Wet Etching Research Articles

Composition Engineering of Indium Zinc Oxide Semiconductors for Damage-Free Back-Channel Wet Etching Metallization of Oxide Thin-Film Transistors.

In contrast to lift-off and shadow mask processes, the back-channel wet etching (BCWE) process is suitable for industrial-scale metallization processes for the large-area and mass production of oxide thin-film transistors (TFTs). However, chemical attacks caused by the corrosive metal etchants used in the BCWE process cause unintended performance degradation of oxide semiconductors, making it difficult to implement oxide TFT circuits through industrial-scale metallization processes. Herein, we propose composition engineering of oxide semiconductors to enhance the chemical durability and electrical stability of oxide semiconductors. The chemical durability of InZnO against Al etchants can be improved by increasing the content of indium oxide, which has a higher chemical resistance than zinc oxide. As a result, A damage-free BCWE-based metallization process was successfully demonstrated for oxide TFTs using In-rich InZnO semiconductors. Furthermore, In-rich InZnO TFTs with wet-etched Al electrodes exhibited electrical performance comparable to that of lift-off Al electrodes, without chemical attack issues.

Ultra‐Stable Zinc Metal Anodes Enabled by Uniform Zn Deposition on A Preferential Crystal Plane

AbstractThe unsatisfactory electrochemical performance of Zn metal batteries (ZMBs) caused by uncontrollable Zn dendrite growth and detrimental parasitic reactions has significantly hindered their large‐scale applications. Herein, periodic hemispherical structures with a preferential exposure of Cu (100) crystal plane are designed and obtained using facile photolithography, which is followed by wet etching treatment. An exposed zincophilic Cu (100) crystal plane with low nucleation barriers acts as the preferred deposition site to induce homogeneous Zn deposition. Additionally, the periodic hemispherical structure with an enlarged surface area not only suppresses the Zn dendrite growth by reducing the local current density, but also synergistically buffers the volume expansion during the cycling process. As a result, the as‐prepared faceted Cu hemispherical electrodes achieve ultra‐stable Coulombic efficiency of over 99.9% for 1500 cycles at a current density of 5 mA cm−2. This work has significant potential for the rational design of dendrite‐free Zn anodes to boost their potential for practical applications.

High-Q trenched aluminum coplanar resonators with an ultrasonic edge microcutting for superconducting quantum devices

Dielectric losses are one of the key factors limiting the coherence of superconducting qubits. The impact of materials and fabrication steps on dielectric losses can be evaluated using coplanar waveguide (CPW) microwave resonators. Here, we report on superconducting CPW microwave resonators with internal quality factors systematically exceeding 5 × 106 at high powers and 2 × 106 (with the best value of 4.4 × 106) at low power. Such performance is demonstrated for 100-nm-thick aluminum resonators with 7–10.5 um center trace on high-resistivity silicon substrates commonly used in Josephson-junction based quantum circuit. We investigate internal quality factors of the resonators with both dry and wet aluminum etching, as well as deep and isotropic reactive ion etching of silicon substrate. Josephson junction compatible CPW resonators fabrication process with both airbridges and silicon substrate etching is proposed. Finally, we demonstrate the effect of airbridges’ positions and extra process steps on the overall dielectric losses. The best quality factors are obtained for the wet etched aluminum resonators and isotropically removed substrate with the proposed ultrasonic metal edge microcutting.

Fabrication of BixY3-xFe5O12 thin films by thermal decomposition of metal oleates

Metal organic decomposition (MOD) for the metal oxides thin films fabrication has been used in various studies over the past 30 years. MOD is a simple and inexpensive method for manufacturing optical grade polycrystalline thin films, including bismuth-substituted yttrium iron garnet. We present a variant of the previously described MOD method and discuss a chemical route for fabricating thin porous oxide films with a thicknesses of more than 100 nm, which non-epitaxially crystallized on fused silica substrates into BixY3-xFe5O12 layers (x = 0.5 and 1.5). Studies of the structural and magneto-optical properties of the layers show that the garnet layers crystallized under optimal conditions are characterized by high values of the specific Faraday rotation. Importantly, crystallization can be done locally by laser irradiation, and wet etching makes possible to remove non-crystallized areas, thus extending microfabrication capabilities.

