Effect of Various Copper Salt Precursors on Metal-Assisted Chemical Etching of Silicon

Metal-assisted chemical etching (MACE) of Si with unstable metals like Cu can be described with reference to the oxidation effect of aqueous Cu2+ metal cations, in combination with the dissolution of Si species by HF. For 1-step MACE, salt precursors introduce possibly oxidizing additional anionic species, such as NO3− ions from Cu(NO3)2, that cannot be ignored when explaining MACE. This study for the first time unites Cu(NO3)2-MACE with isotropic hydrofluoric/nitric/acetic acid (HNA) Si etching, by purposely avoiding extra oxidants like H2O2, and contrasting the etching characteristics with etchants containing CuSO4, Cu(OAc)2, or CuCl2 precursors. A new model involving Cu2+-promoted autocatalysis of reactive nitrogen species (RNSs) is discussed, which explains the high etch rates, the NO3− anion concentration dependence, and the induction period specific to Cu(NO3)2 etchants. These results highlight the importance of Cu salt precursor selection when rationally designing new Cu-containing wet chemical etchants for key applications like MEMS and black silicon.