Etching In KOH Solution Research Articles

Faceting mechanisms of GaN nanopillar under KOH wet etching

This study presents strategies for achieving GaN pillars with the desired m- (or even a-) oriented nonpolar facets through a top-down approach that combines plasma etching followed by room temperature KOH wet treatment processes. Indeed, GaN etching in KOH solution is an anisotropic process, meaning that it allows the appearance of stable facets at the macroscopic scale, while atomic processes, such as step-flow, drive the fundamental mechanisms of the wet etching at the microscopic scale. Our study highlights the key role played by both the shape (circular or hexagonal, aligned with either the m-planes or the a-planes) and the roughness of the hard mask in determining the resulting crystalline facet formation and their associated roughness. Furthermore, it underscores the importance of the GaN pillar profiles (reentrant, straight, tapered) after plasma patterning, as they strongly influence the subsequent wet etching mechanisms. Ultimately, the article demonstrates that smooth m-oriented facets can be achieved by using room temperature wet KOH (44 wt%) on slightly sloped GaN profiles after plasma etching, in conjunction with the use of hexagonal m-oriented hard masks.

Die level thinning after bonding on the substrate in 3D packaging

This paper aims to realize heterogeneous integrated packaging technology for high integration by applying a thinning process technology for controlling the thickness of chipsets used in packaging, i.e., A technology for processing silicon dies with a thickness of 100 um or less and a technology for handling thinning ultrasmall dies. We combine wet etching with dry etching to fit an effective and practical experimental plan. Anisotropic etching in KOH solutions and ICP etching of silicon die bonding on a glass substrate are investigated. Die with copper wiring is fabricated and pasted on the substrate to achieve selective etching of the whole device. The topography of the die surface and corner is analyzed to prevent the influence on the second layer stacking process.

Morphological and Optical Study of Black P- Silicon Textured in KOH Bath

This paper reports the temperature and the concentration dependence of anisotropic etching for (100) p-Silicon in an aqueous KOH solution etching rate of wet etching has been experimentally determined with varying concentrations and the temperature of the KOH solution. The texturing process was managed at different etching durations ( 20 min, 40 min, and 60 min). XRD test showed that the lowest value of grain size was 5.0 nm (obtained with the highest porosity percentage of 50% with 4.5% KOH concentration for 60 min). FESEM test showed that the pore diameter increased with increasing etching time. The lowest reflectance value was (2.8 % at 550 nm wavelengths for samples treated with 4.5% KOH concentration for 60 min etching time. The refractive index value was 1.8 for the same black Si sample, also Hall test is introduced.

Fabrication of ultra-high aspect ratio silicon grating using an alignment method based on a scanning beam interference lithography system.

The high-aspect-ratio silicon grating (HARSG) is an important X-ray optical device that is widely used in X-ray imaging and spectrum detection systems. In this paper, we propose a high-precision alignment method based on the scanning beam interference lithography (SBIL) system to realize precise alignment between the <111> orientation on the (110) wafer plane and the grating lines direction, which is an essential step in HARSG manufacture to obtain the high-aspect-ratio grating structure. After the location of the <111> orientation through fan-shaped mask etching and reference grating fabrication, a two-step method that combines static preliminary alignment and dynamic precision alignment is used to align the reference grating lines direction with the interference field fringes of the SBIL system through the interference of the diffracted light from the reference grating near the normal direction, which can realize a minimal alignment error of 0.001°. Through the overall alignment process, HARSGs with groove densities of 500 gr/mm, 1800 gr/mm, and 3600 gr/mm were fabricated through anisotropic wet etching in KOH solution, producing ultra-high aspect ratios and etch rate ratios greater than 200.

