Solid-state interdiffusion reactions at Al/Ni interfaces in multilayer films have been studied using differential scanning calorimetry, cross-sectional transmission electron microscopy/microanalysis, and thin-film x-ray diffraction. Multilayer films with various modulation periods and an overall atomic concentration ratio of three Al to one Ni were prepared by alternate electron-beam evaporation in high- and ultrahigh-vacuum systems. We show calorimetric, microstructural, and compositional evidence that interdiffusion of Al and Ni leading to solid solutions precedes the formation of intermetallic crystalline compounds. Isothermal calorimetry indicates that Al3Ni subsequently nucleates in the interdiffused region at preferred sites. Calorimetric analyses also suggest that nucleation sites quickly saturate in the early stage of Al3Ni formation and that the nucleation site density strongly depends on the grain sizes of the deposited films. After coalescence into a continuous layer at the interface, Al3Ni thickens through a diffusion-limited process, in agreement with previous reports. A kinetic model is developed which yields calculated calorimetric traces in good agreement with experimental data. Our results suggest the importance of prenucleation interdiffusion, in addition to nucleation, in the selection of the first phase during thin-film reactions.

Self-propagating explosive reactions, with a reaction front speed of about 4 m/s, have been observed in free-standing polycrystalline Al/Ni multilayer thin films. The resultant phases and microstructures are compared with those obtained by conventional thermal annealing. We show evidence which indicates that melting occurred in the explosive reactions of films with an atomic concentration ratio of 3Al:1Ni. It is also observed that the propensity of multilayer films to undergo explosive reactions is dependent on the modulation length of the film as well as on the ambient temperature. These observations are interpreted with a simple model based on the rate balance between the rates of heat generation and heat dissipation.

The reaction between solid layers to form a product phase has been studied using scanning calorimetry of multilayer Nb/Al and Ni/amorphous-Si thin films. The most striking feature for both materials systems is the occurrence of two maxima in the reaction rate during the formation of a single product phase, suggesting a two step growth process. A model has been developed in which the first step is taken to be the nucleation and two-dimensional growth to coalescence of the product phase, in the plane of the initial interface. The second step is taken to be the thickening of the product layer by growth perpendicular to the interface plane. The success of this simple model in describing the principal features of the experimental results on two different materials systems suggests that nucleation is an important aspect of phase formation and selection in these thin-film reactions.

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We report that the ion implantation of a small dose of Mo into a silicon substrate before the deposition of a thin film of Ti lowers the temperature required to form the commercially important low resistivity C54 -TiSi 2 phase by 100-150°C. A lesser improvement is obtained with W implantation. In addition, a sharp reduction in the dependence of C54 formation on the geometrical size of the silicided structure is observed. The enhancement in C54 formation observed with the ion implantation of Mo is not explained by ion mixing of the Ti/Si interface or implant-induced damage. Rather, it is attributed to an enhanced nucleation of C54 -TiSi 2 out of the precursor high resistance C49-TiSi 2 phase.

We demonstrate that the temperature at which the C49 TiSi2 phase transforms to the C54 TiSi2 phase can be lowered more than 100 °C by alloying Ti with small amounts of Mo, Ta, or Nb. Titanium alloy blanket films, containing from 1 to 20 at. % Mo, Ta, or Nb were deposited onto undoped polycrystalline Si substrates. The temperature at which the C49–C54 transformation occurs during annealing at constant ramp rate was determined by in situ sheet resistance and x-ray diffraction measurements. Tantalum and niobium additions reduce the transformation temperature without causing a large increase in resistivity of the resulting C54 TiSi2 phase, while Mo additions lead to a large increase in resistivity. Titanium tantalum alloys were also used to form C54 TiSi2 on isolated regions of arsenic doped Si(100) and polycrystalline Si having linewidths ranging from 0.13 to 0.56 μm. The C54 phase transformation temperature was lowered by over 100 °C for both the blanket and fine line samples. As the concentration of Mo, Ta, or Nb in the Ti alloys increase, or as the linewidth decreases, an additional diffraction peak appears in in situ x-ray diffraction which is consistent with increasing amounts of the higher resistivity C40 silicide phase.

As the minimum VLSI feature size continues to scaie down to the 0.1-0.2-ju.m regime, the need for iow-resistance iocai interconnections wiii become increasingiy critical. Although reduction in the MOSFET channel length will remain the dominant factor in achieving higher circuit performance, existing local interconnection materials will impose greater than acceptable performance limitations. We review the state-of-the-art salicide and polycide processes, with emphasis on work at IBM, and discuss the limitations that pertain to future implementations in high-performance VLSI circuit applications. A brief review of various silicide-based and tungsten-based approaches for forming local interconnections is presented, along with a more detailed description of a tungsten-based "damascene" local interconnection approach.

Self-propagating explosive reactions can occur in multilayer thin films. Explosive silicidation in nickel/amorphous-silicon multilayer thin films has been investigated using a combination of high-speed photography, high-speed temperature measurements, plan-view transmission electron microscopy, and thin film x-ray diffraction. The multilayer films had an atomic concentration ratio of 2 Ni atoms to 1 Si atom. The silicide phase formed by explosive silicidation was Ni2 Si. This was the same phase formed by conventional thermal annealing of the multilayer thin film. The temperature of the explosive reaction front was measured to be approximately 1565 K. The reaction-front velocity was found to vary from 22 to 27 m/s and to be at most weakly dependent on the modulation period and the total film thickness. The resulting Ni2 Si grain structure formed by explosive silicidation is less defective than Ni2 Si formed by conventional thermal annealing. This was attributed to the higher reaction temperatures and the shorter reaction times of explosively formed Ni2 Si as compared to Ni2 Si formed via conventional annealing.