Pulsed Etching Research Articles

Demonstration of irradiation-resistant 4H-SiC based photoelectrochemical water splitting

4H silicon carbide (4H-SiC) has gained a great success in high-power electronics, owing to its advantages of wide bandgap, high breakdown electric field strength, high carrier mobility, and high thermal conductivity. Considering the high carrier mobility and high stability of 4H-SiC, 4H-SiC has great potential in the field of photoelectrochemical (PEC) water splitting. In this work, we demonstrate the irradiation-resistant PEC water splitting based on nanoporous 4H-SiC arrays. A new two-step anodizing approach is adopted to prepare 4H-SiC nanoporous arrays with different porosity, that is, a constant low-voltage etching followed by a pulsed high-voltage etching. The constant-voltage etching and pulsed-voltage etching are adopted to control the diameter of the nanopores and the depth of the nanoporous arrays, respectively. It is found that the nanoporous arrays with medium porosity has the highest PEC current, because of the enhanced light absorption and the optimized transportation of charge carriers along the walls of the nanoporous arrays. The performance of the PEC water splitting of the nanoporous arrays is stable after the electron irradiation with the dose of 800 and 1600 kGy, which indicates that 4H-SiC nanoporous arrays has great potential in the PEC water splitting under harsh environments.

Nanosecond pulsed laser etching anti-reflective gratings and synchronous laser annealing for performance optimization of Ag/AZO thin films

Nanosecond pulsed laser etching for fabricating anti-reflective gratings and inducing synchronous laser annealing was performed on Ag/AZO films. The optimum grating pitch was determined by performance analyses of laser-etched unidirectional gratings, based on which laser-etched bidirectional gratings were fabricated to further determine the optimum grating tilt angle. The results revealed that with the increase of grating pitch, the grating ridge total-side-area was gradually decreased to cause a gradually weakened anti-reflective effect, but the superimposed heat-affected zones between adjacent grooves became smaller or even disappeared, which brought about different synchronous laser annealing effects. The unidirectional grating-textured film with a medium grating pitch of 200 μm showed the best overall performance under the combined influence from anti-reflective gratings and synchronous laser annealing. For the bidirectional gratings, although increasing the grating tilt angle did not change the grating ridge total-side-area, it would enable the laser energy to be more uniformly distributed on the whole film surface, producing a better synchronous laser annealing effect. The bidirectional grating-textured film with a grating tilt angle of 90° had the best overall performance with the highest FOM value of 2.18 × 10−2 Ω−1 that was ∼3.2 times that of the as-prepared film. The results are instructive for preparing high-performance TCO-based films.

Research on the surface characteristics of 42CrMo alloy steel pulse electrochemical machining

Electrochemical processing, due to its unique material removal mechanism compared to mechanical processing, often results in significantly different micro-morphologies. It is widely used in areas such as aerospace component processing, surface finishing, passivation, micro-structure mold processing, etc. The complexity of alloy steel composition makes uniform etching difficult to achieve, and the post-etch micro-morphology is also complex, making it challenging to describe and analyze. Therefore, it is difficult to analyze the surface morphology of electrochemical etching under different parameters. In this study, a self-developed electrochemical etching device with adjustable DC voltage, pulse width voltage, and pulse width positive and negative voltage output was used. Different parameters of pulse voltage were used to etch the surface of 42CrMo alloy steel. Due to the strong relationship between surface roughness parameters and the chosen measurement scale, as well as the surface morphology and waviness errors introduced by specimen preparation and etching processes, a wavelet filtering method was used to remove the low frequency components in the surface morphology, and only the surface roughness part was retained for fractal parameter calculation and analysis. The research showed that fractal analysis can also be applied to analyze the surface morphology after electrochemical etching. Pulse voltages with characteristic voltages, characteristic pulse frequencies, and duty ratios can achieve better surface quality during positive pulse etching. Positive and negative pulse etching can obtain better surface morphology than positive pulse etching, but the etching amount is lower, making it suitable for precision finishing.

The Effects of Etchant on via Hole Taper Angle and Selectivity in Selective Laser Etching.

