Anisotropic Wet Etching Of Silicon Research Articles

Facile and scalable fabrication of Ni cantilever nanoprobes using silicon template and micro-electroforming techniques for nano-tip focused electrohydrodynamic jet printing

Electrohydrodynamic jet (E-Jet) printing is a powerful technique for micro/nanostructure fabrication with high resolution and efficiency. However, conventional E-Jet printing are still limited in printing accuracy and ink adaptability due to the nozzle clogging effect. In this paper, we develop a nano-tip focused electrohydrodynamic jet (NFEJ) method to print high-resolution structures. The Ni cantilever nanoprobes with nanoscale radius of curvature (ROC) on their tips were manufactured by a facile and scalable method using silicon template and micro-electroforming technique. Scanning electron microscope was used to analyse the micromorphology of the silicon template with inverted pyramid pits, which was obtained from anisotropic wet etching of silicon. Electroforming mold was obtained by photolithography and plasma etching which divide the top side of Ni film into isolated cantilever pits. Ni cantilever nanoprobes with an average tip ROC of about 48 nm were achieved by the subsequent micro electroforming process. High-resolution droplets array with an average diameter of about 890 ± 93 nm were printed by the NFEJ printing head equipped with these Ni nanoprobes, which verified the practicality of the developed Ni nanoprobes for NFEJ printing.

3D-fabrication of tunable and high-density arrays of crystalline silicon nanostructures

In this report, a procedure for the 3D-nanofabrication of ordered, high-density arrays of crystalline silicon nanostructures is described. Two nanolithography methods were utilized for the fabrication of the nanostructure array, viz. displacement Talbot lithography (DTL) and edge lithography (EL). DTL is employed to perform two (orthogonal) resist-patterning steps to pattern a thin Si3N4 layer. The resulting patterned double layer serves as an etch mask for all further etching steps for the fabrication of ordered arrays of silicon nanostructures. The arrays are made by means of anisotropic wet etching of silicon in combination with an isotropic retraction etch step of the etch mask, i.e. EL. The procedure enables fabrication of nanostructures with dimensions below 15 nm and a potential density of 1010 crystals cm−2.

3D Fractals as SERS Active Platforms: Preparation and Evaluation for Gas Phase Detection of G-Nerve Agents.

One of the main limitations of the technique surface-enhanced Raman scattering (SERS) for chemical detection relies on the homogeneity, reproducibility and reusability of the substrates. In this work, SERS active platforms based on 3D-fractal microstructures is developed by combining corner lithography and anisotropic wet etching of silicon, to extend the SERS-active area into 3D, with electrostatically driven Au@citrate nanoparticles (NPs) assembly, to ensure homogeneous coating of SERS active NPs over the entire microstructured platforms. Strong SERS intensities are achieved using 3D-fractal structures compared to 2D-planar structures; leading to SERS enhancement factors for R6G superior than those merely predicted by the enlarged area effect. The SERS performance of Au monolayer-over-mirror configuration is demonstrated for the label-free real-time gas phase detection of 1.2 ppmV of dimethyl methylphosphonate (DMMP), a common surrogate of G-nerve agents. Thanks to the hot spot accumulation on the corners and tips of the 3D-fractal microstructures, the main vibrational modes of DMMP are clearly identified underlying the spectral selectivity of the SERS technique. The Raman acquisition conditions for SERS detection in gas phase have to be carefully chosen to avoid photo-thermal effects on the irradiated area.

Reactive ion etching of silicon using low-power plasma etcher

The paper is devoted to the study of deep reactive ion etching of silicon using diode plasma etcher system with a low-power source. Silicon wafers were etched in a sulfur hexafluoride plasma and sulfur hexafluoride/oxygen plasma. The maximum achieved silicon etch rate was about 2 μm/min. The expediency of using dry reactive ion etching in combination with wet anisotropic etching of silicon for manufacturing of microelectromechanical systems (MEMS) was demonstrated.

