(Invited) Low Temperature SiGe:B Epitaxy for Next-Generation Logic Devices
Authors: Muehlenbein Lutz , D'Costa Vijay , Casey Daniel , Khazaka Rami 
Published: December 2024

Epitaxy at low temperatures (< 400°C) will be essential to enable advanced node CMOS logic architectures such as gate-all-around and complementary field effect transistors [1]. Beyond complying with the related thermal budget limitations, the non-thermal-equilibrium conditions drastically increase the attainable active doping level in epitaxially grown source/drain (S/D) materials. This is a crucial benefit for minimizing the contact resistivity ρC between S/D and contact metal, which, with shrinking device dimensions, becomes the major contributor to parasitic resistance [1]
. Another strategy for ρC reduction in p-type S/D is reducing the Schottky barrier height (SBH), for which SiGe:B layers with very high Ge content, up to Ge:B, have been considered [2]. In this work, we report on achieving unprecedented active doping NA = 4.2×1021 cm-3 (Hall Scattering Factor [HSF] = 1) with Ge contents as high as 80% - a dual benefit for ρC reduction. In this context, we furthermore demonstrate two in-line methods for non-contact and non-destructive characterization of composition and electrical parameters in highly doped SiGe:B thin films.

We first discuss the general benefits of low temperature epitaxy, highlighting the improvements in attainable active doping and layer resistivity depicted inFig. 1. We then demonstrate how the high concentration of dopants enables metrology techniques for characterization of composition and electrical parameters of low-temperature SiGe:B thin films, that were so far mainly used for thicker layers or different material systems: Wavelength-dispersive x-ray fluorescence (WDXRF) and Fourier-transform infrared spectroscopy (FTIR). Fig. 2 demonstrates how the WDXRF detection of Ge was calibrated with SiGe films and transferred to SiGe:B. Fig. 3 demonstrates the application of the WDXRF for LT-SiGe:B films, and the direct detection of B concentration by XRF. In Fig. 4, the results of FTIR measurements based on a Drude dispersion model approach are shown for layer resistivity and NA. Systematic differences between the active doping are explained by respective assumptions of HSF equal to 1 and the effective hole mass as 0.37me. FTIR shows better agreement with the substitutional B concentration as extracted from x-ray diffraction, calculated from a three-component Vegard’s law assuming aB = 3.806 Å, as displayed in Fig. 5.

We proceed to demonstrate the usefulness of WDXRF and FTIR in process development. We previously reported on low-temperature selective epitaxial growth of SiGe:B with Ge = 60%, NA = 2.9×1021 cm-3 (Hall, HSF=1) with resulting contact resistivity below 5×10-10 Ω-cm2 measured by ladder transmission line method [3

,4], based on conventional precursors. In a fundamental change to the process, we introduced novel precursor material. Fig. 6 demonstrates, for a similar Ge content around 70%, the drastic increase of total incorporated B per supplied B precursor when using the new precursor configuration by use of WDXRF. The reduction in ‘apparent’ Ge (strain compensated by B) content, reduction in attainable resistivity and increase in NA measured by FTIR prove enhanced substitutional (active) Boron concentration. The NA as of Hall measurement is also drastically increased in absolute value, but in saturation, which may be explained by the HSF increasing for high concentrations of B [5]. The newly developed process exhibits drastic enhancement of achievable NA in conjunction with increased Ge concentration of the layer up to 80% compared to previously reported process types [1,3

,4], as shown in Fig. 7. Fig. 8 displays the relation between NA and hole mobility (HSF=1) for the process types involving the new precursor, and conventional precursors, proving that electrical conductivity is generally improved with introduction of the new precursor.

Reciprocal space maps for two ~15nm thick layers with Ge = 80% in Fig. 9 prove the transition from initial signs of relaxation to a fully strained layer by doubling the B precursor flow, while SIMS results show a 1.7x increase in incorporated B, highlighting the strain compensation effect of B. A transmission electron micrograph demonstrates perfect quality of the 15nm thick SiGe:B thin film, and defect-free substrate-to-film interface.

Finally, fully selective growth on a full patterned wafer with a Si/SiNx/SiO2 structure is depicted in a transmission electron micrograph in Fig. 10.

[1] C. Porret et al., 2022 IEDM, 34.1.1-34.1.4,

[2] O. Gluschenkov et al., 2016 IEDM,
17.2.1-17.2.4,

[3] R. Khazaka et al., 2022 ECS Trans. 109 87

[4] G. Zheng et al., 2023 VLSI, 1-2

[5] F. Severac et al., J. Appl. Phys. 105, 043711 (2009)

Figure 1