KOH Anisotropic Etching Research Articles

Silicon flow stop frame for encapsulation of CMOS microsensor chips by wet anisotropic etching in KOH

Bulk micromachining in silicon is governed by the etching process where anisotropic (wet) etching in KOH can yield complex structures beyond that achievable with isotropic (dry) etching techniques. One example is the miniaturised frame reported herein with an area of 2.9 to 7.5 mm2, walls that are 1/10 mm thick, a height of 525 μm equipped with sloping walls that takes advantage of the 54.7° angle of the (111) planes to the horizontal (100) top surface of the wafer. Convex corners liable to damage are protected by sacrificial bridge structures which are etched thin to a point where the frame can be easily removed from the bulk substrate material. Frames made from isotropic (dry) etching processes have been made for comparison. Although the frame structure has different applications in microfabrication, the intended use is a flow stop barrier preventing liquid resins from entering the active area of a CMOS chemical sensor chip during encapsulation for use in aqueous or gaseous media. Beyond this specific proof-of-concept, the strategy is expected to be of general interest for all who treasures KOH etching and wants to explore new avenues based on this process.

Polarity Inversion of GaN via AlN Oxidation Interlayer Using Metal–Organic Vapor Phase Epitaxy

The polarity inversion of GaN films grown on vicinal sapphire substrates is demonstrated using the metal–organic vapor phase epitaxy (MOVPE) method via an AlN oxidation interlayer for application in wavelength conversion devices. By annealing the N‐polar AlN surface at 900 °C in air, the pit‐free Ga‐polar GaN layer can be regrown. Anisotropic etching in KOH and transmission electron microscopy confirms the formation of a GaN polarity‐inverted structure. X‐ray diffraction measurements reveal an increase in threading dislocations due to the nucleation island formation by low‐temperature GaN layer and strain relaxation at the regrown interface. The simple fabrication process of the GaN polarity‐inverted structure using an AlN oxidation interlayer will pave the way for applications in new devices that require polarity‐controlled and/or polarity‐inverted stacking structures.

Wafer-scale synthesis of a morphologically controllable silicon ordered array as a platform and its SERS performance.

In this study, we fabricated four different structures using single crystal silicon wafers for surface-enhanced Raman spectroscopy (SERS) applications. For single crystal silicon, different crystal orientations exhibit different physical and chemical properties. In chemical etching, the etching speed of different crystal planes also exhibits significant differences. We first used reactive ion etching (RIE) to process the surface of the substrate, and subsequently used KOH anisotropic wet etching technology to modify the surface of silicon wafers with different crystal orientations and produced four different results. In the RIE stage in an O2 atmosphere, the (110) silicon wafer formed a hexagonal hole structure, and the (100) silicon wafer formed an inverted pyramid hole structure; however, in the RIE-treated substrates in O2 and SF6 atmosphere, the (110) silicon wafer formed a pyramid with a diamond-shaped base, and the (100) silicon wafer showed a columnar structure with a "straw hat" at the top. The formation mechanisms of these four structures were elucidated. We also performed structure-related SERS characterizations of the four different structures and compared their performance differences.

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

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

Wafer-level fabrication of individual solid-state nanopores for sensing single DNAs

For biomolecule sensing purposes a solid-state nanopore platform based on silicon has certain advantages as compared to nanopores on other substrates such as graphene, silicon nitride, silicon oxide etc Capitalizing on the developed CMOS technology, nanopores on silicon are scalable without any requirement for additional processing, the devices are low cost and the process can be repeatable with a high yield. One of the essential requirements in biomolecule sensing is the ability of the nanopore to interact with the analyte. In this work, we present a method for processing high aspect ratio, single nanopores in the range of 10–30 nm in diameter and approximately 700 nm in length on a silicon-on-insulator (SOI) wafer. The presented method of manufacturing the high aspect ratio individual nanopores combines optical lithography and anisotropic KOH etching with a final electrochemical etching step to form the nanopores and is repeatable and can be processed in batches. We demonstrate electrical detection of dsDNA translocation, where the characteristic time of the process is in the millisecond range. We also analyse the translocation parameters and correlate the enhanced length of the nanopore to a longer translocation time as compared to other substrates.

