Enabling Ultrahigh-Aspect-Ratio Silicon Nanowires Using Precise Experiments for Detecting the Onset of Collapse.

Top-down patterning along with metal-assisted chemical etching (MACE) can enable the fabrication of highly controlled wafer-scale silicon nanowires (Si-NWs). Maximizing the NW aspect ratio, while avoiding collapse, can enable many important applications. A precise experimental technique has been developed here to study the onset of Si-NW collapse. This experimental approach has resulted in unexpectedly tall Si-NWs for oversized wires separated by sub-50-nm gaps. As compared to known theory, a factor of 4.5 increase in maximum aspect ratio was achieved for uncollapsed nanowires with 200-nm pitch and 25-nm spacing. This discrepancy between known theory and experimental results was eliminated when the gold-resist caps (which are a feature of our MACE process) on top of these nanowires were removed. This led us to incorporate electrostatic repulsion into known theoretical formulations, which matched the experimental results. In summary, this work provides new experimental and theoretical insights into nanowire collapse behavior.