Formation of GaN nanostructures with various morphologies by photo-assisted electroless chemical etching

Interaction of GaN crystal faces with chemicals is crucial to understand why various nanostructures are formed during the etching process. We have prepared GaN nanostructures by a photo-assisted electroless chemical etching method in solutions containing KOH and K₂S₂O₈. Morphology nanostructure GaN layers grown by molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE) were studied. For the GaN layers grown by MBE, the etching reaction process starts at grain boundaries and dislocation domains on the surface and inverted hexagonal pyramids are eventually formed. For the GaN layers grown by HVPE, scattered etch pits with well-defined hexagonal facets are observed after the etching process.     

Hybrid Anodic and Metal‐Assisted Chemical Etching Method Enabling Fabrication of Silicon Carbide Nanowires

Silicon carbide (SiC) is one of the most important third‐generation semiconductor materials. However, the chemical robustness of SiC makes it very difficult to process, and only very limited methods are available to fabricate nanostructures on SiC. In this work, a hybrid anodic and metal‐assisted chemical etching (MACE) method is proposed to fabricate SiC nanowires based on wet etching approaches at room temperature and under atmospheric pressure. Through investigations of the etching mechanism and optimal etching conditions, it is found that the metal component plays at least two key roles in the process, i.e., acting as a catalyst to produce hole carriers and introducing band bending in SiC to accumulate sufficient holes for etching. Through the combined anodic and MACE process the required electrical bias is greatly lowered (3.5 V for etching SiC and 7.5 V for creating SiC nanowires) while enhancing the etching efficiency. Furthermore, it is demonstrated that by tuning the etching electrical bias and time, various nanostructures can be obtained and the diameters of the obtained pores and nanowires can range from tens to hundreds of nanometers. This facile method may provide a feasible and economical way to fabricate SiC nanowires and nanostructures for broad applications.     

Dissolution kinetics of MgO crystals in aqueous acidic salt solutions

The revelation and morphology of dislocation etch pits as well as the rates of macroscopic dissolution and selective etching on the {1 0 0} plane of MgO crystals in aqueous solutions of various inorganic salts are investigated in relation to the nature and concentration of salt in solution and the etching temperature. It is found that addition of a salt generally facilitates etch pit formation and that the rates of surface dissolution and selective etching increase with additive concentration, etchant temperature and character and ageing of dislocations, while the etch pit morphology depends on the concentration and chemical nature of an impurity, etching temperature and the ageing of the dislocations. It is also observed that some fast etching solutions produce very shallow etch pits at screw dislocations. The results are discussed from a consideration of solution pH, standard electrode potentials of metals and stability of complexes present in solution. The importance of the surface entropy factor in revealing etch pits at screw dislocations is pointed out.     

OWL-Based Nanomasks for Preparing Graphene Ribbons with Sub-10 nm Gaps

We report a simple and highly efficient method for creating graphene nanostructures with gaps that can be controlled on the sub-10 nm length scale by utilizing etch masks comprised of electrochemically synthesized multisegmented metal nanowires. This method involves depositing striped nanowires with Au and Ni segments on a graphene-coated substrate, chemically etching the Ni segments, and using a reactive ion etch to remove the graphene not protected by the remaining Au segments. Graphene nanoribbons with gaps as small as 6 nm are fabricated and characterized with atomic force microscopy, scanning electron microscopy, and Raman spectroscopy. The high level of control afforded by electrochemical synthesis of the nanowires allows us to specify the dimensions of the nanoribbon, as well as the number, location, and size of nanogaps within the nanoribbon. In addition, the generality of this technique is demonstrated by creating silicon nanostructures with nanogaps.     

Formation and mechanism of silicon nanostructures by Ni-assisted etching

Instead of noble metal like Pt, Au and Ag, cheap Ni nanoparticles (Ni NPs) were used to fabricate silicon nanostructures. Ni was found to be etched off during the etching process, while forming silicon nanostructures with very low reflectance of 1.59 % from 400 to 900 nm. The formation mechanism of silicon nanostructures by Ni-assisted etching was presented from the point of view of the low electronegativity of Ni. The Ni NPs were found being etched off during the assisted etching process, which implies that the transfer rate of electrons from Si to Ni is slower than that from Ni to O^? in the case of using Ni as assisted metal. The reason of sparser and deeper silicon nanostructures etched in lower H₂O₂ concentration solution is that the Ni NPs can be lasted for longer time in the etching solution with lower H₂O₂ concentration so that more silicon atoms will be oxidized and then removed for those under Ni NPs due to the hole transfer and those where uncovered by Ni NPs due to the hole diffusion.     

