Model for the Mass Transport during Metal-Assisted Chemical Etching with Contiguous Metal Films As Catalysts

Metal-assisted chemical etching is a relatively new top-down approach allowing a highly controlled and precise fabrication of Si and Si/Ge superlattice nanowires. It is a simple method with the ability to tailor diverse nanowire parameters like diameter, length, density, orientation, doping level, doping type, and morphology. In a typical metal-assisted chemical etching procedure, a Si substrate is covered by a lithographic noble metal film and etched in a solution containing HF and an oxidant (typically H2O2). In general, the function of the metal is to catalyze the reduction of H2O2, which delivers electronic holes necessary for the oxidation and subsequent dissolution of the Si oxide by HF. However, the details of the etching process using contiguous metal thin films, especially the mass transport of reactants and byproducts are still not well understood. In this study, the etching mechanism was systematically investigated. Several models of metal-assisted chemical etching using a contiguous metal film as a catalyst were developed and tested by performing different well-controlled etching experiments. The experiments helped to identify two processes fundamental for the formation of Si nanowires. First, a thin porous layer is formed beneath the metal film during etching, which facilitates the transport of the electrolyte (HF and H2O2). Second, the porous layer is continuously etched away in an electropolishing process, which occurs in direct contact with the metal film. Our results lead to an improved understanding of the fundamentals of the metal-assisted chemical etching on a microscopic scale. It potentially paves a way to integrate lithography with metal-assisted chemical etching for fabrication of Si nanowires with adjustable surface patterns.