Anisotropic Wet Etching Research Articles

Multi-electrode silicon microprobes fabrication process for brain-computer interface

Multi-electrode microprobes fabrication process based on silicon substrate was developed using surface micromachining and anisotropic wet etching. The process flow consists of 20 main operations, including 4 lithography steps using 4 photomasks. The minimum size of the elements is 2 μm. The effect of the solution concentration (from 10 to 40% at 80°C) on the etching rate and surface roughness was studied. The optimal value of solution concentration leading to the formation of surface with the lowest root mean square roughness value was determined. The etching rates of monocrystalline silicon (100) face and silicon oxide were 1.5 μm/min and 10 nm/min, respectively. Rapid thermal annealing at 600 °C for 3 min increased the resistance of silicon oxide to the action of an alkaline solution by 2 times. As a result, the neural probe structure including two microprobes and electrical interface of 10 electrodes was fabricated.

Research of the mechanical parameters of silicon membranes for acoustic sensors

This paper presents the results of modeling and fabrication of silicon membranes for acoustic sensors with resonant frequency from 2 kHz to 60 kHz and pressures from 0.1 to 14 Pa. An analysis of the constructive and mechanical parameters of monocrystalline silicon membranes is performed. The range of membrane edge length (6-20 mm) at a fixed thickness of 50 μm determined. The etching process of a silicon wafer by anisotropic wet etching was studied. A process flow for manufacturing silicon membranes is proposed. The results of experimental studies of amplitude-frequency characteristic of membranes are obtained.

Sapphire-supported nanopores for low-noise DNA sensing

Solid-state nanopores have broad applications from single-molecule biosensing to diagnostics and sequencing. The high capacitive noise from conventionally used conductive silicon substrates, however, has seriously limited both their sensing accuracy and recording speed. A new approach is proposed here for forming nanopore membranes on insulating sapphire wafers to promote low-noise nanopore sensing. Anisotropic wet etching of sapphire through micro-patterned triangular masks is used to demonstrate the feasibility of scalable formation of small (<25 μm) membranes with a size deviation of less than 7 μm over two 2-inch wafers. For validation, a sapphire-supported (SaS) nanopore chip with a 100 times larger membrane area than conventional nanopores was tested, which showed 130 times smaller capacitance (10 pF) and 2.6 times smaller root-mean-square (RMS) noise current (18–21 pA over 100 kHz bandwidth, with 50–150 mV bias) when compared to a silicon-supported (SiS) nanopore (~1.3 nF, and 46–51 pA RMS noise). Tested with 1k base-pair double-stranded DNA, the SaS nanopore enabled sensing at microsecond speed with a signal-to-noise ratio of 21, compared to 11 from a SiS nanopore. This SaS nanopore presents a manufacturable nanoelectronic platform feasible for high-speed and low-noise sensing of a variety of biomolecules.

A Study of the Geometrical Correction Factor and the Membrane Thickness on the Sensitivity of the Transversal Piezoresistive Pressure Sensor

The sensitivity of four-terminal devices like Hall and piezoresistive sensors is very dependent on its geometric parameters. This dependence is modeled by the Geometrical Correction Factor (G). The majority of these studies take very time-consumption analytical calculations for the analysis of G. In order to simplify this analysis, we present numerical analyses using FEM (Finite Element Method) for the most common geometrical forms of four-terminal devices. This result is general for any fourterminal-shaped sensors and can be used to optimize the sensor aspect ratio leading the maximization of G. In addition, FEM was also used to evaluate the membrane´s thickness influence over the sensor sensitivity by analyzing the in-plane mechanical stress behavior on the sensor active area. Experimental result of a new topology of pressure sensor is also presented, which maximizes G in comparison with conventional four-terminal devices and also improves its sensitivity. A special designed anisotropic wet etching system was used to post-process the sensor membrane. This anisotropic wet etching system allows the fabrication of well-defined membranes with thickness of 20 μm ± 3 μm and roughness as low as 90 nm rms.