Optimization of corner compensations in wet etching of silicon for a MEMS Z-axis accelerometer

Fabrication of planar MEMS accelerometers using micromachining and wet etching of surface structures on monocrystalline silicon wafer substrates poses several problems. One such problem is the preferential undercut of convex corners in the structure during chemical etch of silicon wafers. Resultant structures after etching are no longer the same as the design structure, leading to degradation in the performance of the device. The preferential undercut arises due to ‘wet anisotropic etching’ wherein the etchant has different etch rates along different crystal planes of the substrate. One of the methods to minimise this preferential undercut is to use compensation extension structures in the planar geometry of the device. In this paper, an accelerometer structure using a square extension has been studied in detail and the resulting dimensions are experimentally validated through fabrication and tests. Design steps and selection of optimal dimensions of the square compensation geometry are discussed considering etch depth with minimal undercut using KOH etching solution. Several samples of structures with three different dimensions of square extensions were fabricated and resultant device geometry was studied to establish the optimal extension dimensions with minimal undercut for the desired depth of etch.

Effect on nanoscale damage precursors of fused silica with wet etching in KOH solutions

We investigate the nanoscale damage precursors that will cause laser damage initiation on fused silica surface during KOH-based wet etching. Some nanoscale damage precursors, like impurity contamination and chemical structure defects on different etched surface with a KOH solution, are explored through a variety of testing methods at nanoscale spatial resolution. The etched surface roughness and photothermal absorption level are also studied. The results show that KOH-based etching can keep a good surface roughness, reduce impurity contamination significantly, and thus decrease surface photothermal absorption level. However, it can mitigate little chemical structure defect and has a risk of secondary pollution induced by residual deposition such as K2SiO3. The work can be a reference on using KOH-based wet etching technology to mitigate nanoscale damage precursors of fused silica ultraviolet optics.

Optical properties of InGaN/GaN QDs nanorods by top-down fabrication after KOH treatment

In this paper, we demonstrate and investigate the process of wet chemical etching in KOH solutions of InGaN quantum dots/GaN nanorods with the average height of heterostructure 330 nm and lateral size up to 120 nm. Morphological studying showed a significant thinning of nanorods to 40-80 nm and the shape transformation from trapezoidal to cylindrical, which led to the formation of an optimal resonator for the light-based GaN material. In this case, the photoluminescence intensity of the structure with nanorods increased in comparison with the initial planar LED structure, decreasing the impact of sidewall defects, which in turn led to the dramatic increase of the photoluminescence intensity. Further etching leads to a thinning of the GaN-p type block of the nanorods and the formation of a sharpened top, which is accompanied by a decrease in the photoluminescence intensity.

Investigation of the Electrochemical Stop Etching of Silicon Upon the Fabrication of Cantilevers

Anisotropic etching in potassium hydroxide (KOH) solution with electrochemical stop is commonly used in the formation of thin silicon membranes of sensitive elements in various microelectromechanical devices. However, there are almost no data on investigating the process of the electrochemical stop etching of elements for p–n junctions formed on a relief silicon surface. Here, the process of electrochemical stop etching in KOH solution for structures using n-silicon layers formed by diffusion on a p-silicon substrate with smooth and relief surfaces is studied. It is found that etching stops at the p–n-junction boundary when using a two-electrode circuit with a positive (relative to the solution) voltage applied to the n-type layer for a structure with an n-type layer formed on a smooth surface. As a result, a membrane, which is reasonably uniform in thickness over the entire area of wafer, manages to form. When using a two-electrode circuit for a structure with a relief surface, the current density is 70–100 times higher during etching than in the case of a smooth surface; etching is nonuniform over the wafer and stops long before achieving a p–n junction. It is found that etching stops at the p–n junction for a structure with a relief surface when using a circuit with two voltage sources and an additional negative (relative to the solution) voltage of –2.0 V applied to the p-type substrate. In this case, it is possible to obtain a silicon membrane, which is fairly uniform in thickness over the entire area of the wafer. The dependences of the density of current between the n-type layer of silicon and the solution on the etching time are obtained for structures with smooth and relief surfaces, which make it possible to determine the end of the etching process.

Structural characteristics of α-Ga2O3 films grown on sapphire by halide vapor phase epitaxy

While the importance of α-Ga2O3 crystals is increasingly gaining much interest, the structural behavior of halide vapor phase epitaxy (HVPE) α-Ga2O3 crystals in wet chemical etching has not been explored yet. In this study, we investigate the structural characterization of HVPE α-Ga2O3 materials via alkali KOH solution etching. Further, the activation energy of the etch rate for the materials, etch pit, and surface roughness after wet chemical treatment have been examined. The cross-sectional TEM analysis demonstrates that the triangular-shaped etch pits with (11 2‾ 6) plane are caused by the propagation of threading dislocations in the HVPE α-Ga2O3 crystals.

