Interaction of molecular contaminants with high-k dielectric surfaces

Abstract

As the device feature size shrinks, films of silicon oxide (SiO₂) will become unsuitable for MOSFET gate dielectric applications and have to be replaced by thicker films of a high-k dielectric material. Among the high-k materials, hafnium oxide (HfO₂) and zirconium oxide (ZrO₂) are the most promising candidates. Molecular contamination can affect the quality of the new gate dielectric films in a manner similar to ultrathin SiO2 films. Therefore, characterization of contaminant adsorption behavior of these high-k films should assist in deciding their potential for successful integration in silicon MOS technology. The interactions of moisture and organic (in particular IPA) contamination with ALCVD(TM) deposited 5-nm HfO₂ and ZrO₂ films were investigated using mass spectrometry. HfO₂ and ZrO₂ were found to have similar moisture adsorption loadings, but significantly higher than that of SiO₂. The new high-k materials also retained a higher portion of the adsorbed moisture after an isothermal nitrogen purge. Almost all the adsorbed moisture could be removed from SiO₂ and HfO₂ after a 300°C bake under nitrogen purge, whereas ZrO₂ surfaces retained significant amounts of the adsorbed moisture. Experiments with ppb-levels of IPA showed that the adsorption loading on the three surfaces had the following order: ZrO₂ > HfO₂ > SiO₂. The relatively slow desorption kinetics of H2O and IPA highlighted the difficulty in removal of these contaminants from HfO₂ and ZrO₂ surfaces. Presence of pre-adsorbed moisture increased IPA adsorption on SiO₂, but reduced adsorption on HfO₂ and ZrO₂. Isotope labeling studies with D₂O showed that IPA reacted with surface hydroxyl groups to form a chemisorbed alkoxy species on all oxides. A multilayer model for adsorption of water and IPA was developed to understand the mechanism of interactions of contaminants with these surfaces. Results indicated that ZrO₂ formed the strongest surface-hydroxyl bond and also physisorbed IPA stronger than HfO₂ and SiO₂. The practical application of the adsorption model is also demonstrated. The results of this work should aid in the selection of the most appropriate dielectric film and design of process/equipment so that it can be more readily integrated into silicon technology.