Wet Chemical Etching Process Research Articles

Fabrication of luminescent disc-shaped microstructures via wet-chemical etching of hybrid sol–gel layers for potential photonic applications

Sol–gel materials based on SiO2 and TiO2 precursors are attractive as a new platform for planar photonics. Particularly interesting are those based on organically modified silica (ORMOSIL), which may improve the luminescent properties of organic dyes. However, their microstructurization remains a challenge as it requires optimization of various technological stages. Here, we report the structurization of thin layers based on ORMOSIL precursor and titanium(IV) ethoxide (TET) containing luminescent rhodamine B (RhB) dye. Films were fabricated using sol–gel synthesis and dip-coating method. Depending on the time of annealing performed at 200 °C, layers with different thicknesses (300–760 nm) and refractive indices (RI) (1.51–1.68) were obtained. Combining photolithography and wet-chemical etching processes made it possible to fabricate well-separated sol–gel waveguides and discs of different diameters. The etching time in buffered hydrofluoric acid (BHF) affected the depth of the etched luminescent microstructures. Additionally, it was found that a longer layer’s annealing time increased the etching selectivity of the substrate over the sol–gel layer. This enabled the obtaining of under-etched sol–gel goblet microstructures. Selected samples were investigated using scanning electron microscopy (SEM). UV–Vis photoluminescence measurements showed that long heat treatment also influenced the emission spectrum’s shape. The stability of the films under ambient conditions was established using spectroscopic ellipsometry. It was proven that films heat-treated at 200 °C did not change their properties during storage time of around 2 months. Relatively high RI, luminescent properties, and structurization potential make these microstructures interesting for application in integrated photonic devices, e.g., light amplifiers or sensing systems.

Design and Fabrication of Quartz MEMS Based Double-ended Tuning Fork Resonator

This paper presents the design and fabrication of double-ended tuning fork (DETF) resonator based on quartz. DETF resonator was designed with two tines structure, due to this configuration has the advantage of providing dynamic balance. Both surface and side electrode were designed on DETF tines in order to enhance the piezoelectric effect. DETF resonator was fabricated from a quartz substrate by chemical wet etching and photolithography processes, the key feature of fabrication is to create Cr/Au metal films, which should be strong enough to withstand the aggressive quartz etchant, meanwhile, the wet etching parameters was optimized in order to reduce the sidewall structure in +X orientation which can greatly deteriorate vibrating characteristics of DETF tines. Experimental results demonstrated that the Q value would be lowered and motional resistance be increased with the force sensitivity increase of DETF tines.

Development and Performance Evolution of Medium-Pressure He/SF6/O2-Based Plasma and Wet Chemical Etching Process for Surface Modification of Fused Silica

Development and Performance Evolution of Medium-Pressure He/SF6/O2-Based Plasma and Wet Chemical Etching Process for Surface Modification of Fused Silica

Nonsaturating Linear Magnetoresistance Manifesting Two-Dimensional Transport in Wet-Chemical Patternable Bi2O2Te Thin Films.

Two-dimensional (2D) materials with exotic transport behaviors have attracted extensive interest in microelectronics and condensed matter physics, while scaled-up 2D thin films compatible with the efficient wet-chemical etching process represent realistic advancement toward new-generation integrated functional devices. Here, thickness-controllable growth and chemical patterning of high-quality Bi2O2Te continuous films are demonstrated. Noticeably, except for an ultrahigh mobility (∼45074 cm2 V-1 s-1 at 2 K) and obvious Shubnikov-de Hass quantum oscillations, a 2D transport channel and large linear magnetoresistance are revealed in the patterned Bi2O2Te films. Investigation implies that the linear magnetoresistance correlates with the inhomogeneity described by P. B. Littlewood's theory and EMT-RRN theory developed recently. These results not only reveal the nonsaturating linear magnetoresistance in high-quality Bi2O2Te but shed light on understanding the corresponding physical origin of linear magnetoresistance in 2D high-mobility semiconductors and providing a pathway for the potential application in multifunctional electronic devices.

Hybrid Dry and Wet Etching of LiNbO3 Domain-Wall Memory Devices with 90° Etching Angles and Excellent Electrical Properties.

