Atmospheric Pressure Selective Epitaxial Growth of Heavily in-situ Phosphorous-Doped Si(:C) Raised Sources and Drains
Authors: Hartmann Jean-Michel , Aubin Joris , Barraud Sylvain , Samson Marie-Pierre 
Published: February 2020

Raising the sources and drains regions of Fully Depleted Silicon-On-Insulator devices is mandatory in order to have enough material for silicidation and obtain low contact resistances. Doping was until the 28 nm technology node carried out using ion implantation. For a variety of reasons, in-situ doping is mandatory in future node transistors, however. We have thus focused on the in-situ phosphorous doping of the Si Raised Sources and Drains (RSDs) used in n-type devices. We have shown in [1] that reduced pressures (e.g. 20 Torr) were harmful in order to obtain at 750°C-800°C high P+ ions concentrations with reasonable Si:P Growth Rates (GR). With SiH2Cl2 + PH3 + HCl, we indeed saturated at [P+] around 1019 cm-3, with a surface poisoning by P atoms that drastically decreased the Si:P GR. Wanting to have a straightforward Si:P co-flow process instead of a more complex Cyclic Deposition/Etch process [2], we have thus studied the in-situ phosphorous doping of Si and Si1-yCy at Atmospheric Pressure (ATM) and in the 700°C – 800°C range.

To that end, we have proceeded as follows. In order to gain access to the Si1-yCy:P layer thickness and confirm that layers were indeed single crystalline, we have grown {Si0.8Ge0.2 / Si1-yCy:P} stacks on Si(001) and measured them with X-Ray Reflectivity (which gives access to thickness) and X-Ray Diffraction (crystalline quality assessment), as shown in Figure 1. We have otherwise grown the very same Si1-yCy:P layers on slightly p-type substrates to have pn junctions and determine through four point probe measurements their sheet resistances. Resistivities and thus P+ ions concentrations were extracted by multiplying those by the Si1-yCy:P layer thicknesses (from XRR). We have plotted in Figure 2 the Si:P growth rate and [P+] at 750°C, ATM, this for increasing F(PH3)/F(SiH2Cl2) Mass-Flow Ratios (MFRs). When the SiH2Cl2 mass-flow was fixed, we had a significant increase of the Si:P GR (up to 13.3 nm/min.) and of [P+] (at most 7x1019 cm-3). We have tried to reach higher MFRs by reducing, for the highest PH3 1% in H2 flow deliverable in our tool, the SiH2Cl2 mass-flow. This led to (i) a Si:P growth rate decrease and (ii) to a slight [P+] decrease, instead of the increase hoped for. We have studied in Figure 3 the impact of growth temperature on the Si:P GR and on [P+] for the optimum point of Fig. 2. The activation energy associated with the Si:P growth rate increase with temperature in the 700°C-800°C range was equal to 53 kcal. mol.-1 (e.g. close to the Si-H bond strength), while [P+] was steady at 7x1019 cm-3.

We have thus moved over to a PH3 5% in H2 bottle and quantified, at 700°C, ATM, the impact of the PH3 flow on the Si:P growth rate and n-type doping for fixed SiH2Cl2 and HCl mass-flows. We have in Figure 4 a sharp increase, a stabilization then a Si:P GR decrease as the PH3 flow increases, which might be due to surface poisoning. Such a behaviour is associated with a P+ ion concentration that sharply increases then reaches a plateau close to 1020 cm-3.

We have used such a co-flow processes to thicken at 725°C, ATM the sources and drains regions of stacked Si nanowires with 50 nm of Si:P. The surface of those RSDs was slightly rough (as for Si:P layers on blanket substrates). Full selectivity versus SiO2 (Buried OXyde) and SiN internal spacers was achieved, however, as illustrated by the SEM images of Figure 5.

Finally, we have studied the feasibility of adding SiCH6 to the gaseous mixture in order to have C atoms that would supress B diffusion (p+ pockets to control short channel effects in bulk nMOS transistors) and hopefully inject tensile strain in short Si channels (if the substitutional C content is high enough). Because of the rather high temperature (700°C), we were not able to obtain [Csubst.] above 0.55%, as shown by Figure 6 XRD profiles. We were otherwise faced with a Si1-yCy:P GR that decreased and a layer resistivity that increased as the SiCH6 mass-flow increased (competition between P and C atoms for incorporation in the lattice and electron mobility degradation because of the small size of the C atoms), as shown in Figure 7. The best trade-off was achieved for a F(SiCH6)/F(PH3) MFR close to unity.

[1] J.M. Hartmann et al, J. Cryst. Growth 264 (2004) 36 and J. Cryst. Growth 310 (2008) 62.

[2] J.M. Hartmann et al., Semicond. Sci. Technol. 28 (2013) 025017 and 025018.

Figure 1