Wet Etching Method Research Articles

Resistive NO2 gas sensor based on GaN hexagonal pits at room temperature

Although fantastic milestones of Gallium nitride (GaN)-based materials in optoelectronic devices had been reached, the focus on the optimization of their geometrical structure for gas-sensing applications was relatively scarce. In this work, we demonstrated NO2 gas sensor based on p-type GaN hexagonal pits (GaN-HP) with high surface-to-volume ratio by wet etching method. The fabricated NO2 gas sensor had high sensitivity, high selectivity, fast response/recovery time. The response of GaN-HP with etching time for 10 min to 2 ppm NO2 gas was 6.2%, which also have quick response and recovery time at room temperature. UV light drastically enhanced the response of GaN-HP sensor by 1.3 times to 100 ppm NO2. The gas sensing mechanism of GaN-HP gas sensor was explored, which provided a theoretical and experimental basis for further research and development of high-performance gas sensors based on GaN material.

Femtosecond Laser Fabrication of Submillimeter Microlens Arrays with Tunable Numerical Apertures.

In recent years, the demand for optical components such as microlenses has been increasing, and various methods have been developed. However, fabrication of submillimeter microlenses with tunable numerical aperture (NA) on hard and brittle materials remains a great challenge using the current methods. In this work, we fabricated a variable NA microlens array with submillimeter size on a silica substrate, using a femtosecond laser-based linear scanning-assisted wet etching method. At the same time, the influence of various processing parameters on the microlens morphology and NA was studied. The NA of the microlenses could be flexibly adjusted in the range of 0.2 to 0.45 by changing the scanning distance of the laser and assisted wet etching. In addition, the imaging and focusing performance tests demonstrated the good optical performance and controllability of the fabricated microlenses. Finally, the optical performance simulation of the prepared microlens array was carried out. The result was consistent with the actual situation, indicating the potential of the submillimeter-scale microlens array prepared by this method for applications in imaging and detection.

A Simple and Scalable Wet Etching Method for Graphene Patterning By Using Hypochlorite and Ultraviolet Light

Graphene patterning via etching is important for enhancing or controlling the properties of devices and supporting their applications in micro- and nano-electronic fields. Herein, we present a simple, low-cost, and scalable wet etching method for graphene patterning. The technique uses hypochlorite solution combined with ultraviolet light irradiation to rapidly remove unwanted graphene areas from the substrate. Raman spectroscopy, atomic force microscopy, scanning electron microscopy, and optical microscopy results showed that well-patterned graphene with micrometer scale regions was successfully prepared. Furthermore, graphene field effect transistor arrays were fabricated, and the obtained devices exhibited good current-voltage characteristics, with maximum mobility of ~1600 cm2/Vs, confirming the feasibility of the developed technique.

Optical Thin Films Fabrication Techniques-Towards a Low-Cost Solution for the Integrated Photonic Platform: A Review of the Current Status.

In the past few decades, several methods concerning optical thin films have been established to facilitate the development of integrated optics. This paper provides a brief depiction of different techniques for implementing optical waveguide thin films that involve chemical, physical, and refractive index modification methods. Recent advances in these fabrication methods are also been presented. Most of the methods developed for the realization of the thin-films are quite efficient, but they are expensive and require sophisticated equipment. The major interest of the scientists is to develop simple and cost-effective methods for mass production of optical thin films resulting in the effective commercialization of the waveguide technology. Our research group is focused on developing a silica-titania optical waveguide platform via the sol-gel dip-coating method and implementing active and passive optical elements via the wet etching method. We are also exploring the possibility of using nanoimprint lithography (NIL) for patterning these films so that the fabrication process is efficient and economical. The recent developments of this platform are discussed. We believe that silica-titania waveguide technology developed via the sol-gel dip-coating method is highly attractive and economical, such that it can be commercialized for applications such as sensing and optical interconnects.