Ferroelectric domain walls, agile nanoscale interfaces of polar order, can be selectively controlled by electric fields for their position, conformation, and function, which is ultimately the key to realizing novel low-energy memory and computing structures. LiNbO3 single-crystal domain wall memory has the advantages of high operational speed, high integration density, and virtually unlimited endurance cycles, appearing as a good solution for the next generation of highly miniaturized low-energy memories. However, the etching process poses significant challenges in the nanofabrication and high-density integration of LiNbO3 domain-wall memories. Here, we employed a hybrid etching technique to achieve smooth sidewalls with a 90° inclined angle, leading to a 24% reduction in the coercive field and a 2.5-fold increase in the linear domain wall current density with a retention time of more than 106 seconds and endurance of over 105 writing cycles. Combined with the results of X-ray diffraction patterns and X-ray photoelectric spectra, it is concluded that the excellent electrical performance is related to the formation of an oxygen-deficient LiNbO3-x layer on the sidewall surface during the wet chemical etching process, which is a conductive layer that reduces the thickness of the "dead" layer between the side electrodes and the LiNbO3 cell and rectifies the diode-like wall currents with an onset voltage reduced from 1.23 to 0.28 V. These results prove the high-density integration of ferroelectric domain-wall memories at the nanoscale and provide a new strategy applicable to the development of LiNbO3 photonic devices.

Selective Wet and Dry Etching of NiO over β-Ga2O3

In this study, a reaction-limited chemical wet etching process of NiO by an HNO3-based solution, with a complete selectivity over β-Ga2O3 was investigated. The threshold ion energies for inductively coupled plasmas (ICP) dry etching of NiO in BCl3 discharges, its selectivity to β-Ga2O3, and the cleaning effect for chlorine-based residues remaining on the surface after plasma exposure was also examined. One of the major drawbacks of β-Ga2O3 device design is the lack of p-type dopants. An alternative approach is to use a p-type oxide, NiO, to form heterojunctions with n-type Ga2O3. Therefore, a selective wet and dry etching process development is in demand for the optimized pattern transfer process. A solution of 1 HNO3: 4 H2O exhibited measurable etch rates for NiO above 40°C and wet etching activation energy of 172.9 kJ.mol-1 (41.3 kCal.mol-1, 1.8 eV/atom), which is firmly in the reaction-limited regime. The selectivity over β-Ga2O3 was infinite for temperatures up to 55 °C. The strong negative enthalpy for producing the etch product Ga(OH)4 suggests that HNO3-based wet etching of NiO occurs via the formation and dissolution of hydroxides. The BCl3/Ar ICP etching produced maximum etch rates up to 300 Å.min-1 for NiO and 800 Å.min-1 for β-Ga2O3 under moderate plasma power conditions. The selectivity for NiO: Ga2O3 was < 1 under all conditions. The ion energy threshold for the initiation of etching NiO was between 35-60 eV, depending on the plasma power conditions. And its etching mechanism was ion-driven, as determined by the linear dependence between etch rate and the square root of ion energy incident on the surface. By sharp contrast, the etching of Ga2O3 had a stronger chemical component without a well-defined ion energy threshold. After being exposed to BCl3 plasmas, both NiO and Ga2O3 surfaces show chlorine residues, which can both be removed by the standard 1NH4OH: 10H2O or 1HCl: 10H2O solution used for native oxide removal. In addition, according to the location of the Cl 2p3/2 peak in the XPS analysis on the surface, the chlorine particles were ionically bonded. Figure 1

Reaction Mechanism Analysis of Si Selective Etching for Gate-All-Around Transistors by Molecular Simulations

The selective etching of Si in multi-stacked Si and SiGe structures is a key process to fabricate the next-generation of FET, Gate-all-around FET. In this study, we investigated the mechanism of wet chemical etching process at the molecular level for two common ionic solutions, potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH). One of the important factors in the etching process is the reaction rate between the water and Si surface. Therefore, the water dynamics in i) bulk system and ii) Si wall confined system were mainly analyzed using molecular dynamics simulations. As a feature of bulk, TMAH showed the larger hydration structure around cation and the lower mobility of water in the hydration shell compared to KOH. In the Si wall confined system, the water and ion dynamics on the OH-terminated Si surface were distinctive. TMAH showed the lower mobility of water as in bulk system. Furthermore, the concentration and long stay of cation near the Si surface were observed in TMAH. This behavior of cation may directly prevent water from contacting surface. These characteristics of TMAH may slow down the Si etching process. However, if the blocking effect for etching depends on the surface composition, it will be useful for selective etching.

Highly Responsive Near-Infrared Si/Sb2Se3 Photodetector via Surface Engineering of Silicon.

