Characterization of HfO 2 /Si Exposed to Water Vapor at Room Temperature
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0917-E11-02
Characterization of HfO2/Si Exposed to Water Vapor at Room Temperature Carlos Driemeier1, Elizandra Martinazzi1, Israel J. R. Baumvol2, and Evgeni Gusev3 1 UFRGS, Porto Alegre, Rio Grande do Sul, 91501-970, Brazil 2 UCS, Caxias do Sul, Rio Grande do Sul, 95070-560, Brazil 3 T. J. Watson Research Center, IBM Research Division, Yorktown Hights, New York, 10598
ABSTRACT HfO2-based materials are the leading candidates to replace SiO2 as the gate dielectric in Si-based metal-oxide-semiconductor filed-effect transistors. The ubiquitous presence of water vapor in the environments to which the dielectric films are exposed (e.g. in environmental air) leads to questions about how water could affect the properties of the dielectric/Si structures. In order to investigate this topic, HfO2/SiO2/Si(001) thin film structures were exposed at room temperature to water vapor isotopically enriched in 2H and 18O followed by quantification and profiling of these nuclides by nuclear reaction analysis. We showed i) the formation of strongly bonded hydroxyls at the HfO2 surface; ii) room temperature migration of oxygen and waterderived oxygenous species through the HfO2 films, indicating that HfO2 is a weak diffusion barrier for these oxidizing species; iii) hydrogenous, water-derived species attachment to the SiO2 interlayer, resulting in detrimental hydrogenous defects therein. Consequences of these results to HfO2-based metal-oxide-semiconductor devices are discussed. INTRODUCTION Scaling Si-based metal-oxide-semiconductor field-effect transistors (MOSFETs) pushed the usual SiO2 gate dielectric layer to its ultrathin limits [1-3]. Hafnium oxide emerged as a leading candidate for SiO2 replacement because of several desirable HfO2 properties, including a dielectric constant much higher than that of SiO2 ( k HfO2 = 22, k SiO2 = 3.9 ) [2]. This allows employing substantially thicker HfO2 films while keeping the same metal-oxide-semiconductor capacitance achieved with ultrathin SiO2. Hence, HfO2 gate dielectrics allow further downscaling of Si-based MOSFETs for several forthcoming device generations. However, since for every generation device requirements become progressively stricter [1], previously negligible factors will become increasingly important for device understanding at the appropriate level of detail. The ubiquity of water vapor and its potential role on detrimental Si oxidation, threshold voltage shifts and hydrogenous defects [4-6] highlights the importance of the here reported room temperature water uptake in HfO2 films on Si. EXPERIMENT HfO2 films 2.5, 5, and 9 nm thick were deposited by metal-organic chemical vapor deposition on 1.5 nm thick SiO2 films thermally grown on p-type Si(001). Although such “thick” SiO2 interlayer would not be of practical use for state-of-the-art devices, it was here employed in
order to provide a barrier to uncontrolled Si oxidation. The resulting HfO2/SiO2/Si structures were annealed for 30 min at 800 °C in vacuum (10-7 mbar) employing a LN2 cold finger to remove residual water va
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