Selective Epitaxial Si:P Film for nMOSFET Application: High Phosphorous Concentration and High Tensile Strain

Semiconductor industry is in the era of transition from 2D transistors to 3D transistors (for example, FinFETs). The scaling–down of transistor source/ drain (S/D) contact area causes more challenges in reducing the S/D parasitic resistance which becomes comparable to (or even higher than) the channel resistance itself. A highly phosphorous doped Si epitaxial film on S/D is crucial to reduce the parasitic resistance in nMOSFET transistors.  Besides that, 2D/3D nMOSFET transistors favor the tensile strain induced in the channel to enhance the electron channel mobility. Here we present a selective Si:P epitaxial film growth which provides both high phosphorous concentrations (>1E+21 at/cc) and high tensile strain (comparable to ~1 at.% of C_sub in Si:CP).

This selective Si:P epitaxial process was performed using dichlorosilane (DCS), phosphine (PH₃), and hydrochloride (HCl) gases in Applied Materials Centura RP Epi system. Grown highly concentrated, highly tensile-strained Si:P (called HS Si:P) films were analyzed by techniques of high-resolution XRD (HR-XRD), four-point Rs probe, SEM, TEM, and SIMS, etc.

Table 1 compares two types of Si:P epitaxial films: conventional Si:P Epi versus HS Si:P Epi. In Fig. 1(a), the total [P] by SIMS in HS Si:P epitaxial film is 1.75E+21 at/cc (~ 3 at.% in silicon), about one magnitude order higher than in conventional Si:P film, much higher than the solid solubility (~3E+20 at/cc) of phosphorous in silicon at 700°C [1]. The 0.6 mΩ-cm resistivity in HS Si:P epitaxial film indicates that only ~1.3E+20 at/cc phosphorous atoms are electrically active. In Fig. 1(b), the HR-XRD profile from HS Si:P film shows a strong tensile strain equivalent to ~0.8 at.% C_sub from a Si:CP film. We assume that majority of phosphorous atoms are covalently bonded with adjacent Si atoms in a stable Si-P compound phase –pseudocubic Si₃P₄ which is energetically favored relative to other Si₃P₄ phases [2]. With the Vegard’s law (of linear relationship between lattice parameter and alloy concentration) applied between Si and pseudocubic Si₃P₄ (which has a smaller lattice constant than Si)_, the tensile strain induced in HS Si:P film matches well the HR-XRD data, as previously reported by Z. Ye et. al.[3].

Fig. 2 plots out selective HS Si:P epitaxial process sensitivity to growth temperature (675-775°C) regarding resistivity, strain, and total [P]. HS Si:P epitaxial films are stable without obvious phosphorous out-diffusion in this temperature range, which is confirmed by SIMS measurements. Both total [P] level and tensile strain decrease with the increasing temperature. Meanwhile, resistivity drops from 0.7 mΩ-cm @675°C to 0.5 mΩ-cm @725°C as more phosphorous atoms are electrically activated at higher temperature.

Fig. 3 characterizes the HS Si:P film epitaxial growth on the (110) orientated substrate. The HR-XRD profile in Fig. 3(a) indicates a well-ordered HS Si:P epitaxial film grown on the (110) substrate. The TEM image in Fig. 3(b) shows a HS Si:P epitaxial film grown on (110) substrate without defects at interface, which is very significant for epitaxial growth around non-planar structures. Furthermore, Fig. 4(a) and (b) present two high-quality HS Si:P films with few defects, epitaxially grown on a planar structure and a Si fin structure, respectively.

HS Si:P epitaxial films have been studied after the millisecond annealing treatment @900-1300°C in Fig. 5. After annealing, resistivity drops from 0.65 mΩ-cm @1050°C to ~ 0.3 mΩ-cm @1150°C and above. Tensile strain in film is stable up to 1200°C. At 1250°C and above, HS Si:P epitaxial film obviously starts to partially lose the strain likely because some phosphorous atoms are released from the structure of pseudocubic Si₃P₄ at such high temperatures.

Overall, this selective HS Si:P epitaxial process demonstrates its great potential in 2D/3D nMOSFET application.