Mechanism Of Selective Etching Research Articles

Crystal-Phase-Selective Etching of Heterophase Au Nanostructures.

Selective oxidative etching is one of the most effective ways to prepare hollow nanostructures and nanocrystals with specific exposed facets. The mechanism of selective etching in noble metal nanostructures mainly relies on the different reactivity of metal components and the distinct surface energy of multimetallic nanostructures. Recently, phase engineering of nanomaterials (PEN) offers new opportunities for the preparation of unique heterostructures, including heterophase nanostructures. However, the synthesis of hollow multimetallic nanostructures based on crystal-phase-selective etching has been rarely studied. Here, a crystal-phase-selective etching method is reported to selectively etch the unconventional 4H and 2H phases in the heterophase Au nanostructures. Due to the coating of Pt-based alloy and the crystal-phase-selective etching of 4H-Au in 4H/face-centered cubic (fcc) Au nanowires, the well-defined ladder-like Au@PtAg nanoframes are prepared. In addition, the 2H-Au in the fcc-2H-fcc Au nanorods and 2H/fcc Au nanosheets can also be selectively etched using the same method. As a proof-of-concept application, the ladder-like Au@PtAg nanoframes are used for the electrocatalytic hydrogen evolution reaction (HER) in acidic media, showing excellent performance that is comparable to the commercial Pt/C catalyst.

Effect of Refresh Time on XeF2 Gas-assisted FIB Milling of GaAs

Focused ion beam (FIB) machining can be used to fabricate gallium arsenide-based devices, which have a surface finish of several nanometers, and the FIB machining speed and surface finish can be greatly improved using xenon difluoride (XeF2) gas-assisted etching. Although the refresh time is one of the most important parameters in the gas-assisted etching process, its effect on the machining quality of the surface finish has rarely been studied. Therefore, in this work, we investigated the effect of the refresh time on the etching process, including the dissociation process of XeF2, the refresh time dependency of the sputter in yield under different incident angles, and the surface finish under different refresh times. The results revealed that a selective etching mechanism occurred at different refresh times. At an incidence angle of 0°, the sputtering yield increased with the refresh time and reached its maximum value at 500 ms; at an incidence angle of 30°, the sputtering yield reached its minimum value at a refresh time of 500 ms. For surface roughness, the incident angle played a more important role than the refresh time. The surface finish was slightly better at an incidence angle of 30° than at 0°. In addition, both F and Xe elements were detected in the processed area: Xe elements were evenly distributed throughout the processing area, while F elements tended to accumulate in the whole processing area. The results suggest that the optimum surface can be obtained when a larger refresh time is employed.

Gas-phase etching mechanism of silicon oxide by a mixture of hydrogen fluoride and ammonium fluoride: A density functional theory study

We report the selective etching mechanism of silicon oxide using a mixture of hydrogen fluoride (HF) and NH4F gases. A damage-free selective removal of native oxide has been used in semiconductor manufacturing by forming and removing the ammonium fluorosilicate [(NH4)2SiF6] salt layer. A downstream plasma of NF3/NH3 or a gas-phase mixture of HF and NH4F was used to form (NH4)2SiF6. We modeled and simulated the fluorination of silicon oxide and the salt formation by density functional theory calculation. First, we simulated the successive fluorination of silicon oxide using SiO2 slab models. The fluorination reactions of SiO2 surfaces by the mixture produced a volatile SiF4 molecule or a surface anion of –OSiF4−* with an NH4+ cation with low activation energies. Unlike HF, NH4F produced surface salt species consisting of a surface anion and an ammonium cation. Next, we simulated the (NH4)2SiF6 formation from the two reaction products on fluorinated SiO2 surfaces. (NH4)2SiF6 can be formed exothermally with low activation energies (0.27 or 0.30 eV). Finally, we compared silicon with SiO2 to demonstrate the inherently selective etching of silicon oxide. The fluorination reactions of silicon by the mixture showed the activation energies significantly higher than the SiO2 cases, 1.22–1.56 eV by HF and 1.94–2.46 eV by NH4F due to the less stable transition state geometries. Therefore, the selective salt formation on silicon oxide, not on silicon, is expected in near-room temperature processing, which enables selective etching of silicon oxide.

