Scientific direction Development of key enabling technologies
Transfer of knowledge to industry

PhD : selection by topics

Engineering science >> Solid state physics, surfaces and interfaces
5 proposition(s).

3D reconstruction of nanometric objects from stereoscopic electron microscope images

Département Technologies Silicium (LETI)




Keywords : Images treatment, GPU programmation, optimisation, inverse problem, stereovision, neural networks. Robust, non-destructive and fast 3D metrology is a world-wide major challenge of microelectronics industry for defects inspection, optical lithography fidelity and process control. Fast reconstruction methods from stereoscopic electron microscope (SEM) images based on geometrical considerations allow to reconstruct 3D topography of micronic objects. However, those technics cannot be applied on nanometric objects because of local physical phenomena which disturb the placement of the points of interest [2]. Alternative methods based on the resolution of inverse problem have been already prototyped. Significant improvements of the computation time are expected after their implementation on the GPUs of our group. Model calibration methods must also be developed, potentially based on neural networks. 3D metrology based on SEM images arouses the interest of several LETI industrial partners, and this thesis is intended to be a key element for present and future collaborations in this field. The objective of this thesis is to develop a 3D metrology from SEM images the most precise and robust as possible. For this, the PhD student will initially use the group's theoretical and simulation resources to improve and develop new SEM imaging models. The scope of these models is broad, from the simulation of micrometric objects to nanometric structures. CEA-LETI has a new generation of SEM that allows to image patterns at different points of view. These multi-stereo images allow an increase of data on the image structure which facilitates its 3D reconstruction, compared to the case of a single SEM image taken in top view. The PhD student will train the SEM models with a collection of stereoscopic SEM images of patterns, which 3D topographies will be known from reference 3D metrology. The student will investigate, in a second time, different mathematical strategies for the 3D reconstruction, allowing fast and precise convergence. Eventually, 3D reconstruction will be applied on different customer products of interest.

Epitaxial growth and nanosecond laser annealing of GeSn/SiGeSn heterostructures

Département Technologies Silicium (LETI)




Since 2015, CEA LETI has the capacity of depositing GeSn/SiGeSn heterostructures on 200 mm substrates. We are currently at the state-of-the-art in several of their application domains. In ordre to fabricate electically-pumped lasers able to operate at room temperature and performant Infra-Red photodetectors, we will explore during this PhD thesis the n-type and p-type doping of such layers, be it by ion implantation or in-situ during the epitaxial growth itself. In order to take full advantage of those doped layers, we will perform recristallisation and electrical activation anneals. With standard annealing techniques, we would be faced with the significant instability of GeSn / SiGeSn stacks (tin precipitation / surface segregation). This is why we will evaluate, during this PhD thesis, the interest of using nanosecond laser anneals and their impact on the structural and electrical properties of those stacks. Those studies, which will be conducted in our brand new SCREEN-LASSE LT3100 tool, will be among the first ever conducted on this type of semiconductors. We will notably focus on the evolution of cristalline quality, doping level, surface roughness, tin agglomeration / segregation and chemical content with the various process parameters (epitaxy and laser anneals). Such a know-how will be put to good use for the fabrication of innovative optoelectronics devices.

Li/S batteries with sulfide solid electrolyte

Département de l'Electricité et de l'Hydrogène pour les Transports (LITEN)

Laboratoire Matériaux



To further improve the performance of lithium batteries, all-solid design has been proposed employing inorganic or polymeric solid electrolytes. Best room-temperature ionic conductivity is demonstrated by sulfides approaching the values typical for liquid electrolytes. This family of ionic conductors is also interesting regarding the low synthesis temperature (compared to that of oxides) and facility of implementation by cold pressing (due to soft particles). Current limitations for sulfide solid electrolytes arise from their electrochemical reactivity with electrodes. Thus, glass-ceramic sulfides of composition Li2S-P2S5, passivate the surface of metallic Li and decompose in contact with charged cathode at high potential. The PhD project aims at evaluation of compatibility of sulfide solid electrolyte with low-potential cathode, such as elemental sulfur. This active material works at potentials as low as 2V but, in return, it has a high capacity of 1675mAh/g. To benefit from high capacity and low voltage, the best candidate for counter electrode is metallic Li. Accordingly, Li/S system with liquid electrolyte has been studied in LITEN for 10 years. The developed techniques allowed to produce cells with encouraging performances. However, the presence of liquid electrolyte leads to limitations on cycle-life of Li counter electrode. Accordingly, the replacement of liquid electrolyte by solid conductors can be a solution for this low-voltage system. Based on our previous experience in Li/S systems and sulfide solid electrolyte, the PhD project will place the first bricks into the Li/sulfide/S solid system. In particular, one of the targets will be to develop the cathode formulation adapted to volume and morphology change of all-solid composite cathode during cycling. Another objective of this PhD project will deal with optimization of interface between lithium and solid electrolyte. In this task coating as well as passivation solutions will be considered. In the end of PhD study, a proof of concept will be presented as a functional pouch cell. The improvement of security due to solid electrolyte will be evaluated in cooperation with a PhD project on characterization proposed by STB/SAMA.

Silicon optomechanical nanoresonators pour DNA strands detection

Département Composants Silicium (LETI)

Laboratoire Composants Micro-Capteurs



The MEMS sensor laboratory at CEA LETI has a very strong expertise and an international recognition in the field of resonating MEMS/NEMS, and more recently for optomechanical resonators. The market of DNA sequencing has been exponentially growing and has reached impressive volumes for biomedical devices realized with micro- and nanotechnologies. The last sequencer generation dominating the market is based on fluorescence techniques. The requirement for external optical readout is an obstacle for faster analysis and miniaturization. Among several solutions investigated for sensing in liquid medium, the NEMS group at LETI recently performed a world first: a biological detection with an optomechanical resonator immersed in liquid. Beside extraordinary sensing performance, this technique allows a very local containment of light, and potentially a significant simplification of optical sequences. The PhD candidate will build on this know-how to study the identification of DNA pairs by a multi-parametric sensing technique: gravimetric on one hand (mass sensing by biological functionnalization), as well as optical. The PhD will study the possibility to monitor in real time and at the single DNA strand level the amplification of a DNA sequence by PCR. This will be done by taking into account field analysis constraints: portability, drastic reduction of analysis time, sample quantity to be analyzed, and thus reagent quantity. The candidate will have to contribute in all aspects, from design, fabrication to experimental proof of concept.

Study and fabrication of ceramic metal assemblies by Spark Plasma Sintering

Département des Technologies des NanoMatériaux (LITEN)

Laboratoire Recyclage et Valorisation des Matériaux



Functionnaly graded materials (FGMs) have received considerable attention in the past decade for several application such as Aerospace, armour, energy, optoelectronics, biomedicine etc. The benefits of such materials are : reduced thermal stress upon usage, adapted CTE, suppress the need of brazing, chemical adaptation (in organic tissues), local fuction etc. Although they have proven their potentiality, FGMs fabricated by powder metallurgy are yet to be optimized and most of the investigated examples are made out of stepwise layered structure. The subject we propose will focus on the mecanical aspects of ceramic-metals gradients and will have the following challenges : - Optimize and characterize continuous ceramic metal powder gradients - Develop Spark plasma sintering thermal cycles and toolings in order to densifiy the FGMs - Investigate failure criteria (thermal stress coupled with energy threshold) in the obtained materials While largely focusing on the spark plasma sintering technique, comparison with HIP will be attempted Contacts :,,

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