Xenon-plasma focused ion beam processing of photonic microstructures with GaAs-based quantum dots

So far, successful focused ion beam (FIB) based fabrication of photonic structures with quantum dots (QDs) has been limited to cases with above 1 µm thick cap, usually in a form of a distributed Bragg reflector of a vertical cavity, which simultaneously protects the active region from the destructive influence of the ion beam. Here, we propose optimized xenon-plasma FIB (Xe-PFIB) technology as a fast and cost-efficient solution alternative to the commonly used combination of electron beam lithography and etching. We demonstrate a 3D processing of GaAs-based photonic microstructures with InGaAs QDs emitting close to the telecom O-band for cylindrical mesas with different cap thicknesses (50-650 nm) obtained by using two approaches: (i) Xe-PFIB for both reducing the cap thickness as well as the in-plane microstructure size, and (ii) wet chemical etching for cap layer removal and subsequent Xe-PFIB for the in-plane milling. The latter appeared more efficient when judging by photoluminescence intensity. Utilizing an additional protecting layer of platinum or carbon was also tested. Eventually, we for the first time show successful FIB-based fabrication of photonic microstructures with bright emission from single QDs capped with only 200 nm layer, which indicates the prospects of this technology for processing of efficient QD-based single-photon sources for quantum communication.

3D structured liquid/gas diffusion layers with flow enhanced microchannels for proton exchange membrane electrolyzers

Porous transport layers (PTLs) work as key water/gas transport components in proton exchange membrane electrolyzer cells. In PTLs, the gas accumulation under the lands of bipolar plate (BP) has been widely recognized, which decreases the local water saturation and blocks some active areas. Especially for some 2D structured PTLs, the absence of in-plane transport ability raises transport concerns at high current densities. In this study, a wet etching method is introduced to fabricate 3D-structured PTLs, named flow enhanced liquid/gas diffusion layer (FELGDL) for promoting multiphase transport under BP lands. In-situ performance evaluation validates a 330-mV voltage drop and a 7.9% efficiency improvement at 6 A/cm2 compared with the 2D-structured LGDL, which are mainly attributed to the improved reactant supply to the pores under BP lands. Additionally, the mass transport limitation is extended from 6.3 to 9.0 A/cm2 with an anode water flow rate of 20 mL/min. By the in-situ visualization method, the enhanced in-plane mass transport under BP lands is directly visualized. The successful fabrication and performance validation of FELGDL opened a new direction to high-performance 3D structured PTL manufacturing.

Femtosecond laser constructed anti-pollution silica glass surfaces with high-viscosity-oil adhesion prevention

Transparent surfaces with anti-pollution and anti-adhesive properties show promise for optical windows. However, there is currently no effective way to create glass surfaces that are impervious to highly viscous and sticky oils, like crude oil. In this research, an electrical-heating silica glass with resistance to high viscosity oil was prepared using a femtosecond laser process. An inner micro-channel system was fabricated by the femtosecond laser wet etching method, and the surface was treated to be superhydrophilic/underwater superoleophobic through femtosecond laser irradiation. The prepared sample could be electrically heated by its internal heating unit, which could reduce the viscosity of the crude oil. Thus, the prepared sample is resistant not only to typically low-viscosity oils, but also prevents the adhesion of crude oil, which would be widely applied in anti-pollution optical windows.

Wet‐Etching‐Boosted Charge Storage in 1D Nitride‐Based Systems for Imitating Biological Synaptic Behaviors

AbstractNitride materials for memristors are benefited from their high response speed and high power density. The memristive effects on 1D nitride structures has not yet been elucidated. Hence, the activation of the memristive capability of nanowire (NW)‐based nitride memristors using an uncomplicated fabrication as single‐step anisotropic wet etching is proposed. Among nitrides, gallium nitride, a third‐generation semiconductor, exhibits properties potentially suitable for neuromorphic applications. The wet etchant considerably alters the chemisorbed molecules and dangling bonds associated with the surface states of the nanowires. A device based on such NWs which exhibits low power consumption with no required compliance current and forming voltage for operation is demonstrated. It can integrate all memristive capabilities, including multiple state switching, nonvolatile bipolar memory, and Ca2+ dynamics‐imitating synaptic actions. The examination of the memristive process also highlights the significance of altering surfaces in the devices, in addition to the shared principles that underlie biological and artificial synapses. The operating mode of the nitride‐nanowire devices can be controlled by controlling the formation/dissolution of the oxygen‐conductive path along the nanowires. Thus, the study realizes nanowire memristors based on a nitride material framework, that is promising for application in the 1D–1D system downsizing required for the bio‐inspired artificial synapse.

