How the Oxidation Stability of Metal Catalysts Defines the Metal-Assisted Chemical Etching of Silicon

Abstract

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.

Using HCl to Control Silver Dissolution in Metal-Assisted Chemical Etching of Silicon

“…Results from our own previous work showed a correlation between the amount of Ag + left in a MACE bath after etching, and the initial H 2 O 2 concentration used . This explains the trend in the literature, where the majority of Ag‐MACE SiNW protocols employ high molar ratios ( ρ , defined as [HF]/([HF] + [H 2 O 2 ])).…”

“…[9] Results from our own previous work showed a correlation between the amount of Ag þ left in a MACE bath after etching, and the initial H 2 O 2 concentration used. [22] This explains the trend in the literature, where the majority of Ag-MACE SiNW protocols employ high molar ratios (ρ, defined as [HF]/ ([HF] þ [H 2 O 2 ])). As high-ρ values imply lower concentrations of H 2 O 2 , these protocols may have inadvertently avoided much of the complication involved with increased silver dissolution.…”

“…In the test cases, we added 0.1 mL of various HCl stock solutions to the baths prior to etch start. This gave the etch baths an HCl concentration of either 0, 2.0, 20, 200, or 2000 × 10 –6 m. Our choice of HCl concentration covers several orders of magnitude, both above and below the expected Ag + concentration for this experimental setup, predicted by our previously published Ag + concentration studies . This range also overlaps with the maximum possible concentration of silver of 18 × 10 –6 M, as calculated in Supporting Information Section S1.1.…”

Ultralow Optical and Electrical Losses via Metal‐Assisted Chemical Etching of Antireflective Nanograss in Conductive Mesh Electrodes

“…In the thin (<20 nm) metal layer, pinholes were spontaneously formed during MacEtch by the ionization and redistribution of the metal catalyst through reduction of H 2 O 2 . [ 26,27 ] Next, MacEtch was performed, where the metal was in contact with the Si substrate, and nanograss was fabricated in the pinhole region. The MacEtch process was implemented as follows: H 2 O 2 reacts with Ag to form electronic holes, which are then injected into Si to oxidize it.…”

“…As an alternative to RIE, metal‐assisted chemical etching (MacEtch) has been suggested. [ 15–28 ] It can fabricate high‐aspect‐ratio semiconductor nanostructures without causing crystal defects because it uses wet‐based chemical processes. Patterned metal catalysts are deposited on a semiconductor that is immersed in an etchant composed of an acid and oxidant.…”