X-ray verification of sol-gel resist shrinkage in substrate-conformal imprint lithography for a replicated blazed reflection grating

Surface-relief gratings fabricated through nanoimprint lithography (NIL) are prone to topographic distortion induced by resist shrinkage. Characterizing the impact of this effect on blazed diffraction efficiency is particularly important for applications in astrophysical spectroscopy at soft x-ray wavelengths (λ ≈ 0.5 − 5 nm) that call for the mass-production of large-area grating replicas with sub-micron, sawtooth surface-relief profiles. A variant of NIL that lends itself well for this task is substrate-conformal imprint lithography (SCIL), which uses a flexible, composite stamp formed from a rigid master template to imprint nanoscale features in an inorganic resist that cures thermodynamically through a silica sol-gel process. While SCIL enables the production of several hundred imprints before stamp degradation and avoids many of the detriments associated with large-area imprinting in NIL, the sol-gel resist suffers shrinkage dependent on the post-imprint cure temperature. Through atomic force microscopy and diffraction-efficiency testing at beamline 6.3.2 of the Advanced Light Source, the impact of this effect on blaze response is constrained for a ∼160-nm-period grating replica cured at 90°C. Results demonstrate a ∼2° reduction in blaze angle relative to the master grating, which was fabricated by anisotropic wet etching in 〈311〉-oriented silicon to yield a facet angle close to 30°.

Transition metal dichalcogenide metamaterials with atomic precision

The ability to extract materials just a few atoms thick has led to the discoveries of graphene, monolayer transition metal dichalcogenides (TMDs), and other important two-dimensional materials. The next step in promoting the understanding and utility of flatland physics is to study the one-dimensional edges of these two-dimensional materials as well as to control the edge-plane ratio. Edges typically exhibit properties that are unique and distinctly different from those of planes and bulk. Thus, controlling the edges would allow the design of materials with combined edge-plane-bulk characteristics and tailored properties, that is, TMD metamaterials. However, the enabling technology to explore such metamaterials with high precision has not yet been developed. Here we report a facile and controllable anisotropic wet etching method that allows scalable fabrication of TMD metamaterials with atomic precision. We show that TMDs can be etched along certain crystallographic axes, such that the obtained edges are nearly atomically sharp and exclusively zigzag-terminated. This results in hexagonal nanostructures of predefined order and complexity, including few-nanometer-thin nanoribbons and nanojunctions. Thus, this method enables future studies of a broad range of TMD metamaterials through atomically precise control of the structure.

Microconical silicon mid-IR concentrators: spectral, angular and polarization response.

It is widely discussed in the literature that a problem of reduction of thermal noise of mid-wave and long-wave infrared (MWIR and LWIR) cameras and focal plane arrays (FPAs) can be solved by using light-concentrating structures. The idea is to reduce the area and, consequently, the thermal noise of photodetectors, while still providing a good collection of photons on photodetector mesas that can help to increase the operating temperature of FPAs. It is shown that this approach can be realized using microconical Si light concentrators with (111) oriented sidewalls, which can be mass-produced by anisotropic wet etching of Si (100) wafers. The design is performed by numerical modeling in a mesoscale regime when the microcones are sufficiently large (several MWIR wavelengths) to resonantly trap photons, but still too small to apply geometrical optics or other simplified approaches. Three methods of integration Si microcone arrays with the focal plane arrays are proposed and studied: (i) inverted microcones fabricated in a Si slab, which can be heterogeneously integrated with the front illuminated FPA photodetectors made from high quantum efficiency materials to provide resonant power enhancement factors (PEF) up to 10 with angle-of-view (AOV) up to 10°; (ii) inverted microcones, which can be monolithically integrated with metal-Si Schottky barrier photodetectors to provide resonant PEFs up to 25 and AOVs up to 30° for both polarizations of incident plane waves; and iii) regular microcones, which can be monolithically integrated with near-surface photodetectors to provide a non-resonant power concentration on compact photodetectors with large AOVs. It is demonstrated that inverted microcones allow the realization of multispectral imaging with ∼100 nm bands and large AOVs for both polarizations. In contrast, the regular microcones operate similar to single-pass optical components (such as dielectric microspheres), producing sharply focused photonic nanojets.

Anisotropic wet etching on the β‐phase poly(vinylidene fluoride) film

The ability to fabricate micro patterns or micro structures directly on a flexible and piezoelectric β-phase poly(vinylidene fluoride) (PVDF) film, has potential applications in the field of micro-electro-mechanical systems. The authors realised several tens to several hundred of micro patterns directly on a 110-μm thick flexible β-phase PVDF film by wet-etching technology. Through experiments, the authors also found that the β-phase PVDF film shows anisotropic etching characteristic in the proposed wet-etching process. In this Letter, the authors reported β-phase PVDF film micropattern fabrication process and its anisotropic wet-etching properties for the first time.

