Ultrathin Strained-SOI by Stress Balance on Compliant Substrates and FET Performance

Ultrathin, strained-silicon-on-insulator (s-SOI) structures without a residual silicon-germanium (SiGe) underlayer have been fabricated using stress balance of bi-layer structures on compliant borophosphorosilicate glass (BPSG). The bi-layer structure consisted of SiGe and silicon films, which were initially pseudomorphically grown on a silicon substrate and then transferred onto BPSG by a wafer bonding and SmartCut process. The viscous flow of the BPSG during a high-temperature anneal then allowed the SiGe/Si bi-layer to laterally coherently expand to reach stress balance, creating tensile strain in the silicon film. No dislocations are required for the process, making it a promising approach for achieving high-quality strained-silicon for device applications. To prevent the diffusion of boron and phosphorus into the silicon from the BPSG, a thin nitride film was inserted between the bi-layer and BPSG to act as a diffusion barrier, so that a lightly doped, sub-10-nm s-SOI layer (0.73% strain) was demonstrated. N-channel MOSFETs fabricated in a 25-nm silicon layer with 0.6% strain showed a mobility enhancement of 50%.