Fabrication of Micro-Lens Array Obtained by Anisotropic Wet Etching of Silicon

This paper presents the fabrication of a micro-lens array surface with a single-mask process and two etching steps with KOH water solution. Numerical analysis of optics was used to determine the optimal design parameters such as curvature sagitta and radius. The dimension of each lens is 20μm x 20μm. We used anisotropic etching of <100> silicon through a circular and squar mask to produce a pyramidal pit formed by four (111) planes. The oxide mask is stripped and the immersion of the sample in the etchant solution favors the etching of (411) plane transforming the pit into a smooth hemispherical cavity. An intermediate stage exists when a wider 19.470 <411> - face pyramid replaces the initial 54.740 inverted pyramid. The dependence of surface roughness on concentration and temperature of KOH is investigated in the range of 25%-40% and 60°C-80°C, respectively, and compared between them. The surface profiles and roughness was characterized by AFM. The etching depth and radius of micro-lens array was obtained from the SEM images and AFM data. Also, the array of concave depressions was directly used as a mould for replication of KER-2500 transparent polymeric silicon from Shin-Etsu with a refractive index n=1.41. The perfectly matched array of micro-lenses can be detached from substrate and used as a local solar concentrator. Optical properties such as the focal length of the plano-convex micro-lens array, obtained by replication, are measured and analyzed.

Diffractive optics based on modulated subwavelength-domain V-ridge gratings

We study the properties of reflection-type V-ridge gratings in the subwavelength domain and describe a method to realize diffractive optical elements by using such gratings as signal carriers. In particular, we utilize a coding scheme based on position modulation of a high-frequency V-ridge carrier grating. We design and demonstrate beam splitting elements using this coding scheme, electron-beam lithography, anisotropic wet etching of silicon, hot embossing of polymer, and metal deposition. These elements have the outstanding property of operating over a large spectral range from 406 to 520 nm. The measured diffraction patterns show excellent agreement with theoretical results given by rigorous diffraction theory.

A novel ZnO piezoelectric microcantilever energy scavenger: Fabrication and characterization

This novel piezoelectric zinc oxide (ZnO) thin film microcantilever was fabricated by micromachining technique. To release the cantilever, wet anisotropic etching of Silicon (Si) was performed by tetramethyl ammonium hydroxide (TMAH). The transverse piezoelectric coefficient d31 of the ZnO film, obtained from the deflection of the cantilever with influence of applied voltage, was calculated as 3.32pC/N. The observed dynamic characterization of the novel piezoelectric microcantilever had linear response with the applied driving voltage. The obtained values of Young Modulus and Hardness were 208±4GPa and 4.84±0.1GPa respectively. This inexpensive novel method provides additional design flexibility to fabricate vibrational energy harvesters. The easy steps of fabrication and cost effectiveness of this method may be preferred it over DRIE. The voltage induced due to deformation of ZnO cantilever were measured ~230mV. This microcantilever energy scavenger may be used to power the nano devices and sensors for medical and agricultural applications as a replacement of traditional bulky batteries.

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.

Orientation-dependent nanostructure arrays based on anisotropic silicon wet-etching for repeatable surface-enhanced Raman scattering.

Repeatable fabrication of sensitive plasmonic substrates through a simple procedure has become a major challenge for SERS-based sensing and imaging. Herein, a new class of high-performance SERS substrates, including pyramid, ridged-hexagon, and quasi-triangle nanostructures, is successfully fabricated based on the nanosphere lithography technique and anisotropic wet etching. Using the wafer-scale Cr-hole array as the etching mask, cavity-templates of various configurations are fabricated by the orientation-dependent wet etching technique, from where the nanostructure arrays are finally peeled-off. The anisotropic wet etching on (100), (110), and (111) silicon wafers has been systematically studied at the nanoscale revealing the formation mechanism of these cavity-templates. The peeled-off nanostructure arrays provide high-density tips and/or gaps (about 2.5 × 10(7) mm(-2)) and thus facilitate the generation of "hot spots". The distribution of the electromagnetic field is visualized by the finite difference time domain calculation. And the calculation results are validated by SERS characterization. The SERS enhancement factors of these substrates are in the order of 10(6)-10(7), with the maximum enhancement factor of 1.32 × 10(7) yielded by the ridged-hexagon arrays. The proposed nanostructure arrays present excellent homogeneity and reproducibility (with the largest relative standard deviation of 16.43%) for the reason that the SERS-active substrates are peeled-off from an identical template. The cost-effective fabrication, high sensitivity, good homogeneity and well-performed reproducibility demonstrate that these orientation-dependent NSs are good candidates for SERS-based in vitro and in situ detection and biosensing.