Fabrication of microfluidic channels with various cross-sectional shapes using anisotropic etching of Si and self-alignment

A novel and simple fabrication method was proposed to produce microfluidic channels with various cross-sectional shapes, such as parallelogram, rhombus, pentagon and hexagon. The present study has the advantages of not only fabricating the microfluidic channel shapes that have not been reported before, but also the fabrication process is simple, flexible and robust. Microfluidic channels were fabricated using anisotropic wet etching of Si wafer and self-alignment between Si structure and PDMS mold. In this regard, (100) single crystal Si wafer was used to fabricate the Si microchannel and the master for PDMS mold using photolithography and anisotropic KOH etching. The Si structure for the microchannel and master were formed from the same Si wafer by KOH etching, and the PDMS mold was made from the Si master. Finally, the microchannels with various cross-sectional shapes could be easily formed through self-alignment of the Si microchannel and PDMS mold. They were permanently bonded using O2 plasma treatment. It is expected that the fabricated microchannel with various cross-sectional shapes can be used in wide fields such as heat transfer, microscale transport of particle and fluid, and particle separation based on inertial focusing.

Micromolding in Capillaries와 Microtransfer Molding법을 이용한 역테이퍼 구조물을 가지는 이방성 소유성 표면 제작

Fabrication of inverse-tapered structure remains as a problem in the fabrication of oleophobic surface mostly due to the complications and the high cost of processes. In this paper, we propose a simple and low-cost fabrication method of inverse-tapered structured oleophobic surface using micromolding in capillaries (MIMIC) and microtransfer molding followed by MEMS processes. Silicon wafer molds for the formation of inverse-tapered structure were made using MEMS processes such as photolithography and anisotropic KOH etching of silicon wafer. The geometry of structure could be changed by controlling the etching depth of the silicon wafer mold. After covering the surface of the mold using flat UV tape, the formed space between mold and UV tape was filled with pre-cured PDMS by capillary force and then cured in oven. The tapered structure on UV tape was transferred and bonded to glass wafer by O₂ plasma treatment. The fabricated inverse-tapered structure was coated with a fluoroalkylsilane monolayer to reduce its surface energy. The wetting behaviors were investigated by the contact angle (CA) measurement of hexadecane droplets. This study demonstrates that an inversetapered structure can be fabricated on a substrate using micromolding in capillaries and microtransfer molding, whose surface shows the superoleophobicity.

실리콘 이방성 습식 에칭과 자가정렬을 이용한 평행사변형 단면을 가지는 마이크로채널 제작

In this paper, we propose a novel and simple fabrication microchannel with parallelogram cross-section using anisotropic wet etching of Si wafer, and self-alignment between Si channel and PDMS mold. Single crystal Si wafer was used to fabricate microchannel and master for PDMS mold, using photolithography and anisotropic KOH etching. Si structure for microchannel and master were formed on the same Si wafer by KOH etching, and the PDMS mold was made from Si master. Thus, we could fabricate the Si microchannel and PDMS mold, with same structural height. Finally, a microchannel with parallelogram cross-section could be easily formed, through self-alignment between them. Si microchannel and PDMS mold were permanently bonded, using O₂ plasma treatment. It is expected that the fabricated microchannel with parallelogram cross-section, can be used to study inertial focusing, widely used to separate particles continuously and with high-throughput.

Batch microfabrication and testing of a novel silicon-base miniaturized reference electrode with an ion-exchanging nanochannel array for nitrite determination.

The reference electrode (RE) provides a stable potential for electrochemical detection; therefore, the RE plays an important role in environmental monitoring. In this paper, a novel batch of microfabricated silicon-base miniaturized Ag/AgCl RE was reported. A specially designed mini-tank for saturated KCl solution storage and a nanochannel array for ion-exchange were fabricated on a 4 inch (100) silicon wafer using a two-step KOH anisotropic etching process. An Ag/AgCl electrode was fabricated on a 4 inch Pyrex 7740 glass substrate. Finally, the finished silicon and glass substrates were anode bonded to form the entire system. By comparing with a conventional solid-state Ag/AgCl RE in electrochemical microsensors, a pre-packaged saturated KCl solution in the mini-tank provided a stable working environment for the Ag/AgCl electrode to ensure a constant reference potential. Compared with a routine glass-structured RE and by replacing the ion-exchange membrane with a nanochannel array, the miniaturized RE achieved a longer lifetime. The size of the finished miniaturized RE electrode was 11 mm × 14 mm. The reference potential variation was only 0.1 mV under continuous testing for 3000 s. The standard deviation in the reference potential was only 1.314 mV in different Na2SO4 buffer concentrations ranging from 3 mM to 30 mM. To verify the practicality of the novel silicon-base miniaturized RE, the fabricated RE was applied to measure the amount of nitrite in a water sample and achieved a better linearity of R2 = 0.998. This miniaturized RE showed better reference potential stability and consistency because of the batch fabrication technique. This novel strategy for the design and manufacture of the miniaturized RE shows a bright future in the wide use of electrochemical sensors in online monitoring of water pollutants.