High-performance p-hexafluoroisopropanol phenyl functionalized multi-walled carbon nanotube film on surface acoustic wave device for organophosphorus vapor detection

A delay line-type surface acoustic wave (SAW) gas sensor based on p-hexafluoroisopropanol phenyl (HFIPPH) functionalized multi-walled carbon nanotube (MWCNT) film is developed to detect organophosphorus dimethyl methylphosphonate (DMMP) vapor (a simulant of chemical nerve agent sarin). Inspired by the transfer process of Cu-based graphene, a uniform and size-controllable HFIPPH-MWCNT film is successfully prepared on the SAW device via a wet-etching transfer method. For the first time, we use the method of measuring the change of the sensor’s insertion loss to achieve the detection of ultra-low concentration DMMP vapor. The designed sensor exhibits a fast response/recovery time about 3 s/50 s, and a low detection limit of 0.1 ppm. Additionally, the stability and selectivity of the sensor and the influence of humidity on its response are evaluated through experiments. The acoustoelectric effect is proved to be the sensing mechanism of the sensor insertion loss response.

Anisotropic Charge Transport Enabling High-Throughput and High-Aspect-Ratio Wet Etching of Silicon Carbide.

Wet etching of silicon carbide typically exhibits poor etching efficiency and low aspect ratio. In this study, an etching structure that exploits anisotropic charge carrier flow to enable high-throughput, external-bias-free wet etching of high-aspect-ratio SiC micro/nano-structures is demonstrated. Specifically, by applying a catalytic metal coating at the bottom surface of a SiC wafer while introducing patterned ultraviolet light illumination from its top surface, spatial charge separation across the wafer is achieved, i.e., photogenerated electrons are channeled to the bottom to participate in the reduction reaction of an oxidant in the etchant solution, while holes flow to the top to trigger oxidation of SiC and subsequent etching. Such design largely suppresses recombination-induced charge losses, and when used in combination with a top metal catalyst mask, the structure yields a remarkable vertical etching rate of 0.737µmmin-1 and an aspect ratio of 3.2, setting new records for wet-etching methods for SiC.

Influence of different etching methods on the structural properties of porous silicon

PurposePorous silicon (Si) was fabricated by using three different wet etching methods, namely, direct current photo-assisted electrochemical (DCPEC), alternating CPEC (ACPEC) and two-step ACPEC etching. This study aims to investigate the structural properties of porous structures formed by using these etching methods and to identify which etching method works best.Design/methodology/approachSi n(100) was used to fabricate porous Si using three different etching methods (DCPEC, ACPEC and two-step ACPEC). All the samples were etched with the same current density and etching duration. The samples were etched by using hydrofluoric acid-based electrolytes under the illumination of an incandescent lamp.FindingsField emission scanning electron microscopy (FESEM) images showed that porous Si etched using the two-step ACPEC method has a higher porosity and density than porous Si etched using DCPEC and ACPEC. The atomic force microscopy results supported the FESEM results showing that porous Si etched using the two-step ACPEC method has the highest surface roughness relative to the samples produced using the other two methods. High resolution X-ray diffraction revealed that porous Si produced through two-step ACPEC has the highest peak intensity out of the three porous Si samples suggesting an improvement in pore uniformity with a better crystalline quality.Originality/valueTwo-step ACPEC method is a fairly new etching method and many of its fundamental properties are yet to be established. This work presents a comparison of the effect of these three different etching methods on the structural properties of Si. The results obtained indicated that the two-step ACPEC method produced an etched sample with a higher porosity, pore density, surface roughness, improvement in uniformity of pores and better crystalline quality than the other etching methods.