Freely Tailorable Yolk‐Shell Encapsulation: Versatile Applications in Ultralow‐k Dielectric, Drug Delivery Systems, and Catalysts

AbstractHerein, a facile, controllable, and versatile method is reported to prepare monodisperse yolk‐shell and yolk‐multishell silica nanoparticles (NPs) with mesoporous shells by a novel selective etching strategy. The mechanism of selective etching based on fluoride‐silica chemistry is investigated in detail and thus provides a fundamentally novel principle for the fabrication of yolk‐shell NPs. Specifically, this unprecedented and versatile synthesis strategy can be used to encapsulate essentially any silica‐based, carbon‐based, metal, metal oxide, or other possible NPs. Noteworthy is that most of the yolk‐shell mesoporous silica (mSiO2) NPs are prepared for the first time. To demonstrate the major structural and compositional advantages of the designed yolk‐shell NPs, their applications in the fields of ultralow‐dielectric constant (k) materials, drug delivery systems, and catalysts were explored. In detail, the lowest k value of the prepared yolk‐shellordered mesoporous silica@mSiO2/fluorinated polybenzoxazole composite films is 2.02; The obtained yolk‐shell mSiO2/C@mSiO2/C NPs possess high hydrophilicity and pH‐responsive sensitivity; The conversion of the catalytic reaction of the designed magnetic yolk‐shell hollow Fe3O4@SiO2/Au@mSiO2 NPs at 20 min is 97% with a high conversion rate (92%) and recyclability even after 10 reuses. This innovative work lays a solid foundation for freely tailorable yolk‐shell encapsulation and will greatly stimulate more efforts devoted to relevant research and development.

Selective etching mechanism of silicon oxide against silicon by hydrogen fluoride: a density functional theory study.

Selective etching of silicon oxide (SiO2) against silicon (Si) using anhydrous hydrogen fluoride (HF) vapor has been used for semiconductor device fabrication. We studied the underlying mechanism of the selective etching by density functional theory (DFT) calculation. We constructed surface slab models of SiO2 or Si with different degrees of fluorination and simulated the four steps of fluorination. The calculations show relatively low activation energies of 0.72-0.79 eV for the four steps of fluorination of SiO2, which are close to ∼0.69 eV observed in the experiment. The four-membered ring structure of -Si-O-H-F- in all transition states stabilized the system, resulting in relatively low activation energies. Thus, continuous etching of SiO2 by HF is plausible at near-room temperature. In contrast, the fluorinations of Si showed relatively high activation energies ranging from 1.22 to 1.56 eV due to the less stable transition state geometries. Thus, negligible etching of silicon by HF is expected by the near-room temperature process. Our calculation results explain well the experimental observation of the selective etching of SiO2 against Si by HF vapor.

Template-free lithography for cross-scale channels towards enhancing nanofluidic devices

Nanofluidic devices act a critical role in many inter-/multidisciplinary research fields including single-molecule DNA sequencing and (bio-) chemical detection because of their unique chemical and physics phenomena. Several template-assisted lithography techniques have been individually or synergistically applied to fabricate channels with nanometer to micrometer scales. However, realizing integrated fabrication of cross-scale channels with an atomic precision for emerging demand for device miniaturization and integration remains a significant challenge. Herein, channel with single atom layer depth, which was regarded as ultimate precision of silicon manufacturing, was realized using mechano-chemical scanning probe lithography. From atom- to micro-scale, the channels with hybrid features were also achieved, which can meet the fabrication requirements for all components of nanofluidic devices. The involved selective etching mechanism was addressed based on the proposed dissolution model. The excellent applicability in the field of cross-scale fabrication was demonstrated by label-free enzyme detection using prepared nanofluidic device. This study can significantly promote the development of integrated fabrication and miniaturization for nanofluidic devices.