Large Red Shift of Emission of PbS Quantum-dot Superlattice with Butylamine Ligands

Background: Quantum-dot (QD) superlattice solar cells can attain quite high energy conversion efficiencies. The formation of epitaxial QD superlattice is very difficult since extremely delicate control is required for the QD stacking during the growth to avoid QD deformation and dislocation generation. It is well known that CQDs are self-assembled to form superlattice in electron microscope area after sedimentation in a solvent. To apply the colloidal QD superlattice to solar cell, superlattice size must be expanded from electron microscope order to centimeters or more. Method: Long-periodic packing was done by slowly depositing PbS QDs with short-chain butylamine ligands in a solvent into a pyramidal-hole array processed by anisotropic KOH etching of a (001)Si substrate. Results: QDs' ground state energy reduced by 116 meV together with 1.5-time increase in luminescence lifetime after the film formation. Conclusion: We demonstrate that the film prepared by close packing of colloidal QDs with shortchain ligands exhibited the large red shift of emission wavelength with the increase in emission lifetime. The results suggest that the carrier wavefunction was well delocalized in the long-periodic superlattice film on the template. Keywords: Quantum dot superlattice, photoluminescence, photovoltaic cells, PbS, butylamine ligands, large red shift.

25 nm Single-Crystal Silicon Nanowires Fabricated by Anisotropic Wet Etching.

We report a top-down method for fabricating ultra-high aspect ratio single-crystal silicon nanowires. The fabrication method is based on the standard photolithography technique and anisotropic wet etching of the single-crystal silicon in KOH solution. SiO₂ mask nanolines used for patterning single-crystal silicon nanowires are formed by the undercut etching of thin SiO₂ layer in buffered hydrofluoric solution. The minimum width of the SiO₂ mask nanolines are 50 nm. The length of SiO₂ mask nanolines is 2 cm. The single-crystal silicon nanowires have been successfully transferred from the SiO₂ mask nanolines by KOH anisotropic wet-chemical etching. The minimum width of the silicon nanowire has obtained to be 25 nm. The fabricated single-crystal silicon nanowires have trapezoidal and triangular cross sections, which are useful for applications in nanoelectronic and nanophotonic elements.

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.

Fabrication of single-crystal silicon nanowires based on surface wet adhesion

In this paper, we present a top–down fabrication method of single-crystal silicon nanowires. The method employs the popular photolithography technique and etching-rate dependent on the crystal orientation of single-crystal silicon in KOH solution. Using surface wet adhesion and reduced silicon dioxide etching, nanoscale SiO2 mask line patterns with width from 45nm to 200nm and length up to 120µm are successfully patterned. The interspace between nanoscale SiO2 mask lines is 750nm, which is narrower than the 1.2µm feature resolution of obtainable photolithography process. A mechanism for explaining the creation of nanoscale SiO2 mask lines based on surface wet adhesion due to the capillary force is suggested and discussed. The single-crystal silicon nanowires have been successfully fabricated by transferring the nanoscale SiO2 mask line patterns into the top silicon layer of SOI wafer by KOH anisotropic wet-chemical etching.

Simple Microcantilever Release Process of Silicon Piezoresistive Microcantilever Sensor Using Wet Etching

In this paper, a simple microcantilever release process using anisotropic wet etching is presented. The microcantilever release is conducted at the final stage of the fabrication of piezoresistive microcantilever sensor. Issues related to microcantilever release such as microscopic roughness and macroscopic roughness has been resolved using simple technique. By utilizing silicon oxide (SiO2) as the etch stop for the wet etching process, issues related to microscopic roughness can be eliminated. On the other hand, proper etching procedure with constant stirring of the etchant solution of KOH anisotropic etching significantly reduces the notching effect contributed by the macroscopic roughness. Upon the completion of microcantilever release, suspended microcantilever of 2μm thick is realized with the removal of SiO2 layer using Buffered Oxide Etching (BOE).

Developing barbed microtip-based electrode arrays for biopotential measurement.

This study involved fabricating barbed microtip-based electrode arrays by using silicon wet etching. KOH anisotropic wet etching was employed to form a standard pyramidal microtip array and HF/HNO3 isotropic etching was used to fabricate barbs on these microtips. To improve the electrical conductance between the tip array on the front side of the wafer and the electrical contact on the back side, a through-silicon via was created during the wet etching process. The experimental results show that the forces required to detach the barbed microtip arrays from human skin, a polydimethylsiloxane (PDMS) polymer, and a polyvinylchloride (PVC) film were larger compared with those required to detach microtip arrays that lacked barbs. The impedances of the skin-electrode interface were measured and the performance levels of the proposed dry electrode were characterized. Electrode prototypes that employed the proposed tip arrays were implemented. Electroencephalogram (EEG) and electrocardiography (ECG) recordings using these electrode prototypes were also demonstrated.