Morphology-Graded Silicon Nanowire Arrays via Chemical Etching: Engineering Optical Properties at the Nanoscale and Macroscale.

We report on a quick, simple, and cost-effective solution-phase approach to prepare centimeter-sized morphology-graded vertically aligned Si nanowire arrays. Gradients in the nanowire diameter and shape are encoded through the macroscale substrate via a “dip-etching” approach, where the substrate is removed from a KOH etching solution at a constant rate, while morphological control at the nanowire level is achieved via sequential metal-assisted chemical etching and KOH etching steps. This combined approach provides control over light absorption and reflection within the nanowire arrays at both the macroscale and nanoscale, as shown by UV–vis spectroscopy and numerical three-dimensional finite-difference time-domain simulations. Macroscale morphology gradients yield arrays with gradually changing optical properties. Nanoscale morphology control is demonstrated by synthesizing arrays of bisegmented nanowires, where the nanowires are composed of two distinct segments with independently controlled lengths and diameters. Such nanowires are important to tailor light–matter interactions in functional devices, especially by maximizing light absorption at specific wavelengths and locations within the nanowires.

Fabrication of free-standing nanoscale SiNx membranes with enhanced burst pressure via improved etching process

Fabrication of free-standing nanoscale SiNx membranes by the wet chemical etching of a thick Si substrate with KOH was investigated. During the long wet-etching process, localized etch rate differences within the substrate occur toward the end of the etching process, due to rapid stress increases at the edges, leaving a Si island at the center of the SiNx membrane. The thickness of this island, which is defined as the thickness of the central portion of the Si island in time at which the SiNx membrane begins to be exposed along the island boundaries, was monitored whilst varying the etching parameters. The parameters varied were the etching temperature, KOH concentration, and the concentration of an anionic surfactant. Bulge testing on SiNx membranes fabricated under various etching conditions indicated that the island thickness is a very useful figure of merit for monitoring etching uniformity and fracture strength of the fabricated membrane. The addition of a surfactant to the KOH solution at a concentration below its critical micelle concentration resulted in a decrease in the island thickness, thereby enhancing the fabrication yield for the etching process as well as the mechanical strength of the final membrane. The membrane fabricated under processing conditions including the use of a 35 wt% KOH etching solution, including 0.1 wt% Aerosol MA80, at 60 °C resulted in a yield of 100% and the average burst pressure of 1422 Pa, which was four times higher than that obtained for the membranes fabricated under one of the non-optimized conditions.

Studi Efektifitas Masking SnCl2.2H2O terhadap Pembentukan Profil QCM Menggunakan Teknik Etsa Basah dengan Larutan KOH

SnCl2.2H2O has been successfully utilized to produce isotropic profile on the surface of a SiO 2 crystall by a wet etching procces. The etching process was carried out by using a 40% KOH solution. In this research, the wet etching method is carried out by optimizing the etch time which was varied for 0,5 hour, 1 hour, 1,5 hour. The profile was determined by applying a SnCl 2 .2H2O on the surface of a QCM. The mask was applied by using a spray coating technique. The SnCl 2 .2H2O material was chosen as the masking material because it has a high adhesive property to SiO 2 , does not react to the KOH etching solution and is corrosion resistance as well as low cost. The etching profile surface results and etch depth (Dz) were determined using topography measurement system (TMS 1200). The etching rate with 40% KOH during etching time of 30 minutes, 60 minutes, 90 minutes were 4.36 μm, 4.56 μm and 5.79 μm respectively. The optimum QCM surface isotropic profile was obtained at the etching rate of 4.56 μm / h by 60 minutes etching.

