Showing papers on "Isotropic etching published in 2018"

3D Printing of Bioinspired Liquid Superrepellent Structures.

Abstract: Bioinspired re-entrant structures have been proved to be effective in achieving liquid superrepellence (including anti-penetration, anti-adhesion, and anti-spreading). However, except for a few reports relying on isotropic etching of silicon wafers, most fluorination-dependent surfaces are still unable to repel liquids with extreme low surface energy (i.e., γ < 15 mN m-1 ), especially those fluorinated solvents. Herein, triply re-entrant structures, possessing superrepellence to water (with surface tension γ of 72.8 mN m-1 ) and various organic liquids (γ = 12.0-27.1 mN m-1 ), are fabricated via two-photon polymerization based 3D printing technology. Such structures can be constructed both on rigid and flexible substrates, and the liquid superrepellent properties can be kept even after oxygen plasma treatment. Based on the prepared triply re-entrant structures, micro open capillaries are constructed on them to realize directional liquid spreading, which may be applied in microfluidic platforms and lab-on-a-chip applications. The fabricated arrays can also find potential applications in electronic devices, gas sensors, microchemical/physical reactors, high-throughput biological sensors, and optical displays.

High-throughput microchannel fabrication in fused silica by temporally shaped femtosecond laser Bessel-beam-assisted chemical etching.

Abstract: We proposed combining temporally shaped (double-pulse train) laser pulses with spatially shaped (Bessel beam) laser pulses. By using a temporally shaped femtosecond laser Bessel-beam-assisted chemical etching method, the energy deposition efficiency was improved by adjusting the pulse delay to yield a stronger material modification and, thus, a higher etching depth. The etching depth was enhanced by a factor of 13 using the temporally shaped Bessel beam. The mechanism of etching depth enhancement was elucidated by localized transient-free electrons dynamics-induced structural and morphological changes. Micro-Raman spectroscopy was conducted to verify the structural changes inside the material. This method enables high-throughput, high-aspect-ratio microchannel fabrication in fused silica for potential applications in microfluidics.

Optimisation of ultrafast laser assisted etching in fused silica.

Abstract: Ultrafast laser assisted etching (ULAE) in fused silica is an attractive technology for fabricating three-dimensional micro-components. ULAE is a two-step process whereby ultrafast laser inscription (ULI) is first used to modify the substrate material and chemical etching is then used to remove the laser modified material. In this paper, we present a detailed investigation into how the ULI parameters affect the etching rate of laser modified channels and planar surfaces written in fused silica. Recently, potassium hydroxide (KOH) has shown potential to outperform the more commonly used hydrofluoric acid (HF) as a highly selective etchant for ULAE. Here we perform a detailed comparison of HF and KOH etching after laser inscription with a wide range of ultrafast laser irradiation parameters. Etching with KOH is found to be significantly more selective, removing the laser modified material up to 955 times faster than pristine material, compared with up to 66 when using HF. Maximum etching rates for the two etchants were comparable at 320 μm/hour and 363 μm/hour for HF and KOH respectively. We further demonstrate that highly selective, isotropic etching of non-planar surfaces can be achieved by controlling the polarization state of the laser dynamically during laser inscription.

Method of processing a substrate and a device manufactured by the same

Abstract: Provided is a substrate processing method capable of preventing over-etching of a part of a stair-case structure due to an etching solution, when a barrier layer is selectively formed on a VNAND device having the stair-case structure. The substrate processing method includes: alternately stacking a first insulating layer and a second insulating layer; forming a stepped structure having an upper surface, a lower surface, and a side surface connecting the upper surface to the lower surface by etching the first insulating layer and the second insulating layer that are stacked; densifying the stepped structure; forming a barrier layer on the densified second insulating layer; and performing isotropic etching on at least a part of a sacrificial word line structure including the second insulating layer and the barrier layer. During etching the barrier layer at the isotropic etching step, the second insulating layer is not etched or etched a little to an ignorable degree.

Condition optimization for exfoliation of two dimensional titanium carbide (Ti3C2T x ).