This research focuses on the manufacturing of a glass interposer that has gone through glass via (TGV) connection holes. Glass has unique properties that make it suitable for 3D integrated circuit (IC) interposers, which include low permittivity, high transparency, and adjustable thermal expansion coefficient. To date, various studies have suggested numerous techniques to generate holes in glass. In this study, we adopt the selective laser etching (SLE) technique. SLE consists of two processes: local modification via an ultrashort pulsed laser and chemical etching. In our previous study, we found that the process speed can be enhanced by changing the local modification method. For further enhancement in the process speed, in this study, we focus on the chemical etching process. In particular, we try to find a proper etchant for TGV formation. Here, four different etchants (HF, KOH, NaOH, and NH4F) are compared in order to improve the etching speed. For a quantitative comparison, we adopt the concept of selectivity. The results show that NH4F has the highest selectivity; therefore, we can tentatively claim that it is a promising candidate etchant for generating TGV. In addition, we also observe a taper angle variation according to the etchant used. The results show that the taper angle of the hole is dependent on the concentration of the etchant as well as the etchant itself. These results may be applicable to various industrial fields that aim to adjust the taper angle of holes.

Improvement of electrical properties by insertion of AlGaN interlayer for N-polar AlGaN/AlN structures on sapphire substrates

In this study, we propose nitrogen-polar (N-polar) Al0.1Ga0.9N/Al0.9Ga0.1N/aluminum nitride (AlN) structures. N-polar Al0.1Ga0.9N/Al0.9Ga0.1N/AlN was grown on a sapphire substrate with a misorientation of 2° with respect to the m-axis using metal–organic vapor deposition. The effects of varying the Al0.9Ga0.1N interlayer thickness from 30 nm to 1 μm using pulsed H2 etching on the planarity and current–voltage characteristics of the samples were investigated. The current first improved upon increasing the interlayer thickness from 30 to 300 nm, owing to the reduction in interfacial impurities between (aluminum) gallium nitride (Al)GaN) and Al0.9Ga0.1N, but subsequently decreased upon further increasing the thickness because of the relaxation growth of the interlayer. Furthermore, pulsed H2 etching of the Al0.9Ga0.1N interlayer suppressed step bunching and improved planarity. Subsequently, the proposed method was employed to fabricate N-polar Al0.1Ga0.9N/Al0.9Ga0.1N/AlN heterostructure FETs, which demonstrated five times higher source-drain current (I DS) than that of conventional structures without an interlayer.

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.

Electrochemical Etching Method to Fabricate Tips for Scanning Tunnelling Microscopes using Tungsten

The goal of this project was to create a tungsten tip for use in STM machines. This project started with the research of existing methods for creating STM tips through electrochemical etching. During initial testing, the electrolyte solution was identified as a powerful variable. Once a suitable solution had been found and optimized, we made changes to the etching procedure while observing the difference between outcomes. In this way we developed a method, which could create consistently sharp tips. The focus moved from finding a method for constant power etching to pulse etching. The advantage of constant power etching was that it gave a good indication of how sharp the tips could get while being very fast, allowing for rapid testing of variables and their impact on the outcome. The advantage of pulsed etching was the ability to control how sharp the tips created were. The final procedure was to pass 4V through the equipment (see figure 9), pulsed on for 1 second and off for 8 seconds. When the wire breaks the voltage is to be turned off and the tip retrieved. Excellent tips have been produced using this method, as evidenced in this report.

Surface enlargement of tantalum capacitor foils by pulsed direct current etching and laser cladding

Electrochemical etching for the surface enlargement of tantalum capacitor foils has always been a challenge, due to the ultra-high chemical stability of tantalum foils. Herein, pulsed direct current etching of tantalum capacitor foils assisted by laser cladding was investigated and high surface area enlargement was achieved. The effects of electrolyte composition, power supply parameters and external environment on the etching process of tantalum foil were systematically studied, to screen out the best experimental conditions. Furthermore, iron particles with diameters of 3–5 μm were successfully laser cladded on the surface of tantalum foils. These particles will be easily etched in the electrochemical process to form more previous pores, which can serve as etching sites for further tantalum foil etching. Under the best conditions, high-specific capacitance tantalum foils with a capacitance of 155.1 nF/mm2 were obtained at a high forming voltage of 100 V, which were 8 times larger than the unetched light foils. The surface enlargement of tantalum capacitor foils is expected to realize the development of novel tantalum capacitors with both high operating voltage and high capacitance.