ZnO/Ge core–shell nanowires and Ge nanotubes fabricated by chemical vapor deposition and wet etching

One-dimensional germanium (Ge)-related nanostructures including core–shell nanowires and nanotubes with high specific surface area show enhanced performance in energy storage and electronic devices, and their structural control is important for further improving their performance and stability. In this work, we fabricated vertically formed ZnO/Ge core–shell nanowires with different shell thicknesses. The dependence of morphology, crystallinity, and internal stress of the nanowires on the shell growth time and temperature was investigated. By applying the wet-etching method to the ZnO/Ge core–shell heterojunction nanowires, we demonstrated the Ge nanotube fabrication and stress relaxation in Ge after ZnO core removal.

Length effect on the plastic deformation of SiO2 microcantilevers

SiO2 films are of much interest for the realization of suspended micromechanical structures such as microcantilevers and micro-bridges owing to their superior characteristics when compared to conventional c-Si. However, presence of residual stress induced bending and structural plastic deformation in these devices especially at elevated temperatures hinder their potential for real-time field applications. Moreover, the impact of micro-scale dimensional effects on the magnitude of deformation in these devices is not well understood. In this work, we have investigated the effect of post-release high temperature annealing on the bending and resonance frequency characteristics of SiO2 microcantilevers (MCs) with a specific focus on the length effect. For this purpose, SiO2 MCs of various lengths (Length: 50–190 μm, Width: 37 μm and thickness: 0.98 μm) were fabricated using direct laser writer and wet chemical etching methods and were subjected to high temperature annealing under N2 and O2 environments. We demonstrate through our experiments that the curvature of SiO2 MCs increases with both annealing temperature and length square of MCs. Our experiments revealed that, apart from onset of plastic deformation, stress accumulation at the fixed end of SiO2 MCs is also responsible for the observed curvature increase. From the measured curvature, mean (σ0) and gradient (σ1) residual stress values were estimated and found to increase with annealing temperature. Interestingly, there exists a critical length of MCs (90 μm) below which both σ0 and σ1 were found to increase further with reducing MC length and is shown to be due to the existence of large transverse curvature at these dimensions. High temperature annealing was also found to increase the resonance frequency of MCs and the increase was maximum ∼ 8% for 130 μm long MC. Various possible reasons for the frequency shift is discussed in detail.

Fabrication and flow rate characterization of a DRIE process based valveless piezoelectric micropump

Micropumps have become one of the major research topics in the field of microfluidics. Different actuators have been used in active micropumps including piezoelectric ones, which convert electrical energy to mechanical energy. In this study, a piezoelectric disc was designed as an actuator and was integrated into a fabricated piezoelectric micropump. Unlike wet etching methods, which is generally used to etch layers of relatively high thickness in the silicon wafer, dry etching was used in this study. Aluminum was used as a mask material during the fabrications-steps, and the fabrication process was shortened with the use of the masking steps. The etching process of the slot, in which the piezoelectric disc was placed, and etching of the inlet/outlet channels were performed simultaneously. Thus, the processing time was significantly shortened. The fabrication of this silicon-based valveless micropump was accomplished by using the DRIE (deep reaction-ion etching) technique, which provided controlled etching. Experiments were conducted on the fabricated micropump with the use of the bulk micromachining technology to deliver the desired pumping action. The leakage and the air entrapment between the consecutive micropump structural layers were satisfactorily eliminated. This micropump is capable of delivering a promising flow rate of 52 (µl min−1) for deionized water, which corresponds to a 150 [Hz] square wave type and a peak-to-peak voltage of 60 (V) Vp -p . No moving mechanical valves were included so that the risks of clogging, reduced performance, and reduced reliability due to wear and fatigue were minimized. Not only gases and liquids but also fluids containing particles could be used as the working fluid for this micropump. The piezoelectric micropump design was optimized to achieve a high time-averaged flow rate (>50 (µl min−1)) with a relatively low excitation voltage (<100 (V) Vp -p ) as opposed to the use of a high excitation voltage.

Investigation of the Integration of Strained Ge Channel with Si-Based FinFETs.

