Jury :

Olivier THOMAS, Professor, IM2NP CNRS - Aix-Marseille Université, France - Reviewer 

llaria ZARDO, Professor, University of Basel, Switzerland - Reviewer 

Valentine GIORDANO, Research Fellow, Institut Lumière Matière, CNRS, Lyon, France - Examiner 

Damien DELLEREUYLE, Professor, INSA Lyon, France - Examiner 

Gabriele NAVARRO, Senior R&D scientist, CEA-LETI, Grenoble, France - Examiner 

Emmanuel DUBOIS, Research Director, IEMN, CNRS, Lille, France - Examiner 

Jean-François ROBILLARD, Associate Professor, Junia, Lille, France - Thesis director 

Simon JEANNOT, Doctoral Engineer, STMicroelectronics, Crolles, France - Thesis co-director 

Philippe BOIVIN, Doctoral Engineer, STMicroelectronics, Rousset, France - Invited 

Summary:

Phase change materials (PM) have been the basis of memory storage since they were first applied to optical data storage in 1987. In 2015, 3D XPointTM memory based on phase change materials is 1,000 times faster and more durable than NAND, and 10 times denser than DRAM. The flexibility of PM memories based on chalcogenides offers a combination of faster speed, higher endurance and thermal stability depending on the stoichiometry used. These characteristics are crucial for non-volatile memories to fill performance gaps and partially replace the current memory hierarchy. Extensive engineering of primary memories based on GeSbTe (GST) alloys has resulted in stoichiometries with high thermal stability, suitable for phase change memories (PCMs) embedded in automotive MCUs. Alloys such as Ge-rich doped GeSbTe exhibit high temperature data retention (~300°C) due to a high crystallisation temperature. In PCMs, reversible switching between phases is initiated thermally. This operation uses < 1 % de l'énergie d'entrée pour le changement de phase, alors que la majeure partie de l'énergie est perdue par d'autres voies de dissipation de la chaleur dans la cellule PCM. Il est donc essentiel de comprendre les propriétés thermiques de ces matériaux en fonction de l'évolution de la structure sur l'ensemble de la température de fonctionnement de la cellule PCM pour améliorer la compréhension thermique et l'optimisation de la mémoire. L'alliage phare GST-225 a été largement caractérisé, mais l'état actuel des connaissances ne permet pas de caractériser thermiquement les stœchiométries nouvellement conçues. Des méthodes de caractérisation thermique telles que 3w, la thermo-réflectance et la radiométrie photo-thermique ont été mises en œuvre pour la caractérisation thermique du GST. Ces méthodes nécessitent une microfabrication supplémentaire de l'élément chauffant ou du transducteur, certaines méthodes exigeant des coûts d'installation élevés accompagnés d'un post-traitement complexe des données. Pour les mesures à haute température (>300°C), they can suffer from inter-diffusion of transducer elements into the TPS. For GSTs, it is also crucial to monitor structural evolution simultaneously with thermal characterisation. Raman thermometry is an optical characterisation technique that does not require microfabrication and can offer the advantage of simultaneous structural study. In this work, we studied particles such as GeTe, Ge-rich GeSbTe and N-doped GeSbTe (GGSTN) using Raman thermometry. This was made possible by studying the stability and sensitivity of the vibrational modes present in GeSbTe-based alloys to temperature and laser input power, using finite element simulations. For the first time, we have succeeded in extracting the thermal properties of these materials as a function of temperature and phase at higher temperatures (~400°C) using Raman thermometry. This provides a better understanding of the behaviour of these materials at higher temperatures and the effect of doping. It broadens the scope of thermal characterisation techniques, as Raman thermometry can provide thermal analysis with simultaneous structural characterisation, which is crucial for particles. It would be of progressive interest that PMs have also been implemented for neuromorphic devices and RF switches.

Abstract:

Phase change materials (PMs) have been the basis of memory storage, since the beginning of its application for optical data storage in 1987. Fast forward to 2015, 3D XPoint^TM memory product based on PMs performed 1000 times faster along with greater endurance than NAND; and 10 times denser than DRAM. The flexibility of PMs based on chalcogenides provide a combination of faster speed, higher endurance and thermal stability depending on the stoichiometry implemented. These characteristics are crucial for non-volatile memories to address the performance gaps and partially replace the existing memory hierarchy. Extensive engineering of PMs based on GeSbTe (GST) alloys led to realization of stoichiometry's with high temperature stability, which are suitable for embedded phase change memories (PCMs) application in automotive MCU. Alloys like doped Ge-rich GeSbTe presented high temperature data retention (~300°C) due to high crystallization temperature. In PCM, the reversible switching between phases is thermally initiated. This operation uses < 1% of input energy for phase change, whereas most of the energy is lost via other heat dissipation pathways in the PCM cell. So, understanding of the thermal properties of these materials as a function of structural evolution over the entire operation temperature of PCM cell is crucial for better thermal understanding and optimization of the memory. The flag-ship GST-225 alloy has been extensively characterized but the current state of art falls short on thermal characterization of the newly engineered stoichiometry's.  

Thermal characterization methods like 3w, thermo-reflectance and photo-thermal radiometry have been implemented for thermal characterization of GST. These methods require additional microfabrication of heater or transducer with some methods requiring high setup costs accompanied with complex post-processing of data. For high temperature measurement (>300°C), it can suffer from inter-diffusion of transducer elements into GST. For GST, it is also crucial to monitor the structural evolution simultaneously, during thermal characterization. Raman thermometry is an optical characterization technique which requires no microfabrication and can provide an advantage of simultaneous structural investigation. In this work, we investigated PMs like GeTe, Ge-rich GeSbTe and N-doped Ge-rich GeSbTe (GGSTN) using Raman thermometry. This was possible by studying the stability and sensitivity of vibrational modes present in GeSbTe based alloys to temperature and input laser power aided with finite element simulations. We demonstrate successful extraction of temperature and phase dependent thermal properties of these materials to higher temperature (~400°C) by Raman thermometry, for the first time. It provides a better understanding of the behavior of these materials at higher temperature and the effect of doping. This extends of the scope of thermal characterization techniques as Raman thermometry can provide thermal analysis accompanied by simultaneous structural characterization which is crucial for PMs. It would be of progressive interest as PMs has also been implemented for neuromorphic devices and RF switches.