Anisotropic characteristics and morphological control of silicon nanowires fabricated by metal-assisted chemical etching

Low-cost fabrication methods enabling the morphological control of silicon nanowires are of great importance in many device application fields. A top-down fabrication method, metal-assisted chemical etching, is proved to be a feasible solution. In this paper, some novel approaches based on metal-assisted chemical etching, alkaline solution etching, and electrochemical anodic etching are presented for fabricating micro- and nano-structures, which reveal the anisotropic characteristics of metal-assisted chemical etching in silicon. A new model is proposed to explain the motility behavior of Ag particles in metal-assisted chemical etching of silicon. It is shown that Ag particle forms a self-electrophoresis unit and migrates into Si substrate along [100] direction independently. Diameter and length control of silicon nanowires are achieved by varying Ag deposition and etching durations of metal-assisted chemical etching, respectively, which provide a facilitation to achieve high-aspect-ratio silicon nanowires at room temperature in a short period. These results show a potential simple method to microstructure silicon for devices application, such as solar cells and sensors.     

两步法腐蚀硅锥阴极阵列的工艺研究

详细研究了利用硅的各向异性腐蚀、各向同性腐蚀制备硅锥阴极阵列的工艺。采用快速腐蚀与缓蚀削尖两步工艺，制备出了均匀、尖锐的硅锥阵列。其阵列密度可达106／cm2，硅锥曲率半径可做到20nm。     

Damage-free top-down processes for fabricating two-dimensional arrays of 7 nm GaAs nanodiscs using bio-templates and neutral beam etching

The first damage-free top-down fabrication processes for a two-dimensional array of 7 nm GaAs nanodiscs was developed by using ferritin (a protein which includes a 7 nm diameter iron core) bio-templates and neutral beam etching. The photoluminescence of GaAs etched with a neutral beam clearly revealed that the processes could accomplish defect-free etching for GaAs. In the bio-template process, to remove the ferritin protein shell without thermal damage to the GaAs, we firstly developed an oxygen-radical treatment method with a low temperature of 280?°C. Then, the neutral beam etched the defect-free nanodisc structure of the GaAs using the iron core as an etching mask. As a result, a two-dimensional array of GaAs quantum dots with a diameter of ～ 7 nm, a height of ～ 10 nm, a high taper angle of 88° and a quantum dot density of more than 7 × 10(11) cm(-2) was successfully fabricated without causing any damage to the GaAs.     

Fabrication of slantingly-aligned silicon nanowire arrays for solar cell applications

Large-area slantingly-aligned silicon nanowire arrays (SA-SiNW arrays) on Si(111) substrate have been fabricated by wet chemical etching with dry metal deposition method and employed in the fabrication of solar cells for the first time. The formation of SA-SiNW arrays possibly results from the anisotropic etching of silicon by silver catalysts. Superior to the previous cells fabricated with vertically-aligned silicon nanowire arrays (VA-SiNW arrays), the SA-SiNW array solar cells exhibit a highest power conversion efficiency of?11.37%. The improved device performance is attributed to the integration of the excellent anti-reflection property of the arrays and the better electrical contact of the cell as a result of the special slantingly-aligned structure. The high surface recombination velocity of minority carriers in SiNW arrays is still the main limitation on cell performance.     

Fabrication and Characterization of Silicon (100) Membranes for a Multi-beam Superconducting Heterodyne Receiver

We fabricated silicon (100) membranes of 3 mm in diameter on the surface of silicon-on-insulator (SOI) substrates and investigated the characteristics of the membranes. The handle layer of one SOI substrate was etched using deep reactive ion etching process with the buried oxide (BOX) layer that remained together with the device layer. The BOX layer of the other SOI substrate was removed using C₄F₈-based plasma etching after the handle layer etching. The surfaces of both silicon (100) membranes were observed using the scanning white light interferometer system at room temperature. Both silicon (100) membranes have dome-like deformations. The silicon (100) membranes are effectively flattened by etching the BOX layer under the device layer. Both silicon (100) membranes were cooled from room temperature to 4 K by a Gifford–McMahon refrigerator. Wrinkles appeared on the surfaces of both silicon (100) membranes when the temperature dropped to about 200 K. However, the wrinkles disappeared below about 180 K. This phenomenon indicates the wrinkles at low temperature would depend on the properties of the silicon (100) of the device layers and independent of the properties of the BOX layers under the silicon (100) membranes.     

