Reaction kinetics of silicon etching in HF-K2Cr2O7-H2O solution

The reaction kinetics of silicon etching in HF-K₂Cr₂O₇-H₂O solution was studied experimentally. The etch rates were measured with varying HF and K₂Cr₂O₇ concentrations, agitation speed reaction temperature and time. The etch rates of n- and p-Si (100) were both similar. The etchec surfaces consisted mainly of silicon and showed a relatively smooth and planar morphology. At suffi ciently high HF concentration, the etch rate was increased with increasing K₂Cr₂O₇ concentratior due to the increase of hole formation on the silicon surface. However, at low HF concentration the etch rate maintains low value and increases very slowly because of insufficient hole concentratior for etching reaction. The apparent activation energy was about 7.8 kcal/g-mole, and the rate equatior for the silicon etching reaction in HF-K₂Cr₂O₇-H₂O solution was obtained as-r_Si = 600 exp(-3900/T) $$ - r_{Si} = 600{\text{ exp( - 3900/T) C}}_{{\text{K}}_{\text{2}} {\text{Cr}}_{\text{2}} {\text{O}}_{\text{7}} } ^{05} {\text{ C}}_{{\text{HF}}} ^3 $$ C_hk ³ at HF concentrations greater than 8 M.     

Porous Si layers—Preparation, characterization and morphology

Porous silicon layers (PSL) of nano- and micro-structures were prepared by metal-assisted electroless etching of silicon in HF-oxidizing agent aqueous solutions. The effect of oxidizing agent and HF content on the characteristics of the formed porous layers was investigated. A thin Pt film was electroless deposited on p-Si〈1 0 0〉 prior to immersion in the etching solution. The properties and morphology of the PSL formed by this method were investigated by electrochemical impedance spectroscopy (EIS) scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) technique. The characteristics of the PSL were found to be affected by the constituents of the etching medium and also, the etching time. Potassium bromate (KBrO₃), potassium iodate (KIO₃), and potassium dichromate (K₂Cr₂O₇) have been used as oxidizing agents. Pt-assisted etching of p-Si for 1 h in an etching solution consisting of 22.0 M HF and 0.05 M of KBrO₃, results in the formation of nano- and micro-pores on the Si surface. The use of 0.05 M KIO₃ or K₂Cr₂O₇ as oxidizing agent has led to the formation of a deposit on the silicon surface. At relatively higher concentration [>0.05 M] of K₂Cr₂O₇ the surface deposit becomes clear and was found to consist of an insoluble passive solid-phase of K₂SiF₆ which increases the film impedance and blocks the porous structure formation. The use of higher concentration [>22 M] of HF in the etching electrolyte is accompanied by an increase in the dissolution rate of the insoluble K₂SiF₆ layer and a decrease in the PSL passivity. The experimental impedance data were fitted to theoretical data according to a proposed equivalent circuit model which accounts for the mechanism of the porous film formation at the Si/electrolyte interface.     

Metal-assisted chemical etching of silicon in HF-H2O2

Metal-assisted etching of silicon in HF/H₂O₂/H₂O solutions with Ag nanoparticles as catalyst agents was investigated. SEM observations and etch rate measurements were carried out as a function of the etching solution composition. Depending on the relative amount of HF and H₂O₂, different regimes of dissolution take place and a strong similarity with the etching in HF-HNO₃ solution is evidenced, for the first time. Formation of meso- and macroporous Si, etched craters and polished Si are observed as the HF/H₂O₂ ratio decreases. The dissolution mechanisms are discussed on the basis of a localized hole injection from the Ag nanoparticles into Si and in terms of the well known chemistry of Si dissolution in HF-based chemical and electrochemical systems. At high HF/H₂O₂ ratio, there is no formation of oxide at the surface. Hole injection and Si dissolution occur at the level of the Ag nanoparticle only, resulting in the formation of meso and macropores depending on the Ag nanoparticle size. At low HF/H₂O₂ ratio, the Si surface is oxidized, the injected holes are homogeneously distributed and thus polishing occurs. There is an intermediate range of composition in which injected holes diffuse away from the Ag nanoparticles and cone-shaped macropores, several tens of nm in diameter are formed.     