Optical Properties of Black Silicon Using the Combination Method of KOH and Gold-Assisted HF Etching

A kind of hill-like black silicon have been designed and fabricated by using the combination method of KOH anisotropic etching and gold-assisted HF etching. Pillars array on the surface of a silicon sample was obtained by KOH etching with a SiNx film dots pattern used as a mask. The sample was then etched in the oxidant HF solution catalyzed by Au nanoparticles for 5 minutes. The etched sample appears dark black. This black silicon is orderly hill-like textures in micro-scale with density nanopores on them. It can suppress the reflection to less than 4% in wavelength range from 250nm to 1000nm, and to about 2.5% at the wave number of 2000-4000 cm-1. It also has high additional absorption in IR range.

Fabrication and application of ordered Si nanopore array induced by Au

Size-controlled Si nanopore array with a pore size less than 100 nm is fabricated on Si (100) substrates by using monolayer self-assembled and KOH anisotropic wet etching technique. Morphology and structure of the pores are characterized by SEM and AFM. Results show that a large area of defect-free polystyrene (PS) monolayer film can be obtained when the volume ratio of PS solution to methanol solution is 9:11. A larger volume ratio or a smaller volume ratio will induce similar bilayer structure and defects (point and line) in the PS film, respectively. The lateral size and depth of the nanopore will increase with the etching time, and its morphology will change from circular to inverted pyramid type gradually. But the orderly arranged structure will be destroyed as the etching time is over 10 min. On the other hand, ordered Ge/Si nanoislands and nanorings will be grown on nanopore-patterned Si (100) substrates (inverted pyramid and circular nanopores, respectively) by ion beam sputtering. In addition, reasonable interpretations have been proposed for the formation mechanism of the ordered Ge/Si nanostructure.

A novel sandwich capacitive accelerometer with a double-sided 16-beam-mass structure

A novel sandwich capacitive accelerometer with a double-sided, 16-beam-mass structure is presented. In this design, the proof mass is supported by 16 tiny beams distributed uniformly on both sides, which aims to dramatically reduce the cross-axis response. Parameters of the beam-mass structure are analyzed and optimized by analytical modeling and the finite element analysis (FEA) method. The micro-accelerometer is fabricated by bulk micromachining technology, and the proof mass and tiny beams are released by KOH anisotropic wet etching from both sides of the silicon wafer, simultaneously. The resonance frequency and the quality factor of the accelerometer are 4.34kHz and 311, respectively, which are measured in an open-loop system. The measurement results show that the accelerometer has a full-scale (FS) range of 30g, a close-loop sensitivity of 80mV/g, and a nonlinearity of 0.27% of FS. The cross-axis sensitivities are 0.353% (x/z axis) and 0.045% (y/z axis), respectively. The bias stability is 0.63mg for an hour. The accelerometer can withstand high shock of over 10,000g.

Periodic arrays of nanopores made on single-crystalline silicon substrates with a self-assembled lithographic process

We report here the fabrication of periodic arrays of size- shape-, and spacing-controllable Si nanopores on single-crystalline (110)Si and (111)Si substrates by using our proposed approach, which is based on the self-assembled polystyrene nanosphere lithography in conjunction with the use of oxygen plasma treatments and KOH anisotropic etching processes. Compared with other works, the facile approach proposed here offers a much simpler and low-cost scheme which does not require the use of additional metal-film hard masks deposition and stripping processes. By adjusting the KOH etching duration, the Si nanopore size can be effectively tuned and controlled. The Si nanopores formed on (110)Si and (111)Si were found to be heavily faceted, and their faceted morphologies were mainly determined by the crystal orientations of the Si substrates used. Furthermore, the results of the ultraviolet-visible spectroscopic measurements revealed that the (110)Si substrate with nanopore arrays exhibited significant antireflection properties, and its optical reflectance was found to decrease with increasing the etched nanopore size. The obtained results present the exciting prospects that the combined approach presented here could have significant potential for use in Si-based optoelectronic devices.

A Novel Z-Axis Capacitance Accelerometer with Highly Symmetrical 16-Beam Structure

A novel highly symmetrical 16-beam sandwich structure Z-axis differential capacitance accelerometer is presented. In this design, the proof mass is suspended symmetrically by double-side of 16 straight beams with highly uniform dimension which can reduce the cross-axis sensitivity and rotational influences dramatically. Parameters of the beam-mass structure were analyzed and optimized by finite element analysis (FEA) software. The micro accelerometer is based on bulk-micromachining by DRIE and KOH anisotropic wet etching technologies. The beam-mass structure was released by anisotropic wet etching on both device layer sides simultaneously. The fabricated accelerometer was measured over the maximum range of 30g gravity field, results of measurement show that the close-loop sensitivity is 80mV/g, the nonlinearity is 0.27%, and the bias stability is 0.63mg for an hour.