Microfluidic device with chemical gradient for single-cell cytotoxicity assays

Here, we report the fabrication of a chemical gradient microfluidic device for single-cell cytotoxicity assays. This device consists of a microfluidic chemical gradient generator and a microcavity array that enables entrapment of cells with high efficiency at 88 ± 6% of the loaded cells. A 2-fold logarithmic chemical gradient generator that is capable of generating a serial 2-fold gradient was designed and then integrated with the microcavity array. High density single-cell entrapment was demonstrated in the device without cell damage, which was performed in 30 s. Finally, we validated the feasibility of this device to perform cytotoxicity assays by exposing cells to potassium cyanide (0-100 μM KCN). The device captured images of 4000 single cells affected by 6 concentrations of KCN and determined cell viability by counting the effected cells. Image scanning of the microcavity array was completed within 10 min using a 10× objective lens and a motorized stage. Aligning cells on the microcavity array eases cell counting, observation, imaging, and evaluation of singular cells. Thus, this platform was able to determine the cytotoxicity of chemicals at a single-cell level, as well as trace the cytotoxicity over time. This device and method will be useful for cytotoxicity analysis and basic biomedical research.     

Amorphous hydrogenated carbon films used as masks for silicon microtips fabrication in a reactive ion etching with SF6 plasma

A room temperature fabrication process for silicon microtips has been developed using amorphous hydrogenated carbon films as masks for silicon etching. Reactive ion etching using an SF₆ plasma has been employed to sharpen the microtips without any thermal oxidation technique.     

Fabrication of a porous polyimide membrane using a silicon nanowire array as a template

We demonstrate that a porous polyimide membrane can be fabricated by curing liquid polyimide on a vertically oriented silicon nanowire array and selectively etching away the nanowire-array-template using xenon difluoride (XeF₂). Pore size and density using the described technique are controllable. The former is dependent on nanowire diameter and the duration of etching, whereas pore density is determined by silicon nanowire density. We believe that the described porous membrane fabrication method can be applied to various polymer and nanowire systems.     

Metal‐Assisted Chemical Etching of Silicon: A Review

This article presents an overview of the essential aspects in the fabrication of silicon and some silicon/germanium nanostructures by metal‐assisted chemical etching. First, the basic process and mechanism of metal‐assisted chemical etching is introduced. Then, the various influences of the noble metal, the etchant, temperature, illumination, and intrinsic properties of the silicon substrate (e.g., orientation, doping type, doping level) are presented. The anisotropic and the isotropic etching behaviors of silicon under various conditions are presented. Template‐based metal‐assisted chemical etching methods are introduced, including templates based on nanosphere lithography, anodic aluminum oxide masks, interference lithography, and block‐copolymer masks. The metal‐assisted chemical etching of other semiconductors is also introduced. A brief introduction to the application of Si nanostructures obtained by metal‐assisted chemical etching is given, demonstrating the promising potential applications of metal‐assisted chemical etching. Finally, some open questions in the understanding of metal‐assisted chemical etching are compiled.     

Patterning of various silicon structures via polymer lithography and catalytic chemical etching

We demonstrate a facile fabrication of a rich variety of silicon patterns with different length scales by combining polymer lithography and a metal-assisted chemical etching method. Several types of polymer patterns were fabricated on silicon substrates, and silver layers were deposited on the patterned silicon surfaces and used to etch the silicon beneath. Various silicon patterns including topographic lines, concentric rings, and square arrays were created at a micro-?and nanoscale after etching the silicon and subsequent removal of the patterned polymer masks. Alternatively, the arrays of sub-30?nm silicon nanowires were produced by a chemical etching of the silicon wafer which was covered with highly ordered polystyrene-block-polyvinylpyridine (PS-b-PVP) micellar films. In addition, silicon nanohole arrays were also generated by etching with hexagonally packed silver nanoparticles that were prepared using PS-b-PVP block copolymer templates.     