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.

Surface-adaptable all-metal micro-four-point probe with unique configuration

In this paper, we propose a surface-adaptable all-metal micro-four-point probe (μ4PP) with a unique configuration. The μ4PP consists of four independent metallic sub-cantilevers with sharp Cu tips, and an SU-8 body structure to support the sub-cantilevers. The tip height is approximately 15 μm, and the tips are fabricated by anisotropic wet-etching of silicon followed by Cu electroplating. Each metallic cantilever connected to the SU-8 body structure acts as a flexible spring, so that the conducting tip can make gentle, non-destructive contact with fragile surfaces. To enhance the adhesion between the metallic sub-cantilevers and the SU-8 body, mushroom-shaped Cu structures were fabricated using an under-baked and under-exposed photolithography process. Various μ4PPs were designed and fabricated to verify their diverse range of applications, and preliminary experiments were performed using these fabricated μ4PPs. The resultant flexibility and reliability were experimentally confirmed on several samples, such as a polymer cantilever, a graphene flake, and curved metallic surfaces. We also expect that the proposed μ4PP will be suitable for measuring the anisotropic characteristics of crystal materials or the Hall effect in semiconductors.

Fabrication of silicon nanostructures with large taper angle by reactive ion etching

Micro- and nanostructures with a tapered sidewall profile are important for antireflection and light trapping applications in solar cell, light emitting diode, and photodetector/imager. Here, the authors will show two etching processes that offer a large taper angle. The first process involved a mask-less etching of pre-etched silicon structures having a vertical profile, using a recipe that would give a vertical profile when masked. The authors obtained a moderate taper angle of 14° using CF4/O2 etching gas. The second process involved a one-step etching step with Cr as mask using a recipe that was drastically modified from a nonswitching pseudo-Bosch process that gives a vertical profile. The gas flow ratio of C4F8/SF6 was greatly increased from 38/22 to 59/1 to result in a taper angle of 22°. Further reduction of the RF bias power led to an unprecedented large taper angle of 39° (at the cost of greatly reduced etching rate), which is even higher than the angle obtained by anisotropic wet etching of silicon.

Silicon wet etching: Hillock formation mechanisms and dynamic scaling properties

Surface roughening due to anisotropic wet etching of silicon was studied experimentally and modeled using the Monte Carlo method. Simulations were used to determine the consequences of site-dependent detachment probabilities on surface morphology for a one- and two-dimensional substrate models, focusing on the formation mechanisms of etch hillocks. Dynamic scaling properties of the 1D model were also studied. Resorting to the height–height correlation function and the structure factor, it is shown that the model presents conventional and anomalous scaling (faceted) depending on the stability of the hillocks tops. We also found that there is an intermediate regime that cannot be described by the Family–Vicsek or anomalous scaling ansatz.

J026033 微小孔アレイを有するSiN製円形細胞培養膜の開発([J026-03]細胞および分子のマイクロ・ナノスケール解析(3))

We developed circular cell-culture membranes which are made of SiN (silicon nitride) films of 1 μm in thickness and have multiple square or circular holes of several micrometers in size. The membranes were fabricated using SiN film formation with PECVD (plasma-enhanced chemical vapor deposition) and anisotropic wet etching of silicon with a surfactant-added TMAH (tetramethylammonium hydroxide) solution. Mechanical properties of the membranes were evaluated using a bulge test, and it was found that a circular membrane with circular holes has much larger fracture strength than a square membrane with square holes. Also, we demonstrated that optical transmittance of the membranes decreases with decreasing size of microholes and increasing porosity of the membranes, which gives useful design rules for their practical applications to cell culturing.

Surface tension and its role for vertical wet etching of silicon

In this paper, we present for the first time a systematic investigation of the role of the surface tension in anisotropic wet etching of silicon, focusing on the sidewall angle. We show that in the KOH and NaOH solutions with high surface tension a reliable fabrication of vertical sidewalls is possible. Etching experiments on (1 0 0)-Si and surface tension measurements via bubble pressure tensiometry were conducted using inorganic etchants in a wide range of concentrations. Finally, the surface tension of the etching solutions was identified as an important quantity that determined the etching behavior, while concentration dependences were eliminated by controlling the surface tension using temperature.

