Papers by Author: Jean Bernardini

Abstract: The use of nanometric size materials as embedded clusters, nanometric films, nanocrystalline layers and nanostructures is steadily increasing in industrial processes aiming to produce materials and devices. This is especially true in today Si-based microelectronics with transistors made of a multitude of different thin film materials (B-, As-, and P-doped Si, NiSi (Pt), poly-Si, W, TiO_x, LaO, SiO₂, Al, H_fO₂), and exhibiting a characteristic lateral size of 32-22 nm. Size reduction leads to an increasing role of surfaces and interfaces, as well as stress and nanoscale effects upon important phenomena driving fabrication processes, such as atomic diffusion, phase nucleation, phase growth, and coarsening. Consequently, nanotechnology related to Material Science requires an investigation at the nanometric (or atomic) scale of elementary physical phenomena that are well-known at the microscopic scale. This paper is focused on nanosize effects upon diffusion in Si and Si reactive diffusion. We present recent results showing that the kinetic of lattice diffusion is enhanced in semiconductor nanometric (nano) grains, while grain boundary (GB) diffusion is not changed in nanoGBs. It is also shown that diffusion in triple-junction (TJ) is several orders of magnitude faster than GB diffusion, and that its effect cannot be neglected in nanocrystalline (nc) layers made of 40 nm-wide grains. Experimental results concerning Si sub-nanometric film reaction on Ni (111) substrate are also presented and compared to theoretical results giving new prospects concerning nanosize effects on reactive diffusion at the atomic scale.

Abstract: Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and scanning tunnelling microscopy (STM) were used to study the reactive diffusion of one monolayer of silicon deposited at room temperature onto a Ni (111) substrate. We have done isochronal and isothermal kinetics by AES, and we observed in both cases a kinetics blockage on a plateau corresponding to around one third of a silicon monolayer. STM images and LEED patterns both recorded at room temperature just after annealing, reveal formation of an ordered hexagonal superstructure corresponding probably to a two-dimensional surface silicide.

Abstract: With the development of nanotechnologies, the number of industrial processes dealing with the production of nanostructures or nanoobjects is in constant progress (microelectronics, metallurgy). Thus, knowledge of atom mobility and the understanding of atom redistribution in nanoobjects and during their fabrication have become subjects of increasing importance, since they are key parameters to control nanofabrication. Especially, todays materials can be both composed of nanoobjects as clusters or decorated defects, and contain a large number of interfaces as in nanometer-thick film stacking and buried nanowires or nanoislands. Atom redistribution in this type of materials is quite complex due to the combination of different effects, such as composition and stress, and is still not very well known due to experimental issues. For example, it has been shown that atomic transport in nanocrystalline layers can be several orders of magnitude faster than in microcrystalline layers, though the reason for this mobility increase is still under debate. Effective diffusion in nanocrystalline layers is expected to be highly dependent on interface and grain boundary (GB) diffusion, as well as triple junction diffusion. However, experimental measurements of diffusion coefficients in nanograins, nanograin boundaries, triple junctions, and interfaces, as well as investigations concerning diffusion mechanisms, and defect formation and mobility in these different diffusion paths are today still needed, in order to give a complete picture of nanodiffusion and nanosize effects upon atom transport. In this paper, we present recent studies dealing with diffusion in nanocrystalline materials using original simulations combined with usual 1D composition profile measurements, or using the particular abilities of atom probe tomography (APT) to experimentally characterize interfaces. We present techniques allowing for the simultaneous measurement of grain and GB diffusion coefficients in polycrystals, as well as the measurement of nanograin lattice diffusion and triple junction diffusion. We also show that laser-assisted APT microscopy is the ideal tool to study interface diffusion and nanodiffusion in nanostructures, since it allows the determination of 1D, 2D and 3D atomic distributions that can be analyzed using diffusion analytical solutions or numerical simulation.

Abstract: Silicide sequential phase formation during tens-of-nanometer-thick metallic film reaction on Si substrate has been extensively studied. Nevertheless, the reasons of sequential phase formation are still under debate, and have been poorly studied at the atomic scale. Using atomistic kinetic Monte Carlo simulations, we show that considering a binary fcc non-regular solid solution, without diffusion asymmetries, the diffusive reaction of a sub-nanometer-thick film (~5 atomic monolayers) on a semi-infinite substrate leads to the sequential formation of all the phases present in the binary phase diagram, starting with the film atom richest phase. These predictions are supported by experimental observations: the dissolution of a 4 monolayer-thick Si film on a Ni(111) substrate, during in-situ ultra high vacuum Auger electron spectroscopy, shows delays and kinetic changes in the dissolution process that may correspond to the sequential formation of the Ni-Si compounds, i.e. NiSi₂, NiSi, Ni₃Si₂, Ni₂Si, Ni₃₁Si₁₂ and Ni₃Si.