An Ultra‐Thin, Ultra‐High Capacitance Density Tantalum Capacitor for 3D Packaging

AbstractAlthough embedded capacitors have been applied and researched for many years, their large‐scale application still faces challenges such as low capacitance density, high thickness, high cost, and incompatibility with integrated processes. In this work, a breakthrough has been made in the fabrication of ultra‐thin tantalum (Ta) capacitors with ultra‐high capacitance density that can be used for 3D packaging. The key to these excellent performances is the application of Ta foil with nano‐porous structure to the anode of the capacitor. The Ta foil with high specific surface area (SSA) is successfully prepared by direct current pulse etching. At a current density of 15 mA cm−2, a pulse frequency of 50 Hz, and a duty cycle of 30%, the SSA of Ta foil is increased by 76 times after etching in an electrolyte with 0.01 v% TOA for 30 min. Based on this, Ta capacitors with a form factor of less than 40 µm, showing a capacitance density of 750 nF mm−2 and a leakage current of less than 2.1 × 10−7 A at 8 V is successfully fabricated. To the best of the authors’ knowledge, this is the highest capacitance density reported to date for the mentioned form factors.

Nanosecond laser high-precision fabrication of microgrooves on TC4 surface: Morphology simulation and drag reduction performance of microgrooves

With the increasing global awareness of environmental protection, higher requirements are also placed on the fuel consumption rate of aircraft. Microgrooves can reduce the near-wall frictional resistance of aircraft to save fuel, which has become the focus of research in the aviation industry. To realize the low-cost and high-precision fabrication of microgrooves with drag reduction effect, an efficient and high-precision numerical simulation method for nanosecond pulsed laser etching TC4 titanium alloy is studied. A laser processing strategy is formulated. Furthermore, an efficient and high-precision computational fluid dynamics simulation method is studied to verify the drag reduction effect of microgrooves. The research results show that the size error of prepared microgrooves is less than 2%, and their drag reduction rate is as high as 9.6%, which verifies the validity and reliability of the research method in this paper. This work can guide the design of high-density functional microstructures and their high-efficiency and high-precision fabrication. The research results can provide technical parameters for aircraft drag reduction, which has essential engineering practical value.

Quasi-periodic micro-lens array via laser-assisted wet etching

A close-packed micro-lens array (MLA) is widely used not only in novel optical systems but also in various engineering fields, such as semiconductors and display devices. In this paper, we present a simple and efficient method for fabricating MLAs on a glass substrate via laser ablation with a single femtosecond laser pulse and chemical wet etching in a hydrofluoric acid solution. The shapes of concave micro-lenses were optimized for laser pulse energy and etching time, and ∼70 000 micro-lenses with 15 µm diameter were formed. The shape of the micro-lens varies significantly with respect to the etching time in accordance with an initial ablated area (or laser energy), which is a key feature in fabricating micro-lenses of several tens of μm. Using the glass MLA as a mold, a polydimethylsiloxane convex-plano lens array was replicated, and the performance of the optical imaging and beam integrator was then examined.

Effect of different pulse modes during Cl2/Ar inductively coupled plasma etching on the characteristics of nanoscale silicon trench formation

The etch characteristics of silicon trenches masked with various SiO2/Si3N4pattern distances were investigated using synchronously and asynchronously pulse modes in addition to the continuous wave (CW) mode for Ar/Cl2inductively coupled plasmas. By using synchronously and asynchronously pulse modes instead of CW mode, the selectivity between Si and the mask layer was increased by 2 and 10 times, respectively. Also,the etch rate differences between wide and narrow pattern distance patterns (aspect ratio dependent etching, ARDE) was decreased from 35% to 21 and to 8%,respectively. The increased etch selectivity andreduction of ARDE for the synchronously pulsed mode were related to the increased conduction of Cl radicals/byproductsthrough the high aspect ratio trench, therefore, increased chemisorption/byproduct removal on the silicon trench surface during the pulse-off period. For the asynchronously pulsed mode, the further improved etch selectivity and reduced ARDE were believed to be related to the time separated etch cycle composed of Cl chemical adsorption on the silicon trench surface and the removal of the chemisorbed species by ion bombardment. It is believed that, the asynchronously pulsed plasma etching method can be applicable to various next generation deep nanoscale device fabrication requiring high etch selectivity and low ARDE.

