Anisotropic Etching Research Articles

Super-Resolution Nanolithography of Two-Dimensional Materials by Anisotropic Etching.

Nanostructuring allows altering of the electronic and photonic properties of two-dimensional (2D) materials. The efficiency, flexibility, and convenience of top-down lithography processes are, however, compromised by nanometer-scale edge roughness and resolution variability issues, which especially affect the performance of 2D materials. Here, we study how dry anisotropic etching of multilayer 2D materials with sulfur hexafluoride (SF6) may overcome some of these issues, showing results for hexagonal boron nitride (hBN), tungsten disulfide (WS2), tungsten diselenide (WSe2), molybdenum disulfide (MoS2), and molybdenum ditelluride (MoTe2). Scanning electron microscopy and transmission electron microscopy reveal that etching leads to anisotropic hexagonal features in the studied transition metal dichalcogenides, with the relative degree of anisotropy ranked as: WS2 > WSe2 > MoTe2 ∼ MoS2. Etched holes are terminated by zigzag edges while etched dots (protrusions) are terminated by armchair edges. This can be explained by Wulff constructions, taking the relative stabilities of the edges and the AA' stacking order into account. Patterns in WS2 are transferred to an underlying graphite layer, demonstrating a possible use for creating sub-10 nm features. In contrast, multilayer hBN exhibits no lateral anisotropy but shows consistent vertical etch angles, independent of crystal orientation. Using an hBN crystal as the base, ultrasharp corners can be created in lithographic patterns, which are then transferred to a graphite crystal underneath. We find that the anisotropic SF6 reactive ion etching process makes it possible to downsize nanostructures and obtain smooth edges, sharp corners, and feature sizes significantly below the resolution limit of electron beam lithography. The nanostructured 2D materials can be used themselves or as etch masks to pattern other nanomaterials.

Etalon for Nanometrology Produced by Anisotropic Etching

Possibilities and results of using relief structures produced by anisotropic etching as etalons for linear measurements in nanorange are discussed. Two types of the structures with different profiles and with two approaches to estimation of influence of its sophistication degree on measurement results are considered. Analyze of methods and means of measurements of its geometric parameters and comparison of uncertainness of measurement results are conducted. The important condition for diminishing the measurements uncertainness is taking into account irregularities of a sample surface. These irregularities were evaluated by TEM and CD-AFM. Results of harnessing these types of structures are different also. The bureau International des Poids et Mesures recommended the structure with rectangular profile and its attestation procedure as an example for all national committees on linewidth metrology in nanorange. The structure with trapezoid profile that induced hopes for decision of basic tasks of nanometrology in past is occurred to be poorly in demand even on national scale.

Atomic layer etching of Si3N4 with high selectivity to SiO2 and poly-Si

Atomic layer etching (ALE) is usually classified into ion-driven anisotropic etching or thermally driven isotropic etching. In this work, we present a thermal ALE process for Si3N4 with high selectivity to SiO2 and poly-Si. This ALE process consists of exposure to a CH2F2/O2/Ar downstream plasma to form an (NH4)2SiF6-based surface-modified layer, followed by infrared (IR) annealing to remove the modified layer. CH2F2-based chemistry was adopted to achieve high selectivity to SiO2 and poly-Si. This chemistry was expected to reduce the number density of F atoms (radicals), which contributes to decreasing the etching rate of SiO2 and poly-Si films. X-ray photoelectron spectroscopy analysis confirmed the formation of an (NH4)2SiF6-based modified layer on the surface of the Si3N4 after exposure to the plasma and subsequent removal of the modified layer using IR annealing. An in situ ellipsometry measurement revealed that the etch per cycle of the ALE process saturated with respect to the radical exposure time at 0.9 nm/cycle, demonstrating the self-limiting nature of this etching process. In addition, no etching was observed on SiO2 and poly-Si films, successfully demonstrating the high selectivity of this ALE process. This high selectivity to SiO2 and poly-Si is attributed to the fact that the spontaneous etching rates of these films are negligibly small and that there is no surface reaction to etch these films during the IR annealing step.

