Selective Epitaxial Phosphorus-Doped SiGe Layers for Short-Channel Effect Reduction

Epitaxially grown silicon germanium layers are utilized in very high performance short channel MOSFETs. To reduce short-channel effects, the substrate doping concentration must be increased at the edges of the source and drain. These regions commonly called halos are typically created by ion implantation but the precise positioning of the dopant is a challenge. As devices with embedded SiGe source/drain regions continue to scale, the proximity of the source and drain regions to each other increases the challenge of proper halo placement. This work proposes incorporation of halo placement into the epitaxial deposition process so that dopants are automatically placed where they are needed. Segregation of ion-implanted phosphorus halos into the boron regions is observed after annealing as shown in Figure 1. This segregation reduces the localization and effectiveness of the halo implant. Figure 2 shows the application of the proposed concept in a device with embedded source/drain regions. For a p-FET device, an in-situ SiGeP epitaxy growth is performed to circumvent difficulties with halo placement. After SiGeP deposition, a SiGeB layer is deposited to form the embedded source/drain. Figure 3 shows the phosphorus concentration in SiGe observed by SIMS, as a function of phosphine flow rate on blanket p-Si 300mm wafers at a low growth temperature and a reduced process pressure. At high phosphine flow, the phosphorus doping level approaches saturation. Phosphine (PH3) diluted with H2 was used as the sources of phosphorus. A mixed gas line configuration was used with H2 to achieve successful localization of the phosphorus with low doping concentration. For phosphorus-doped SiGe, the P concentration increases monotonically with the PH3 flow then saturates at 4×1019 cm-3. This saturation at a rather low value is most probably due to the well-known surface segregation behavior of phosphorus atoms during epitaxial growth [1], and also to a parasitic gas phase reaction (2PH3↔P2+3H2) at high PH3 partial pressures that partially inhibits mono-atomic P incorporation into SiGe [2]. Figure 4 shows a SIMS profile to verify control. A SiGeP film was grown without any interruption using pure dichlorosilane, diluted germane and diluted phosphine with H2. The phosphorus concentration is uniform through the entire film. Figure 5 shows an SEM cross-section for a multilayer structure (Si-cap (10nm)/SiGeB (50nm)/SiGe (7nm)/SiGeP (3nm)/Si-substrate) as proposed in Figure 2. As can be seen, high quality multilayer epitaxial films were obtained in this novel approach to replacing ion implanted halos with in-situ SiGeP epitaxy growth. Previous attempts in the literature to localize a P doped epitaxial layer have been unsuccessful [3]. In order to verify localization in this work, two different SIMS techniques were used on a multilayer structure (Si-cap/5X alternating SiGeB and SiGe/SiGeP/Si substrate): regular quadrupole SIMS and Cameca IMS 6f SIMS with high mass resolution (Figure 6). Phosphorus artifacts are observed at the B peak positions with a regular quadrupole SIMS as shown in Figure 6(a). However, there are no artificial P peaks in Figure 6(b) using the Cameca IMS 6f and the result clearly shows the desired halo nature of the P profile. In summary, a new epitaxial deposition processes with ideal doping profiles was demonstrated to reduce short channel effects in scaled devices.