Dichlorosilane (SiH₂Cl₂) is essential for the Selective Epitaxial Growth of Si and SiGe in the windows of patterned wafers. However, it is plagued by relatively slow growth rates at low temperatures. Meanwhile, hydrogenated silicon precursors (such as silane (SiH₄), disilane (Si₂H₆) or trisilane (Si₃H₈)), yield significantly higher Si and SiGe Growth Rates (GR) at low temperatures. Such precursors are intrinsically non selective versus dielectric masking materials, but this difficulty can be overcome with advanced (Cyclic) Deposition / Etch processes.

Liquid Si precursors such as trisilane have however several drawbacks: (i) they are expensive to produce with electronic-grade quality; (ii) being liquid, they require dedicated bubblers to be delivered into the growth chamber. Taking full advantage of their decomposition at very low temperatures is also not always possible, as the resulting films might be of lesser crystalline and electronic quality (gas phase nucleation and/or an adatom surface mobility which is too low).

By contrast, disilane is gaseous and gives higher GR than silane (the Si-Si bond strength, 33 kcal mol.^-1, is indeed weaker that the Si-H bond strength, 47 kcal. mol^.-1). With the proper process parameters, it is possible to have reasonable growth rates down to 450°C only for SiGe(:B). It is thus a good choice of silicon precursor for the low temperature growth of Si and SiGe layers. This is illustrated in the following, where we have benchmarked SiH₄ and Si₂H₆ against SiH₂Cl₂ for the low temperature growth of Si and SiGe in 300mm industrial Reduced Pressure – Chemical Vapour Deposition (RP-CVD) chambers from major Suppliers A and B.

Si growth rates are, for T < 575°C, approximately ten times higher with Si₂H₆ than with SiH₄, which are in turn roughly ten times higher than with SiH₂Cl₂ (see Figure 1). Activation energies, 49-52 kcal. mol.^-1 (SiH₄), 51-56 kcal. mol.^-1 (Si₂H₆) or 60-61 kcal. mol.^-1 (SiH₂Cl₂), are in-between the Si-H and Si-Cl bond strengths. For given GeH₄ and Si precursor mass-flow ratios, lower Ge contents and significantly higher SiGe growth rates are obtained at 550°C, 20 Torr with Si₂H₆ than with SiH₄ and especially SiH₂Cl₂ (see Figure 2). The dependency of the Ge content x on the F(GeH₄) / F(Si precursor), linear for a SiH₄ - based chemistry (x / (1-x) = 2.27–2.50 * F(GeH₄) / F(SiH₄)), is parabolic for Si₂H₆ and SiH₂Cl₂ based chemistries (x² / (1-x) = 0.51–0.58 * F(GeH₄) / 2*F(Si₂H₆) and x² / (1-x) = 2.22-2.37 * F(GeH₄) / F(SiH₂Cl₂), respectively). The associated SiGe GR increase linearly with the GeH₄ mass-flow irrespective of the Si precursor used.

With SiH₄ and SiH₂Cl₂, growth rates (Ge concentrations) are typically lower (slightly lower) in Supplier A than in Supplier B chamber. The situation is the opposite with Si₂H₆. Growth rates (and Ge contents) are then higher in the former reactor than in the latter. This is assigned to (i) a ~ 5°C offset between the two and (ii) effective precursor flows which are different (likely due to chamber geometry differences). Growth rate activation energies and relationships linking Ge concentration to precursor mass-flow ratios are nevertheless similar, as shown in Figure 3 for the SiH₂Cl₂ + GeH₄ chemistry and various temperatures. Process transfer between the two should be rather easy,

We have otherwise assessed the interest of having an in-situ surface preparation chamber connected to the N₂-purged transfer chamber of our cluster tool on interfacial contamination and epitaxial quality. There are indeed two ways of removing native oxide on a Si wafer: (i) thanks to an ex-situ “HF-last” wet cleaning followed by a short queue time loading in the N₂-purged load-locks or (ii) thanks to an in-situ exposure of the wafer surface to a remote NH₃/NF₃ plasma which transforms native oxide into a salt which is sublimated at low temperature. As the oxide-free surface is exposed for minutes up to tens of minutes to the air in the former case, while it stays under N₂ in the latter case, we would expect the second to be better.

We have therefore compared both processes for Si surface preparation prior to epitaxy. Surfaces are with the in-situ process always under high purity N₂. This results in a threshold H₂ bake temperature (above which there is no O interfacial contamination anymore) which is shifted downwards by ~ 25°C (from 775°C down to 750°C). Below that temperature, O sheet concentrations are with the in-situ process typically one third those associated with “HF-Last” wet cleanings and epitaxial surfaces are smoother, as shown in Figure 4.