3D sequential integration of CMOS transistors, which consists in stacking transistors on top of one another, has received a lot of attention in recent years. Top level transistor process temperatures have then to be drastically reduced in order to avoid any degradation of the bottom transistor performances and preserve interconnects. Selective epitaxy, which is used for the formation of the in-situ doped raised sources and drains of the top level transistor, has then to be conducted at temperatures around 500°C (i.e. 150°C to 250°C below that of conventional processes).

With conventional silicon precursors such as silane (SiH₄) or, to a lesser extent, disilane (Si₂H₆), the epitaxial growth rate of Si is not high enough at 500°C and below^[1]. We present here our latest results on the epitaxy of Si, SiGe and SiGe:B layers in the 450°C to 550°C range, with a Liquid Silicon (LS) source which is benchmarked against Si₂H₆. Digermane (Ge₂H₆) and Diborane (B₂H₆) were used as the precursors of Germanium (Ge) and Boron (B). A 200mm Reduced-Chemical Vapor Deposition (RP-CVD) cluster tool was used for this study. A liquid precursor gas delivery system was connected to our RP-CVD chamber, with a pressure and temperature controlled bubbler used to transform our liquid silicon source into a controlled gas flow.

Surface roughness was measured with Atomic Force Microscopy (AFM). X-Ray Reflectivity and X-Ray Diffraction were used to determine layer thickness and germanium content. The atomic composition of the layers was determined with Secondary Ion Mass Spectrometry (SIMS). Finally, four Point Probe (4PP) was used to measure the layer resistivity. Si layers were grown on several tens of nanometers thick SiGe marker layers on Si (001), while SiGe and SiGeB layers were grown directly on n-type Si(001).

Figure 1 shows as expected an exponential increase of the Si growth rate with temperature, with a 37 kcal.mol^-1 activation energy. At 550°C, 20 Torr, the Si growth rate is ten times higher with LS than with SiH₄ (for a Si atomic flow ~ 9 times lower). Figure 2 shows the growth rate of SiGe at 500°C, 20 Torr, as a function of the germanium concentration (fixed Si₂H₆ and LS flows, variable Ge₂H₆ flows). We observe a slightly over linear increase of the SiGe growth rate with the Ge content for both chemistries. This increase is due to the catalysis of H desorption from the growing surface by Ge atoms. For the same growth rate, the Ge content is significantly lower with LS than with disilane, although the atomic Si flow was less than half (1.15x10^-3 for LS, versus 2.50x10^-3 with disilane). The sub-linear increase of the Ge content with the Ge₂H₆ flow was fitted with x²/(1-x) = n x F(Ge)/F(Si) relationships, with n=0.50 (LS) and 2.19 (disilane). For a given Ge₂H₆ flow, the SiGe growth rate was otherwise barely higher with LS than with Si₂H₆. Figure 3 shows an exponential increase of the SiGe growth rate with the temperature (in the 450°C–550°C range). A 10.5 kcal.mol^-1 activation energy was extracted. There is also a significant Ge content decrease as temperature increases, with a -1.95%/10°C slope. These values are comparable to those obtained with a Si₂H₆+Ge₂H₆ chemistry, E_a=14.8 kcal.mol^-1 and -2.25%/10°C^[2].

Figure 4 shows that, at 500°C, 20 Torr, the SiGeB growth rate increases for high B₂H₆ mass-flows, as with other chemistries. A significant drop of the apparent Ge concentration from XRD is also noticed as the B₂H₆ flow increases. This is due to (i) compressive strain compensation by small Boron atoms and (ii) a drop in the atomic Ge content. Figure 5 shows a linear increase of the Boron atomic concentration with the B₂H₆ mass-flow (x_B/(1-x_B) = 4.0 x F(Ge)/(F(Si)+F(Ge)) and (ii) an atomic Ge content which is stable then goes down as the B₂H₆ mass-flow goes up. As expected from strain compensation, atomic Ge content from SIMS is higher than apparent Ge concentration from XRD.

As the B₂H₆ flow increases, we observe in Figure 6 a SiGeB resistivity decrease, a stabilization at ~0.3 mOhm.cm then a re-increase as the layer becomes polycrystalline (confirmed by XRD). AFM showed that SiGe:B surfaces were rather flat, with small islands which disappeared when the B₂H₆ flow became high. This could be due to a lower compressive strain in high boron concentration layers. XRD profiles were however characteristics of pseudomorphic layers with a high crystalline quality.

[1] J.M. Hartmann et al., Thin Solid Films 520, 3185 (2012).

[2] J.M. Hartmann et al., ECS Trans. 75 (8), 281 (2016).