Investigating the formation of ultra-thin Sc silicide layers by medium and low energy ion scattering

Metal silicides (i.e., compounds of a metal with Si) are widely used as contact materials in CMOS devices. Moreover, they provide an ideal platform for studying thin-film reactions, whereby ion beam analysis (IBA) has been playing a key role throughout the years. As device dimensions aggressively shrink and film thicknesses approach only several nanometer, a number of new challenges emerge. Firstly, the formation of such very thin films is to a large extent governed by surface and interface effects. Moreover, transient (metastable) phases can play an important role – and despite their transient nature determine the properties of the final silicide layer. Therefore, in order to understand the formation mechanisms of ultra-thin silicides, one has to focus on the initial stages of the reaction, i.e., the very first nanometers. However, resolving the various reaction stages requires significantly improved depth resolution. Whereas Rutherford backscattering spectrometry has played a paramount role in silicide studies in the past, taking advantage of the drastically enhanced depth resolution of medium and low energy ion scattering (MEIS and LEIS) is the preferred approach for the current thickness regime [1, 2].

In line with current evolutions in device research, our study focuses on the formation of ScSi, which has been reported to be an excellent material for lowering the contact resistance on to phosphorus-doped Si, as used in NMOS technology [3, 4]. The use of scandium, however, brings along yet another challenge: the extremely high reactivity of the rare-earth like metal requires the use of a protective cap, and/or performing the layer growth and its analysis in situ.

In this work, 15 nm Sc thin films were sputter-deposited and capped with 10 nm TiN without breaking the vacuum, using either bare Si(100) substrates or Si(100) with a 40 nm, highly P-doped epitaxial Si:P layer (3%P), known to improve contact resistivity [3]. The exact film thickness and composition/purity were confirmed by Rutherford backscattering spectrometry (using 2 MeV He ions), heavy ion elastic recoil detection analysis (using 36 MeV I⁸⁺ for recoil generation), and transmission electron microscopy with energy-dispersive spectroscopy. Silicide formation was studied after ex situ annealing in forming gas (N₂ + 5% H₂) between 310°C and 450°C using time-of-flight medium energy ion scattering (MEIS) with a pulsed 60 keV He⁺ beam [5]. Alternatively, the formation of 3 nm thick films was investigated in situ by time-of-flight low energy ion scattering (LEIS) using a 4 keV He⁺ beam. LEIS spectra were collected both with or without applying a 2 kV post-acceleration voltage.

Already in the as-deposited state, a thin intermixed Sc–Si interfacial layer is detected. This layer is amorphous and grows during the initial annealing stages, indicating solid-state amorphization. With increasing temperature, a gradually thicker (amorphous) silicide layer develops (Fig. 1). From the MEIS analysis, it is deduced that this silicide is slightly off-stoichiometric compared to ScSi. For both substrate types, growth proceeds in a layer-by-layer fashion, consistent with diffusion-limited kinetics. Via isothermal annealing (as a function of annealing duration) at various temperatures, the activation energy for diffusion is deduced. In the case of reaction with a Si:P layer, phosphorus redistributes non-uniformly during the reaction. Additionally, P incorporation leads to a slight reduction of the reaction rate (compared to pure Si), most pronounced at higher annealing temperatures.

Fig.1 60 keV He MEIS from <Si>/Si:P(3 at.%)/Sc/TiN as a function of annealing temperature.

Quantitative analysis of MEIS spectra is challenged by the lack of reference stopping power data for He in Sc, in particular for this energy range [6]. Therefore, dedicated measurements were performed over a broad energy range. Preliminary results indicate that the stopping power at 60 keV is approximately 20-25% higher than predicted by SRIM, which allows accurate analysis of the MEIS data.

Since a capping layer is not compatible with LEIS investigations, we also endeavored in in situ Sc deposition from a high-purity Sc rod, using electron bombardment. Films with a thickness of approximately 3 nm have been deposited on both types of substrates (bare Si and P-doped Si). For each substrate, two cleaning procedures were used, i.e., a standard ex situ chemical cleaning including HF dip and an in situ procedure consisting of repeated cycles of sputtering/annealing. After Sc deposition, the layers were in situ annealed to induce silicide formation. LEIS backscattering allowed to disentangle the overall reaction, whereas polar and azimuthal scans show that the silicides are epitaxially aligned with the Si substrate (Fig. 2). The epitaxial lattice match is further confirmed by high-resolution TEM.

Fig.2 4 keV He LEIS polar scans from annealed <Si>/Sc samples. Two types of substrates are used (bare Si, i.e., non-doped, and P (3 at.%) doped Si), as well as two cleaning procedures (ex situ HF dip vs. in situ sputter/annealing cycles). Pronounced minima are observed at 0°, 45° and 90° in the silicide signal as well as the substrate signal, consistent with epitaxial alignment.

References.

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[6] https://www-nds.iaea.org/stopping