Inscription of surface waveguides in glass by femtosecond laser writing for enhanced evanescent wave overlap

Near-surface optical waveguides were fabricated in alkaline earth boro-aluminosilicate glass (Eagle2000), by femtosecond laser direct writing, using two distinct approaches. First, the capability of directly inscribing optical waveguides close to the surface was tested, and then, compared to the adoption of post writing wet etching to bring to the surface waveguides inscribed at greater depths. Laser ablation was found to limit the minimum surface to core center distance to 6.5 µm in the first method, with anisotropic wet etching limiting the latter to 3 µm without any surface deformation; smaller separations can be achieved at the cost of the planar surface topography. Furthermore, the waveguide’s cross-section was seen to vary for laser inscription nearing the surface, observations that were also corroborated by its distinct guiding characteristics when compared to the adoption of post writing wet etching. The spectral analysis (in the 500–1700 nm range) also evidenced an increase in insertion loss for longer wavelengths and smaller surface to core center separations, caused, most likely, by coupling loss due to the interaction between the propagating mode and the surface. Different lengths of waveguide exposed to the surface were also tested, revealing that scattering loss due to surface roughness is not an issue at the centimeter scale.

Simulation-based optimization of out-of-plane, variable-height, convoluted quartz micro needle arrays via single-step anisotropic wet etching

Variable-height, out-of-plane, convoluted microneedle arrays with high aspect ratio, tapered shafts, sharp tips, and smooth surfaces are required for applications where the microneedle heights must conform to a specific target geometry. Here, we show that it is possible to achieve such convoluted microneedle arrays on a Z-cut quartz substrate by a single wet etching step in saturated NH4HF2 at 80 °C. Due to the low etch rate of quartz, the procedure is based on carefully controlling the dimensions of every needle-like structure by properly dimensioning the masked area on top. In particular, we show that the mask dimensions for all the microneedles can be optimized via a combination of Level Set (LS) simulations of wet etching and an Evolutionary Algorithm (EA) to search the optimal dimensions. As an example, we fabricate two different microneedle arrays, where the needle heights follow two different spherical surfaces. Our experiments confirm that (i) the LS method correctly predicts the evolution of the etch front for microneedles etched on AT, BT and Z quartz cuts; and (ii) the LS + EA approach provides a suitable procedure to determine an optimal collection of mask sizes for the fabrication of convoluted microneedle arrays on the Z cut.

Effect of Etching Temperature on the Surface Texturing of Si{100} in an Extremely Low Concentration Tmah for Minimized Reflectivity

Among various renewable energy resources, solar energy is one of the most abundant energy available in nature. The solar energy can be utilized by converting photon energy into electrical energy by using solar cells as energy harvesters. Silicon-based solar cells dominate the current photovoltaic market because of its higher efficiency, excellent stability, non-toxicity, lower processing cost and excellent reliability in the outdoor conditions.1 However, the silicon solar cell efficiency can be further increased by reducing the front surface reflection from silicon, which is greater than 35%. In order to reduce the reflection from silicon surface, surface texturing is done where silicon surface is intentionally made rough. Wet anisotropic etching is most widely used in surface texturing of silicon because of its high throughput and lower cost. Potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH) are two most extensively used alkaline solution for surface texturing of silicon. 2-4 In these two etchants, TMAH is preferred because it is fully compatible with CMOS process and also provide high etch selectivity between silicon and silicon dioxide. Surface texturing using low concentration TMAH (<5%) results in high surface roughness. This is a remarkable observation that can be applied in solar cell industries to minimize the reflection from silicon front surface.In this work, we have studied the effect of very low concentration TMAH (1.0-0.1 wt%) without adding any additive and agitation during etching for surface texturing of Si{100} surface. The main objective of this study is to achieve an optimized etched surface topography at lowest possible etching concentration and etching time to minimize the reflectance for application in solar cells. The average solar weighted reflectance (Rsw) of around 10% is obtained in a very mild etching conditions. Compared to the previous results reported in literatures and also the mild etching conditions, reflectance values obtained in this work are quite promising for large area surface texturing of silicon solar cell panels for enhancement in their efficiency. In addition, the use of very low concentration TMAH minimizes the chemical waste. This also helps in adopting the technique for fabricating anti-reflective silicon surfaces at large scale.

High speed etching of silicon in KOH + NH 2 OH solution at lower temperatures for the fabrication of through holes in silicon wafer

In the fabrication of complicated MEMS inertial sensors, there are many instances where metallisation cannot be done by usual methods such as lift off or conventional metal etch. In such cases, a shadow mask needs to be used. A silicon shadow mask can be realised by the fabrication of through holes in silicon. This also can be used as a cover wafer for wafer level encapsulation by fabricating a cavity in the device region. The fabrication of the shadow mask is carried out by doing double side wet anisotropic etching using potassium hydroxide (KOH) + hydroxylamine (NH2 OH) solution at low temperatures of 50–60°C. The low temperature of etching is preferred so as to reduce the etching of thermal oxide used as a mask layer. This work focuses on the etching characteristics of the KOH + NH2 OH solution at low temperatures in terms of etch rate, undercutting, surface morphology, and selectivity of an oxide layer with silicon. This is very useful in through hole etching using silicon dioxide as a mask. The effect of aging on these characteristics is also studied and this is useful in continuous etching spread over few days especially during pilot production. The results are compared with those obtained using a pure KOH solution.

Reducing light reflection by processing the surface of silicon solar cells

This study discussed a surface processing technique for improving the energy conversion rate of solar cells with silicon as the substrate. The technique involves texturing the surface of a silicon substrate and coating it with an antireflective layer to enhance its antireflective property and thereby its photoelectric conversion efficiency. Double surface texturing (DST) texture was formed through photolithography and anisotropic wet etching, and radio frequency magnetron sputtering was performed to deposit the SiO2/TiO2 double-layer antireflective coating to reinforce the surface texture of a silicon solar cell. Subsequently, a scanning electron microscope was employed to analyze the DST texture before and after the surface was coated with a double-layer antireflective coating (ARC). An UV–VIS spectrophotometer attached with an integrating sphere was then applied to measure the light absorption rate of the processed surface at different wavelengths. After optimum DST and double-layer ARC treatments, the average reflectance within the visible light reduce to 9.94%, which means the absorption of incident light into the absorption layer was enhanced to improve the energy conversion efficiency of silicon solar cells.

Bactericidal Characteristics of Bioinspired Nontoxic and Chemically Stable Disordered Silicon Nanopyramids.

Controlling bacterial growth using artificial nanostructures inspired from natural species is of immense importance in biomedical applications. In the present work, a low cost, fast processing, and scalable anisotropic wet etching technique is developed to fabricate the densely packed disordered silicon nanopyramids (SiNPs) with nanosized sharp tips. The bactericidal characteristics of SiNPs are assessed against strains implicated in nosocomial and biomaterial-related infections. Compared to the bare silicon with no antibacterial activities, SiNPs of 1.85 ± 0.28 μm height show 55 and 75% inhibition of Escherichia coli (Gram-negative) and Bacillus subtilis (Gram-positive) bacteria, whereas the silicon nanowires (SiNWs) fabricated using a metal-assisted chemical etching method show 50 and 58% inhibition of E. coli and B. subtilis. The mechanistic studies using a scanning electron microscope and live/dead bacterial cell assay reveal cell rupture and predominance of dead cells on contact with SiNPs and SiNWs, which confirms their bactericidal effects. Chemical stability and cell viability studies demonstrate the biocompatible nature of SiNP and SiNW surfaces. Owing to their capability to kill both Gram-negative and positive bacteria and minimal toxicity to murine fibroblast cells, SiNPs can be used as an antibacterial coating on medical devices to prevent nosocomial and biomaterial-related infections.

HfOx-based nano-wedge structured resistive switching memory device operating at sub-μA current for neuromorphic computing application

We fabricated a silicon based nano-wedge resistive switching memory device with the stack of Ti/HfOx/p+-Si. By using 25% tetra-methyl-ammonium hydroxide (TMAH) aqueous solution, the anisotropic wet etching process is carried out to minimize the tip structure of the silicon bottom electrode to a width of 4 nm, and the structure was validated through TEM analysis. Due to the minimized device area, low read current levels (<1 μA) were obtained in the nano-wedge RRAM while the opposites were measured in large size RRAM devices. In addition, the fabricated nano-wedge RRAM exhibited low power consumption during the DC switching process. Additionally, pulse measurement and retention tests were performed to demonstrate the synaptic behaviors of long-term potentiation and depression. Software neural network simulation was followed to test the feasibility of nano-wedge RRAM’s array implementation. These results demonstrate the fabricated nano-wedge RRAM devices’ potential usage as a synaptic device in neuromorphic computing systems.

Extreme Ultraviolet and Soft X-Ray Diffraction Efficiency of a Blazed Reflection Grating Fabricated by Thermally Activated Selective Topography Equilibration