Sub-quarter micrometer periodically poled Al0.68Sc0.32N for ultra-wideband photonics and acoustic devices

In this study, we demonstrate the ability of polarity inversion of sputtered aluminum scandium nitride thin films through post-fabrication processes with domain widths as small as 220 nm at a periodicity of 440 nm. An approach using photo- and electron-beam lithography to generate sub-quarter micrometer feature size with adjustable duty cycle through a lift-off process is presented. The film with a coercive field Ec+ of 5.35 MV/cm was exercised first with a 1 kHz triangular double bipolar wave and ultimately poled with a 0.5 kHz double monopolar wave using a Radiant Precision Premier II tester. The metal polar (M-polar) and nitrogen polar (N-polar) domains were identified and characterized through potassium hydroxide wet etching as well as piezoresponse force microscopy (PFM). Well-distinguished boundaries between the oppositely polarized domain regions were confirmed through the phase diagram of the PFM results. The relationship between the electrode width, poling voltage, and domain growth was experimentally studied and statistically analyzed, where 7.96 nm/V domain width broadening vs escalating poling voltage was observed. This method produces extremely high domain spatial resolution in III-nitride materials via poling and is transferable to a CMOS-compatible photolithography process. The spatial resolution of the periodically poled Al0.68Sc0.32N is suitable for second-harmonic generation of deep ultraviolet through quasi-phase-matching and RF MEMS operating in the X-Band spectrum.

Fabrication of YAG ceramics surface without damage and grain boundary steps using catalyzed chemical wet etching

Achieving an atomically smooth surface without damage or grain boundary steps is crucial for yttrium aluminum garnet (YAG) ceramics to deliver high-power laser oscillation and stable output. We propose a wet etching method that utilizes a thin-film catalyst, enabling the topmost crystal grain to be selectively removed through a catalyzed chemical reaction. This study demonstrates the effectiveness of the proposed method by polishing Nd- and Yb-doped YAG ceramic substrates. We realized damage-free surfaces having steps with a height of less than 1.0 nm. Such surfaces can effectively deliver high-power laser oscillation with high output stability.

Temperature, pressure, and humidity SAW sensor based on coplanar integrated LGS

This paper presents a surface acoustic wave (SAW) sensor based on coplanar integrated Langasite (LGS) that is fabricated using wet etching, high-temperature bonding, and ion beam etching (IBE) processes. The miniaturized multiparameter temperature‒pressure-humidity (TPH) sensor used the MXene@MoS2@Go (MMG) composite to widen the humidity detection range and improve the humidity sensitivity, including a fast response time (3.18 s) and recovery time (0.94 s). The TPH sensor was shown to operate steadily between 25–700 °C, 0–700 kPa, and 10–98% RH. Coupling issues among multiple parameters in complex environments were addressed by decoupling the Δf-temperature coupling factor to improve the accuracy. Therefore, this work can be applied to simultaneous measurements of several environmental parameters in challenging conditions.

Non-linear pH responses of passivated graphene-based field-effect transistors

Graphene-based field-effect transistors (FETs) are suitable for pH sensors due to their outstanding surface chemical properties and its biocompatibility. To improve the devices' stability and pH sensitivity, different sets of dielectric passivation layers composed of monolayer hexagonal boron nitride with and without aluminum oxide layers were evaluated. Non-linearities of the pH response were observed. Heterostructure FETs were derived from subtractive manufacturing of commercially transferred two-dimensional materials on four-inch SiO2/Si wafers via stainless steel and polypropylene masking. Phosphate solutions (10 mM) of varying pH were incubated on bare devices, whereby liquid-gating elucidated linear changes in the Dirac voltage of hBN/graphene (−40 mV/pH) that was smaller than a device consisting only of monolayer graphene (−47 mV/pH). Graphene-based FETs were passivated with aluminum oxide nanofilms via electron beam or atomic layer deposition and were observed to have distinct Raman spectral properties and atomic force microscopy topologies corroborating the hypothesis that morphological differences of the deposited aluminum oxide influence the pH-dependent electrical properties. Atomic layer deposition of aluminum oxide on the 2D sensing areas resulted in non-linear shifting of the Dirac voltage with respect to pH that evolved as a function of deposition thickness and was distinct between graphene with and without hexagonal boron nitride as a capping monolayer. The non-linear response of varying thickness of AlxOy on graphene-based FETs was progressively reduced upon basic wet etching of the AlxOy. Overall, passivated graphene-based transistors exhibit deposition-dependent pH responses.

