Etching In KOH Solution Research Articles

Low-damage direct patterning of silicon oxide mask by mechanical processing.

To realize the nanofabrication of silicon surfaces using atomic force microscopy (AFM), we investigated the etching of mechanically processed oxide masks using potassium hydroxide (KOH) solution. The dependence of the KOH solution etching rate on the load and scanning density of the mechanical pre-processing was evaluated. Particular load ranges were found to increase the etching rate, and the silicon etching rate also increased with removal of the natural oxide layer by diamond tip sliding. In contrast, the local oxide pattern formed (due to mechanochemical reaction of the silicon) by tip sliding at higher load was found to have higher etching resistance than that of unprocessed areas. The profile changes caused by the etching of the mechanically pre-processed areas with the KOH solution were also investigated. First, protuberances were processed by diamond tip sliding at lower and higher stresses than that of the shearing strength. Mechanical processing at low load and scanning density to remove the natural oxide layer was then performed. The KOH solution selectively etched the low load and scanning density processed area first and then etched the unprocessed silicon area. In contrast, the protuberances pre-processed at higher load were hardly etched. The etching resistance of plastic deformed layers was decreased, and their etching rate was increased because of surface damage induced by the pre-processing. These results show that etching depth can be controlled by controlling the etching time through natural oxide layer removal and mechanochemical oxide layer formation. These oxide layer removal and formation processes can be exploited to realize low-damage mask patterns.

A multi-probe micro-fabrication apparatus based on the friction-induced fabrication method

A novel multi-probe micro-fabrication apparatus was developed based on the friction-induced fabrication method. The main parts of the apparatus include actuating device, loading system, and control system. With a motorized XY linear stage, the maximum fabrication area of 50 mm × 50 mm can be achieved, and the maximum sliding speed of probes can be as high as 10 mm/s. Through locating steel micro balls into indents array, the preparation of multi-probe array can be realized by a simple and low-cost way. The cantilever was designed as a structure of deformable parallelogram with two beams, by which the fabrication force can be precisely controlled. Combining the friction-induced scanning with selective etching in KOH solution, various micro-patterns were fabricated on Si(100) surface without any masks or exposure. As a low-cost and high efficiency fabrication device, the multi-probe micro-fabrication apparatus may encourage the development of friction-induced fabrication method and shed new light on the texture engineering.

A novel oxidation-based wet etching method for AlGaN/GaN heterostructures

A novel wet etching method for AlGaN/GaN heterojunction structures is proposed using thermal oxidation followed by wet etching in KOH solution. It is found that an AlGaN/GaN heterostructure after high temperature oxidation above 700°C could be etched off in a homothermal (70°C) KOH solution while the KOH solution had no etching effects on the region of the AlGaN/GaN heterostructure protected by a SiO2 layer during the oxidation process. A groove structure with 150 nm step depth on an AlGaN/GaN heterostructure was formed after 8 h thermal oxidation at 900°C followed by 30 min treatment in 70°C KOH solution. As the oxidation time increases, the etching depth approaches saturation and the roughness of the etched surface becomes much better. The physical mechanism of this phenomenon is also discussed.

Fabrication of Silicon Pyramid-Nanocolumn Structures with Lowest Reflectance by Reactive Ion Etching Method

We have developed nanosized structures on a silicon surface by the reactive ion etching method. The purpose of this work is to fabricate pyramid-nanocolumn structures for crystalline silicon solar cells using a reactive ion etching (RIE) system with a metal mesh and to simulate surface texturing structures using the PC1D program. The reflectance of the structures was lower than that of only-pyramid or only-nanocolumn structure arrays. We formed micropyramids by general anisotropic saw damage removal (SDR) etching using potassium hydroxide (KOH) solution, as well as nanocolumn structures by RIE with a metal mesh and SF6/O2 gas (35/30 sccm). We studied the fundamentals and morphologies of structures (micropyramids, nanocolumn structures, and pyramid-nanocolumn structures). Owing to anisotropic SDR etching in KOH solution for a single-crystalline silicon wafer, the whole wafer surface is covered with pyramid structures with a size range of 1–5 µm. After RIE texturing, nanocolumn structures with a size range of 20–50 nm were formed on the pyramid structures. Owing to binary surface texturing with pyramid and nanocolumn structures, spectra with weighted average reflectances below 3% in the wavelength range from 350 to 1100 nm were obtained under optimized condition. In particular, the average reflectance was 1.16% in the wavelength range of 400 to 1000 nm without an antireflection coating (ARC). The combination of the pyramid and nanostructures has a very low reflectance. Therefore, ARC is not required in the solar cell process in the case of using pyramid-nanocolumn structures.