Abstract Future observatories utilizing reflection grating spectrometers for extreme ultraviolet (EUV) and soft X-ray (SXR) spectroscopy require high-fidelity gratings with both blazed groove facets and custom groove layouts that are often fanned or feature a slight curvature. While fabrication procedures centering on wet anisotropic etching in monocrystalline silicon produce highly efficient blazed gratings, the precision of a nonparallel groove layout is limited by the cubic structure of the silicon crystal. This motivates the pursuit of alternative techniques to grating manufacture, namely thermally activated selective topography equilibration (TASTE), which uses gray-scale electron-beam lithography to pattern multilevel structures in resist followed by an optimized polymer thermal reflow to smooth the 3D patterns into continuous surface relief profiles. Using TASTE, a mold for a reflection grating with a periodicity of 400 nm and grooves resembling an asymmetric sawtooth was patterned in 130 nm thick poly(methyl methacrylate) resist on a silicon substrate over a 50 mm by 7.5 mm area. This structure was coated with 15 nm of gold by electron-beam physical vapor deposition using titanium as an adhesion layer and then tested for EUV and SXR diffraction efficiency at beamline 6.3.2 of the Advanced Light Source synchrotron facility. Results demonstrate a quasi-blaze response characteristic of a 27◦ blaze angle with groove facets smooth to 1.5 nm rms. Absolute peak-order efficiency ranges from 75% to 25%, while total relative efficiency measures ⪆90% across the measured bandpass of 15.5 nm &gt; λ &gt; 1.55 nm.

Etching of Uncompensated Convex Corners with Sides along and in 25 wt% TMAH at 80 °C.

This paper presents etching of convex corners with sides along <n10> and <100> crystallographic directions in a 25 wt% tetramethylammonium hydroxide (TMAH) water solution at 80 °C. We analyzed parallelograms as the mask patterns for anisotropic wet etching of Si (100). The sides of the parallelograms were designed along <n10> and <100> crystallographic directions (1 < n < 8). The acute corners of islands in the masking layer formed by <n10> and <100> crystallographic directions were smaller than 45°. All the crystallographic planes that appeared during etching in the experiment were determined. We found that the obtained types of 3D silicon shape sustain when n > 2. The convex corners were not distorted during etching. Therefore, no convex corner compensation is necessary. We fabricated three matrices of parallelograms with sides along crystallographic directions <310> and <100> as examples for possible applications. Additionally, the etching of matrices was simulated by the level set method. We obtained a good agreement between experiments and simulations.

Reduction of threading dislocation density in top-down fabricated GaN nanocolumns via their lateral overgrowth by MOCVD

Reduction of threading dislocation density in top-down fabricated GaN nanocolumns (NCs) via their successive lateral shrinkage by anisotropic wet etch and lateral overgrowth by metalorganic chemical vapor deposition is studied by transmission electron microscopy. The fabrication process involves a combination of dry and wet etches to produce NC arrays of a low fill factor (&amp;lt;5%), which are then annealed and laterally overgrown to increase the array fill factor to around 20%–30%. The resulting NC arrays show a reduction in threading dislocation density of at least 25 times, allowing for the reduction in material volume due to the array fill factor, with dislocations being observed to bend into the voids between NCs during the overgrowth process.

The Level-Set Method for Multi-Material Wet Etching and Non-Planar Selective Epitaxy

We present numerical methods to enable accurate and robust level-set based simulation of anisotropic wet etching and non-planar epitaxy for semiconductor fabrication. These fabrication techniques are characterized by highly crystal orientation-dependent etch/growth rates, which lead to non-convex Hamiltonians in their description by the level-set equation. As a consequence, instable surface propagation may emerge, leading to unphysical results. We propose a calibration-free Stencil Lax-Friedrichs scheme and an advanced adaptive time-stepping approach, tailored to the level-set speed functions associated with anisotropic etching and epitaxy. The scheme calculates the numerical dissipation based on information about the local geometry and the nature of the etch rates/growth function, which enables an optimized trade-off between overly rounding of sharp geometric features and stable surface propagation. Furthermore, we introduce the deposition top layer method, which allows for robust handling of multiple material regions in non-planar epitaxy simulations. Both methods are demonstrated in a prototypical implementation, which is used to validate the capability and accuracy of our approaches. In particular, two-dimensional wet etching and three-dimensional epitaxy simulations are performed and characteristic geometry parameters are compared to the ideally expected values, showing robustness and high accuracy.

Simplified and cost-effective technique to enhance optical properties of microstructured silicon

Microstructured/nanostructured Silicon (Si) is widely used in several MEMS and Nanotechnology applications. Wet anisotropic etching of Si is a well-known cost-effective approach where alkaline chemicals can etch the Si surface. In the current work, a facile and cost-effective way of improving the conventional potassium hydroxide (KOH) based etching process is presented for obtaining uniform large area microstructuring on (1 0 0) Si. In order to achieve the goal, a systematic study of the etching process without the use of any external agitation has been accomplished with different types of alcohol and surfactant additives of varying concentrations. Various modifications and optimization of the etching process has finally resulted in enhanced anti-reflection property of Silicon.