Template-free fabrication of silicon micropillar/nanowire composite structure by one-step etching

A template-free fabrication method for silicon nanostructures, such as silicon micropillar (MP)/nanowire (NW) composite structure is presented. Utilizing an improved metal-assisted electroless etching (MAEE) of silicon in KMnO₄/AgNO₃/HF solution and silicon composite nanostructure of the long MPs erected in the short NWs arrays were generated on the silicon substrate. The morphology evolution of the MP/NW composite nanostructure and the role of self-growing K₂SiF₆ particles as the templates during the MAEE process were investigated in detail. Meanwhile, a fabrication mechanism based on the etching of silver nanoparticles (catalyzed) and the masking of K₂SiF₆ particles is proposed, which gives guidance for fabricating different silicon nanostructures, such as NW and MP arrays. This one-step method provides a simple and cost-effective way to fabricate silicon nanostructures.     

Effect of fluorine chemistry in the remote plasma enhanced chemical vapor deposition of silicon films from Si2H6-SiF4-H2

SiF₄ was added into Si₂H₆-H₂ to deposit polycrystalline silicon films at low temperatures around 400°C in a remote plasma enhanced chemical vapor deposition reactor. It was found out that the fluorine chemistry obtained from SiF₄ addition had an influence on the chemical composition, crystallinity, and silicon dangling bond density of the film. The fluorine chemistry reduced the amount of hydrogen and oxygen incorporated into the film and also suppressed the formation of powders in the gas phase, which helped the crystallization at low temperatures. Effect of SiF₄ concentration as well as the deposition temperature was also significant.     

The role of silanol groups on the reaction of SiO2 and anhydrous hydrogen fluoride gas

It is essential to etch SiO₂ for producing silica glass components, semiconductor devices, and so on. Although wet-etching with hydrogen fluoride (HF) solutions is usually employed for this purpose, it faces a drawback that microstructures stick during the drying of the solution. To overcome this problem, we have developed a dry-etching technique with gaseous HF at high temperatures. In the present study, an interesting phenomenon was found that silicon thermal oxides were much less etched than vitreous silica by gaseous HF. Such difference had not been found in wet- or humid HF gas etching. Because their bulk chemical formulae are the same (SiO₂), it was suggested that the surface species affected the reaction rate. In fact, preprocessing with water vapor plasma remarkably increased the etching rate on the thermal oxides layer, and vacuum heating almost completely suppressed the reaction on the vitreous silica and the plasma-treated thermal oxides. These results indicate that the surface silanol groups enhance the reaction between SiO₂ and gaseous HF. Based on the results, a model of chain reaction for SiO₂ and gaseous HF was proposed, where the surface silanol groups act as the reaction center.     

Synthesis of K2SiF6:Mn4+ Phosphor from SiO2 Powders via Redox Reaction in HF/KMnO4 Solution and Their Application in Warm‐White LED

In this article, we propose a facile method for synthesis of K₂SiF₆:Mn⁴⁺ phosphor and discuss its promising application in warm‐white light emitting diodes (LED). The K₂SiF₆:Mn⁴⁺ was synthesized from SiO₂ powders through redox reaction in HF/KMnO₄ solution. The optical properties of LEDs containing different ratios of K₂SiF₆:Mn⁴⁺ phosphor and commercial Ce³⁺‐doped garnets (YAG‐40) yellow–green phosphor were studied. A warm‐white LED, with color temperature of 3510 K and color rendering index of 90.9 and efficacy of 81.56 lm/W was demonstrated.     

Cycle of two-step etching process using ICP for diamond MEMS applications

Inductively coupled plasma (ICP) etching was performed to etch diamonds, and a new diamond etching technique was established in order to obtain the high selective etching rate of diamond with respect to the silicon oxide etching mask and the smooth etched surface. Scanning electron microscope (SEM) observations and energy dispersive X-ray (EDX) characterizations of the etched diamond surfaces show that the silicon oxide etching mask particles were re-deposited on the fresh etched diamond surface. This results in un-intentional whisker formation. In order to obtain both the high selectivity and the smooth etched surface, a sequential cycle of ICP etching was, for the first time, applied for diamond. Diamond is etched with O₂ plasma during the first step and, in the second step, silicon oxide was removed using (CF₄ + O₂) plasma. Both the selective etching rate of diamond to silicon oxide was improved and the smooth surface was obtained. By applying the two-step etching process, diamond gene-surgery tips for atomic force microscopy (AFM) have been fabricated.     

