Epitaxial Formation of a Metastable Hexagonal Nickel–Silicide K. De Keyser, a,z C. Van Bockstael, a C. Detavernier, a R. L. Van Meirhaeghe, a J. Jordan-Sweet, b and C. Lavoie b a Department of Solid State Physics, Ghent University, Belgium 9000 b IBM T.J. Watson Research Center, Yorktown Heights, New York 10598, USA The growth of epitaxial layers of hexagonal -nickel–silicide on Si100 and Si111 substrates is reported. They form at 370°C on Si100 and 360°C on Si111, from codeposited Ni/Si mixtures, containing 37 to 42 atom % Si and the equivalent of a 50 nm Ni layer. These codeposited layers model the Ni/Si mixing layer at the interface in sputter-deposited films. The occurrence and stability at room temperature conflict with the phase diagram for bulk Ni/Si. Congruent crystallization is shown to initiate the growth of this metastable phase. © 2008 The Electrochemical Society. DOI: 10.1149/1.2955580 All rights reserved. Manuscript submitted April 28, 2008; revised manuscript received June 16, 2008. Published July 16, 2008. Nickel–silicides have important applications in current deep sub- micrometer microelectronics technology as contacting materials on source and drain regions. 1,2 Traditionally, the reaction of nickel with silicon on Si100 is believed to result in the sequential formation of orthorhombic Ni 2 Si, orthorhombic NiSi, and cubic NiSi 2 . 3 However, detailed in situ X-ray diffraction XRD measurements show addi- tional peaks which this phase sequence cannot explain. 4 Typically, transmission electron microscopy studies on quenched samples or XRD measurements in Bragg–Brentano geometry were carried out, where certain phases can be overlooked, due to the small tempera- ture range in which they occur, or due to their epitaxial texture. As a detailed interest in the texture of nickel–silicides is only recent, 5-7 this would not be surprising. In this article, we report the formation of the high-temperature, hexagonal -nickel-silicide phase, which grows epitaxially on both Si100 and Si111, by codepositing a nickel/silicon mixture. These codeposited layers model the thin amorphous mixing layer at the interface. Such a layer is invariably produced when sputter depositing pure Ni on Si wafers, 8 which is currently being done in the fabrication of NiSi for microelectronic applications. As the interface reaction plays a crucial role in the phase formation sequence in thin films, we believe our results are valuable for the understanding of the properties and fabrication of nickel–silicides. Nickel and silicon were cosputtered on standard RCA cleaned and HF dipped Si100 and Si111 wafers, at room temperature. The sputtering power and time for the nickel deposition were kept constant, corresponding to a thickness of the nickel layer of a con- stant 50 nm if no Si would have been cosputtered, and the sputtering power for the silicon was chosen to result in a layer containing between 37 and 42 atom % silicon. In this article, results are shown for samples with 40% silicon. The samples were studied using in situ XRD at the X20C beam line at the National Synchrotron Light Source NSLS at Brookhaven National Laboratory BNL, using monochromatic X-rays with a wavelength of 1.78 nm, with an inci- dence angle of  = 27° to the surface normal. While heating each sample from 100 to 700°C at a rate of 3°C /s in a high-purity helium atmosphere, the diffracted X-ray intensity is recorded every 0.5 s, in a2 range from 48 to 61°. Figure 1 shows the results for the 40 atom % silicon samples. Three regions can be distinguished. In the first region, at temperatures below 370°C for Si100 or 360°C for Si111, one sees a faint and broad peak at 2 = 53°, caused by the as-deposited nickel–silicon layer. The low crystallinity of this layer results in the weak and broad peak. In a second region, between 370 and 480°C Si100 or 360 and 510°C Si111, hardly anything is visible, hinting at either an epitaxial or amorphous phase being formed. At 480°C Si100 or 510°C Si111, the third region starts, where nickel–monosilicide peaks are visible. On the Si111 substrate, these peaks have a very low intensity, once again hinting at a strongly textured or largely amorphous layer of NiSi. To study the phase existing in the second region, quenches were made at 420°C. X-ray reflectivity measurements showed the thickness of the samples to be 100 nm after anneal, and the composition of both the as-deposited and annealed films was analyzed using Rutherford backscattering spectroscopy RBS. The as-deposited film was found to contain 40  2% Si, uniformly distributed over the thick- ness of the layer. After annealing, RBS measurements showed about 44  2% Si in the layer, also uniformly distributed. From these results, there seems to be a slight increase in silicon content during the annealing. However, based on the current RBS measurements and their error, we cannot be completely sure whether this increase is significant. It has been shown that electron backscatter diffraction EBSD can provide valuable information about silicides: 6 it allows one to distinguish between materials with different Laue groups, and pro- vides local crystal orientation information and grain sizes. Measure- ments were carried out on the samples quenched at 420°C, using an FEI Quanta 200F field emission gun scanning electron microscope, and an HKL Channel 5 system attached. The EBSD patterns, which consist of bands corresponding to certain lattice planes, were ana- lyzed using five different phases: the hexagonal -nickel–silicide, -Ni 3 Si 2 , -Ni 2 Si, NiSi, and Ni 31 Si 12 . -nickel–silicide has a hexagonal lattice space group P6 3 /mmc, with variable composition. Typically, it has been reported containing 33% Si, resulting in lattice parameters a = 0.3805 nm and c = 0.489 nm. 9 The unit cell has a series of positions the nickel atoms z E-mail: koen.dekeyser@ugent.be Figure 1. In situ XRD measurement on Ni/Si alloy with 40% Si on Si100 subfigure A and Si111subfigure B substrates. Electrochemical and Solid-State Letters, 11 9 H266-H268 2008 1099-0062/2008/119/H266/3/$23.00 © The Electrochemical Society H266 Downloaded 30 Sep 2010 to 130.199.3.140. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp