Direct Visualization of Etching Trajectories in Metal-Assisted Chemical Etching of Si by the Chemical Oxidation of Porous Sidewalls

Structuring of Si into Multiple Scales by Metal-Assisted Chemical Etching

Structuring Si, ranging from nanoscale to macroscale feature dimensions, is essential for many applications. Metal-assisted chemical etching (MaCE) has been developed as a simple, low-cost, and scalable method to produce structures across widely different dimensions. The process involves various parameters, such as catalyst, substrate doping type and level, crystallography, etchant formulation, and etch additives. Careful optimization of these parameters is the key to the successful fabrication of Si structures. In this review, recent additions to the MaCE process are presented after a brief introduction to the fundamental principles involved in MaCE. In particular, the bulk-scale structuring of Si by MaCE is summarized and critically discussed with application examples. Various approaches for effective mass transport schemes are introduced and discussed. Further, the fine control of etch directionality and uniformity, and the suppression of unwanted side etching are also discussed. Known application examples of Si macrostructures fabricated by MaCE, though limited thus far, are presented. There are significant opportunities for the application of macroscale Si structures in different fields, such as microfluidics, micro-total analysis systems, and microelectromechanical systems, etc. Thus more research is necessary on macroscale MaCE of Si and their applications.

Spontaneous Symmetry Breaking of Nanoscale Spatiotemporal Pattern as the Origin of Helical Nanopore Etching in Silicon

Nanometric chiral objects such as twisted or helical nanoribbons represent a new class of objects having important potential in a large panel of applications, taking advantage, for example, of electromechanical or optical chirality, local chiral environment for catalysis, and chiral recognition. Supramolecular chemistry has played a central role in the production of such structures through either chiral macromolecules/foldamers or the self-assembly of chiral molecules; the latter can also be used as templates for the sol-gel transcription to silica materials, offering them polymorphisms with further structural stability. Here, we report a totally different and dynamic approach to produce helical mesostructures. This study focuses on helical nanopores that are spontaneously formed in the platinum-assisted chemical etching of silicon by dynamic self-organization under a nonequilibrium state. The symmetry breaking of a helical nanopore formation is achieved by the spatial symmetry breaking of a spatiotemporal pattern at the nanoscale and without incorporation of chiral molecules. Rotational motion of the platinum nanocatalyst, which is regarded as a spatiotemporal pattern at the etching frontier (the platinum/silicon interface), induces precession movement of the nanocatalyst, and movement of the catalyst during etching forms helical nanopores in the silicon. We consider that this study is an important milestone to understand the close relation between spatiotemporal pattern formation and the dynamic emergence of symmetry breaking in chemical reactions.

Minimizing Isolate Catalyst Motion in Metal-Assisted Chemical Etching for Deep Trenching of Silicon Nanohole Array

The instability of isolate catalysts during metal-assisted chemical etching is a major hindrance to achieve high aspect ratio structures in the vertical and directional etching of silicon (Si). In this work, we discussed and showed how isolate catalyst motion can be influenced and controlled by the semiconductor doping type and the oxidant concentration ratio. We propose that the triggering event in deviating isolate catalyst motion is brought about by unequal etch rates across the isolate catalyst. This triggering event is indirectly affected by the oxidant concentration ratio through the etching rates. While the triggering events are stochastic, the doping concentration of silicon offers a good control in minimizing isolate catalyst motion. The doping concentration affects the porosity at the etching front, and this directly affects the van der Waals (vdWs) forces between the metal catalyst and Si during etching. A reduction in the vdWs forces resulted in a lower bending torque that can prevent the straying of the isolate catalyst from its directional etching, in the event of unequal etch rates. The key understandings in isolate catalyst motion derived from this work allowed us to demonstrate the fabrication of large area and uniformly ordered sub-500 nm nanoholes array with an unprecedented high aspect ratio of ∼12.

Ultra-high aspect ratio functional nanoporous silicon via nucleated catalysts

Large scale, sub-10 nm high aspect ratio nanoporous silicon is fabricatedviascalable sputtering and a solution-based process.

Catalyst Self-Assembly for Scalable Patterning of Sub 10 nm Ultrahigh Aspect Ratio Nanopores in Silicon

Nanoporous silicon (NPSi) has received significant attention for its potential to contribute to a large number of applications, but has not yet been extensively implemented because of the inability of current state-of-the-art nanofabrication techniques to achieve sufficiently small pore size, high aspect ratio, and process scalability. In this work we describe the fabrication of NPSi via a modified metal-assisted chemical etching (MACE) process in which silica-shell gold nanoparticle (SiO2-AuNP) monolayers self-assemble from solution onto a silicon substrate. Exposure to the MACE etchant solution results in the rapid consumption of the SiO2 spacer shell, leaving well-spaced arrays of bare AuNPs on the substrate surface. Particles then begin to catalyze the etching of nanopore arrays without interruption, resulting in the formation of highly anisotropic individual pores. The excellent directionality of pore formation is thought to be promoted by the homogeneous interparticle spacing of the gold core nanocatalysts, which allow for even hole injection and subsequent etching along preferred crystallographic orientations. Electron microscopy and image analysis confirm the ability of the developed technique to produce micrometer-scale arrays of sub 10 nm nanopores with narrow size distributions and aspect ratios of over 100:1. By introducing a scalable process for obtaining high aspect ratio pores in a novel size regime, this work opens the door to implementation of NPSi in numerous devices and applications.