Anisotropic Wet Etching Process Research Articles

Design and Fabrication of Silicon-Blazed Gratings for Near-Infrared Scanning Grating Micromirror

Blazed gratings are the critical dispersion elements in spectral analysis instruments, whose performance depends on structural parameters and topography of the grating groove. In this paper, high diffraction efficiency silicon-blazed grating working at 800–2500 nm has been designed and fabricated. By diffraction theory analysis and simulation optimization based on the accurate boundary integral equation method, the blaze angle and grating constant are determined to be 8.8° and 4 μm, respectively. The diffraction efficiency is greater than 33.23% in the spectral range of 800–2500 nm and reach the maximum value of 85.62% at the blaze wavelength of 1180 nm. The effect of platform and fillet on diffraction efficiency is analyzed, and the formation rule and elimination method of the platform are studied. The blazed gratings are fabricated by anisotropic wet etching process using tilted (111) silicon substrate. The platform is minished by controlling etching time and oxidation sharpening process. The fillet radius of the fabricated grating is 50 nm, the blaze angle is 7.4°, and the surface roughness is 0.477 nm. Finally, the blazed grating is integrated in scanning micromirror to form scanning grating micromirror by MEMS fabrication technology, which can realize both optical splitting and scanning. The testing results show that the scanning grating micromirror has high diffraction efficiency in the spectral range of 810–2500 nm for the potential near-infrared spectrometer application.

Anisotropic wet etching of a novel micro-texture structure for an Al/n-Si/Al metal–semiconductor–metal photodetector fabrication

Micro-electro-mechanical system fabrication involves molding on a single substrate. Chemical wet etching is common in these systems because it can provide a very high etch rate and selectivity. This optical device has been successfully completed fabrication by using the single-step lithography process. The surface was coated with PR and patterned using a single mask to create U-shaped structures by a wide electrode gap on a silicon substrate. Then it was etched using anisotropic wet etching process in potassium hydroxide, tetramethylammonium hydroxide (TMAH), silicic acid and dissolved silicon powder. The etchant created a 58% increase in the light sensitive area of the U-shaped trench in the structure higher compared to the planar structure. Random micro-pyramids, formed on the silicon surface in the U-shaped structure by the silicic acid-added TMAH solution, led to a ∼254 times higher ratio between photon generated current and dark current at reverse bias. This processing technique is a promising technology for improving performance for next-generation optical sensors or silicon photodetector.

Controllable synthesization of Au nanoparticles by laser enhanced wet KOH etching process

In Si substrate, anisotropic KOH etchants are mainly utilized to form pyramids like on the Si surface . However, this process is not well controlled way owing to the different and random etching pathway. In this work, we applied laser radiation during the anisotropic KOH wet etching process to modifies the topographical properties of Si substrate, as an efficient,simple and low cost texturing process for Si substrate. This approach employs different laser wavelength to modify the topographical features from a crater like structures to Si nanocrystallites in the form of pillars like structures on the Si surface. In order to investigate the formation of plasmonics species, gold nanoparticles was incorporated into Si surfaces by simple ion reduction process. The Si topographical features was studied with atomic scanning microscopy (AFM) images of Si before and after laser irradiation process. The irradiation with 405 laser wavelength, show the formation of thin and high density of Si nano pillars-like structures compared with more thick depther Si nano pillars like structures layer.

High signal collection efficiency in a 3D SERS chip using a micro-reflector.

To improve the sensitivity of surface-enhanced Raman spectroscopy (SERS) detection, we propose a three-dimensional (3D) SERS chip based on an inverted pyramid micro-reflector (IPMR) that converges Raman scattering light signals to improve the signal collection efficiency. The influence of the geometric parameters of the inverted pyramid structure on the Raman signal collection efficiency was analyzed by simulation for the determination of the optimal design parameters. The inverted pyramid through-hole structure was prepared on the silicon wafer through an anisotropic wet etching process, followed by the sputtering of a gold film to form the IPMR. The 3D SERS chip was constructed by bonding the IPMR and the active substrate that assembled with silver nanoparticles. Using Rhodamine 6G molecules, the Raman intensity measured with the 3D SERS chip was threefold greater than that of the silicon-based SERS substrate under the same test conditions. These experimental results show that the 3D SERS chip can significantly improve the SERS signal intensity. Its 3D structure is convenient for integration with microfluidic devices and has great potential in biochemical detection applications.

