Improvement of Boron Doping in SiGe Raised Sources and Drains for FD-SOI Technology by Carbon Incorporation
Authors: Labrot Maxime , Cheynis Fabien , Barge David , Juhel Marc , Müller Pierre 
Published: February 2020

In semiconductor industry, the continuous scaling reduction of CMOS components improves devices performances and reduces manufacturing cost. However, the electrostatic control of the channel must be very accurate. To address this requirement, Fully-Depleted Silicon-On-Insulator (FD-SOI) technology, relying on the use of ultra-thin SOI films (less than 10 nm), is a promising option for the technical nodes beyond 14nm.
This option requires the realization of Raised Source & Drain (RSD) by epitaxial Chemical-Vapor Deposition (CVD) of silicon alloys [1, 2]. The quality of the RSD epitaxy depends on the cleanliness of the surface before the deposition. For standard SOI technology, the state of the art generally consists in a high temperature treatment of the surface (>1000°C) in presence of hydrogen. However, such a thermal budget cannot be used for ultra-thin SOI films that dewets and agglomerates into 3D islands in this temperature range [3, 4].
Therefore, for ultra-thin SOI films, an efficient low temperature surface preparation process is required to avoid improper growth of Si or SiGe:B ( resp. SiC:P) layers, used as RSD material for pFET (resp. nFET).
This works focus on the surface preparation prior to the low-temperature epitaxy of Si0.7Ge0.3:B layers on full-sheet and patterned wafers. In a first stage different processes of surface preparations are evaluated including standard wet cleans (HF-last, HF-RCA), plasma-based clean (SiCoNiTM
setup [5, 6]) and their combination. For SiGe channels, the so-called HFRCA + SiCoNi process without any subsequent baking is recommended to reach clean surfaces [7].
In a second stage, we show that the Si1-xGex growth rate increases linearly when increasing the B2H6 boron-precursor gas flow. However the quality of the SiGe:B epitaxial films is degraded for high boron doping (≈1021at.cm-3) with B accumulation at the interface (Fig. 1). We report on the conditions necessary to reach high boron concentrations while maintaining a uniform boron concentration by incorporating carbon (≈1%) in the RSD deposition step.
A wide set of characterization tools are used to assess the sample quality in terms of defect density, chemical analysis, roughness and surface morphology: AFM, SEM, TEM, Spectrometric Ellipsometry (SE), EDX, XPS, Secondary Ion Mass Spectroscopy (SIMS), Model Based Infrared Reflectometry (MBIR), Atom Probe Tomography (APT), Spectroscopy Infrared Reflectometry (SIR) and Low-Energy Electron Microscopy (LEEM).

Keywords: Si, SiGe, SiCoNiTM Preclean, FD-SOI, SiGe:B, WET clean, RSD

References:

[1] N. Loubet et al. Ultra-Low resisitivity in-situ phosphorus doped Si and SiC epitaxy for source/drain formation in advanced 20 nm n-type field effect transistor devices, Thin Solid Films 520 (2012) 3149-3159.
[2] J.M et al. Very Low Temperature (Cyclic) Deposition / Etch of In Situ Boron-doped SiGe Raised Sources and Drains. ECS Journal of Solid State Science and Technology, 3 (11) P382-P390 (2014).
[3] F.Cheynis et al. Dynamics and instability of solid-state dewetting. Comptes Rendus Physique 14 578-589 (2013).
[4] D.Dutartre et al. Defect free Stranski Krastanov growth of strained Si1-xGex layers on Si, Journal of Crystal Growth 142, p. 78-86 (1994).
[5] H Yang, X Lu, CT Kao, M Chang - US Patent 7,780,793, (2010).
[6] R Arghavani, CT Kao, X Lu - US Patent 7,955,510, (2011).
[7] M.Labrot et al. Low thermal budget for Si and SiGe surface preparation for FD-SOI technology. Applied Surface Science 371, p. 436–446 (2016).

Figure 1