AlGaN/GaN HEMTs transfert for enhanced electrical and thermal performances
Due to their large critical electric field and high electron mobility, gallium nitride (GaN) based devices emerge as credible candidates for power electronic applications. In order to face the large market needs and benefit from available silicon manufacturing facilities, the current trend is to fabricate those devices, such as aluminum gallium nitride (AlGaN)/GaN high electron mobility transistors (HEMTs), directly on (111) silicon substrates. However, this pursuit of economic sustainability negatively affects device performances mainly because of self-heating effect inherent to silicon substrate use. New substrates with better thermal properties than silicon are desirable to improve thermal dissipation and enlarge the operating range at high performance.
A Ph.D. student in the lab. has developed a method to replace the original silicon material with copper, starting from AlGaN/GaN HEMTs fabricated on silicon substrates. He has demonstrated the interest of the postponement of a GaN power HEMT on a copper metal base with respect to self heating without degrading the voltage resistance of the component. But there are still many points to study to improve the power components.
Post-doc objectives : We propose to understand what is the best integration to eliminate self-heating and increase the voltage resistance of the initial AlGaN/GaN HEMT. The impact of the component transfer on the quality of the 2D gas will be analyzed.
The same approach can be made if necessary on RF components.
Different stacks will be made by the post-doc and he will be in charge of the electrical and thermal characterizations. Understanding the role of each part of the structure will be critical in choosing the final stack.
This process will also be brought in larger dimensions.
This post-doc will work if necessary in collaboration with different thesis students on power components.
Physisorption of chemical species on sensitive surfaces during transfer in controlled mini-environment in microelectronics industry
A characterization platform based on the connection concept between process and characterization tools through the use of a transfer box under vacuum was implemented allowing a quasi in-situ characterization of substrates (wafers) of the microelectronics. Currently, this transfer concept based simply on static vacuum inside a carrier box is satisfactory regarding the residual O or C on the surface of especially sensitive materials (Ge, Ta, Sb, Ti…) and the MOCVD layers growth on GST or III/V surfaces. Its optimization for more stringent applications (molecular bonding, epitaxy…) in terms of contamination surface prevention requires studies the understanding of the physico-chemical evolution of the surfaces.
The proposed work will be focused on physico chemical studies of the evolution and molecular contamination of surfaces during transfers and will take place in clean room. XPS, TD-GCMS and MS coupled to the carrier itself (to be implemented) will be used to address the sources (wall, seals, gaseous environment…) of the adsorbed chemical species implied and to determine the physisorption mechanisms on the substrates. The studied surfaces will be sensitive to the contaminants in such a way than the box environment impact will be extracted and studied parameters will be the nature of polymer seal used, the carrier box thermal conditioning, the vacuum level, the use of low pressure gaseous environment in the carrier (gas nature, pressure level…).
Development of flexible solar panel for space application
Traditional solar panels used to power satellites can be bulky with heavy panels folded together using mechanical hinges. Smaller and lighter than traditional solar panels, flexible solar array consists of a flexible material containing photovoltaic cells to convert light into electricity. Being flexible, the solar array could roll or snap using carbon fiber composite booms to deploy solar panels without the aid of motors, making it lighter and less expensive than current solar array designs.
On the other hand, satellite trends are shifting away from one-time stints and moving towards more regular use in a constellation setting. In the last years, the desire increased to mass-produce low-weight satellites. Photovoltaic arrays companies are challenged on their capacity to face these new needs in terms of production capacity and versatility. And this is exactly where space photovoltaics can learn from terrestrial photovoltaics where this mass production and low-cost shift occurred years ago.
To tackle these new challenges, the Liten institute started to work on these topics two years ago. In the frame of this post-doc, we propose the candidate to work on the development of an innovative flexible solar panel architecture, using high throughput assembly processes. We are looking for a candidate with a strong experience in polymers and polymers processing, along with an experience in mechanics. A previous experience in photovoltaic will be greatly appreciated.
Development of lead free piezoelectric actuator
At CEA-Tech, the LETI Institute creates innovation and transfers it to industry. The micro-actuator component laboratory (LCMA) is working on the integration of piezoelectric materials into microsystems that allow electromechanical transduction. Lead zirconate titanate (PZT) is today the most powerful piezoelectric material for micro-actuator applications. However, the introduction in the near future of a new standard regarding the lead amount allowed in chips (European RoHS directive) leads us to evaluate alternative lead-free materials to PZT for piezoelectric actuator applications. The development of lead-free materials has thus become a major focus of piezoelectric research. This research led to revisit and modify some classical piezoelectric such as KNbO3 and BaTiO3. In particular, the KNaxNb1-xO3 (KNN) family has been identified as promising. The objective of the postdoc is therefore to evaluate some lead-free piezoelectric materials and to compare their properties with that of the reference material, PZT. Suitable test vehicles will be fabricated in LETI’s clean rooms for electrical and piezoelectric characterizations by mean of dedicated tools already available at lab. For this work the candidate will lean on a solid experience developed at LETI for more than 20 years on piezoelectric thin films.