HfOx-based nano-wedge structured resistive switching memory device operating at sub-μA current for neuromorphic computing application

We fabricated a silicon based nano-wedge resistive switching memory device with the stack of Ti/HfOx/p+-Si. By using 25% tetra-methyl-ammonium hydroxide (TMAH) aqueous solution, the anisotropic wet etching process is carried out to minimize the tip structure of the silicon bottom electrode to a width of 4 nm, and the structure was validated through TEM analysis. Due to the minimized device area, low read current levels (<1 μA) were obtained in the nano-wedge RRAM while the opposites were measured in large size RRAM devices. In addition, the fabricated nano-wedge RRAM exhibited low power consumption during the DC switching process. Additionally, pulse measurement and retention tests were performed to demonstrate the synaptic behaviors of long-term potentiation and depression. Software neural network simulation was followed to test the feasibility of nano-wedge RRAM’s array implementation. These results demonstrate the fabricated nano-wedge RRAM devices’ potential usage as a synaptic device in neuromorphic computing systems.

Efficient detecting of TNT molecules using palladium nanoparticles/ cross shape pores like structure porous silicon

A set of porous silicon PSi samples with different cross-shaped pores like structures are successfully synthesized by using double successive wet anisotropic etching process: KOH wet chemical process and laser-assisted etching (LAE) process. Four different forms of palladium nanoparticles are deposited on PSi substrate to fabricate PdNPs/PSi heterostructures. Surface-enhanced Raman scattering (SERS)-active PdNPs/PSi heterostructures are fabricated and investigated for detecting Trinitrotoluene (TNT) molecules. Dipping plating process of PSi in 0.05 M concentration of PdCl2 solution at room temperature for 15 min is used to assemble spread of surface morphologies PdNPs layer over PSi. The results show that the SERS of heterostructures for TNT molecules expos strong reliance on the density of hot-spot junctions more than the local surface plasmonic of PdNPs. Analytical Enhancement factor (EF) of the Raman signal is varied according to the formed density of hot spots regions.

Graphenic carbon as etching mask: patterning with laser lithography and KOH etching

The wet anisotropic etching process is generally used in the field of micromachining (MEMS), particularly for commercial products such as accelerometers. Hard masks like oxide or nitride play a key role in the transfer of patterns to the substrate during the lithography process. This work reports on the use of polycrystalline graphenic carbon as an etch mask for wet chemical processing and outlines a simple method to create patterned structures on (100) silicon wafers. Graphenic carbon (GC) was deposited on the silicon substrate by chemical vapor deposition (CVD) using C2H4 as precursor. The desired pattern was written in the spin-coated negative photoresist using UV laser lithography. Different geometrical shapes were printed on the substrate with dimensions ranging from 10 to 50 micrometers. In the next stage, the O2 plasma etched away the carbon from the area not covered by the photoresist, acting as an additional mask for this and the subsequent processing steps. Finally, the sample was immersed in the KOH bath saturated with isopropanol and the etching rate was evaluated for each crystal plane. Compared to the use of a sacrificial oxide mask, this technique is simpler and produces more reliable results.

Redesigned t-PFF-v device fabricated with one-step anisotropic chemical wet etching process: enhanced separation efficiency for colloidal particles in continuous-mode

A pinched flow fractionation device with trapezoid-shaped pinched segment and cross-flow side channels (t-PFF-v) was redesigned and fabricated with a one-stop anisotropic chemical wet etching process and its enhanced separation capability was demonstrated for spherical polystyrene (PS) particles with diameter ranges of 2–8 µm. The tilted sidewalls and vertical focusing channels of the t-PFF-v device led to an improved separation resolution (Rm,n) via enhanced separation distance of PS particles at the pinched segment (Δxm,n) and reduced effluent particle distribution width (sn). Using this t-PFF-v device with Wp (pinched segment width) of 25 µm, PS particles with diameters of 4, 6, and 8 µm were simultaneously separated with good separation resolution (R4,6 = 3.2 and R6,8 = 3.4). Moreover, by adapting even smaller pinched segment width (Wp = 15 µm), PS particles with diameters of 2 and 4 µm, not well separated in the previous studies, were clearly separated with 9.5-fold improvement in R2,4 compared with the normal PFF device (n-PFF). The effects of inlet flow rate ratio (Q2/Q1) on the separation efficiency were also carefully investigated and provided additional insight on the separation processes within the pinched segment of t-PFF-v device. The drain flow through the cross-flow v-channels helps better alignment of particles at the sidewall of pinched segment and resulted in significantly reduced distribution of effluent particles, even under the operating conditions with low Q2/Q1 values.

