Boron redistribution during reactive diffusion in Ni–Si contacts O. Cojocaru-Mirédin a, * , C. Perrin-Pellegrino b , D. Mangelinck b , D. Blavette a,1 a Groupe de Physique des Matériaux, Université de Rouen, UMR 6634 CNRS, Avenue de l’université, BP 12, 76801 Saint Etienne du Rouvray, France b Laboratoire Matériaux et Microélectronique de Provence, UMR 6137 CNRS, Université Paul Cézanne, Faculté de Science et Techniques, Case 142-13397 Marseille cedex 20, France article info Article history: Received 20 March 2009 Received in revised form 12 June 2009 Accepted 22 June 2009 Available online 26 June 2009 Keywords: Laser atom probe tomography Microelectronics Ni silicides Boron abstract The NiSi silicide that forms by reactive diffusion between Ni and Si-rich active regions of nanotransistors is currently used for contacts in nanoelectronics because of its low resistivity. The redistribution of boron during reactive diffusion between Ni (30 nm) and B doped-Si has been investigated by laser assisted wide-angle tomographic atom probe (LAWATAP). Two states were characterized (room temperature and rapid thermal annealing at 450 °C for 1 min). LAWATAP shows that after deposition of Ni (30 nm) at room temperature a very thin film (7 nm) of Ni silicide was formed. The initial boron distribution in silicon is almost unchanged. After a heat treatment in vacuum at 450 °C (1 min) the nickel monosilicide NiSi was formed. Boron distribution at this stage is very different from that at room temperature. Boron is shown to accumulate at NiSi/Si interface due to snowplow effect. Very small amounts of boron were also found in NiSi phase close to the surface. Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved. 1. Introduction The nickel monosilicide NiSi has replaced TiSi 2 and CoSi 2 as con- tacts in microelectronic devices for sub-90 nm nodes because of its low resistivity, relatively low formation temperature, low silicon consumption and no resistivity increase on narrow lines [1]. This silicide is formed trough the self-aligned salicidation which is an important key technology in modern CMOS fabrication processes. In this process, the metal is deposited on the actives parts areas of the transistor (source, gate and drain) and during a subsequent annealing procedure the silicides will be formed. During the silicide formation dopants redistribute in the silicide/doped silicon struc- ture and this can change the source/drain junctions. It is also very important to know how dopants are affected in the vicinity of the silicide/Si interface in order to optimize the device performance. Over the last few years, there has been extensive works on determining the effects of doping impurities on the formation and stability of Ni silicides [2–4]. For example, Xu et al. [3] show that the transition from NiSi to NiSi 2 was retarded by the presence of boron for thin silicide films. Moreover, Ahmet et al. [2] indicate that the agglomeration of NiSi is delayed in Ni silicides films formed on heavily doped B + Si substrate (level 1.5  10 20 at/cm 3 ) in comparison with Ni silicide formed on As + implanted silicon substrate (level 7.5  10 19 at/cm 3 ). However, the redistribution of doping impurities during the formation of the silicide is not well understood. One of the reasons is the difficulty to map out the dis- tribution of doping impurities at the nanometric scale. Advanced characterization methods with very high spatial resolution are thus required to analyze the redistribution at the nanoscale. Laser assisted atom probe tomography (APT) is a powerful approach for such investigations [5–8]. Zaring et al. [9] showed with secondary ion mass spectrometry (SIMS) technique that boron accumulates at the surface, and in the silicide at the silicide/silicon interface. The profile has a nearly exponential shape. Jiang et al. [10] confirmed the results obtained by Zaring et al. in the case of NiSi formation after annealing at 450 °C. Two significant dopant peaks are re- vealed: one is located near the NiSi/Si interface and the other is found to be several nanometers below the silicide film surface. In this paper, the redistribution of boron during formation of Ni silicides using the laser assisted wide-angle tomographic atom probe (LAWATAP) has been investigated. 2. Experimental A thin film of polycrystalline Ni, 30 nm thick, was deposited at room temperature by sputtering on {1 0 0} boron-doped Si sub- strate. A high energy and a high dose (10 keV and 5  10 15 at/ cm 2 ) were chosen for implantation in silicon substrate in order to be close to that of ultra shallow junctions. The Ni film was deposited by sputter deposition simultaneously on high aspect ra- tio flat-topped {1 0 0} silicon posts for APT analysis. In the APT technique, the sample made in the shape of a thin tip is evaporated atomic layer by atomic layer and analyzed by time-of-flight mass spectrometry, allowing a small volume of material (typically 70  70  200 nm 3 ) to be reconstructed in the three dimensions 0167-9317/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.06.018 * Corresponding author. E-mail address: oana.cojocaru@etu.univ-rouen.fr (O. Cojocaru-Mirédin). 1 Also with the Institut Universitaire de France. Microelectronic Engineering 87 (2010) 271–273 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee