This paper reviews the recent advances in reaction-ion etching (RIE) for application in high-aspect-ratio microfabrication. High-aspect-ratio etching of materials used in micro- and nanofabrication ...has become a very important enabling technology particularly for bulk micromachining applications, but increasingly also for mainstream integrated circuit technology such as three-dimensional multi-functional systems integration. The characteristics of traditional RIE allow for high levels of anisotropy compared to competing technologies, which is important in microsystems device fabrication for a number of reasons, primarily because it allows the resultant device dimensions to be more accurately and precisely controlled. This directly leads to a reduction in development costs as well as improved production yields. Nevertheless, traditional RIE was limited to moderate etch depths (e.g., a few microns). More recent developments in newer RIE methods and equipment have enabled considerably deeper etches and higher aspect ratios compared to traditional RIE methods and have revolutionized bulk micromachining technologies. The most widely known of these technologies is called the inductively-coupled plasma (ICP) deep reactive ion etching (DRIE) and this has become a mainstay for development and production of silicon-based micro- and nano-machined devices. This paper will review deep high-aspect-ratio reactive ion etching technologies for silicon, fused silica (quartz), glass, silicon carbide, compound semiconductors and piezoelectric materials.
Silicon carbide (SiC) is a wide bandgap third‐generation semiconductor well suited for harsh environment power electronics, micro and nano electromechanical systems, and emerging quantum technology ...by serving as hosts for quantum states via defect centers. The chemical inertness of SiC limits viable etching techniques to plasma‐based reactive ion etching methods; however, these could have significant undesirable effects for electronic and photonic devices. This paper presents a plasma‐free, open‐circuit, photo‐induced metal‐assisted chemical etch for fabricating micro and nanoscale features without the inherent high energy ion‐related surface damage. The method presented herein utilizes above bandgap ultraviolet light, patterned noble metal (Pt), and a solution consisting of an oxidant potassium persulfate (K2S2O8) and an acid, hydrofluoric acid, to spatially define the etching morphology. The parameter space is comprehensively explored to demonstrate the controllability and versatility of this technique to produce ordered arrays of micro and nanoscale SiC structures with porous or solid sidewalls, and to elucidate the etching mechanism.
Silicon carbide periodic hole arrays are produced via photo‐enhanced‐metal‐assisted chemical etching. This is an open‐circuit, plasma‐free, metal‐catalyzed highly‐anisotropic etching method. The surface morphology including porosity and smoothness can be tuned by the etch condition, potentially useful for power electronics, MEMS, and quantum technologies.
A new dual linearly polarised phase-shifting cell for reflectarray applications is presented. It consists of two orthogonal sets of parallel rectangular slots etched in a ground plane. Each set may ...be loaded with a combination of variable and fixed capacitances to provide a reconfigurable 2-bit phase-shifter for both polarisations. A passive version of the proposed cell has been fabricated and characterised in C-band with four fixed states. For each polarisation only two variable capacitances are needed in order to control the phase of the reflected wave. PUBLICATION ABSTRACT
Both alkali and metal‐catalyzed chemical etching (MCCE) of multicrystalline silicon (mc‐Si) wafer show anisotropic etching behavior, resulting in different morphologies among the different grains. ...However, by combining alkali etching, MCCE, and a post‐etching process, homogeneous microstructures can be obtained on the surface of mc‐Si wafer. After the first alkali etching, there are three typical morphologies of upward pyramids, terraces, and tilt planes, and relative to the initial Si(100), Si(110), and Si(111) dominated grains, these show low, moderate, and high reflection, respectively. After MCCE and the post‐etching process, the microstructures on the different grains have converged to a similar morphology and reflection. Mc‐Si solar cells fabricated by complementary alkali etching, MCCE, and post‐etching have a good appearance and high efficiency of ~19.4%. Moreover, the cells with submicrometer texture have the advantages of reverse current‐voltage characteristics and weak light response over traditional cells with micrometer texture.
We succeeded in obtaining the homogeneous microstructures and the less color difference among grains on the surface of mc‐Si wafer by anisotropic alkali etching, MCCE, and post‐etching, which show complementary etching behavior on certain grains. The mc‐Si solar cells made from the complementary etching behavior show good appearance and high efficiency ~19.4%, and have higher reverse breakdown voltages and better weak light performances than that of H‐DRE cells.