Abstract: The new class of 2D MXene material exfoliated from transition metal carbides receives increasing research interest due to its extraordinary properties and high potential in energy and environmental applications. However, the exfoliation of Ti3C2T x , a widely studied MXene, from its precursor Ti3AlC2 by chemical etching in HF solution remains to be optimized. This study investigated the optimum exfoliation condition through systematic evaluating potential effects of reaction parameters, including the weight ratio of Ti3AlC2 in HF solution, etching time, reaction temperature, repeating etching, and sonication, on the yield, purity, and structure of produced Ti3C2T x . Results show that a high weight percentage (5 wt%) of Ti3AlC2 etching at 50 °C for 36 h produced highly exfoliated MXene material. Etching at lower weight percentages (0.6-2.5 wt%) of Ti3AlC2 resulted in observable byproduct (AlF3). Degradation of MXene layers with AlF3 enrichment was observed under prolonged etching or higher temperatures. Room temperature etching failed to exfoliate Ti3AlC2 and the repeated etching denatured the MXene material. Introduction of controlled sonication during the etching produced highly exfoliated MXene with minimum etching time, which can be a promising alternative for high quality MXene production.

Enhanced Performance of Ge Photodiodes via Monolithic Antireflection Texturing and α-Ge Self-Passivation by Inverse Metal-Assisted Chemical Etching.

Abstract: Surface antireflection micro and nanostructures, normally formed by conventional reactive ion etching, offer advantages in photovoltaic and optoelectronic applications, including wider spectral wavelength ranges and acceptance angles. One challenge in incorporating these structures into devices is that optimal optical properties do not always translate into electrical performance due to surface damage, which significantly increases surface recombination. Here, we present a simple approach for fabricating antireflection structures, with self-passivated amorphous Ge (α-Ge) surfaces, on single crystalline Ge (c-Ge) surface using the inverse metal-assisted chemical etching technology (I-MacEtch). Vertical Schottky Ge photodiodes fabricated with surface structures involving arrays of pyramids or periodic nano-indentations show clear improvements not only in responsivity, due to enhanced optical absorption, but also in dark current. The dark current reduction is attributed to the Schottky barrier height increase and self-passivation effect of the i-MacEtch induced α-Ge layer formed on top of the c-Ge surface. The results demonstrated in this work show that MacEtch can be a viable technology for advanced light trapping and surface engineering in Ge and other semiconductor based optoelectronic devices.

One-Dimensional Porous Silicon Nanowires with Large Surface Area for Fast Charge⁻Discharge Lithium-Ion Batteries.

Abstract: In this study, one-dimensional porous silicon nanowire (1D⁻PSiNW) arrays were fabricated by one-step metal-assisted chemical etching (MACE) to etch phosphorus-doped silicon wafers. The as-prepared mesoporous 1D⁻PSiNW arrays here had especially high specific surface areas of 323.47 m²·g-1 and were applied as anodes to achieve fast charge⁻discharge performance for lithium ion batteries (LIBs). The 1D⁻PSiNWs anodes with feature size of ~7 nm exhibited reversible specific capacity of 2061.1 mAh·g-1 after 1000 cycles at a high current density of 1.5 A·g-1. Moreover, under the ultrafast charge⁻discharge current rate of 16.0 A·g-1, the 1D⁻PSiNWs anodes still maintained 586.7 mAh·g-1 capacity even after 5000 cycles. This nanoporous 1D⁻PSiNW with high surface area is a potential anode candidate for the ultrafast charge⁻discharge in LIBs with high specific capacity and superior cycling performance.