Facile fabrication of sub-20-nm nanochannels based on crystallinity-dependent anisotropic etching of silicon

We developed a novel method to fabricate nanochannels with simple and facile process utilizing the crystallinity-dependent anisotropic wet etching of silicon. The nanochannels were formed in bulk (111)-oriented single-crystalline silicon wafer by single step of microscale lithography and sealed by thin film deposition. Hence the massively parallel and wafer scale nanochannel fabrication was easily achieved without the necessity of nanolithography or complicated process. The dimension of the fabricated nanochannels was down to 16nm in depth and 175nm in width, and up to 8cm in length.

Application of KOH Anisotropic Etching in the Fabrication of MEMS Devices

It is known that the wet chemical etching of silicon in alkaline solution has attracted wide attention due to its advantages such as lower cost, simpler setup, higher rate, smoother surface at micro level, higher degree of anisotropy, and lower pollution. In this paper, the key processes of fabricating vacuum microelectronic accelerometer and slits are presented. The cone curvature radius of the silicon tip arrays less than 30nm was fabricated with wet anisotropic etching of silicon in 33wt. % KOH solution at 70°C added potassium iodine (KI) and Iodine (I2) as additive and the cone aspect ratio was about 0.7. Smooth surface after etching in 33wt. %KOH solution added isopropyl alcohol (IPA) at 80°C was obtained and lateral etching was less than 5um after etching several hours for etching depth over 400um. Scalar slits with bottom width 25um and depth 500um were attained. A constant etch rate lead to precise and reproducible production. The test result reveals that the process to a specific occasion can reach practical requirements.

Investigation of Simple Process Technology for the Fabrication of Valveless Micropumps

This paper presents a simple process technique for the fabrication of valveless micro-pumps. The process design utilizes standard MEMS process using double-sided anisotropic silicon wet etching process with an additional adhesive bonding technique. The diffuser and nozzle element of the pump with depth of 50 µm, as well as a 150 µm thick silicon membrane are designed and fabricated using only 3 patterning process steps. A piezoelectric plate working at the frequency range from 0.1 kHz to 2 kHz is bonded on to the back side of the silicon membrane to create the membrane actuation. The patterning process of thick photoresist used as the adhesive layer for the substrate bonding is also discussed in detail. The fluid flow is observed and the process reproducibility is proven which show a good prospect for the future development of miniaturized valveless pump for biomedical application.

A Method to Fabricate a Template With a Long Range Ordered Dense Array of True Nanometer Scale Pits

We present a method to form a patterned template having a dense array of subnanometer scale pits that is useful for numerous applications that require significant size reduction. The polystyrene-polymethylmethacrylate diblock copolymer nanopatterned sample has been used in order to form a hexagonal symmetry as a starting scaffold. The ordered copolymer pattern is then transferred to the oxide layer to form a hard mask for subsequent anisotropic silicon wet-etching process. With a controlled wet etching, we obtained inverted pyramidal pits on a silicon substrate of which the tips can be a few or even a single nanometer in its lateral dimension. The most common phase change material Ge2Sb2 Te5 (GST225) was deposited on this substrate and isolated within the pits using chemical mechanical polishing approach, followed by X-ray photoelectron spectroscopy elemental analysis.

Fabrication of a MEMS micro-hotplate

With the development of microelectronics and micromachining technology, micro-hotplate as a micro-heater is a very important component in micro-sensor. So far, it has wide applications in pressure detection, gas detection and so on. In this paper, a micro-hotplate based on micro-bridge structure was designed and fabricated. Here we present a micro-hotplate with SOI (silicon-on-insulator) as the substrate and metal Au as the heating electrode material. It shows low thermal conductivity coefficient, good electrical insulation, low power consumption, and excellent resistance to high temperature and low pressure. Some IC (integrated circuit) processes such as magnetron sputtering of Ni/Cr, ion beam sputtering of Au, dry and wet etching processes are employed to fabricate the device. Anisotropic wet etching of silicon and thin film deposition were mainly studied. To protect the front side micro structure during the wet etching process, a special apparatus was designed and fabricated. The optimization process for anisotropic wet etching was obtained for releasing the micro-bridge after a large number of experiments and the micro-hotplate was successfully fabricated. The electrical, thermal characteristics of the micro-hotplate were tested and approved it a well functional device.