Effect of NH2OH on Etching Characteristics of Si{100} in KOH Solution

Potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH) are most extensively used etchants for wet anisotropic etching process. Amongst these two etchants, KOH is a low cost etchant and provides high etch rate anisotropy between Si{111} and Si{100} planes. Increasing the etch rate is an important research problem for both academic and industrial applications. In this research, we study the effect of hydroxylamine (NH2OH) in 20 wt% KOH on the etching characteristics of Si{100}. This type of wafer orientation (i.e. Si{100}) is selected owing to its popularity in fabricating planar semiconductor devices (e.g. diode, transistor, complementary metal oxide semiconductor (CMOS), etc.) and MEMS components (e.g. cantilever, diaphragm, etc.). A systematic parametric analysis of various concentrations of hydroxylamine (from 0 to 20% in step of 5%) added 20 wt% KOH is carried out and its effect on the etching characteristics is discussed. We have found that the etch rate of Si{100} is increased threefold when 15% NH2OH is added to 20 wt% KOH solution. Additionally, this concentration also exhibits improved etching characteristics including undercutting rate, etch selectivity, and surface morphology. We have obtained improved etching characteristics when NH2OH is added to 20 wt% KOH. Due to an improvement in etching characteristics, present research is very useful to pave the path toward application in microfabrication industries as well as academic research laboratories.

Development of an FPW Biosensor with Low Insertion Loss and High Fabrication Yield for Detection of Carcinoembryonic Antigen

In the last two decades, various flexural plate-wave (FPW)-based biosensors with low phase velocity, low operation frequency, high sensitivity, and short response time, have been developed. However, conventional FPW transducers have low fabrication yield because controlling the thickness of silicon/isolation/metal/piezoelectric multilayer floating thin-plate is difficult. Additionally, conventional FPW devices usually have high insertion loss because of wave energy dissipation to the silicon substrate or outside area of the output interdigital transducers (IDTs). These two disadvantages hinder the application of FPW devices. To reduce the high insertion loss of FPW devices, we designed two focus-type IDTs (fan-shaped and circular, respectively) that can effectively confine the launched wave energy, and adopted a focus-type silicon-grooved reflective grating structure (RGS) that can reduce the wave propagation loss. To accurately control the thickness of the silicon thin-plate and substantially improve the fabrication yield of FPW transducers, a 60 °C/27 °C two-step anisotropic wet etching process was developed. Compared with conventional FPW devices (with parallel-type IDTs and without RGS), the proposed FPW devices have lower insertion loss (36.04 dB) and higher fabrication yield (63.88%). Furthermore, by using cystamine-based self-assembled monolayer (SAM) nanotechnology, we used the improved FPW device to develop a novel FPW-based carcinoembryonic antigen (CEA) biosensor for detection of colorectal cancer, and this FPW-CEA biosensor has a low detection limit (5 ng/mL), short response time (<10 min), high sensitivity (60.16–70.06 cm2/g), and high sensing linearity (R-square = 0.859–0.980).

A MEMS thermal shear stress sensor produced by a combination of substrate-free structures with anodic bonding technology

By combining substrate-free structures with anodic bonding technology, we present a simple and efficient micro-electro-mechanical system (MEMS) thermal shear stress sensor. Significantly, the resulting depth of the vacuum cavity of the sensor is determined by the thickness of the silicon substrate at which Si is removed by the anisotropic wet etching process. Compared with the sensor based on a sacrificial layer technique, the proposed MEMS thermal shear-stress sensor exhibits dramatically improved sensitivity due to the much larger vacuum cavity depth. The fabricated MEMS thermal shear-stress sensor with a vacuum cavity depth as large as 525 μm and a vacuum of 5 × 10−2 Pa exhibits a sensitivity of 184.5 mV/Pa and a response time of 180 μs. We also experimentally demonstrate that the sensor power is indeed proportional to the 1/3-power of the applied shear stress. The substrate-free structures offer the ability to precisely measure the shear stress fluctuations in low speed turbulent boundary layer wind tunnels.

