Effect of TSV presence on BEOL reliability for 3-layer stacked CMOS image sensor (CIS)
Because conventional downsizing based on the empirical Moore's law has reached its limitations, an alternative integration technology, such as three-dimensional integration (3DI) is becoming the mainstream. The 3rd generation of CMOS image sensor (CIS) stacks up to 3 die interconnected by hybrid bonding and High Density Through Silicon Vias (HD-TSVs). Devices and circuits good functioning and integrity have to be maintained in such an integration especially in the close neighborhood of TSVs. Thermal budget, copper pumping, thin wafer warpage can lead to electrical yield and reliability concerns and must be investigated.
The work consists in evaluating the impact of TSV processing and proximity on BEOL and FEOL performance and reliability. Acquired data sets will help to define design rules and in particular a potential Keep-Out Zone (KOZ) and calibrate a finite element model (FFM).
Development and application of Inverse Uncertainty Quantification methods in thermal-hydraulics within the new OECD/NEA activity ATRIUM
Within the Best Estimate Plus Uncertainty methodologies (BEPU) for the safety analysis of the Nuclear Power Plants (NPPs), one of the crucial issue is to quantify the input uncertainties associated to the physical models in the code. Such a quantification consists of assessing the probability distribution of the input parameters needed for the uncertainty propagation through a comparison between simulations and experimental data. It is usually referred to as Inverse Uncertainty Quantification (IUQ).
In this framework, the Service of Thermal-hydraulics and Fluid dynamics (STMF) at CEA-Saclay has proposed a new international project within the OECD/NEA WGAMA working group. It is called ATRIUM (Application Tests for Realization of Inverse Uncertainty quantification and validation Methodologies in thermal-hydraulics). Its main objectives are to perform a benchmark on relevant Inverse Uncertainty Quantification (IUQ) exercises, to prove the applicability of the SAPIUM guideline and to promote best practices for IUQ in thermal-hydraulics. It is proposed to quantify the uncertainties associated to some physical phenomena relevant during a Loss Of Coolant Accident (LOCA) in a nuclear reactor. Two main IUQ exercises with increasing complexity are planned. The first one is about the critical flow at the break and the second one is related to the post-CHF heat transfer phenomena. A particular attention will be dedicated to the evaluation of the adequacy of the experimental databases for extrapolation to the study of a LOCA in a full-scale reactor. Finally, the obtained input model uncertainties will be propagated on a suitable Integral Effect Test (IET) to validate their application in experiments at a larger scale and possibly justify the extrapolation to reactor scale.
Thermo-aeraulic numerical simulation of an incineration reactor
An incineration and vitrification process devoted to the treatment of apha contaminated organic/metallic wastes originating from MOX production facilities is currently under development at the LPTI laboratory (Laboratoire des Procédés Thermiques Innovants) from the CEA of Marcoule. The development program relies on full scale mock-up investigation tests as well as 3D numerical simulation studies.
The thermo-aeraulic model of the incinerator reactor, developed with the Ansys-Fluent commercial software, is composed of several elementary bricks (plasma, pyrolysis, combustion, particle transportation).
The proposed work consists in improving the model, in particular as regards the pyrolysis and combustion components : chemical reactions, unsteady process… The degree of representativeness of the model will be assessed on the basis of a comparative study using experimental data coming from experiments carried out on the prototype reactor. Besides this development work, various parametric studies will be performed in order to evaluate the impact of various reactor design modifications.
So as to investigate the radiologic behaviour of the reactor during incineration of alpha contaminated wastes, a particle transport model (DPM) associated to a parietal interaction model will be implemented. The simulation results will be compared to experimental data obtained from the analysis of deposits collected on reactor walls (experimental tests are performed with actinides inactive surrogates).
Theoretical and experimental studies of the polarized light's propagation into OLED structure
In collaboration with chemists from CEA Saclay and the University of Rennes, Leti's LCEM laboratory is interested in new chiral molecules for OLED (Organic Light Emitting Device) sources able to emit circularly polarized light (CP). The interest of these CPOLED sources is multiple and encompasses both micro-screens and healthcare applications. While the state of the art is quite extensive on the chemical part, few studies have looked at the generation and transport of light in CPOLEDs components.Likewise, the conditions for measuring the polarity of the light emitted are not very detailed in the existing literature.
