Ge-on-Insulator (GeOI) substrates for photonics
The induction of tensile strain in intrinsic and doped Germanium (Ge) is one approach currently explored to transform the Ge indirect bandgap into a direct one. To take full advantage of Ge, we study the Ge CMOS photonics platform with Ge-on-Insulator (GeOI) structure, which enables strong 2D optical confinement in the Ge photonic-wire devices. One recent study in our lab showed the interest of a method of incorporation of mechanical stress into Ge, one of the essential ingredients of the laser. In particular, the method could be applied to the massive Ge, making compatible gap direct and crystalline quality.
Post-doc objectives : Development of GeOI substrates from massive Ge donors with tensile strain inside the Ge film. These developments will be realized from the existing Smart Cut / thinning processes, combined with technological steps to overcome their current limits (SAB bonding). The substrates obtained will be characterized to determine their state of deformation as well as their damage (Raman / XRD) and final GeOI substrates will be provided to the application laboratories for the production of photonic components.
Eco-innovation of insulating materials by AI, for the design of a future cable that is long-lasting, resilient, bio-sourced and recyclable.
This topic is part of a larger upcoming project for the AI-powered creation of a new electrical cable for future nuclear power plants. The goal is to design cables with a much longer lifetime than existing cables in an eco-innovative approach.
The focus is on the cable insulation because it is the most critical component for the application and the most sensitive to aging. The current solution is based on adding additives (anti-rad and antioxidants) to the insulation to limit the effects of irradiation and delay aging as much as possible. However, there is another solution that has never been tested before: self-repairing materials.
The project to which this topic is attached aims to design and manufacture several test model of insulation specimens. With several test characterization protocols, in order to verify the gain in terms of reliability and resilience. The results obtained will begin to fill a future database for the AI platform Expressif, developed at CEA List, which will be used to design the future cable.
Aging study of silicon nanowires used as piezoresistive detection gauges for achieving inertial MEMS sensors.
Today’s sensors are present in all areas: housing, automotive... Thanks to recent developments in microelectronics, new generations of sensors combine high performance, small size and low cost. In this context, CEA-LETI has proposed an innovative concept called M&NEMS for the realization of inertial sensors such as accelerometers, magnetometers and gyroscopes. The M&Nems concept combines MEMS and NEMS to take advantage of the great inertia generated by a MEMS mass and the high detection sensitivity of piezoresistive silicon nanowires. Demonstrators have already been carried out and have shown the good potential of the M&Nems concept. One of the main challenges which remain to overcome is the reliability of sensors based on this concept and specifically the reliability of the piezoresistive nanowires. The research work will be mainly focused on the study of the failure modes of these piezoresistive nanowires gauges i.e. the identification of physical phenomena and the development of failure models. In order to do this study, a first preliminary work will be focused on the physical mechanisms which manage the electrical conduction in the nanowires: piezoresistivity, charge trapping, relaxation field effect ... The work will then continue by the study of the failure modes of nanowires, the goal will be to understand and model the physical aging of these nanowires: it will be possible to rely on the knowledge of the physics of nanowires conduction but also play with the physical parameters of these nanowires such as silicon doping, the process fabrication, the packaging technique, the thermomechanical stresses, the scale effect due to surface / volume ratio, or the surface condition. Finally, models of aging will allow proposing and validating technological choices to ensure the nanowires lifetime depending on operating conditions of the sensors.
Dual layer transfer of piezoelectric films for advanced RF devices
The aim of these workds is to study and develop an innovating concept of piezoelectric thin films multilayer transfer for RF applications.The applicant will be responsible for the development of the entire fabrication sequence of these multilayer structures and of the 3D RF components. To this end, he or she must master the physical mechanisms involved in the film transfer technology and design the complete architecture through simulation of the expected RF filters properties. Once the structure is defined and the technology backbone is mastered, the candidate will collaborate with Leti technology experts to identify the necessary process developments. He or she will then ensure their implementation in the fabrication technology platform and support the achievement of the most critical steps.
The development of this fabrication sequence will allow the generation of substrates whose features meet device specifications. The functionality of the substrates will then be demonstrated through the fabrication and characterization of RF devices that are relevant to the target applications.
Evolution of the surface layers resulting from the physico-chemical interactions between low pH concrete and clays: experiments and modeling
The design of an industrial facility for storage of radioactive wastes in geological environment is an important issue taken into account in the French nuclear energy sector. In this context the cementitious materials are an important (packages, structures).
The main objective of the proposed study is to characterize alterations of the materials in the concrete-clay interfaces, caused by chemical exchanges. At the current stage, a comprehensive approach was initiated taking into account simultaneously the chemistry of the storage site and concrete considered for this application, based on commercial cements or innovative binders (low pH) formulated specifically. On these low pH materials in particular, questions remain as to their mineralogical and microstructural evolution. An experimental program (dedicated testing, microscopic characterization), supplemented by digital simulations, will increase the essential knowledge for use of these materials.
This project will involve both of the specialists of cementitious materials of the CEA, and researchers at the laboratory Hydrasa of the University of Poitiers.
Development of a solid electrolyte to optimise lithium microbatteries
All-solid-state microbatteries are developing for nomad applications (RFID, autonomous sensors…). Their main advantages are due to the electrolyte, a thin film inorganic material allowing a very good cyclability (>10000 cycles), long life, good thermal resistivity and excellent safety. These thin film microbatteries have to response to a challenge of integration: decreasing of size but increasing of performances.