The effect of monohydric and polyhydric alcohols on silicon anisotropic etching in KOH solutions

Tensioactive compounds, such monohydric or polyhydric alcohols cause that with an increase in their concentration in solutions, surface tension at the liquid-gas interface – γLG, and wetting angle at the interface silicon-solution – θLS decrease. It means that on both the surfaces an adsorption layer of the alcohol occurs. It results in lowering of etch rates of some silicon crystallographic planes. In this work, the values of γLG and θLS for selected monohydric and polyhydric alcohols (diols) have been compared and etch rates and morphologies of silicon substrates Si(100) and Si(110) etched in KOH solutions containing these alcohols have been determined. Advantages of diols and possibilities of their using in silicon anisotropic etching have been considered.

Preparation of structured surfaces for full-spectrum photon management in photovoltaic-thermoelectric systems

Micro-structured surfaces for broadband and omnidirectional photon management in photovoltaic-thermoelectric systems are fabricated on the silicon wafer by inductively coupled plasma etching with self-assembly polystyrene spheres mask, KOH solution etching and magnetron sputtering method. The Si antireflection structure coated with SiO2 film is fabricated on the top surface of Si substrate to enhance the absorption of the photons with wavelengths of 0.3–1.1µm is increase the electron-hole pairs. The TiO2/SiO2 films as the back-antireflection coating are deposited at the back of Si wafer to improve the transmission of solar energy within the wavelength range of 1.1–2.5µm. The absorbed photons within the wavelength range of 0.3–1.1µm can directly be converted into electricity power by Si solar cells while the photons with wavelengths of 1.1–2.5µm transmit through the cells and are utilized by thermoelectric modules. The effects of etching parameters on the spectral features are investigated to get the optimal etching parameters. With the optimal etching parameters, the surface absorption for the wavelengths of 0.3–1.1µm is significantly increased to above 0.95 due to the effective graded refractive index and the transmission is obviously enhanced for the wavelengths of 1.1–2.5µm. The incident angle-sensitivity and polarization-sensitivity of the fabricated structured surfaces are analyzed. The results indicate that these surfaces can be selected for photovoltaic-thermoelectric systems due to their high omnidirectional absorption and transmission of the photons as well as the favorable polarization-insensitivity within the expected wavelength ranges.

Effect of molecular structure of alcohols on wet anisotropic etching of silicon

The study concerns anisotropic etching of silicon (100) and (110) planes in potassium hydroxide aqueous solutions saturated with different monohydric alcohols (C3H7OH, C4H9OH and C5H11OH). The main goal of the research is to systematically examine the influence of length and branching of alcohol molecules alkyl chains on etching process. The etch rate ratio R(100)/R(110)>2 and (110) planes having zigzag pattern are obtained as a result of etching in KOH solutions with all considered alcohols. Additionally, it is demonstrated that convex corners of spatial structures etched in aforementioned solutions are probably formed by exact or vicinal {221} planes. It is shown that for alcohols whose molecules have more than 3 carbon atoms the Si (100) surface is covered with pyramidal hillocks after etching. Moreover, the etch rate of (110) plane decreases as the alcohol molecules alkyl chain becomes longer and more branched. The attempt to explain the impact of molecular structure of different alcohols on the Si etch rates is made, based on results of surface tension and contact angle measurements.

Nondestructive nanofabrication on Si(100) surface by tribochemistry-induced selective etching.

A tribochemistry-induced selective etching approach is proposed for the first time to produce silicon nanostructures without lattice damage. With a ~1 nm thick SiOx film as etching mask grown on Si(100) surface (Si(100)/SiOx) by wet-oxidation technique, nano-trenches can be produced through the removal of local SiOx mask by a SiO2 tip in humid air and the post-etching of the exposed Si in potassium hydroxide (KOH) solution. The material removal of SiOx mask and Si under low load is dominated by the tribochemical reaction at the interface between SiO2 tip and Si/SiOx sample, where the contact pressure is much lower than the critical pressure for initial yield of Si. High resolution transmission electron microscope (HRTEM) observation indicates that neither the material removal induced by tribochemical reaction nor the wet etching in KOH solution leads to lattice damage of the fabricated nanostructures. The proposed approach points out a new route in nondestructive nanofabrication.