At the LCEM laboratory, where these chiral molecules are integrated into CPOLED devices, the goal is to design OLED architectures that can better preserve the polarization of light. To do this, it is essential to understand the propagation of light in OLED stacks from a theoretical and experimental point of view. This work is part of a larger collaboration set up in the ANR "i-chiralight" project.
In this context, we are proposing a study which will take place in two phases.
- Study of simple emitting materials: The materials to be studied will be thin layers deposited under vacuum using evaporation's system of thin layers available in the laboratory. The organic materials used will be supplied by our chemical partners in Saclay or Rennes. Optical characterizations such as ellipsometry,photoluminescence, etc. will be carried out in order to assess the performance of molecules in terms of emission efficiency but also in terms of the rotational power of light. For this last point, a model able to calculate all the terms of the Müller matrices is under development and the validation of this one will be a work to be carried out by the post-doctoral fellow.
- Study of complete OLED components: In the second phase of this work, we will focus on the complete OLED system by studying the propagation of optical modes in the stack of the different layers const
Experimentation and numerical simulation of lithium battery thermal runaway
In the current Energy transition context, the lithium battery is an essential technology to address the strong challenge of the electrical energy storage. However, Li battery severe solicitations/loadings can lead to a thermal runaway phenomenon, which can cause an outbreak of fire, even an explosive combustion of the cell or of the whole battery pack. If this phenomenon is well known, the research and development dedicated to the battery safety is emerging and must be consolidated. The post-doctorate global objective is to develop a numerical modelling and simulation strategy for thermal runaway occurring when a Li battery is subjected to mechanical/thermal/electrical abuse, in order to gain an understanding of the phenomenon, estimate the thermal spreading risk as a result of gas combustion, or study the runaway mechanical consequences (fluid structure interaction). This strategy relies on physical testing campaigns carried out as part of the post-doctorate, and on numerical tools developed by CEA (EUROPLEXUS, Cast3M). The work will be organised into three main content areas: Understanding and modelling of the phenomena on the basis of experimental tests (shock tube, abusive tests), Development of a numerical model representative of identified phenomena, Modelling including fluid-structure interaction (case deformation due to pressure increase).
Development and optimization of adaptive mesh refinement methods for fluid/structure interaction problems in a context of high performance computing
A new simulation code for structural and compressible fluid mechanics, named Manta, is currently under development at the french CEA. This code aims at both unifying the features of CEA’s legacy implicit and explicit codes and being natively HPC-oriented. With its many numerical methods (Finite Elements, Finite Volumes, hybrid methods, phase field, implicit or explicit solvers …), Manta enables the simulation of various static or dynamic kinds mechanical problems including fluids, structures, or fluid-structure interactions.
When looking for optimizing computation time, Adaptive Mesh Refinement (AMR) is a typical method for increasing numerical accuracy while managing computational load.
This postdoctoral position aims at defining and implementing parallel AMR algorithms in a high performance computing context, for fluid/structure interaction problems.
In a preliminary step, the functionalities for hierarchical AMR, such as cell refinement and coarsening, field transfers from parents to children cells, refinement criteria or hanging nodes management, will be integrated in Manta. This first work will probably rely on external libraries that should be identified.
In a second step, the distributed-memory parallel performances will be optimized. Especially, strategies for load balancing between the MPI processes should be studied, especially for fluid/structure interaction problems.
Finally, especially for explicit in time computations, one will have to define and implement spatially adapted time stepping to cope with the several levels of refinement and the different wave propagation velocities.
These last 2 points will give rise to some publications in specialized scientific journals.
Improvement by thermodynamic calculations of the modeling for the joint oxyde-gaine and the fuel cladding chemical interaction into the fuel performance code GERMINAL
This work is proposed in the frame of studies on the physico-chemical behaviour of the (U,Pu)O2 fuel during irradiation considered for the future reactors of 4th generation. Indeed, this kind of fuel is subject in particular to two specific specific phenomena that can have an impact on its behaviour:
- the formation of a JOG (Joint Oxyde-Gaine), a fission products layer localised between the external surface of the fuel pellet and the inner surface of the cladding material ;
- the FCCI (Fuel-Cladding Chemical Interaction), which leads to the formation of a corrosion layer on the internal surface of the clad containing fission products and elements constituting the cladding material.