Monolithic-integrated piezoresistive MEMS accelerometer pressure sensor with glass-silicon-glass sandwich structure

A monolithic composite MEMS sensor with sandwich structure is designed, simulated, fabricated and characterized. It consists of a rectangular diaphragm piezoresistive pressure sensor and a double-cantilever-mass piezoresistive accelerometer. The professional MEMS software, Intellisuite 8.5, is used to simulate the performances of the composite sensor. The composite sensor is fabricated on a (100)—silicon wafer by using MEMS bulk-micromachining and anodic bonding technology. One-step wet anisotropic etching process on the backside of the wafer can form the main backside shape of the composite sensor including the mass of accelerometer and the pressure sensing diaphragm. The glass-silicon-glass sandwich structure is formed with α-Si (amorphous silicon)-glass anodic bonding on the top surface of the wafer and Si-glass anodic bonding at the bottom. The fabricated composite sensor is measured, resulting in 33.0 μV/V kPa sensitivity for the 450 kPa-ranged pressure sensor, as well as, 11.2 μV/V g sensitivity for the 125 g-ranged accelerometer. The die size of the composite sensor chip is 2.5 mm × 2.5 mm × 1.4 mm. The measured results show that the composite sensor is appropriate for the application fields such as automobile, aerospace and environment monitoring.

Silicon template preparation for the fabrication of thin patterned gold films via template stripping

Metallic nanostructures play an important role in the vast field of modern nanophotonics, which ranges from the life sciences to biomedicine and beyond. Gold is a commonly-used and attractive material for plasmonics in the visible wavelength range, most importantly due to its chemical stability. In the present work, we focused on the different methods of plasmonic nanostructure fabrication that possess the greatest potential for cost-efficient fabrication. Initially, reusable (1 0 0) silicon templates were prepared. For this purpose, three different lithography methods (i.e. e-beam, optical, and nanoparticle lithography) were used that correspond to the desired structural scales. The application of a subsequent anisotropic crystal orientation-dependent wet etching process produced well-defined pyramidal structures in a wide variety of sizes, ranging from several microns to less than 100 nm. Finally, a 200 nm-thick gold layer was deposited by means of confocal sputtering on the silicon templates and stripped in order to obtain gold films that feature a surface replica of the initial template structure. The surface roughness that was achieved on the stripped films corresponds well with the roughness of the template used. This makes it possible to prepare cost-efficient high-quality structured films in large quantities with little effort. The gold films produced were thoroughly characterized, particularly with respect to their plasmonic response.

Two-Step Texture Process for High-Efficiency Crystalline Silicon Solar Cell Applications

Anisotropic etching of single-crystalline solar cells is used to increase the light absorption and surface area, which can improve the conversion efficiency. However, the conventional anisotropic etching process is limited for increasing surface area. For high-efficiency solar cells, unique surface structures are necessary. We present a new two-step texture process that involves combining dry etching and wet etching to produce a high-aspect-ratio surface structure for high-efficiency solar cells. Using this process, we achieved pillar-type surface structure with 1:1.9 aspect ratios in reactive ion etching (RIE), and the aspect ratio was increased further to 1:2.6 by the anisotropic wet etching process. The reflectance of the c-Si wafer was reduced from 24% to 12% by this two-step texturing process. This new technique can be used to increase the aspect ratio and surface area for high-efficiency c-Si solar cells.

Effect of texturing process involving saw-damage etching on crystalline silicon solar cells

For high efficiency silicon solar cells, surface texturing is used to increase the short circuit current by reducing the surface reflection loss. Surface texturing is an anisotropic wet chemical etching process commonly used to form random pyramids. We investigated how the process is affected by surface conditions. We also compared the texturing behavior and cell performances of as-cut, polished and saw-damage etched wafers. Textured samples with different processing times were analyzed to detect pyramids and determine weighted reflectances. After the texturing process, conventional screen-printed solar cells were fabricated to observe the cell performance. The pseudo I–V curves and quantum efficiency for the samples were analyzed. Performance of samples with different surface conditions makes no difference. Thus, the processing-cost of solar cells can be reduced by omitting the saw-damage etching process.