In this manuscript, the integration of a strained Ge channel with Si-based FinFETs was investigated. The main focus was the preparation of high-aspect-ratio (AR) fin structures, appropriate etching topography and the growth of germanium (Ge) as a channel material with a highly compressive strain. Two etching methods, the wet etching and in situ HCl dry etching methods, were studied to achieve a better etching topography. In addition, the selective epitaxial growth of Ge material was performed on a patterned substrate using reduced pressure chemical vapor deposition. The results show that a V-shaped structure formed at the bottom of the dummy Si-fins using the wet etching method, which is beneficial to the suppression of dislocations. In addition, compressive strain was introduced to the Ge channel after the Ge selective epitaxial growth, which benefits the pMOS transport characteristics. The pattern dependency of the Ge growth over the patterned wafer was measured, and the solutions for uniform epitaxy are discussed.

Direct visualization of structural defects in 2D semiconductors

Direct visualization of the structural defects in two-dimensional (2D) semiconductors at a large scale plays a significant role in understanding their electrical/optical/magnetic properties, but is challenging. Although traditional atomic resolution imaging techniques, such as transmission electron microscopy and scanning tunneling microscopy, can directly image the structural defects, they provide only local-scale information and require complex setups. Here, we develop a simple, non-invasive wet etching method to directly visualize the structural defects in 2D semiconductors at a large scale, including both point defects and grain boundaries. Utilizing this method, we extract successfully the defects density in several different types of monolayer molybdenum disulfide samples, providing key insights into the device functions. Furthermore, the etching method we developed is anisotropic and tunable, opening up opportunities to obtain exotic edge states on demand.

Single-side micromachined ultra-small thermopile IR detecting pixels for dense-array integration

This paper presents a technology to fabricate ultra-small (100 × 100 μm) thermopile infrared (IR) sensing devices with a novel front-side micromachining process for integration of dense IR detection array. The thermopile IR detection unit is constructed into double-level suspending films, where the IR absorber and n/p-polysilicon thermopile are located at the central-supported upper level and the surrounding-supported lower level, respectively. The separated design for IR absorber and thermopile temperature-detection helps to achieve a high thermocouple fill-factor of 0.9 in terms of the structural area. A single-step wet etching method is proposed to simultaneously release the umbrella-shaped upper-level absorber and the thermal insulation cavity beneath the lower-level thermopile layer. By pre-depositing a polysilicon sacrificial layer, the etching method breakthroughs the restriction of anisotropic over-etch to the single crystalline silicon substrate, thereby maximizing the thermocouple number in the lower-layer film and minimizing the opened area of the etching holes. Finite-element-method simulation results show that the detector with 20 pairs of thermocouples exhibits maximized responsivity than those with more or less pairs of thermocouples. Benefit from the optimized heat dissipation design, the thermopile detection unit in atmospheric environment achieves a high normalized detectivity of 2.1 × 107 cm Hz1/2 W−1, a high responsivity of 52 V W−1 and sound respond time of 1.4 ms. Thanks to the ultra-small unit area, many unites can be densely integrated into IR detection arrays of various scales. As an example, an integrated 4 × 4 IR sensing array is herein fabricated. The 16 integrated pixels demonstrate good consistency in responsivity, with the deviation being less than ±7%. Featuring the tiny-size pixels for dense-array integration and the single-sided micromachining process that is compatible with integrated circuit fabrication, the proposed thermopile IR sensing devices are promising in IR detection applications including consumer-electronic level IR focal plane arrays.

Surface texturisation for the reduction of light reflection in ZnO/Si heterojunction

ABSTRACT In this paper, the impact of pyramidal texture on a silicon substrate in ZnO/p-Si heterojunction was investigated. The texturisation of p-type silicon (100) substrate was obtained using the KOH anisotropic wet chemical etching method for different etching times. The RF magnetron sputtering technique was used to deposit ZnO thin films on textured Si substrates and planar Si substrates to form ZnO/Si heterojunction. The surface morphology was studied with field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). Optical properties were investigated using UV-Visible spectroscopy and photoluminescence (PL). The results show that the PL intensity in the visible region of the electromagnetic spectrum increases with the etching time, while a significant reduction is observed in the reflectance. Due to impressive anti-reflection response, ZnO/Si (textured silicon-TS) heterojunction can be effective in improving the efficiency of solar cells.

