Very Low Temperature Tensile and Selective Si:P Epitaxy for Advanced CMOS Devices

Nowadays, “more Moore” and “more than Moore” device architectures are becoming more and more complex. In CEA-Leti, we work on the “CoolCubeTM” 3D sequential integration which is based on the stacking of FDSOI devices [1]. We present solutions, at T<500°C, for the integration of SiP Raised Sources & Drains (RSD) in the upper devices without degrading the electrical performances of the bottom ones. We also target a lowering of the RSD resistance and an increase of the electron mobility in the channel of NMOS devices thanks to tensile strain [2]. Such a know-how will be useful to minimize the contact resistance and fabricate other types of devices such as FINFETs or h-GAA (e.g. nano-sheet devices) with Si (110) surfaces. Experiments were carried out in an Applied Materials epitaxy reactor featuring (i) liquid precursor delivery, together with H2, N2 or He carrier gas capability, enabling the use of Cl2; (ii) “High Precision Temperature Control (HPTC)”, allowing excellent LT control and enabling flexible rotation speeds; (iii) “precision flow distribution PFD-III”, enhancing uniformity performances. Selective Epitaxial Growth (SEG) is usually obtained with “co-flow” processes at rather high temperatures (>600°C). Chlorinated precursors (SiH2Cl2 (+ GeH4) + HCl, typically) are then sent simultaneously into the growth chamber. At LT (<500C°C), HCl cannot decompose, however. To overcome those limitations, we used a Cyclic Deposition Etch (CDE) strategy, with non-selective depositions followed by selective chemical vapor etches, to obtain SiP SEG. This strategy allowed us to obtain high quality films, as shown in Fig.1. The Omega-2Theta scans around the (004) X-Ray Diffraction order for tensile SiP (t-SiP) layers grown at T < 500°C with different Phosphorus concentrations were indeed typical of monocrystalline layers, with well-defined and intense peaks together with numerous thickness fringes. The substitutional P contents in those ~ 60 nm thick t-SiP layers were in the 1.02% - 5.42% range. The good layer uniformity in terms of thickness and P content, over a 300mm wafer radius, is shown in figure Fig.2. These layers grown at T <500°C were smooth, as shown in Fig.3, with a 0.21 nm Root Mean Square (RMS) roughness for a 60nm thick Si:P layer, i.e. a value close to the typical RMS roughness for t-SiP layers grown at high temperature with a chlorinated chemistry. The electrical resistivity in various t-SiP layers is plotted in Fig.4 as function of the substitutional phosphorus concentration and for various growth temperatures in the 450°C – 525°C range. Reducing the temperature by 75°C halved the electrical resistivity. We were able to achieve a resistivity as low as 0.21 mOhm.cm for a t-SiP layer with 5.8% of P grown at 450°C. We then evaluated, at first on tests structures without gates, our process selectivity. A top view Scanning Electron Microscopy image of a t-SiP layer grown non-selectively, with numerous amorphous SiP nuclei on SiO2, is shown in Fig 5.a. After some careful optimization, we succeeded in having fully selective processes versus SiO2, as shown in Fig 5.b. We then tested such optimized processes on low density FD-SOI devices with 28 nm design rules. A top-view SEM image of transistors after such a growth is shown in Fig.6. The growth selectivity was excellent, with nitride spacers and hard masks as well as isolations free of a-SiP nuclei for 31 nm of t-SiP with 4.5% of P deposited in the Sources/Drains. The surface was smooth, with a RMS roughness as low as 0.30nm on active areas, as shown in Fig.7. Thanks to High Resolution Reciprocal Space Maps (HR-RSM), we measured a Phosphorus concentration of 4.5% for that SiP layer grown on SOI. The very high quality of that epitaxy layer, with well-defined thickness fringes, is obvious in Fig.8. Cross-sectional Transmission Electron Microscopy (TEM) images such as the one shown in Fig. 9 enabled us to confirm, at the nanoscale, the excellent quality of such layers in RSDs. To sum up, we were able to develop a tensile Si:P process which was shown to be selective, at a temperature lower than 500°C, against SiO2 and SiN. Such t-SiP layers were successfully integrated in the Sources/Drains regions of FD-SOI 28nm devices. The very low material resistivity and the high phosphorus content should yield, notably because of tensile strain, performant NMOS devices in the near future. [1] C. Fenouillet-Beranger et al., IEEE TED 68, 3142-3148 (2021) [2] V. Chan et al., IEEE 2005 Custom Integrated Circuits Conference 2005, pp. 667-674 Figure 1