The development of imaging technology and optical communication demands a photodetector with high responsiveness. As demonstrated by microfabrication and nanofabrication technology advancements, recent progress in plasmonic sensor technologies can address this need. However, these photodetectors have low optical absorption and ineffective charge carrier transport efficiency. Sb2Se3 is light-sensitive material with a high absorption coefficient, making it suitable for photodetector applications. We developed an efficient, scalable, low-cost near-infrared (NIR) photodetector based on a nanostructured Sb2Se3 film deposited on p-type micropyramidal Si (made via the wet chemical etching process), working on photoconductive phenomena. Our results proved that, at the optimized thickness of the Sb2Se3 layer, the proposed Si micropyramidal substrate enhanced the responsivity nearly two times, compared with that of the Sb2Se3 deposited on a flat Si reference sample and a glass/Sb2Se3 sample at 1064 nm (power density = 15 mW/cm2). More interestingly, the micropyramidal silicon-based device worked at 0 V bias, paving a path for self-bias devices. The highest specific detectivity of 2.25 × 1015 Jones was achieved at 15 mW/cm2 power density at a bias voltage of 0.5 V. It is demonstrated that the enhanced responsivity was closely linked with field enhancement due to the Kretschmann configuration of Si pyramids, which acts as hot spots for Si/Sb2Se3 junction. A high responsivity of 47.8 A W-1 proved it suitable for scalable and cost-effective plasmonic-based NIR photodetectors.

Probing the depth inhomogeneity of spray pyrolyzed CZTS thin films via chemical etching

The surface of the Cu2ZnSnS4(CZTS) absorber layer has a critical role to attain higher photovoltaic(PV) performance. Hence, the study of inhomogeneity in the absorber layer is imperative to enhance absorber layer properties. In this work, the structural and compositional inhomogeneity between the surface and bulk region of CZTS thin film has been investigated using a wet chemical etching process. The Bromine (Br) combined etchant is used to remove the surface of the CZTS thin film to examine the inhomogeneity in the bulk region. The X-ray diffraction study reveals the reduction of crystallinity and the magnitude of Cu-Zn ordering for the etched-CZTS thin film. The magnitude of Cu-Zn order for the surface as well as the bulk region of CZTS thin film has been determined using near-resonance Raman spectroscopy. Further, the UV–Vis spectroscopy shows a band gap of 1.5 eV for the etched-CZTS thin film. The field-emission scanning electron microscopy displays the reduction of spherical grains on the surface of etched-CZTS thin film which is beneficial for the device’s performance. The energy-dispersive X-ray spectroscopy confirms the compositional inhomogeneity through the reduction of the Zn/Sn ratio in etched-CZTS thin films. The presented results from this study demonstrate the inhomogeneity between the surface and bulk region of CZTS thin films and offer a path to improve the device's performance.

Is 3D surface structuring always a prerequisite for effective SERS?

In this work, we systematically investigate and compare the need for 3D vis-‘a-vis 2D surface morphologies of SERS templates by considering all major aspects of uniformity, reproducibility, sensitivity and cost-effectiveness. Controlled Si nanostructured surfaces varying largely in their local height fluctuations are fabricated by adopting an effortless wet chemical etch process. To evaluate the SERS behavior, an Au (50%)-Ag (50%) alloy plasmonic layer is grown over these substrates owing to its high and stable SERS response. An initial assessment of the SERS activity is done by using Rh6G as a probe molecule. Apparently, the surface with 3D morphology shows a slightly higher response than the surface possessed with a few nm of surface roughness (2D). But from signal uniformity and reproducibility considerations the 2D morphology gives the lowest value of RSD of 5.56% in contrast to a high value of 27.13% for the 3D morphology. The 2D substrate also allows a LOD up-to 10−15 M and an EF of 7.1 × 107, thereby indicating a high sensitivity at par the best values reported till date. Further, detection of methylene blue and methyl parathion up to nM concentrations bear testimony to its universal applicability and ultra-sensitivity.

Rapid anisotropic chemical etching for quick formation of novel octagonal pyramids on silicon surface for photovoltaics

A novel octagonal micro pyramid structure was formed by etching a monocrystalline silicon wafer (100) by employing a wet chemical anisotropic etching process. The main objective is to reduce the silicon wafer's front surface reflectance, improve surface morphology, enhance wettability, and increase the coverage area of the octagonal pyramids on the surface of silicon wafers. The reaction time of the etching process was kept precise under strict observation and control. The experimental results demonstrate a significant reduction in silicon wafers' optical surface reflectance, achieving the lowest reflectance of 8.87%, along with improved surface morphology, periodicity, and coverage of more than 88%. While adding hydrofluoric acid together with magnetic stirring (mechanical agitation) at 300 rpm was advantageous in forming the octagonal silicon pyramid with a high etch rate of 0.41 um/min and significantly reducing the reaction time.