Adhesionable flexible GaN-based microLED array film to brain surface for in vivo optogenetic stimulation

A development of a biocompatible, optical stimulation device capable of adhering to the brain surface and activating spatially separated brain regions is necessary for in vivo optogenetic applications. In this study, a hollow structure for isolating the microLED epitaxial layer was fabricated using the anisotropic KOH wet-etching method. Using a thermal release sheet, a method to transfer microLEDs onto a biocompatible parylene film was established without rotation or misalignment of the microLEDs while retaining their characteristics. Accordingly, a flexible microLED array film was fabricated, which adhered to the surface of the brain of a mouse and exhibited blue emission.

Patterning of graphene using wet etching with hypochlorite and UV light

Graphene patterning via etching is important for enhancing or controling the properties of devices and supporting their applications in micro- and nano-electronic fields. Herein, we present a simple, low-cost, and scalable wet etching method for graphene patterning. The technique uses hypochlorite solution combined with ultraviolet light irradiation to rapidly remove unwanted graphene areas from the substrate. Raman spectroscopy, atomic force microscopy, scanning electron microscopy, and optical microscopy results showed that well-patterned graphene with micrometer scale regions was successfully prepared. Furthermore, graphene field effect transistor arrays were fabricated, and the obtained devices exhibited good current–voltage characteristics, with maximum mobility of ~ 1600 cm2/Vs, confirming the feasibility of the developed technique.

Amorphous-crystalline heterostructure for simulated practical water splitting at high-current–density

Heterostructure (WO 2 -Ni 17 W 3 )/NiFe(OH) x /NF with amorphous-crystalline interface as bifunctional catalyst for efficient water splitting at high current density. • Amorphous-crystalline interface modifies the electronic structure of catalyst for accelerating water electrolysis. • The formation of NiFe(OH) x stabilizes the catalyst for efficient H 2 and O 2 evolution at high current density. • Ultra-high stability with 120 h at 1000 mA cm −2 under simulated industrial condition (6.0 M KOH solution at 60 ℃). The catalyst for practical water electrolysis needs to maintain long-term stability and work effectively withstanding high-intensity gas evolution, which is extremely challenging. Heterostructure with an amorphous-crystalline interface has been reported recently to availably enhance the catalytic performance of materials at high output. Herein, a wet chemical etching method realizes the hierarchical amorphous/crystalline heterostructure [(WO 2 -Ni 17 W 3 )/NiFe(OH) x ]. It induces modulated electronic structure and abundant active sites with chemical stability, resulting in extraordinary catalytic activity for hydrogen evolution reaction ( η 10 = 31 mV, η 1000 = 258 mV), oxygen evolution reaction ( η 50 = 240 mV, η 1,000 = 336 mV), and overall water splitting (1.54/1.84 V at 50/1,000 mA cm −2 ). Besides, it can maintain at 1,000 mA cm −2 stably for 120 h under simulated industrial condition (6.0 M KOH solution at 60 ℃) as a bifunctional catalyst. The coupled active phases and the in-situ formed heterojunction ensure an excellent performance at high current density. This strategy could be promoted to fabricate catalysts for practical water electrolysis.

