Decoding Directional Control in Metal-Assisted Chemical Etching via Catalyst Architecture.

Metal-assisted chemical etching (MaCE) has emerged as a promising technique for fabricating silicon nanostructures, yet the presence of anomalous isotropic etching poses significant challenges for precise dimensional control. Here, it is demonstrated that catalyst morphology, particularly its aspect ratio, plays a crucial role in determining etching directionality. Through systematic investigation of the initial stages of MaCE, it is revealed that significant undercutting occurs within seconds of etching initiation, persisting across all solution compositions. This phenomenon is quantitatively analyzed using the Degree of Undercutting (DoU) and Degree of Anisotropy (DoA) metrics, establishing that conventional solution chemistry control alone cannot suppress lateral etching. These findings reveal that high-aspect-ratio dendrite catalysts, formed at elevated AgNO concentrations, undergo physical separation during etching, leading to residual catalysts that promote localized isotropic etching. To address this, a thermal treatment approach is developed that effectively transforms these problematic structures into stable, low-aspect-ratio catalysts. A critical transition at 450 °C, where enhanced silver atom mobility coincides with surface defect formation, enables nearly perfect vertical etching. This work not only provides fundamental insights into the relationship between catalyst geometry and etching behavior but also presents a practical solution for achieving precise control over silicon nanostructure fabrication.