Understanding plasma-surface interactions in cryogenic etching for advanced patterning applications – PSICRYO

Low-pressure plasmas are widely used to transfer high resolution patterns into functional materials because they offer many advantages (anisotropy, selectivity) compared to wet processes. First introduced to achieve silicon deep etching, cryogenic etching consists in etching materials at very low temperatures, the substrate being cooled down with liquid nitrogen to typically -100°/-150°C. Cryogenic processes have the advantage to be polymer-free and clean (passivation layers only form on cooled surfaces), which avoids process drift and makes them suitable for new applications (3D IC/Si integration, More than Moore MEMS demonstrators) where smooth sidewalls or reduced plasma-induced damage are required. Therefore, scientific and technological interest in cryoetching has recently increased. Applications to (atomic layer) etching of conventional materials like Si, Ge, GaN, InP and emerging 2D materials (graphene, MoS2), but also their oxide and nitride ceramic compounds, are envisaged. Research activities are also launched on porous SiOCH low-K materials, low-damage or high-selective plasma processes for sub-20 nm features etching. However, many questions remain about the fundamental mechanisms involved in cryoetching. Mechanisms behind the formation, the stoichiometry or the thermal desorption of passivation layers are not fully understood. And little is known about the differences between elementary surface processes at room and cryogenic temperature. In this context, the project aims at investigating the fundamental mechanisms of plasma-surface interactions (PSI) at cryogenic temperature and to develop innovative cryo-ALE processes of various materials for advanced patterning applications. Three main objectives are targeted. The first is to provide an overview of the reactions (physisorption, chemisorption) involved at the atomic scale. The second is to retrieve quantitative information (nature, energy, flux, residence times) on the plasma species (ions, radicals, stable molecules) interacting with the surface, and to compare their surface reaction probabilities (sticking, sputtering, by-product formation, etc.) at room and cryogenic temperature. The third is to correlate both the plasma conditions and surface temperature with the modification of exposed materials. In terms of strategy, the project will first focus on cryoetching experiments of Si with SF6/O2 plasmas (with or without SiF4/O2 steps), a case study but an essential step to better understand the fundamental mechanisms of PSI involved at low temperature. Then, cryo-ALE of SiO2 (with C4F8 gas adsorption and Ar plasma steps) will be performed to explore this new concept and investigate the ion-induced reactions of C4F8 physisorbed molecules with SiO2 at low temperature. Finally, cryo-ALE will be extended and tested to etch new materials (TiN, TiO2) of interest for high-K gate oxide and contact nanostructuration. To achieve our goals, we will develop atomistic simulations (molecular dynamics), coupled with in situ advanced diagnostics of both the plasma gas-phase (probes, UV-VUV absorption spectroscopy, mass spectrometry, ellipsometry) and the exposed materials (in situ XPS analysis). The project will involve partners of 3 research labs (LTM, GREMI, IMN) with complementary backgrounds in modelling and diagnostics. Thanks to this expertise, we aim to bring new and fundamental insights to an existing technology only partially explored (namely Si cryoetching in F chemistries) and tackle emerging cryo-ALE processes of alternative materials for innovative applications in 3D microelectronics. The project should thus have an important impact on both etch technology and ICT in general.