Retarded Growth of Sputtered HfO 2 Films on Germanium
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Retarded Growth of Sputtered HfO2 Films on Germanium Koji Kita, Masashi Sasagawa, Masahiro Toyama, Kentaro Kyuno and Akira Toriumi Department of Materials Science, The University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. ABSTRACT HfO2 films were deposited by reactive sputtering on Ge and Si substrates simultaneously, and we found not only the interface layer but the HfO2 film was thinner on Ge substrate compared with that on Si substrate. A metallic Hf layer has a crucial role for the thickness differences of both interface layer and HfO2 film, since those thickness differences were observed only when an ultrathin metallic Hf layer was predeposited before HfO2 film deposition. The role of metallic Hf is understandable by assuming a formation of volatile Hf-Ge-O ternary compounds at the early stage of film growth. These results show an advantage of HfO2/Ge over HfO2/Si systems from the viewpoint of further scaling of electrical equivalent thickness of the gate oxide films. INTRODUCTION Germanium CMOS is historically one left behind, but recently its research has come to life again from the trend of using deposited high-k film than thermally grown SiO2 for further scaling [1]. On the other hand, it is also very attractive that Ge has intrinsically higher carrier mobility than silicon. However, it is really needed to clarify what disadvantages are, and what other advantages are in Ge CMOS. There are several critical differences between high-k/Ge and high-k/Si systems. The chemically and thermodynamically unstable characteristics of Ge native oxides will affect the interface layer formation processes and the interface reactions during thermal treatments. The purpose of this study is to clarify these differences by a comprehensive comparison between HfO2/Ge and HfO2/Si systems through physical and electrical characterizations. This paper also reports an advantage we have found in HfO2/Ge MOS system. This fact will become remarkably important in the sub-nm equivalent oxide thickness (EOT) gate dielectrics region. EXPERIMENTAL HfO2 films were processed on (100) Ge and (100) Si wafers simultaneously except for the wafer pre-cleaning steps. Ge wafers were degreased by methanol, immersed in HCl solution to remove the native oxides, and re-oxidized in H2O2 solution, followed by dipping in diluted HF solution and rinsing in deionized (DI) water. Almost no Ge oxides were left on Ge surface after these cleaning steps, which was confirmed by x-ray photoelectron spectroscopy (XPS). Si wafers were cleaned with H2SO4/H2O2 and HCl/H2O2 solutions, and immersed in diluted HF solution. The HfO2 films (2 - 15 nm-thick) were deposited by a 2-step sputtering of Hf metal target. First, 1.5nm-thick Hf metal layer was deposited in Ar, followed by a HfO2 reactive sputtering deposition in O2/Ar gas flow. (O2 : Ar = 1 : 4). The ultra-thin metallic Hf layer was expected to be oxidized during the reactive sputtering to restrict the growth of the interface layer [2]. For comparison, HfO2 films without the predeposition o
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