Papers by Keyword: Reactive Diffusion

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: Nanostructures used to build current technology devices are generally based on the stack of several thin films (from few nanometer-thick to micrometer-thick layers) having different physical properties (conductors, semiconductors, dielectrics, etc.). In order to build such devices, thin film fabrication processes compatible with the entire device fabrication need to be developed (each subsequent process step should not deteriorate the previous construction). Solid-state reactive diffusion allows thin film exhibiting good interfacial properties (mechanical, electrical…) to be produced. In this case, the film of interest is grown from the reaction of an initial layer with the substrate on which it has been deposited, during controlled thermal annealing. In the case of the reaction of a nano-layer (thickness < 100 nm) with a semi-infinite substrate, nanoscale effects can be observed: i) the phases appear sequentially, ii) not all the thermodynamic stable phases appear in the sequence (some phases are missing), and iii) some phases are transient (they disappear as fast as they appear). The understanding of the driving forces controlling such nanoscale effects is highly desired in order to control the phase formation sequence, and to stabilize the phase of interest (for the targeted application) among all the phases appearing in the sequence.This chapter presents recent investigations concerning the influence of atomic transport on the nanoscale phenomena observed during nano-film reactive diffusion. The results suggest that nano-film solid-state reaction could be controlled by modifying atomic transport kinetics, allowing current processes based on thin-film reactive diffusion to be improved.

Abstract: Silicide growth via reaction between a metallic film and a Si substrate has been well documented. In general, atomic transport kinetic during the growth of silicides is considered to be the same as during equilibrium diffusion, despite the reaction and its possible injection of point-defects in the two phases on each side of the interface. To date, the main studies aiming to investigate atomic transport during silicide growth used immobile markers in order to determine which element diffuses the fastest during growth and in which proportion. The quantitative measurements of effective diffusion coefficients during growth was also performed using Deal-and-Groove-type of models, however, these effective coefficients are in general not in agreement with the interdiffusion coefficients calculated using the equilibrium diffusion coefficients measured during diffusion experiments. In general, atomic transport kinetic measurements during growth and without growth are performed using different types of samples for experimental reasons. In this paper, we discuss the possible use of ultrahigh vacuum in situ Auger electron spectroscopy in order to measure the effective diffusion coefficient during growth, as well as the equilibrium self-diffusion coefficients, in the same samples, in the same experimental conditions. The first results on the Pd-Si system show that atomic transport during Pd₂Si growth is several orders of magnitude faster than at equilibrium without interfacial reaction.

Abstract: Kinetics and structural evolution of the intermediate phase layer formation during reactive diffusion is revisited. Main new input is an account of limited efficiency of vacancy sinks/sources at the moving interfaces and in the bulk, leading to nonequilibrium vacancy concentration gradients. Competition of the two types of vacancy sinks/sources is studied – K-sinks (providing Kirkendall lattice shift) and F-sinks (Frenkel-Kirkendall voids formation). Reactive diffusion with limited vacancy sources/sinks power in isothermal regime as well as in adiabatic SHS-regime is considered.

Abstract: The growth of phases by reactive diffusion in Mo-Si and W-Si systems are compared. The crystal structures of MSi₂ and M₅Si₃ phases (M = Mo, W) are similar in these two systems. However, the diffusion rates of the components change systematically with a change in the atomic number. Integrated diffusion coefficients in both phases increase with an increasing atomic number of refractory elements i.e. from Mo to W. On the other hand, the ratio of diffusivities of the components decreases. This indicates a relative increase in the diffusion rates of the metal components with increasing atomic number and a difference in defects concentrations in these two systems.

Abstract: The problem of phase competition during reactive diffusion is revisited. Nucleation of an intermediate phase at an interface under external fluxes in the neighboring phases is considered in the frame of kinetic Fokker-Planck approach. Effective nucleation barrier depending on the divergence of external fluxes is introduced. New suppression/growth criteria are obtained.