MASS TRANSFER EFFECTS ON THE ETCH RATE OF GAAS IN FLOW SYSTEMS

In industrial wet etching reactors, the fluid contacts the substrate surface as a spray of flowing stream, thus introducing mass-transfer resistances to the reaction rate. The etching of gallium arsenide in H₂O₂-NH₄OH-H₂O solutions was studied using an open-channel flow reactor to simulate the industrial conditions. The etch rate was always lower than that obtained under kinetic control, and the dependence of etch rate on H₂O₂ concentration shifted closer to first order. From the calculation of the ratio of rate constant to mass-transfer coefficient, the reaction-rate and mass-transfer resistances were both significant in this system. When the mass-transfer coefficient was calculated from equations for flow past a flat plate, the prediction of etch rate was good, particularly when the starting length for velocity boundary layer development ahead of concentration boundary layer development was taken into account. Another approach for the calculation of mass-transfer coefficient, based on the assumptions for flow between parallel plates, best represented the relative insensitivity of etch rate to fluid velocity.     

Nanofabrication on monocrystalline silicon through friction-induced selective etching of Si3N4 mask

A new fabrication method is proposed to produce nanostructures on monocrystalline silicon based on the friction-induced selective etching of its Si₃N₄ mask. With low-pressure chemical vapor deposition (LPCVD) Si₃N₄ film as etching mask on Si(100) surface, the fabrication can be realized by nanoscratching on the Si₃N₄ mask and post-etching in hydrofluoric acid (HF) and potassium hydroxide (KOH) solution in sequence. Scanning Auger nanoprobe analysis indicated that the HF solution could selectively etch the scratched Si₃N₄ mask and then provide the gap for post-etching of silicon substrate in KOH solution. Experimental results suggested that the fabrication depth increased with the increase of the scratching load or KOH etching period. Because of the excellent masking ability of the Si₃N₄ film, the maximum fabrication depth of nanostructure on silicon can reach several microns. Compared to the traditional friction-induced selective etching technique, the present method can fabricate structures with lesser damage and deeper depths. Since the proposed method has been demonstrated to be a less destructive and flexible way to fabricate a large-area texture structure, it will provide new opportunities for Si-based nanofabrication.     

Fabrication and photocatalytic properties of silicon nanowires by metal-assisted chemical etching: effect of H2O2 concentration

In the current study, monocrystalline silicon nanowire arrays (SiNWs) were prepared through a metal-assisted chemical etching method of silicon wafers in an etching solution composed of HF and H₂O₂. Photoelectric properties of the monocrystalline SiNWs are improved greatly with the formation of the nanostructure on the silicon wafers. By controlling the hydrogen peroxide concentration in the etching solution, SiNWs with different morphologies and surface characteristics are obtained. A reasonable mechanism of the etching process was proposed. Photocatalytic experiment shows that SiNWs prepared by 20% H₂O₂ etching solution exhibit the best activity in the decomposition of the target organic pollutant, Rhodamine B (RhB), under Xe arc lamp irradiation for its appropriate Si nanowire density with the effect of Si content and contact area of photocatalyst and RhB optimized.     

The influence of oxidizing agents on etching and passivation of silicon in KOH solution

The effect of oxidizing agents (MnO₄⁻ and CrO₄²⁻) on the electrochemistry, etching kinetics and etching morphology of p-type silicon in KOH solution has been studied. Electrochemical results show that reduction of MnO₄⁻ occurs via hole injection into the valence band of oxide-covered silicon, while the rate of cathodic reduction of CrO₄²⁻ is very low. The current density measured with MnO₄⁻ at negative potential drops to a low value when the surface oxide is removed and chemical etching starts. It is suggested that during etching, a surface intermediate is formed which can interact chemically with the oxidizing agents. This is confirmed by optical absorption measurements. At potentials positive with respect to the open circuit potential both oxidizing agents modify the formation of surface oxide. In addition, both oxidizing agents influence the surface morphology by ‘preventing’ pyramid formation. These results are discussed in terms of a model previously suggested for chemical etching of silicon in alkaline solutions.     