Deep ultraviolet emission from multiple quantum wells on flat N-polar AlN templates fabricated using periodical pulsed H2 etching

In a previous study, we successfully grew flat N-polar AlN layers on a c-plane sapphire substrate with a misorientation angle of 2.0° by metal-organic vapor phase epitaxy. However, its surface had undulations due to step bunching, and therefore further improvement of the surface flatness is required. In this study, we employed pulsed H2 etching during the growth of N-polar AlN layers to improve the surface flatness. Atomic force microscopy results indicated that the surface flatness was significantly improved, exhibiting a root mean square value of 0.4 nm. Further, the deep ultraviolet emission from AlGaN-based multiple quantum wells (MQWs) on the N-polar AlN layers was characterized, and the effect of the surface flatness on the optical characteristics was investigated. The surface flatness was found to play a crucial role in improving the optical characteristics of MQWs on N-polar AlN layers.

Pulse-Modulated Plasma Etching of Copper Thin Films via CH₃COOH/Ar.

Pulse-modulated plasma etching of copper masked using SIO₂ films was conducted via a CH₃COOH/Ar. The etch characteristics were examined under pulse-modulated plasma. As the duty ratio of pulse decreased and the frequency of pulse increased, the etch selectivity and etch profile were improved. X-ray photoelectron spectroscopy and indicated that more copper oxides (Cu₂O and CuO) and Cu(CH₃COO)₂ were formed using pulse-modulated plasma than those formed using continuous-wave (CW) plasma. As the concentration of CH3COOH gas in pulse-modulated plasma increased, the formation of these copper compounds increased, which improved the etch profiles. Optical emission spectroscopy confirmed that the active ingredients of the plasma increased with decreasing pulse duty ratio and increasing frequency. Therefore, the optimized pulsed plasma etching of copper via a CH₃COOH/Ar gas provides better etch profile than that by CW plasma etching.

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.

LRTM effect and electronic crystal imaging on silicon surface

Some interesting phenomena have been observed in the laser reflecting Talbot magnification (LRTM) effect discovered at first, in which the high-order nonlinear imaging and the plasmonic structures imaging occur. The LRTM effect images were obtained on the 1D and 2D photonic crystals fabricated by using nanosecond pulsed laser etching on silicon surface, where the high-order nonlinear imaging on the 1D and 2D photonic crystals was observed interestingly. The theory result is consistent with the experimental one, which exhibits that the suitable wave-front shape of injection beam selected in optical route can effectively enlarge the magnification rate and elevate the resolution of the Talbot image. Especially the periodic plasmonic structures on silicon surface have been observed in the LRTM effect images, which have a good application in the online detection of pulsed laser etching process. The temporary reflecting Talbot images exhibit that the electrons following with photonic frequency float on plasma surface to form electronic crystal observed on silicon at first, which is similar with the Wigner crystal structure.

The Effects of Psi Morphology on the Detection Efficiency of the Au-Nps Hot Spot SERS Sensors

This work is related to the investigation of the effects of porous silicon (PSi) morphologies on the performance of plasmonic gold nanoparticles (Au-NPs) hot spot SERS sensors for the detection of amoxicillin molecules. Two Si wafers with different resistivity values of 10 and 100 Ω.cm were used to synthesize a PSi layer of pores- and mud-like structures, respectively, by pulsed photo chemical etching process. The hot spot SERS sensors were synthesized by incorporating the Au-NPs within the PSi morphologies of pores- and mud- like structures which are characterized by high density of nucleation sites. Plasmonic Au- NPs with different sizes and hot spot regions were incorporated into the porous structures by the ion reduction process. It was recognized that the PSi morphologies have a considerable influence on the fabrication process of the detection sensors and, consequently, on the detection process. The plasmonic Au-NPs hot spot SERS sensor based on the PSi morphology of the mud-like structures shows higher sensitivity than that of pores-like structures, even at very low concentrations of amoxicillin solutions.

