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.     

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.     

电催化金属辅助化学刻蚀法制备硅纳米线/多孔硅复合结构

量子限制效应使硅纳米线具有良好的场致发射特性,结合多孔硅的准弹道电子漂移模型可提高场发射器件的性能。传统的金属辅助化学刻蚀法制备硅纳米线的效率较低,本研究在传统方法的基础上引入恒流源,提出电催化金属辅助化学刻蚀法,高效制备了硅纳米线/多孔硅复合结构。在外加30mA恒定电流的条件下,硅纳米线的平均制备速率可达308nm/min,较传统方法提升了173%。研究了AgNO₃浓度、刻蚀时间和刻蚀电流对复合结构形貌的影响规律;测试了采用电催化金属辅助化学刻蚀法制备样品的场发射特性。结果显示样品的阈值场强为10.83 V/μm,当场强为14.16 V/μm时,电流密度为64μA/cm²。     

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.     

电化学腐蚀多孔硅表面形貌的结构特性

多孔硅作为微电子机械系统中重要的热绝缘层和牺牲层材料,其表面形貌结构特性是影响多孔硅上薄膜器件性能的重要因素,为此,利用双槽电化学腐蚀方法制备了多孔硅薄膜,并通过原子力显微镜和场发射扫描电子显微镜对制备多孔硅的表面形貌和孔径大小分布进行了观察．结果发现：腐蚀初期,在硅表面会有大量的硅柱形成,硅柱的直径、高度、分布密度与电流密度成正比关系;硅柱在进一步腐蚀过程中会消失,多孔硅的表面粗糙度随着腐蚀的进行,先减小再增大,最后达到稳定值0．52nm;多孔硅孔径大小分布区间随腐蚀时间增加变窄．     

用化学腐蚀制备多孔硅太阳电池减反射膜的研究

采用HF HNO3溶液化学腐蚀 ,在硅片上制备减反射效果优良的多孔硅太阳电池减反射膜 ,借助原子力显微镜 (AFM)和X光电子谱 (XPS)对其表面形貌和成分进行观察 ,发现该膜与电化学阳极腐蚀得到的多孔硅具有相似性 ,其主要成分为非化学配比的硅的氧化物SiOx(X <2 )。采用带积分球的光度分光计 ,测得形成多孔硅减反射膜后 ,硅片表面反射率大大下降 ,,在波长 330～ 80 0nm范围反射率只有 1 5～ 2 9%。研究指出这种强减反射作用 ,与多孔硅具有合适的折射率及其多孔特性的光陷阱作用有关     

Wafer-Scale Fabrication of Hierarchically Porous Silicon and Silica by Active Nanoparticle-Assisted Chemical Etching and Pseudomorphic Thermal Oxidation (Small 22/2023)

Many biological materials exhibit a multiscale porosity with small, mostly nanoscale pores as well as large, macroscopic capillaries to simultaneously achieve optimized mass transport capabilities and lightweight structures with large inner surfaces. Realizing such a hierarchical porosity in artificial materials necessitates often sophisticated and expensive top-down processing that limits scalability. Here, an approach that combines self-organized porosity based on metal-assisted chemical etching (MACE) with photolithographically induced macroporosity for the synthesis of single-crystalline silicon with a bimodal pore-size distribution is presented, i.e., hexagonally arranged cylindrical macropores with 1 µm diameter separated by walls that are traversed by pores 60 nm across. The MACE process is mainly guided by a metal-catalyzed reduction–oxidation reaction, where silver nanoparticles (AgNPs) serve as the catalyst. In this process, the AgNPs act as self-propelled particles that are constantly removing silicon along their trajectories. High-resolution X-ray imaging and electron tomography reveal a resulting large open porosity and inner surface for potential applications in high-performance energy storage, harvesting and conversion or for on-chip sensorics and actuorics. Finally, the hierarchically porous silicon membranes can be transformed structure-conserving by thermal oxidation into hierarchically porous amorphous silica, a material that could be of particular interest for opto-fluidic and (bio-)photonic applications due to its multiscale artificial vascularization.     

Facile Synthesis of Vanadium Metal‐Organic Frameworks for High‐Performance Supercapacitors

Compared to traditional metal oxides, metal‐organic frameworks exhibit excellent properties, such as a high surface area, significant thermal stability, low density, and excellent electrochemical performance. Here, a simple process is proposed for the fabrication of rod‐like vanadium metal‐organic frameworks (V^IV(O)(bdc), bdc = 1,4‐benzenedicarboxylate, or MIL‐47), and the effect of the structure on the electrochemical performance is investigated via a series of electrochemical measurements. The V^IV(O)(bdc) electrode exhibits a maximum specific capacitance of 572.1 F g^?1 at current densities of 0.5 A g^?1. More significantly, aqueous and solid‐state asymmetric supercapacitors are successfully assembled. The solid‐state device shows an excellent energy density of 6.72 mWh cm^?3 at a power density of 70.35 mW cm^?3. This superior performance confirms that V^IV(O)(bdc) electrodes are promising materials for applications in supercapacitors.     

化学刻蚀法制备多孔硅的表面形貌研究

本工作首先采用化学刻蚀法制备出各种特征的多孔硅,然后通过扫描电镜（ＳＥＭ）技术,对多孔硅的表面形貌进行了表征,并分析了多孔硅表面微结构的形成过程     

Metal–Organic Framework‐Derived Materials for Sodium Energy Storage

Recently, sodium‐ion batteries (SIBs) are extensively explored and are regarded as one of the most promising alternatives to lithium‐ion batteries for electrochemical energy conversion and storage, owing to the abundant raw material resources, low cost, and similar electrochemical behavior of elemental sodium compared to lithium. Metal–organic frameworks (MOFs) have attracted enormous attention due to their high surface areas, tunable structures, and diverse applications in drug delivery, gas storage, and catalysis. Recently, there has been an escalating interest in exploiting MOF‐derived materials as anodes for sodium energy storage due to their fast mass transport resulting from their highly porous structures and relatively simple preparation methods originating from in situ thermal treatment processes. In this Review, the recent progress of the sodium‐ion storage performances of MOF‐derived materials, including MOF‐derived porous carbons, metal oxides, metal oxide/carbon nanocomposites, and other materials (e.g., metal phosphides, metal sulfides, and metal selenides), as SIB anodes is systematically and completely presented and discussed. Moreover, the current challenges and perspectives of MOF‐derived materials in electrochemical energy storage are discussed.