Studies on metal/benzocyclobutene (BCB) interface and adhesion

Interfacial characteristics such as chemical reaction, metal diffusion, and morphology were investigated for Cu/BCB, Cr/BCB and Ti/BCB structures. Using Auger and XPS depth profiling, the formation of titanium carbide and chromium oxide was confirmed at the metal/BCB interface. Annealing at 250°C for extended periods resulted in the diffusion of Cu, Cr and Ti into the BCB and subsequent formation of Cu-Si, CrSi₂ and Ti-Si compound precipitates. The reaction is a thermal diffusion controlled process which is dependent on time and temperature. Ar sputtering treatment of BCB film before metallization was found to roughen the surface, resulting in metal spikes which penetrate into the roughened BCB film. However, the peel strength of metals on BCB was only about 177 g cm^_1presumably due to the brittleness of the BCB film. The etch rates of the BCB film in a reactive ion etcher (RIE) and a plasma etcher were measured using Ar, O₂, O₂ + CF₄, and O₂ + SF₆ gas mixtures. Faster etch rates were obtained when CF₄ and SF₆ were added to oxygen, since the presence of atomic fluorine enhances the etch rate of organics, while also etching Si and SiO₂ formed by exposure of Si-containing BCB film to oxygen gas. Surface compositional changes on the BCB film were observed by XPS after plasma modification. Pure O₂ and O₂ + CF₄ plasmas oxidized the carbo-siloxane linkage (C_—Si_—O) of the BCB, resulting in the formation of SiO₂ on the surface. The O₂ + SF₆ plasma, however, did not produce the surface SiO₂, because of its faster Si and SiO₂ etch rates.     

Hydrophobic surface modification toward highly stable K2SiF6:Mn4+ phosphor for white light-emitting diodes

K₂SiF₆:Mn⁴⁺ phosphor is well known for its excellent red emission performance which is vital for improving the color rendering of white light-emitting diodes. However, the poor moisture resistance limits its application in optical devices. In this paper, K₂SiF₆:Mn⁴⁺ phosphor is coated with an inorganic hydrophobic protective layer to obtain good moisture resistance. Chemical vapor deposition method was used to decompose acetylene at high temperature, and the generated nanoscale carbon layer worked as a hydrophobic protective coating on the surface of the phosphor. Microstructure, compositions and properties of the synthesized K₂SiF₆:Mn⁴⁺@C phosphor were investigated in detail. It is found that most of the deposited carbon is coated on the surface of phosphor crystals in amorphous state. The carbon atoms are bonded with the fluorine element in K₂SiF₆:Mn⁴⁺ phosphor, forming carbon-fluorine (C–F) covalent bonds. The moisture resistance of K₂SiF₆:Mn⁴⁺@C phosphor is improved owing to the protection of the hydrophobic carbon. The relative emission intensity of K₂SiF₆:Mn⁴⁺@C phosphor could maintain 73% of the initial luminous intensity after immersing in the aqueous solution at room temperature for 8 h, whereas K₂SiF₆:Mn⁴⁺ phosphor without carbon coating was only 0.7% remaining of the initial value under the same conditions.     

Etch rate studies of base catalyzed hydrolysis of polyimide film. II

The influence of a potassium carbonate (K₂CO₃) additive on the base catalyzed hydrolysis of polyimide (Kapton?) film in aqueous potassium hydroxide (KOH) was determined experimentally. The etch rate is significantly greater for KOH/K₂CO₃ solutions compared with solutions composed of KOH only. The experimental order of the reaction with respect to the KOH concentration was found to be 1.5. In addition, the rate was found to increase linearly with respect to the K₂CO₃ concentration at a fixed KOH concentration. Visual observations of a thinner gel layer on the surface of the film, combined with increased solubility of the etch by-products in KOH/K₂CO₃ relative to KOH, help to explain the difference in etch rate. It appears the ability of K₂CO₃ to enhance the solubility of the hydrolyzed polyimide (polyamic acid salt) results in faster etch rates.     