Controlled ferroelectric switching in ultrawide bandgap AlN/ScAlN multilayers

Ultrawide bandgap ferroelectric nitride semiconductors have shown promising applications in electronic, micromechanical, and optical devices. Current studies, however, have largely been focused on single layer ferroelectric nitrides. Controlled polarization switching in artificial multilayer composite structures, such as ferroelectric/piezoelectric or ferroelectric/dielectric heterostructures, provides additional dimension for engineering their properties and improving device performance and functionality. Here, we demonstrate controlled ferroelectric switching in an AlN/ScAlN/AlN trilayer structure grown by molecular beam epitaxy. The trilayer showed large switchable polarization with reasonable endurance and retention performance. Polarity-sensitive wet etching further confirmed the controlled switching in the clamped ScAlN layer. The ability to control the polarity switching in ScAlN/AlN multilayers offers an intriguing avenue for the design and development of next-generation electronic, piezoelectronic, and ferroelectric devices.

Localized anodization of 4H–SiC by electrochemical etching with constant current mode in KOH solutions

With the increasing application of SiC, it has become increasingly evident that high hardness and difficulty in etching are major issues. Wet etching is a popular method due to its low cost and convenience. Electrochemical (EC) etching is one such wet-etching technique used for removing SiC material. Currently, most studies on EC focus on constant voltage mode with few investigations into constant current mode operation. In this study, we investigate the EC etching of 4H–SiC under a constant current regime by analyzing basic reaction processes through changes in voltage curves while studying large-period pulse etching modes as well as examining how different magnitudes of electric currents affect the process outcome. We hope our systematic research will contribute towards advancing knowledge about using EC techniques under continuous-current conditions thereby promoting wider application possibilities for this type of surface modification approach.

Wet etching of gold on graphene for high-quality resist-free graphene surfaces

Wet etching of gold on graphene is challenging due to the weak adhesion of the resist mask to graphene. We report an operating procedure for alkali ion-free wet etching of gold on graphene using a mixture of hydrochloric and nitric acids (aqua regia) with a high lateral resolution down to 100 nm. We investigate the role of positive and negative resists, electron beam lithography (EBL) dose, hard-bake, oxygen etching, aging, and sensitivity to the etch parameters, such as the freshness of dilute aqua regia, etch time, and the order of etched samples. The negative-tone resist provides the best results. The over-dosed EBL exposure can enhance the resist adhesion, as hard-bake below the glass-transition temperature and well-defined wet etch of the resist-residua-free gold surface. We also present a cleaning procedure to avoid bubble formation after the hard bake. Our results demonstrate that wet etching of gold on graphene using aqua regia is a viable method for achieving high-quality resist-free graphene surfaces. This method has potential applications in graphene nanoelectronics and nanophotonics, where high-quality graphene surfaces are essential for device performance.

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

Fabrication and Characterization of Self-Aligned WSe2 p-Type Field-Effect Transistor

Semiconducting transition metal dichalcogenides (TMDCs) have been growing interests as channel materials in field effect transistors (FET) for next generation low power digital electronics. [1]. Since a complementary metal oxide-semiconductor (CMOS) inverter constructed from pairs of n-type and p-type FETs is a fundamental building blocks in modern digital electronics [2], implementation of both n-type and p-type FETs with semiconducting TMDCs is of particular importance [3]. Among various semiconducting TMDCs, tungsten diselenide (WSe2) is a promising candidate for constructing a CMOS inverter because of its high mobility, symmetric electron and hole effective mass and ambipolar transport [4]. These prominent features of WSe2 is suitable for an application to CMOS inverter.One of the significant challenges is to develop a top gate and top contact FET structure with WSe2 for CMOS device integration on the same wafer. In addition, it is desirable to minimize the access region between channel and S/D contact because this access region served as a series resistance to channel resistance, resulting in the lower drain current in FET. This study reports a self-aligned process to fabricate top gate and top contact WSe2 p-type FET. In this self-aligned structure, the gate electrode and source/drain contact edges are automatically positioned and hence there are no overlap or access region between the gate and source/drain. The feature of this proposed method is that the gate stacks was utilized as a mask for self-aligned formation of WOx, which is source/drain contact for efficient hole injection [5]. The focus on this study is the impact of self-aligned structure of p-type WSe2 FET on the electrical characteristics. The effectiveness of proposed process was experimentally demonstrated by the fabrication and characterization of device.Figures 1 shows the self-aligned fabrication process of p-type WSe2 FET. 20 nm-thick SiO2 was formed by dry oxidation of p+-Si substrate. The mechanically exfoliated multi-layer WSe2 was transferred by PDMS stamp. Next, 15 nm-thick Al2O3 was prepared by ALD at 200 oC with H2O. Then, Nickel (Ni) metal was deposited by RF sputtering on Al2O3. Ni metal was patterned for gate electrode with conventional lithography process. After that, A2O3 layer was removed by wet etching. Subsequently, substrate was exposed to oxygen radicals to form WOx used as source/drain contact at surface of WSe2. Second layer of Al2O3 was deposited as an encapsulating passivation layer. After the contact hole opening, the electrical contact pad was fabricated. Finally, forming gas (N2 : H2 = 97 % : 3 %) annealing was performed at 200 oC for 30 min. The top gate length was 5 μm. The gate width was estimated to be 40 μm by optical microscope. The electrical characteristics were measured with a manual probe station in an atmospheric pressure without inert gas purge at room temperature using a precision semiconductor parameter analyzer (Agilent 4156 C).Figures 2 shows the Id–Vg characteristics of fabricated FETs for (a) back gate operation and (b) top gate operation. Representative Id–Vg characteristics of p-type FETs were observed irrespective of back gate and top gate operation. The on/off ratio of about 106 ~ 107 order were obtained for both back gate and top gate operation. Furthermore, the threshold voltage and sub-threshold slope were modulated by varying the substrate bias during gate sweep.This study opens up interesting directions for the research and development of TMDC-based devices.AcknowledgmentsThis study was supported by a JSPS Grant-in-Aid for Scientific Research (C) (Grant No. 20K04616), a research grant for Tokyo Tech Challenging Research Award, the Samco Foundation and a research grant for Suematsu Award.

