Isotropic wet etching of molybdenum revealed by liquid-phase transmission electron microscopy

Advances in semiconductor technology have driven structural evolution, including size reduction and increased structural complexity. Highly integrated devices require interconnect only a few nanometers in size. Copper (Cu) is widely used material for this purpose; however, its high electrical resistance at the sub-10 nm scale has prompted the search for alternatives. Molybdenum (Mo) is a promising candidate because of its superior electrical performance. However, realizing sophisticated Mo interconnects is challenging as anisotropic wet etching roughens the Mo surface. To overcome this problem, we propose an isotropic wet-etching method facilitated by the accumulation of etching reaction products. These accumulated reaction products inhibit the diffusion of etching ions from the bulk solution to the metal surface, inducing diffusion-limited etching that is independent of grain orientation. To investigate this accumulation effect, we compare two etchants, one that causes accumulation and one that does not, and verify their etching isotropy using ex-situ scanning transmission electron microscopy (STEM) and in-situ liquid-phase transmission electron microscopy (LPTEM). Consequently, we successfully achieve a smooth Mo surface through isotropic wet etching via controlled reaction product accumulation. Given the demand for high-quality etching in sub-10 nm interconnects, these findings hold considerable potential for next-generation electronic device fabrication.