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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.

For the analysis of thin films, with high aspect ratio (HAR) structures, time-of-flight secondary ion mass spectrometry (ToF-SIMS) overcomes several challenges in comparison to other frequently used techniques such as electron microscopy. The research presented herein focuses on two different kinds of HAR structures that represent different semiconductor technologies. In the first study, ToF-SIMS is used to illustrate cobalt seed layer corrosion by the copper electrolyte within the large through-silicon-vias (TSVs) before and after copper electroplating. However, due to the sample’s surface topography, ToF-SIMS analysis proved to be difficult due to the geometrical shadowing effects. Henceforth, in the second study, we introduce a new test platform to eliminate the difficulties with the HAR structures, and again, use ToF-SIMS for elemental analysis. We use data image slicing of 3D ToF-SIMS analysis combined with lateral HAR test chips (PillarHall™) to study the uniformity of silicon dopant concentration in atomic layer deposited (ALD) HfO2 thin films.

This study adopted femtosecond laser with a wavelength of 515 nm to drill high-aspect-ratio micro through holes on a 500-μm thickness single-crystal SI-type 4H-SiC wafer. Firstly, through holes with a high aspect ratio of 20 were fabricated in air. However, the heat affect zone (HAZ), cracks, and surface material shedding around entrances and exits of the holes are inevitable in air even after chemical corrosion post-processing. In order to remove these defects, the water-assisted femtosecond laser drilling of 4H-SiC was investigated. The high-quality through holes free of cracks, surface material shedding, and HAZ were obtained under the action of internal scour and heat diffusion of water. Besides, the water layer thickness and the laser repetition frequency have a great influence on the processing quality and efficiency of the micro-holes. Finally, high-quality high-ratio-rate through micro-hole arrays on 4H-SiC were fabricated with the optimal process parameters, which is significant for the development of SiC electronic devices and the high-quality micro-fabrication of other hard and brittle materials.

The challenging machinability of silicon carbide (SiC) ceramic, due to its hardness and brittleness, has traditionally constrained its machined quality and the creation of functional surfaces. Compared to direct laser machining (DLM), waterjet-assisted laser micromachining (WJALM) is an alternative technique for SiC ceramic that is capable of reducing thermal-induced damages. In this paper, high-aspect-ratio (HAR) microchannels are fabricated on silicon carbide ceramic by WJALM, and its effectiveness is verified through comparative experiments with DLM. The effects of the parametric combination of waterjet and laser parameters on machining responses of geometric structural features and sidewall surface quality are investigated by controlled variable experiments. The results revealed that HAR microchannels with almost no recast layers could be obtained when the SiC workpiece was fabricated by a nanosecond laser under the flowing water medium layer, and higher average laser power of 27 W, lower scanning speed of 600 m/s, and medium waterjet velocity of 12/16 m/s contributed to larger aspect ratio, more ablation area and superior sidewall quality of HAR microchannels.

Background The EU-project GRACIOUS developed an Integrated Approach to Testing and Assessment (IATA) to support grouping high aspect ratio nanomaterials (HARNs) presenting a similar inhalation hazard. Application of grouping reduces the need to assess toxicity on a case-by-case basis and supports read-across of hazard data from substances that have the data required for risk assessment (source) to those that lack such data (target). The HARN IATA, based on the fibre paradigm for pathogenic fibres, facilitates structured data gathering to propose groups of similar HARN and to support read-across by prompting users to address relevant questions regarding HARN morphology, biopersistence and inflammatory potential. The IATA is structured in tiers, allowing grouping decisions to be made using simple in vitro or in silico methods in Tier1 progressing to in vivo approaches at the highest Tier3. Here we present a case-study testing the applicability of GRACIOUS IATA to form an evidence-based group of multiwalled carbon nanotubes (MWCNT) posing a similar predicted fibre-hazard, to support read-across and reduce the burden of toxicity testing. Results The case-study uses data on 15 different MWCNT, obtained from the published literature. By following the IATA, a group of 2 MWCNT was identified (NRCWE006 and NM-401) based on a high degree of similarity. A pairwise similarity assessment was subsequently conducted between the grouped MWCNT to evaluate the potential to conduct read-across and fill data gaps required for regulatory hazard assessment. The similarity assessment, based on expert judgement of Tier 1 assay results, predicts both MWCNT are likely to cause a similar acute in vivo hazard. This result supports the possibility for read-across of sub-chronic and chronic hazard endpoint data for lung fibrosis and carcinogenicity between the 2 grouped MWCNT. The implications of accepting the similarity assessment based on expert judgement of the MWCNT group are considered to stimulate future discussion on the level of similarity between group members considered sufficient to allow regulatory acceptance of a read-across argument. Conclusion This proof-of-concept case-study demonstrates how a grouping hypothesis and IATA may be used to support a nuanced and evidence-based grouping of ‘similar’ MWCNT and the subsequent interpolation of data between group members to streamline the hazard assessment process.

