Hydrogen dissociation and diffusion near the Si/a-SiO2 interface: Understanding degradation in MOSFETs

Abstract

Hydrogen atom dissociations at the gate/dielectric interface is known to be a degrading process in MOSFET devices. In this work we use molecular dynamics and the ReaxFF forcefield to study hydrogen dissociations as well as diffusion near the gate/dielectric interface. We show that any process that induces hydrogen dissociations (such as radiation) damages the transistors in a fashion very similar to NBTI aging by knocking the hydrogen atoms out of the Si−H bond. We computed the likelihood of dissociation as a function of the initial kinetic energy of hydrogen. We further computed diffusion activation energies, which are in close agreement with previous studies. The analysis of the diffusion paths near the interface reveals mechanisms that are not present in bulk SiO2. In particular, diffusion via oxygen and silicon vacancies with high energy barriers are more prevalent in the vicinity of the interface. This explains the rather high diffusion activation energy of 0.8 ev near the interface compared to the 0.2–0.4 ev in bulk material [1,2].