Surface texturing for suppressing the reflection losses is the first and foremost step in the solar cell fabrication process. Over the years, multi-crystalline silicon (mc-Si) wafer solar cells ...dominated the PV market due to their cost-effectiveness. This article reviews various etching methods reported for texturing mc-Si wafers under the light of basic reaction mechanism, general composition of chemicals used, merits, drawbacks, and the progress made towards their commercialization. The article starts with introducing the alkaline etching process used in initial days of mc-Si solar cell production along with its process limitations. Subsequently, evolution of acid texturing process to address the challenges associated with alkaline etching and its journey as an established industrial process are discussed. Thereafter, progressive but expensive texturing methods such as reactive ion etching, laser etching, mechanical grooving and colloidal lithography are also reviewed. Later, the challenges imposed by introduction of diamond wire sawn (DWS) mc-Si wafers on established acid texturing process are presented. Further, remarkable approaches reported towards the effective texturing of DWS mc-Si wafers are discussed. Finally, a summary of reaction chemistry, recent process advances and future roadmap of proposed next-generation texturing scheme known as metal assisted chemical etching is being compiled and presented.
Metal‐assisted chemical etching (MacEtch) of silicon in oxidizing hydrofluoric acid (HF) solutions has emerged as a prominent top‐down micro/nanofabrication approach for a wide variety of silicon ...micro/nanostructures. The popularity of the process is due to its simplicity, rapidity, versatility, and scalability. In recent years, there has been a surge of interest in developing MacEtch silicon micro/nanostructures for advanced energy conversion and storage applications, such as photovoltaic devices, thermoelectric devices, lithium‐ion rechargeable batteries, and supercapacitors. Particularly, MacEtch has emerged as a powerful surface micro/nanostructuring method for low‐cost and scalable production of commercial black silicon (b‐Si) with excellent light trapping properties. This review on MacEtch processing of silicon in oxidizing HF solutions provides a critical description of its history and origin including how it evolved into what it is today, the understanding of its mechanism and important technical advances in the field. As regards MacEtch‐fabricated b‐Si, its initial discovery and further improvements to its large‐scale deployment in silicon photovoltaic industry are traced. Some fundamental challenges and perspectives in this exciting field are also discussed.
Metal‐assisted chemical etching of silicon in oxidizing hydrofluoric acid solutions is attracting considerable interest as an emerging top‐down micro/nanofabrication approach owing to its simplicity, rapidity, versatility, and scalability. This article reviews its development, mechanism, and advances in fabrication techniques, and highlights its development for black silicon solar cell applications.
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.
Metal‐assisted chemical etching (MaCE) is a versatile method for the facile fabrication of Si and other semiconductors into structures having widely different scales. The recent advances in MaCE are introduced, with special emphasis on the macroscale structuring of Si and its applications to a variety of fields, such as microelectromechanical systems, microfluidics, and micro‐total analysis systems.
This study proposes a top-down approach for fabricating high aspect ratio AlN pillars with m-oriented nonpolar sidewalls, which will serve as the first building block for the fabrication of ...core-shell UV-LEDs. Such structures are achieved through a two-step process, combining a chlorine plasma etching, followed by a wet chemical etching with KOH. In this work, the mechanisms driving the AlN etching in chlorine plasma are discussed. In particular, we highlight the impact of the ratio between ion flux and radical flux on AlN etch rates, pillar profiles, and crystal orientation-dependent etching. We also identify two mechanisms of passivation layer formation on the pillar sidewalls that contribute to the pattern slope: redeposition of the carrier wafer etch by-products and Aluminium line of sight redeposition, both phenomena are also driven by the ionic bombardment. Low ionic bombardment (either low ion over radical flux ratio or low ion energy) has been identified as plasma conditions allowing the patterning of anisotropic AlN pillars but with the formation of a-nonpolar facets. Due to the inability to obtain m-oriented sidewalls using Cl2 plasma etching alone, we show that the use of a KOH wet etching treatment allows verticalizing the pillars and revealing smooth m-facets on the pillars’ sidewalls if suitable m-oriented hard mask is used. In this wet etching step, we highlight the key role played by the hard mask and its initial shape during the wet etching if straight and m-oriented pillars are desired.
Bearings are vital components that are widely used in modern machinery. Although usually manufactured with high-strength steels, bearings still suffer from rolling contact fatigue where unique ...microstructural alterations take place beneath the contact surface as a result of the complex stress state. Studying these microstructural alterations is a hot research topic with many efforts in recent decades. In this respect, the key information regarding four major types of microstructural alterations, white etching areas/white etching cracks, dark etching regions, white etching bands and light etching regions is reviewed regarding the phenomenology and formation mechanisms. Then, classical and state-of-the-art models are established to predict their formation and are summarised and evaluated. Based on the current research progress, several key questions and paradoxes for each type of microstructural alteration are raised, suggesting possible research directions in this field.
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.
A hybrid anodic and metal‐assisted chemical etching method is proposed to etch silicon carbide, one of the hardest to etch wide bandgap materials, to form nanowire arrays. Through investigations of the etching mechanism and optimal etching conditions, the required electrical bias is significantly lowered while the etching efficiency enhanced. By tuning the etching conditions, various nanostructures can be obtained.
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