Single-step reactive ion etching process for device integration of hafnium-zirconium-oxide (HZO)/titanium nitride (TiN) stacks

The integration of new materials such as high-k dielectrics or metals into advanced CMOS gate stacks has led to major developments in plasma etching. The authors present a study which is dedicated to the etching of amorphous hafnium zirconium oxide (HZO) and titanium nitride (TiN) layers with Ar/Cl2 chemistry in one single step. By adjusting the gas ratio and the inductively coupled plasma power, the etching process is shown to have a slow and well controllable etch rate for HZO and TiN. Additionally, a high selectivity between both materials and SiO2 can be achieved. Gate stack etching was successfully demonstrated and transmission electron microscopy-images revealed good anisotropic etching for HZO and TiN with an etch stop in SiO2 without damaging the silicon underneath. The process is further applied for the fabrication of metal-ferroelectric-metal capacitors, here TiN-HZO-TiN, and the feasibility of the chosen material combination is proven by electrical characterization. The strategy of using low temperature plasma-enhanced atomic layer deposition for TiN-deposition and forming gas anneal after structuring leads to high remanent polarization-values.

ПРОЕКТИРОВАНИЕ КРЕМНИЕВОГО МИКРОЗОНДА ДЛЯ МАЛОИНВАЗИВНОГО НЕЙРОИНТЕРФЕЙСА

Microprobes have become an important tool in the study of brain activity. Research and developmentin the field of invasive neurointerfaces is aimed at reducing the characteristic damageto the nervous tissue by reducing the diameter of the implanted probes to less than 100 μm. Suchstructures are produced by micromachining, in particular, by types of anisotropic etching. In thiscase, the size and shape of the probe are influenced by the etching conditions. The latter should betaken into account at the designing of the probe. The paper evaluates the ranges of the geometricparameters of a silicon microprobe taking into account the etching conditions and the number ofelectrodes. Analytical calculations were carried out for the structure of the probe, represented byfour regions of different widths, carrying up to seven electrodes. The dependences of bottom basewidth of a trapezoidal section of the probe and the size of the mask on the thickness and width ofthe top base are received. The admissible ranges of sizes for the proposed case of the four-levelmicroprobe are established; in particular, the minimum values of the width of the top base were17, 28, 39 and 50 microns, and the corresponding ranges of permissible values of the probe thicknessfor cases with 1 electrode – 30–58 microns, 2 and 3 electrodes – 30–51 microns, 4 and 5electrodes – 30–43 microns, and for the case of a probe with 6 and 7 electrodes – 30–35 microns.The correction value of the mask size is estimated, reflecting the effect of etching conditions on theprobe geometry.

Fabrication and Characterization of Inverted Silicon Pyramidal Arrays with Randomly Distributed Nanoholes

We report the fabrication, electromagnetic simulation and measurement of inverted silicon pyramidal arrays with randomly distributed nanoholes that act as an anti-reflectivity coating. The fabrication route combines the advantages of anisotropic wet etching and metal-assisted chemical etching. The former is employed to form inverted silicon pyramid arrays, while the latter is used to generate randomly distributed nanoholes on the surface and sidewalls of the generated inverted silicon pyramidal arrays. We demonstrate, numerically and experimentally, that such a structure facilitates the multiple reflection and absorption of photons. The resulting nanostructure can achieve the lowest reflectance of 0.45% at 700 nm and the highest reflectance of 5.86% at 2402 nm. The average reflectance in the UV region (250–400 nm), visible region (400–760 nm) and NIR region (760–2600 nm) are 1.11, 0.63 and 3.76%, respectively. The reflectance at broadband wavelength (250–2600 nm) is 14.4 and 3.4 times lower than silicon wafer and silicon pyramids. In particular, such a structure exhibits high hydrophobicity with a contact angle up to 132.4°. Our method is compatible with well-established silicon planar processes and is promising for practical applications of anti-reflectivity coating.

Structural and Optical Properties of Textured Silicon Substrates by Three-Step Chemical Etching.