Bulk Micromachining of Si by Metal‐assisted Chemical Etching

Bulk micromachining of Si is demonstrated by the well‐known metal‐assisted chemical etching (MaCE). Si microstructures, having lateral dimension from 5 μm up to millimeters, are successfully sculpted deeply into Si substrate, as deep as >100 μm. The key ingredient of this success is found to be the optimizations of catalyst metal type and its morphology. Combining the respective advantages of Ag and Au in the MaCE as a Ag/Au bilayer configuration leads to quite stable etch reaction upon a prolonged etch duration up to >5 h. Further, the permeable nature of the optimized Ag/Au bilayer metal catalyst enables the etching of pattern features having very large lateral dimension. Problems such as the generation of micro/nanostructures and chemical attacks on the top of pattern surface are successfully overcome by process optimizations such as post‐partum sonication treatment and etchant formulation control. The method can also be successful to vertical micromachining of Si substrate having other crystal orientations than Si(100), such as Si(110) and Si(111). The simple, easy, and low‐cost nature of present approach may be a great help in bulk micromachining of Si for various applications such as microelectromechanical system (MEMS), micro total analysis system (μTAS), and so forth.     

Fluid-kinetics enhanced selective etching process of NiPt film by piranha chemistry in silicide formation for complementary metal oxide semiconductor fabrication

NiPtSi is a prime candidate for the complementary metal-oxide-semiconductors CMOS self-aligned silicidation process beyond the 22 nm node. The formation of NiPt silicide in smaller geometries demands more Platinum (Pt) additive to control the silicide quality and a more capable NiPt selective etching process to remove surface residual metals for complementing the formation of silicide. Both higher Pt selective etch rate and lower surface material loss are desired in NiPt selective etching process.High temperature (> 150 °C) sulfuric acid base piranha chemistry in fresh dispensing on wafer can etch Pt with less damage to the exposed wafer surface. By using (1) a larger mass-to-charge density Pt redox reaction zone in the electrochemistry spectrum of the Pt redox behavior, (2) stronger chemical fluid kinetics and (3) intensified voltammetric cycles, the Pt selective removal rate can be boosted. Two types of wet chemical processors are used to examine the fluid-chemical kinetics effect on the Pt selective etching rate. It is shown that higher chemical flow rates and stronger fluid-kinetics can enhance the Pt transport behavior. The collateral wafer surface material loss rate also increases by higher chemical flow rates, but the amount of total material loss actually reduces due to a greater reduction in the required process time. The fluid-kinetics enhanced selective etching process can cover a wider range of NiPt film conditions (5% Pt, up to 200 Å, 10% Pt up to 180 Å).     

Etching of MgO crystals in acids: kinetics and mechanism of dissolution

Kinetics of etching of MgO crystals have been studied in H₂SO₄, HNO₃ and HCl. The effects of etching time, acid concentration and temperature on the growth of hillocks, on the selective etch rate and on the rate of overall dissolution are demonstrated. It is observed that etch rates are independent of time, but are determined by the temperature and concentration of the acid. The etch rate-concentration curves show maxima which are characteristic of an acid. The values of activation energy for the processes of dissolution, selective etching and hillock growth and the corresponding frequency factors are computed. It is established that the process of dissolution in concentrated H₂SO₄ is diffusion controlled, while in H₂SO₄ with concentrations below 18 N and in HNO₃ and HCl it is reaction rate controlled. The pre-exponential factor is found to be a function of acid concentration. The results are discussed from the standpoint of chemistry. A comment on the data published on MgO by previous workers is made.     

On improving the track development properties of soda glass detectors

A detailed study of track development properties of soda glass detectors is carried out by varying the etching parameters, i.e. etching time, etchant temperature and the composition of the etching solution. The etch product layer (EPL) which mainly limits the sensitivity of these detectors is dissolved chemically as soon as it is formed. The largest observable etch pit diameter by the use of this method is almost double the one obtained using HF. Room temperature (39 ± 1)°C appears to be the optimum temperature for etching of soda glasses in this solution. The spectrometric response of soda glasses to fission fragments of ²⁵²Cf has also been studied, using this solution. The energy resolution of soda glass detectors is seen to improve and become comparable with the energy resolution of phosphate glass detectors.     