Fabrication of large-area and highly uniform interlaced silicon grating arrays for high-performance SERS substrates

Surface-enhanced Raman spectroscopy (SERS) provides super ability for detecting trace analytes. However, effective plasmonic architectures and optimal fabrication routes for SERS substrates remained to be further revealed in terms of the cost and performance to promote their applications. In this work, mechano-chemical scanning probe lithography (SPL) was developed to fabricate large-area and high-homogeneity interlaced silicon grating arrays that are hardly achieved by conventional lithography towards high-performance SERS substrates. It was found that this route presented excellent ability for fabricating high-quality groove and hillock structures by controlling the mechanically-driven defects and subsequent wet etching, thereby providing unlimited design diversity for optimizing SERS substrates. Upon designing the layout by finite-difference time-domain simulations, optimizing fabricating processes, and investigating involved selective etching mechanism, various grating patterns were controllably manufactured using the mechano-chemical SPL. SERS spectra of malachite green absorbed on interlaced grating substrates demonstrated an impressive sensitivity and excellent reproducibility with relative deviations of up to 4.09 and 6.08% for batch-to-batch substrate and intra-substrate sampling. This study provides a promising route for customizable and cost-effective fabrication of SERS substrates for chemical and biological sensing.

Hollow Channel Formation inside Sodium Aluminoborate Glass by Femtosecond Laser Writing and Distilled Water Etching.

Recently, the effect of nanograting formation was demonstrated for binary sodium borate glass with the possibility of data storage with an enhanced level of security. The obvious disadvantage of such glass is poor chemical stability, which limits real applications. In this paper, we show that the introduction of Al2O3 allows preserving the possibility of nanograting formation with a significant increase of chemical resistance and thus to preserve optical memory applications. On the other hand, the possibility of selective etching of laser-written tracks by means of distilled water is revealed, which was not demonstrated for other types of glasses. The dependence of retardance of nanogratings form birefringence on laser writing parameters is established and discussed. Structural features of laser-modified microdomains are studied via Raman spectroscopy which revealed an increase of three-coordinated boron content. A possible mechanism of selective etching is discussed.

Nanoscale directional etching features and mechanism of HF/HNO3 etchant

Large-scale and periodic silicon nanostructure arrays are an essential component for various functional devices. However, the conventional selective etching mechanism of specific crystal faces for monocrystalline silicon generally achieves slope or vertical morphology, which greatly limits the preparation of other morphology of silicon nanostructures. In this work, the conventional isotropic etchant HF/HNO3 has successfully achieved nanoscale directional etching and cambered morphology on monocrystalline silicon surface for the first time by changing the synchronous process of oxidation and dissolution into an asynchronous process. Based on the self-assembled monolayer colloidal crystal templates, large-area and high-quality nanopore arrays are prepared on the monocrystalline silicon surface. Particularly, the directional etching ability of HF/HNO3 etchant is further confirmed by combining with electron beam lithography and scanning probe lithography technologies. Hence, this nanoscale directional etching mechanism and etchants provide a promising strategy to achieve cambered morphology nanopores on the monocrystalline silicon surface.

Husbandry waste derived coralline-like composite biomass material for efficient heavy metal ions removal

The resource utilization of biological solid waste is crucial for practical environmental remediation. By comprehensively utilizing LiBr treatment and dopamine chemistry, herein the cow dung waste was successfully converted into the composite biomass material for efficient heavy metal ions removal. A selective etching mechanism of cellulose was discovered in the LiBr treatment process, achieving the large-scale preparation of coralline-like porous biomass material with hundred times increased specific surface. Benefiting from the co-deposition of polyethyleneimine and Fe3O4, the fabricated material showed significantly higher adsorption capacity (183.82 and 231.48 mg·g−1 for Cu2+ and Cd2+) than that of raw cow dung (0.95 and 1.25 mg·g−1 for Cu2+ and Cd2+). Furthermore, this composite biomass adsorbent also exhibited rapid adsorption equilibrium, magnetic separation capability, monolayer chemisorption feature and feasible recycling use. Collectively, this work contributes to both the resource utilization of husbandry solid waste and the development of advanced biomass adsorbent.

