Atomistic simulations of H₂ and He plasmas modification of thin-films materials for advanced etch processes

This PhD thesis focuses on technological challenges related to the development of advanced transistors (FinFET, FDSOI), where the etching of thin films reveals several issues. In particular, the etching of silicon nitride spacers should be achieved with a nanoscale precision without damaging the underlayers, a step which cannot be addressed by conventional CW plasmas. To overpass this limitation, an innovative approach was recently developed (so-called Smart Etch), which is based on light ion implantation and composed of two steps. First, the material to be etched is modified by exposure to a hydrogen (H₂) or helium (He) ICP or CCP plasma; in a second step, the modified layer is selectively removed using wet etching or gaseous reactants only. To support the fundamental understanding of the first step and assist the development of this new technology, molecular dynamics (MD) simulations were performed to study the interaction between silicon/silicon nitride films and hydrogen/helium plasmas. MD was used to investigate how the substrates modification is affected by the ion energy, the ion dose, the ion composition or the radical-to-ion flux ratio (in the case of a H₂ plasma). In agreement with experiments, simulations of He⁺ or H^x+ (x=1-3) ion bombardment of Si/SiN show that a self-limited ion implantation takes place with a surface evolution composed of two stages: a rapid volume modification (with no etching) followed by a slow saturation and the formation of a stable He- or H- implanted layer at steady state. The mechanisms of ion-induced damage (Si-Si or Si-N bond breaking, He or H₂ trapping/desorption, SiH_x (x=1-3) complex creation) are investigated and allow to bring new insights to both the Smart Cut and Smart Etch technologies. Si/SiN exposure to various H₂ plasma conditions (with both H^x+ ions and H radicals) was then studied. In this case, a self-limited transformation is observed but the H-modified layers are simultaneously etched during the ion implantation, at a rate ∼10 times smaller for SiN compared to Si. Simulations show that to modify Si/SiN thin films with a nanoscale precision by H₂ or He plasmas, both the ion energy and the ion flux have to be controlled very cautiously. In particular, low ion doses, where the substrate evolution is in rapid modification stage, must be avoided since the substrate evolution cannot be precisely controlled. In H₂ plasmas, high ion energies induce thicker modified layers but smaller and less homogeneous hydrogenation rates. The ion composition and the radical-to-ion flux ratio G must be considered as well, since the etch rate increases with G, compromising even the possibility to achieve a Smart Etch of silicon. The MD simulations performed in this thesis enable to clarify various unexplained phenomena seen in the Smart-Etch experimentally, and reveal some possible issues in this new process. In the end, a range for plasma parameters is proposed to optimize this first step of the Smart Etch process and to control the modification of SiN with a sub-nanoscale precision. (author)