INITIAL REACTION AND INTERFACIAL LAYER CONTROL OF HAFNIUM OXIDE THIN FILM GROWN BY REMOTE PLASMA ATOMIC LAYER DEPOSITION

Abstract

Many high-k dielectric materials have been studied extensively to replace current gate dielectric materials such as SiO₂ and SiO_(x)N_(y). Among the high-k dielectric materials, hafnium oxide is considered to be one of best choices for 45 nm technology and beyond. However, most of high-k oxides such as HfO₂, ZrO₂, Ta₂O₃, and TiO₂ are transition metal oxides with the ionic nature and have poor thermal stability with Si substrate. In addition, they are easily crystallized compared to SiO₂ and SiO_(x)N_(y), and exhibit high oxide traps and interface state densities, and have large amount of oxygen vacancies. To use the high-k oxides as gate dielectrics, the technologies for growing high quality high-k oxides and improving the interface properties between high-k oxide and Si substrate are required. In this dissertation, HfO₂ was selected as high-k gate dielectrics and remote plasma atomic layer deposition (RPALD) was used to deposit it without charging damages caused by the plasma. The RPALD process of HfO₂ was experimentally investigated and theoretically modeled, and it was found that the initial phase of the HfO₂ growth was the formation of amorphous Hf-silicate. The HfO₂ grew on top of the initial Hf-silicate layer. However, the interfacial Hf-silicate layer or SiOx formed through reaction at HfO₂/Si interface limits the scaling benefits of the high-k dielectric in the devices. The interfacial layer is also critical for the degradation of high-k dielectric materials. Therefore, several different buffer layers were grown, before HfO₂ growth, to suppress the formation of Hf-silicate or interfacial layer and to improve the film quality. The buffer layers were ultrathin SiO₂, SiO_(x)N_(y), Al₂O₃, RPN-treated Al₂O₃, RPN-treated HfO₂, and HfOxNy. Remote plasma oxidation (RPO) and nitridation (RPN) were carried out to form the buffer layers. The dominant emission species in the RPO and RPN processes were O*and O2*, and N* and N2*, respectively. They can reach down to the Si substrate and react with the hydroxyl groups (-OH) or with plasma oxide formed on the Si substrate. The HfO₂ films with buffer layers suppressed silicate formation or growth of an interfacial layer more effectively than those without buffer layers. The HfO₂ films with buffer layers showed lower effective oxide thickness (EOT), lower effective fixed oxide charge density (Qf,eff.), and lower leakage current density compared to those without buffer layers. The surface preparation before deposition can obtain good physical and electrical characteristics of the HfO₂ films grown by RPALD.