Study on wet etching of dummy polysilicon in narrow pattern gap using alkaline solution

The polysilicon etching in a patterned wafer is an important process. In this process, the polysilicon must be completely removed without damaging the gaps made from SiO2 or Si3N4. In this study, a wet etching method was used to completely remove polysilicon while reducing it to SiO2. Alkaline solutions known as etching polysilicon, i.e., ammonium hydroxide (AH) and tetramethylammonium hydroxide (TMAH), were used. The etch rate was confirmed according to various factors such as temperature, chemical concentration, and rotation speed, but the etch rate was most affected by the chemical concentration. Accordingly, the concentration of hydroxide ions was the main factor for etching polysilicon. The etch rate of TMAH had the same tendency as AH for various factors, but the results indicated a difference of approximately eight times (1744 Å/min for TMAH and 217 Å/min for AH, condition: 6 wt%, 70 °C, 100 RPM). The electrical double layer (EDL) was calculated by using the pH, which was measured at each concentration of AH and TMAH solution, to etch the polysilicon in the narrow gap. The EDL was affected by the hydroxide concentration; thus, the TMAH solution etched the polysilicon in the narrow gap better than AH did. Furthermore, the experiment was conducted using an additive that could decrease the SiO2 etch rate and increase the selectivity between polysilicon and SiO2. Although the etch rate of polysilicon was reduced by 23%, the etch rate of SiO2 was also reduced by 43%.

Demonstration of using HF-etchant mixed into CO2 to lift-off thin-film GaAs layers by supercritical fluid technology

The epitaxial lift-off (ELO) process based on selectively etching a thin sacrificial AlAs layer from GaAs substrate was performed using high-concentrated aqueous hydrofluoric (HF) etchant. However, because of using the wet etching method, the traditional ELO process has many drawbacks and limitations. Supercritical fluids (SCFs) naturally have the characteristics of low viscosity, high diffusivity, and zero surface tension. Therefore, the development of a gas-phase-like dry etching method based on mixing HF into CO2 and operating the mixture of HF/CO2 in SCFs condition as etchant is hereby proposed to overcome those bottlenecks existing in traditional wet ELO processes. However, there are no available experimental results for etching AlAs layers by HF in SCFs yet. Therefore, a HF-compatible corrosion-resistant high-pressure system was designed and built up to perform the idea. The capabilities of etching sample in supercritical CO2 (scCO[Formula: see text] had been systemically investigated under various pressures (2000–3000 psi) and temperatures (40–60[Formula: see text]C). Besides, the etching performances separately conducted by using aqueous-HF and anhydrous HF/Pyridine as the source etchant and mixing with scCO2 at a fixed temperature, pressure and etching time were also examined and compared under different equivalent HF concentrations. An evaluation of using acetone as the co-solvent mixed with HF/scCO2 mixture for enhancing the etch rate in different volume ratio of HF/co-solvent was further investigated and discussed. With this system, we demonstrate releasing a size of [Formula: see text] ([Formula: see text]) and 3 [Formula: see text]m-thick free-standing GaAs sheet from a 150 nm AlAs sacrificial layer by the etching sample in HF/scCO2 mixture. The released GaAs sheet was also successfully transferred to a flexible PET substrate by using a PDMS stamp and an adhesive layer of NOA61.

Enhancing SERS Intensity by Coupling PSPR and LSPR in a Crater Structure with Ag Nanowires

The sensitive characteristics of surface-enhanced Raman scattering (SERS) can be applied to various fields, and this has been of interest to many researchers. Propagating surface plasmon resonance (PSPR) was initially utilized but, recently, it has been studied coupled with localized surface plasmon resonance that occurs in metal nanostructures. In this study, a new type of metal microstructure, named crater, was used for generating PSPR and Ag nanowires (AgNWs) for the generation of LSPR. A crater structure was fabricated on a GaAs (100) wafer using the wet chemical etching method. Then, a metal film was deposited inside the crater, and AgNWs were uniformly coated inside using the spray coating method. Metal films were used to enhance the electromagnetic field when coupled with AgNWs to obtain a high SERS intensity. The SERS intensity measured inside the crater structure with deposited AgNWs was up to 17.4 times higher than that of the flat structure with a deposited Ag film. These results suggest a new method for enhancing the SERS phenomenon, and it is expected that a larger SERS intensity can be obtained by fine-tuning the crater size and diameter and the length of the AgNWs.