Thèses

Mardi 27 Juin 2023 à 14h00.

Overcoming the thermal stability limit with innovative chalcogenide alloys for embedded phase-change memory applications

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Directeur de thèse / thesis director : Valentina M. Giordano (ILM), Pierre Noe (CEA-LETI), Daniel Benoit (ST Microelectronics)

Membres du jury / jury members :

Olivier Bourgeois, I. Neel, Grenoble (referee)
Vittoria Pischedda, ILM, Villeurbanne
Magali Putero, Université Aix-Marseille et CNRS (referee)
Massimo Longo, CNR/IMM, Rome Italie

Résumé / Abstract :

The present thesis aims at exploring the properties and evaluate the potential of innovative chalcogenide thin films for future embedded phase-change memory (PCM) applications requiring high-temperature data retention. The investigated films, deposited by sputtering technique, are the GeSe1-xTex alloys, located on the GeSe-GeTe pseudo-binary line.

The manuscript is structured in 5 chapters. Chapter 1 presents the memory technology market and its current limitations, providing the context for the development of emerging memories. The working principle of a PCM is described in details, focusing on the link between device performance and phase-change material properties. Particular attention is given to the embedded PCM application requirements. The most known phase-change materials, i.e. GeTe and Ge2Sb2Te5 (GST225), their crystalline structure and properties in relation to device performances are presented. The mechanism of metavalent bonding (MVB) and the properties of MVB materials are then introduced. The use of Ge-rich GST alloys as a strategy to improve thermal stability in embedded PCM applications is discussed, together with the beneficial role of N doping in these alloys. A state of the art of the literature on chalcogenide GeSe1-xTex alloys is then provided, with particular emphasis on the structure of their crystalline phase.

In chapter 2, the experimental techniques used to characterize GeSe1-xTex thin films are presented. They are grouped according to the physical mechanism on which they are based: electrical resistivity measurements as a function of temperature, optical characterization techniques, scanning electron microscopy, X-ray analysis and composition measurements by ion beam techniques.

Chapter 3 is devoted to the sample synthesis. It first presents the principle of the sputtering deposition method. This deposition technique offers the possibility to easily explore a wide range of compositions and produce films with nanometer thicknesses on the order of those integrated in memory devices. Several GeSe1-xTex thin films, with x varying from 0 to 1, were deposited by magnetron co-sputtering of pure GeTe and GeSe targets in industrial sputtering tools. Nitrogen was also incorporated in GeSe1-xTex films. The sputtering equipment, the deposition parameters and the elaboration process are described in this chapter. A list of the deposited samples and their composition is also provided.

Chapter 4 is devoted to the experimental characterization of GeSe1-xTex thin films. The evolution of their physical properties and crystalline structure with Te concentration is investigated. The study of their thermal stability and resistive behavior is reported, as well as the optical and electrical contrast between the crystalline and amorphous states. The crystalline phase of GeSe1-xTex thin films is analyzed by room temperature and temperature-resolved X-ray diffraction (XRD), as well as room-temperature RAMAN spectroscopy measurements. The obtained results are discussed and compared with the state of the art and literature results published during the thesis.

The study of nitrogen incorporation in GeSe1-xTex thin films is provided in chapter 5, focusing on the influence of N on the phase-change properties and structure of the films. The first integration of N-doped and undoped GeSe1-xTex films in PCM memory test vehicles of CEA-LETI is presented in the second part of the chapter. The choice of compositions to be integrated is discussed and the issues encountered during the integration process are highlighted. Some preliminary electrical characterizations in device are also reported.

Finally, the Conclusions and the Perspectives of this work are presented.

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