We implemented the fabrication of hybrid structures, including pyramids, etching holes, and inverted pyramidal cavities on silicon substrates, by three-step chemical etching. To achieve this, we utilized anisotropic wet etching as the first-step etching to form pyramids of various sizes. Subsequently, metal-assisted chemical etching was performed to develop aligned etching holes on the pyramidal structure. Ultimately, anisotropic wet etching was used again as the third-step etching for the etchant to penetrate holes to form inverted pyramidal cavities. Optimizing the three-step etching treatments, large-scale textured structures with low reflectance could be obtained, and they show potential for applications in sensors, solar cells, photovoltaics, and surface-enhanced Raman scattering (SERS). Examples of using the textured silicon substrates for SERS applications were given.

Study of Black Silicon Wafer through Wet Chemical Etching for Parametric Optimization in Enhancing Solar Cell Performance by PC1D Numerical Simulation

Black silicon (BSi) fabrication via surface texturization of Si-wafer in recent times has become an attractive concept regarding photon trapping and improved light absorption properties for photovoltaic applications. In this study, surface texturization has been conducted on mono-crystalline Si(100) wafer using a wet chemical anisotropic etching process with IPA:KOH solution to form micro-pyramidal surface structures. Moreover, the optimized properties of the fabricated BSi wafers are used for numerical simulation using PC1D software to analyze the performance of the solar cell and establish the correlation among relevant parameters. Effects such as doping concentration, texturization, passivation, and anti-reflection coating of BSi on the solar cell performance have numerically been investigated. Results show that textured surface obtained from the wet chemical anisotropic etching process has successfully reduced the reflectance of the BSi wafer and surpassed the solar cell efficiency by 2%, which is mainly attributed to the optical confinement of the textured pyramids on the surface with a height of 1–2 μm and angles of 70 degrees. Furthermore, the doping concentration of the p-type wafer and n-type emitter were optimized to be 1 × 1016 cm−3 and 1 × 1018 cm−3, respectively. In the case of device optimization, the SiO2 passivation layer with a refractive index of 1.48 and the Si3N4 ARC layer with a refractive index of 2.015 has been identified as the best combination for the solar cell performance. These optimized parameters eventually result in 23.14% conversion efficiency from numerical simulation for solar cells that use black silicon wafers as fabricated in this study.

The role of etching anisotropy in the fabrication of freestanding oxide microstructures on SrTiO3(100), SrTiO3(110), and SrTiO3(111) substrates

The release process for the fabrication of freestanding oxide microstructures relies on appropriate, controllable, and repeatable wet etching procedures. SrTiO3 (STO) is among the most employed substrates for oxide thin films growth and can be decomposed in HF:water solution. Such a process is strongly anisotropic and is affected by local defects and substrate cut-planes. We analyze the etching behavior of SrTiO3 substrates having (100), (110), and (111) cut-planes during immersion in a 5% HF:water solution. The etching process over the three substrates is compared in terms of pitting, anisotropy, macroscopic etch rate, and underetching effects around HF-resistant (La,Sr)MnO3 thin film micropatterns. The release of targeted structures, such as the reported (La,Sr)MnO3 freestanding microbridges, depends on the substrate crystallographic symmetry and on the in-plane orientation of the structures themselves along the planar directions. By comparing the etching evolution at two different length scales, we distinguish two regimes for the propagation of the etching front: an intrinsic one, owing to a specific lattice direction, and a macroscopic one, resulting from the mixing of different etching fronts. We report the morphologies of the etched SrTiO3 surfaces and the geometries of the underetched regions as well as of the microbridge clamping zones. The reported analysis will enable the design of complex MEMS devices by allowing to model the evolution of the etching process required for the release of arbitrary structures made of oxide thin films deposited on top of STO.

