Tojo RASOANARIVO (PCM) thesis defense
2026-04-08 10:00 (Europe/Paris) | Amphi IMN
Wednesday, April 8 at 10:00 a.m.

Multiscale modeling of GaN atomic layer etching in chlorinated plasmas
Summary: Gallium nitride (GaN) is a key material for power electronics devices operating in the radio frequency domain. The miniaturization of architectures makes nanometric control of etching critical. Conventional etching processes using continuous cold plasmas in chlorinated chemistries offer high speeds, but can induce defects in the material, impacting device performance and reliability. Atomic layer etching (ALE), a cyclic process alternating chemical surface modification and activation by low-energy ion bombardment, aims to control etching at the atomic scale while improving surface finish. This thesis focuses on the development of a multiscale model of GaN FTA in Cl₂/Ar plasmas. The simulator couples a dynamic global model describing plasma species fluxes and densities with a Monte-Carlo sheath model providing ion energy and angular distribution functions. The results of these first two models are then exploited by a kinetic Monte-Carlo surface model to describe chlorine-induced modification reactions and ion bombardment selectivity at the GaN surface during the ALE process. This surface model is fed by an ab initio study describing Cl-GaN(0001) interactions. Ultimately, the simulator package makes it possible to link mesoscopic parameters to plasma/surface interactions at the atomic scale, to gain a deeper understanding of the mechanisms involved and optimize process performance, such as post-etch GaN surface finish and control of etched thickness per cycle. Ultimately, this work can be integrated into a digital twin of GaN ALE processes, to guide the optimization and industrialization of atomic-scale etching.
Keywords: semiconductor, cold plasma process, etching, modeling, multiscale

Multiscale modelling of Atomic Layer Etching of GaN in chlorinated plasmas
Abstract: Gallium nitride (GaN) is a key material for power electronics devices operating in radiofrequency ranges. The downscaling of architectures makes nanometric control of etching critical. Conventional etching processes using continuous plasmas in chlorinated chemistries offer high etch rates but can induce defects in the material, degrading device performance and reliability. Atomic layer etching (ALE), a cyclic process alternating between chemical surface modification and activation by low-energy ion bombardment, aims to control etching at the atomic scale while improving the surface state. This thesis focuses on the development of a multiscale model for ALE of GaN in Cl₂/Ar plasmas. The simulator combines a global dynamic model describing plasma species fluxes and densities with a Monte Carlo sheath model providing ions energy and angular distribution functions. The results from these first two models are then used as inputs for a kinetic Monte Carlo surface model to describe chlorine-induced modification reactions and ion bombardment selectivity on the GaN surface during ALE processing. This surface model relies on ab initio study describing Cl-GaN(0001) interactions. The simulator associates mesoscopic parameters to plasma/surface interactions at the atomic scale in order to deepen understanding of the involved mechanisms and optimize process performances, such as the GaN surface state after etching and etched thickness per cycle. Ultimately, this work can be integrated into a digital twin for GaN ALE processes to guide the optimization and industrialization of atomic-scale etching.
Keywords: semiconductor, low temperature plasma process, etching, modelling, multiscale

Journées des Doctorants - JDD 2026
2026-04-09 09:00 - 2026-04-10 17:00

Christophe VALLEE seminar
2026-04-09 13:30 - 14:45
Thursday, April 9 at 1:30 pm – Amphi IMN

Christophe VALLEE
University at Albany, CNSE, USA

More than 50 years of plasma etching innovation: from RIE to ALE… and what’s next?

Abstract
In 1974, in their paper entitled “RF sputter-etching by fluoro-hloro-hydrocarbon gases”, N. Hosokawa et al. proposed to combine energetic ions with volatile products for plasma etching [1]. They came up with the idea of replacing argon with fluoro-chloro-hydrocarbon gases in rf diode sputtering equipment. They observed very high sputter-etching speeds, 10 to 20 times higher than those normally encountered with Si or Al. This marked the beginning of plasma etching combining ions and radicals, Reactive Ion Etching (RIE).
With this presentation I will discuss the following 50 years of work that followed the pioneer work of Hosokawa et al, in understanding, designing and optimizing plasma etching processes and reactors. I will show how plasma etching was modified to enable the transistor scaling areas: geometrical scaling (1975-2002), equivalent scaling (2003-2024), and 3D power scaling with transition to complete vertical structures (2025-…). The first scaling area saw the emergence of new reactors enabling the decoupling of flow and energy. The second was accomplished thanks to the transition from continuous flow processes to pulsed processes (pulsed plasma etching, Atomic Layer Etching). As we enter the third scaling area, selectivity will become predominant and temperature control will be a key parameter for the plasma etching processes, as illustrated by the recent development of cryogenic etching. Finally, future challenges for etching processes to meet sustainability and environmental concerns will also be addressed.
Having completed my thesis at IMN, I will take a little time at the beginning of my presentation to explain my career path since my thesis and what my current research activity is at CNSE.

Jeudis de la Chimie du Solide seminar - Julien Varignon (CRISMAT Caen)
2026-04-09 13:30 - 15:00 | Videoconferencing
Thursday, April 9 at 1:30 p.m. by videoconference

Julien Varignon
CRISMAT Laboratory, CNRS UMR 6508, ENSICAEN, Normandie University

Charge and orbital orders drive superconductivity in complex oxides

Abstract: Superconductivity, characterized by zero electrical resistance and expulsion of the magnetic field, remains limited by relatively modest critical temperatures Tc, reaching 135 K at best in cuprates at ambient pressure. In complex oxides, understanding the mechanisms by which Tc can be increased is a major challenge, particularly from the point of view of electronic and structural chemistry.
Oxides derived from Ruddlesden-Popper An+1BnO3n+1-type phases play a central role in this issue. In these systems, superconductivity frequently appears close to a transition between an insulating and a metallic phase. This transition is often associated with the disappearance of electronic orders such as charge orders (cationic dismutation) or orbitals, frequently linked to Jahn-Teller-type structural instabilities.
In this seminar, I show, using first-principles simulations based on density functional theory (DFT), that superconductivity emerges systematically in the vicinity of these ordered phases. Even if these orders disappear in the metallic phase, their fluctuations persist and generate sufficient electron-phonon coupling to initiate superconductivity.
These results thus suggest that ordered insulating and superconducting phases share a common origin1,2. Finally, I highlight that structural distortions of O₆ octahedral lattices, such as rotations, allow modulation of this electron-phonon coupling. This paves the way for an oxide structural engineering strategy to optimize Tc.
______________________
1. J. Varignon, M. Bibes, and A. Zunger, Nat. Comm. 10, 1658 (2019)
2. A. A. Carrasco-Alvarez, S. Petit, W. Prellier, M. Bibes, and J. Varignon, Nat. Comm. 16, 1458 (2025)

https://univ-nantes-fr.zoom.us/j/88146334543?pwd=[REDACTED]
Meeting ID: 881 4633 4543
Secret code: 468276