Unraveling the selective etching mechanism of silicon nitride over silicon dioxide by phosphoric acid: First-principles study

Highly selective etching of a target silicon compound is essential in semiconductor fabrication. Searching for alternatives to conventional etchant that contain hazardous chemicals is challenging, largely due to the unclear understanding of the chemical reaction. In this study, we elucidate etching machinery of phosphoric acid and its outstanding selectivity toward silicon nitride (Si3N4) over silicon dioxide (SiO2) surfaces in atomistic level. Ab-initio thermodynamic and kinetic formalisms integrated with density functional theory computation propose that pyrophosphoric acid (H4P2O7), a condensed form of orthophosphoric acid.(H3PO4) at high concentration and temperature, is even more reactive toward Si3N4 than H3PO4 which has been regarded as a dominant species for decades. We demonstrate that the superior etching selectivity derives from reaction mechanism of thermodynamic control, where H4P2O7 is much more exergonic than H3PO4. Notably, we find that water molecules close to the H4P2O7 assist sequential etching process in two ways: catalysis via structural proton transfer, and hydrolysis of diphosphate group in the Si3N4 surface. Our study guides a quick and accurate screening as well as designing efficient and safe etchants, which facilitates the fabrication of nanoscale semiconductor devices that accompanies selective etching of alternately stacked hundreds of atomic layers of Si3N4 and SiO2.

Selective chemical wet etching of Si1-xGex versus Si in single-layer and multi-layer with HNO3/HF mixtures

A gate-all-around device, considered as a key structural design for improving performance and density, requires selective etching to obtain nanostructure channels. We investigated the wet etching of epitaxial Si1-xGex films with various HNO3/HF-based chemical etchants to elucidate the mechanism of selective etching of Si1-xGex versus pure Si. Blanket etching was performed on Si1-xGex single-layer and lateral etching was performed on Si/Si1-xGex/Si multi-layer with three different Ge concentrations (x = 0.13, 0.24, and 0.30). In both cases of etching, our results show that the Si1-xGex layer with higher Ge concentration was etched faster. We found that the Si1-xGex etch rate decreased consistently with decreasing HF in both cases, while the slope of the etch rate lowers as the Ge concentration decreases with the blanket etching of Si1-xGex. We postulated that the decrease in the slope of the Si1-xGex etch rate with low Ge concentration occurs due to the difficulty of hole injection into the surface of Si1-xGex, which has a lower valence band offset. In contrast, for the case of lateral etching of Si1-xGex, we found that the slopes of the etch rate are similar to each other irrespective of Ge concentrations. We suggest that the similarity of the slopes is due to the sufficiency of holes, which is supplied not only by HNO3 but also from adjacent Si layers into the Si1-xGex layer. Furthermore, we investigated the effect of structural factors on the lateral etching of Si1-xGex to apply a selective etching process in Si/Si1-xGex/Si multi-layer to obtain Si nanostructures. We found that the lateral etch rate of Si1-xGex in Si/Si1-xGex/Si multi-layers can be reduced owing to the oxide of adjacent Si layers in the etched tunnel, which is not completely removed by HF during the etching process.

The Underlying Chemical Mechanism of Selective Chemical Etching in CsPbBr3 Nanocrystals for Reliably Accessing Near-Unity Emitters.