A microfabricated ion trap chip with a sloped loading slot to minimize exposing trapped ions to stray charges

A novel microfabricated ion trap chip that can geometrically minimize exposing trapped ions to stray charges on dielectric surfaces is developed. The new design utilizes a sloped loading slot to make the dielectric layers of the loading slot sidewalls invisible to the trapped ions. The designed loading slot is realized by applying silicon anisotropic etching processes, and the ion trap chip that contains the sloped loading slot is fabricated using microelectromechanical system processes. Stray charges induced by injecting an ultraviolet laser on the surface of the fabricated ion trap chip are experimentally characterized by measuring stray electric fields at the location of the trapped ions. The measured results are compared to those of a previous ion trap chip with a vertical loading slot. The magnitude of the measured stray electric field in the axial direction is within 1.5 V m−1 for the proposed ion trap chip, whereas it is approximately 10 V m−1 for the previously reported ion trap chip.

Anisotropic wet etching of a novel micro-texture structure for an Al/n-Si/Al metal–semiconductor–metal photodetector fabrication

Micro-electro-mechanical system fabrication involves molding on a single substrate. Chemical wet etching is common in these systems because it can provide a very high etch rate and selectivity. This optical device has been successfully completed fabrication by using the single-step lithography process. The surface was coated with PR and patterned using a single mask to create U-shaped structures by a wide electrode gap on a silicon substrate. Then it was etched using anisotropic wet etching process in potassium hydroxide, tetramethylammonium hydroxide (TMAH), silicic acid and dissolved silicon powder. The etchant created a 58% increase in the light sensitive area of the U-shaped trench in the structure higher compared to the planar structure. Random micro-pyramids, formed on the silicon surface in the U-shaped structure by the silicic acid-added TMAH solution, led to a ∼254 times higher ratio between photon generated current and dark current at reverse bias. This processing technique is a promising technology for improving performance for next-generation optical sensors or silicon photodetector.

Fabrication of a silicon μDicer for uniform microdissection of tissue samples

Generating uniform tissue microfragments is important in many applications, including disease diagnostics, drug screening, spatial-omics, and fundamental wound healing and tissue regeneration studies. Common mechanical dissection methods, such as manual mincing, are imprecise and result in fragments with a broad range in size. This work aims to develop a microscale dicing device, referred to as the “μDicer,” consisting of a hollow array of blades spaced hundreds of micrometers apart. A tissue pushed through this array is diced into many microfragments simultaneously. The focus of this paper is on the fabrication process of the μDicer using a combination of isotropic and anisotropic etching in silicon. A single silicon oxide etch mask is used in a dry silicon etcher for both a tapered etch to form the microblades, and an anisotropic etch to form the through-holes in the hollow blade array. The use of a single mask reduces the mask fabrication time by more than twofold compared with two-mask approaches often used to generate similar etch features. The etch parameters and the design of the etch mask control the blade angles and the edge profiles of the blades. Specifically, the incorporation of “notches” in the two-dimensional mask design generates three-dimensional microserrated features on the blade edges. A custom, open-source etching model is also developed to facilitate the prediction of the etch profiles. Finally, a proof-of-concept application of the μDicer to dissect soft materials and tissues is demonstrated.

Controllable synthesization of Au nanoparticles by laser enhanced wet KOH etching process

In Si substrate, anisotropic KOH etchants are mainly utilized to form pyramids like on the Si surface . However, this process is not well controlled way owing to the different and random etching pathway. In this work, we applied laser radiation during the anisotropic KOH wet etching process to modifies the topographical properties of Si substrate, as an efficient,simple and low cost texturing process for Si substrate. This approach employs different laser wavelength to modify the topographical features from a crater like structures to Si nanocrystallites in the form of pillars like structures on the Si surface. In order to investigate the formation of plasmonics species, gold nanoparticles was incorporated into Si surfaces by simple ion reduction process. The Si topographical features was studied with atomic scanning microscopy (AFM) images of Si before and after laser irradiation process. The irradiation with 405 laser wavelength, show the formation of thin and high density of Si nano pillars-like structures compared with more thick depther Si nano pillars like structures layer.