Bio-compatible, bio-resorbable microbatteries for medical applications
In the framework of its activities dedicated to embedded micro-batteries, LETI initiates prospective research in the field of micro-batteries for medical applications, and in particular as energy power sources for implantable micro-devices. In this context, a collaborative project, including LETI labs and an academic Partner (ICMCB, Bordeaux), is aiming at designing, manufacturing and studying prototypes of bio-resorbable primary microbatteries.
The main tasks will include (i) a contribution to the design of the thin film electrochemical cell by the selection of adequate biocompatible materials (able to generate the targeted electrical power, corrodible and able to solubilize in the body), (ii) the manufacture of the cell constituents (electrodes, electrolyte, substrate) as thin films (sputtering, electrochemical plating, doctor blade coating) and their characterization,(iii) the achievement of full prototype cells and the study of their in vitro behaviour.
The work will be carried out at ICMCB (Bordeaux) in a joint CEA/ICMCB team, in collaboration with LETI labs in Grenoble.
Cluster dynamic simulations of materials under irradiation
Alloys used in nuclear applications are subjected to neutron irradiation, which introduces large amounts of vacancy and interstitial defects. Over time, these defects migrate, recombine and agglomerate with minor alloying elements to form small clusters. This affects the mechanical properties of ferritic steels and weakens them. In this context, the microstuctural evolution is to be simulated using the rate equation cluster dynamic method. However, this approach becomes ineffecient when several minor alloying elements need being taken into account. The difficulty comes from the huge number of cluster variables to describe. The project aims at optimizing the code efficiency on a distributed parallel architecture by implementing parallelized vector and matrix functions from SUNDIALS library. This library is used to integrate the ordinary differential equation describing the reactions between clusters. Another aspect of the work is more theoretical and involves reformulating the non-linear root-finding problem by taking advantage of the reversibility of most chemical reactions. This property should facilitates the implementation of direct and gradients iterative sparse solvers for symmetric definite positive matrices, such as the multi-frontal Cholesky factorization and the conjugate gradient methods, respectively. One avenue of research will consists of combining direct and iterative solvers, using the former as a preconditioner of the latter.
High entropy alloys determination (predictive thermodynamics and Machine learning) and their fast elaboration by Spark Plasma Sintering
The proposed work aims to create an integrated system combining a computational thermodynamic algorithm (CALPHAD-type (calculation of phase diagrams)) with a multi-objective algorithm (genetic, Gaussian or other) together with data mining techniques in order to select and optimize compositions of High entropy alloys in a 6-element system: Fe-Ni-Co-Cr-Al-Mo.
Associated with computational methods, fast fabrication and characterization methods of samples (hardness, density, grain size) will support the selection process. Optimization and validation of the alloy’s composition will be oriented towards two industrial use cases: structural alloys (replacement of Ni-based alloys) and corrosion protection against melted salts (nuclear application)
Nonlinear ultrasonic testing for the assessment of adhesive bonding properties
The CEA-LIST carries out Non Destructive Testing (NDT) projects in partnership with various industrial sectors. A strong collaboration with Airbus Group Innovations (AGI) had led to a common entity through the NDT laboratory for Aeronautics Applications (LC2A).
With the increasing portion of composite materials in the aerospace industry, assessment of the adhesive bonding properties of such composite structures is a key issue. Various aspects could decrease the quality of bonding, such as the surface contamination, non-optimal thermal cycle or external mechanical stresses. However, conventional NDT techniques are often not sensible to such damages in the adhesive bonds.
Non-linear ultrasonic methods such as wave mixing, harmonic generation or non- linear imaging appear as promising techniques to detect kissing bonds and pre-damaging that could occur in adhesive bonds. The objective of this postdoc position is to develop NDT innovative solutions for the assessment of the adhesion quality by means of experimental techniques based on such non-linear methods.
This post-doc position will be carried out in the framework of an international research program on the adhesion bonding. The candidate will work in the NDE laboratory for Aeronautics Applications located in Toulouse. Strong skills in experimental physics, instrumentation, and non-linear ultrasonics would be appreciated.
New packaging for power electronics : application to SiC components
In continuity of ongoing work (PhD thesis) on the 3D assembly of vertical Silicon power components, the purpose of the post-doc proposal is to develop a similar assembly on vertical wide-gap SiC power components. The required work will be to define the components (high frequency / high voltage) with the supplier and to adapt them to the best vertical integration (Cu finishing, topology,...), to adjust the metal leadframe design for the 3D assembly, and to develop the transfer layer technology adapted to this new material. The candidate will also take care of the electrical characterizations of the final stack to validate the interest of this 3D packaging on wide-gap power devices.
Active medical implants encapsulated using hermetic glass package
Microelectronics extends its range of applications via the micro-systems with high level of integration including sensors, energy scavenger, and communication modules. Medical implants such as pacemakers and defibrillators medical implants, drug dispensers, intra-cranial probes are many possible applications for these modules. The use of glass offers a wide field of investigation. Moreover, recent innovations for the glass material (interconnections, thinning, and functionalization) reinforce its relevance to the medical field: biocompatibility, stability, transparency, and potentially lower cost.
The objective of this work is to design and validate technological steps to integrate high level of microsystems encapsulated in glass material.