The goal of this post-doc is to develop the electrolyte performances. The first part will focus on the optimisation of the actual electrolyte and the second part on the development of a new electrolyte material. The electrolyte deposition will be done by Physical Vapor Deposition.
Le position is in the CEA/LITEN/DTNM/LCMS for a common laboratory CEA/ST Microelectronics.
Selective removal of metal alloy for advanced silicidation applied to sub-20nm CMOS transistors
CMOS transistor performances depend on electrical contact resistivity reduction. Thus, self aligned silicidation (salicide) is one of the key processes which have to be improved to meet the ITRS requirements for the future technological nodes. Nowadays, solid state reaction between thin metal layer (Ni1-yPty < 10nm) and a silicon substrate allows to decrease access resistances of transistor source & drain. The metal is currently deposited by physical vapor deposition method all over the wafer surface. Under heat treatment, metal reacts preferably with semiconductor areas rather than dielectrics ones. Then, unreacted metal layer is selectively etched with an appropriate acidic solution; only metal silicide remains.As new specifications (use of ultra-thin Ni-alloy,very low temperature process leading to partial salicidation, use of various additive metals ...)are required for advanced nodes (C20nm & C14nm), the capability to chemically remove the excess of metal on dielectric areas has to be investigated. In the clean room environment of CEA-LETI (Grenoble, France), the candidate will work on innovative wet chemistries to remove selectively the different metallic layers (Ni, Pd, NiCo, NiPd…). In a first time preliminary test will be conducted on sample in manual tank in order to check removal kinetic and global selectivity on structures devices… Based on several characterization techniques (TXRF, XRR, AFM, SEM, TEM, XRD…), residual additive interaction with dielectric and chemical mixture behavior towards the metal rich phase on silicided area (roughness, resistivity) will be studied. Different semi-conductor (Si, SiGe…) and dielectrics surfaces (SiO2, SixNy…) will be investigated. Afterwards the most promising selective processes will be selected to be installed on a fully automatic 300mm tool. Finally, best processes will be integrated on critical patterned wafer architectures for morphological and electrical characterizations.
Development of a Metal Supported Cell for Hydrogen production by High Temperature Steam Electrolysis
The development of Metal Supported Cells (MSC) for High Temperature Steam Electrolysis (HTSE) constitutes an interesting innovation able to reduce the degradation of this component under operation. An increase in the cell life time would be a valuable contribution to cost reduction and is able at positioning HTSE as an alternative process to other hydrogen production technologies. However, some progresses in the elaboration of MSCs are still required. Within the current process, functional ceramic layers of the MSC are joined to the metallic substrate at elevated temperature (> 1000 °C). Mismatch of the mechanical properties of the materials as well as the reducing conditions fixed by the metal substrate during sintering lead to MSCs having insufficient electrochemical performances. The post-doctorate aims, on the one hand, at obtaining a better understanding of the mechanisms that occur in the multilayer structure during sintering and, on the other hand, at proposing and testing technological solutions able to improve to reliability of MSC elaboration.
Droplets motion through modulation of surface energy gradient
Droplet motion through electro-wetting is nowadays largely studied and used in several systems and applications. In order to be useful, this technique needs an electrical field to monitor the droplet. For this post-doctoral fellowship, the main objective is to define an alternative method to the using of the electro-wetting technique in order to generate a droplet motion. The elaboration of surfaces with energy gradients conceived by thin film deposition or by laser ablation will be realized inside this study. The main difficulty is related to the patterns realization in order to obtain the appropriate hydrophilic/hydrophobic resolution. Apart from these “classical” techniques, an innovative method will be studied here by using switchable molecules. These molecules could modify the contact angle between a surface and a droplet, when acting on the potential of hydrogen (pH) or the wall temperature. For all the defined surfaces, the post-doctoral fellow will also analyze the coupling effect between the surface energy gradient and a thermal energy gradient on the droplet motion dynamics.
Porous layer integration for advanced temporary substrates
Double transfer of thin single crystalline processed layer can be very interesting for all technologies that require front and back side engineering of the silicon active film. With the increase of the electronical system complexity, this alternative technology can offer new opportunities to miniaturize the semiconductor devices. To fulfill such requirements, a recent alternative technology was developed at the CEA-LETI, based on the use of porous silicon substrates [1]. This new technology will be of a great interest for 3D integration, back-side imager but also MEMS or photovoltaics applications. This technology should now be validated at a larger scale, and we need to focus on all involved mechanisms such as the porous silicon layer rupture.
In a first place, the applicant should comprehend the specification of porous materials in thin film configuration, including elaboration steps and distinctive properties of transferred porous layers. Subsequently he/she may need to interact with Leti’s technological experts to determine process improvements to be implemented to reach pre-established specifications of desired prototypes. In order to evaluate and recommend appropriate materials and equipments, he/she may need to extensively focus on the behaviour of porous material under specific stress conditions such as chemical, thermal or mechanical solicitations. The purpose is to ensure compliance to Smart Cut or Smart stacking technologies that involve amongst others processes molecular bonding technology.
Later, the effort should be focused on the development of a specific technology to induce the mechanical separation inside the buried porous silicon layer. One line of approach would be to trigger the mechanical separation by ultrasound solicitation. Understanding the mechanisms of the splitting and characterising the resulting structures are part of the expected work to be completed in this project.
[1] A-S.Stragier et al., JECS,158 (5) H595-H599 (2011)