Metal carbide-induced negative flatband voltage shift in TaC x and HfC x /HfO 2 gate stacks Wataru Mizubayashi a, *, Koji Akiyama b , Wenwu Wang a , Minoru Ikeda b , Kunihiko Iwamoto b , Yuuichi Kamimuta b , Akito Hirano b , Hiroyuki Ota a , Toshihide Nabatame b , Akira Toriumi a,c a MIRAI Project, Advanced Semiconductor Research Center (ASRC), National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba SCR Building, 16-1 Onogawa, Tsukuba 305-8569, Japan b MIRAI Project, Association of Super-Advanced Electronics Technologies (ASET), AIST Tsukuba SCR Building, 16-1 Onogawa, Tsukuba 305-8569, Japan c The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan 1. Introduction Metal gate electrodes are required in high-k metal-oxide- semiconductor field-effect-transistors (MOSFETs) to eliminate gate depletion in conventional poly-Si gates. Metal carbides (MeC x ) such as TaC x [1,2] and HfC x [3], with sufficient thermal stability and low effective work function (F m,eff ), have been reported as promising gate electrodes for n-MOSFETs. Although no flatband voltage (V FB ) shift in the case of MeC x gate/HfO 2 /SiO 2 with respect to MeC x /SiO 2 was observed [3], its origin has not yet been understood. We previously found that V FB for high-k MOS devices is predominantly determined by the high-k/SiO 2 -bottom-interface dipole [4]. Given these results, the V FB shift for MeC x /HfO 2 gate stacks could be caused by not only the dipole formation at the bottom interface but also a potential offset at the top interface. However, the top interface effect for MeC x /HfO 2 MOS devices on V FB shift has been hardly investi- gated. In this work, using a SiN insertion layer at the gate/HfO 2 and HfO 2 /SiO 2 interfaces, we systematically studied the role of the top interface for TaC x and HfC x /HfO 2 gate stacks on the F m,eff shift. 2. Experimental The fabrication flow for n-MOS capacitors with the TaN, TaC x , and HfC x gate electrodes is shown in Fig. 1(a). HfO 2 layers with thickness of 2–6 nm were deposited on a 4-nm-thick thermal SiO 2 film by atomic layer deposition (ALD), followed by post deposition annealing at 800 8C in N 2 . As shown in Fig. 1(b), a 1.5- nm-thick SiN layer was inserted into the gate/HfO 2 or HfO 2 /SiO 2 interfaces by ALD. To compare with pure SiO 2 MOS gate-stack characteristics, a thermal SiO 2 film with thickness of 3–10 nm was formed on a hydrogen-terminated Si substrate at 1000 8C in O 2 . A 20-nm-thick TaN film, a TaC x (C: 44.5 at.%) film, and a HfC x (C: 45.2 at.%) film were then deposited on SiO 2 and HfO 2 by DC sputtering, in which carbon content was estimated by Rutherford backscattering spectroscopy (RBS). Next, TiN/NiSi stacks were formed on the TaN, TaC x , and TaC x /HfC x layers. Finally, forming gas annealing (FGA) was performed at 400 8C in H 2 . To avoid additional reactions between the metal gates and HfO 2 , the maximum temperature after fabrication of the metal gates was kept under 500 8C. Applied Surface Science 254 (2008) 6123–6126 ARTICLE INFO Article history: Accepted 2 February 2008 Available online 12 March 2008 Keywords: Metal carbide HfO 2 Effective work function ABSTRACT We systematically investigated the role of the top interface for TaC x and HfC x /HfO 2 gate stacks on the effective work function (F m,eff ) shift by inserting a SiN layer at the gate/HfO 2 top interface or HfO 2 /SiO 2 bottom interface. We found that F m,eff of the TaN gate electrode on HfO 2 was larger than that on SiO 2 because of the HfO 2 /SiO 2 -bottom-interface dipole. On the other hand, we found that F m,eff values of the TaC x and HfC x gate electrodes on HfO 2 agree with F m,eff on SiO 2 . This is because the potential offset of the opposite direction with respect to the bottom interface dipole appears at the metal carbide/HfO 2 interface. It is thus concluded that the top interface in the metal carbide/HfO 2 gate stacks causes the negative F m,eff shift. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +81 29 849 1629; fax: +81 29 849 1529. E-mail address: w.mizubayashi@aist.go.jp (W. Mizubayashi). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.02.159