On the Operation Modes of Dual-Gate Reconfigurable Nanowire Transistors

We investigate the operation of dual-gate reconfigurable field-effect transistor (RFET) in the programgate at drain (PGAD) and program-gate at source (PGAS) configurations. To this end, dual-gate silicon nanowire (SiNW) FETs are fabricated based on anisotropic wet chemical silicon etching and nickel silicidation yielding silicide-SiNW Schottky junctions at source and drain. Whereas in PGAD-configuration ambipolar operation is suppressed, switching is deteriorated due to the injection through a Schottky-barrier. Operating the RFET in PGAS configuration yields a switching behavior close to a conventional MOSFET. This, howewer, needs to be traded off against strongly non-linear output characteristics for small bias.

High-efficiency passivated emitter and rear cells with nano honeycomb structure

In this paper, a light-trapping nano honeycomb (NH) structure with a polyhedral interior was prepared on a c-Si wafer surface, which has a much lower reflectance and better passivation effect than that of a pyramid-textured surface. The NH structure was prepared using the metal-assisted chemical etching method together with the nanostructure rebuilding (NSR) method. Through reflectance measurement, internal quantum efficiency characterization and finite-difference time-domain simulation, the advantages of the NH structure were systematically studied. The formation mechanism of the NH structure was considered to be due to both the anisotropic and isotropic etching properties of Si crystal, and its main affecting factor was the NSR reaction time. To further demonstrate its potential in monocrystalline silicon cells, passivated emitter and rear cells (PERC) using NH-textured c-Si wafers were fabricated. A high efficiency of 22.80% was achieved, which was 0.3% and 0.16% higher compared to the normal upward pyramid-textured PERC and the inverted pyramid-textured PERC, respectively. The NH structure is expected to have a great potential application in the mass production of c-Si solar cells.

Microscale patterning of semiconductor c-Si by selective laser-heating induced KOH etching

Laser patterning has been used for the micro-scale fabrication of semiconductor devices like solar cells, photodetectors, LEDs, and also for modification of surfaces for wettability, reflected colors, initial bacterial adhesion, etc, due to its several advantages of patterning flexibility, spatial resolution, and mask-free operation over complex conventional lithography. Currently, laser-induced ablation is a promising patterning method in silicon solar cell fabrication; however, laser-induced defects, and thermal stresses remain a significant concern. In this paper, we demonstrate a laser ablation-free method for patterning c-Si based on the observation that the anisotropic etching of c-Si by KOH is highly temperature-dependent as the etching rate is about 100 times faster at 80 °C compared to room temperature. The laser heating-induced chemical etching (LHICE) of crystalline silicon can help alleviate such laser-induced damage by providing the necessary low temperature on the localized area(s) on the silicon substrate. We investigated the micro-second pulsed laser-assisted chemical etching of c-Si substrate for microscale patterning and showed that laser-induced damage could be eliminated as indicated by the minority carrier lifetime preservation. We also present results of the effect of laser processing parameters such as laser power, scan speed, and duty cycle on etching depth and surface morphology. The optical, surface morphology, depth profile, and LCPSim simulation results are also presented to optimize and understand the LHICE process. This versatile methodology of temperature-selective chemical etching could be applied to various thin-film and bulk materials used in diverse device fabrication.

Single-Step Dual-Layer Photolithography for Tunable and Scalable Nanopatterning.

Conventional photolithography, due to its scalability, robustness, and straightforward processes, has been widely applied to micro- and nanostructure manufacturing in electronics, optics, and biology. However, optical diffraction limits the ultimate resolution of conventional photolithography, which hinders its potential in nanoscale patterning for broader applications. Here, we introduce a derivative of conventional photolithography for nanoscale patterning called dual-layer photolithography (DLPL), which is based on the controlled exposure and development of overlapping positive and negative photoresists. In a typical experiment, substrates are sequentially coated by two layers of photoresists (both positive and negative). Then, we purposefully control the exposure time to generate slightly larger features in the positive photoresist than those in the negative photoresist. After development, their overlapping areas become the final features, which outline the original features. We demonstrate line widths down to 300 nm here, which can be readily improved with more precise control. By adjusting the lithography parameters and material deposition, the feature sizes, shapes (e.g., rings, numbers, letters), line widths (300-900 nm), and materials (e.g., SiO2, Cr, and Ag) of these features can be independently controlled. Combined with anisotropic etching, more complex three-dimensional nanostructures can be fabricated as well, as we demonstrate here with Si. We further fabricate photodetectors as an example application to show that these nanostructures fabricated by DLPL can be used to promote light-trapping MAPbI3 perovskite films to achieve good photoelectric properties. This strategy is not limited to ultraviolet photolithography and may also be incorporated into other energetic beam-based lithographic approaches, including deep and extreme ultraviolet photolithographies and electron beam lithography, to enhance their resolution.

