Columnar and Equiaxed Solidification of Al-7 wt.% Si Alloys in Reduced Gravity in the Framework of the CETSOL Project

International audienceDuring casting, often a dendritic microstructure is formed, resulting in a columnar or an equiaxed grain structure, or leading to a transition from columnar to equiaxed growth (CET). The detailed knowledge of the critical parameters for the CET is important because the microstructure affects materials properties. To provide unique data for testing of fundamental theories of grain and microstructure formation, solidification experiments in microgravity environment were performed within the European Space Agency Microgravity Application Promotion (ESA MAP) project Columnar-to-Equiaxed Transition in SOLidification Processing (CETSOL). Reduced gravity allows for purely diffusive solidification conditions, i.e., suppressing melt flow and sedimentation and floatation effects. On-board the International Space Station, Al-7 wt.% Si alloys with and without grain refiners were solidified in different temperature gradients and with different cooling conditions. Detailed analysis of the microstructure and the grain structure showed purely columnar growth for nonrefined alloys. The CET was detected only for refined alloys, either as a sharp CET in the case of a sudden increase in the solidification velocity or as a progressive CET in the case of a continuous decrease of the temperature gradient. The present experimental data were used for numerical modeling of the CET with three different approaches: (1) a front tracking model using an equiaxed growth model, (2) a three-dimensional (3D) cellular automaton–finite element model, and (3) a 3D dendrite needle network method. Each model allows for predicting the columnar dendrite tip undercooling and the growth rate with respect to time. Furthermore, the positions of CET and the spatial extent of the CET, being sharp or progressive, are in reasonably good quantitative agreement with experimental measurements

On the Deformation of Dendrites During Directional Solidification of a Nickel-Based Superalloy

Abstract: Synchrotron X-ray imaging has been used to examine in situ the deformation of dendrites that takes place during the solidification of a nickel-based superalloy. By combining absorption and diffraction contrast imaging, deformation events could be classified by their localization and permanence. In particular, a deformation mechanism arising from thermal contraction in a temperature gradient was elucidated through digital image correlation. It was concluded that this mechanism may explain the small misorientations typically observed in single crystal castings

Электронный компас

Разработка электронного компаса на основе датчиков: магнитометра, акселерометра и гироскопа. Данное устройство будет определять направление на северный магнитный полюс как при горизонтальном положении, так и при отклонениях от горизонтальной плоскостиDevelopment of electronic compass based on sensors: magnetometer, accelerometer and gyroscope. This device will determine the direction to the North magnetic pole both in the horizontal position and in deviations from the horizontal plan

Etude pour la reconnaissance de paysages agricoles sur des images satellitaires

National audienc

Nitrogen enhanced thermal stability of nickel monosilicide

International audienceThe effect of annealing ambient during rapid thermal processing on thermal stability of NiSi has been investigated. Nickel films deposited onto Si(100) were annealed in vacuum, Ar and N-2 at pressures up to 40 Torr. Rapid thermal annealing in vacuum or in Ar at 750 degrees C resulted in the formation of NiSi2 with concomitant increase in sheet resistance and surface roughness. However, annealing in N-2 increases the NiSi2 nucleation temperature and thus the thermal stability of NiSi. Higher the N-2 pressure, higher was the annealing temperature required to initiate NiSi2 nucleation. Experimental results were analyzed using a simple thermodynamic model. It suggests dissolved nitrogen impurities lower the chemical potential and increase thermal stability of NiSi phase. Phase stability observed experimentally was in agreement with that predicted by the model. [GRAPHICS] Threshold nitrogen pressure exists for stabilization of nickel monosilicide during rapid thermal annealing. (C) 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Reconnaissance de paysages modèles sur images satellitaires

National audienc

Nanoscale effect on the formation of the amorphous Ni silicide by rapid thermal annealing from crystalline and pre-amorphized silicon