Reliably accessing nanocrystal luminophores with near-unity efficiencies aids in the ability to understand the upper performance limits in optoelectronic applications that require minimal nonradiative losses. Constructing structure-function relationships at the atomic level, while accounting for inevitable defects, allows for the development of robust strategies to achieve near-unity quantum yield luminophores. For CsPbBr3 perovskite nanocrystals, bromine vacancies leave behind undercoordinated lead atoms that act as traps, limiting the achievable optical performance of the material. We show that selective etching represents a promising path for mitigating the consequences of optical defects in CsPbBr3 nanocrystals. A mechanistic understanding of the etching reaction is essential for developing strategies to finely control the reaction. We report a study of the selective etching mechanism of CsPbBr3 nanocrystal cubes by controlling the etchant chemical potential. We observe optical absorption and luminescence trajectories while varying the extent and rate of lead removal, removing in some cases up to 75% of the lead from the original nanocrystal ensemble. At modest etchant chemical potentials, the size and shape uniformity of the nanocrystal ensemble improves in addition to the quantum yield, proceeding through a layer-by-layer etching mechanism. Operating with excessively high etchant chemical potentials is detrimental to the overall optical performance as the etching transitions to nonselective, while too low of a chemical potential results in incomplete etching. Through this general approach, we show how to finely control selective etching to consistently access a steady state or chemical stability zone of near-unity quantum yield CsPbBr3 nanocrystals postsynthetically, suggesting a practical framework to extend this treatment to other perovskite compositions and sizes.

Ion-Exchange-Induced Selective Etching for the Synthesis of Amino-Functionalized Hollow Mesoporous Silica for Elevated-High-Temperature Fuel Cells.

As differentiated from conventional synthetic processes, amino-functionalized hollow mesoporous silica (NH2-HMS) has been synthesized using a new and facile strategy of ion-exchange-induced selective etching of amino-functionalized mesoporous silica (NH2-meso-silica) by an alkaline solution. Nuclear magnetic resonance (NMR) spectroscopy and in situ time-resolved small-angle X-ray scattering (SAXS) reveal that ion-exchange-induced selective etching arises from the gradient distribution of OH- in the NH2-meso-silica nanospheres. Moreover, the ion-exchange-induced selective etching mechanism is verified through a successful synthesis of hollow mesoporous silica. After infiltration with phosphotungstic acid (PWA), PWA-NH2-HMS nanoparticles are dispersed in the poly(ether sulfone)-polyvinylpyrrolidone (PES-PVP) matrix, forming a hybrid PWA-NH2-HMS/PES-PVP nanocomposite membrane. The resultant nanocomposite membrane with an optimum loading of 10 wt % of PWA-NH2-HMS showed an enhanced proton conductivity of 0.175 S cm-1 and peak power density of 420 mW cm-2 at 180 °C under anhydrous conditions. Excellent durability of the hybrid composite membrane fuel cell has been demonstrated at 200 °C. The results of this study demonstrated the potential of the facile synthetic strategy in the fabrication of NH2-HMS with controlled mesoporous structure for application in nanocomposite membranes as a technology platform for elevated-temperature proton exchange membrane fuel cells.

Influence of annealing on the optical properties, structure, photosensitivity and chemical stability of As30S70 spin-coated thin films

Chalcogenide glass (ChG) thin films prepared by solution based deposition technique offer important advantages over vacuum deposited films, which are useful for many applications especially in infrared optics. Their fabrication, which comprises dissolving the ChG in volatile organic solvent and then depositing thin film by spin coating the solution, is simpler and significantly cheaper. However, contaminants from the solvent are incorporated. The residual organic content in the films is significantly reduced by appropriate thermal treatment. This work reports the influence of specific annealing conditions and UV light exposure on the structure, opto-physical and chemical properties of As30S70 spin-coated ChG films. The results show increase in refractive index with increasing annealing temperature, whereas the thickness and optical bandgap decrease. The UV exposed regions of the samples exhibit positive etching in amine based solvents. The HS-LEIS spectroscopy is used to determine the elemental concentration profile of annealed thin film, which shows almost stable As/S ratio with composition close to that of the source bulk glass. Thus UV light induced structural changes and the mechanism of selective etching are proposed.

