Cross-sectional Transmission Electron Microscope Studies on Intrinsic Breakdown Spots of Thin Gate Oxides

Abstract

Cross-sectional transmission electron microscopy was used to study the intrinsic breakdown spots of 10-nm-thick gate oxides thermally grown on (001) Si substrates. At a breakdown spot with a breakdown field of 15 MV/cm, (111) twin planes are confirmed by electron diffraction to exist in the surface region of the Si substrate. High resolution observation also reveals Si lattice fringes in the gate oxide, penetrating from the Si substrate and gate electrode of polycrystalline Si, and significant roughening of the anode interface between oxides and Si substrates. Also, similar growth twins and anode interface roughening are observed for a lower field breakdown spot with a breakdown field of 7 MV/cm. The presence of growth twins indicates that a Si substrate is locally molten during dielectric breakdown. These results are well explained by the local thermal breakdown model of intrinsic oxides. The interface roughening, presumably caused by deoxidization at the SiO2/Si interface in a local hot spot, is indicative of an initial stage of intrinsic dielectric breakdown of thin oxide.