Iso- and Anisotropic Etching of Micro Nanofibrillated Cellulose Films by Sequential Oxygen and Nitrogen Gas Plasma Exposure for Tunable Wettability on Crystalline and Amorphous Regions.

The surface of cellulose films, obtained from micro nanofibrillated cellulose produced with different enzymatic pretreatment digestion times of refined pulp, was exposed to gas plasma, resulting in a range of surface chemical and morphological changes affecting the mechanical and surface interactional properties. The action of separate and dual exposure to oxygen and nitrogen cold dielectric barrier discharge plasma was studied with respect to the generation of roughness (confocal laser and atomic force microscopy), nanostructural and chemical changes on the cellulose film surface, and their combined effect on wettability. Elemental analysis showed that with longer enzymatic pretreatment time the wetting response was sensitive to the chemical and morphological changes induced by both plasma gases, but distinctly oxygen plasma was seen to induce much greater morphological change while nitrogen plasma contributed more to chemical modification of the film surface. In this novel study, it is shown that exposure to oxygen plasma, subsequently followed by exposure to nitrogen plasma, leads first to an increase in wetting, and second to more hydrophobic behaviour, thus improving, for example, suitability for printing using polar functional inks or providing film barrier properties, respectively.

Geometry of Structures Developed by Anisotropic Etching in the Nanometer Range

Geometric disadvantages of nanostructure surface developed by anisotropic etching of silicon are discussed. These disadvantages increase uncertainness of its sizes and impeded its using as an etalons for linear measurements. The greatest uncertainness are observed for structures with trapezoid profile. They make up due to defects on sidewalls of etched structures. The surface of a sidewall is proposed to be disposed in the [111] plane and so be absolutely flat. Really parts of a sidewall surface are deflected in stepwise way from the plane [111]. This phenomenon leads to deflection of angles between converging flat sections of etched structure from its known values for silicon. Consequence of this is most drastic to a measure MShps-2K due to its structure, technology of anisotropic etching and absence of required control. Sources of these surface disadvantages induced by anisotropic etching are discussed. Possibilities to decrease disadvantages are evaluated.

Surface damage formation during atomic layer etching of silicon with chlorine adsorption

As semiconductor device structures continue to approach the nanometer size range, new challenges in the fabrication of such devices have arisen. For example, the need for high-aspect-ratio, highly selective, controllable, and isotropic or anisotropic etching at the nanometer scale are some of them. Recently, atomic layer etching (ALE) has attracted much attention as an alternative to the conventional reactive ion etching (RIE) to address these issues. In comparison with RIE, ALE offers highly uniform etching over a large area with a precise etched depth and little damage to the underlying material surface. However, the extent of the surface damage formation in ALE processes has not been extensively reported yet. In this study, molecular dynamics simulation is used to examine the surface damages and reaction mechanisms during plasma-assisted (PA-) ALE of silicon (Si) with chlorine (Cl) radical adsorption and low-energy Ar+ ion irradiation for desorption. Several ALE cycles have been simulated and reproducible etched depths per cycle have been obtained. Based on the depth profiles, a damaged surface layer with a thickness of about 1.5 nm is found to be caused by the ALE process even at a very low ion incident energy of 20 eV in the simulation. The thickness of a damaged-layer on the etched surface slightly increases with the ion incident energy for the energy range examined in this study (20–60 eV), and Cl atoms deposited on the surface in the ALE adsorption step are transported deeper in the damaged-layer by the ion bombardment. Our simulation results indicate that a certain damage formation cannot be avoided on the “as-etched” surface of a PA-ALE process and, if the damaged-layer inadvertently affects the device performance, further action to mitigate the damage needs to be taken.