Abstract
The combination of metal-assisted chemical etching (MACE) with colloidal lithography has emerged as a simple and cost-effective approach to nanostructure silicon. It is especially efficient at synthesizing Si micro- and nanowire arrays using a catalytic metal mesh, which sinks into the silicon substrate during the etching process. The approach provides a precise control over the array geometry, without requiring expensive nanopatterning techniques. Although MACE is a high-throughput solution-based approach, achieving large-scale homogeneity can be challenging because of the instability of the metal catalyst when the experimental parameters are not set appropriately. Such instabilities can lead to metal film fracture, significantly damaging the substrate and thus compromising the nanowire array quality. Here, we report on the critical parameters that influence the stability of the metal catalyst layer for achieving large-scale homogeneous MACE: etchant composition, metal film thickness, adhesion layer thickness, nanowire diameter and pitch, metal film coverage, Si/Au/etchant interface length, and crystalline quality of the colloidal template (grain size and defects). Our results investigate the origin of the catalyst film fracture and reveal that MACE experiments should be optimized for each Si wire array geometry by keeping the etch rate below a certain threshold. We show that the Si/Au/etchant interface length also affects the etch rate and should thus be considered when optimizing the MACE experimental parameters. Finally, our results demonstrate that colloidal templates with small grain sizes (i.e., <100 μm2) can yield significant problems during the pattern transfer because of a high density of defects at the grain boundaries that negatively affects the metal film stability. As such, this work provides guidelines for the large-scale synthesis of Si micro- and nanowire arrays via MACE, relevant for both new and experienced researchers working with MACE.
Highlights
Micro- and nanostructuring silicon is a crucial process for the fabrication and development of transistors, micro-electromechanical systems, optical sensors, metamaterials, and Sibased batteries.[1−4] Vertically aligned silicon nanowire (VA-SiNW) arrays are a interesting class of nanostructured silicon because of their strong and tunable interaction with light that can arise from wave-guiding,[5,6] Mie resonance excitation,[7] diffractive effects,[8] and near-field coupling.[9]
Without the aluminum-doped zinc oxide (AZO) adhesion layer, the gold film either does not etch through the substrate properly or delaminates after the tape removal and during Metal-assisted chemical etching (MACE), while the presence of an AZO layer leads to homogeneous etching of the silicon substrates (Figure 2)
We demonstrated that the etch rate is strongly affected by the geometry of the etching mask and that unfavorable MACE conditions can lead to local fractures of the gold film, which can significantly reduce the homogeneity of the SiNW arrays produced
Summary
Micro- and nanostructuring silicon is a crucial process for the fabrication and development of transistors, micro-electromechanical systems, optical sensors, metamaterials, and Sibased batteries.[1−4] Vertically aligned silicon nanowire (VA-SiNW) arrays are a interesting class of nanostructured silicon because of their strong and tunable interaction with light that can arise from wave-guiding,[5,6] Mie resonance excitation,[7] diffractive effects,[8] and near-field coupling.[9]. VA-SiNW arrays have been first prepared via vapor−liquid−solid (VLS) synthesis.[18−20] At the time, VLS afforded a versatile route to prepare single-crystal Si nanowires. Dispersed Si nanowires can be prepared via the solution−liquid−solid (SLS) method, which has the advantage of being low cost and has potential for large-scale production.[22,23] the preparation of highly ordered VA-SiNW arrays via SLS remains to be seen. Metal-assisted chemical etching (MACE) on the other hand is a cost-effective, solution-based, and highthroughput technique to micro- and nanostructure silicon with high structure fidelity and purity.[5,20,26−40] It involves a nanostructured metal catalyst layer, an oxidant, and hydrofluoric acid (HF).[26−28,41] The oxidizing agent, usually H2O2, is preferentially reduced at the surface of the metal catalyst, Received: September 7, 2020 Revised: October 14, 2020 Published: October 26, 2020
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