Role of lanthanum in the gate stack: Co-sputtered TaLaN metal gates on Hf-based dielectrics Brian Coss * , Hyun-Chul Kim, Francisco S. Aguirre-Tostado, Robert M. Wallace, Jiyoung Kim * Materials Science and Engineering, University of Texas at Dallas, Richardson TX 75080, United States article info Article history: Received 27 September 2007 Received in revised form 1 March 2008 Accepted 19 May 2008 Available online 4 June 2008 Keywords: High-k dielectrics Metal gate TaN Lanthanum Hafnium Oxide EWF abstract We examine the characteristics of TaLaN metal gates in direct contact with HfO 2 dielectric, in particular focusing on the effect of La in the gate stack for NMOS applications. Effective work functions (EWF) and vacuum work functions (WF) are measured as a function of lanthanum content in TaLaN without any intentional heating using X-ray photoelectron spectroscopy, U-V photoelectron spectroscopy, and electri- cal C–V measurements. We find that the addition of lanthanum to tantalum nitride lowered both the EWF and the WF of the metal gate by 0.2 eV and 0.9 eV, respectively. Furthermore, XPS indicates that lan- thanum in TaLaN at the interface with HfO 2 is primarily bonded to nitrogen rather than oxygen and not reacting with the dielectric. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Intense scaling has put enormous demand on the electrical and physical properties of the conventional complementary metal oxide semiconductor devices (CMOS). In order to continue transis- tor scaling, high-k gate dielectrics are being investigated. The use of a high-k gate dielectric poses several challenges, one of which is a suitable choice in the gate material [1]. In order to meet the threshold voltage requirements, an effective metal gate solution for n- or p-channel metal oxide field effect transistors (MOSFETs) should have an effective work function (EWF) near the conduction or valence band edges [2,3]. Many metal systems have been inves- tigated [3–5], but finding a thermally stable, low work function metal for n-channel metal oxide semiconductor devices (NMOS) has proven quite challenging. The properties of La-doped TaN me- tal gate are studied as a potential replacement to n + poly-Si gates. The addition of lanthanum to the gate stack near the interface of mid-gap metal TaN and HfO 2 has been shown to shift the EWF close to conduction band edge for NMOS and is also thermally sta- ble on silicon [2,3,6]. In order to explain this effect, several mech- anisms, including a dipole model [2,6] have been proposed. The dipole model involves diffusion of a lanthanum based dielectric cap through the HfO 2 dielectric to the SiO 2 interfacial oxide during annealing. Once at the SiO 2 /high-k interface, it forms a lanthanum silicate, and this highly polar bond (i.e. dipole) close to the channel provides the mechanism for tuning V t [2]. In this study, we employ three different techniques to understand the role of La on tuning the work function of a TaLaN electrode. The chemical bonding at the interface between TaLaN films and HfO 2 dielectrics are investi- gated using in-situ X-ray photoelectron spectroscopy (XPS). Mea- surements of the vacuum work functions (WF) of TaLaN films are made by ultra-violet photoelectron spectroscopy (UPS) and EWF by XPS through a 2 nm thick TaLaN thin layer directly on top of HfO 2 dielectrics. We also investigate EWF using C–V measurement after 400 °C forming gas anneal. 2. Experiment In-situ reactive-ion sputter depositions of La-doped TaN were carried out in an ultra-high vacuum (UHV) cluster tool with a sput- ter chamber and analysis chamber connected by a UHV transfer system (Fig. 1) [7]. Lanthanum content was varied and the proper- ties were investigated by XPS and UPS, and subsequently by ex-situ C–V electrical measurement. EWF measurements are performed using 8 00 terraced SiO 2 wafers, ranging in thickness from 1 nm to 4 nm [8,9], which also have a 2 nm ALD HfO 2 layer provided by SEMATECH. The SEMATECH terrace wafer substrates have not been annealed in NH 3 or N 2 as is typically done and are only treated with SC1 (NH 4 OH:H 2 O 2 :H 2 O = 1:1:10) to remove surface contaminants prior to loading in UHV. Lanthanum is extremely reactive and read- ily bonds with O or OH. La target oxygen contamination was con- trolled by pre-sputter cleaning of the target and checking for trace 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.05.027 * Corresponding authors. E-mail addresses: brain.coss@student.utdallas.edu (B.S. B. Coss), jiyoung.kim@ utdallas.edu (J. Kim). Microelectronic Engineering 86 (2009) 235–239 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee