Modification mechanisms of silicon thin films in low-temperature hydrogen plasmas

To achieve quasi-atomic precision etching of thin film materials in advanced transistors, a method based on light ion implantation and consisting of two sequential steps-(1) surface modification in hydrogen ICP or CCP plasmas, and (2) selective removal of modified layers in wet solutions or remote plasmas-was recently proposed. In this paper, to better understand this process (called Smart Etch thereafter), molecular dynamics simulations are performed to study the modification of crystalline Si (100) substrates under low-energy H-x(+) (x = 1-3) ion implantation and mixed H-x(+) ion/H radical bombardment. In agreement with the experiments, simulations of H-x(+)(x = 1-3) ion bombardment of Si show a self-limited implantation 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 [a: Si-H] layer at a steady state. The mechanisms of ion-induced damage (Si-Si bond breaking, formation of H-2 molecules, creation of SiHx groups) are investigated and allow us to bring new insights to both the wellknown Smart Cut(TM) and more recent Smart Etch technologies. Si exposure to both H-x(+) ions and H radicals (H-2 plasma) also shows a self- limited transformation but the modified layers are simultaneously etched during the implantation. Both the modified layer thickness and the ion dose required to reach the steady state increase with the ion energy. The ion composition and the radical-to-ion flux ratio (F) must be considered as well. In particular, the etch rate strongly increases with F, compromising even the possibility to achieve a Smart Etch of silicon.