Nanoscale etching and flattening of metals with ozone water

Etchants used for metal etching are generally harmful to the environment. We propose an environmentally friendly method that uses ozone water to etch metals. We measured the dependencies of ozone water etching on the temperature and ozone concentration for several metals and evaluated the surface roughness of the etched surfaces. The etching rate was proportional to the dissolved ozone concentration, and the temperature and the surfaces were smoothed by etching.     

Laser Chemical Etching of Metals in Liquids

Laser-assisted chemical etching of Co, Cr, Cu and Ti w;is investigated using aqueous solutions of phosphoric acid and KOH at different concentrations. Thin metal films on glass substrates and thin foils were etched upon irradiation with a focussed Ar-laser operating at 514 nm and an output power of about 1 W. Static etch rates of the order of 10 pmμmsol;s were obtained at measured background etch rates less than 103 nm/s. The influence of the laser power on the etch rate suggests dominating thermally activated etch reactions. Due to the thermal nature of the etch process etched lines of about two times smaller width than the estimated laser spot diameter could be obtained. Etching of lines in thin Ti films on glass subsu'ates was performed by laser direct writing at speeds of about 1 mm/s and a laser power of about 0.3 W. Cutting of thin Ti foils was obtained at cutting velocities of about 30 um/s and a power of 0.8 W. The width of the etched lines was found to be controlled by laser power and writing speed. Some applications of the method are mask fabrication for lithography, drilling of small holes into metal parts and cutting of thin metal foils. Fabrication of microparts by laser etching of Ti foils is demonstrated.     

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.     

Guided three-dimensional catalyst folding during metal-assisted chemical etching of silicon

In recent years metal-assisted chemical etching (MaCE) of silicon, in which etching is confined to a small region surrounding metal catalyst templates, has emerged as a promising low cost alternative to commonly used three-dimensional (3D) fabrication techniques. We report a new methodology for controllable folding of 2D metal catalyst films into 3D structures using MaCE. This method takes advantage of selective patterning of the catalyst layer into regions with mismatched characteristic dimensions, resulting in uneven etching rates along the notched boundary lines that produce hinged 2D templates for 3D folding. We explore the dynamics of the folding process of the hinged templates, demonstrating that the folding action combines rotational and translational motion of the catalyst template, which yields topologically complex 3D nanostructures with intimately integrated metal and silicon features.     

Controlled core removal from a D-shaped optical fiber

The partial removal of a section of the core from a continuous D-shaped optical fiber is presented. In the core removal process, selective chemical etching is used with hydrofluoric (HF) acid. A 25% HF acid solution removes the cladding material above the core, and a 5% HF acid solution removes the core. A red laser with a wavelength of 670 nm is transmitted through the optical fiber during the etching. The power transmitted through the optical fiber is correlated to the etch depth by scanning electron microscope imaging. The developed process provides a repeatable method to produce an optical fiber with a specific etch depth.     

Wet etching of GaN,AlN, and SiC: a review

The wet etching of GaN, AlN, and SiC is reviewed including conventional etching in aqueous solutions, electrochemical etching in electrolytes and defect-selective chemical etching in molten salts. The mechanism of each etching process is discussed. Etching parameters leading to highly anisotropic etching, dopant-type/bandgap selective etching, defect-selective etching, as well as isotropic etching are discussed. The etch pit shapes and their origins are discussed. The applications of wet etching techniques to characterize crystal polarity and defect density/distribution are reviewed. Additional applications of wet etching for device fabrication, such as producing crystallographic etch profiles, are also reviewed.     

