Metal-assisted chemical etching (MACE) is done with different metal species. The resulting silicon nanostructures appear strongly dependent on the choice of metal, but a deeper understanding of the MACE process is still missing. We report here direct evidence that the etching solution composition plays a major role in the chemical stability of the metal catalyst used. We show from an elemental analysis of post-MACE etch baths that dissolved silver is found in the bath with concentrations up to 3 orders of magnitude larger than when gold is used. Furthermore, the dissolved silver content also correlates with the amount of H2O2, either in different initial conditions, or as would be expected from its decomposition over time. We also show that silver dissolution leads to unintended etching elsewhere on the substrate. This species-dependent behavior of the metal catalyst is responsible for the different kinds of control possible over the nanostructures produced with silver- and gold-based MACE.

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.

Metal assisted chemical etching (MACE) is a viable route to cheaply and easily produce large‐scale arrays of SiNW for, e.g., solar cell applications. However, control over nanostructure dimensions such as length, width, and angle is not simple, particularly with the cheaper silver‐based MACE. Even an initially well‐defined silver catalyst will dissolve uncontrollably under the influence of a variety of conditions acting concurrently, including H2O2 concentration. The authors prove here that HCl can be used, not just as an etch stop, but also as a method for controlling the nanowire etching. The authors suggest that the mechanism of this effect involves the prevention of silver catalyst dissolution, and not only the trapping of already‐dissolved Ag+ ions via the formation of AgCl, as previously thought. The authors further highlight the importance of the silver dissolution rate as a control parameter, in contrast to an earlier focus on the initial thickness alone. This new knowledge will improve the design of etching, through the use of the independent HCl concentration parameter to moderate silver dissolution.