Locally non‐uniform oxidation in self‐terminating thermal oxidation assisted wet etching technique for AlGaN/GaN heterostructure

The oxidation mechanism in self-terminating wet etching technique with thermal oxidation of AlGaN layer followed by etching in KOH solution is investigated. Spike-shape remnants of oxidised AlGaN are observed at the initial stage of wet etching in KOH solution, which could be completely etched away after enough etching time. Transmission electron microscope/energy dispersive spectroscopy analysis indicates the existence of crystalline AlGaN inside the remnants. Finally, a possible explanation is given that the oxide channels from AlGaN surface towards AlGaN/GaN interface generated during thermal oxidation are firstly etched away at the initial stage of KOH wet etching, then after enough time these remnants with non-c axis crystal orientation surfaces exposed to KOH solution could be completely etched away leaving GaN layer beneath unaffected, which realises self-terminating etching.

Two-step sharpening process for silicon probe in quartz-based atomic force microscopy sensor

A two-step sharpening process for a Si pillar in a quartz tuning fork sensor was developed for use in frequency modulation atomic force microscopy (FM-AFM). By combining electrochemical etching in HF solution and anisotropic etching in KOH solution, the tip radius was decreased to 120 nm. The Si-probe-attached sensor showed a higher resonance frequency than a tungsten-probe attached sensor, which would lead to a higher force sensitivity. We demonstrated FM-AFM imaging on a cleaved mica surface in aqueous solution by using the fabricated sensor, and atomic resolution was successfully achieved.

Preparation and characterization of GaN micro/nano- structures by the electrodeless photo-assisted chemical etching

GaN micro/nano-structures of different morphology were successfully preparated by the electrodeless photo-assisted chemical etching in KOH solution, with K2S2O8 as the oxidizing agent. The structure, crystalline quality, stress state and optical property of both normal GaN and etched GaN were provided by the combined results of scanning electron microscopy (SEM) images, cathodoluminescence mapping (CL mapping), high-resolution X-ray diffraction (HRXRD) detection, micro-Raman spectra and photoluminescence (PL), respectively. The results show that pits, micro/ nanopillars and nanowires of high crystalline quality are formed in the high KOH concentrations (1 mol/L) and the low light intensity; GaN pyramid arrays have been fabricated in 0.4 mol/L KOH solution with high UV light intensity, and CL mapping indicates that these pyramids are composed of crystalline GaN surrounding a dislocation; Whiskers can also be obtained in 0.1 mol/L KOH with high UV light intensity after etching. The whiskers are believed to be directly correlated with line-type defects of edge and mixed character. The formation mechanism and the feature of GaN different morphology were rationally explained. And the etching temperature and the polarity of GaN surface have an evident influence on the surface morphology of GaN.

FABRICATION OF PYRAMIDAL CAVITY STRUCTURE WITH MICRON-SIZED TIP USING ANISOTROPIC KOH ETCHING OF SILICON (100)

Microelectromechanical System (MEMS) are systems of micron-sized structures and typically integrated with microelectronic components. Bulk micromachining using wet anisotropic etching is able to etch silicon substrates to a desired three-dimensional (3D) structure, depending on the silicon crystallographic orientation. To date, MEMS components i.e. thermal, pressure, mechanical, bio/chemical sensors have been fabricated with wet anisotropic etching of silicon. This paper presents the fabrication of a 3D pyramidal cavity structure with micron-sized tip of silicon (100) using anisotropic KOH etching of w/w 45 % at 80 oC temperature. Volume percent of 10 % IPA as a less polar diluent is added to the KOH etching solution in saturating the solution and controlling the etching selectivity and rate. Smooth etched silicon surface of hillock free is able to be achieved with IPA addition to the KOH etching solution. A characteristic V-shaped cavity with side angle of 54.8 degrees has successfully been formed and is almost identical to the theoretical structure model. Comparison of two different silicon nitride window masks on the micron-size tip formation is also investigated. Under etch, over etch and etching selectivity, as common problems effecting the micron-tip size variation, are also addressed in this work. In conclusion, anisotropic KOH etching as a simple, fast and inexpensive bulk micromachining technique, in fabricating 3D MEMS structure using silicon (100), is validated in this work.