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 (HNO) and buffered oxide etching (BOE) was investigated in detail. It was found that the HNO 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 HNO concentration. As a result, at 31.5% HNO concentration, the relative etched amount per cycle (REPC) and the etching selectivity of SiGe 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.