Abstract: The measurement of diffusion coefficients in today’s materials is complicated by the down scaling of the studied structures (nanometric effects in thin films, nano-crystalline layers, etc.) and by the complex production process conditions of industrial samples or structures (temperature variations, complex solute and point defect distributions, stress gradients, etc.). Often diffusion measurements have to be performed in samples for which initial experimental conditions do not offer the possibility of using conventional diffusion analytical solutions. Furthermore, phenomena involved with diffusion are sometimes so numerous and complex (stress, matrix composition inhomogeneities, time dependence of point defect generation sources, electrical effects, clustering effects, etc…) that the use of analytical solutions to solve the observed diffusion behavior is difficult. However, simulations can be of use in these cases. They are time consuming compared to the use of analytical solutions, but are more flexible regarding initial conditions and problem complexity. The use of simulations in order to model physical phenomena is quite common nowadays, and highly complex models have been developed. However, two types of simulations have to be considered: i) simulations aiming to understand and predict phenomena, and ii) simulations for measurement purposes, aiming to extract the (average) value of a physical parameter from experimental data. These two cases have different constrains. In the second case, that is the subject of this article, one of the most important stress is that the simulation has to precisely scale the experiment (sample size, experiment duration, etc.), sometimes preventing the measurement due to computational time consumption. Furthermore, the simpler the model (small number of parameters) used in the simulation, the more relevant the measurement (minimum error). In this paper, examples of recent works using two- and three-dimensional finite element simulations for diffusion coefficient measurements in thin polycrystalline films and nano-crystalline layers are presented. The possible use of simulations for diffusion coefficient measurements considering GB migration, GB segregation, or triple junctions is also discussed.

Abstract: In this paper, examples of some of the most challenging features of GB diffusion are considered covering selected problems, strongly related to the research activity at our Laboratories and to the scientific interest of Boris Bokstein too. The following problems (and still open questions related to them) are addressed: i) Diffusion in a random network of grain boundaries with different structures and diffusion coefficients in polycrystalline materials; ii) Segregation effects; iii) Stress effects and iv) Effect of the presence of moving and/or non-equilibrium grain boundaries.

Abstract: A method is presented to measure lattice and grain boundary diffusion coefficients using secondary ion mass spectroscopy and 2-dimensional diffusion simulations. SIMS is used to measure concentration profiles of implanted species before and after annealing. The as-implanted concentration profile is used as the initial condition for 2-dimensional diffusion simulations using the finite element method. The geometry of the simulation is based on the microstructure of the sample observed by transmission electron microscopy. Both lattice and grain boundary diffusion are simulated. The final 2-dimensional concentration distribution is projected on the depth axis to obtain a simulated depth profile. The diffusion coefficients are adjusted to fit the profiles measured after annealing. We find that this method allows to determine simultaneously and independently the lattice and grain boundary diffusion coefficients from the same profiles. This method is used to measure the diffusion coefficients of As in polycrystalline Ni2Si thin films. The simulations are found to fit the measured profiles with accuracy. The coefficients are measured between 550 and 700°C. An activation energy ratio Qgb/Qv is found greater than one. This result is corroborated by existing data in silicides and is compared to results in other materials for discussion.

Abstract: Diffusion controlled processes play a crucial role in the degradation of technical materials. At low temperatures the most significant of them is the diffusion along grain boundaries. In thin film geometry one of the best methods for determining the grain boundary (GB) diffusion coefficient of an impurity element is the Hwang-Balluffi method, in which a surface sensitive technique is used to follow the surface accumulation kinetics. Results of grain boundary diffusion measurements, carried out in our laboratory by this method in three different materials systems (Ag/Pd, Ag/Cu and Au/Ni) are reviewed. In case of Ag diffusion along Pd GBs the surface accumulation was followed by AES method. The data points can be well fitted by an Arrhenius function with an activation energy Q=0.99eV

Abstract: We present an experimental study by Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) of the dissolution of about one monolayer of silicon previously deposited at room temperature on Cu (001). The isochronal dissolution has been recorded in the temperature range [50-320°C] (annealing rate 1.5°C/min). The plateau observed in the kinetics dissolution for temperatures between 95°C and 240°C, reveals the formation of an intermetallic two dimensional superficial phase thermally stable in this range of temperature. On the plateau, LEED patterns show the formation of a (5x3) superstructure. Above 255°C, we observe a very fast dissolution of the surface alloy characteristic of a first-order surface transition. Isothermal dissolutions kinetics have been recorded above and under the surface transition temperature (250°C and 270°C). From these measurements, we have evaluated bulk diffusion coefficients of Si in Cu assuming a local equilibrium. The diffusion coefficients measured within this hypothesis at 250°C and 270°C are respectively higher and lower than those extrapolated from high temperature measurements for Ge in Cu.

Abstract: Depending on the thermodynamic, structural and diffusion properties of the system, a thin deposit dissolves into a substrate by different mechanisms. In this communication these different behaviours, investigated by surface analytical techniques (AES, XPS, STM, UPS, etc) [ - ], are reviewed. The experiments were also supported by computer simulations. The obtained results are compared and it is summarized how different parameters influence the dissolution of a thin film in a substrate. Furthermore, it is show that i) the volume dissolution kinetics is different on the atomic-/nano-scale than on the microscopic scale due to the diffusion asymmetry ii) the volume and GB diffusion in one measurement can be separated and iii) pure (C-kinetic) GB diffusivities can be determined from thin film kinetics measurements performed under adequate conditions.