The goal is this work is to improve the modelling of the JOG and of the FCCI into the fuel performance code (FPC) GERMINAL, dedicated dedicated to the calculation of the thermo-mechanical and physico-chemical behaviour of fast reactor fuel irradiated in normal and off-normal conditions. For that purpose, the candidate will work on the dedicated calculation scheme of GERMINAL which uses the thermochemical software OpenCalphad and on the comparison of the JOG and of the internal cladding corrosion widths obtained to experimental observations obtained for some irradiation experiments. Complementary stand-alone thermodynamic calculations will be performed with the TAFID, thermodynamic database on nuclear materials developed in an international framework, in order to analyse the thermochemistry JOG/FCCI versus parameters of interest.
This work will be performed in collaboration with a team specialised in thermodynamic modelling, in charge of the TAFID project. The student will thus have the opportunity to exchange on his results in a collaborative frame with international partners. In addition, he will be able to highlight his work through publications and presentations at conferences.
Implementation of a sensor allowing the online monitoring of the corrosion of stainless steels in a hot and concentrated nitric acid medium
The control of materials (mainly stainless steel) aging of the spent nuclear fuel reprocessing plant is the subject of permanent attention. Some installations at La Hague plant will have to be replaced very soon. In this context, it is important for the industry to develop sensors that are resistant to concentrated nitric acid (˜ 2.5 mol / L) and temperature (from ambient to 130 °C), allowing the online monitoring of the corrosion.
The aim of this work is to manufacture one sensor for the detection of corrosion of the steel intended for handling by the operators of the plant. In case of a positive response, the second sensor is used.
The challenges of this work are essentially technological since it will develop or use materials adapted to concentrated and hot nitric acid media.
The laboratory is specialized in the corrosion study in extreme conditions. It is composed of a very dynamic and motivated scientific team.
Environmental dosimetry: study, design and implementation of a calibration facility for low dose equivalent rates
In order to meet the calibration needs of the European radioactivity monitoring network, the Laboratoire national Henri Becquerel, part of CEA List, is installing a calibration facility for low dose equivalent rates, below 1 µSv/h. The work includes a study of the performance of the existing radiation beams and the design, installation and dosimetric characterization of a shielded facility to reduce the radiative background, in which low activity photon sources will be installed.
Micro-scale modelling of the mass transfer induced by a vaporization-condensation process in a ceramic material under thermal gradient
The post-doctoral work concerns the mechanism of mass transfer induced by evaporation-condensation under a thermal gradient. In nuclear fuels, the presence of porosities, the very high temperatures combined with the strong thermal gradient activate this evaporation-condensation phenomenon. This results in a displacement of porosities towards the central hot part and a transfer of material in the opposite direction towards the external cold part. This phenomenon is currently modeled by a 2D homogenized approach at the fuel pellet scale in which the material transfer is computed by solving the advection equation coupled to the heat equation by the finite element method.
The post-doctoral fellow will have to set up a microscopic modeling of the vapor phase transfer phenomenon. This work will allow to improve the simulation of free volumes associated to cracks and thus, to justify the assumptions of the velocity law of porosities migration used in the 2D homogenized model.
The work to be carried out is decomposed in two main steps which are on the one hand, the formulation and the numerical implementation of the constitutive equations of the microscopic model, and on the other hand, the justification of the homogenized model. The post-doctoral fellow will work at the CEA Cadarache site in the framework of a collaboration between the research teams of the Department of Fuel Studies and the IUSTI of Aix-Marseille University on the simulation of material transfer in the vapor phase under a thermal gradient. A major advance expected from this work is to take into account the evolution of the geometry of porosities, induced by the material transfer, with techniques for tracking the movement of solid-gas interfaces. The results will be valorized by publications in scientific journals and participation in conferences.