Densification of ∼5 nm-thick SiO2 layers by nitric acid oxidation

Abstract

Low-temperature nitric acid (HNO3) oxidation of Si (NAOS) has been used to improve the interface and electrical properties of ∼5nm-thick SiO2/Si layers produced by plasma-enhanced chemical vapor deposition (PECVD). Investigations of the physical properties and electrical characteristics of these thin films revealed that although their thickness is not changed by NAOS, the leakage current density at a gate bias voltage of −1V decreases by about two orders of magnitude from 1.868×10−5A/cm2. This leakage current density was further reduced by post-metallization annealing (PMA) at 250°C for 10min in a 5 vol.% hydrogen atmosphere, eventually reaching a level (5.2×10−8A/cm2) approximately three orders of magnitude less than the as-grown SiO2 layer. This improvement is attributed to a decrease in the concentration of suboxide species (Si1+, Si2+ and Si3+) in the SiO2/Si interface, as well as a decrease in the equilibrium density of defect sites (Nd) and fixed charge density (Nf). The barrier height (Фt) generated by a Poole-Frenkel mechanism also increased from 0.205 to 0.371eV after NAOS and PMA. The decrease in leakage current density is therefore attributed to a densification of the SiO2 layer in combination with the removal of OH species and increase in interfacial properties at the SiO2/Si interface.