(Invited) Selective Etch of Si and SiGe for Gate All-Around Device Architecture

 The next era in CMOS technology scaling seeks not only to introduce SiGe or Ge [1] but also aims at replacing the FINFET technology by a gate all around concept, requiring selective removal of Si_1-xGe_x to Si_1-yGe_y(with 0 ≤ x, y ≤ 1).  Gate All-Around (GAA) integration schemes are investigated for improved short channel control. In this paper we will show how alkaline solutions can be used for selective etch of Si and SiGe up to 50% and that the alkaline etch behavior of SiGe is not merely an extension of Si, even at low to moderate Ge concentrations (Ge ≤ 50%). 

The exponential decrease in alkaline etch rate of SiGe with increasing Ge concentration enables the selective removal of Si to Si_0.75Ge_0.25. [2] Alkaline etching of Si has been studied extensively and is very well understood. [3] The oxidation of Si by water is catalyzed by the hydroxyl anion and results in anisotropic etch properties. The Si (001) and (110) directions etch much faster compared to the Si (111) crystallographic planes. The alkaline dissolution mechanism of SiGe is much less studied and understood.

Figure 1 shows a Si-Si_0.75Ge_0.25 multilayer stack after alkaline selective etch in 5% TMAH at 60¢ªC. The stack was made by FIN patterning of an epitaxial Si_0.75Ge_0.25 / Si / Si_0.75Ge_0.25 / Si multilayer. The TEM picture shows the complete removal of the top Si layer. The selective etch of the Si layer sandwiched between two Si_0.75Ge_0.25 layers is limited by the slowly etching Si (111) planes, resulting in the formation of inverted pyramids. Figure (1) shows that the Si_0.75Ge_0.25 layer is also limited by the (111) crystallographic planes. The shape of convex corners results from the crystallographic planes with the highest etch rate. [4] The pyramidal shape of the Si_0.75Ge_0.25 layer indicates that the Si_0.75Ge_0.25(111) plane etches faster than (110). This is very different from Si where (111) planes etch much slower than the (110) planes.  

In this paper we will show how SiGe (Ge ≤ 50%) alkaline etching behaves differently compared to Si and its implications to Si or SiGe selective etch. In addition we will show how improper control of process conditions can change SiGe etching from anisotropic to isotropic.

[1] M.L. Lee et al. Appl. Phys. Rev. 97 (2005) 011101.

[2] Seidel et al. J. Electrochem. Soc. 137 (1990) 3626.

[3] M.A. Hines, Annu Rev Phys Chem 54 (2003) 29-56.

[4] P. Pal et al. J. Micromech. Microeng 17 (2007) R111-R133.