 23 different energies have been used, ranging from 1 to 2 MeV. The samples have then been annealed 30 min at temperatures ranging from 100 to 1100P°C and examined by transmission electron microscopy (TEM). Only two types of planar defects lying in {111} planes have been observed: fault-like defects (F) and loop-like defects (L). The present paper reports a detailed TEM study of the structure of the F and L defects observed in a (111) crystal. We propose a model which gives a coherent interpretation of the various characteristics of these defects.

Interfacial reaction rates and free energy of cubic clusters

A new formulation of interfacial reaction rates for clusters in binary alloys is presented. It accounts for the matrix structure and the topological properties of the clusters at the atomic scale. It is shown that the probabilities per unit time that a solute atom be captured or released by a cluster are functions not only of the partition function but also of a transition function. The principles of calculation of these functions are general but only the case of cubic clusters is treated here (results can be used for L12 clusters in fcc matrices). Exact calculations have been done for small clusters (size<10), followed by a Monte-Carlo sampling method for intermediate sizes as a function of temperature and interaction energy (a material characteristic). Finally, it is shown that generic results can be extrapolated at higher cluster size in a large range of temperature and/or interaction energy.A new formulation of interfacial reaction rates for clusters in binary alloys is presented. It accounts for the matrix structure and the topological properties of the clusters at the atomic scale. It is shown that the probabilities per unit time that a solute atom be captured or released by a cluster are functions not only of the partition function but also of a transition function. The principles of calculation of these functions are general but only the case of cubic clusters is treated here (results can be used for L12 clusters in fcc matrices). Exact calculations have been done for small clusters (size<10), followed by a Monte-Carlo sampling method for intermediate sizes as a function of temperature and interaction energy (a material characteristic). Finally, it is shown that generic results can be extrapolated at higher cluster size in a large range of temperature and/or interaction energy.

Dislocation mechanisms and steady states in the cyclic deformation of facecentred cubic crystals

Abstract A model is presented which describes cyclic saturation in a structure consisting of a hard (wall) phase and a soft (channel) phase. The processes leading to a saturation of the density of screw dislocations in channels are investigated in detail; emphasis is put on multiplication processes which have not, up to now, been incorporated in such models. It is shown that all the quantities which characterize a state of dynamic equilibrium (the mean free paths of screw dislocations, multiplication or annihilation rates, irreversibility of slip) can be estimated in terms of a few parameters accessible to experiment: the density of screw dislocations at saturation, the plastic shear strain amplitude, the critical annihilation distance and the geometry of a dislocation loop expanding from one wall to an opposite wall. Numerical estimates are presented, for copper single crystals, and the model is applied to the transition between static and dynamic equilibria, to the matrix structure and to the persistent...

Dynamic and Static Recrystallization

Dynamic or static recrystallizations are certainly the main mechanisms responsible for the microstructure and texture changes occurring during or after deformation. New experimental devices to produce large strains (Equal Channel Angular Extrusion, ECAE) and to investigate the microstructures (orientation mapping by electron backscattering diffraction, EBSD), are used for investigations in this field. Recent results are presented here as well as the development of analytical modeling and numerical simulations.

Comparing kinetic Monte Carlo simulations with cluster dynamics: What can we learn about precipitation? Application to AlZr alloys

Kinetic Monte Carlo simulations of precipitation in AlZr alloys are compared with predictions of cluster dynamics based on an enhanced thermodynamic model. A methodology and various tools are proposed to learn as much as possible from such comparisons. Important deviations between the two methods are investigated and interpreted through the role of different mechanisms.

Cu Grain Growth in Damascene Narrow Trenches

Since the end of the last century, Cu damascene integration scheme has been the favoured choice for advanced interconnect technologies. Indeed, due to the lowest Cu bulk resistivity and to it higher resistance to electromigration, performances enhancement with respect to Al have been obtained. Nevertheless, dimensional scaling considerably reduces these performances. At line width below 150nm, grain size is usually measured around line dimension and thus decreases with down scaling. Moreover, columnar grain morphology perpendicular to line sidewall is frequently observed. This results in a large increase of Cu resistivity and in degradation of resistance to electromigration. Optimization of Cu microstructure in damascene architecture is then required. This is the topic of this work. Direct measurements of microstructure through electron microscopy (TEM, EBSD) and X ray diffraction methods are performed. Indirect measurement of grain size evolutions via resistivity characterization is done. Complementary grain growth simulations are performed using vertex method. It is observed that, depending on line dimension, anneal conditions and electrolyte, different type of microstructures are achieved. As expected, certainly due grain boundary pinning on sidewall, for long time anneal, a stable situation is reached. We evidence that Cu line microstructure results from an interaction between grain growth inside the trenches and grain growth in the Cu overburden. On one hand, for the larger lines, the grain size is directly related to the grain growth in the overburden, on the other hand, for the narrowest lines, interfaces limit the impact of this layer on the inline grain growth. A quantitative measurement of overburden microstructure extension in trenches is reported. It could be used to optimize the in-line microstructure with respect to resistivity and electromigration resistance.

On the Nucleation of Abnormal Grain Growth: A 2D Vertex Simulation

The conditions for nucleation of abnormal grain growth have been investigated using a 2D vertex simulation allowing for the presence of pinning centers. The topological characteristics of an “almost pinned” structure, compared to the classical grain growth with no pinning force, show a striking correlation between the grain size and the size of the neighbors of this potential abnormal grain. The main finding of this contribution is to stress the crucial importance of pinning particles, and to point to the stochastic nature of the nucleation event in abnormal grain growth.