Hierarchical Porous Structure of Niobium Prepared By an Electrochemical Method

Fabrication of meso-/macro-porous electrode materials is known to be one of the effective ways to improve the performance of functional electrochemical devices since the porous structure would provide large surface area and allow rapid transport of atoms and ions. Recently, niobium (Nb) or Nb-based porous material has drawn much attention because of its highly versatile chemical and physical properties. There are various ways to form the porous Nb structure, such as hydrothermal, sol-gel, anodic oxidation and electrochemical etching methods. Among them, an electrochemical etching method is noteworthy because it is cost-effective, and the porous structure of the surface is easily tunable by adjusting etching conditions. Nevertheless, the general use of a noxious chemical (e.g. HF) for Nb porous structure formation needs to be avoided and then the creation of highly-porous Nb structure using F--free, more environmentally-benign chemical is still a challenge.In this work, the novel hierarchical Nb porous structure is created by an electrochemical method by using F--free etching solution. In particular, it is confirmed that the pulse etching is far more effective to form a hierarchical or self-similar fractal porous structure of Nb than conventional continuous etching, most likely due to the repetitive and incessant supply of aggressive anion. It is further proved experimentally that the initial concentration of aggressive anion is one of the essential factors in forming well-defined, high-order hierarchical Nb porous structure. Under the optimal etching condition, the porous Nb structure with more than 3 hierarchical order can be successfully created by single etching process.On the basis of the experimental observation of pore shape, the formula for the surface area of porous Nb is derived as a function of the hierarchical order of pore. From the comparison of the theoretically calculated surface area with the experimentally estimated one, the actual pore shape along with the hierarchical order of pores can be reasonably analyzed. In this presentation, the governing factors to construct a hierarchical Nb porous structure are suggested. Moreover, the utility of the formula between surface area and hierarchical pore order is discussed in terms of the non-uniformity of porous structure and hypotheses used to derive it.

Synthesis of SiC Nanowires via Controllable Anodic Etching Time

Porous SiC (PSC) was successfully synthesized via UV-assisted pulsed current anodic etching of hexagonal n-type silicon carbide (6H–SiC) substrate using different etching times. The micromorphological and structural characterizations of PSC were reported. Field-emission scanning electron microscopy (FESEM) results established that the etching time can be considered as a significant etching parameter that controls the micromorphology aspects of the porous SiC specimens. Furthermore, the adjustment of etching time can convert SiC pores into nanowire structures. Raman spectroscopy characterization was performed as well, where the shape of the Raman spectra was analyzed, in precise the transverse optical E1 (TO) and the longitudinal optical A1 (LO) peaks, which correlate powerfully with the porous SiC micromorphology. This simplistic synthesis may be considered as a potential and affordable technique to synthesize SiC nanowires for extensive applications.

Using Laser Duty Cycles for Modifying the Performance of Au-NPs/Si Nano Column Hot Spot SERS Sensors

In the current study, the Si nano column layer via pulsed photo chemical etching with different laser pulse duty cycle 30 and 60% using short laser wavelength (405 nm) and laser intensity (100 mW/cm2) was formed and studied. Two types of Si nano column-based plasmonic Au-NP hot spot layers were synthesized and examined successfully as an efficient SERS layer for the detection of the ultra-low concentration of amoxicillin. Si nano columns exposed a great effect on the performance of the Au-NP hot spot SERS sensor showing a strong dependence on the density of the hot spot gaps within the sensitive layer. Enhancement factor (EF) of the Raman signal improved considerably with increasing the density of the hot spot gaps due to the coupling efficiency among the plasmonic Au-NPs and the molecules of amoxicillin within the hot spot regions. EF increased by about four orders of magnitude with decreasing the laser duty cycles due to the increase of the integrated plasmonic Au-NPs into the Si nano column layer.