Mechanistic studies of titanium dioxide and ruthenium atomic layer deposition by in situ techniques [electronic resource]
Keywords: Chemical Engineering.

The demand of smaller, higher capacity and higher performance devices in microelectronics has driven the necessity of uniform, conformal, and pinhole-free thin film production. Furthermore, the design toward more complex structures and higher aspect ratios requires the processes to be highly controllable, down to the nanoscale. Atomic layer deposition (ALD) is a powerful technique that produces thin films with these desired properties, through a series of alternating self-limited surface reactions. The self-saturated nature of the technique allows for precise thickness control at the atomic scale. Despite increasing interest in ALD, there is still a lack of understanding of the mechanisms behind the process at a molecular level. The nucleation and growth fundamentals are crucial for better control and development of the process and, hence, need to be systematically studied. Due to the vulnerability of the reactions to ambient conditions, ex situ analysis techniques alone may not provide complete information on the surface chemistries needed to elucidate the mechanisms governing the processes. In situ analysis techniques, which allow surface investigation without disruption from contaminants and other species, are required. Therefore, in this work we have designed and constructed various in situ systems for this purpose. The in situ systems are ALD reactors integrated with different analysis tools, able to operate as fully functional deposition system so as to replicate the actual conditions of typical ALD reactors. Through in situ X-ray photoelectron spectroscopy (XPS), we studied ALD of TiO2 at 100 °C using titanium tetrachloride (TiCl4) and water (H2O) on two different surfaces. The initial growth rate on hydroxyl-enriched silicon dioxide (SiO2) is found to be higher than on hydrogen-terminated silicon. The XPS results provide evidence of Si-O-Ti bonds on the SiO2 surface and Si-Ti bonds on the hydrogen-terminated Si surface, without a trace of interfacial oxide. However, a silicon oxide layer forms at the interface between Si and TiO2 after vacuum annealing, concurrent with the reduction of TiO2. The results hence suggest TiO2 as an oxygen source for silicon oxidation under these conditions. In addition, we studied ruthenium thermal ALD using a new precursor, bis(2,4-dimethylpentadienyl) ruthenium, and oxygen. The process is achievable at a low operating temperature of 185 °C. Variation in the exposure time and pressure of oxygen has significant effects on the nucleation, growth rate and composition of the deposited ruthenium films. We propose that the subsurface oxygen formation, which involves slow diffusion of oxygen, is a rate-limiting step in the RuO2 formation process. The crystal growth and structures of Ru and RuO2 deposited on amorphous SiO2 by the same ALD process were measured by ex situ and in situ synchrotron X-ray diffraction. Interestingly, in situ XRD studies reveal that RuO2 films initially nucleate as metallic Ru crystallites. The hindered formation of subsurface oxygen in small nanocrystals is hypothesized as the cause that prohibits the growth of the initial oxide. Although metallic ruthenium films are textured with a (002) preference in the growth direction, RuO2 films nucleating on the metallic Ru nanoparticles have no preferential orientation. We also studied surface chemistries of Ru reactions during half ALD cycles via in situ synchrotron photoemission spectroscopy (PES). After long oxygen exposures, Ru oxide and carbon-oxygen species, which localize near the top surface, were detected. The peak intensities of these species noticeably decreased after reaction with the Ru precursor, indicating the reactions of Ru precursor with both O-Ru and O-C species. In brief, we fabricated and utilized in situ ALD/analysis systems, together with ex situ analysis tools, for studies of TiO2 ALD and Ru/RuO2 ALD. The studies not only demonstrate the power of the in situ systems for mechanistic studies, but also provide information on possible bond formation, surface reactions, and nucleation and growth mechanisms in the ALD processes.