This article presents an overview of the essential aspects in the fabrication of silicon and some silicon/germanium nanostructures by metal-assisted chemical etching. First, the basic process and mechanism of metal-assisted chemical etching is introduced. Then, the various influences of the noble metal, the etchant, temperature, illumination, and intrinsic properties of the silicon substrate (e.g., orientation, doping type, doping level) are presented. The anisotropic and the isotropic etching behaviors of silicon under various conditions are presented. Template-based metal-assisted chemical etching methods are introduced, including templates based on nanosphere lithography, anodic aluminum oxide masks, interference lithography, and block-copolymer masks. The metal-assisted chemical etching of other semiconductors is also introduced. A brief introduction to the application of Si nanostructures obtained by metal-assisted chemical etching is given, demonstrating the promising potential applications of metal-assisted chemical etching. Finally, some open questions in the understanding of metal-assisted chemical etching are compiled.

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Metal-Assisted Chemical Etching of Silicon: A Review

Metal-assisted chemical etching of silicon in HF–H2O2

Au, Pt, or Ag particles with particle sizes of ca. 1 μm were used as catalysts for boring pores in p-type Si(100) wafers by wet etching in aqueous solutions containing hydrofluoric acid and hydrogen peroxide. Boring speed was fastest when Pt particles were used as the catalyst. However, the sidewalls of the pores and the surface of the wafer were covered with a nanoporous silicon layer of ca. 500 nm in thickness, and the pore showed a tapered structure. When micrometre-sized Ag particles were used, no deep pores were formed because the particles were unstable in the solution. In contrast to Pt and Ag particles, Au particles bored straight pores under some conditions. However, the morphology of pores depended on the shape of the Au particles. Spherical Au particles formed straight pores, whereas non-spherical Au particles formed pores with spiral sidewalls. When Au particles formed aggregates consisting of a small number of particles (<10 particles), crooked pores tended to be formed. In contrast, when the aggregates were composed of a larger number of particles, straight pores were formed and the boring speed was faster than the pores formed with isolated Au particles.

Pore formation in silicon by wet etching using micrometre-sized metal particles as catalysts

Recent advances in the production of Si nanostructures from electroless etching are reviewed, including stain etching, metal-assisted etching and chemical vapour etching. A brief review of the explosion in applications of porous silicon over the past 18 months is also given. The stain film that results from the etching of (poly- or single-)crystalline Si is composed of a porous network of nanocrystalline silicon. Few mechanistic studies of electroless etching have been performed, but the more extensively studied anodic etching of silicon in fluoride solutions provides many clues as to how porous films are formed. Intriguing recent results have shown that control over the properties of the film can be obtained by exercising control over the composition of the etchant.

Silicon nanostructures from electroless electrochemical etching

This review is devoted to the analysis of the problems related to fabrication of the Si porous layers. The review was motivated by a great interest to Si-based porous materials from nano- to macro-scale for various applications in electronics, optoelectronics, photonics, chemical sensors, biosensors, etc. The peculiarities of the silicon porosification and the principles of preparing porous layers are considered in the present article. Various methods used for Si porosification such as chemical stain etching, chemical vapor etching, laser-induced etching, metal-assisted etching, spark processing and reactive ion (plasma) etching were analyzed. However, the main attention was focused on electrochemical porosification of Si. The review discusses in detail the influence of parameters such as electrolyte composition and pH, current density, etching time, temperature, wafer doping and orientation, lighting, magnetic field, and ultrasonic agitation on the process of Si porosification. It was shown that the stru...

Silicon Porosification: State of the Art

The application of porous silicon (PS) as a filter substrate in microstructured systems is a promising field of research. Based on this approach, metal-assisted chemical etching is applied to produce PS in a cheap and fast way. This simple and effective method enables the fabrication of PS with different morphologies, pore distribution, and thickness by varying the deposited metal type and thickness, and the Si doping type and level. The metal (Au, Pt) is sputtered in varied thicknesses on patterned p- or n-doped silicon wafers. The etching solution consists of an 1:1:1 mixture of HF:H 2 O 2 :EtOH. Etching time and temperature are varied showing a direct effect on the pore depth. To obtain a thin permeable porous Si layer, advanced silicon etching is used to microstructure the back side of the patterned wafer. Hence, the resulting PS layers turn to thin membranes ∼150 μm thick, maintaining the desired mechanical stability at enhanced gas-permeation rates and thus face a very auspicious career as gas diffusion substrates.

https://iopscience.iop.org/article/10.1149/1.1914747/pdf

Arne Hönig-d’Orville

Papers

Fabrication and Optimization of Porous Silicon Substrates for Diffusion Membrane Applications