Properties of nanopillar structures prepared by dry etching of undoped GaN grown by maskless epitaxial overgrowth

Nanopillar structures were prepared by dry etching of maskless epitaxial lateral overgrowth (MELO) GaN samples using a mask of Ni nanoparticles formed upon annealing thin Ni films deposited on top of SiO2/GaN. Under our experimental conditions the average nanopillars dimensions were close to 170nm, with the nanopillars density close to 109cm−2. The nanopillars formation was random and not correlated with the threading dislocation density in MELO GaN, as evidenced by comparing the size and density of nanopillars in the wing and seed regions of MELO GaN differing in dislocation density by an order of magnitude. After dry etching the luminescent intensity of nanopillars became actually lower than the intensity from the unetched matrix due to the impact of defects introduced in the sidewalls during nanopillars formation. The intensity greatly increased, together with a decrease in the leakage current of Schottky diodes, after rapid thermal annealing of nanopillar structures at 900°C and further increased after additional etching in KOH solution. These changes are attributed to annealing of radiation defects introduced by dry etching and further removal of the damaged region by KOH etching. The results suggest that, in nanopillar structures produced by dry etching, some increase of internal quantum efficiency alongside improvement of light extraction efficiency are responsible for the observed luminescence intensity changes.

The effect of isopropyl alcohol concentration on the etching process of Si-substrates in KOH solutions

In this paper the process of silicon anisotropic etching in KOH solutions containing isopropyl alcohol in a wide concentration range is extensively studied. Though the alcohol does not take part in the etching process itself, it strongly affects the etching results. Both etch rates and the roughness of etched surfaces depend on the alcohol concentration in the etching solution, which is connected with the adsorption phenomena on the etched surface. The surface coverage with alcohol depends on the level of saturation of the etching solution and crystallographic orientation of an etched surface. It was observed that the best morphology of (1 1 0) surface was achieved just below saturation level with IPA whereas the (1 0 0) surfaces were improving above the saturation. A model, which explains these phenomena, was proposed. Based on this model, a simple way of selection of the composition of KOH solutions with alcohol additives, assuring optimization of etching results was suggested. The method is restricted to surface tension measurements and allows one to avoid elaborated etching experiments.

표면 거칠기와 분포 상태에 따른 Si-셀 효율에 관한 연구

실리콘 태양전지는 KOH 에칭 용액을 이용하여 texturing 시간을 다르게 하는 방법으로 표면의 texturing 상태를 다양하게 제작하였다. 그리고 <TEX>$POCl_3$</TEX> 공정을 이용하여 n형 불순물을 도핑하여 pn접합을 만들었으며, 알루미늄 후면전극과 은 전면전극을 이용하여 Si-cell을 제작하였다. 피라미드 구조가 가장 크고 표면에 고르게 형성된 태양전지 셀에서 F.F. 계수가 높게 나타났으며, 효율도 높게 나타났다. texturing 형성이 잘 이루어진 셀인 경우 빛을 많이 흡수할 수 있고 회로 내부적으로는 직렬저항성분이 감소하여 단락전류성분이 현저히 증가하게 되어 최종적으로 효율이 증가되는 것을 확인하였다. Si-cell was prepared with various types owing to the etching times textured by the KOH etching solution. The pn junction for solar cell was prepared on p-type Si wafer by the furnace using the <TEX>$POCl_3$</TEX> and oxygen mixed precursor, and the metalization was done using by the Al back electrode and Ag front electrode. Textured Si surface was etched by the pyramid formation. The efficiency and the fill factor was increased in the Si-cell with a large size of pyramids, because of the series resistances decrease depending on the increasing of the photon absorbance. Increasing of the absorbance occurred the induction of the short current and open voltage, and then the efficiency was increased.

Investigation of yellow luminescence intensity of N-polar unintentionally doped GaN**

This paper reports that the yellow luminescence intensity of N-polar GaN Epi-layers is much lower than that of Ga-polar ones due to the inverse polarity, and reduces drastically in the N-polar unintentionally-doped GaN after etching in KOH solution. The ratio of yellow luminescence intensity to band-edge emission intensity decreases sharply with the etching time. The full width at half maximum of x-ray diffraction of (10–12) plane falls sharply after etching, and the surface morphology characterized by scanning electron microscope shows a rough surface that changes with the etching time. The mechanism for the generation of the yellow luminescence are explained in details.

The etching of a-plane GaN epilayers grown by metal-organic chemical vapour deposition

Morphology of nonpolar a-plane GaN epilayers on r-plane sapphire substrate grown by low-pressure metal-organic vapour deposition was investigated after KOH solution etching. Many micron- and nano-meter columns on the a-plane GaN surface were observed by scanning electron microscopy. An etching mechanism model is proposed to interpret the origin of the peculiar etching morphology. The basal stacking fault in the a-plane GaN plays a very important role in the etching process.

A low damage GaN-based light-emitting diode with textured/inclined sidewalls and an air–buffer layer

A GaN-based light-emitting diode (LED) with an air–buffer layer and textured sidewalls is fabricated and investigated. After the deposition of a 1-μm silicon dioxide (SiO 2) passivation layer on the whole device region except for the sidewall, a hot KOH etching solution is used to develop the desired textured sidewall. Many sidewalls with triangular-like shape are produced along the specific direction of 〈 1 1 ¯ 0 0 〉 , where photons introduced within multiple-quantum-well (MQW) exhibit more opportunities to scatter into the air. Based on the lateral etching, many triangular-shaped cones are formed on the patterned sapphire substrate (PSS). The device sidewall inclines with a 24° angle which results in an improved reemission of photons in the vertical/lateral direction. At 20 mA, as compared with the LED without KOH treatment, the studied device exhibits improved output power by 11.7% without deteriorating its electrical characteristics. This phenomenon can be attributed to the enhanced output light from vertical and lateral directions due to the formation of an air–buffer layer, textured and inclined sidewalls.

Silicon microstructure for bringing closer parallel beams of laser diodes

A spatial silicon structure containing parallel {111} faces forming two pairs of micromirrors used for making close two parallel optical beams at micrometric distance has been designed. The structure was fabricated in a single (113) oriented Si wafer. The fabrication process involved one stage double-side anisotropic etching in KOH solution. The structure employs natural arrangement of crystallographic planes in silicon, which assures an ideal parallelism of beam axes of reflected light. The reduction of the distance between the laser beams is possible without the need of setting up and adjusting a complex optical system.

Characteristics of a Low-Damage GaN-Based Light-Emitting Diode Using a KOH-Treated Wet-Etching Approach

A GaN-based light emitting diode (LED) with an air-buffer layer and textured sidewalls is fabricated and investigated. A hot KOH etching solution is used to develop the desired textured sidewalls. Many sidewalls with triangular-like shape are produced along the specific direction of <1100 >, where photons introduced within multiple-quantum-well (MQW) exhibit more opportunities to scatter into the air. Based on the lateral etching, many triangular-shaped cones are formed on the patterned sapphire substrate (PSS). At 20 mA, as compared with the LED without KOH treatment, the studied device exhibits improved output power by 11.7% without deteriorating its electrical characteristics. This phenomenon can be attributed to the enhanced output light from vertical and lateral directions due to the formation of an air-buffer layer as well as textured sidewalls. The thermal reliability of the studied device is also comparable to the one without KOH-treated process.

The influence of the AlN film texture on the wet chemical etching

The influence of the aluminum nitride (AlN) film texture on the chemical etching in KOH solution was invested. The AlN films with the different texture and crystal quality were prepared by sputtering. It is found that the chemical etching behaviors, including the etch rate, the activation energy, the surface morphology and the anisotropy, are strongly dependent on the film texture. There is a faster etching in the case of mixed (100) and (002) texture and a lower rate in the case of only (002) texture. The etch rate also decreases with the crystal quality. The sample with the only (002) texture forms discontinuous column structure after etching and exhibits lower porosity compared to that of the mixed (100) and (002) texture. Due to the strong anisotropy of the AlN wurtzite structure, the morphology of the film deposited at 700°C shows the homogeneous pyramid shape after etching. The cross-section micrographs of etching patterns indicate that the anisotropy of the chemical etching is improved with the improving of the crystal quality.

Wet Chemical Etching of Silicon {111}: Autocatalysis in Pit Formation

Accuracy and reproducibility of etching is essential to obtaining the smallest-scale devices in silicon. The present study focuses on the etching of silicon {111}, the slowest etching face, using different aqueous solutions of KOH. In all cases, shallow, point-bottomed etch pits are formed. It was found that etchant concentration, temperature, transport limitation, and addition of isopropyl alcohol (IPA) change the number, density, and morphology of the pits to a large extent. Sirtl delineation etching proved that the pits are not related to dislocations or stacking faults in the crystals. We propose that etch pit formation is autocatalytic in nature and is triggered by accumulation of reaction products at the bottom of the pits, locally enhancing the downward etch rate. This autocatalytic process also explains the formation of the concave-shaped pits, obtained after etching in KOH solution with IPA additive.

