Metal‐Assisted Chemical Etching of Silicon: A Review

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.     

Tailored Height Gradients in Vertical Nanowire Arrays via Mechanical and Electronic Modulation of Metal‐Assisted Chemical Etching

In current top‐down nanofabrication methodologies the design freedom is generally constrained to the two lateral dimensions, and is only limited by the resolution of the employed nanolithographic technique. However, nanostructure height, which relies on certain mask‐dependent material deposition or etching techniques, is usually uniform, and on‐chip variation of this parameter is difficult and generally limited to very simple patterns. Herein, a novel nanofabrication methodology is presented, which enables the generation of high aspect‐ratio nanostructure arrays with height gradients in arbitrary directions by a single and fast etching process. Based on metal‐assisted chemical etching using a catalytic gold layer perforated with nanoholes, it is demonstrated how nanostructure arrays with directional height gradients can be accurately tailored by: (i) the control of the mass transport through the nanohole array, (ii) the mechanical properties of the perforated metal layer, and (iii) the conductive coupling to the surrounding gold film to accelerate the local electrochemical etching process. The proposed technique, enabling 20‐fold on‐chip variation of nanostructure height in a spatial range of a few micrometers, offers a new tool for the creation of novel types of nano‐assemblies and metamaterials with interesting technological applications in fields such as nanophotonics, nanophononics, microfluidics or biomechanics.     

锰掺杂锯齿型石墨烯纳米带电磁学特性研究

基于密度泛函理论的第一性原理,计算了锰原子单空位掺杂锯齿型石墨烯纳米带6种不同位置时的电磁学特性。结果表明:锰原子掺杂石墨烯纳米带的能带结构对掺杂位置十分敏感。随着锰掺杂位置的变化,掺杂石墨烯纳米带分别表现出半导体性和金属性特征。锰原子掺杂石墨烯纳米带改变了原本的磁性特征,掺杂位置不同,结构磁性特点也不相同,掺杂位置在4号位置时,纳米带实现了由反铁磁态的锯齿型石墨烯纳米带向铁磁性的转化。锰原子掺杂锯齿型石墨烯纳米带可以调制其磁性和能带特性,为石墨烯纳米带在电磁学领域应用提供一定的理论依据。     

Controllable and Facile Fabrication of Gold Nanostructures for Selective Metal‐Assisted Etching of Silicon

A method with the combination of organic‐vapor‐assisted polymer swelling and nanotransfer printing (nTP) is used to manufacture desirable patterns consisting of gold nano‐clusters on silicon wafers for Au‐assisted etching of silicon. This method remarkably benefits to the size control and regional selection of the deposited Au. By tuning the thickness of the Au films deposited on the polydimethylsiloxane (PDMS) stamps, along with the swelling of PDMS stamps in acetone atmosphere, the Au films are cracked into diverse nanostructures. These nanostructures are covalently transferred onto silicon substrates in a large scale and enable to accelerate the chemical etching of silicon. The etched areas are composed of porous structures which can be readily distinguished from the surroundings on optical microscope. PDMS stamps and the Au clusters provide the control over the feature of the etched areas and the porous silicon, respectively. The silicon surfaces with patterned porous features offer a platform for exploiting new functional templates, for example, they present a diversity of antireflective and fluorescent performance.     

Bulk Micromachining of Si by Metal‐assisted Chemical Etching

Bulk micromachining of Si is demonstrated by the well‐known metal‐assisted chemical etching (MaCE). Si microstructures, having lateral dimension from 5 μm up to millimeters, are successfully sculpted deeply into Si substrate, as deep as >100 μm. The key ingredient of this success is found to be the optimizations of catalyst metal type and its morphology. Combining the respective advantages of Ag and Au in the MaCE as a Ag/Au bilayer configuration leads to quite stable etch reaction upon a prolonged etch duration up to >5 h. Further, the permeable nature of the optimized Ag/Au bilayer metal catalyst enables the etching of pattern features having very large lateral dimension. Problems such as the generation of micro/nanostructures and chemical attacks on the top of pattern surface are successfully overcome by process optimizations such as post‐partum sonication treatment and etchant formulation control. The method can also be successful to vertical micromachining of Si substrate having other crystal orientations than Si(100), such as Si(110) and Si(111). The simple, easy, and low‐cost nature of present approach may be a great help in bulk micromachining of Si for various applications such as microelectromechanical system (MEMS), micro total analysis system (μTAS), and so forth.     

Hybrid Anodic and Metal‐Assisted Chemical Etching Method Enabling Fabrication of Silicon Carbide Nanowires

Silicon carbide (SiC) is one of the most important third‐generation semiconductor materials. However, the chemical robustness of SiC makes it very difficult to process, and only very limited methods are available to fabricate nanostructures on SiC. In this work, a hybrid anodic and metal‐assisted chemical etching (MACE) method is proposed to fabricate SiC nanowires based on wet etching approaches at room temperature and under atmospheric pressure. Through investigations of the etching mechanism and optimal etching conditions, it is found that the metal component plays at least two key roles in the process, i.e., acting as a catalyst to produce hole carriers and introducing band bending in SiC to accumulate sufficient holes for etching. Through the combined anodic and MACE process the required electrical bias is greatly lowered (3.5 V for etching SiC and 7.5 V for creating SiC nanowires) while enhancing the etching efficiency. Furthermore, it is demonstrated that by tuning the etching electrical bias and time, various nanostructures can be obtained and the diameters of the obtained pores and nanowires can range from tens to hundreds of nanometers. This facile method may provide a feasible and economical way to fabricate SiC nanowires and nanostructures for broad applications.