Design and evaluation of in-plane silicon microneedles fabricated with post-CMOS compatible processes

In this paper, a comprehensive study was carried out on in-plane silicon (Si) microneedles, a useful tool for transdermal drug delivery and sample collection. Microneedles with eleven designs were investigated by post-complementary metal-oxide-semiconductor (CMOS) compatible microfabrication processes and characterized via pricking tests by insertion in chicken breast flesh. Mechanical strength of all designs were also evaluated by theoretical calculation and finite element modeling (FEM) for bending and buckling analysis. To efficiently improve the sharpness and insertion, the wedge-shaped needle tips with thickness determined by Si wafer thickness were sharpened by a wet chemical etching process. Insertion forces recorded from pricking tests and bending and buckling from theoretical calculation and FEM analysis before and after etching were compared. The results showed that the insertion force, free bending force and the maximum buckling force were all reduced and the maximum bending stress were improved after tip sharpening. Furthermore, the buckling safety factor of all eleven designs was great than 1 and the maximum bending stress was less than the fracture strength of Si, indicating that our in-plane Si microneedles are robust enough for insertion into human skin.

Microfabrication of Alkali Vapor MEMS Cells for chip-scale atomic clock

The technology of MEMS atomic cells containing rubidium or caesium vapors in an atmosphere of neon buffer gas has been developed. Two-chamber silicon cells containing an optical cavity, shallow filtration channels and a technical container for a solid-state alkali source have been implemented in a single-step process of anisotropic wet chemical etching. To prevent significant undercutting of the filtration channels during etching of the through silicon cavities, the shapes of the compensating elements at the convex corners of the silicon nitride mask have been calculated and the composition of the silicon etchant has been experimentally found. The sealing of the cells has been carried out by silicon-glass anodic bonding at a temperature of 250 °C. For this purpose the LK5 glass which has an increased ionic conductivity in comparison with the conventional glass Borofloat 33 was used. The best microfabricated cells allowed us to obtain estimates of the relative instability of the coherent population trapping resonance frequency at the level of 5 · 10-11 at 1 s.

Quasi-Self-Aligned Organic Thin-Film Transistors in Coplanar Top-Gate Configuration

Besides charge carrier mobility and contact resistance, parasitic overlap capacitance is a key factor limiting the dynamic behavior of organic thin-film transistors. The most effective away to reduce parasitic overlap capacitances between source/drain and gate electrode is to employ self-aligned transistor architectures. However, so far, self-alignment in organic transistors has only been achieved using nonscalable or complex fabrication processes. Here we demonstrate quasi-self-aligned OTFTs in a coplanar top-gate architecture fabricated with reliable and scalable state-of-the-art fabrication techniques such as wet-chemical etching. Self-alignment is achieved by carrying out a wet-chemical etching process on top of the channel interface. We prove that for the proper choice of etchant, the charge carrier transport properties of the channel interface are not deteriorated. The benefit of the self-aligned design lies in the reduction of the specific overlap capacitances which are 2 orders of magnitude (0.13 nF cm–2) lower than for nonself-aligned devices (15 nF cm–2). Furthermore, we highlight the potential of this approach for high-frequency operation of organic transistors and quantify the gain in cutoff frequency compared to non-self-aligned devices.

Etching-Free Transfer and Nanoimaging of CVD-Grown MoS2 Monolayers

The transfer of two-dimensional materials from grown substrates onto target substrates is critical for device applications and postgrown analysis. The traditional transfer method for as-grown two-dimensional materials requires a wet chemical etching process that damages the crystals and the substrates. These issues deteriorate the electrical and optical performances of two-dimensional-material-based devices fabricated afterward. Herein, we developed an etching-free method and nanoimaging for transferring and analyzing monolayers of MoS2 onto arbitrary substrates using polyurethane as a sacrifice polymer. The polymer layer and the MoS2 crystals can be peeled off from the substrate by tweezers and transferred to any substrate. We analyzed the transferred samples to a glass coverslip substrate with optical microscopy, atomic force microscopy, tip-enhanced Raman spectroscopy, and tip-enhanced photoluminesce spectroscopy. Also, we transferred monolayers of MoS2 to a transmission electron microscopy grid and acquired scanning electron microscopy together with high-resolution scanning transmission electron microscopy images. The results of nanoimaging indicate that the method preserved the sample’s optical and structural properties and avoid undesirable cracks, wrinkles, and polymer residues. The etching-free transfer method of as-grown two-dimensional materials will improve the quality of transferred samples enhance the devices’ performance.