Nano-fabrication with a flexible array of nano-apertures

We report fabrication and use of a flexible array of nano-apertures for photolithography on curved surfaces. The batch-fabricated apertures are formed of metal-coated silicone tips. The apertures are formed at the end of the silicone tips by either electrochemical etching of the metal or plasma etching of a protective mask followed by wet chemical etching. The apertures are as small as 250 nm on substrates larger than several millimeters. We demonstrate how the nano-aperture array can be used for nano-fabrication on flat and curved substrates, and show the subsequent fabrication steps to form large arrays of sub-micron aluminum dots or vertical silicon wires.     

微静电陀螺仪的结构设计与工艺实现

为了探索微机械陀螺突破精度极限的新途径,设计了一种基于环形转子、体硅加工工艺、转子5自由度悬浮的硅微静电陀螺仪.采用玻璃-硅-玻璃键合的三明治式微陀螺结构,提出了包括双边光刻、反应离子刻蚀(RIE)、电感耦合等离子体(ICP)刻蚀、玻-硅静电键合、硅片减薄、多层金属溅射等关键工艺的加工路线.在工艺设计中采用铝牺牲层对转子进行约束,在第2次玻-硅键合后再通过湿法去除牺牲层,以得到可自由活动的转子.基于提出的体硅工艺路线,成功加工出了微陀螺敏感结构,并完成了转子5自由度悬浮和加转实验,测试结果表明大气环境下转子转速可达73.3 r/min.     

3D Nanofabrication of Fluidic Components by Corner Lithography

A reproducible wafer‐scale method to obtain 3D nanostructures is investigated. This method, called corner lithography, explores the conformal deposition and the subsequent timed isotropic etching of a thin film in a 3D shaped silicon template. The technique leaves a residue of the thin film in sharp concave corners which can be used as structural material or as an inversion mask in subsequent steps. The potential of corner lithography is studied by fabrication of functional 3D microfluidic components, in particular i) novel tips containing nano‐apertures at or near the apex for AFM‐based liquid deposition devices, and ii) a novel particle or cell trapping device using an array of nanowire frames. The use of these arrays of nanowire cages for capturing single primary bovine chondrocytes by a droplet seeding method is successfully demonstrated, and changes in phenotype are observed over time, while retaining them in a well‐defined pattern and 3D microenvironment in a flat array.     

Fabrication of p-type silicon nanowire arrays with a high aspect ratio using electrochemical and alkaline etching

We report on the fabrication of silicon nanostructures with a high aspect ratio that were created using a combination of electrochemical etching and alkaline etching. With this technique, we were able to fabricate nano- and/or micro-wire structures that are perfectly periodic over large areas of 3.14 cm2. After porous silicon was created by electrochemical etching, the effect of post-alkaline etching was investigated to determine how changes in the etching time, solution concentration and temperature of the etchant influenced the silicon morphology. As a result, periodic silicon wire arrays with good vertical alignment were obtained, and these arrays had a width of less than 500 nm and/or a high aspect ratio of more than 20.     

Fabrication of silicon dioxide submicron channels without nanolithography for single biomolecule detection

We successfully fabricated nanochannel arrays with silicon dioxide (SiO(2)) surfaces for single biomolecule detection. The SiO(2) nanochannel fabrication is based on the combination of anisotropic etching by potassium hydroxide (KOH) solution, local oxidation of silicon (LOCOS) and plasma etching of silicon. This fabrication technique is easily controllable and is a simple and practical solution for low-cost and high-throughput fabrication of nanofluidic channels. Thus, this technique enables the generation of nanochannels with various nanoscale dimensions without using nanolithography.     

Step voltage with periodic hold-up etching: A novel porous silicon formation

A novel etching method for preparing light-emitting porous silicon (PS) is developed. A gradient steps (staircase) voltage is applied and hold-up for different periods of time between p-type silicon wafers and a graphite electrode in HF based solutions periodically. The single applied staircase voltage (0–30 V) is ramped in equal steps of 0.5 V for 6 s, and hold at 30 V for 30 s at a current of 6 mA. The current during hold-up time (0 V) was less than 10 μA. The room temperature photoluminescence (PL) behavior of the PS samples as a function of etching parameters has been investigated. The intensity of PL peak is initially increased and blue shifted on increasing etching time, but decreased after prolonged time. These are correlated with the study of changes in surface morphology using atomic force microscope (AFM), porosity and electrical conductance measurements. The time of holding-up the applied voltage during the formation process is found to highly affect the PS properties. On increasing the holding-up time, the intensity of PL peak is increased and blue shifted. The contribution of holding-up the applied steps during the formation process of PS is seen to be more or less similar to the post chemical etching process. It is demonstrated that this method can yield a porous silicon layer with stronger photoluminescence intensity and blue shifted than the porous silicon layer prepared by DC etching.     