Thermal and chemical stabilities of In‐ and N‐polar InN surfaces

AbstractWe have investigated the thermal and the chemical stabilities of In‐ and N‐polar InN surfaces by treating InN films in several environments: annealing in vacuum, etching in KOH solution, and irradiating with atomic hydrogen (H*). For the annealing case, In‐polar films mostly disappeared after 5 min at 550 °C and In‐droplets of various sizes were left on the surface of the samples, whereas N‐polar films showed no noticeable change in thickness and only tiny In‐droplets were formed on the surface. For the KOH case, N‐polar films developed rough surface morphologies by the emergence of hexagonal pyramids, whereas In‐polar films were not etched noticeably. In‐polar films also exhibited a much lower reactivity to H* than that of N‐polar films. As a result, the N‐polar surface was thermally more stable than the In‐polar surface, whereas the In‐polar surface appeared to be chemically more stable than the N‐polar surface. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Miniaturized fluorescence excitation platform with optical fiber for bio-detection chips

This paper presents a new research study on the fabrication of a fluorescence bio-detection chip with an optical fiber transmission platform. Anisotropic wet etching on (100) silicon wafers to fabrication V-groove for optical fiber alignment and micro-mirror were included. Combined with anodic bonding technique to adhere glass, silicon structure and optical fiber for a fluorescence excitation platform was completed. In this study, a 40% KOH etching solution was used to study the parameter effect. The results show that the working temperature is the main parameter that significantly effects the etch rate. The anisotropic etching resulted in 54.7° reflective mirrors and the reflectivity for the optical beam was also examined. The surface roughness of the micro-mirror is Ra 4.1 nm measured using AFM providing excellent optical reflection. The incident light and beam profiles were also examined for further study. This study shows that this micro-platform is adaptable for fluorescence bio-detection.

Fabrication of the slanted electrode matrix on tilting 4.5° (1 1 1) silicon

The slanted low electrode matrix is designed and fabricated on one tilting 4.5° (1 1 1) silicon wafer to reduce the actuating voltage of 8×8 micro-electromechanical systems (MEMS) optical switch matrix. Due to compact size of the upper electrode chip and (1 1 1) silicon anisotropic etching in KOH solution, photomask is designed which is to fabricate the slanted low electrode matrix that can be matched with the upper electrode chip and every slanted low electrode has enough space for actuating cantilever. The experimental results show that all of the applied voltages for the full range of actuating micromirrors of 8×8 MEMS optical switch matrix are in the range of 67.2±0.5 V. It is demonstrated that the fabricated slanted low electrode matrix has good consistency and every slanted low electrode can be precisely aligned with one-to-one corresponding upper electrodes.

Wet chemical etching of AlN in KOH solution

AbstractIn this work we investigated the influence of the AlN material quality on the etching rate in KOH‐based solutions. Thus, AlN layers were deposited by three different methods on sapphire and silicon substrates (i) by metal organic chemical vapor deposition (MOCVD), (ii) by molecular beam epitaxy (MBE), and (iii) by reactive sputter deposition. The etch rate is strongly dependent on crystal quality and etch temperatures. The high quality MBE‐AlN could be etch anisotropic with a preferred lateral component in [11$ \bar 2$0] direction at 60 °C while the polycrystalline AlN layers we etched isotropic and homogenously already at room temperature. The wet chemical etching in KOH solution is mainly an etching along defects and grain boundaries. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Low stress PECVD—SiNx layers at high deposition rates using high power and high frequency for MEMS applications

The paper reports a new fabrication method for low stress SiNx layers at high deposition rates in a PECVD reactor using high power and high frequency (13.56 MHz) and their MEMS applications. By increasing the deposition power at 600 W in high frequency mode, a decrease of the tensile residual stress to values between 0 and 20 MPa was noticed. Meanwhile, the high power, which promotes a high dissociation of gases, generates a high deposition rate (in the range 250 to 350 nm min−1). In addition, the paper presents the influence of other important parameters that affect the residual stress and deposition rate such as pressure and SiH4, NH3 and N2 flow rates. SiNx layers fabricated at high power in high frequency mode were successfully used in two classical MEMS applications: fabrication of the masking layer for anisotropic wet etching in KOH solution and fabrication of a low stress cantilever.

Etching characteristics of a silicon surface induced by focused ion beam irradiation

The etching resistance characteristics of a Focused Ion Beam (FIB) irradiated silicon surface against KOH are investigated in this study. An FIB irradiated silicon surface can withstand etching in KOH solution, whereas the non-irradiated area is etched and consequently, a protruding nanostructure can be fabricated on the irradiated area. Height dependence of the nanostructure on the FIB irradiating conditions is investigated in order to control the shape of the nanostructure for application to three-dimensional nanofabrication. As a consequence, it was found that the height of the nanostructure can be controlled by FIB irradiating conditions such as dose and acceleration voltage. The mechanism is investigated by a selective etching method using HF solution. The results of a simulation indicate that the amorphous layer induced by ion irradiation is strongly related to this phenomenon. In addition, surface roughness and line width dependence was investigated, and these results indicate the potential use of this method as a novel three-dimensional nanofabrication process.