The Effect of Doping on the Digital Etching of Silicon-Selective Silicon-Germanium Using Nitric Acids

Gate-all-around (GAA) field-effect transistors have been proposed as one of the most important developments for CMOS logic devices at the 3 nm technology node and beyond. Isotropic etching of silicon-germanium (SiGe) for the definition of nano-scale channels in vertical GAA CMOS and tunneling FETs has attracted more and more attention. In this work, the effect of doping on the digital etching of Si-selective SiGe with alternative nitric acids (HNO₃) and buffered oxide etching (BOE) was investigated in detail. It was found that the HNO₃ digital etching of SiGe was selective to n⁺-Si, p⁺-Si, and intrinsic Si. Extensive studies were performed. It turned out that the selectivity of SiGe/Si was dependent on the doped types of silicon and the HNO₃ concentration. As a result, at 31.5% HNO₃ concentration, the relative etched amount per cycle (REPC) and the etching selectivity of Si_0.72Ge_0.28 for n⁺-Si was identical to that for p⁺-Si. This is particularly important for applications of vertical GAA CMOS and tunneling FETs, which have to expose both the n⁺ and p⁺ sources/drains at the same time. In addition, the values of the REPC and selectivity were obtained. A controllable etching rate and atomically smooth surface could be achieved, which enhanced carrier mobility.