Diffusion Foundations Vol. 21

Abstract: Silicide formation by reactive diffusion is of interest in numerous applications especially for contact formation and interconnections in microelectronics. Several reviews have been published on this topic and the aim of this chapter is to provide an update of these reviews by focusing on new experiment results. This chapter presents thus some progress in the understanding of the main mechanisms (diffusion/reaction, nucleation, lateral growth…) for thin and very thin films (i.e. comprised between 4 and 50 nm). Recent experimental results on the mechanisms of formation of silicide are presented and compared to models and/or simulation in order to extract physical parameters that are relevant to reactive diffusion. These mechanisms include nucleation, lateral growth, diffusion/interface controlled growth, and the role of a diffusion barrier. The combination of several techniques including in situ techniques (XRD, XRR, XPS, DSC) and high resolution techniques (APT and TEM) is shown to be essential in order to gain understanding in the solid state reaction in thin films and to better control these reaction for making contacts in microelectronics devices or for other application.

Abstract: Interdiffusion studies conducted in group IVB, VB and VIB metal-silicon systems are discussed in detail to show a pattern in the change of diffusion coefficients with the change in atomic number of the refractory metal (M) component. MSi₂ and M₅Si₃ phases are considered for these discussions. It is shown that integrated diffusion coefficients increase with the increase in atomic number of the refractory component when the data are plotted with respect to the melting point normalized annealing temperature. This indicates the increase in overall defect concentration facilitating the diffusion of components. This is found to be true in both the phases. Additionally, the estimated ratios of tracer diffusion coefficients indicate the change in concentration of antisite defects in certain manner with the change in atomic number of the refractory components.

Abstract: Thermodynamic and diffusion models are given to describe morphological evolution of the reaction zone during diffusion-limited interaction between non-oxide Si-containing ceramics (SiC and Si₃N₄) and transition metals (Cr, Mo, Ti, Ni, Co, Pt). In the case of diffusion-controlled process in the ternary metal-ceramic systems, reaction phenomena can be rationalized using chemical potential diagrams. However, in some cases, a periodic layered morphology is found in the transition zone, which is not fully understood, and it is difficult to predict a priori. Silicide formation in systems based on dense Silicon Nitride and non-nitride forming metals can be explained by assuming a nitrogen pressure build-up at the contact surface. This pressure determines the chemical potential of Silicon at the interface, and hence, the product phases in the diffusion zone. Traces of Oxygen in the ambient atmosphere might affect the interaction in non-oxide ceramic/transition metal systems. The thermodynamic stability of the condensed phases in the systems where volatile species may form can be interpreted using predominant area-type diagrams.

Abstract: The oxidation behavior of Mo, Nb, and Ti-silicides has received significant attention in past few decades for their potential to be used as high temperature structural materials. These Si-bearing intermetallic alloys have the ability to form an oxide scale containing SiO₂, which is protective if formed as a continuous and impervious layer, so that the ingress of oxygen from atmosphere to the underneath alloy is restricted. To form a continuous and stable SiO₂ scale, it is important to have sufficient activity of Si along with thermodynamic and kinetic conditions favoring its growth in comparison to that of oxides of other alloying elements. MoSi₂ has superior oxidation resistance compared to that of Mo₃Si or Mo₅Si₃, because of its higher Si content. Furthermore, a continuous film of SiO₂ is able to form at temperatures in the range of 800-1700 ^oC on MoSi₂ due to vaporization of MoO₃, but not on NbSi₂ or TiSi₂ due to competitive growth of Nb₂O₅ or TiO₂, respectively. During past two decades, a significant effort has been devoted to development of Mo-Si-B alloys containing Mo-rich solid solution, Mo₃Si and Mo₅SiB₂ as constituent phases, due to their ability to form a protective borosilicate scale. The presence of B₂O₃ contributes to fluidity of borosilicate scale, thereby contributing to closure of porosities. Efforts have been also made to develop multicomponent Nb-silicide based alloys with optimum combination of mechanical properties and high temperature oxidation resistance with limited success. There have been efforts to develop silicide based coatings for protection oxidation for Mo-rich Mo-Si-B alloys and Nb-Si based ternary or multicomponent alloys with inadequate oxidation resistance. Oxidation behavior of selected silicides with potential for structural application, along with mechanisms for protection against oxidation has been reviewed and discussed.

Abstract: Periodic layered morphology may occur during displacement solid-state reactions in ternary (and higher-order) silicide and other material systems. This periodic layered structure consists of regularly spaced layers (bands) of particles of one reaction product embedded in a matrix phase of another reaction product. The number of systems that is known to produce the periodic layered structure is rather small but increasing and includes metal/metal and metal/ceramic semi-infinite diffusion couples. The experimental results on different systems, where the periodic pattern formation has been observed are systematized and earlier explanations for this peculiar diffusion phenomenon are discussed. Formation of the reaction zone morphologies periodic in time and space can be considered as a manifestation of the Kirkendall effect accompanying interdiffusion in the solid state. The patterning during multiphase diffusion is attributed to diverging vacancy fluxes within the interaction zone. This can generate multiple Kirkendall planes, which by attracting in situ-formed inclusions of “secondary-formed phase” can result in a highly patterned microstructure.