Observation and Origin of Vertical Segment and Its Transition to Stable Slant Wire Formation via Metal-Mesh-Assisted Chemical Etching.

We report the temporal variation of slant angles across three distinct segments within a single wire at a fixed HF-to-H2O2 volume ratio R during metal-mesh-assisted chemical etching (MACE). This intrawire change in slant angle at a constant R differs fundamentally, in both phenomenon and mechanism, from the wire-to-wire variation in slant angles observed under varying R values. MACE of (100) Si substrate over extended durations typically yields tilted wires when using a medium-to-high R. Interestingly, while these structures appear slanted macroscopically, electron microscopy reveals a microscopic vertical segment at the uppermost portion of the wires. The underlying mechanism remains poorly understood. In this work, the existence of bubble-induced two-phase flow was confirmed indirectly by comparing stirred and unstirred MACE. The hydrogen produced by MACE initially remains dissolved, and after some time, bubbles start to nucleate and grow until they detach from the etching front. This characteristic three-stage gas evolution behavior was found to align well, in chronological order, with the vertical, curved, and slanted segments observed in the etched wires. To the best of our knowledge, this study offers the first interpretation linking gas evolution phases to intrawire morphology of macroscopically oblique wires produced by MACE.