Highly enhanced response of MoS2/porous silicon nanowire heterojunctions to NO2 at room temperature

Abstract: Molybdenum disulfide/porous silicon nanowire (MoS2/PSiNW) heterojunctions with different thicknesses as highly-responsive NO2 gas sensors were obtained in the present study. Porous silicon nanowires were fabricated using metal-assisted chemical etching, and seeded with different thicknesses. After that, MoS2 nanosheets were synthesized by sulfurization of direct-current (DC)-magnetic-sputtering Mo films on PSiNWs. Compared with the as-prepared PSiNWs and MoS2, the MoS2/PSiNW heterojunctions exhibited superior gas sensing properties with a low detection concentration of 1 ppm and a high response enhancement factor of ∼2.3 at room temperature. The enhancement of the gas sensitivity was attributed to the layered nanostructure, which induces more active sites for the absorption of NO2, and modulation of the depletion layer width at the interface. Further, the effects of the deposition temperature in the chemical vapor deposition (CVD) process on the gas sensing properties were also discussed, and might be connected to the nucleation and growth of MoS2 nanosheets. Our results indicate that MoS2/PSiNW heterojunctions might be a good candidate for constructing high-performance NO2 sensors for various applications.

Difference in anisotropic etching characteristics of alkaline and copper based acid solutions for single-crystalline Si

Abstract: The so called inverted pyramid arrays, outperforming conventional upright pyramid textures, have been successfully achieved by one-step Cu assisted chemical etching (CACE) for light reflection minimization in silicon solar cells. Due to the lower reduction potential of Cu2+/Cu and different electronic properties of different Si planes, the etching of Si substrate shows orientation-dependent. Different from the upright pyramid obtained by alkaline solutions, the formation of inverted pyramid results from the coexistence of anisotropic etching and localized etching process. The obtained structure is bounded by Si {111} planes which have the lowest etching rate, no matter what orientation of Si substrate is. The Si etching rate and (100)/(111) etching ratio are quantitatively analyzed. The different behaviors of anisotropic etching of Si by alkaline and Cu based acid etchant have been systematically investigated.

Designing effective Si/Ag interface via controlled chemical etching for photoelectrochemical CO2 reduction

Abstract: Photoelectrochemical reduction of CO2 to value-added chemicals represents a promising approach for artificial photosynthesis, but often suffers from limited selectivity and stability. Improving its performance would require proper design of semiconductor and co-catalyst materials, along with a strategy for effective coupling. Here, we report that controlled chemical etching of Si wafer by Ag+ ions yields effective semiconductor/co-catalyst interface for photoelectrochemical CO2 reduction. Resultant photocathodes exhibit large photocurrent density (∼10 mA cm−2 under 0.5 sun), great CO faradaic efficiency (90% at −0.5 V versus reversible hydrogen electrode), and impressive operational stability (little activity or selectivity loss within 8 h). Further enhancement (by ∼20%) of photocurrent density is achieved by combining photolithography patterning with chemical etching. Our study applies long-known chemistry as an unexpected solution and may provide a new strategy for high-performance photoelectrochemical CO2 reduction.

Isotropic atomic layer etching of ZnO using Acetylacetone and O2 plasma

Abstract: Atomic layer etching (ALE) provides Angstrom-level control over material removal and holds potential for addressing the challenges in nanomanufacturing faced by conventional etching techniques. Recent research has led to the development of two main classes of ALE: ion-driven plasma processes yielding anisotropic (or directional) etch profiles and thermally driven processes for isotropic material removal. In this work, we extend the possibilities to obtain isotropic etching by introducing a plasma-based ALE process for ZnO which is radical-driven and utilizes acetylacetone (Hacac) and O2 plasma as reactants. In situ spectroscopic ellipsometry measurements indicate self-limiting half-reactions with etch rates ranging from 0.5 to 1.3 A/cycle at temperatures between 100 and 250 °C. The ALE process was demonstrated on planar and three-dimensional substrates consisting of a regular array of semiconductor nanowires (NWs) conformally covered using atomic layer deposition of ZnO. Transmission electron microscopy studies conducted on the ZnO-covered NWs before and after ALE proved the isotropic nature and the damage-free characteristics of the process. In situ infrared spectroscopy measurements were used to elucidate the self-limiting nature of the ALE half-reactions and the reaction mechanism. During the Hacac etching reaction that is assumed to produce Zn(acac)2, carbonaceous species adsorbed on the ZnO surface are suggested as the cause of the self-limiting behavior. The subsequent O2 plasma step resets the surface for the next ALE cycle. High etch selectivities (∼80:1) over SiO2 and HfO2 were demonstrated. Preliminary results indicate that the etching process can be extended to other oxides such as Al2O3.