Realization of vertical silicon nanowire networks with an ultra high density using a top-down approach

In this paper, we demonstrate the top-down fabrication of vertical silicon nanowires networks with an ultra high density (4 x 10(10) cm(-2)), a yield of 100%, and a precise control of both diameter and location. Firstly, dense and well-defined networks of nanopillars have been patterned by e-beam lithography using a negative tone e-beam resist Hydrogen SylsesQuioxane (HSQ). A very high contrast has been obtained using a high acceleration voltage (100 kV), very small beam size at a current of 100 pA and a concentrated developer, 25% Tetramethylammonium Hydroxide. The patterns were transferred by reactive ion etching. Using chlorine based plasma chemistry and low pressure, etching anisotropy was guaranteed while avoiding the so-called 'grass effect'. This approach enabled the production of vertical silicon nanowires networks with a 20 nm diameter and a pitch of 30 nm. Lastly, the self-limited oxidation phenomenon in 1D structure has been used to perfectly control the shrinking of NWs and to obtain a Si surface free of defects induced by reactive ion etching. The silicon nanowires networks have been tapered by wet oxidation (850 degrees C) down to a diameter of 10 nm with a high aspect ratio 11.     

Porosity control in metal-assisted chemical etching of degenerately doped silicon nanowires

We report the fabrication of degenerately doped silicon (Si) nanowires of different aspect ratios using a simple, low-cost and effective technique that involves metal-assisted chemical etching (MacEtch) combined with soft lithography or thermal dewetting metal patterning. We demonstrate sub-micron diameter Si nanowire arrays with aspect ratios as high as 180:1, and present the challenges in producing solid nanowires using MacEtch as the doping level increases in both p- and n-type Si. We report a systematic reduction in the porosity of these nanowires by adjusting the etching solution composition and temperature. We found that the porosity decreases from top to bottom along the axial direction and increases with etching time. With a MacEtch solution that has a high [HF]:[H(2)O(2)] ratio and low temperature, it is possible to form completely solid nanowires with aspect ratios of less than approximately 10:1. However, further etching to produce longer wires renders the top portion of the nanowires porous.     

Large-area 2D periodic crystalline silicon nanodome arrays on nanoimprinted glass exhibiting photonic band structure effects

Two-dimensional silicon nanodome arrays are prepared on large areas up to 50 cm2 exhibiting photonic band structure effects in the near-infrared and visible wavelength region by downscaling a recently developed fabrication method based on nanoimprint-patterned glass, high-rate electron-beam evaporation of silicon, self-organized solid phase crystallization and wet-chemical etching. The silicon nanodomes, arranged in square lattice geometry with 300 nm lattice constant, are optically characterized by angular resolved reflection measurements, allowing the partial determination of the photonic band structure. This experimentally determined band structure agrees well with the outcome of three-dimensional optical finite-element simulations. A 16% photonic bandgap is predicted for an optimized geometry of the silicon nanodome arrays. By variation of the duration of the selective etching step, the geometry as well as the optical properties of the periodic silicon nanodome arrays can be controlled systematically.     

Scaling down lateral dimensions of silicon nanopillars fabricated by reactive ion etching with Au/Cr self-assembled clusters as an etch mask

Nanodot and nanopillar structures and precisely controlled reproducible fabrication thereof are of great interest in common nanoelectronic devices, including photonic crystals and surface plasmon resonance instruments. In this work, fabrication process of the silicon nanopillar structures is described. It includes self-organization of gold and chromium clusters at thickness close to that of one atomic diameter to serve as etching masks followed by the reactive ion etching to form silicon nanopillars. Scanning electron microscopy and X-ray photoelectron spectroscopy were used to characterize self-organized gold and chromium clusters as well as the final silicon nanopillars. This method was found to produce silicon nanopillars of sub-10 nm lateral dimensions and the diameter-to-height aspect ratio of up to 1:14.