Abstract: An alternative solution for producing logic devices in microelectronics is spintronics (SPIN TRansport electrONICS). It relies on the fact that in a magnetic layer, the electrical current can be spin polarized. To fabricate such components, a material whose electronic properties depend on its magnetic state is needed. The Mn-Ge system presents a lot of phases with different magnetic properties, which can be used for spintronics. The most interesting phase among the Mn-Ge system is Mn₅Ge₃ because of its stability at high temperatures, its Curie temperature which is close to room temperature and its ability of injecting spin-polarized electrons into semiconductors. In this paper, we have combined Reflection High-Energy Electron Diffraction (RHEED) and X-ray Diffraction (XRD), to study the sequence of formation of Mn_xGe_y phases during reactive diffusion of both a 50 nm and a 210 nm thick Mn films deposited by Molecular-Beam Epitaxy (MBE) on Ge (111).

Abstract: The use of nanometric size materials as embedded clusters, nanometric films, nanocrystalline layers and nanostructures is steadily increasing in industrial processes aiming to produce materials and devices. This is especially true in today Si-based microelectronics with transistors made of a multitude of different thin film materials (B-, As-, and P-doped Si, NiSi (Pt), poly-Si, W, TiO_x, LaO, SiO₂, Al, H_fO₂), and exhibiting a characteristic lateral size of 32-22 nm. Size reduction leads to an increasing role of surfaces and interfaces, as well as stress and nanoscale effects upon important phenomena driving fabrication processes, such as atomic diffusion, phase nucleation, phase growth, and coarsening. Consequently, nanotechnology related to Material Science requires an investigation at the nanometric (or atomic) scale of elementary physical phenomena that are well-known at the microscopic scale. This paper is focused on nanosize effects upon diffusion in Si and Si reactive diffusion. We present recent results showing that the kinetic of lattice diffusion is enhanced in semiconductor nanometric (nano) grains, while grain boundary (GB) diffusion is not changed in nanoGBs. It is also shown that diffusion in triple-junction (TJ) is several orders of magnitude faster than GB diffusion, and that its effect cannot be neglected in nanocrystalline (nc) layers made of 40 nm-wide grains. Experimental results concerning Si sub-nanometric film reaction on Ni (111) substrate are also presented and compared to theoretical results giving new prospects concerning nanosize effects on reactive diffusion at the atomic scale.

Abstract: The formation of intermetallic compounds (IMC) at the solder-substrate interface is required to initiate the metallurgical bond. However, rapid growth of IMCs may degrade joint strength through i) the increased presence of a low toughness phase, ii) the consumption of the solderable surface (void formation) and iii) generation of primary and secondary stresses. Knowledge of mass transport and reaction processes during joint formation and service life are essential for solder system design. The mathematical description of inter-and reactive diffusion in open system presented here is based on Darken method (bi-velocity), involving the different molar volumes in the system and Wagner boundary conditions. It combines the interdiffusion, reactive diffusion and the effective flux constraints to couple processes occurring at different time scales. The rCADiff software serves as a tool to simulate simultaneous growth of the two Cu-Sn IMCs.

Abstract: Problems of reactive diffusion at the solid phase and melt contact are studied theoretically. The rate constant is a fundamental parameter characterizing the dissolving rate at a certain configuration of experiment. Relationships between the solid phase dissolving rate, i.e. the solid phase interface boundary movement in the melt, and rates of growth of intermetallic phases in the metal (Cu) are observed. This procedure enables the creation of surface and subsurface layers of regulated thickness in metallic materials by means of reactive diffusion. The main intention was an experimental study of copper dissolving in melts of various solder alloys and the related reactive diffusion. We used Sn, SnCu, SnAgCu, SnZn and SnIn alloys as a solder material. The problems that need to be solved preferentially are emphasized. It concerns especially the determination of the rate constant of dissolving and verifying whether the proposed model equations can be used for this constant determination in cases of cylindrical and planar dissolving. Rapid growth of phases in the metal (Cu) and determination of the thickness of layers with these phases pose considerable time demands to X-ray microanalyses (WDX, EDX, BSE, SEM) of specimens after their long-time heating.