Direct Hydrothermal Synthesis of Carbonaceous Silver Nanocables for Electrocatalytic Applications

This study demonstrates a facile but efficient hydrothermal method for the direct synthesis of both carbonaceous silver (Ag@C core–shell) nanocables and carbonaceous nanotubes under mild conditions (<180 °C). The carbonaceous tubes can be formed by removal of the silver cores via an etching process under temperature control (60–140 °C). The structure and composition are characterized using various advanced microscopic and spectroscopic techniques. The pertinent variables such as temperature, reaction time, and surfactants that can affect the formation and growth of the nanocables and nanotubes are investigated and optimized. It is found that cetyltrimethylammonium bromide plays multiple roles in the formation of Ag@C nanocables and carbonaceous nanotubes including: a shape controller for metallic Ag wires and Ag@C cables, a source of Br^? ions to form insoluble AgBr and then Ag crystals, an etching agent of silver cores to form carbonaceous tubes, and an inducer to refill silver particles into the carbonaceous tubes to form core–shell structures. The formation mechanism of carbonaceous silver nanostructures depending upon temperature is also discussed. Finally, the electrocatalytic performance of the as‐prepared Ag@C nanocables is assessed for the oxidation reduction reaction and found to be very active but much less costly than the commonly used platinum catalysts. The findings should be useful for designing and constructing carbonaceous‐metal nanostructures with potential applications in conductive materials, catalysts, and biosensors.     

Application of level set method in simulation of surface roughness in nanotechnologies

One of the limiting factors in applications of plasma etching in nanotechnologies in general will be the control of plasma induced roughness or perhaps control of surface roughness by plasma etching. In this paper we consider roughening of nanocomposite materials during plasma etching for two etching modes (isotropic and anisotropic) by using a level set method. It was found that the presence of two phases with different etch rates (the ratio of the two etch rates is s and the abundance of one phase is p) affects the evolution of the surface roughness and that the etch rate is higher during the isotropic process as compared to the anisotropic process for all values of s and p. At the same time, in case of isotropic process, the higher s leads to a higher overall etch rate. The obtained results apart from their theoretical relevance, have practical implications for surface treatment of nanocomposite materials.     

GaN etch rate and surface roughness evolution in Cl2/Ar based inductively coupled plasma etching

Cl₂/Ar based inductively coupled plasma (ICP) etching of GaN is investigated using photoresist mask in a consequential restricted domain of pressure < 1.2 Pa and radio frequency (RF) sample power < 100 W, for selective mesa etching. The etch characteristics and root-mean-square (rms) surface roughness are studied as a function of process parameters viz. process pressure, Cl₂ percentage in total flow rate ratio, and RF sample power at a constant ICP power, to achieve moderate GaN etch rate with anisotropic profiles and smooth surface morphology. The etch rate and resultant surface roughness of etched surface increased with pressure mainly due to dominant reactant limited etch regime. The etch rate and surface roughness show strong dependence on RF sample power with the former increasing and the later decreasing with the applied RF sample power up to 80 W. The process etch yield variation with applied RF sample power is also reported. The studied etch parameters result in highly anisotropic mesa structures with Ga rich etched surface.     

Fast release process of metal structure using chemical dry etching of sacrificial Si layer

An ultra-fast removal process of a silicon sacrificial layer for the selective release of a metal structure on a Si substrate was studied, which uses a chemical dry etching method. The chemical dry etching of a Si layer was performed in an NF₃ remote plasma with the direct injection of additive nitric oxide (NO) gas. When the NO gas was injected into the chamber into which F radicals were supplied from a remote plasma source using NF₃ input gas, the silicon layer was removed selectively and the metal structure could be released easily. It was found that the etch rate on the sidewall (up to ≅ 18.7 μm/min for an opening width of 100 μm) and the bottom (up to ≅ 24.5 μm/min for an opening width of 100 μm) depends on the NO/(NO + Ar) gas flow ratio, time duration, and opening width. The developed dry etching process could be used to release a Ni structure with near infinite selectivity in a very short time. The process is well suited for fabricating various devices which require a suspended structure, such as in radio-frequency microelectromechanical system switches, tunable capacitors, high-Q suspended inductors and suspended-gate metal-oxide semiconductor field-effect transistors.     

Controlling the Length of Conical Pores Etched in Ion‐Tracked Poly(ethylene terephthalate) Membranes

An etching procedure that allows for reproducible control of the length of conically shaped pores etched into poly(ethylene terephthalate) (PET) membranes is developed. At the lower etch temperature used (20 °C), the length of the pore is found to be linearly related to etch time. At the higher etch temperature (30 °C) the etch rate is five times faster and the pores quickly propagate through the entire thickness of the PET membrane. Hence, the lower etch temperature is best for controlling the pore length. Pores etched at this temperature are used to prepare arrays of gold cones where the length of the cones is controlled from 1 to 10 µm. The track‐etch rates and the radial‐etch rates at both of the etch temperatures used are also reported.