Abstract
We characterize composition and structure of ultrathin nickel silicide during formation from 3 nm Ni films on Si(100) using in-situ high-resolution ion scattering and high-resolution transmission electron microscopy. We show the transition to occur in discrete steps, in which an intermediate phase is observed within a narrow range of temperature from 230 °C to 290 °C. The film composition of this intermediate phase is found to be 50% Ni:50% Si, without evidence for long-range structure, indicating the film to be a homogeneous monosilicide NiSi phase. The final phase is resemblant of the cubic disilicide NiSi2, but with slightly off-stoichiometric composition of 38% Ni and 62% Si. Along the [100] axis, the lattices of the film and the substrate are found in perfect alignment. Due to the epitaxial growth of the silicide, a contraction of the c lattice constant of the film by 0.7–1% is detected.
Highlights
Nickel (Ni) silicide is an important local contact material for silicon (Si) transistors due to its low resistivity, immunity to the fine-line effect and low Si consumption [1]
The normal phase transition sequence and the formation temperature of nickel silicides formed from Ni films with thickness >4nm is δ-Ni2Si (~250 °C) → nickel monosilicide (NiSi) (~350 °C) → NiSi2 (~800 °C) [1,2]
As compared to the 260°C spectrum, the backscattering spectrum for the 290°C in Fig. 1b is almost similar, except for a small shoulder and broader energy distribution at the lowenergy edge of the Ni peak. This observation suggests that while interdiffusion from Ni and Si can be observed from 230°C, a clear crystal lization of the silicide film starts to occur at the interface at the an nealing temperature of 290°C
Summary
Nickel (Ni) silicide is an important local contact material for silicon (Si) transistors due to its low resistivity, immunity to the fine-line effect and low Si consumption [1]. The normal phase transition sequence and the formation temperature of nickel silicides formed from Ni films with thickness >4nm is δ-Ni2Si (~250 °C) → NiSi (~350 °C) → NiSi2 (~800 °C) [1,2]. The intermediate phase, nickel monosilicide (NiSi), is usually desirable for high-perfor mance devices. Several studies have suggested that the transition of the Ni-Si system is more complex than the thermodynamic sequence because a metastable silicide, namely the θ phase, plays an important role in the transition. Due to texture inheritance from the δ-Ni2Si phase it is metastable at around 300 °C. The presence of the θ phase facilitates the transition to the NiSi phase because this process requires little movement of the atoms from their equili brium positions [3,4]
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