Study of Germanium Diffusion in HfO 2 Gate Dielectric of MOS Device Application
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Study of Germanium Diffusion in HfO2 Gate Dielectric of MOS Device Application Qingchun Zhang, Nan Wu, L.K. Bera1, and Chunxiang Zhu* Silicon Nano Device Lab, Dept of Electrical and Computer Engineering, National University of Singapore, Singapore 119260 (*E-mail: [email protected]) 1 Institute of Microelectronics, Singapore, 117685 ABSTRACT Significant germanium incorporation into HfO2 gate dielectrics has been found after thermal annealing in germanium MOS device. The dependences of germanium incorporation in HfO2 with dielectric deposition method, annealing temperature and annealing ambient were extensively studied by means of physical analytical methods such as SIMS and XPS. MOCVD (metal organic chemical vapor deposition) technique shows stronger germanium incorporation than PVD (physics vapor deposition) while surface nitridation of germanium can effectively suppress the Ge-incorporation. In addition, the results indicate that a thermal budget higher than 500oC in device fabrication results in apparent Ge out-diffusion. And the germanium out-diffusion is found to be enhanced under oxygen environment. INTRODUCTION Historically, the first transistors were Ge based, but the low thermal stability and water solubility of the GeO2 gate dielectric caused people to abandon Ge and choose Si because of the superior physical and electrical properties of silicon dioxide. During the past several decades, scaling MOSFETs to improve performance results in higher gate leakage current as the gate dielectric becomes thinner. To solve this problem, a major approach is to replace the conventional SiO2 gate dielectric with a thick dielectric while maintaining the same capacitance density. To achieve this, there has been much interest in hafnium-based dielectrics as a potential high permittivity replacement for SiO2. With the recent successful development of high-κ dielectric deposition, germanium MOSFET draws a great attention again. Several works about HfO2 deposition on germanium substrate have been reported. For the HfO2 film deposited with MOCVD technique, a large amount of germanium was found to be incorporated inside the HfO2 film [1]. Similar germanium incorporation was also observed in the PVD HfO2 after high temperature annealing [2]. However, there is no detail investigation about the dependence of germanium incorporation on the process. In this paper, the germanium incorporation into HfO2 is extensively evaluated on bulk germanium wafers with varied process conditions. EXPERIMENTAL DETAILS HfO2 was directly deposited on (100) germanium substrates by MOCVD using Hf t-butoxide precursor and O2 at 400oC in the MOCVD chamber of Jusung gate cluster. Before deposition, the
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wafers were cleaned by diluted-HF and some samples were applied with surface nitridation (600ºC 1min in NH3). The as-deposited HfO2 thickness is 5 nm measured by ellipsometer. To evaluate the dependence of Ge incorporation with deposition methods, reactive sputtering method, by using a pure Hf target in O2 ambient at room temperature, w
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