Effect of OH− on chemical mechanical polishing of β-Ga2O3 (100) substrate using an alkaline slurry

Abstract: β-Ga2O3, a semiconductor material, has attracted considerable attention given its potential applications in high-power devices, such as high-performance field-effect transistors. For decades, β-Ga2O3 has been processed through chemical mechanical polishing (CMP). Nevertheless, the understanding of the effect of OH− on β-Ga2O3 processed through CMP with an alkaline slurry remains limited. In this study, β-Ga2O3 substrates were successively subjected to mechanical polishing (MP), CMP and etching. Then, to investigate the changes that occurred on the surfaces of the samples, samples were characterised through atomic force microscopy (AFM), three-dimensional laser scanning confocal microscopy (LSCM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). LSCM and SEM results showed that β-Ga2O3 is highly vulnerable to brittle fracture during MP. AFM revealed that an ultrasmooth and nondamaged surface with a low Ra of approximately 0.18 nm could be obtained through CMP. XPS results indicated that a metamorphic layer, which mainly contains soluble gallium salt (Ga(OH)4−), formed on the β-Ga2O3 surface through a chemical reaction. A dendritic pattern appeared on the surface of β-Ga2O3 after chemical etching. This phenomenon indicated that the chemical reaction on the β-Ga2O3 surface occurred in a nonuniform and selective manner. The results of this study will aid the optimization of slurry preparation and CMP.

Chiral visible light metasurface patterned in monocrystalline silicon by focused ion beam

Abstract: High refractive index makes silicon the optimal platform for dielectric metasurfaces capable of versatile control of light. Among various silicon modifications, its monocrystalline form has the weakest visible light absorption but requires a careful choice of the fabrication technique to avoid damage, contamination or amorphization. Presently prevailing chemical etching can shape thin silicon layers into two-dimensional patterns consisting of strips and posts with vertical walls and equal height. Here, the possibility to create silicon nanostructure of truly tree-dimensional shape by means of the focused ion beam lithography is explored, and a 300 nm thin film of monocrystalline epitaxial silicon on sapphire is patterned with a chiral nanoscale relief. It is demonstrated that exposing silicon to the ion beam causes a substantial drop of the visible transparency, which, however, is completely restored by annealing with oxidation of the damaged surface layer. As a result, the fabricated chiral metasurface combines high (50–80%) transmittance with the circular dichroism of up to 0.5 and the optical activity of up to 20° in the visible range. Being also remarkably durable, it possesses crystal-grade hardness, heat resistance up to 1000 °C and the inertness of glass.

Nanoscale groove textured β-Ga2O3 by room temperature inverse metal-assisted chemical etching and photodiodes with enhanced responsivity

Abstract: β-Ga2O3 is an emerging wide band-gap semiconductor that holds great promise for next generation power electronics and optoelectronics. β-Ga2O3 based ultraviolet photodetectors have been the subject of active research for the last few years. However, no micro and nanostructure surface texturing has been demonstrated for efficient light management in β-Ga2O3 optoelectronic applications yet. We hereby present nanoscale groove textured β-Ga2O3 metal-semiconductor-metal photodiodes, enabled by the unique metal-assisted chemical etching (MacEtch) method at room temperature in liquid. Although the textured surface stoichiometry shows ∼10% oxygen deficiency which results in a reduced Schottky barrier height and increased dark current, clear enhancement of the responsivity is demonstrated, compared to the planar untreated surface. The realization of MacEtch's applicability to β-Ga2O3 opens the door for producing more sophisticated device structures for this material, without resorting to conventional dry etch and potential damage.

Broadband luminescence in defect-engineered electrochemically produced porous Si/ZnO nanostructures.

