Mechanical Properties of Silicon Based Compounds: Silicides

Abstract

In this chapter the silicides are defined. They are the product of the reaction between silicon and metal. Almost all metals can react with silicon forming various silides. Of particular interest are the MSi2 (M stands for metal) silicides. In this book the CoSi2, NiSi2, FeSi2, WSi2, TiSi2 and MoSi2 are considered either as bulk or thin films. Silicides are binary compounds of silicon with other more electropositive elements. Chemical bonds in silicides may exhibit primarily covalent or primarily ionic characteristics, depending on the electronegativity of the participating elements. Similar to borides and carbides, the composition of silicides cannot be easily specified as covalent molecules, since the chemical bonds in silicides range frommetal like structures to covalent or ionic. The metal-like structure provides to many of the silicides a conductive character. Of the silicides the transition metal silicides are usually inert to aqueous solutions with exception of hydrofluoric acid. Silicides are a group of compounds, comprising silicon in combination with one or more metallic elements. These hard, crystalline materials are closely related to inter-metallic compounds and have, therefore, many of their physical and chemical characteristics and some of the mechanical properties of metals. Further they do not appear as natural products and therefore must be synthesized. A metallurgical technique for silicide formation is by directly depositing a refractory metal on a silicon surface to form the required silicide layer by reaction between them. After the metal is deposited on the silicon, this system is exposed to high temperatures that promote the chemical reactions between the metal and the silicon. In such a metallurgical reaction, metal-rich silicides generally form first, and continue to grow until all themetal is consumed.When themetal has been consumed, silicides of lower metal content start appearing, which can continue to grow simply by consuming the metal-rich silicides. At the final point the system attains stability usually when the silicide is a MSi2, where M stands for a metal. Clearly, enough silicon must be available for silicide formation by the direct metallurgical reaction between metal and silicon. This requirement is generally fulfilled when the metal is deposited on silicon substratewhich is sufficient even if the silicon is consumed during the reaction forming the silicide. © Springer Nature Switzerland AG 2019 J. Pelleg,Mechanical Properties of Silicon Based Compounds: Silicides, Engineering Materials, https://doi.org/10.1007/978-3-030-22598-8_1 1 2 1 What Are the Silicides? Almost all metals-definitely a majority-react with silicon. However, not all silicides are of interest for practical engineering and structural applications. The silicides for high temperature applications are limited to the refractory metals of groups IVA, VA and VIA. Included in this category are the silicides of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, and tungsten. However, in the microelectronic industry, the silicides of the VIIIA group metals of the periodic table are of great interest as contacts, gate and interconnection in silicon integrated circuits. Their low resistivity is the main characteristic of these silicides among them, FeSi2, CoSi2, NiSi2, PdSi and PtSi. Silicide classification of elements in the periodic table is presented in Table 1.1 after Murarka. Various silicides are formed during the process of fabrication when a metal is deposited on silicon. The phases of the silicides form sequentially, but often more than one phase is present which makes the identification of the silicide more difficult requiring skill for the operation of various techniques in addition to X-ray diffraction (XRD), such as Rutherford backscattering (RBS) and Auger electron spectroscopy (AES). In the thin film silicide formation resistivity measurements is an important tool to evaluate the end-product of the silicidation, which exhibits usually the lowest resistivity. Care must be taken—in the processes of thin silicide film formation for the silicon based microelectronic industry—to avoid SiO2 formation between metal and silicon which inhibits reaction between the silicon and the deposited metal. The misfit between the depositedmetal and the silicon introduces stress in the silicide film Table 1.1 Silicides of elements in the periodic table. Murarka (1995). With kind permission of Elsevier 1 What Are the Silicides? 3 formed, thus the mechanical properties are of interest also in thin film silicides not only in bulk silicide formation. The temperature of the silicide formation is critical because the reaction betweenmetal and silicon is diffusion controlled and as such the diffusion of themetallic atoms into the silicon or vice versa is temperature dependent. It is thus understood why cleanliness of metal-silicon interface is important, since SiO2 if formed inhibits interdiffusion of the participating entities in the reaction. As mentioned in the preface of all possible silicides formation the book is considered with the mechanical properties of only the end-phases of a metal-silicon reaction, specifically those of CoSi2, NiSi2, FeSi2, WSi2, TiSi2 and MoSi2.