Selective wet etching in fabricating SiGe nanowires with TMAH solution for gate-all-around MOSFETs

In this work, a selective wet etching process of Si to Si_0.7Ge_0.3 with TMAH solution to fabricate SiGe nanowires is systematically investigated. Initially, the 2.3% TMAH solution at 20 °C is applied for the as grown Si_0.7Ge_0.3/Si multilayers stack and a “rectangular” profile is achieved at the Si_0.7Ge_0.3 extremity due to its high selectivity. However, the reduction of Ge concentration due to the Ge interdiffusion caused by the high temperature anneal treatment will attain a reduced selectivity of Si to Si_0.7Ge_0.3 and a “rounding” Si_0.7Ge_0.3 extremity profile. Moreover, the profile of the upper Si_0.7Ge_0.3 extremity is clearly worse than the bottom one. This is because the Ge concentration of upper Si_0.7Ge_0.3 is 1.5% lower and the Ge interdiffusion of the upper Si_0.7Ge_0.3 is slightly worse. After increasing the TMAH concentration to 25%, both the upper and bottom Si_0.7Ge_0.3 extremity can achieve a “rectangular” extremity profile with an almost the same Si_0.7Ge_0.3 loss ~ 1.5 nm per side. Compared with 2.3% TMAH concentration, the Ge concentration of the etching off SiGe film can be reduced from lower than 29% to lower than 24% using the 25% TMAH solution at 20 °C. Therefore, the 25% TMAH solution at 20 °C is chosen as the optimal selective etching condition for the thermal treated Si_0.7Ge_0.3/Si multilayers stack samples. Finally, a vertical stacked double SiGe nanowire structure is successfully prepared by utilizing this optimal process condition. This indicates that it is a practicable technique for the selective wet etching of Si in fabricating SiGe nanowires.