Over the past few years, target detectors that utilize Convolutional Neural Networks have gained extensive application in the domain of remote sensing (RS) imagery. Recently, optimizing bounding boxes has consistently been a hot topic in the research field. However, existing methods often fail to take into account the interference caused by the shape and orientation changes of RS targets with high aspect ratios during training, leading to challenges in boundary perception when dealing with RS targets that have large aspect ratios. To deal with this challenge, our study introduces the Adaptive Boundary Perception Network (ABP-Net), a novel two-stage approach consisting of pre-training and training phases, which enhances the boundary perception of CNN-based detectors. In the pre-training phase, involving the initialization of our model’s backbone network and the label assignment, the traditional label assignment with a fixed IoU threshold fails to fully cover the critical information of slender targets, resulting in the detector missing lots of high-quality positive samples. To overcome this drawback, we design a Shape-Sensitive (S-S) label assignment strategy that can improve the boundary shape perception by dynamically adjusting the IoU threshold according to the aspect ratios of the targets so that the high-quality samples with critical features can be divided into positive samples. Moreover, during the training phase, minor angle differences of the slender bounding box may cause a significant change in the value of the loss function, producing unstable gradients. Such drastic gradient changes make it difficult for the model to find a stable update direction when optimizing the bounding box parameters, resulting in difficulty with the model convergence. To this end, we propose the Robust–Refined loss function (R-R), which can enhance the boundary localization perception by focusing on low-error samples and suppressing the gradient amplification of difficult samples, thereby improving the model stability and convergence. Experiments on UCAS-AOD and HRSC2016 datasets validate our specialized detector for high-aspect-ratio targets, improving performance, efficiency, and accuracy with straightforward operation and quick deployment.

Superhydrophobic surfaces (SHS), with their exceptional water‐repellent properties, have attracted great interest due to their versatile applications. The robustness of SHS has emerged as an essential issue for practical applications, as SHS are directly exposed to various harsh environments, such as continuous raindrop impact, corrosive media, and extreme temperatures. Polyimide (PI) is an ideal candidate for robust SHS due to its superior mechanical, thermal, and chemical properties. However, the low processability of PI in surface microstructuring has limited its application in SHS. In this study, a high‐yield and cost‐effective fabrication method for constructing high‐aspect‐ratio PI microstructures has been developed by controlling the template surface treatment, precursor molecular weight, and vacuum process. This approach achieves an exceptional yield rate of 99.8% and an aspect ratio of 10.7, enabling the construction of various microstructures. The SHS is demonstrated by fabricating microstructures on PI surfaces using the proposed method. The PI SHS exhibits a water contact angle of up to 162° and a roll‐off angle of less than 9°. The water repellency withstands 100 tape peeling tests and remains stable after continuous exposure to temperatures up to 250 °C and various chemical reagents for 60 days, which presents excellent robustness against environmental factors. This study presents robust polyimide superhydrophobic surfaces with excellent mechanical durability, thermal stability, and chemical stability. A high‐yield and cost‐effective method to fabricate high‐aspect‐ratio polyimide surface microstructures is developed by controlling the template surface treatment, precursor molecular weight, and vacuum process time. The proposed method is applied for the realization of robust superhydrophobic surfaces.

Creating micrometer-resolution high-aspect-ratio three-dimensional (3D) structures remain very challenging despite significant microfabrication methods developed for microelectromechanical systems (MEMS). This is especially the case when such structures are desired to be metallic to support electronic applications. Here, we present a microfabrication process that combines two-photon-polymerization (2PP) printing to create a polymeric high-aspect-ratio three-dimensional structure and electroless metal plating that selectively electroplates only the polymeric structure to create high-aspect-ratio 3D metallic structures having micrometer-resolution. To enable this, the effect of various 2PP processing parameters on SU-8 photoresist microstructures were first systematically studied. These parameters include laser power, slicing/hatching distances, and pre-/post-baking temperature. This optimization resulted in a maximum aspect ratio (height to width) of ~ 12. Following this polymeric structure printing, electroless plating using Tollens’ Reagent were utilized to selectively coat silver particles only on the polymeric structure, but not on the silicon substrate. The final 3D metallic structures were evaluated in terms of their resistivity, reproducibly showing resistivity of ~ 10–6 [Ω·m]. The developed 3D metallic structure microfabrication process can be further integrated with conventional 2D lithography to achieve even more complex structures. The developed method overcomes the limitations of current MEMS fabrication processes, allowing a variety of previously impossible metallic microstructures to be created.

High aspect ratio (HAR) structures are widely utilized in diverse applications, including the defense industries, medical treatment, and aerospace. Studies show that the micro-milling has superiorities such as high efficiency and high flexibility over other methods. Currently, the biggest problem of micro-milling is the influence of burrs on the milling performance, which is an enormous challenge to remove. In the present study, numerical simulations through the finite element method (FEM) and experiments are carried out to optimize cutting parameters of the specimen made of Cr12MoV alloy. Moreover, an improved constitutive model is proposed by considering the size effect of the micro-milling. The proposed model is based on the J-C constitutive model, and it is concluded that the cutting force error is 4.6% through the comparison of experiment and FEM, which further proves the convergence of the improved constitutive model. It is also found that the side burr of HAR slots and the top burr of shallow slots are the biggest factors affecting the surface quality. The depth of cut (DoC) affects the maximum bending angle of the tool, the feed per tooth (FpT) affects the size of the unremoved area, and the spindle speed (SpS) affects the dynamic balance of the micro-mill, through the exploration of the above cutting parameters which can effectively improve the cutting state of the tool and finally achieve the purpose of curbing burr.

PurposeGround vibration testing is critical for aircraft design and certification. Fast relaxed vector fitting (FRVF) and Loewner framework (LF), recently extended to modal parameter extraction in mechanical systems to address the computational challenges of time and frequency domain techniques, are applied for damage detection on aeronautically relevant structures.Design/methodology/approachFRVF and LF are applied to numerical datasets to assess noise robustness and performance for damage detection. Computational efficiency is also evaluated. In addition, they are applied to a novel damage detection benchmark of a high aspect ratio wing, comparing their performance with the state-of-the-art method N4SID.FindingsFRVF and LF detect structural changes effectively; LF exhibits better noise robustness, while FRVF is more computationally efficient.Practical implicationsLF is recommended for noisy measurements.Originality/valueTo the best of the authors’ knowledge, this is the first study in which the LF and FRVF are applied for the extraction of the modal parameters in aeronautically relevant structures. In addition, a novel damage detection benchmark of a high-aspect-ratio wing is introduced.