Nanoscale‐Sculptured Al Microparticles as Universal Hierarchical Adhesion Promoters

A combination of properties (e.g., biocompatibility, abrasion resistance, chemical inertness, etc.) from different polymers is desirable in many applications such as in medical and marine fields. However, joining nonstick polymers is still challenging. Herein, a facile and environmentally friendly approach is demonstrated to improve the adhesion between nonstick polymers through mechanical interlocking using a low amount of nanoscale‐sculptured Al microparticles. The hook‐like structures for mechanical interlocking are created on Al microparticles by nanoscale sculpturing, that is, a wet chemical etching process. By the incorporation of nanoscale‐sculptured Al microparticles at a polydimethylsiloxane (PDMS)/polythiourethane (PTU) interface serving as just one demonstrator example, the peeling strength of PDMS from PTU is 30 times higher compared with that without Al microparticles and 4 times higher than that with untreated Al microparticles, respectively. The significant increase in adhesion can be attributed to the penetration of both polymers into the hook‐like structures of nanoscale‐sculptured Al microparticles, forming a tight and strong hierarchical mechanical interlocking.

Quantum efficiency and oscillator strength of InGaAs quantum dots for single-photon sources emitting in the telecommunication O-band

We demonstrate experimental results based on time-resolved photoluminescence spectroscopy to determine the oscillator strength and the internal quantum efficiency (IQE) of InGaAs quantum dots (QDs). Using a strain-reducing layer, these QDs can be employed for the manufacturing of single-photon sources emitting in the telecom O-Band. The oscillator strength and IQE are evaluated by determining the radiative and non-radiative decay rates under the variation of the optical density of states at the position of the QD for InGaAs QDs emitting at wavelengths below 1 μm. For this purpose, we perform measurements on a QD sample for different thicknesses of the capping layer realized by a controlled wet-chemical etching process. From numeric modeling of the radiative and non-radiative decay rates dependence on the capping layer thickness, we determine an oscillator strength of 24.6 ± 3.2 and a high IQE of (85 ± 10)% for the long-wavelength InGaAs QDs.

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.

Fabrication of single-mode circular optofluidic waveguides in fused silica using femtosecond laser microfabrication

We demonstrate fabrication of single-mode circular optofluidic waveguides in fused silica glass substrates using femtosecond (fs) laser assisted chemical etching based on slit beam shaping. The fabrication procedure consists of four steps: slit-assisted fs laser direct writing of fused silica, selective chemical etching, polydimethylsiloxane film bonding, and vacuum-assisted liquid filling. The combination of slit beam shaping and high numerical aperture objectives ensures very narrowly modified lines with nearly circular cross-sections during laser direct writing. Introduction of a string of extra-access ports allows production of uniform circular microchannels with the diameters of ~10 µm and the lengths at the centimeter scale due to the improvement of wet chemical etching process. By vacuum-assisted filling a mixture of liquid paraffin and decane into a microchannel structure, a single-mode circular optofluidic waveguide embedded in glass can be obtained.

Aluminium-assisted chemical etching for fabrication of black silicon

This paper investigates aluminium-assisted chemical etching (AACE) process to fabricate b-Si absorber for the first time. The AACE process combines deposition of a thin aluminium (Al) film with thickness of 12–30 nm on crystalline silicon (c-Si) wafers, followed by subsequent thermal annealing and wet chemical etching processes. The AACE process produces random nanopores on the b-Si surface. From the results, thicker Al thickness leads to lower depth of nanopores, lower surface coverage of nanopores on the b-Si and exhibits higher weighted average reflection (Ravg) within 300–1100 nm wavelength region. The b-Si fabricated by Al film of 12 nm thickness demonstrates the lowest Ravg (11.9%) and the highest broadband light absorption. The b-Si reveals nanopores with average depth of 200 nm and surface coverage of 45.1%. The findings from this work demonstrate the potential of the AACE process to produce b-Si with superior broadband light absorption, which is crucial for photovoltaic (PV) applications.