Fabrication of Silicon Nanopore Arrays with Three-Step Wet Etching

Solid-state nanopore arrays are realized with a well-controlled, three-step, anisotropic wet etching process with different KOH bath temperatures. The influence of etchant concentration and temperature on the etching rate is investigated. Nanopore arrays with an average size of 100 nm and single pores with a feature size of 30 nm in a silicon membrane are successfully fabricated at low cost using this method. This study paves the way towards the controllable, large-scale production of arrays of nanopores with desired size.

Fabrication of Inverted-Pyramid Silicon Nanopore Arrays with Three-Step Wet Etching

Inverted-pyramid silicon nanopore arrays are realized with a well-controlled, three-step, anisotropic wet etching process with different KOH bath temperatures. The influence of etchant concentration and temperature on the etching rate is investigated. Nanopore arrays with an average size of 100 nm and individual nanopores with feature sizes as small as 30 nm were successfully fabricated at low cost using this method. This study paves the way toward the controllable, large-scale production of arrays of inverted-pyramid nanopores with desired size.

Fabrication of various cross-sectional shaped polymer microchannels by a simple PDMS mold based stamping method

We describe a facile and simple stamping method to fabricate various cross-sectional shaped microfluidic channels in transparent polymer materials such as polydimethylsiloxane (PDMS), poly(methylmethacrylate) (PMMA), polystyrene (PS), and cyclic olefin copolymer (COC). First, microchannels of circular, rectangular, and triangular shape and different size were fabricated on a silicon wafer through an isotropic or anisotropic wet etching process, and the resultant microchannels on silicon wafers were transferred to PDMS through a replication technique. The produced PDMS with replicated convex microchannels served as a master mold. A variety of polymer solutions were pressed down against the PDMS master mold, and cured until the solvent was evaporated, generating circular, rectangular, and triangular shaped PDMS, PMMA, PS, and COC microchannels. The microchannels could be repeatedly prepared with a narrow standard deviation, thus demonstrating high reproducibility of the proposed method. The microchannel dimensions, shape, and surface roughness could be controlled by tuning the channel width of the mask, the wet etching direction, the etchant solution, and the concentration of KOH, respectively, when the microchannels were fabricated on a silicon wafer. This simple but efficient PDMS mold based stamping method can be widely used for fabricating different shaped microchannels on diverse polymer materials with high reproducibility, low cost, and high speed.

Numerical investigation and comparison with experimental characterisation of side gate p-type junctionless silicon transistor in pinch-off state

A side gate p-type junctionless silicon transistor is fabricated by atomic force microscopy nanolithography using a anisotropic potassium hydroxide wet etching process on low doped (105 cm−3) silicon-on-insulator wafer. The structure is a gated resistor and turns off based on a pinch-off effect principle, when essential positive gate voltage is applied and made a sufficiently large barrier in the gating region. Negative gate voltage is unable to make a significant impact on drain current to drive the device into accumulation mode. The experimental transfer characteristic is investigated and compared with the simulation result for positive gate voltage. ‘On/off’ ratio and subthreshold swing were also measured. The numerical study of the device in ‘off’ state is investigated based on the variation of majority and minority carriers’ density and recombination generation in the active region of the device, which offers more understanding of the device operation and also for previous works.

A ${\rm Z}$-Axis Quartz Tuning Fork Micromachined Gyroscope Based on Shear Stress Detection

This paper proposes a novel quartz micromachined gyroscope. The structure is designed by using shear stress detection method which can simplify the sidewall electrodes obviously. Furthermore, a tuning fork is introduced by the structure to obtain better differential vibrations. In order to increase the sensitivity of the sensor, the sense beam is designed to be a symmetric tapered beam. The device was fabricated using quartz anisotropic wet etching process. The drive mode frequency is 14.99 kHz, and the quality factor is 7600 in air. The sense mode frequency is 14.25 kHz, and the quality factor is 600 in air. Therefore, this gyroscope can operate at atmosphere pressure properly. The sensor is tested on a rate table through a specially designed readout circuitry. The sensitivity is 23.9 and the nonlinearity is 1.1% in range of . The noise floor is 0.1 .