Characterization of Chemical Bonding Features and Interfacial Reactions in Ge-MIS Structure with HfO2/TaGexOy Dielectric Stack

Control of the chemical reactions at both high-k/Ge and metal/high-k interfaces is of great importance to realize the high quality Ge-MIS capacitor with a thinner EOT.  So far, we have fabricated Ge-MIS structures with HfO₂/TaGe_xO_y gate stack, in which TaGe_xO_y is used as an interfacial control layer between HfO₂ and Ge substrate from the viewpoint of relatively high dielectric constant (ε = ~9ε₀), and CET of 1.4 nm has been obtained [1].  As for the metal/dielectric interface, it has been clarified that thermally grown GeO₂ are reduced depending on element metals formed by thermal evaporation, and this reduction reaction reflects the Gibbs free energy difference [2, 3].  In this work, we mainly focused on the chemical reactions in the TiN/HfO₂/TaGe_xO_ygate stack caused by post metallization annealing (PMA), and the chemical structures of the MIS samples were examined by using hard x-ray photoemission spectroscopy (HAXPES) excited synchrotron radiation (hν = 7939 eV).

A 2 nm-thick TaO_x layer was deposited on wet-cleaned p-type Ge(100) surface at 400 ºC in a layer-by-layer fashion in which Ta-TTT was introduced intermittently using a N₂ bubbling method.  Subsequently, a ~2 nm-thick HfO₂ layer was deposited at 280 ºC by an ALD method using TEMA-Hf and O₃.  Then, the post deposition annealing (PDA) was performed at 400 °C in diluted O₂ ambience to densify the dielectrics.  A formation of the TaGe_xO_y interfacial layer by PDA was confirmed from XPS analyses.  After that, a TiN thin layer with a typical thickness of ~15 nm was deposited by RF sputtering.  In some samples, PMA in the temperature range from 300 to 600 °C was carried out in N₂ambience at atmospheric pressure.

Figure 1 shows the HAXPES core-line signals taken for TiN/HfO₂/TaGe_xO_y/Ge structure before and after PMA.  Although a slight increase in the chemically shifted Ge 2p_3/2 signals due to the incorporation of Ge atoms into high-k dielectric layers was observed after PMA at 400 °C, change in the chemical structure near the interface between TiN and HfO₂/TaGe_xO_y dielectrics were hardly detected.  Even in the reference sample of TiN/GeO₂(21nm)/Ge structure, chemical bonding features at the top interface show a good thermal stability against PMA at 400 °C.  After the PMA at temperatures over 500 ºC, chemically shifted Ge 2p_3/2 signals were drastically increased.  From the photoelectron take-off angle dependence of core-line signals for the sample after PMA at 600 °C, the formation of Ge oxide component was confirmed at the sample surface.  In contrast, significant diffusion of Hf atoms into TiN was hardly detected.  Electrical properties of MIS capacitors also show a significant increase in the gate leakage current and a decrease of accumulation capacitance by PMA at temperatures over 500 ºC.  These results suggest that dissociation of Ge-O bonds proceeds in the region near the TiN at temperatures over 500 ºC and it makes a degradation of the electrical properties.

Acknowledgements

This work was supported in part by JSPS Core-to-Core Program of International Collaborative Research Center on Atomically Controlled Processing for Ultralarge Scale Integration.  The Ta-TTT precursor was provided by TOSOH Corporation.  A part of this work was supported by Research Institute for Nano-device and Bio Systems (RNBS), Hiroshima University, Japan.  HAXPES experiments were done at a beam line of BL47XU in SPring-8 with the approval of JASRI as a Nanotechnology Support Project of the Ministry of Education, Culture, Sports, Science, and Technology (Proposal No. 2013A1696/BL47XU).  We would like to thank Prof. S. Higashi (Hiroshima Univ.) and Dr. E. Ikenaga (JASRI) for the fruitful discussion and contribution in assisting the experiments.

References

[1] K. Hashimoto et al., IEICE Trans. Electron., E96-C(2013) 674.

[2] H. Murakami, et al., ECS Trans., 33(2010) 253.

[3] A. Ohta, et al., Jpn. J. Appl. Phys., 50(2011) 10PE01.