Comparative analysis of barium titanate thin films dry etching using inductively coupled plasmas by different fluorine-based mixture gas

In this work, the inductively coupled plasma etching technique was applied to etch the barium titanate thin film. A comparative study of etch characteristics of the barium titanate thin film has been investigated in fluorine-based (CF₄/O₂, C₄F₈/O₂ and SF₆/O₂) plasmas. The etch rates were measured using focused ion beam in order to ensure the accuracy of measurement. The surface morphology of etched barium titanate thin film was characterized by atomic force microscope. The chemical state of the etched surfaces was investigated by X-ray photoelectron spectroscopy. According to the experimental result, we monitored that a higher barium titanate thin film etch rate was achieved with SF₆/O₂ due to minimum amount of necessary ion energy and its higher volatility of etching byproducts as compared with CF₄/O₂ and C₄F₈/O₂. Low-volatile C-F compound etching byproducts from C₄F₈/O₂ were observed on the etched surface and resulted in the reduction of etch rate. As a result, the barium titanate films can be effectively etched by the plasma with the composition of SF₆/O₂, which has an etch rate of over than 46.7 nm/min at RF power/inductively coupled plasma (ICP) power of 150/1,000 W under gas pressure of 7.5 mTorr with a better surface morphology.     

Investigation of mask selectivities and diamond etching using microwave plasma-assisted etching

Diamond etching is characterized using a microwave ECR plasma reactor with regard to etch rate selectivity, surface morphology, and feature size. Etching is performed on diamond substrates using a variety of etch mask materials including aluminum, titanium, gold, silicon dioxide and silicon nitride. The etch feed gases are combinations of oxygen, sulfur hexafluoride and argon. Aluminum masks provided the highest selectivity ratio of diamond etch rate to mask etch rate, both with and without SF₆ in the oxygen/argon feedgas. Selectivity was not found to be dependent on mask feature size. Gold masks produced the least degree of micromasking.     

Formation of carbonaceous deposits on metallic Ni from methane-water vapor mixtures

The influence of water vapor on the formation of carbonaceous deposits formed in the course of CH₄-H₂O reaction on metallic Ni was studied at 1000 K. It was found that a very low concentration of water vapor (x_H2O = 0.05) increased the quantity of carbon deposits on the metallic Ni-surface. Only with higher contents of water in the reaction gas mixture an inhibitory effect of this component was established. The reaction rate of C-formation varied with contact time showed a minimum. The reaction mechanism of C-deposition on metallic Ni-surface in the course of CH₄-H₂O reaction is proposed.     

A Facile Route to BaSiF6:Mn4+ Phosphor with Intense Red Emission and Its Humidity Stability

Red phosphor BaSiF₆:Mn⁴⁺ has been synthesized by a hydrothermal method at 120°C for 24 h, in which either Si or SiO₂ is used as silicon source. HF as weak acid performs a complex agent for the formation of anion groups [SiF₆]^2? and [MnF₆]^2?. The luminescence properties of undoped BaSiF₆ have been firstly observed. The dependence of luminescence intensities of BaSiF₆:Mn⁴⁺ on the concentrations of HF and KMnO₄ in precursory solution has been investigated. The as‐prepared BaSiF₆:Mn⁴⁺ exhibits high chemical stability even in deionized water.     

Domain structure of Bi4Ti3O12 ceramics revealed by chemical etching

Chemical etching of bismuth titanate (Bi₄Ti₃O₁₂) ceramics was developed using a mixture of different agents based on HF, NH₄F and H₂O. Thermal etching modifies the surface morphology and this give place to rounded plate-like grains. Suitable etching of Bi₄Ti₃O₁₂ samples could be however achieved with chemical agents, because it preserves the original shape of the platelets. Furthermore with this kind of etching two relevant features of the microstructure arise in scanning electron microscopy (SEM) micrographs: the domain structure of the etched grains and the presence of square-shaped holes inside the platelets.