(Invited) Fabrication and Device Performance of 2.7 Kv/2.5A NiO/Ga2O3 Heterojunction Power Rectifiers

In this study, The fabrication of a p-n junction NiO/Ga2O3 rectifier in which we employ a bilayer NiO structure with two different doping levels to achieve both high breakdown voltage and low on-state resistance was described. The band alignment of the NiO to the Ga2O3 was measured. Selective wet and dry etch processes for removing NiO from the underlying Ga2O3 was developed. Finally, the dc and switching performance of the rectifiers was characterized. The maximum breakdown voltage was 2.7 kV for a 1 mm diameter rectifier, with 2.5 A forward current and on/off ratio > 108 over a broad range of switching voltages. The reverse recovery time was limited by the measurement circuit but was < 100 µs. These results show the promise of heterojunction rectifiers based on Ga2O3 for future generations of power switching applications. Figure 1

Effect of wet chemical etching on femtosecond laser fabricated surface microstructure of polycrystalline ZnS

In this paper, wet chemical etching is employed to treat femtosecond (fs) laser fabricated surface microstructure on zinc sulfide (ZnS) to improve its surface quality (surface roughness) and regulate its size (depth/width ratio). Dilute nitric acid (HNO3) solution is selected as the etchant. First, the surface morphology of microgrooves before and after etching is observed as a function of etching time. Surface microcracks are also observed. Then, under different etchant concentrations, the size evolution of the microgrooves is measured for different etching time. Finally, an explanation for the changes in surface quality and size caused by etching is provided.

Wet Chemical and Plasma Etching of Photosensitive Glass

Photosensitive glasses for radiation-induced 3D microstructuring, due to their optical transparency and thermal, mechanical, and chemical resistance, enable the use of new strategies for numerous microscale applications, ranging from optics to biomedical systems. In this context, we investigated the plasma etching of photosensitive glasses after their exposure and compared it to the established wet chemical etching method, which offers new degrees of freedom in microstructuring control and microsystem fabrication. A CF4/H2 etching gas mixture with a constant volumetric flow of 30 sccm and a variable H2 concentration from 0% to 40% was utilized for plasma-based etching, while for wet chemical etching, diluted hydrofluoric acid (1% ≤ cHF ≤ 20%) was used. Therefore, both etching processes are based on a chemical etching attack involving fluorine ions. A key result is the observed reversion of the etch selectivity between the initial glassy and partially crystallized parts that evolve after UV exposure and thermal treatment. The crystallized parts were found to be 27 times more soluble than the unexposed glass parts during wet chemical etching. During the plasma etching process, the glassy components dissolve approximately 2.5 times faster than the partially crystalline components. Unlike wet chemical etching, the surfaces of plasma etched photostructured samples showed cone- and truncated-cone-shaped topographies, which supposedly resulted from self-masking effects during plasma etching, as well as a distinct physical contribution from the plasma etching process. The influences of various water species on the etching behaviors of the homogeneous glass and partially crystallized material are discussed based on FTIR-ATR and in relation to the respective etch rates and SNMS measurements.