Investigation of selective etching mechanism and its dependency on the particle size in preparation of hollow silica spheres

In this paper, the core–shell SiO2@mSiO2/CTAB spheres were fabricated by choosing the solid silica spheres of different sizes as the internal cores, then a series of etching experiments were conducted. For different etching time and particle size, different rattle-type or hollow silica spheres were obtained. Investigation showed that CTAB in the mesoporous shell played a very important role to the selectivity of the etching. Both the rattle-type and hollow structures were successfully fabricated by adjusting the etching time in the presence of CTAB but without obvious etching of mesoporous silica shell. More importantly, the morphology transformation depended on the particle size of the solid silica cores. Generally, when the particle size of the internal cores was very small (d 1 μm), the rattle-type microspheres with at least double-shell structure were first obtained after a certain period of etching time, then the hollow spheres with multi-shell structure were obtained with prolonging etching time. It is further revealed that both the rattle-type and hollow silica spheres with multi-shell structure could be prepared successfully if the particle size of the solid silica core was large enough.

Mechanism of Selective Etching of SiGe Layers in SiGe/Si Systems

Selective lateral etching of the SiGe layer for various Si/SiGe/Si systems was studied. We showed the different etching rates for SiGe layers with the same thickness and Ge concentration, but with different structures of the Si/SiGe/Si stacked layer. We described a mechanism that could explain the experimental results of selective etching of SiGe layers in Si/SiGe/Si systems. According to the mechanism, we were able to summarize the following 3 key points in realizing high selectivity SiGe etching. 1) Should be the SiGe layer in a higher oxidation state (enough number of holes), 2) a wider area is necessary for Si substrate surface in contact with the solution, and 3) the current path should be maintained at the SiGe/Si interface.

Mechanism of Selective Etching of SiGe Layers in SiGe/Si Systems

not Available.

In-situ examination of the selective etching of an alkanethiol monolayer covered Au{111} surface

An examination of the selective etching mechanism of a 1-alkanethiol self-assembled monolayer (SAM) covered Au{111} surface using in-situ atomic force microscopy (AFM) and molecular resolution scanning tunnelling microscopy (STM) is presented. The monolayer self-assembles on a smooth Au{111} surface and typically contains nanoscale non-uniformities such as pinholes, domain boundaries and monatomic depressions. During etching in a ferri/ferrocyanide water-based etchant, selective and preferential etching occurs at SAM covered Au(111) terrace and step edges where a lower SAM packing density is observed, resulting in triangular islands being relieved. The triangular islands are commensurate with the Au(111) lattice with their long edges parallel to its [11¯0] direction. Thus, SAM etching is selective and preferential attack is localized to defects and step edges at sites of high molecular density contrast.

The growth and characterization of diamond cone arrays formed by plasma etching

Diamond cone arrays with high aspect ratio are formed by plasma etching on diamond film grown by hot-filament chemical vapor deposition (HFCVD) method. The cone angle and orientation of etched cones with tip diameter of about 10–20 nm are uniform, and the cone density of cone arrays formed on diamond film can be controlled by changing the etching time during plasma etching. Energy dispersion X-ray analysis (EDX) and Raman spectroscopy show that the cone arrays are composed of carbons in the chemical states of diamond, nanodiamond and amorphous carbons. High-resolution transmission electron microscopy (HRTEM) analysis indicates that diamond cone has an amorphous outer layer and an inner core with certain crystallization. It can be concluded that the outer amorphous layer is formed by the bombardment of carbon containing ions on the diamond surface, and the nanodiamond is formed by the dimensional reduction of original diamond crystalline into nanometer scale. Selected area electrons diffraction (SAED) pattern analysis is used to study the structures of diamond cones. The diamond cone formation is interpreted using the selective etching mechanism of grooves on a rough surface of diamond film.