International audienceThe Ni monosilicide alloyed with Pt is widely used as contact material in advanced microelectronics devices and a good knowledge of silicide formation kinetics is required for the process control. In this work, the nature, and the growth kinetics of the first silicide obtained during the solid-state reaction between the Ni0.9Pt0.1 and the Si are studied for different pre amorphization implant (PAI) conditions as well as for a reference sample without PAI. Reactions between a 10 nm thick Ni0.9Pt0.1 film and Si (100) substrate are analyzed after several rapid thermal anneals (RTA). The nature of the first silicide is determined by Fourier Transform of TEM images and by chemical TEM-EDX analyses. The silicide growth behavior is determined by measuring the silicide thickness by X ray reflectivity (XRR) after the partial reaction induced by RTAs at different temperatures and times. To determine the growth law, the linear parabolic model is first considered but a nonlinear reactive diffusion model must be developed to accurately reproduce the experimental results. From this model, the effective diffusion coefficient as well as its activation energy were determined for the three samples with PAI and the reference sample without PAI. The influence of the driving force on the nonlinear diffusion for thin films is proved, and the impact of the amorphous substrate on the kinetics parameters is quantified and compared to the literature

On the influence of Ni(Pt)Si thin film formation on agglomeration threshold temperature and its impact on 3D imaging technology integration

Ni(10 at.% Pt) monosilicide is used as contact in microelectronics but suffers from degradation at relatively low temperatures due to agglomeration. Recent results obtained on 28 nm-FDSOI microelectronics devices have demonstrated severe yield loss after an anneal at 550 °C/2 h linked to Ni(Pt)Si film dewetting. Such agglomeration thermal budget is 100 °C lower than the ones measured on blanket wafers with in-situ or exsitu four-point probe measurements. In this context, the aim of this paper is to investigate the effect Ni(Pt)Si formation process on the Ni(Pt)Si agglomeration using different approaches as (i) the classical one in which one anneal is applied to form silicide and leads also to agglomeration, (ii) the silicide formation through the standard SALICIDE process, "Self-Aligned Silicide", and a subsequent anneal to induce agglomeration, and (iii) the standard SALICIDE process for silicide formation followed by an encapsulation of the top silicide surface by a SiN layer as applied in devices, and submitted finally to the agglomeration anneal. Our work demonstrated that the film thermal stability is influenced by the sequencing of the selective etch (SE) in the formation process and whether it is formed by a single or a double anneal. Another conclusion of this work is that four-point probe measurements are not sensitive enough to well estimate the real starting point of agglomeration phenomenon which is detrimental for devices (holes formation at the triple junctions). Some additional characterizations such as tilted Scanning Electron Microscopy (tilted SEM) are deeply needed for an accurate determination of agglomeration thermal budget. This study allows clarifying the main parameters leading to agglomeration: the film thickness and the grain size appear to be the more important ones

Impact of the pre amorphization by Ge implantation on Ni0.9_{0.9}Pt0.1_{0.1} silicide

International audienceThe impact of the Pre Amorphization by Ge Implantation (PAI) on Ni0.9Pt0.1 silicide is studied. Reactions between a 10 nm thick Ni0.9Pt0.1 film and Si (100) substrate are analyzed as a function of the induced amorphous-Si thicknesses. In view of being compatible with the integration constraint of the 28 nm CMOS technologies, the Ge implantation dose is fixed at a low level. The relative position of the amorphous-Si/crystalline-Si interface and the silicide growth front is defined for each sample. Then, in-situ XRD analyses, X-Ray Reflectometry (XRR) and Sheet resistance (Rs) measurements are achieved to provide a deep study of silicide growth kinetics and silicide properties. First, a clear relationship is established between the silicide growth rate and the amorphous-Si thickness. Secondly, an easier NiSi nucleation and a decrease of its resistivity is observed when NiSi nucleates at the θ-Ni2Si/a-Si interface. These observations are discussed considering the impact of the amorphous-Si layer on the driving force, the nucleation barrier, the lateral growth rate, and NiSi roughness

Modeling and experimental characterization of the microstructure and grains structure of Al-7wt%Si directionally solidified

International audienceControlling the solidification microstructure of metallic materials is the main objective of several researches developed nowadays due to its strong influence on the mechanical properties. The natural convection in the bulk liquid caused by the action of the gravitational field during solidification process can modify the final structural morphology of the materials. The objective of this study is to develop a numerical and experimental approach to verify the evolution of the grain morphology, segregation of the eutectic phase and predictions about the action of the natural convection over the final grain structure of an Al-7wt%Si alloy in the presence of refining particles (0,5wt% Al-Ti-B). The experiments were carried out by using Bridgmann technique and the numerical analysis was done by using the cellular automaton with finite element model. The experimental results presented good agreement with the simulations