Abstract: The fabrication, by an all electrochemical process, of porous Si/ZnO nanostructures with engineered structural defects, leading to strong and broadband deep level emission from ZnO, is presented. Such nanostructures are fabricated by a combination of metal-assisted chemical etching of Si and direct current electrodeposition of ZnO. It makes the whole fabrication process low-cost, compatible with Complementary Metal-Oxide Semiconductor technology, scalable and easily industrialised. The photoluminescence spectra of the porous Si/ZnO nanostructures reveal a correlation between the lineshape, as well as the strength of the emission, with the morphology of the underlying porous Si, that control the induced defects in the ZnO. Appropriate fabrication conditions of the porous Si lead to exceptionally bright Gaussian-type emission that covers almost the entire visible spectrum, indicating that porous Si/ZnO nanostructures could be a cornerstone material towards white-light-emitting devices.

Printing of complex free-standing microstructures via laser-induced forward transfer (LIFT) of pure metal thin films

Abstract: A combined approach of laser-induced forward transfer (LIFT) and chemical etching of pure metal films is studied to fabricate complex, free-standing, 3-dimensional gold structures on the few micron scale. A picosecond pulsed laser source with 515 nm central wavelength is used to deposit metal droplets of copper and gold in a sequential fashion. After transfer, chemical etching in ferric chloride completely removes the mechanical Cu support leaving a final free-standing gold structure. Unprecedented feature sizes of smaller than 10 μm are achieved with surface roughness of 0.3 to 0.7 μm. Formation of interfacial mixing volumes between the two metals is not found confirming the viability of the approach.

Low-loss bent channel waveguides in lithium niobate thin film by proton exchange and dry etching

Abstract: We propose and demonstrate an efficient method combining proton exchange with dry etching for the fabrication of low-loss bend channel waveguides in lithium niobate (LN) thin film. Our proposed method introduces the chemical etching caused by F+ ion to increase the etching rate. Our fabricated straight and bent channel waveguides have a trapezoid cross section with a top width of ~1.0 µm, a height of ~900 nm, and a slope of ~20° with respect to the vertical direction. To the best of our knowledge, this is the largest etching depth but with a small slope reported up to now. Mode intensity distributions and insertion losses were measured at 1.55 µm wavelength and bending losses were deduced. The results show that our fabricated bent channel waveguide with a radius of 20 μm can achieve low bending losses of 0.455 dB/90° and 0.488 dB/90° for the fundamental quasi-TE (qTE) and quasi-TM (qTM) modes, respectively. Compared with the fabrication methods reported so far, our method can realize a faster etching rate and a larger etching depth while maintaining a high etching quality.

Enhanced bending-tuned magnetic properties in epitaxial cobalt ferrite nanopillar arrays on flexible substrates

Abstract: Herein, large-scale epitaxial (111) CoFe2O4 nanopillar arrays with an average nanopillar diameter of ∼40–60 nm and thicknesses of 26–700 nm have been obtained on flexible fluorophlogopite substrates by chemically etching the vertically aligned self-assembled CoFe2O4:MgO nanocomposite thin films. The chemical etching process has not affected the crystalline quality of the CoFe2O4 phase, but results in volume shrinkage through the removal of the surrounding MgO phase. Compared with the planar CoFe2O4 films, the nanopillar arrays show sharply declined coercivity and enhanced saturation magnetization. Even the thinnest nanoisland-shaped arrays (∼26 nm) retain a relatively high saturation magnetization (∼90 emu cc−1), nonzero coercivity (∼250 Oe), and remanence (∼30 emu cc−1), which are promising for the requirements of weak ferromagnetism in flexible devices. With an increase in the bending radius, a strong and monotonous increase in saturation/remanent magnetization has been found in the nanopillar arrays. This reveals that the bending-induced shape anisotropy as well as the intrinsic magnetocrystalline anisotropy mainly dominate the tunable magnetic properties in the CoFe2O4 nanopillar arrays. With strong bending, the increment of remanent magnetization in the nanopillar arrays can be as high as 98%, exhibiting the huge potential of these nanopillar arrays in future applications such as in bending sensors and related devices.